Fine-tuning of Pb-210-based methods for dating vegetated saltmarsh sediments

Fine-tuning of 210 Pb-based methods for dating vegetated saltmarsh sediments A.R. Iurian a , c , G. Millward b , W. Blake b , c , J.M. Abril Hern´andez d , * a Terrestrial Environment Laboratory, IAEA Environment Laboratories, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Vienna International Centre, PO Box 100, Vienna, 1400, Austria b Consolidated Radioisotope Facility (CoRiF), University of Plymouth, UK c School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth, PL4 8AA, United Kingdom d Departamento de Física Aplicada I, ETSIA Universidad de Sevilla, Carretera de Utrera km 1, Seville, D.P. 41013, Spain Keywords: 210 Pb radiometric dating Saltmarsh sediment Mineral sediment fraction Accretion rate Organic matter ABSTRACT Saltmarshes, wetlands, inter-tidal mudflats are highly productive natural ecosystems with significant ecological value. 210 Pb and 137 Cs have been used for establishing chronologies at a centennial scale in these sediments. They are relevant for assessing how saltmarshes are vulnerable to climate change, natural hazards and contamination from human inputs. This paper aims to review and adapt existing methods for the 210 Pb-based radiometric dating of these sediments where the subterranean production and decay of OM are sources of achronicity. This limits the use of the basic assumption of considering the sediment as a continuous medium. From the available evidence, 210 Pb fluxes onto sediments may be mediated by the activities bound to the mass flows, suggesting that models assuming a constant flux should be handled with care. The activity concentrations of excess 210 Pb required by CFCS, CIC and TERESA radiometric models must be those of the sediment mineral fraction, while CRS and PLUM can work with the composite sediment matrix. A review of these issues and application of the models is illus-trated with new data from two recent saltmarsh sediment cores. The effects of non-ideal boundary conditions in radiometric dating and the system time-average of 137 Cs fluxes are discussed. Although specific to vegetated saltmarshes, this topic and the present results have generic applications for other vegetated coastal sediments. 1. Introduction Saltmarshes are vegetated coastal wetlands of great ecological value being commonly characterized by accreting mudflats stabilized by spreading salt tolerant (halophytic) vegetation (Allen and Pye, 1992). They are of wide interest, including paleoecology inferring sea level changes (Barnett et al., 2017), carbon storage (Livesley and Andrusiak, 2012), marsh restoration and land reclamation (Bakker et al., 2002), anthropogenic impacts and environmental forcing (Kirwan and Megonigal, 2013), marsh contaminant storage and water quality (Cundy and Croudace, 2017), and responses to changing environmental controls and sea level change (Kolker et al., 2009). Establishing accurate dates and accretion rates in saltmarsh ecosystems is important, among other environmental studies, for i) the quantification of their carbon sequestration capacity (Kristensen et al., 2008; Nellemann et al., 2009; Marchand, 2017); ii) assessing how the acceleration in sea-level-rise linked to global warming could surpass the accretion capacity of vegetated saltmarsh sediments, resulting in an overall degradation of these habitats (Kennish, 2001; Hughes, 2004; Crosby et al., 2016); iii) decoding past anthropogenic impacts and environmental forcing (Kirwan and Megonigal, 2013). Estimation of sedimentation rates at a centennial scale have been reported using the 210 Pb-based radiometric dating method (Cundy and Croudace, 1995; Wilson et al., 2014; Yang et al., 2014; Ruíz-Fern´ andez et al., 2018). The 210 Pb-based ages may be supported by independent chronostratigraphic markers provided by the 137 Cs specific activity versus depth profile, or other radionuclides such as 241 Am and 207 Bi (Kim et al., 1997; Pedersen et al., 2007; Bellucci et al., 2007; Abbreviations: SWI, sediment-water interface; NID, non-ideal deposition; ECL, early compaction limit; SAR, sediment accretion rates; CR, concentration ratio; OM, organic matter; STA, System-Time-Average; CRS, Constant Rate of Supply; CF–CS, Constant Flux-Constant Sedimentation; CIC, Constant Initial Concentration; TERESA, Time Estimates from Random Entries of Sediments and Activities. * Corresponding author. E-mail address: [email protected] (J.M. Abril Hern´ andez). 2 new data from two saltmarsh sediment cores sampled in two marshes situated on Lynher Estuary and, respectively, Tamar-Tavy Estuary (South West England). 2. Materials and methods 2.1. Regional setting The Lynher Estuary forms a confluence with the River Tamar (Fig. 1) which in turn drains into Plymouth Sound (Natural England, 2016). The Lynher Estuary has a tidal reach of ~14 km and its coastline is dominated by upper saltmarsh communities. It is designated as a Site of Special Scientific Interest and a Special Protection Area and it has valuable salmonid and brown trout fisheries (English Nature, 2006). The land surrounding the Lynher is mainly under grass and accommodates rough grazing with mixed arable and dairy farming. The Tamar-Tavy Estuary is microtidal and flood dominant, reaching a spring tidal range of ~6.5 m. It is surrounded by mudflats and salt marshes in its lower and middle section, being granted a number of nature conservation designations for the richness of its natural diversity. The catchments of the rivers Lynher, Tamar and Tavy had experienced extensive metal mining for metals during the 18th and 19th centuries (Dines, 1956). Mining in the region reached its peak in the 19th century but had largely ceased by 1950, leaving piles of mine waste near adjacent rivers or channels providing contaminated fine sediments into the local rivers and streams over time (Mighanetara et al., 2009). The factors taken into account when choosing a representative saltmarsh sampling area were primarily the longevity (>100 years) of the habitat, which was obtained from historical maps collections, the extent and stability of the saltmarsh, the potential for undisturbed sediment accretion and accessibility for sampling. Antony Marsh, lying on the southern bank of the Lynher, is a large, unaltered expanse of saltmarsh in the Lynher/Tamar riverine complex, divided by the main creek channel. Bere Ferrers Marsh is an embayment marsh, laying on the western bank of Tavy river (Fig. 1). Historical maps of Tamar/Lynher estuarine system (Ordnance Survey, 1809) show that in 1809, the two marshes were not mapped, while the marshes appear on the maps from early 20th century (Edina Digimap collection UK, 2015). The marshes may have formed, or expanded, in the middle of 19th century, during a period of increased sediment availability from mining operations in the upper catchments of the Lynher/Tamar riverine system and in the period that followed their cessation. Both marshes have typical halophytic vegetation near the foremarsh e.g. common cordgrass (Spartina anglica) and common saltmarsh-grass (Pucinella maritima) (Fig. S1, in electronic supplementary material -ESM), having a cliff at their seaward margins. 2.2. Sampling, sample treatment and physical measurements Two sampling sites were chosen on stable ground in the middle of Antony Marsh and Bere Ferrers Marsh, respectively at reasonable distance from the creek channel (see Fig. S1, in electronic supplementary material - ESM). Depth continuous sediment sequences of 45 cm (Antony Marsh) and 29 cm (Bere Ferrers), respectively, were collected in the form of two monoliths, representing the side wall of a pit. This sampling strategy was appropriate i) for the collection of the OM–rich upper sediment layers without disturbance, ii) for the avoidance of compaction while sampling, and iii) for obtaining sufficient material of the upper sediment layers for laboratory analysis. The monolith cores were stored at −20 ◦C and subsequently processed in a clean laboratory environment (Fig. S2a, in ESM). The monolith sequences were sectioned in 1 cm layers under partially-frozen conditions, which allowed the better processing of the porous material rich in organic matter. Distinct stratigraphic zones (Fig. S2, in ESM) were observed during the processing of the monolith cores, both with an García-Artola et al., 2016). In some cases, the agreement between the 210 Pb and 137 Cs derived ages is poor, or they are incompatible (Madsen et al., 2007; Marshall et al., 2007; García-Artola et al., 