Revisiting five decades of 234Th data: a comprehensive global oceanic compilation

Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License. Revisiting five decades of 234Th data: a comprehensive global oceanic compilation Elena Ceballos-Romero1,2, Ken O. Buesseler2, and María Villa-Alfageme1 1Department of Applied Physics II, University of Sevilla, ETSIE, Av. Reina Mercedes 4, 41012 Seville, Spain 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Clark Building 447, Woods Hole, MA 02543-1541, USA Correspondence: Elena Ceballos-Romero ([email protected]) Received: 29 July 2021 – Discussion started: 1 October 2021 Revised: 29 March 2022 – Accepted: 18 April 2022 – Published: 9 June 2022 Abstract. We present here a global oceanic compilation of 234Th measurements that collects results from researchers and laboratories over a period exceeding 50 years. The origin of the 234Th sampling in the ocean goes back to 1967, when Bhat et al. (1969) initially studied 234Th distribution relative to its parent 238U in the Indian Ocean. However, it was the seminal work of Buesseler et al. (1992) – which proposed an empirical method to estimate export fluxes from 234Th distributions – that drove the extensive use of the 234Th–238U radioactive pair to evaluate the organic carbon export out of the surface ocean by means of the biological carbon pump. Since then, a large number of 234Th depth profiles have been collected using a variety of sampling instruments and strategies that have changed during the past 50 years. The present compilation is made of a total 223 data sets: 214 from studies published in either articles in refereed journals, PhD theses, or repositories, as well as 9 unpublished data sets. The data were compiled from over 5000 locations spanning all the oceans for total 234Th profiles, dissolved and particulate 234Th activity concentrations (in dpm L−1), and POC:234Th ratios (in µmoldpm−1) from both sediment traps and filtration methods. A total of 379 oceanographic expeditions and more than 56600 234Th data points have been gathered in a single open-access, long-term, and dynamic repository. This paper introduces the dataset along with informative and descriptive graphics. Appropriate metadata have been compiled, including geographic location, date, and sample depth, among others. When available, we also include water temperature, salinity, 238U data (over 18 200 data points), and particulate organic nitrogen data. Data source and method information (including 238U and 234Th) is also detailed along with valuable information for future data analysis such as bloom stage and steady-/non-steady-state conditions at the sampling moment. The data are archived on the PANGAEA repository, with the dataset DOI https://doi.org/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). This provides a valuable resource to better understand and quantify how the contemporary oceanic carbon uptake functions and how it will change in future. 1 Introduction For several decades, radioactive tracers have been used to gain a better understanding of different oceanographic processes. In the context of the biological carbon pump (BCP) (Eppley and Peterson, 1979; Volk and Hoffert, 1985), radionuclides such as 210Po and thorium isotopes are extensively used to study the various physical, chemical, or biological processes involved in the particle export and flux attenuation in the oceans (Cochran and Masqué, 2003). Radionuclides are characterized by a unique property: their half-lives, which account for the time it takes for one-half of the atoms of a radioactive element to undergo radioactive decay and thus transform into a different isotope. Halflives are not affected by temperature, physical or chemical state, or any other influence. As a result, the concentration Published by Copernicus Publications. 2640 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data of naturally occurring radioactive elements varies over time at well-characterized decay and production rates. Observations of radionuclides’ distributions in the water column, in time and space, provide valuable insights into the presence and rates of ocean processes on spatial scales from local to basin-wide and timescales of days to millennia depending on the radionuclide employed. The naturally occurring radioisotope 234Th has been commonly used to understand natural aquatic processes in four major areas: particle cycling, horizontal transport, sediment dynamics, and vertical transport (Waples et al., 2006). 234Th has been collected by a variety of sampling procedures since its initial use by Bhat et al. (1969). 234Th chemistry dictates that it is adsorbed onto particle surfaces and is effectively scavenged from the dissolved water phase. Hence, when biological activity is high, 234Th is removed from the surface ocean and transported downward by the sinking particles, thereby generating a deficit relative to its soluble or “conservative” parent 238U. This deficit can be used to calculate the downward flux of 234Th. An excess in 234Th activity – i.e. a daughter concentration higher than the parent one – is attributed to fragmentation processes that result in the conversion of sinking to non-sinking particles, generically termed “remineralization” (Maiti et al., 2010). Due to its short half-life of T1/2=24.1 d (decay constant: λ=ln2/T1/2=0.02876 d−1; mean lifetime: τ=34.8 d) and its particle reactivity in seawater, it is suitable to trace processes occurring in the upper ocean on timescales from days to months (Rutgers van der Loeff et al., 2006) or even shorter when there is high scavenging by particles (Turnewitsch et al., 2008) (see Sect. 4.2). 234Th has been an indispensable tool in many oceanographic field expeditions. The most widespread application of the 234Th approach is to estimate the gravitational settling of carbon as particulate organic carbon (POC) out of the surface layer, which results in a downward flux of POC (see, e.g., review by Le Moigne et al., 2013a, and references therein). To a lesser extent, this radionuclide is also used to estimate the downward flux of other elements to the deep ocean, such as particulate inorganic carbon (e.g., Le Moigne et al., 2013b; Wei et al., 2011), biogenic silica (e.g., Buesseler et al., 2005; Lemaitre et al., 2016; Le Moigne et al., 2013b), particulate organic nitrogen (PON) (e.g., Buesseler et al., 1992; Charette and Buesseler, 2000; Murray et al., 1996), or trace metals fluxes (e.g., Black et al., 2018; Lemaitre et al., 2016, 2020; Weinstein and Moran, 2005). It was in the 1990s that an increasing number of studies for 234Th took place. This increase in use is in part due to a variety of new sampling instruments and strategies that have changed over the years. In 2006, a special issue entitled “Future Applications of 234Th in Aquatic Ecosystems” (FATE; https://www.sciencedirect.com/journal/ marine-chemistry/vol/100/issue/3, last access: 1 June 2022) was published in Marine Chemistry with the purpose of thoroughly reviewing the use of 234Th in aquatic systems. Papers included reviews of the applications and future uses of this radiotracer (Benitez-Nelson and Moore, 2006; Waples et al., 2006), discussions on the techniques and methodologies used for 234Th analyses (Rutgers van der Loeff et al., 2006), the impact of POC:234Th ratios and their sampling methodology on POC flux estimates (Buesseler et al., 2006), and 234Th sorption and export models in the water column (Savoye et al., 2006), among other topics. As one of the most actively used tracers in oceanography, Waples et al. (2006) already reported 237 papers dealing with 234Th published in refereed journals. However, after five decades of extensive use, a unique repository of 234Th measurements has never been compiled, and 234Th data remain scattered when not belonging to major sampling programs (see Sect. 4 for details). Therefore, it is valuable to bring together all existing 234Th data, along with appropriate metadata, in one repository to facilitate further use and analysis. Previous efforts compiling 234Th-based data have been created to access 234Th-derived POC fluxes (see Le Moigne et al., 2013a), total 234Th activity from the surface to 1000 m depth (Le Gland et al., 2019), and, more recently, POC:234Th ratios (see Puigcorbé et al., 2020, and https://doi.org/10.1594/PANGAEA.911424; Puigcorbé, 2019). In contrast, we have compiled the complete results of 234Th measurements in seawater and particles at every depth, location, and time of sampling. The compilation can be found at https://doi.org/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). This article is the report of the compilation, a unique dataset to better understand and quantify how the contemporary oceanic carbon uptake functions and how it will change in future. The goal of this effort is to serve as a basis for an openaccess, long-term, and dynamic oceanic repository of 234Th measurements and valuable metadata to be used in an accessible, easily findable, and inter-operable way that grows from now on from the contribution of other authors involved in 234Th sampling. Moreover, given the great amount of metadata and parameters compiled, the compilation offers multiple ways to use 234Th, even opening the possibility of exploring new applications. For that reason, we have chosen not to compile the derived parameters reported by other authors, such as 234Th downward fluxes or 234Th-derived POC fluxes, but rather provide the data necessary for others to make such analyses, open to the criteria, modeling, and interpretation chosen by each researcher. 2 Data 2.1 Data organization We have gathered data sets consisting of 234Th measurements in oceanic waters sampled between 1967 and 2018. The compilation includes a total of 56 631 data points for 234Th activity concentrations (in dpmL−1, referred to as simply 234Th concentrations from now on), distributed as folEarth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2641 lows: (i) 21 457 for total, (ii) 6591 for dissolved, (iii) 13 977 for particulate 234Th measurements, and (iv) 10856 and (v) 3750 for POC:234Th and PON:234Th ratios respectively (in µmoldpm−1). Note that when total carbon is provided instead of POC:234Th ratios (such as in, e.g., Owens et al., 2015) this is indicated in the “methods” section of the “metadata” sheet (see Sect. 2.1.1). Additionally, 238U concentrations (18 256 data) and POC (7651 data) and PON (1740 data) concentrations (in µmolL−1) are also reported. POC and PON concentrations are referred to as “CHN” (carbon– hydrogen–nitrogen) in the compilation. We are aware that CHN is not the only analytical method that can be used to determine POC and PON concentrations. An elemental analyzer – isotope ratio mass spectrometer (EA-IRMS) is also widely used for this purpose (see, e.g., studies by Lemaitre et al., 2018, and Planchon et al., 2015), but we have used this notation for the sake of simplicity. Please refer to the original source for the analytical method used to measure these data. Temperature and salinity values are also included when possible, making a total of 5652 values compiled for temperature and 12 721 values for salinity. When fields are missing for a given record, the data are entered as “−999”. The data have been extracted from a total of 219 different studies published in refereed journals between 1969 and 2020, 5 PhD dissertations, 9 data sets accessible in four different public repositories, and 9 unpublished ones directly provided by authors (listed in Table 1) spanning all oceanic regions (Fig. 1a) and compiled in a total of 223 data sets. The data repositories include the following: (1) BCO-DMO®(Biological and Chemical Oceanography Data Management Office; https://www.bco-dmo.org/, last access: 1 June 2022), (2) DARWIN®(Data and Sample Research System for Whole Cruise Information in JAMSTEC; https://www.jamstec.go.jp/e/database/, last access: 1 June 2022), (3) EDI Data Portal®(Environmental Data initiative; https://portal.edirepository.org/nis/home.jsp, last access: 1 June 2022), and (4) PANGAEA®(https://www. pangaea.de/, last access 1 June 2022). Additionally, many of the data corresponding to published GEOTRACES ®(https: //www.geotraces.org/, last access: 1 June 2022) cruises can be found on the GEOTRACES website (see the most recent version of its intermediate data product released in 2021; GEOTRACES Intermediate Data Product Group, 2021). Each data set is univocally identified with a unique integer record ID number (denoted as “Reference_USE”) and consists of two tables: (i) the “metadata” and (ii) the “data”. 