2016). Difficulties may arise, at least partially, from the application of radiometric dating methods originally developed for lacustrine (Krishnaswamy et al., 1971) and marine (Koide et al., 1972) sediments, without any tuning for the specific conditions pertaining in vegetated coastal sediments or without proper consideration for the specific assumptions underlying the models. Arias-Ortiz et al. (2018) presented a comprehensive review of 210 Pb-based dating models for recent sediments with focus on vegetated coastal ecosystems. The paper considers and tries to numerically simulate a series of factors (namely mixing, acceleration in mass accumulation rates, erosion, grain size and organic matter decay) which can alter the shape of ideal 210 Pb profiles and then lead to misestimating ages, sediment accretion and carbon accumulation rates in the routinely application of CF-CS and CRS models. Annex A presents a brief description of the 210 Pb dating method and reviews the assumptions adopted by a series of models excluding postdepositional redistribution, namely the models CRS (constant rate of supply), CIC (constant initial concentration), CF-CS (constant flux with constant sedimentation), SIT (sediment isotope tomography), TERESA (time estimates from random entries of sediments and activities), CS (constant sedimentation rate), PLUM (the Bayesian formulation of the constant-flux model), serac (a R-software package including some of the previous models) and STA (system-time-average, for pre-depositional processes affecting man-made radionuclides). Discussions on the limitations in their use arising from a partial or null accomplishment of the involved assumptions related to fluxes of matter onto the sediment and radionuclides behaviour are included in Annex A. Corrections in dating models due to grain-size effects are also considered. All models and the diagenetic equations (Abril, 2003) share the fundamental assumption that the sediment can be treated as a continuous medium, which is characterized by two phases: solids and pore water. The radionuclides of interest are assumed to be particle bound, so they are virtually absent from the pore water, and new inputs are ideally deposited on the sediment-water interface (SWI) over the previously existing material. Scenarios with non-ideal deposition have been described by Abril and Gharbi (2012). They arise from the failure of the above assumption when inputs of radionuclides are in the dissolved phase and sediments have high porosities in their top layers (see Annex A). The above basic assumption of a continuous medium can also fail in vegetated coastal sediments. The major difficulties arise from their high content in organic matter (OM) which represents a distinct phase due to the achronological character of its belowground production and decay (Allen, 2000), its non-negligible radionuclide concentrations, and its distinct advective transport. Thus, while the mineral fraction within the sediment may retain some chronological information, each sediment slice also contains a depth and time dependent amount of OM which may be decoupled from the chronology. This then raises questions about the estimation of accretion rates from the composite mass thickness lying between two dated horizons. A reliable understanding of how early compaction operates in saltmarshes is another key question for estimating reliable linear accretion rates (commonly used in sea level rise studies). Arias-Ortiz et al. (2018) pointed out the effect of OM decay in the increase of 210 Pb concentrations in the composite sediment matrix, but they did not account for belowground production, among other simplifying assumptions. Boyd et al. (2017) distinguished among mineral sediment and organic matter accumulation rates. But the above works still used the composite concentrations in the CF-CS model. This paper is aimed at reviewing and tuning, the methodology for the 210 Pb-based radiometric dating of vegetated coastal sediments, specifically saltmarshes, focussing on the mineral fraction as the physical carrier of the chronological information. The work is illustrated with 3 upper zone (up to 18 cm and 8 cm depth, respectively) with a rich rooting system. The sample processing included freeze-drying, weighing, fine grinding, and sieving to a particle size of <1 mm. The moisture content of each sample was determined by the difference between its wet (mws) and dry (ms) weight. The dry bulk density values, ρ b, were estimated by dividing the total dry weight, ms, by the volume of each centimetre sediment layer: ρ b=ms V(1) For saline pore-fluids m* s is the corrected value of ms by subtracting the amount of precipitated salts. The corrected ρ * b is derived as following (Dadey et al., 1992): ρ * b= ρ bϕs;ϕs=m* s ms =(1−s mws/ms 1−s)(2) where s is the fraction (in weight) of salts in the pore fluid. It is also useful defining fw as the fraction in weight of water (fw= (mws −ms)/ mws). 2.3. Organic carbon (OC) and loss on ignition (LOI) Homogenized sub-samples of every third 1 cm slice were pre-treated with 0.1 M HCl to remove inorganic carbon. Dried sub-aliquots were encapsulated in pre-weighed Sn foil cups, re-weighed when packed and measured for organic carbon content (Shimadzu TOC-V Analyser). Calibration was performed using L–Cystine (30% carbon content), while the Certified Reference Material PACS–1 (National Research Council of Canada) was utilized as Quality Control (QC). An average of three replicate measurements of the PACS–1 gave a relative difference of 2.9% by comparison with the certified value. Organic matter content was determined by the Loss-On-Ignition (LOI) method. Every third sample from both monolith cores was analysed for both LOI and organic carbon. Prior to ignition, sediment samples and empty ceramic crucibles were preconditioned by ovendrying at 105 ◦C overnight. Ignition of ~1 g of sample material was undertaken in a muffle furnace at 440 ◦C for 3.5 h (following the British Standard BS 1377–3:1990: Methods of test for soils for civil engineering purposes. Chemical and electro-chemical tests). A QC sample of known LOI was included in the sample batch. Three samples were prepared in triplicate given the heterogeneous appearance of the material and mean and relative standard deviation (SD) values were determined for these Fig. 1. Location map of Antony Marsh, Lynher estuary, and Bere Ferrers Marsh, Tavy estuary (South-West England). Other saltmarshes on the upper estuary of Lynher (Treluggan Marsh and Port Eliot Marsh) and lower estuary of Tamar (South Tamar Marsh) are also indicated. 4 continental runoff (see the review by Allen, 2000). The composite flow of matter ensures the sediments undergo an ordered growth in time. Nevertheless, plant roots, rhizomes and tubers grow ‘in-situ’ because of non-localized photo-assimilation (biological compounds produced by photosynthesis). They occupy the pore spaces and become the major contributor to sediment structure in the rhizosphere. This contribution from below ground biomass represents a depth-distributed input of matter which is independent from the ages ascribed to any sediment horizon. Fig. 2 illustrates the root zone in vegetated salt-marsh sediments and the two-mass flow components. The below ground productivity of Spartina, a common type of vegetation in saltmarshes, has shown to vary between 0.34 and 6.04 kg m −2 a −1 (dry weight) being broadly similar for other species (Allen, 2000). In the absence of decay, the productivity range above is equivalent to a linear accretion rate of 1–10 mm a −1 (Allen, 2000). These figures are increased by above ground productivity and allochthonous inputs. Nevertheless, a significant proportion of the organic material introduced below ground eventually decays as the result of bacterial and fungal processes, the activities of invertebrate grazers, and the presence of oxygen and moisture, with decay rates largely depending on several factors (Allen, 2000). Values of 12–50% per year have been reported for Spartina in the upper few decimetres of the saltmarsh sediments (Allen, 2000), where unsaturated conditions often prevail. The decay rate of OM decreases as the sediment becomes permanently waterlogged and anoxic conditions prevail. Consequently, the OM is mineralized by an anaerobic food chain whose final by-products are carbon dioxide and methane (Middelburg et al., 1993). The bulk density (Eqs. (1) and (2)) can be divided into its organic ( ρ b,org) and mineral ( ρ b,min) components. ρ b,org =f* org ρ * b=forg ρ b, ρ b,min = ρ * b− ρ b,org (3) where forg =morg/ms, with morg being the mass of organic matter (determined by LOI), and f* org =forg/ϕs is its salt-corrected value. The mass depth m(z) is the dry mass per unit area accumulated from the SWI until the sediment horizon, which during the slicing operation is at actual depth z. It is the physically meaningful depth-scale for radiometric dating since it remains invariant under natural compaction and Fig. 2. Sketch illustrating the root zone in vegetated salt-marsh sediments and the two-mass flow components: i) w 0 is the flow of solids entering through the sediment-water/atmosphere interface, and ii) w(z) is the composite mass flow, which results from the one above and S(z); this last is the net balance of root growth from non-localized photo-assimilation and the mineralization rate with gaseous exchanges. samples. Relative SD ranged from 2.0% to 4.6% (later value given for the first sediment layer). The OM content was calculated as the difference in weight before and after the ignition. 2.4. Radionuclide measurements Gamma-ray energies at 46.5 keV, 351.9 keV, 661.6 keV and 59.5 keV were evaluated for the determination of 210 Pb, 214 Pb (assumed to be in secular equilibrium with its parent 226 Ra) and 137 Cs and, where possible, 241 Am, in saltmarsh sediments for radiometric dating. Homogenized sub-samples of grain-size <1 mm were placed into individual aluminium containers (inner diameter 47.50 mm, filling height to the rim 15.00 mm) or into high density polyethylene vials (inner diameter 12.25 mm, filling height 41.3 mm) depending on the available sample mass, compressed and sealed with glue and electrical tape. Gamma-ray measurements begun after incubation for four weeks during which secular equilibrium was achieved between 222 Rn and its progenies (e.g. 214 Pb). Two Ortec gamma detectors (Ortec, Wokingham, UK) were used for the gamma measurements: an n-type coaxial HPGe detector, with 50% relative efficiency, and a well HPGe detector. Counting live time was set to approximately 48 h to achieve a statistically significant number of counts and a counting statistics of about 10% or less for the 46.5 keV photopeak. Special care was taken to maintain these standard counting conditions for all samples. The commercially available software (GammaVision, Ortec) was used for data acquisition, while data analysis was performed in EXCEL, in combination with Monte Carlo simulation using EFFTRAN (Vidmar, 2005). The detector efficiency was calibrated over the energy range 46.5–1836 keV using a secondary standard soil material in the same geometry as the samples. The standard was created by spiking a soil material of known chemical composition with a gravimetrically-determined quantity of a multi-gamma standard solution (Eckert and Ziegler; traceable to the National Institute of Standards and Technology - NIST). The efficiency transfer approach and EFFTRAN derived efficiency transfer factors were employed to account for differences in chemical composition between the soil standard and saltmarsh samples (Iurian et al., 2018). The chemical composition of the sample matrix, used to derive the efficiency transfer factors, was determined by Wavelength Dispersive X- Ray Fluorescence (WD-XRF) in pressed pellets, using a state-of-art PANalytical XRF Spectrometer and dedicated software for data interpretation. The accuracy of the XRF results was checked by repeated measurements (n = 3) of the CRM SdAR-M2, a metal-rich sediment (US Geological Survey). The maximum deviation from the reference values was 8%. Activity concentrations were decay-corrected to the sampling date and corrected for background fluctuations and moisture content, the latter estimated by oven-drying about 1 g of material at 105 ◦C. The activities are reported with their combined standard uncertainties at a 1 σ confidence level, determined according to the Guide to the Expression of Uncertainty in Measurement JCGM 100:2008 (JCGM, 2008). Quality control was undertaken using IAEA Proficiency Test Materials with similar matrix characteristics to the Antony sediment samples (e.g. IAEA-CU-2009-03 moss-soil and IAEA-TEL-2016-03 soil). External quality control is assured by regular laboratory participation in external proficiency tests: e.g. IAEA (international); National Physical Laboratory (United Kingdom), Max Rbner Institute Karlsruhe (Germany). 2.5. 210 Pb-based radiometric dating models adapted to vegetated saltmarsh sediments 2.5.1. Mass flows in saltmarsh sediments and the continuity equation Flows of solids onto the saltmarsh sediment-water interface (SWI), or onto the sediment-atmosphere interface–under unsaturated conditions, consist of mineral or inorganic particles and detrital organic matter from plant tissues and other inputs associated with suspended loads and 5 m(z) = ∫ z 0 ρ b(z′)dz′;morg(z) = ∫ z 0 ρ b,org(z′)dz′;mmin(z) = ∫ z 0 ρ b,min(z′)dz′ (4) The continuity equation for bulk density, ρ b, expressing the mass conservation for solids in non-vegetated sediments is (Abril, 2003): ∂ρ b ∂ t= − ∂ w ∂ z(5) Eq. (5) applies to a reference framework located at the SWI, with depth z measured positive downwards. At any time, and at the sediment horizon at depth z, the flow of matter, w (z,t) [M L −2 T −1 ], can be written in terms of the linear accretion rate, v (the velocity at which the SWI displaces vertically due to overall processes contributing to sediment accretion) plus the relative velocity of solids with respect to a fixed (at rest) framework, q. Thus, w(z,t) = [v(t) + q(z,t)] ρ b(z,t)(6) The flow of matter driven by the compaction potential vanishes at the early compaction limit (ECL), where the bulk density reaches a virtually constant value ρ b,ECL (Abril, 2003, 2011). Then, from Eq. (6): w(zECL,t) = v(t) ρ b,ECL (7) The steady state solution of Eq. (5) ( ∂ρ b ∂ t=0)implies that the mass flow, w, is constant with depth z. Then the mass flow at the SWI, w 0 , usually known as SAR, satisfies the relationship: w0=v ρ b,ECL (8) In vegetated saltmarsh sediments Eq. (5) can be generalized by separately applying it to the mineral and the organic phases, and by including the depth-dependent net balance of below-ground production and decay, labelled as s(z) [ML −3 T −1 ]: ∂ρ b,min ∂ t= − ∂ wmin ∂ z= − ∂ ∂ z( ρ b,minv+ ρ b,minqmin)(9a) ∂ρ b,org ∂ t= − ∂ worg ∂ z+s(z) = − ∂ ∂ z( ρ b,orgv+ ρ b,orgqorg)+s(z)(9b) Importantly, the linear accretion velocity of the SWI is the same for both components. Here the terms q min and q org can be interdependent, following a more complex law than that reported by Abril (2011), but which falls beyond the scope of the present work. In general, they will take different values, and thus, the chronological information eventually attached to the detrital OM entering through the SWI advances at a different rate when comparing with mineral solids. Moreover, the term s (z) breaks down any chronological information attached to the OM as a whole. Although the situation is complex, it is possible to get some helpful insights from the above physical fundamentals. Below the rhizosphere, a physical environment of water pores and solids of uniform density is progressively re-encountered, allowing for an ECL. Let us assume that by some means, ages t 1 and t 2 have been estimated for the mineral fraction at the sediment horizons z 1 and z 2 . The amount of mineral matter per unit area (mineral mass thickness) comprised between both horizons, Δm min , can be empirically determined. The mean value of the mineral SAR between the above two ages is w min =Δm min /Δt; with Δt =t 2 – t 1 . When after some elapsed time this material reaches the ECL, it will be bounded by horizons at depths z′ 1and z′ 2, with the same absolute difference in ages, Δt, but with dz′=z′ 2–z′ 1< dz =z2−z1 due to self-compaction. Thus, the ratios dz′/Δt and dz/Δt only describe the state (in terms of early compaction) of the mineral mass thickness Δm min at different times of observation (or different times of sediment sampling), and they are not describing the linear accretion velocity of the SWI at any time. At the ECL the mass-flow ρ b,minqmin vanishes, and it can be said that w min , a former mass flow at the SWI, will finally produce a linear accretion velocity of the SWI given by v=wmin/ ρ b,min,ECL (10) The effect of plant growth is reflected in the OM content beyond the ECL, which competes with the mineral fraction for the bulk volume leading to lower values of ρ b,min,ECL than in OM-free sediments, and then to higher v values. 