2.1.1 Metadata sheet The metadata table is a list of the data’s origin in its broadest sense at a glance. It contains information structured in six categories: (1) “REFERENCE_USE”, (2) “INFO”, (3) “DATA”, (4) “METHODS”, (5) “ADDITIONAL_DATA”, and (6) “DATA_SOURCE”. The full list of metadata included in each dataset and a brief description of the table fields can be found in the Supplement (see Table S1). There are data from 379 cruises, covering 5134 locations spanning across all oceanic regions (Fig. 1a). Some stations were part of cruise transects, whereas others were part of small-scale surveys or reoccupation of the same location at different seasons and years. In all cases, sampling region and period – including bloom stage at the sampling moment when indicated by the authors – are given as metadata as part of the “INFO” section, in which a total of 17 fields are reported (see Table S1). Information such as the project name, when sampling took place within an observational program, cruise name, leg details, research vessel, and chief scientist is indicated. A summary of the sampling period, the maximum and minimum latitude and longitude of the region surveyed, and the maximum depth sampled for 234Th is also provided. When available, for the sake of a better interpretation of the data for the accurate POC flux assessment, the stage of the bloom has also been included (categorized as “bloom”, “pre-bloom”, “post-bloom”, and “no bloom”), as recommended by, for example, Ceballos-Romero et al. (2016, 2018). Note that we do not assess the bloom stage, but instead we include the information as indicated by the original authors when provided. To distinguish between the periods before and after the peak in primary production, we identify “development of the bloom” and similar expressions as pre-bloom stage and “decline of the bloom” and similar as post-bloom phase. Only when bloom or non-bloom conditions are stated by authors have we assigned these phases. When none of these stages are referenced, no information is included (noted as “−999”). We acknowledge there could be issues on the way the different authors have decided if the conditions were non-bloom, pre-bloom, bloom, or post-bloom over the different years that are out of the scope of this compilation and were not evaluated. We plan to address this gap in the compilation in future versions of it (see Sect. 5). The “DATA” section provides information of the data set contents at a glance with YES/NO indicators to the basic data of 238U, 234Th (total, dissolved, and particulate phases), and POC(PON):234Th ratios. The fraction size details and the number of total stations and samples are also reported. A total of 13 parameters are detailed in this section. The “METHODS” section is intended to provide useful interpretive information regarding how the sampling and/or measurements were accomplished at a glance. It provides basic information about (i) 238U determination: whether it was directly measured or salinity-derived, in which case the salinity relationship employed is specified; (ii) total 234Th sampling and radiochemical purification methods; sampling methods for the (iii) dissolved and (iv) particulate 234Th phases; and (v) the modeling approach followed when 234Th data were used to estimate POC fluxes (i.e., the assumption of steady- (SS) or non-steady-state (NSS) conditions). A final space for comments of any kind is included in this section. https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2642 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data Table 1. Summary of the studies included in the compilation in chronological order of publication. Four types are distinguished as data source type: publications in refereed journals (J), publication in repository (R), PhD thesis (T), and personal communication (P) for unpublished data sets released in this compilation. Note that in some cases, published data have been reported in different publications, for which all the publications are indicated. Basic information of sampling details (i.e., region and period) is also included. Additionally, availability of seven different categories of 234Th data are indicated with an “×” as follows: (1) total 234Th, (2) dissolved 234Th, (3) particulate 234Th, (4) POC:234Th ratios from filtration methods, (5) PON:234Th ratios from filtration methods, (6) POC:234Th ratios from sediment traps, and (7) 234Th underway sampling. Data source Sampling details Data available Type Reference Sampling region Sampling period 1 2 3 4 5 6 7 Era 1 (1969–1991: the beginning of 234Th as tracer of particle scavenging) J Bhat et al. (1969) Indian Ocean 1 Jan–31 Dec 1967 × J Matsumoto (1975) Pacific Ocean 31 Dec 1969–1 Jan 1970 × J Krishnaswami et al. (1976) Pacific Ocean 3 Aug–9 Nov 1973 × J Knauss et al. (1978) Pacific Ocean 1 Jan–31 Dec 1973 × J Santschi et al. (1979, 1980) Narragansett Bay 16 Jan 1978–21 Feb 1979 × × J Minagawa and Tsunogai (1980) Pacific Ocean 9 Sep 1974–13 Apr 1975 × × J Kaufman et al. (1981) Atlantic Ocean 29 Apr–8 May 1977 × J Tanaka et al. (1983) Pacific Ocean 25 Apr 1979–26 Feb 1980 × J McKee et al. (1984) Yangtze River China 1 Nov–30 Dec 1981 × × × J Coale and Bruland (1985) Pacific Ocean 14 Nov 1978–1 Sep 1980 × × × J Bruland and Coale (1986) Pacific Ocean 1–9 Sep 1980 × × J McKee et al. (1986) Amazon River mouth 8–10 Jun 1983 × J Tsunogai et al. (1986) Pacific Ocean 19 May 1981–15 Mar 1982 × J Coale and Bruland (1987) Pacific Ocean 1 Oct 1981–31 Aug 1983 × × J Huh and Beasley (1987) Pacific Ocean 1–30 Oct 1985 × × J Kershaw and Young (1988) Atlantic Ocean 1–30 Nov 1985 × × J Bacon and Rutgers van der Loeff (1989) Pacific Ocean 3 Jul 1979–17 Aug 1980 × × × J Dominik et al. (1989) Lake Geneva 21 Apr–29 Oct 1986 × × J Moran and Moore (1989) Atlantic Ocean 6–11 Sep 1988 × × J Murray et al. (1989) Pacific Ocean 6–23 Jun 1986 × × J Schmidt et al. (1990) Mediterranean Sea 8 Apr 1986–6 Apr 1988 × × J Lee et al. (1991) Pacific Ocean 24 Jun–8 Oct 1989 × J Rutgers van der Loeff and Berger (1991) Southern Ocean 7 Nov–9 Dec 1987 × × × J Wei and Murray (1991) Atlantic Ocean 30 Jun 1980–1 Jun 1988 × × Era 2 (1992–2000: introduction of the empirical determination of POC export from 234Th profiles) J Baskaran et al. (1992) Atlantic Ocean 1 Oct 1989–30 Jun 1991 × × × J Buesseler et al. (1992), Cochran et al. (1993) Atlantic Ocean 19 Apr–5 Jun 1989 × × × × × × J Moran and Buesseler (1992) Atlantic Ocean 14 May 1991 × × × J Sarin et al. (1992) Indian Ocean 1 Mar–30 Apr 1991 × × J Schmidt et al. (1992) Mediterranean Sea 17 Mar–14 May 1987 × × J Wei and Murray (1992) Dabob Bay 1 Jan–31 Dec 1987 × × × J Baskaran and Santschi (1993) Galveston coast 1 May–31 Dec 1990 × × J Moran and Buesseler (1993) Atlantic Ocean 25 Jun 1991–28 Jul 1992 × × × J Buesseler et al. (1994) Atlantic Ocean 24 Apr 1992–8 May 1992 × × × × J Kuptsov et al. (1994) Arctic Ocean 8 Jan–31 Oct 1993 × × J Sarin et al. (1994a) Indian Ocean 1 Dec–31 Dec 1988 × J Sarin et al. (1994b) Indian Ocean 15 Mar 1991–24 Feb 1992 × J Thunell et al. (1994) Pacific Ocean 7 Jan–2 Jul 1988 × J Buesseler et al. (1995) Pacific Ocean 1 Mar–2 Dec 1992 × × × × J Cochran et al. (1995) Arctic Ocean 19 Jul 1992–11 Aug 1993 × × J Huh and Prahl (1995) Pacific Ocean 7–9 Aug 1993 × × × J Niven et al. (1995) Atlantic Ocean 17 Mar–17 May 1993 × × × J Shimmield et al. (1995) Southern Ocean 7 Dec 1992 × × J Bacon et al. (1996) Pacific Ocean 22 Mar–21 Oct 1992 × × × J Baskaran et al. (1996) Atlantic Ocean 16 Mar–25 Jun 1992 × × × J Murray et al. (1996), Dunne et al. (1997), Murray et al. (2005) Pacific Ocean 7 Feb–13 Sep 1992 × × × × × × J Sarin et al. (1996) Indian Ocean 12 Apr 1994–12 Aug 1995 × × J Guo et al. (1997), Santschi et al. (1995) Atlantic Ocean 1 Jun 1992–31 Jul 1994 × × × J Gustafsson et al. (1997a) Atlantic Ocean 1 Sep 1993–31 May 1994 × × J Gustafsson et al. (1997b) Atlantic Ocean 1 Sep 1993–31 May 1994 × × × J Langone et al. (1997) Southern Ocean 7–13 Dec 1994 × × × J Moran et al. (1997) Arctic Ocean 1 Aug–30 Sep 1994 ××××× J Rutgers van der Loeff et al. (1997), Friedrich and Rutgers van der Loeff (2002) Southern Ocean 2 Oct–22 Nov 1992 ××××× J Buesseler et al. (1998) Indian Ocean 9 Jan–12 Sep 1995 ××××× Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2643 Table 1. Continued. Data source Sampling details Data available Type Reference Sampling region Sampling period 1 2 3 4 5 6 7 J Gustafsson et al. (1998) Atlantic Ocean 26 Sep 1993–22 May 1994 × × J Wei and Hung (1998) Pacific Ocean 1–30 Nov 1991 × × J Kersten et al. (1998) Atlantic Ocean 2 Feb–9 Nov 1994 × × J Charette et al. (1999) Pacific Ocean 1 Feb 1996–28 Feb 1997 × × × × × J Charette and Moran (1999) Atlantic Ocean 15 May–11 Jun 1996 × × × × J Santschi et al. (1999) Atlantic Ocean 17 May 1993–12 Jul 1994 × × × × J Smoak et al. (1999) Pacific Ocean 16 Oct 1993–31 Mar 1996 × J Benitez-Nelson et al. (2000) Atlantic Ocean 1 Mar–31 Aug 1997 × × × J Buesseler et al. (2000) Atlantic Ocean 6 Oct 1997 × × × J Charette and Buesseler (2000) Southern Ocean 9–23 Feb 1999 ××××× J Cochran et al. (2000) Southern Ocean 8 Oct 1996–4 May 1997 × × × × J Gulin (2000) Atlantic Ocean 2 Nov 1998–11 Jun 1999 × × × J Hall et al. (2000) Atlantic Ocean 11–14 Jun 1997 × × × × × J Moran and Smith (2000) Arctic Ocean 1 Aug–30 Sep 1995 ××××× Era 3 (2001–2009: improvements of the 234Th technique) J C. Benitez-Nelson et al. (2001) Pacific Ocean 1 Apr 1999–31 Mar 2000 × × × J Buesseler et al. (2001a) Southern Ocean 24 Oct 1997–15 Mar 1998 ××××× J Cai et al. (2001) Pacific Ocean 1 Apr–1 May 1999 × × × × J Charette et al. (2001) Atlantic Ocean 19 Mar 1995–31 Aug 1997 × × × × × J Dai and Benitez-Nelson (2001) Atlantic Ocean 6 Jul 1996–12 Jul 1997 × × × J Hernes et al. (2001) Pacific Ocean 7 Feb–13 Sep 1992 × × J Kim and Church (2001) Atlantic Ocean 6 Oct 1996–20 Aug 1997 × × × J Porcelli et al. (2001) Atlantic Ocean 1 Jun–31 Jun 1995 × × × J Turnewitsch and Springer (2001) Atlantic Ocean 1 Jul 1997–28 Feb 1998 × × × R Turnewitsch (2001) Indian Ocean 1 Feb–31 Mar 1998 × × J