2.5.2. Radionuclides in sediments and transfer to plants Radionuclides uptake by plants has been extensively studied for the soil-plant system, and it is described as a series of two processes: (1) radionuclides partition between soil and soil solution (defined by the partition coefficient, K d ), and (ii) radionuclides uptake by plants from the soil solution (defined by the Concentration Ratio (CR) parameter as the ratio between activity concentration in plant material and the one in the substrate – typically at the rooting zone i.e. approximately the upper 10 cm layer for grasses –e.g., see IAEA, 2010). A compilation of literature data on concentration ratios (CR) for some radionuclides of interest in radiometric dating has been made (Table 1), and directly compared with site and plant specific data from estuarine saltmarshes in Plymouth Sound (SW, England). Concentration ratios (CR) of 210 Pb and 226 Ra for UK saltmarsh plants seem to be comparable with the TRS 472 (IAEA, 2010) generic values for grasses (stems and shoots) and 2–3 fold higher than rice mean values (under flooded conditions) (Table 1). A significant positive correlation (Pearson Correlation Coefficient r =0.919, p <0.001) was found between 137 Cs CRs and organic matter (%) (Sanchez et al., 1999). However, the difference in CR values seems to be significant only for very high OM content (OM ≥90%) (see Table 1). Any sediment sample with dry mass ms is composed by a mass forg ms of organic matter and a mass [1−forg − (1−ϕs)]ms of mineral mater (without salts). If they could be physically divided, the studied radionuclides would be bound to these fractions with activity concentrations Aorg and Amin, respectively, but in practice we measure the activity concentration Ab in the bulk (composite) sediment sample. The following relationship follows from the above definitions: Table 1 Radionuclide concentration ratios, CR (geometric means), for 226 - Ra, 210 Pb, 137 Cs, 241 Am (dimensionless). Vegetal tissue 226 Ra 210 Pb 137 Cs 241 Am Aerial plant (saltmarshes UK) a 2.0 x 10 ¡1 3.5 x 10 ¡1 Rice b 8.7 × 10 −4 8.4 × 10 −3 8.3 ×10 −3 Root crops (roots) b 7.0 × 10 −2 1.5 × 10 −2 4.2 ×10 −2 6.7 × 10 −4 Root crops (stems and shoots) b 7.1 × 10 −2 6.3 × 10 −2 Grasses (stems and shoots) b 1.3 × 10 −1 3.1 × 10 −1 6.3 ×10 −2 3.3 × 10 −2 Grasses (stems and shoots); organic substrate (OM ≥20%) b 2.8 ×10 −1 Grasses; organic substrate (OM ≥ 90%) c 3.6–4.4 (x 10) Grasses; organic substrate (OM 48%, clay 6.1%) c 7.4 ×10 −1 a Geometric mean of two values (authors’ personal data from estuaries of SW England). b IAEA (2010). c Sanchez et al. (1999). the sediment shortening during coring, storage and extrusion operations. The organic (morg(z)) and mineral (mmin(z)) mass-depth scales can be defined as follows: 6 Ab=Amin(1−forg − (1−ϕs))ms+Aorgforg ms ms =Amin(1−forg − (1−ϕs)) +Aorgforg (11) At this point, and in absence of more precise determinations of the ratio Aorg/Ab, we will assume that it can be approached by a reference CR value (Table 1). The best choice for CR could be a site and plant specific value, preferably determined from root tissues. This could be thought to be a first order approach due to the varying contributions to OM (living tissues and detrital) at different sediment depths. It will be shown below that this approach can be reliable in the tuning radiometric dating models, mainly because of the low order of magnitude of CR. This approach allows estimating Amin from Eq. (11) in terms of the experimentally measured A b : Amin ≅Ab(1−CR forg) 1−forg− (1−ϕs) (12) The partitioning of the studied radionuclide between the mineral and organic phases can be quantified. In a sediment sample of mass m s , the activity contained in the mineral component is A min [1–f org − (1−ϕs)] m s while the total radionuclide activity is A b m s , and their ratio is (from Eq. (12)) 1–CR f org . Thus, in the case of 210 Pb, when using the CR value for plant bodies in UK saltmarshes (Table 1), and with f org =0.5, it results that 82.5% of the activity is bound in the mineral phase. Thus, when A b is used instead of A min (which is the one linked to the chronological information), the major effect of a high OM content is a dilution, or a decrease, of A b (i.e. the flattening in A b often observed in the upper sediment layers occupied by the rhizosphere). This can lead to spurious SARs and chronologies. Equation (12) can be assessed using different values of CR and f org to conclude that for CR of an order of 10 −1 the dominant effect in the A min / A b ratio is the ‘dilution’ given by f org . Furthermore, sensitivity tests will be conducted in the next sections for assessing the effect of a range of values for the CR, situated within 1.5 ×10 −2 and 3.0 ×10 −1 . 3. Results Tables S1 and S2, in the supplementary material, report the raw results from physical measurements (section 2.2), OC and LOI (section 2.3), and radionuclide mass activity concentrations (section 2.4) for the Antony Marsh and Bere Ferrers cores, respectively. 3.1. Bulk density, OM and LOI Fig. 3 shows the results of the water content for both cores, along with bulk densities estimated by (Eq. (1)) and (Eq. (2)), by using s = 0.025 according to the values reported by Ralph (1956) for interstitial salinities of intertidal muds at several locations along the Tamar Estuary. Correction by Eq. (2) implies a maximum absolute deviation of 0.03 g cm −3 , or a maximum relative deviation of 13.9% (data for the Antony Marsh core). Note that propagated relative uncertainties of about 5–10% can be expected in the estimation of bulk density (Eq. (1)), mainly contributed by the associated uncertainties in the bulk volume of the samples. Fig. 3 also reports the percentages of clay +silt (particle sizes <63 μ m) determined for each sediment slice in the two studied cores. Twelve samples from the Antony Marsh core, in the range 0–45 cm, have been measured for C org (%) and LOI (%) to establish a site-specific linear regression, together with data on their silt +clay fraction (<63 μ m). The clay content of the samples ranged from 1.8% to 3.6% and silt was the major constituent. A multiple linear regression analysis (LOI versus C org and particle sizes <63 μ m) was conducted using Statgraphics Centurion XVI. LOI (%) showed a significant positive correlation with C org (%), r =0.917, but no significant correlation with the silt +clay fraction. An atypical residue was found for data from the uppermost sediment slice, which also showed a high relative standard deviation in the replicated analysis (see Table S1). When discarding this point, the correlation coefficient LOI versus C org was r =0.946, with p <0.001. The regression equation being: LOI(%) =-2.68 +2.94 Corg(%) (13) The intercept is compatible with zero, since it is affected by a standard error over 100%. The slope (2.94 ±0.34) is noticeably higher than the van Bemmelen factor of 1.724, but comparable to the factor 2.41 inferred from the correlation reported by Kauffman and Donato (2012) for mangrove forest sediments. High precision (RSD, 1.2%) in TOC on LOI prediction was found by De Vos et al. (2005) in forest floor samples with high organic matter content (LOI >5%). In the Antony Marsh core, C org (%) was determined for ~33% of the samples. For the non-measured slices, linear interpolations were used to complete the record. LOI (%) was determined for 33% of the samples in the 0–25 cm depth range, and for various slices at greater depths. For the non-measured sediment slices, linear interpolations were used in the 0–25 cm depth range, and the empirical Eq. (13) for the rest (Fig. 3). For the Bere Ferrers core it has been assumed that Eq. (13) can be applied for estimating LOI(%) from the C org (%) values. Linear interpolations were used for completing the LOI(%) record (Fig. 3). Fig. 3. Water content (given as the fraction f w ), bulk density, LOI (%) and percentage of clay +slit (particle sizes <63 μ m) in the Antony Marsh and the Bere Ferrers cores. 