Amiel et al. (2002) Atlantic Ocean 1 May 1998–30 Sep 1999 × × × × × J Cai et al. (2002) Pacific Ocean 15 Nov 1997 × × × × J Coppola et al. (2002) Arctic Ocean 7 Jul 1999 × × × × × J Foster and Shimmield (2002) Atlantic Ocean 19–27 Jun 1999 × × × × J Frignani et al. (2002) Atlantic Ocean 17 Sep 1994–28 Jun 1995 × × × J Guo et al. (2002) Atlantic Ocean 5 Jul 2000 × × × × J Rutgers van der Loeff et al. (2002a) Southern Ocean 30 Dec 1995–18 Jan 1996 × × × × J Rutgers van der Loeff et al. (2002b) Arctic Ocean 26 Jun–11 Aug 1997 × × × J Schmidt et al. (2002a, 2009) Mediterranean Sea 9 Feb 1994–29 May 1995 × × × × × J Schmidt et al. (2002b) Atlantic Ocean 1 Jun 1997–31 Jan 1998 × × × × × J Somayajulu et al. (2002) Bay of Bengal 16–23 Dec 1991 × J Usbeck et al. (2002) Southern Ocean 20 Mar–6 May 1999 × × × J Baskaran et al. (2003) Arctic Ocean 24 Jul–10 Oct 1998 × × × × J Chen et al. (2003) Pacific and Arctic oceans 1–31 Aug 1999 × × × × J Moran et al. (2003) Atlantic Ocean 1–31 Jul 1999 × × × × J Radakovitch et al. (2003) Mediterranean Sea 6 Mar–12 Sep 1997 × × × × R Rutgers van der Loeff and Vöge (2003) Southern Ocean 24 Oct–29 Nov 2000 × × × J Santschi et al. (2003) Atlantic Ocean 1–31 May 2001 × × × × × J Sweeney et al. (2003) Atlantic Ocean 1 Mar 1993–30 Sep 1995 × × J Gustafsson et al. (2004) Atlantic Ocean 17 Jan 1998–15 Aug 2000 × × J Hung et al. (2004) Atlantic Ocean 7 Jul 2000–21 May 2001 × × × × J Kawakami et al. (2004), Kawakami (2009), Yang et al. (2004) Pacific Ocean 12 Nov 1997–4 Jun 2000 × × × J Savoye et al. (2004), Buesseler et al. (2006) Southern Ocean 3 Nov–5 Dec 2001 × × × × J Smith et al. (2004) Atlantic Ocean 15 Jun–15 Jul 1998 × × J Smoak et al. (2004) Atlantic Ocean 15 Nov 1995–30 Apr 1996 × J Trimble et al. (2004) Arctic Ocean 1–31 Aug 2000 × × J Waples et al. (2004) Lake Michigan 15 Feb–18 Oct 1999 × × J Aono et al. (2005) Pacific Ocean 7–31 Jul 2001 × × × × × × J Buesseler et al. (2005) Southern Ocean 30 Jan–20 Feb 2002 × × × J Coppola et al. (2005, 2006) Southern Ocean 18 Jan–15 Feb 1999 × × × × J Ma et al. (2005) Arctic Ocean 7 Jan–30 Sep 2003 × × × J Moran et al. (2005), Lepore and Moran (2007) Arctic Ocean 6 May–26 Aug 2002 ××××× J Trimble and Baskaran (2005) Arctic Ocean 1–31 Aug 2000 ××××× J Cai et al. (2006c) Pacific Ocean 21–28 Feb 2004 × × × J Cai et al. (2006a) Pacific Ocean 4–9 May 2005 × × × × J Gustafsson et al. (2006) Atlantic Ocean 15 Aug 2000–31 Mar 2001 × × J Rodriguez y Baena et al. (2006, 2008) Southern Ocean 17 Nov 2003–18 Jan 2004 × × https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2644 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data Table 1. Continued. Data source Sampling details Data available Type Reference Sampling region Sampling period 1 2 3 4 5 6 7 J Schmidt (2006) Atlantic Ocean 26 Dec 1988–8 Sep 1989 × × × × J Speicher et al. (2006) Mediterranean Sea 16 Mar–2 May 2004 × × × × J Thomalla et al. (2006, 2008) Atlantic Ocean 2–26 May 2004 × × J Giuliani et al. (2007) Mediterranean Sea 16 Mar–7 Apr 1997 × × × × J Hung and Gong (2007) Pacific Ocean 18 Aug 2006 × × × × × J Kawakami and Honda (2007) Pacific Ocean 16 Oct 2002–21 Aug 2004 × × × J Lalande et al. (2007), Lepore et al. (2007), Lepore and Moran (2007) Arctic Ocean 24 May–25 Aug 2004 × × × × × × J Morris et al. (2007) Southern Ocean 10 Nov 2004–14 Jan 2005 × × R Rutgers van der Loeff (2007a) Atlantic Ocean 21 Oct–13 Nov 2005 × × × R Rutgers van der Loeff (2007b) Southern Ocean 16–30 Jan 2006 × × × × J Stewart et al. (2007) Mediterranean Sea 4 Mar–30 Jun 2003 × × × T Thomalla (2007) Atlantic Ocean 17 May–4 Oct 2003 × × × J Amiel and Cochran (2008) Arctic Ocean 1 Sep 2002–31 Jul 2004 × × × × J Buesseler et al. (2008) Atlantic Ocean 24 Jun 2004–25 Aug 2005 × × × × × J Cai et al. (2008) Pacific Ocean 4 Apr–4 May 2004 × × × J Chen et al. (2008) Pacific Ocean 1 Jul 2000–30 Nov 2002 × × × × J Lalande et al. (2008) Arctic Ocean 10 Jul 2003–31 May 2005 × × × × × J Lamborg et al. (2008) Pacific Ocean 23 Jun 2004–9 Aug 2005 × J Lampitt et al. (2008) Atlantic Ocean 12 Jul 2003–6 Jul 2007 × × × J Maiti et al. (2008) Pacific Ocean 10–28 Mar 2005 × × × × J Savoye et al. (2008) Southern Ocean 23 Jan–12 Feb 2005 × × × × J Brew et al. (2009), Stewart et al. (2011) Atlantic Ocean 9 May 2006–24 Mar 2007 × × × × J Buesseler et al. (2009) Pacific Ocean 23 Jun 2004–16 Aug 2005 × × × × × R Charette (2009) Southern Ocean 16 Jan–6 Aug 2006 × J Cochran et al. (2009) Mediterranean Sea 10 Mar 1999–30 Apr 2005 × × × J Lepore et al. (2009) Atlantic Ocean 3 Dec 2004–6 May 2005 × × × × × J Schmidt et al. (2009) Mediterranean Sea 3–24 Sep 2004 × × × × × J Szlosek et al. (2009) Mediterranean Sea 6 Mar 2003–1 May 2005 × × J Wei et al. (2009) Pacific Ocean 6–8 Mar 2006 × × × Era 4 (2010–present: GEOTRACES program and a new way to study the ocean) J Buesseler et al. (2010) Southern Ocean 9 Jan 2009 × × × × J Cai et al. (2010) Arctic Ocean 30 Jul–23 Sep 2007 × × × J Hung and Gong (2010) Pacific Ocean 3–4 Dec 2008 × × J Hung et al. (2010) Atlantic and Pacific oceans 1 Aug 2005–16 Jun 2009 × × × × × J Kawakami et al. (2010) Pacific Ocean 26 Sep–17 Oct 2005 × × × × J Sanders et al. (2010) Atlantic Ocean 24 Jul–23 Aug 2007 × × × × J Yu et al. (2010) Arctic Ocean 30 Jul–12 Sep 2003 × × × × J Evangeliou et al. (2011) Mediterranean Sea 1 Jul 2008–17 Jan 2009 × × × × J Jacquet et al. (2011) Southern Ocean 17 Jan–20 Feb 2007 × × × × J Kim et al. (2011) Pacific Ocean 4 Apr 2007–23 Feb 2008 × × J Martin et al. (2011) Atlantic Ocean 1–21 May 2008 × × × × J Rutgers van der Loeff et al. (2011) Southern Ocean 13 Feb–11 Apr 2008 × × × × J Shaw et al. (2011) Southern Ocean 15 Mar–15 Apr 2009 × × × J Stukel et al. (2011) Pacific Ocean 11 May–5 Jun 2006 × × J Wei et al. (2011) Pacific Ocean 1 Oct 2006–31 Dec 2008 × × × × × J Xu et al. (2011) Atlantic Ocean 30 Apr–5 May 2006 × × × J Gustafsson and Andersson (2012) Arctic Ocean 13–31 Jul 2001 × × × × J Hung et al. (2012) Pacific Ocean 2 Aug–12 Dec 2008 × × R Kawakami (2012) Pacific Ocean 9 Sep 2007–28 Oct 2008 ××××× J Moran et al. (2012) Pacific Ocean 29 Mar–31 Jul 2008 × × × J Yu et al. (2012) Arctic Ocean 4 Aug–8 Sep 2008 × × × × J Zhou et al. (2012) Southern Ocean 24 May–8 Jun 2008 × × × J Baumann et al. (2013) Pacific Ocean 29 Mar 2008–15 Jul 2010 × × × J Evangeliou et al. (2013) Mediterranean Sea 1–10 Jan 2010 × × × × J Haskell et al. (2013) Pacific Ocean 1 Feb 2010–3 Apr 2011 × × × R JGOFS-India2013 Indian Ocean 9–11 Feb 1997 × J Le Moigne et al. (2013b) Atlantic Ocean 13 Jul–8 Aug 2009 × × × T Luo (2013) Pacific Ocean 1–30 Jun 2010 × × × × × J Martin et al. (2013) Southern Ocean 12 Jan–6 Mar 2009 × × × × × × T Owens (2013) Atlantic Ocean 10 Sep–5 Oct 2010 × × × × T Owens (2013) Southern Ocean 30 Dec 2009–7 Feb 2010 × × × × × × T Owens (2013) Southern Ocean 16 Mar–1 May 2010 × × × × × × J Planchon et al. (2013) Southern Ocean 21 Feb–14 Mar 2008 × × × J Schmidt et al. (2013) Atlantic Ocean 1–31 May 2002 × × × × Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2645 Table 1. Continued. Data source Sampling details Data available Type Reference Sampling region Sampling period 1 2 3 4 5 6 7 J Stukel et al. (2013) Pacific Ocean 4 Apr 2007–28 Oct 2008 × × × J Zhou et al. (2013) Pacific Ocean 1–31 Aug 2007 × × × J Le Moigne et al. (2014) Atlantic Ocean 19 Oct 2010–24 Jan 2012 × × × × J Luo et al. (2014), Luo (2013) Pacific Ocean 10 Mar 2009–9 Feb 2011 ××××× T Pabortsava (2014) Atlantic Ocean 7 Feb 2011 × × × × J Cai et al. (2015) Pacific Ocean 18 Jul 2009–24 May 2011 × × × J Estapa et al. (2015) Atlantic Ocean 30 Sep 2011–12 Feb 2012 × × J Kawakami et al. (2015) Pacific Ocean 3 Jun–17 Jul 2006 × × × × J Le Moigne et al. (2015) Arctic Ocean 5 Jun–1 Jul 2012 × × × J Owens et al. (2015) Atlantic Ocean 15 Oct 2010–7 Apr 2011 × × × × J and T Planchon et al. (2015), Lemaitre et al. (2016), Lemaitre (2017) Southern Ocean 20 Oct–18 Nov 2011 × × × × × J Puigcorbé et al. (2015) Pacific Ocean 10 Jul–7 Aug 2008 × × × × × R Roca-Martí, et al. (2015) Southern Ocean 26 Jan–2 Mar 2012 × × × × J Rosengard et al. (2015) Atlantic and Indian oceans 14 Jan–20 Mar 2012 × × × J Stukel et al. (2015, 2016, 2017) Pacific Ocean 22 Apr 2009–23 Jul 2010 × × × J Ceballos-Romero et al. (2016) Atlantic Ocean 1 May–7 Aug 2010 × × × × J Haskell et al. (2016) Pacific Ocean 16 Jan 2013–19 Jun 2014 × × × J Le Moigne et al. (2016) Southern Ocean 13 Jan–5 Feb 2013 × × × J Maiti et al. (2016) Atlantic Ocean 1 Mar 2012–31 Mar 2013 × × J Roca-Martí et al. (2016) Arctic Ocean 3 Aug–8 Oct 2012 × × × × J Turnewitsch et al. (2016) Atlantic Ocean 28 Sep–19 Oct 2009 × × × J Zhou et al. (2016) Atlantic Ocean 5 Jun 2008–30 Sep 2009 × × × × J Anand et al. (2017) Indian Ocean 16 Mar–19 Apr 2014 × × × J Puigcorbé et al. (2017a) Atlantic Ocean 2 May–4 Jul 2010 × × J Puigcorbé et al. (2017b) Southern Ocean 7 Jan–11 Mar 2012 × × × × × J Roca-Martí et al. (2017) Southern Ocean 7 Jan–11 Mar 2012 × × × × × J Anand et al. (2018b) Indian Ocean 24 Oct 2013–29 Jan 2014 × × J Anand et al. (2018a) Indian Ocean 9–28 May 2014 × × × J Black et al. (2018) Pacific Ocean 29 Oct–19 Dec 2013 × × × × J Lemaitre et al. (2018) Atlantic Ocean 20 May–26 Jun 2014 × × × J Schlitzer et al. (2018) Mediterranean Sea 4 May–30 May 2013 × J Stukel et al. (2019) Pacific Ocean 9 Aug 2014–11 May 2016 × × × R Stukel and California Current Ecosystem LTER (2019) Pacific Ocean 1–29 Jun 2017 × J Umhau et al. (2019) Pacific Ocean 1 Feb 2014–30 Sep 2015 × × × × × J Alkalay et al. (2020) Mediterranean Sea 25 May 2017–3 Jun 2018 × × × J Buesseler et al. (2020a) Pacific Ocean 14 Aug–10 Sep 2018 × × × × J Xie et al. (2020) Pacific Ocean 13 Apr–23 Jul 2017 × P Ken O. Buesseler (2018) Southern Ocean 6–30 Jan 2009 × × × × P Ken O. Buesseler Southern Ocean 20 Feb–14 Mar 2009 × P Elena Ceballos-Romero (2018) Southern Ocean 1–12 Dec 2013 × × × × P Beat Gasser (2020) Pacific Ocean 27 Oct–11 Dec 2004 × × × P Jennifer Kenyon (2020) Pacific Ocean 25 Sep–23 Oct 2018 × × × × P Jennifer Kenyon (2020) Pacific Ocean 26 Oct–24 Nov 2018 × × × × P Stephanie Owens (2018) Atlantic Ocean 8–26 Nov 2010 × × × P Katsiaryna Pabortsava (2019) Atlantic Ocean 11 Aug–7 Sep 2011 × × × × P Viena Puigcorbé (2019) Mediterranean Sea 8 Mar–20 Sep 2009 × × × The potential of 234Th data increases when combined with methods that account for export episodes over different timeframes of a bloom period, such as 210Po–210Pb, or sediment traps (see, e.g., Ceballos-Romero et al., 2016). For that reason, we included information regarding the availability of some other techniques when combined with 234Th sampling in the “ADDITIONAL_DATA” section. Additional data of interest are specified with YES/NO indicators for the cases of (i) 234Th underway sampling, (ii) sediment trap deployments, (iii) 234Th being paired with 210Pb–210Po disequilibrium sampling, and (iv) CHN data. Note that this section is merely intended as being informative of the sampling methods used complementarily to the 234Th technique. Both 234Th underway data and POC:234Th and PON:234Th ratios from sediment traps are reported when available. However, 210Pb–210Po concentrations are not compiled in this dataset. The availability of these data is indicated as reported by authors. We have only consulted publications and cruise reports when accessible to gather information for these metadata, so we acknowledge that