7 model (Robbins et al., 2000, and Annex A) attempts to account for this effect. The best fit of the STA model for 137 Cs in the mineral fraction of the two cores (by using the atmospheric deposition record from Fig. S3, ESM, data from Wright, 2016) is depicted in Fig. 4. The time parameter is k r =0.068 a −1 for Antony Marsh, and k r =0.070 a −1 for Bere Ferrers cores. These values are comparable to others reported in the scientific literature (e.g., Robbins et al., 2000; Abril et al., 2018). The STA model approximates to the observed recent 137 Cs activity concentrations, which are higher than expected from atmospheric deposition. Consequently, it can be concluded that recent 137 Cs fluxes onto the SWI must be governed by the 137 Cs activity concentration attached to the (mineral) mass flow entering into the SWI. Additionally, the 137 Cs profiles in the sediments extend deeper than expected from the STA model. This can be explained by the non-ideal deposition of 137 Cs fallout reaching the SWI shortly after the bomb-fallout peak (see Annex A). From the data in Tables S1 and S2, the total 137 Cs inventory for the Antony Marsh core is ΣCs =545 ±13 Bqˑm −2 , and 1032 ±12 Bqˑm −2 for the Bere Ferrers core, while the integrated atmospheric deposition at the site, decay-corrected to the date of sampling is 355 ±38 Bq m −2 (error estimates from a uniform relative standard deviation of 16% for each annual data). Thus, ΣCs is greater than the integrated atmospheric deposition. This agrees with the notion of inputs onto the SWI being governed by the mass flow from the catchment. 3.3. Varying 210 Pb exc fluxes onto the SWI The widespread use of the Constant Rate of Supply (CRS) model has resulted in the general acceptance of the assumption of a constant rate of supply as the paradigm describing the functioning of natural aquatic ecosystems. Nevertheless, in all those cases where it has been possible to reconstruct 210 Pb exc palaeofluxes onto the SWI (e.g., in varved sediments for which an independent chronology is available), they show large variations over time and a positive correlation with SAR (Abril and Brunskil, 2014). A more detailed discussion of the two mass-flow components with intrinsic scatter (the conceptual model which explains the available empirical evidence) can be found in the above reference. Fig. 5 shows the 210 Pb exc and 137 Cs inventories in five vegetated saltmarshes sediment cores sampled at different locations within the same Plymouth Sound estuarine environment (UK), plotted versus the SAR value estimated from the composite mass depth and the position of the 137 Cs fallout peak ascribed to 1963. Here the estimation of SARs can be considered only as a proxy, but the fundamental fact is the positive Fig. 4. 137 Cs activity concentrations (in the composite sediment) versus depth in the two studied sediment cores. 137 Cs activity concentrations in the mineral fraction have been estimated after Eq. (12) for CR =1.0 ×10 −2 and CR =1.0 × 10 −1 . In both cases vertical error bars are 1 σ analytical uncertainties (omitted for CR =1.0 ×10 −2 for graphical clarity), while horizontal bars delimit the depth interval of the measured sediment slice. The predicted profiles in the mineral fraction after the system-time-average (STA) model (Robbins et al., 2000; Annex A) are also depicted. Fig. 5. Measured 210 Pb exc and 137 Cs inventories in five saltmarsh sediment cores from the Lynher estuary (see Fig. 1) versus the SAR value estimated from the composite mass depth and the position of the 137 Cs fallout peak ascribed to 1963 (data from authors personal records). 3.2. Results on 137 Cs: preservation of the chronological information and STA model Fig. 4 shows the 137 Cs activity concentrations in the composite sediment versus depth in the two studied sediment cores. The saltcorrected 137 Cs activity concentrations in the mineral fraction (after Eq. (12)) are also depicted for CR = 1.0 × 10 − 2 and CR = 1.0 × 10 − 1 , and using s = 0.025. In both cases, a well-defined 137 Cs peak is observed. This is commonly accepted as a confirmation of the preservation of the chronological information. Applications of 137 Cs as a chronostratigraphic marker in saltmarsh sediments can be found, among others, in Cundy and Croudace (1996); Bellucci et al. (2007); Leorri et al. (2008) and García-Artola et al. (2016). Estimations of 137 Cs concentrations in the mineral fraction in the two cores (after Eq. (12)) reveal that the dilution effect by OM is quite noticeable, being higher in the Antony Marsh core because its higher OM content (Fig. 3), and that here the accurate estimation of the CR factor plays a minor role. It is worth noting that none of the 137 Cs versus depth profiles in Fig. 4 is a mirror-image of the atmospheric deposition record (see Fig. S3, in ESM). Particularly, concentrations in recent years (upper sediment slices) are much higher than expected. However, this is a common feature in most of the sedimentary systems, and the system-time-average (STA) 8 establishing a chronology. Estimations of 210 Pb exc concentrations in the mineral fraction in these cores (after Eq. (12)) reveal that the dilution effect by OM creates a visible variation of these concentrations with depth. The effect is higher in the Antony Marsh core because its higher OM content (Fig. 3). Sensitivity tests for CR reveal the importance of handling site and plant specific values. It is worth noting that other works assumed CR =0 for studying the effect of OM decay in 210 Pbbased dating models (e.g. Arias-Ortiz et al., 2018). 3.4.1. CRS model in vegetated saltmarsh sediments The assumptions adopted for the CRS model and its basic equations are summarized in Annex A. In vegetated saltmarsh sediments the major limitations can be: i) non-random temporal variability in fluxes (Abril, 2019); ii) the system not being old enough to ensure a steady state inventory; iii) the partitioning of the inventory into mineral and organic fractions with different mass flow rates (w min , w org in Eq. (9)), so that Eq. A-2 is not strictly applicable. In cases where the first two limitations do not apply, the third can be surpassed when most of the activity remains bound to the mineral fraction; this is, when CR x f org ≪ 1, as it seems to be feasible for 210 Pb exc (see Section 2.5.2). In these cases Eq. A-2 should produce close chronologies when consistently applied to both the mineral phase or to the composite sediment. By using Eq. A-2 with the composite sediment, the computed CRS ages are depicted in Fig. 7, along with the 137 Cs time-mark, for the two studied cores. When using Eq. A-2 with the mineral phase with the two extreme values for CR, the chronologies closely match the previous ones (Fig. 7). Linear SARs (Fig. 7) show a trend of increasing in recent years in both cores, which can be a spurious effect due to the early compaction (see Section 2.5.1) and to the contribution of achronological belowground production of OM. Physically meaningful palaeofluxes of inorganic solids (or mineral SARs) can be estimated from the mineral mass thickness and the dated horizons. Only the trend-lines or mean values of CRS-SARs over relatively large time periods have a physical meaning (Abril, 2019, 2020). For the Antony Marsh sediment core, the CRS mineral SARs post-dating the 137 Cs peak slightly fluctuate about a value of 0.030 g cm −2 a −1 , with large fluctuations for older layers, and a mean value of 0.043 g cm −2 a −1 over the last century. In the Bere Ferrers core, the mineral SAR also shows acceleration in recent years, which is the expected model outcome to the pronounced flattening of the 210 Pb exc profile (Fig. 6). In this core the CRS chronology does not match well the 137 Cs time mark (Fig. 7); this can be attributable to a weak accomplishment of the model assumptions. Particularly, the assumption of a constant flux can hardly hold under a continuous trend of increasing SAR in scenarios where the inputs of radionuclides are linked to the mass flows, as seem to be the case for these cores (Figs. 4 and 5). Fig. S4, in ESM, shows a comparison among the chronologies produced by the CRS and the PLUM model when applied to the composite sediment in both cores. For PLUM application, the current version for Python has been used (see Annex A), with the default settings. Exception was the parameter Al, involved in the definition of a chronological limit (linked to the analytical detection limit), for which we tested the values Al =0.1 and Al =5.0. Fig. S4 plots the mean and the 95% confidence interval (dashed lines) for PLUM. The two models converge for recent ages, but PLUM overestimates older ages, slightly in Bere Ferrers core, but with strong discrepancies in the Antony Marsh core. PLUM did not fit the 137 Cs time mark in these cores. Its current version for Python may still need some adjustments, so no further implications are considered here. The use of the assumption of a constant flux can be independently tested by the estimation of the equivalent constant fluxes pre- and postdating the 1963 time-mark of 137 Cs from the 210 Pb exc partial inventories (e.g., Abril, 2019). For the Antony Marsh core, the estimated fluxes pre- and post-dating 1963 are 125 ±13 Bq m −2 a −1 and 106.1 ±2.2 Bq m −2 a −1 , respectively, whose difference is not statistically significant at 95% confidence level (Least Significant Difference (LSD) test). Fig. 6. 