information as to the existence of these data might be missing. We therefore recommend using them cautiously when stated “NO (available)” but fully trust https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2646 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data it when stated “YES”. To report an update to the metadata or an error in the data compiled, we encourage authors to contact us, and changes will be included in future versions of the compilation (see Sect. 5). It is also worth mentioning here that the amount of data that could be additionally reported in the compilation is very extensive due to the wide applicability that has been shown for 234Th throughout the years (see, e.g., review by Waples et al., 2006). The inclusion of new parameters in a future version of the compilation will be discussed in Sect. 5 as part of our assessment of steps towards improving the global data set. Finally, several details regarding the data source are included in the eight fields specified in the “DATA_SOURCE” section, including a YES/NO indicator for the publication date. The data owners are always clearly indicated by the first author(s) of the publication or data set. In the case of data published in research articles, the journal and the publishing year are indicated, while in the case of unpublished data obtained from personal communication (as is the case for nine datasets), “np” (for non-published) is given in the journal information, and no data are provided as publication year (indicated as “−999”). If data had been assigned a digital object identifier (DOI), this is included in DOI/others. When no DOI is available, the data URL source – either database or publication links – is included instead. Other URL sources or personal communication from data sources is keyed by the text variable “data_resource” in the “metadata” table, when available. Most of the measurements were obtained from publications. In these cases, if the data were transcribed from tables, the table number is also given as “data localization”. If data were only available graphically, a computer program to digitize the data from plots was used (WebDigitizer, https: //automeris.io/WebPlotDigitizer/, last access: 1 June 2022), and the figure number is also given. In the rare cases in which data were not accessible through any of these procedures, the authors were directly contacted for data. For those cases, the author(s) contacted is (are) indicated in the “data localization” field. Finally, a space for further links or information of interest is provided under the text variable “other DOI/resources”. In the few cases that the same data were reported in another publication (e.g., Murray et al., 1996, Dunne et al., 1997, and Murray et al., 2005, reported the same data from the U.S. JGOFS (Joint Global Ocean Flux Study) program during 1992 in the equatorial Pacific), it was indicated under this text variable. 2.1.2 “Data” sheet The “data” table is the data set core and contains all the data detailed above (more details in Table S2). Each data point is accompanied by cruise and station IDs, location – latitude, longitude, depth, and additionally bottom depth when available – and sampling date – including month–date–year and day-of-year (DOY) formats. All 234Th (total, dissolved, and particulate) concentrations were converted to disintegration per minute per liter (dpm L−1; density 1027 kg m−3) if not already reported in these units. POC:234Th and PON:234Th ratios are given in micromoles per disintegration per minute (µmoldpm−1) and include samplings with sediment traps and filtration methods, in which we report (i) bottles – Go-Flo and Niskin types – (ii) filtration systems, (iii) SPLITT (split flow-thin cell fractionation), and (iv) (large or small volume) in situ pumps for either the entire particulate fraction or two sizes classes (preferably 1–53 and >53 µm) when available. These size classes were chosen largely based on results of Bishop et al. (1977), Clegg and Whitfield (1990), and others, who assumed that the >53 µm size class was responsible for most of the mass flux into traps. Other cutoffs of 51 or 70 µm and other size classes are found and are noted when different than 53 µm. The particles’ sampling method – categorized as “method 1” (for filtration methods) and “method 2” (for sediment traps) – and size fractions – categorized as “small” or “large” – are specified as part of the data. Total, particulate, and sediment trap 234Th sampling depths are separately indicated. When reported, water temperature, salinity, and 238U measurements are included. Moreover, if accessible, POC and PON concentrations (“CHN” data in µmolL−1) are included. In all cases we assume that the originating authors and editors have undertaken steps necessary to control data quality. Measurement uncertainties in the data points are compiled as provided by the original authors (e.g., “uncert_total_234Th”). Please refer to the original source for whether data uncertainty includes only the 1-sigma counting error or other factors, such as uncertainty in volumes, detector efficiency, background, etc. 2.2 Data formats and availability The data are archived on the PANGAEA repository, with the data set’s DOI https://doi.org/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). The data table is available for download either as a unique merged file containing all data sets and metadata or as individual Excel files. Moreover, the template followed to compile the data set (including the “metadata” and “data” tables) along with instructions to fill in this template is available in PANGAEA for any author who wants to either review a data set, complement a data set included in this compilation, or contribute to its extension with a new data set. We strongly encourage authors to contact us to submit suggestions or requests to amend the data sets compiled. 3 Scope of and introduction to the data set In this article, we aim at providing a broad overview of the character of the data sets to be used for different purEarth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2647 Figure 1. (a) Map showing the distribution of sampling stations catalogued as (i) unpublished (yellow diamonds), (ii) published exclusively in repositories (dark blue squares), and (iii) published in refereed journals (magenta dots). (b) Map showing data density by sampling location. (c) Map showing long-term, high-frequency time-series stations (TSSs) either (i) “operational” (blue diamonds), (ii) “registered” for future operation (yellow diamonds), (iii) “inactive” (red asterisks), or (iv) “close” (black crosses) (source: https://www.ocean-ops.org, last access: 1 June 2022) and those locations including 234Th sampling that match a TSS (light blue dots). https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2654 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data that this radioisotope is an ideal particle reactive tracer for studying the scavenging of thorium from surface waters. In fact, a series of papers by Coale and Bruland in the mid1980s (Bruland and Coale, 1986; Coale and Bruland, 1985, 1987) were key for establishing the baseline for future 234Th studies that would use 234Th as a proxy for POC fluxes in the next era. The authors discovered that the 238U–234Th disequilibrium is a direct tracer of the rates of sinking particles from the upper ocean, which led to the acknowledgment of the relevance of the downward 234Th flux. Previously, in a 1-year time-series study, Tanaka et al. (1983) found large variations in the total 234Th concentrations, with the minimum in 234Th inventory coinciding with the early spring bloom, which they proposed was related to biological activity in the surface waters. It was within this context that Eppley (1989) proposed that, if 234Th is scavenged by biogenic particles, 234Th could be used as a tracer of export production. The “Coale and Bruland papers” were followed by many other investigators (e.g., Bacon and Rutgers van der Loeff, 1989; Huh and Beasley, 1987; Kershaw and Young, 1988; Murray et al., 1989; Rutgers van der Loeff and Berger, 1991; Schmidt et al., 1990; Wei and Murray, 1991). A series of milestones have been identified as the most remarkable of this era, which are summarized as follows: – 1969: initial measurement of total 234Th and introduction of the co-precipitation of 234Th with Fe(OH)3(Bhat et al., 1969); – 1972: beginning of GEOSECS program, which would be prolonged until 1978 and transform the field of chemical oceanography as it existed at that time by exploiting new technologies available then (see synthesis of results by Broecker and Peng, 1982); – 1977: introduction of the concept of the “great particle conspiracy” by Turekian (1977); – 1979: initial analysis of particulate and dissolved 234Th in the Narragansett Bay (Santschi et al., 1979); – 1984: – beginning of the Joint Global Ocean Flux Study (JGOFS) program, which grew out of the recommendations of a National Academy of Sciences workshop and would last until 2003; – introduction of the MnO2-impregnated filter cartridge technique (Mann et al., 1984), used with in situ pumps; – 1985: initial analysis of particulate and dissolved 234Th in open ocean and link between biological processes and 234Th deficits clearly demonstrated (Bruland and Coale, 1986; Coale and Bruland, 1985, 1987); – 1987: one TSS project established in the framework of the French MOOSE project (Mediterranean Ocean Observing System for the Environment) and the JGOFSFrance program: DYFAMED: Dynamics of Atmospheric Fluxes in the MEDiterranean sea (43.42◦N, 7.87◦E); – 1988: beginning of JGOFS field work with the establishment of two TSS projects: BATS: the Bermuda Atlantic Time-series Station in the Atlantic Ocean (31.50◦N, 64.10◦W), and HOT: Hawaii Ocean Time-series in the Pacific Ocean at Station ALOHA: A Long-term Oligotrophic Habitat Assessment (22.45◦N, 158◦W); – 1989: 234Th proposed to trace export production (Eppley, 1989); – 1990: one TSS project established in the frame of JGOFS program: OSP – Station PAPA Ocean Weather Station “P” in the Pacific Ocean (50.1◦N, 144.9◦W) – note that observations at Station Papa (formerly Ocean Station Peter and referred to as OSP or Ocean Station “P”) started in 1949, although the larger surveys became a focus of activities in support of JGOFS during the 1990s (Freeland, 2007). There exist two distinct radioanalytical methods for the 234Th extraction and purification from water samples that were initiated during this era: (i) the co-precipitation of 234Th with Fe(OH)3, proposed by Bhat et al. (1969), and (ii) the scavenging of this nuclide onto MnO2cartridges, introduced by Mann et al. (1984). A thorough review of these techniques can be found in Rutgers van der Loeff et al. (2006). Briefly, for the Fe(OH)3technique, 20–30 L seawater samples are treated and beta-counted. The addition of Fe carrier forms a precipitate that removes Th (and other elements) from solutions, so ion exchange purification procedures are required (at sea or quickly after return to shore) to separate 234Th from its parent and other potential beta emitters. For the MnO2cartridge technique, seawater is sequentially pumped through filters and two MnO2impregnated cartridges connected in series to scavenge dissolved Th isotopes. This technique