210 Pb exc vs depth (cm) in the composite sediment and its estimated values in the mineral fraction (from Eq. (12), using LOI from Fig. 3, s =0.025, and CR =1.5 ×10 −2 and CR =3.0 ×10 −1 ) in the two studied cores. Error bars are 1 σ propagated uncertainties. correlation between inventories (related with fluxes onto the SWI) and SARs, which are consistent with results by Abril and Brunskil (2014) and the above insights from 137 Cs. 3.4. Fine-tuning radiogeochronological models for vegetated saltmarsh sediments The key idea for adapting the existing radiometric dating models is the use of specific activity concentrations in the mineral phase (Amin) and the mineral mass depth scale. Determining the mineral mass depth is quite straightforward (Eq. (4)), whereas the former requires further methodological treatment (including the physical separation of the mineral phase). As discussed in Section 2.5 and owing to the relatively low values of CR reported in Table 1, the application of Eq. (12) on a local basis can provide a reasonable proxy for A min . This will be the approach adopted in what follows. Since the aim is a methodological development, rather than an in-depth discussion of the case study, the CR = 7.0 × 10 − 2 value was adopted for 226 Ra, as the reference value for roots of root crops from the IAEA TRS 472, reported in Table 1. For 210 Pb the sensitivity of results was tested using extreme CR values of 1.5 × 10 − 2 and 3.0 × 10 − 1 , corresponding, respectively, to root crops, and to grasses and aerial parts of the plants in the studied saltmarshes (Table 1). Fig. 6 shows the 210 Pb exc profiles for the two studied cores, with mass activity concentrations referred to the composite sediment and to its mineral fraction, the latter being estimated for the above two CR values. The depth distribution function of 210 Pb exc has a major importance in 9 Nevertheless, for Bere Ferrers core, the equivalent constant fluxes pre- and post-dating 1963 are 76 ±9 Bq m −2 a −1 and 134 ±4 Bq m −2 a −1 , respectively, with a statistically significant difference at 95% confidence level (LSD test). 3.4.2. CF–CS model in vegetated saltmarsh sediments The assumptions adopted for the CF-CS model and its basic equations are summarized in Annex A. Fig. 8, using a logarithmic scale, plots 210 Pb exc concentrations in the composite sediment and in the mineral phase for both cores (each one with its corresponding mass depth scale), and considering the two extreme values for CR. It is evident that the assumptions of the CF–CS model clearly fail in the upper and deepest portions of the cores for both mineral and composite concentrations. A piecewise CF–CS model would interpret the flattening of the 210 Pb exc profiles in the upper portion of the cores as the result of very high SARs, as in the case of a single depositional event, what seems unrealistic for these cores. Eq. A-4 has been fitted to the portions of the 210 Pb exc profiles shown in Fig. 8, where a linear trend of decrease is quite apparent. SAR values (-λ/slope) can be estimated from these fits and compared with the average SAR value post-dating the 1963 peak in 137 Cs fallout (see Tables S1 and S2). Results are summarized in Table 2. Although the application of the CF-CS model to the composite sediment leads to numeric values for SAR, they do not have a physical meaning, since sediment masses and their activity concentrations are accompanied by a varying amount of ‘achronological’ organic matter, as explained above. Since mineral SARs have been estimated only for a portion of the core, several constraints must be noted: i) the mean SAR value does not apply to the entire core, nor to the entire portion post- Fig. 7. CRS chronologies for the Antony Marsh and the Bere Ferrers sediment cores, as a function of depth (cm). They have been computed using Eq. A-2 with the composite sediment and with the mineral fraction using two CR values. The 137 Cs time-mark (see Fig. 4), ascribed to 1963, is also depicted. Error-bars for ages are reported only for the composite sediment. Linear SAR (Δz/Δt) and mineral SAR (Δm min /Δt) are represented after normalizing to their arithmetic means in the 0–42 cm interval (0.48 cm/a and 0.043 g cm −2 a −1 , respectively) for Antony Marsh and 0–23 cm interval (0.20 cm/a and 0.048 g cm −2 a −1 , respectively) for the Bere Ferrers core. The mineral mass depth scales can be seen in Tables S1 and S2. Fig. 8. Natural log of 210 Pb exc concentrations in the composite sediment and in the mineral phase for both cores (each one with its corresponding mass depth scale), and considering the two extreme values for CR. The straight lines are the fit of the function from Eq. A-4 to the portion of the 210 Pb exc profiles where the linear trend of decrease is apparent (the plot for CR =3.0 ×10 −1 is not shown for graphical clarity). Results are summarized in Table 2. Table 2 SAR values estimated from the CF-CS model and the 137 Cs peak for the Antony Marsh and the Bere Ferrers cores. SAR Antony Marsh (g cm −2 a −1 ) Bere Ferrers (g cm −2 a −1 ) Composite SAR from 137 Cs (post-dating 1963) 0.0719 ±0.0018 0.088 ±0.004 Composite SAR from fit to Eq. A-4 0.049 ±0.004 0.065 ±0.005 Mineral SAR from 137 Cs (postdating 1963) 0.0270 ±0.0009 0.058 ±0.003 Mineral SAR from fit to Eq. A-4 (CR =0.30) 0.0205 ±0.0007 0.043 ±0.004 Mineral SAR from fit to Eq. A-4 (CR =0.015) 0.0202 ±0.0007 0.045 ±0.004 For all fits p <0.001, with R 2 in the range (0.89, 0.99). 16 high porosities in their upper layers. Radionuclides in solution can penetrate large depths into sediments with high porosity before being irreversibly bound to the solid phase (after what the post-depositional redistribution ceases). Due to the involved time scales, it is possible a mathematical treatment which modifies the diagenetic equation with a depth-distributed source term while the boundary condition at the SWI (involving the 210 Pb exc fluxes) is consistently updated (Abril and Gharbi, 2012). Long penetration tails have often been observed for 137 Cs from the Chernobyl accident and shortly after the bomb-fallout peak. The fingerprint of non-ideal deposition in 210 Pb exc profiles is their flattening at the top sediment and the appearance of sub-surface maxima. For given sediment the extent of the non-ideal deposition effect depends on the physical carriers of the different radionuclides at the SWI. Particularly, it can be different for 210 Pb exc , 137 Cs and 7 Be. Quite often meteoric 7 Be (half-live 53.1 days) is found to penetrate up to few centimetres depth in sediments with high porosities, what has been considered by many authors as an evidence of mixing (e.g. Arias-Ortiz et al., 2018). Nevertheless, Botwe et al. (2017) have shown that this feature can be explained by non-ideal deposition. STA model for pre-depositional processes affecting man-made radionuclides The system-time-average (STA) model (Robbins et al., 2000) is an updated and simpler formulation of the already known idea on the integration of atmospheric fluxes prior reaching the SWI (McCall et al., 1984; Abril and García-Le´ on, 1994). The fluxes onto the SWI, φ, are expressed as a function of the atmospheric deposition, F a , involving the radioactive decay constant of the radionuclide, λ R , and a system’s constant, k r , with physical dimension of T −1 : dFS dt =krFa− (λR+kr)φ(A-6) Intuitively, atmospheric fluxes are accumulated (integrated) in the catchment area while they are transferred towards the sediment at a constant rate k r . For large-surface catchments, fluxes onto the SWI (and the resulting inventories) are larger than the atmospheric deposition (and its integrated value). This anomaly is solved by the STA model which uses a normalization factor, Z N , defined as the ratio between the inventory and the integrated fluxes onto the SWI (decay-corrected to the date of sampling). The simplest version of the model works with a constant (mean) value of SAR, which can be first estimated from the peak position, and then further refined (the age of the peak is only slightly displaced by the STA model). This allows conversion of the sequence of φ into activity concentrations, generating their profiles versus mass