was often used with large volume samples (102–104L), needed primarily for 228Th, 230Th, and 232Th analyses, which required large amounts of ship time given the use of in situ pumps for filtration, limiting the spatial coverage of the 234Th profiles. MnO2cartridges do not adsorb appreciable 238U, which is another advantage as ingrowth after sampling from 238U can be neglected. The large samples represented by a MnO2cartridge also allowed for direct gamma counting, thus eliminating the need for laborious radiochemical purification. A wide variety of methods were used to sample the different 234Th phases within era 1. For the total phase, PVC tubes and Van Dorn samplers (i.e., horizontal water bottle) were Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2655 the prevalent equipment (Bhat et al., 1969, Matsumoto, 1975, McKee et al., 1984, and Minagawa and Tsunogai, 1980, among others), although a few studies used pumping systems (Lee et al., 1991; Tsunogai et al., 1986) and bottles (Bacon and Rutgers van der Loeff, 1989). In the majority of studies measuring dissolved and particulate 234Th phases, the total 234Th concentration was estimated as the sum of them (see, e.g., Coale and Bruland, 1985, and Murray et al., 1989). Regarding particulate 234Th analyses, most of the studies used filtration systems on volumes between 30 and 700 L, a few studies used bottles (with the Go-Flo model more typical than the Niskin one), and one study introduced the use of pumps (Bacon and Rutgers van der Loeff, 1989) Note that the distinction between the dissolved and particulate phases is generally operational, with the term “dissolved” usually comprising all the phases passing through a pore size cutoff of 0.45 µm (i.e., Bacon and Rutgers van der Loeff, 1989; Dominik et al., 1989; Rutgers van der Loeff and Berger, 1991; Santschi et al., 1979; Schmidt et al., 1990; Wei and Murray, 1991). A total of 25 published works in refereed journals comprise the 234Th studies of this era, compiled in a total of 24 datasets. Sampling was characterized by cruises mostly in the Pacific Ocean (see Figs. 2a and 4a), with a reduced number of locations sampled (maximum of 29 locations and averaged value of 6 locations per cruise) and a reduced number of data points per study (rarely over 100, averaged value of 60 samples per cruise). A total of 36 cruises were compiled from this era, with a total of 1493 234Th data points. Most surveys took place solely in the NH (29) between April and September (Fig. 5a). Only one expedition surveyed the SH, but a total of six cruises crossed the Equator and sampled in both hemispheres. Only two studies reported samplings that were part of a selected ocean program, which included the GEOSECS program (Krishnaswami et al., 1976) and the DYFAMED program (Schmidt et al., 1990) (see Table 2 for a summary). Another study took place within the framework of the Joint Chinese–American Field Program (JCAFP) (McKee et al., 1984). A total of nine studies reported sampling TSSs (see, e.g., Tanaka et al., 1983). During this era, 234Th studies were commonly focused on analyzing the parameters influencing the partitioning of a species between dissolved and particulate phases as a way to understand the mechanisms and rates for the scavenging of particle-reactive species. One way to quantify the scavenging of particle-reactive species is the use of distribution coefficients (Kd) which measure the partitioning of a species between dissolved and particulate phases (McKee et al., 1986). As the knowledge of scavenging increased, novel applications of 234Th were developed, reducing the interest in the dissolved phase of 234Th over time and driving changes in the 234Th data type collected during field expeditions (see next sections). The majority of the studies reported 238U concentrations along with at least one 234Th phase concentration (684 238U data points reported in this era), but none of them measured POC:234Th (Fig. 6a) as the importance of this parameter was not evinced until era 2 (see Sect. 4.2). 238U was either measured (a total of 6) or derived from salinity (a total of 10) mostly using the relationship from Ku et al. (1977). It is also worth mentioning that very few studies reported simultaneous sampling of complementary non-thorium measurements, such as sediment traps (a total of six studies; see, e.g., Coale and Bruland, 1987, and Tsunogai et al., 1986) or 210Pb–210Po disequilibrium (five studies; see Krishnaswami et al., 1976; Moran and Moore, 1989; Santschi et al., 1979, 1980; Tanaka et al., 1983). None of the studies reported CHN (i.e., POC and/or PON) data. Finally, in terms of modeling 234Th data, more than half of the studies (56 %, a total of 14) did it, and most of the cases used a two-box model, following Coale and Bruland (1985). Except for Tanaka et al. (1983) who applied both an SS and NSS model to estimate the residence time of 234Th by accounting for the change in the inventory of 234Th with time, all the studies that provided information in this regard assumed SS conditions during sampling, which is not surprising since stations were not usually reoccupied during cruises for the collection of time-series data. The physical advection and diffusion (often referred to as Vterm) were neglected in most of the cases. 4.2 Era 2: “The Sea Around Us” 1992–2000: Introduction of the empirical determination of POC export from 234Th profiles. “When I think of the floor of the deep sea. . . I see always the steady, unremitting, downward drift of materials from above, flake upon flake, layer upon layer. . . the most stupendous “snowfall” the earth has ever seen.” – Rachel Carson, The Sea Around Us. The beginning of era 2 is marked by a new approach that was introduced to estimate POC fluxes from 234Th distributions from samples collected during the U.S. JGOFS North Atlantic Bloom Experiment (NABE) carried out in 1989. During this field experiment, the seminal work of Buesseler et al. (1992) found a clear relationship between the onset of the spring bloom, the subsequent drawdown of nutrients and CO2, the net removal of 234Th, and the flux of POC. Because of the conservative nature of 238U in the ocean, any measurable deficit of 234Th relative to its parent can be assumed to imply a significant removal by scavenging and particle sinking flux over a period of days to weeks before sampling as the mean residence time of 234Th is dictated by its decay constant and removal rate by particles (Coale and Bruland, 1985). Buesseler et al. (1992) postulated the empirical determination of POC export fluxes (in µmolm−2d−1) from https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2656 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data Figure 4. Map showing the distribution of sampling stations reported (a) within era 1 distinguished by the type of data available as follows: (i) dissolved plus particulate 234Th (yellow diamonds), (ii) 238 U plus total 234Th (dark blue squares), and (iii) no 238U (magenta dots); and within eras 2 (b), 3 (c), and 4 (d) distinguished by the type of data available as follows: (i) total 238U plus total 234Th plus POC:234Th ratios (yellow diamonds), (ii) 238U not sampled (dark blue squares), (iii) total 234Th not sampled (magenta dots), and (iv) POC:234Th ratios not sampled (light blue triangles). The “no parameter” categories mean that from the data shown in the maps – i.e., 238U, dissolved, particulate, and total 234Th in panel (a) and 238U, total 234Th, and POC:234Th ratios from both filtration methods and sediment traps for panels (b),(c), and (d) – the “no” category has not been reported by the original authors and therefore is not included in the compilation, but at least one of the other parameters has been reported. Therefore, in panels (b),(c), and (d), only locations that fall into category (i) can be used to estimate POC fluxes. 234Th–238U oceanic disequilibrium following POCflux (z)=POC:234Th(z)·234Thflux(z),(1) where POC:234Th is the ratio of POC to 234Th measured on sinking particles at the desired depth z(in µmoldpm−1), and 234Thflux is the 234Th downward flux measured at the same depth (in dpm m−2d−1). Equation (1) is the base of the so-called 234Th method, and the POC flux obtained is referred to as 234Th-derived POC flux. Note that the concept for this empirical method was introduced in a conference abstract in one much earlier study in the North Pacific (Tsunogai et al., 1976). Buesseler et al. (1992) proposed that the sinking flux of any element – such as carbon, phosphorus, or nitrogen – could be derived from 234Th flux if the ratio of this element to 234Th on sinking particles is known. Element :234Th ratios are directly determined from their in situ measurement and vary with both depth and particle size (Buesseler et al., 2006). The 234Th flux can be calculated by evaluating the change in the corresponding total 234Th concentration with time and the contributions due to horizontal and vertical advection and diffusion processes. The simplest solution to estimate 234Th flux is SS conditions and ignoring advective and diffusive transport. These assumptions are the most commonly used (e.g., Le Moigne et al., 2013a, and references therein) as generally only a single 234Th profile can be measured. The neglection of the physical term became inadequate with the expansion of 234Th research to coastal and more dynamic regimes (Savoye et al., 2006). In the open ocean, the most relevant physical process is vertical upwelling, although there are also areas with downwelling, and it will typically result in underestimations of 234Th export if it is not included (Buesseler et al., 1995). Furthermore, Dunne and Murray (1999) developed a model to estimate advection and diffusion, concluding that incorporating advection to estimate carbon export might overestimate the sinking flux if diffusion is not considered. For the temporal variation in 234Th concentration, alternatively to the SS, an NSS model can be applied when temporal fluctuations in 234Th concentration can be assessed owing to repeated sampling, ideally over the course of 2 to 4 weeks (Resplandy et al., 2012) and only if the same Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2657 Figure 5. Seasonal distribution of annual field expeditions with 234Th sampling reported within eras 1 (a), 2 (b), 3 (c), and 4 (d) separated by hemisphere as follows: (i) Northern Hemisphere (HN), (ii) Southern Hemisphere (SH), and (iii) crossing the Equator and sampling in both hemispheres (both). water mass is sampled (Savoye et al., 2006) (i.e., the NSS approach is difficult to assess in dynamical settings). The study by Buesseler and co-authors motivated an increase in oceanic 234Th measurements in the 1990s, and we consider it one of the main milestones of this era along with the following ones: – 1992: introduction of the empirical method for the POC flux estimate from 234Th concentrations proposed by Buesseler et al. (1992); – 1992–1993: studies on the role of colloidal material (i.e., ∼0.001 <colloids <1 µm; Stumm, 1977) in 234Th scavenging (Baskaran et al., 1992; Moran and Buesseler, 1993); – 1995: – introduction of the use of the 210Pb–210Po pair in a similar manner to the 238U–234Th pair by Shimmield et al. (1995); – development of a regional 3-D 234Th flux model to estimate the physical components of the flux (i.e., the Vterm) by Buesseler et al. (1995); – 1996: beginning of the expansion of the 234Th approach to particulate inorganic carbon fluxes (Bacon et al., 1996); – 1998: one TSS project established in the frame of JGOFS program, operated by Taiwan: SEATS: South East Asia Time-series Study station (18◦N, 116◦E); – 1999: introduction of the 20 L MnO2co-precipitation technique for 234Th by Rutgers van der Loeff and Moore (1999) to allow particulate and dissolved concentrations to be analyzed from a single aliquot and to be measured by beta counting on board. In the earlier experiments of this era, bottle POC data were generally compared to 234Th-derived from individual cartridge filters or other particle collectors, such as in Buesseler et al. (1992). Several options of large-volume in situ pumps that allowed for the measurement of POC and 234Th on the same filter emerged during this era: (i) large volume filtration using Challenger Oceanic’s submersible pumps, which was introduced by Shimmield et al. (1995), (ii) MULVFS (multiple-unit large-volume filtration system) in situ pumping system by Charette et al. (1999), with large volumes (2500–3500 L) passing sequentially through a 1 µm cartridge pre-filter, and (iii) large volume (400 L) samples collected https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2658 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data Figure 6. Histogram of data points reported within eras 1 (a), 2 (b), 3 (c), and 4 (d) separated by data type as follows: (i) 238U, (ii) total 234Th, (iii) dissolved 234Th, (iv) particulate 234Th, (v) POC:234Th ratios, and (vi) PON:234Th ratios. using in situ battery-operated pump deployed on the CTD rosette frame (Charette and Buesseler, 2000). Ratios were normally determined using small-pore-sized filters, either > 0.4–0.5 µm (Huh and Prahl, 1995; Cochran et al., 1995; Laodong et al., 1997; Santschi et al., 1999) or >1 µm (e.g., Benitez-Nelson et al., 2000; Gustafsson et al., 1997a; Rutgers van der Loeff et al., 1997), typically made of glass or quartz fiber. A total of 10 studies sampled two size fractions determined by filtering through 53 µm (Bacon et al., 1996; Buesseler et al., 1998, 1995; Charette and Moran, 1999; Moran et al., 1997; Moran and Buesseler, 1992; Moran and Smith, 2000; Murray et al., 1996) or 70 µm Nitex screens (Charette and Buesseler, 2000; Cochran et al., 2000), but only one of them reported POC:234Th on both small and large particles. In total, only three studies reported POC:234Th ratios measured with filtration methods (Buesseler et al., 1992; Charette et al., 1999; Murray et al., 1996). Additionally, POC:234Th ratios in sinking particles were collected in sediment traps by a total of six studies, including modern designs such as NBSTs (see, e.g., Buesseler et al., 2000) and surface-tethered particle interceptor traps (PITs) (see, e.g., Buesseler et al., 1994). A total 43 data sets comprise this era, extracted from a total of 48 publications. Sampling was focused on the Atlantic Ocean (see Figs. 2a and 4b). Surveys in the Pacific Ocean were limited, while increased sampling in the Southern Ocean and field expeditions to the Arctic were conducted. Both the number of expeditions and the locations sampled increased. Despite its short duration (less than a decade), a total of 70 cruises were reported within this era, indicative of the dedicated programs and experiments that marked this era, resulting in 8739 and 1133 data points for 234Th and 238U respectively. Similar to the first era, cruises mainly took place exclusively in the NH (52 cruises), although expeditions both solely to the SH and to both hemispheres increased, with a total of 7 and 11 cruises respectively. Samplings mainly took place within the first semester of the year (Fig. 5b). As previously mentioned, this period of the 234Th history partially overlapped with the golden years of the JGOFS program, and therefore, many of the studies published during era 2 reported 234Th measurements collected during field expeditions that took place in the frame of this international program. A summary of these activities is provided in Table 2. In the case of U.S. JGOFS, this included 234Th measurements at the BATS TSS (see, e.g., Buesseler et al., 1994, 2000) and during several process studies in well-defined areas at strategic oceanic locations: (1) NABE (North Atlantic Bloom Experiment; http://usjgofs.whoi.edu/research/nabe.html, last access: 1 June 2022) that was one of the first major activities of JGOFS with three cruises along longitude 20◦W in 1989 – it was published 3 years later by Buesseler et al. (1992); (2) EqPac (Equatorial Pacific; http://usjgofs.whoi. Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2659 edu/research/eqpac.html, last access: 1 June 2022) process study that was conducted along 140◦W during year 1992 and included a total of four cruises – it was published between 1994 and 1997 by Bacon et al. (1996), Buesseler et al. (1995), Dunne et al. (1997), and Murray et al. (1996) and extended in the next era by Hernes et al. (2001) and Murray et al. (2005); (3) the Southern Ocean expedition during October and November 1992 published by Rutgers van der Loeff et al. (1997) and expanded in era 3 by Friedrich and Rutgers van der Loeff (2002); (4) Arabian Sea (http://usjgofs.whoi.edu/research/arabian.html, last access: 1 June 2022), beginning in October 1994 and ending in January 1996 and reported a few years later by Buesseler et al. (1998); (5) AESOPS (Antarctic Environment and Southern Ocean Process Study; http://usjgofs.whoi.edu/research/ aesops.html, last access: 1 June 2022), which carried out field work between August 1996 and April 1998 and was published within this era by Cochran et al. (2000) and extended in the next era by Buesseler et al. (2001a). Additionally, studies also reported data from other JGOFS expeditions from the following: (6) Indian program, which completed three major sampling expeditions for 234Th to the eastern and central Arabian Sea in April–May 1994, February–March 1995, and July–August 1995 – it was reported by Sarin et al. (1996) and extended in PANGAEA in 2013 (https://doi.org/10.1594/PANGAEA.807500, JGOFSINDIA, 2013); (7) Canadian program in the northeast Pacific Ocean which had two phases from 1992 to 1994 and from 1995 to 1997, although 234Th was only sampled during the second phase – data were published by Charette et al. (1999); and (8) France DYFAMED program, sampled in 1987 but published in this era by Schmidt et al. (1992) and extended in the next era (see Schmidt et al., 2002a, b). Finally, field work also included (9) the Southern Ocean Iron RElease Experiment (SOIREE; https://www.bco-dmo.org/project/2051, last access: 1 June 2022), which was the first in situ iron fertilization experiment performed in the polar waters of the Southern Ocean. It took place in February 1999 south of the Polar front in the Australian–Pacific sector of the Southern Ocean and was reported in Charette and Buesseler (2000). Data from this iron enrichment experiment were compiled along with others in a common open-access database during the iron SYNTHESIS program (FeSynth; https://www. bco-dmo.org/program/2017, last access: 1 June 2022) started in 2007. In addition to JGOFS, cruises from another major initiative that included 234Th sampling were with the Ocean Margins Program (OMEX; http://po.msrc.sunysb.edu/omp/, last access: 1 June 2022), a large field-based study with extensive physical, chemical, biological, and geological measurements, carried out on a northwest European shelf break that ran in two phases from 1993 to 1996 and from 1997 to 2000. 234Th data were made available by Hall et al. (2000). The number of data points measured per cruise significantly increased within this era, with an average of 125 234Th data points reported per cruise in comparison to the 41 data points averaged in era 1. Such an increase was significantly marked for the dissolved and particulate phases, whose measurements increased more than 5-fold relative to those from era 1 (see Fig. 6b). Additionally, measurements to determine POC and PON to 234Th ratios became routine, therefore allowing the estimate of POC fluxes (see Fig. 4b). However, only half of the studies reported 238U concentrations along with 234Th data, in their majority using a variety of 238U–salinity relationships, with Chen et al. (1986) being the prevalent one. An overall 69 % of the studies (33 out of 48 that comprise this era) modeled 234Th data. Time-series data were collected more often than during the first era, likely with the purpose of following the NSS approach (Buesseler et al., 1992, 1995). Nonetheless, the majority of the studies (70 %) that modeled 234Th data assumed SS conditions (see, e.g., Bacon and Rutgers van der Loeff, 1989; Baskaran et al., 1992, and Gustafsson et al., 1997b). A total of 10 studies applied the NSS model, with seven of them combining it with the SS model (see, e.g., Buesseler et al., 1994, Cochran et al., 2000, and Kersten et al., 1998, for the combined use of both approaches). In terms of additional data, 1 study introduced 234Th underway sampling (Hall et al., 2000), a total of 11 studies reported sampling with sediment traps (see, e.g., Cochran et al., 2000, Murray et al., 1996, and Smoak et al., 1999), 10 studies sampled for 210Pb–210Po disequilibrium (e.g., Moran and Moore, 1989; Santschi et al., 1999; Wei and Murray, 1992), and 13 studies reported CHN data (see, e.g., Niven et al., 1995, Rutgers van der Loeff et al., 1997, and Santschi et al., 1999). 4.3 Era 3: “In the Heart of the Sea” 2001–2009: improvements of the 234Th technique. “At sea, things appear different.” – Nathaniel Philbrick, In the Heart of the Sea. The final boost to the widespread and increasing use of 234Th as a particle flux tracer was motivated by the works of Buesseler et al. (2001b) and Benitez-Nelson et al. (2001), which dramatically increased the number of 234Th measurements and marked the third era with the introduction of the small-volume (2–4 L) technique. This procedure modified the 20 L method developed by Rutgers van der Loeff and Moore (1999) in era 2 and uses the lowest sample volumes of all known 234Th methods to date. It not only allowed immediate onboard beta counting of 234Th concentration and avoided some tedious folding sessions but also enhanced both spatial and temporal resolution of particle export. The revolution that this novel technique brought with it was substantial. The small-volume technique is essential to capture the particle dynamics and export flux variations on scales that could be better related to local biogeochemical conditions. The main advantage is the convenience of handling small volumes and more rapid processing times. Multiple sampling casts are often required for a 20 L sample https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2660 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data profile, whereas a 4 L technique usually allows simultaneous sampling of 234Th with other parameters on a single cast (e.g., nutrients, phytoplankton biomass). As a result, this method can be easily applied at sea using samples obtained through CTD rosette water samplers that are available on most research vessels. For that reason, we have