depths. Thus, STA only involves a free parameter, namely k r , which is selected to get the best agreement between the measured and the modeled profiles. It is worth noting that for radionuclides with a constant atmospheric deposition (as often assumed for 210 Pb exc ), the STA model also leads to a constant flux onto the SWI. Sediments grain size effects in 210 Pb-based dating models He and Walling (1996) conducted experimental studies on the 137 Cs and 210 Pb uptake by suspended mineral soil particles in spiked aqueous solutions (their experiments 1 and 2). They suggested an empirical relationship between activity concentrations, C, and a power of the specific surface area of the particles, S sp, being this last in the range 0.1–6 m 2 /g. Particularly, from their experiment 1, they found for 210 Pb exc that C∝ S0.67 sp . These experiments were later revisited and interpreted in terms of a basic microscopic theory of the distribution, transfer and uptake kinetics of dissolved radionuclides by suspended particulate matter (Abril, 1998a, 1998b). He and Walling (1996) also reported radionuclide concentrations in different size fractions of catchment soils receiving natural fallout inputs, and in overbank sediment deposits. These results showed radionuclide concentrations increasing with a certain power of S sp , being this last in the range 0.1–3.5 m 2 /g. With the approach of particles with spherical shape and uniform density, the above studies accounted for grain sizes roughly below 25 μ m. Clay-normalized 210 Pb activity concentrations have been used for establishing geochronologies of flood events in large tropical river systems (Aalto and Nittrouer, 2012). Sun et al. (2018) found a strong positive correlation between the clay component (<4 μ m) and 210 Pb exc in a wide set of sediment cores from the coastal area of the Yangtze River Estuary. Based upon the above results and other similar studies, some authors have suggested corrections in 210 Pb-based dating models to account for changes in granulometry along the sediment core (e.g., Arias-Ortiz et al., 2018). Nevertheless, it hardly can be assumed that the empirical relationships found by He and Walling (1996) - their Eq. (3), in laboratory conditions (spiked solutions), with a particular soil sample, and studied in the range of sizes <25 μ m, also holds for any natural conditions of coastal sediments and for particles sizes ranging from 4 μ m up to 2000 μ m. Other authors assume that the sand content of a sediment slice is virtually free of 210 Pb exc and then they report and use mud-normalized concentrations for applying the CF-CS model (e.g., Sanders et al., 2012). It is worth noting that the above normalization procedures use the information on particles size distributions, but they do not involve the direct measurements of 210 Pb in the physically-separated grain-size classes These kind of measurements have been reported among others, by Alonso-- Hern´ andez et al. (2006) for coastal sediments in Cienfuegos Bay, and by Mantero et al. (2019) for some estuarine sediments from Spain and Ghana. Their results showed weak or negligible dependence of 137 Cs and 210 Pb exc concentrations with particle grain sizes in the range from 4 μ m up to 2000 μ m. Unlike under ideal experimental conditions, the uptake of radionuclides by suspended particulate matter and surficial sediments in real aquatic environments is the aggregate result of a large variability of chemical-physical conditions and a large diversity of contact times. Consequently, the observed increase in activity concentrations with decreasing particle sizes can be only slight or negligible in the some cases (Mantero et al., 2019). This does not discard stronger relationships in other aquatic environments, but decision on corrections of 210 Pb dating models by particle sizes should be taken on case by case basis, and using site specific empirical equations for corrections. This was not the approach adopted for the present work, but TERESA model already accounts for independent variability of SARs and initial activity concentrations, being this last contributed by both particle size effects and the varying contributions of mas flows with different provenances. References Aalto, R., Nittrouer, C.A., 2012. 210 Pb geochronology of flood events in large tropical river systems. Phil. Trans. R. Soc. A 370, 2040–2074. Abril, J.M., 1998a. Basic microscopic theory of the distribution, transfer and uptake kinetics of dissolved radionuclides by suspended particulate matter. Part I: theory development. J. Environ. Radioact. 41, 307–324. 17 processes in sediments. A reviewed basis for radiometric dating models. J. Paleolimnol. 30, 363–370. Abril, J.M., 2011. Could bulk density profiles provide information about recent sedimentation rates? J. Paleolimnol. 46, 173–186. Abril, J.M., 2015. Why would we use the Sediment Isotope Tomography (SIT) model to establish a 210 Pb-based chronology in recent-sediment cores? J. Environ. Radioact. 143, 40–46. Abril, J.M., 2016. A 210 Pb-based chronological model for recent sediments with random entries of mass and activities: model development. J. Environ. Radioact. 151, 64–74. Abril, J.M., 2019. Radiometric dating of recent sediments: on the performance of 210 Pbbased CRS chronologies under varying rates of supply. Quat. Geochronol. 51, 1–14. Abril, J.M., 2020. Multimodal-TERESA, a 210Pb-based radiometric dating model for recent sediments under largely varying rates of supply. Quat. Geochronol. 55, 101032. Abril, J.M., Brunskil, G.J., 2014. Evidence that excess 210 Pb flux varies with sediment accumulation rate and implications for dating recent sediments. J. Paleolimnol. 52, 121–137. Abril, J.M., García-Le´ on, M., 1994. The integrated atmospheric flux effect in a radiogeochronological model. J. Environ. Radioact. 24, 65–79. Abril, J.M., Gharbi, F., 2012. Radiometric dating of recent sediments: beyond the boundary conditions. J. Paleolimnol. 48, 449–460. Abril, J.M., García-Le´ on, M., García-Tenorio, R., S´ anchez, C.I., El- Daoushy, F., 1992. Dating of marine sediments by an incomplete mixing model. J. Environ. Radioact. 15, 135–151. Abril, J.M., San Miguel, E.G., Ruiz-C´ anovas, C., Casas-Ruiz, M., Bolívar, J.P., 2018. From floodplain to aquatic sediments: radiogeochronological fingerprints in a sediment core from the mining impacted Sancho Reservoir (SW Spain). Sci. Total Environ. 631–632, 866–878. Allen, J.R.L., 2000. Morphodynamics of holocene salt marshes: a review sketch from the atlantic and southern north sea coast of europe. Quart. Sci. Rev. 19, 1155–1231. Allen, J.R.L., Pye, K., 1992. Coastal saltmarshes: their nature and importance. In: Allen, J.R.L., Pye, K. (Eds.), Saltmarshes: Morphodynamics, Conservation and Engineering Significance. Cambridge University Press, pp. 1–18. Alonso-Hern´ andez, C.M., Díaz-Asencio, M., Mu˜ noz-Caravaca, A., Delfanti, R., Papucci, C., Ferretti, O., Crovato, C., 2006. Recent changes in sedimentation regime in Cienfuegos Bay, Cuba, as inferred from 210 Pb and 137 Cs vertical profiles. Continent. Shelf Res. 26, 153–167. Appleby, P.G., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210 Pb to the sediment. Catena 5, 1–8. Aquino-L´ opez, M.A., Blaauw, M., Christen, J.A., Sanderson, N.K., 2018. Bayesian analysis of 210 Pb dating. J. Agric. Biol. Environ. Stat. 23, 317–333. Aquino-L´ opez, M.A., Ruiz-Fern´ andez, A.C., Blaauw, M., Sanchez-Cabeza, J.A.-., 2020. Comparing classical and Bayesian 210 Pb dating models in human-impacted aquatic environments. Quat. Geochronol. 60, 101106. Arias-Ortiz, A., Masqu´ e, P., Garcia-Orellana, J., Serrano, O., Mazarrasa, I., Marb` a, N., Lovelock, C.E., Lavery, P.S., Duarte, C.M., 2018. Reviews and syntheses: 210 Pbderived sediment and carbon accumulation rates in vegetated coastal ecosystems: setting the record straight. Biogeosci. Discuss. https://doi.org/10.5194/bg-2018-78. Bakker, J.P., Esselink, P., Dijkema, K.S., van Duin, W.E., de Jong, D.J., 2002. Restoration of salt marshes in The Netherlands. Hydrobiol. (Sofia) 478, 29–51. Barnett, R.L., Newton, T.L., Charman, D.J., Gehrels, W.R., 2017. Salt-marsh testate amoebae as precise and widespread indicators of sea-level change. Earth Sci. Rev. 164, 193–207. Bellucci, L.G., Frignani, M., Cochran, J.K., Albertazzi, S., Zaggia, L., Cecconi, G., Hopkins, H., 2007. 210 Pb and 137 Cs as chronometers for salt marsh accretion in the Venice Lagoon and links to flooding frequency and climate change. J. Environ. Radioact. 