chosen the introduction of the small-volume technique as a shifting milestone in the 234Th timeline. Further improvements of this technique within this era were carried out some years later by Pike et al. (2005) and Cai et al. (2006b), which are major milestones of this era and whose timeline summaries are as follows: – 2001: introduction of the small-volume technique by Buesseler et al. (2001b) and Benitez-Nelson et al. (2001); – 2002: – one long-term multidisciplinary and moored observatory established in the northwest Atlantic and coordinated by the National Oceanographic Center of Southampton (UK) at the PAP site: Porcupine Abyssal Plain (49–13.5◦E); – expansion of the 234Th approach to biogenic silica fluxes (Friedrich and Rutgers van der Loeff, 2002); – 2005: introduction of the use of yield tracers – double spike technique – for the sake of maintaining the reproducibility (Pike et al., 2005), the advantage of this modified approach being a precise knowledge of 234Th recovery, leading to an enhancement in data quality of 234Th, which is standardly used nowadays and recommended by the GEOTRACES protocol (Maiti et al., 2012); – 2006: reduction of the filtration time and introduction of the alpha spectrometric measurement of 230Th recovery by means of using a combination of water bath heating and a reduction in reagent quantities (Cai et al., 2006b), which also modified the typical ion-exchange chemistry to allow for the alpha spectrometric measurement of 230Th recovery. Moreover, a technological innovation was introduced during era 3 with the development of two in situ pumping systems for high volume filtration for 234Th concentrations and POC:234Th ratios in sinking particles widely used from this era on (1) McLane large-volume water transfer system (WTS-LV) pumps by McLane Laboratories (Falmouth, MA) and (2) SAPS (stand-alone pump system) by Challenger Oceanic (Surrey, UK). Moreover, one study reported the use of a novel split flow-thin cell fractionation (SPLITT) (see Gustafsson et al., 2006). This has been the golden age of the 234Th so far in terms of publications released by year (almost 10, versus 1 and 6 in eras 1 and 2), with a total of 159 cruises reported in a period of 9 years. A total of 19 676 and 4567 data points for 234Th and 238U respectively (see Figs. 4c and 6c) were compiled in 76 datasets, extracted from 87 studies (81 publications in refereed journals, 1 PhD thesis, and 5 repositories). Data reported in the context of selected ocean programs continued (see Table 2 for more details). This included JGOFS through the TSSs of HOT (C. BenitezNelson et al., 2001), BATS (Sweeney et al., 2003), and DYFAMED (e.g., Szlosek et al., 2009), as well as OMEX (Schmidt et al., 2002b). Other new studies, such as the Carbon Retention In A Colored Ocean (CARIACO; https: //www.st.nmfs.noaa.gov/copepod/time-series/ve-10101/, last access: 6 June 2022) time-series program operative between 1995 and 2017 and published by Smoak et al. (2004), were also initiated. Additionally, novel JGOFS initiatives took place: (1) JGOFS-France MEDATLANTE (http://www.obs-vlfr.fr/cd_rom_dmtt/other_main.htm, last access: 6 June 2022) in the Gibraltar Strait and the northeast Atlantic Ocean – data were made available by Schmidt et al. (2009); (2) JGOFSFrance ANTArctic RESearch program (ANTARES; http://www.obs-vlfr.fr/cd_rom_dmtt/an_main.htm, last access: 6 June 2022) – data published by Coppola et al. (2005); (3) JGOFS-France DYNAPROC (http://www.obs-vlfr.fr/cd_rom_dmtt/other_main.htm, last access: 6 June 2022) cruise, reported by Schmidt et al. (2002a, 2009); and (4) the JGOFS-Japan North Pacific Process Study carried out at KNOT station (44◦N, 155◦E; https://www.nodc.noaa.gov/archive/arc0013/0001873/1.1/ data/1-data/general/res_outline/KNOT.html, last access: 6 June 2022) by Kawakami and Honda (2007). Data from a great number of projects and experiments were published during this era (see Table S3 for a chronological summary). The publications included in this era report data covering the entire ocean and with an increased number of data points in all regions, except the Indian Ocean (see Figs. 2a and 4c). Expeditions to the SH increased, with 18 reported, yet were far below those in the NH, where a total of 134 cruises were undertaken (see Fig. 5c). Additionally, a total of seven surveys sampled in both hemispheres. Stations included the long-term observatories previously sampled for 234Th along with novel ones, such as the PAP site (Turnewitsch and Springer, 2001). Fieldwork took place throughout the entire year, with a remarkable number of cruises in March. Both the overall number of data points and the data points measured per year increased due to the large number of field expeditions reported during this era and the widespread use of the small volume technique. An average of 2183 234Th data points were reported per year in comparison to an average of 60 and 971 data points during eras 1 and 2 respectively. As for the 234Th sample types in this era, the total number of measurements significantly increased for all phases (2fold for dissolved and particulate and 4-fold for total) and for POC:234Th ratios, and the number remains constant for PON:234Th (see Fig. 6c). Earth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2661 A great increase in the number of data points also was detected for 238U activities, with an increase of 4-fold in the data points. Once again, derived salinity was the most extended approach, with almost all the studies using Chen et al. (1986). A total of 66 studies out of the 87 that comprise this era (i.e., 76 %) modeled 234Th data: 42 of them applied the SS model (see, e.g., Amiel et al., 2002, Guo et al., 2002, and Speicher et al., 2006), 2 applied the NSS one (Kawakami et al., 2004; Smoak et al., 2004), and 22 studies applied both approaches (e.g., Amiel and Cochran, 2008; Gustafsson et al., 2004; Kim and Church, 2001). In terms of sampling strategy, 2 studies reported 234Th underway sampling (Rutgers van der Loeff, 2007; Schmidt et al., 2002b), and 30 studies reported the use of sediment traps, with 24 of them measuring POC:234Th ratios (Charette et al., 2001, Schmidt et al., 2002a, and Sweeney et al., 2003, among others). Around a third of the studies reported CHN measurement data (see, e.g., Lepore et al., 2009, and Santschi et al., 2003), while a very reduced number of the studies, around 10 %, analyzed 210Po (see Porcelli et al., 2001, and Somayajulu et al., 2002, among others). 4.4 Era 4: “Twenty Thousand Leagues under the Sea” 2010–present: GEOTRACES program and a new way to study the ocean. “The sea is everything. It covers seven tenths of the terrestrial globe. Its breath is pure and healthy. It is an immense desert, where man is never lonely, for he feels life stirring on all sides. The sea is only the embodiment of a supernatural and wonderful existence.” – Jules Verne, Twenty Thousand Leagues under the Sea. The beginning of this era is identified with the first publication of 234Th data from a GEOTRACES cruise, reported by Cai et al. (2010). GEOTRACES is an international study of the marine biogeochemical cycles of trace elements and their isotopes which changed the way to explore the oceans by combining ocean sections, process studies, data syntheses, and modeling. Its launch marked the beginning of internationally dedicated large-scale collaborative projects characterized by long field surveys with very high spatial resolution and many different parameters measured simultaneously. For this reason, the initial 234Th-related GEOTRACES publication was chosen as the milestone that starts the – so far – final era in the 234Th timeline. The history of GEOTRACES dates back to 2000, when the initial idea started with group discussions at international meetings inspired by both the successes and the limitations of GEOSECS and JGOFS. After some years in the planning and enabling phase, the GEOTRACES Science Plan was published in 2006. During 2007–2009 the first GEOTRACES cruises took place, including those that were part of the International Polar Year (IPY; https://www.geotraces. org/geotraces-in-the-international-polar-year/, last access: 1 June 2022) devoted to detecting and understanding a suite of trace elements and isotopes in the Arctic and Antarctic marine environments, and the intercalibration cruises (to the Atlantic and Pacific oceans in 2008 and 2009 respectively). A GEOTRACES transect in the Drake Passage was also carried out in 2008 (GIPY05, Germany, ZERO, and DRAKE) in the framework of IPY. Finally, the GEOTRACES program formally launched its seagoing effort in January 2010 and was fully announced to the scientific community at the Ocean Sciences Meeting that year in Portland (Oregon, USA). Numerous cruises have taken place since then, with many of them including 234Th sampling. Many of these results have been reported in the Intermediate Data Products, the first one released in 2014 (IDP2014 by Mawji et al., 2015), the second one in 2017 (IDP2017 by Schlitzer et al., 2018), and the third and most recent one in 2021 (IDP2021 by GEOTRACES Intermediate Data Product Group, 2021). The GEOTRACES program brought a new philosophy that, in the context of 234Th, drove a shift from mostly deriving POC fluxes only to include trace metal fluxes (see, e.g., Black et al., 2018) and the implementation of standards and intercalibration initiatives to establish procedures and protocols for sampling at sea to ensure that samples are collected, handled, and stored without contamination or other sources of bias (see, e.g., the versions of the Cookbook by GEOTRACES Standards and Intercalibration Committee (Cutter et al., 2014, 2017)). This era includes the development of new technologies to accelerate the collection and analysis of samples, the intercalibration of those technologies to ensure internal consistency among the participating labs, the development of a data management system to facilitate access to the results to the entire oceanographic community, and a broad collaborative effort to model, synthesize, and interpret the results. The most remarkable milestones for this era are as follows: – 2010: – initial publication of 234Th from a GEOTRACES cruise by Cai et al. (2010), which reported data from ARK XXII/2 expedition to the Arctic Ocean in 2007 carried out in the context of the IPY GEOTRACES program (GIPY11, Germany); – intercalibration initiative by Cutter et al. (2010) to ensure that 234Th results produced by different groups were comparable and internally consistent by using deep waters or stored samples as standards in which 234Th and 238U are known to be in secular equilibrium; – 2012: intercalibration initiative by Maiti et al. (2012), which carried out an intercomparison of 234Th measurements in both water and particulate samples between 15 laboratories worldwide; – 2014: release of the first GEOTRACES IDP (Intermediate Data Product; Mawji et al., 2015); https://doi.org/10.5194/essd-14-2639-2022 Earth Syst. Sci. Data, 14, 2639–2679, 2022 2662 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data – 2017: release of the second GEOTRACES IDP (Schlitzer et al., 2018); – 2021: – improvement of the small-volume technique recently carried out by Clevenger et al. (2021), which introduces a revised protocol that decreases sample volumes to 2 L, shortens wait times between steps, and simplifies the chemical recovery process, expanding the ability to more rapidly and safely apply the 234Th method; – release of the third GEOTRACES IDP (GEOTRACES Intermediate Data Product Group, 2021). A total of 80 data sets were compiled from this era, including 9 previously unpublished ones, extracted from a total of 82 studies (65 publications in refereed journals, 4 PhD theses, and 4 data repositories). A total of 114 cruises and 26 755 and 11 872 data points for 234Th and 238U respectively were compiled, mostly distributed between the Atlantic and Pacific oceans and, to a lesser extent, the Southern Ocean (see Fig. 2a). This era is the second most productive, after era 3, in terms of publications released per year (seven per year). It is also the most intense in terms of sampling, with an average of 235 234Th measurements reported by cruise and over 2400 234Th data points reported by year. Once again, the NH dominated the surveys (79 cruises sampled exclusively above the Equator), particularly numerous in early spring and fall (see Fig. S1). Nonetheless, this era reports the highest number of cruises to the SH, with a total of 27 samplings exclusively below the Equator and an additional 8 cruises sampling in both hemispheres. More than half of the expeditions of this era reported data from cruises that took place in the framework of major ocean programs (see Table 2) and some minor projects, such as the (i) Arctic Ocean 2001 (AO-01) expedition, reported by Gustafsson and Andersson (2012); (ii) the second and third Chinese National Arctic Research Expedition (CHINARE-2 and CHINARE-3), carried out in 2003 and 2008 respectively and published by Yu et al. (2010, 2012); (iii) Indo–German iron fertilization experiment LOHAFEX (https://epic.awi.de/ id/eprint/21440/, last access: 1 June 2022) in 2009, published 4 years later by Martin et al. (2013); (iv) second KErguelen Ocean and Plateau compared Study expedition (KEOPS2) of 2011, published by Planchon et al. (2015); and (v) fate of the northwestern Mediterranean open sea spring bloom (FAMOSO; http://digital.csic.es/handle/10261/ 94572, last access: 1 June 2022) project, which took place between March and September 2009 and was personally communicated by Viena Puigcorbé to be included in this compilation. Furthermore, 46 % of the data sets included 234Th data from an established time-series location (see Fig. 1c). An interesting characteristic of this period is that a relatively smaller number of cruises produced a greater amount of data in comparison to previous eras. The most relevant peaks are found in years 2013 and 2015, which coincides with the publication of several GEOTRACES transects (see Sect. 3). Dissolved 234Th measurements reduced drastically (4-fold), while the rest of measurement types increased to some extent, most notably for total 234Th and 238U (see Fig. 6d, note the change in the scale of this plot throughout the eras). Once again, 238U derived from salinity was the most commonly used approach with almost all the studies using the U–salinity relationship of Chen et al. (1986) or Owens et al. (2011). In terms of modeling 234Th data, a total of 64 out of the 82 studies (78 %) modeled 234Th concentrations and specified whether an SS, an NSS, or both models were used. From them, 50 (78 %) studies applied the SS approach (see, e.g., Alkalay et al., 2020, Cai et al., 2010, and Ceballos-Romero et al., 2016), just 1 (2 %) study applied the NSS one (see Kawakami et al., 2015), and a total of 13 (20 %) studies compared both approaches (see Buesseler et al., 2020a, Kim et al., 2011, and Lemaitre et al., 2018, among others). The number of studies carrying out 234Th underway sampling increased to seven (see, e.g., Black et al., 2018, Estapa et al., 2015, and Martin et al., 2013). More than 32 % of the studies reported POC:234Th ratios collected with sediment traps (Haskell et al., 2013, Kawakami et al., 2010, and Maiti et al., 2016, among others). Around a third of the studies (34 %) reported CHN data (see, e.g., Rosengard et al., 2015), while a very reduced number (∼9 %) analyzed 210Po (such as Alkalay et al., 2020; Ceballos-Romero et al., 2016; or Wei et al., 2011). 4.5 Launching of dedicated large-scale collaborative projects: the beginning of era 5? “Dawn begins pushing back the covers of night and the sun rolls out of bed.” – Carl Safina, Song for the Blue Ocean. Significant milestones within recent years are marked by the launching of an unprecedented number of large-scale collaborative projects including 234Th measurements: e.g., (1) PICCOLO: Processes Influencing Carbon Cycling: Observations of the Lower limb of the Antarctic Overturning (UK, 2016–2020, https://roses.ac.uk/piccolo/, last access: 1 June 2022), a project devoted to quantify the crucial processes that determine carbon cycling in the lower limb of the Southern Ocean overturning circulation through observations and models; (2) COMICS: Controls over Ocean Mesopelagic Interior Carbon (UK, 2017–2021, https://www.comics.ac.uk/, last access: 1 June 2022), a research project that aims to quantify the flow of carbon in the ocean’s twilight zone (part of the ocean between 100 and 1000 m below the sea surface) in order to more accurately model global climate change; (3) EXPORTS: EXport Processes in the Ocean from Remote Sensing project (USA, 2017–2022, https://www.us-ocb.org/nasa-exports-phase-i/, last access: 1 June 2022), a large-scale NASA-led field camEarth Syst. Sci. Data, 14, 2639–2679, 2022 https://doi.org/10.5194/essd-14-2639-2022 E. Ceballos-Romero et al.: Revisiting five decades of 234Th data 2663 paign that will provide critical information for quantifying the export and fate of upper-ocean net primary production using satellite observations and state of the art ocean technologies; (4) CUSTARD: Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth project (UK, 2018– 2022, https://roses.ac.uk/custard/, last access: 1 June 2022), an initiative which will examine how seasonal changes in food availability for phytoplankton influences how long carbon is trapped in the ocean in the Southern Ocean; (5) MOBYDICK: Marine Ecosystem Biodiversity and Dynamics of Carbon around Kerguelen: an integrated view (France, 2017–2022, https://mobydick.mio.osupytheas.fr/, last access: 1 June 2022), a project that combines investigation of the BCP and the end-to-end structure of the pelagic food web; (6) OTZ: Ocean Twilight Zone (USA, 2018–2024, https://twilightzone.whoi.edu/, last access: 1 June 2022), a project devoted to the study of this region of the ocean, with particular emphasis on development of new technologies for scientific exploration of the biomass and biodiversity, animal lives and behavior, food web interactions, and flow of carbon through the twilight zone; and (7) SOLACE: Southern Ocean Large Area Carbon Export (Australia, December 2020–January 2021, https://solace2020.net/, last access: 1 June 2022), a 6-week voyage aimed at developing an approach to quantify the changing effectiveness of CO2sequestration by the BCP using the remote sensing of the ocean surface by satellites and its interior by autonomous vehicles (specially BIO-ARGO profiling floats) and MOBYDICK project. Additionally, there is another funded future project that includes 234Th in its methodology: the APERO program – Assessing marine biogenic matter Production, Export and Remineralisation: from the surface to the dark Ocean (France, 2022–2026, with a cruise planned for 2023 in the western North Atlantic, https://www.polemermediterranee.com/Activites-Projets/ Ressources-biologiques-marines/APERO, last access: 1 June 2022). The Joint Exploration of the Twilight Zone Ocean Network (JETZON; https://www.jetzon.org/, last access: 1 June 2022) is a UN Ocean Decade Program that was created in 2020 to serve as a focal point for twilight zone studies. It includes 234Th measurements as one of the key methodologies and aims at acting as an international coordinating umbrella of projects such as COMICS, EXPORTS, CUSTARD, OTZ, SOLACE, APERO, and PICCOLO, among many others, providing a link between researchers working on these projects through JETZON so that cruises, data, results, and conclusions are shared. Note that so far, only data from the first expeditions of the EXPORTS project have been published (Buesseler et al., 2020a) and are available in the compilation (see peak in Fig. 2b and d). The benefits to be derived from the success of these endeavors and the inclusion of the remaining data in the compilation could trigger the beginning of a new era for the 234Th technique (i.e., era 5). Whether or not the publication of the 234Th measurements collected during these dedicated large-scale collaborative projects would mark the beginning of era 5 is yet to be known. 5 Steps towards improving the global data set We present here two perspectives that will improve the 234Th global data set and broaden its applicability: detect ocean regions with low 234Th measurements and identify which parameters could be useful to include in future versions of the compilation for a wider application of 234Th data. 5.1 Recommendations on 234Th sampling During the last few decades, considerable progress has been made towards unraveling the behavior of the BCP and understanding the factors influencing the carbon dynamics and the ocean carbon cycle, e.g., primary production, aggregation, ballasting, and the activities of zooplankton and bacteria (see reviews by, e.g., Buesseler and Boyd, 2009, De La Rocha and Passow, 2007, Sanders et al., 2014, and Turner, 2002). Capturing the spatiotemporal variability in 234Th concentrations could play a key role on our precise quantification of carbon uptake, storage rates, and subsequent ecosystem impacts. Strategies up to now have included time series of process studies in key ocean regions, global surveys of carbon parameters, TSSs, and models and databases built from field observations. However, there are gaps in the ship-based 234Th sampling. In terms of spatial distribution, the SH remains clearly undersampled. Of the total of 5134 locations compiled, 3351 belong to the NH (which accounts for a total of 65 % of the sampling locations) and only 1749 to the SH (35 %), with the 34 remaining ones corresponding to the Equator (<1 %). This is the result of the majority of expeditions taking place in the NH: 78 % solely in the NH and 8 % as part of surveys in both hemispheres. Figure 1a highlights an especially important gap in the South Pacific region and some additional notable gaps in the data set in the Benguela system (although this was included in the COMICS project, and data will be available in the near future), the Mauritanian upwelling, and the southern Indian Ocean. More attention should be paid to these regions when planning future expeditions for 234Th sampling. Moreover, Fig. 1c shows numerous established time-series stations that have never been sampled for 234Th, and we recommend visiting them in the next expeditions. In terms of temporal distribution, research cruises are frequently conducted between January and October, although data are skewed to early spring and summer in both hemispheres (see peak in March and October in Fig. S1b). 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