97, 85–102. Bevington, P.A., Robinson, D.K., 2003. Data Reduction and Error Analysis for the Physical Sciences, third ed. McGraw-Hill, New York. Botwe, B.O., Abril, J.M., Schirone, A., Barsanti, M., Delbono, I., Delfanti, R., Nyarko, E., Lens, P.N.L., 2017. Settling fluxes and sediment accumulation rates by the combined use of sediment traps and sediment cores in Tema Harbour (Ghana). Sci. Total Environ. 609, 1114–1125. Boyd, B.M., Sommerfield, C.K., Elsey-Quirk, T., 2017. Hydrogeomorphic influences on salt marsh sediment accumulation and accretion in two estuaries of the U.S. Mid- Atlantic coast. Mar. Geol. 383, 132–145. Bruel, R., Sabatier, P., 2020. serac: a R package for ShortlivEd RAdionuclide chronology of recent sediment cores. J. Environ. Radioact. 225, 106449. https://doi.org/ 10.1016/j.jenvrad.2020.106449. Carroll, J., Lerche, I., 2003. Sedimentary Processes: Quantification Using Radionuclides. Elsevier, Oxford. Christensen, E.R., 1982. A model for radionuclides in sediments influenced by mixing and compaction. J. Geophys. Res. 87, 566–572. Crosby, S.C., Sax, D.F., Palmer, M.E., Booth, H.S., Deegan, L.A., Bertness, M.D., Leslie, H. M., 2016. Salt marsh persistence is threatened by predicted sea-level rise. Estuar. Coast Shelf Sci. 181, 93–99. Cundy, A.B., Croudace, I.W., 1995. Sedimentary and geochemical variations in a salt marsh/mud flat environment from the mesotidal Hamble estuary, southern England. Mar. Chem. 51, 115–132. Cundy, A.B., Croudace, I.W., 1996. Sediment accretion and recent sea-level rise in the Solent, Southern England: inferences from radiometric and geochemical studies. Estuar. Coast Shelf Sci. 43, 449–467. Cundy, A.B., Croudace, I.W., 2017. The fate of contaminants and stable Pb isotopes in a changing estuarine environment: 20 years on. Environ. Sci. Technol. 51, 9488–9497. Dadey, K.A., Janecek, T., Klaus, A., 1992. Dry-bulk density: its use and determination. In: Taylor, B., Fujioka, K., et al. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 126, pp. 551–554. De Vos, B., Vandecasteele, B., Deckers, J., Muys, B., 2005. Capability of loss-on-ignition as a predictor of total organic carbon in non-calcareous forest soils, commun. Soil Sci. Plan. 36, 2899–2921. Dines, H.G., 1956. The Metalliferous Mining Region of South-West England, vol. I. Her Majesty’s Stationary Office, BGS, London. England, Natural, 2016. Plymouth Sound and Estuaries SAC/SSSI: 2013 Saltmarsh Condition Assessment Survey Report (Available online). Accessed March 2016. http://publications.naturalengland.org.uk/publication/5738783394234368. García-Artola, A., Cearreta, A., Irabien, M.J., Leorri, E., Sanchez-Cabeza, J.A., Corbett, D. R., 2016. Agricultural fingerprints in salt-marsh sediments and adaptation to sealevel rise in the eastern Cantabrian coast (N. Spain). Estuar. Coast Shelf Sci. 171, 66–76. Goldberg, E.D., 1963. Geochronology with Pb-210. Proceedings of a Symposium of Radioactive Dating. International Atomic Energy Agency, Vienna, pp. 121–131. He, Q., Walling, D., 1996. Interpreting particle size effects in the adsorption of 137 Cs and unsupported 210 Pb by mineral soils and sediments. J. Environ. Radioact. 30 (2), 117–137. Hughes, R.G., 2004. Climate change and loss of saltmarshes: consequences for birds. Ibis 146, 21–28. IAEA, 2010. Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Terrestrial and Freshwater Environments. International Atomic Energy Agency, TRS No. . 472. Iurian, A.R., Millward, G.E., Sima, O., Taylor, A., Blake, W., 2018. Self-attenuation corrections for Pb-210 in gamma-ray spectrometry using well and coaxial HPGe detectors. Appl. Radiat. Isot. 134, 151–156. JCGM, 2008. Evaluation of Measurement Data - Guide to the Expression of Uncertainty in Measurement (GUM 1995 with Minor Corrections), vol. 100, 2008. Joint Committee for Guides in Metrology. http://www.bipm.org/utils/common/docum ents/jcgm/JCGM_100_2008_E.pdf. Kauffman, J.B., Donato, D.C., 2012. Protocols for the measurement, monitoring and reporting of structure, biomass and carbon stocks in mangrove forests. In: Working Paper, vol. 86. CIFOR, Bogor, Indonesia. Kennish, M.J., 2001. Coastal salt marsh systems in the U.S.: a review of anthropogenic impacts. J. Coast Res. 17, 731–748. Kim, G., Hussain, N., Church, T.M., Carey, W.L., 1997. The fallout isotope 207 Bi in a Delaware salt marsh: a comparison with 210 Pb and 137 Cs as a geochronological tool. Sci. Total Environ 196, 31–41. Kirwan, M.L., Megonigal, J.P., 2013. Tidal wetland stability in the face of human impacts and sea-level rise. Nature 504, 53–60. Klubi, E., Abril, J.M., Nyarko, E., Laissaoui, A., Benmansour, M., 2017. Radioecological assessment and radiometric dating of sediment cores from dynamic sedimentary systems of Pra and Volta estuaries (Ghana) along the Equatorial Atlantic. J. Environ. Radioact. 178–179, 116–126. Koide, M., Soutar, A., Goldberg, E.D., 1972. Marine geochronology with 210 Pb. Earth Planet Sci. Lett. 14, 442–446. Kolker, A.S., Goodbred, S.L., Hameed, S., Cochran, J.K., 2009. High-resolution Records of the response of coastal wetland systems to long-term and short-term sea level. Variability. Estuar. Coast. Shelf Sci. 84, 493–508. Krishnaswamy, S., Lal, D., Martin, J.M., Meybek, M., 1971. Geochronology of lake sediments. Earth Planet Sci. Lett. 11, 407–414. Kristensen, E., Bouillon, S., Dittmar, T., Marchand, C., 2008. Organic carbon dynamics in mangrove ecosystem. Aquat. Bot. 89, 210–219. Laissaoui, A., Benmansour, M., Ziad, N., Ibn Majah, M., Abril, J.M., Mulsow, S., 2008. Anthropogenic radionuclides in the water column and a sediment core from the Alboran Sea: application to radiometric dating and reconstruction of historical water column radionuclide concentration. J. Paleolimnol. 40, 823–833. Leorri, E., Horton, B.P., Cearreta, A., 2008. Development of a foraminifera-based transfer function in the Basque marshes, N. Spain: implications for sea-level studies in the Bay of Biscay. Mar. Geol. 251, 60–74. Livesley, S.J., Andrusiak, S.M., 2012. Temperate mangrove and salt marsh sediments are a small methane and nitrous oxide source but important carbon store. Estuar. Coast Shelf Sci. 97, 19–27. Mabit, L., Benmansour, M., Abril, J.M., Walling, D.E., Meusburger, K., Iurian, A.R., Bernard, C., Tarjan, S., Owens, P.N., Blake, W.H., Alewell, C., 2014. Fallout 210 Pb as a soil and sediment tracer in catchment sediment budget investigations: a review. Earth Sci. Rev. 138, 335–351. Madsen, A.T., Murray, A.S., Andersen, T.J., Pejrup, M., 2007. Temporal changes of accretion rates on an estuarine salt marsh during the late Holocene — reflection of local sea level changes? The Wadden Sea, Denmark. Mar. Geol. 242, 221–233. Mantero, J., Abril-Hern´ andez, J.M., Garcia-Tenorio, R., Klubi, E., Nyarko, E., 2019. Experimental study on the use of granulometric speciation for the radiometric dating of recent sediments. J. Environ. Radioact. 208–209, 106016. https://doi.org/ 10.1016/j.jenvrad.2019.106016. Marchand, C., 2017. Soil carbon stocks and burial rates along a mangrove forest chronosequence (French Guiana). For. Ecol. Manag. 384, 92–99. Marshall, W.A., Gehrels, W.R., Garnett, M.H., Freeman, S.P.H.T., Maden, C., Xu, S., 2007. The use of ‘bomb spike’ calibration and high-precision AMS 14 C analyses to date saltmarsh sediments deposited during the past three centuries. Quat. Res. 68, 325–337. McCall, P.L., Robbins, J.A., Matisoff, G., 1984. 127 Cs and 210 Pb transport and geochronologies in urbanized reservoirs with rapidly increasing sedimentation rates. Chem. Geol. 44, 33–65. Abril, J.M., 1998b. Basic microscopic theory of the distribution, transfer and uptake kinetics of dissolved radionuclides by suspended particulate matter. Part II: applications. J. Environ. Radioact. 41, 325–341. Abril, J.M., 2003. A new theoretical treatment of compaction and the advective-diffusive 18 isotopes. In: Nriago, J.P. (Ed.), Biochemistry of Lead in the Environment. Elsevier, Amsterdam, pp. 285–393. Robbins, J.A., Edgington, D.N., 1975. Determination of recent sedimentation rates in Lake Michigan using 210 Pb and 137 Cs. Geochem. Cosmochim. Acta 39, 285–304. Robbins, J.A., Holmes, C., Halley, R., Bothner, M., Shinn, E., Graney, J., Keeler, G., ten Brink, M., Orlandini, K.A., Rudnick, D., 2000. Time-averaged fluxes of lead and fallout radionuclides to sediments in Florida Bay. J. Geophys. Res. 105, 28805–28821. Ruíz-Fern´ andez, A.C., Carnero-Bravo, V., Sanchez-Cabeza, J.A., P´ erez-Bernal, L.H., Amaya-Monterrosa, O.A., Boj´ orquez-S´ anchez, S., L´ opez-Mendoza, P.G., Cardoso- Mohedano, J.G., Dunbar, R.B., Mucciarone, D.A., Marmolejo-Rodríguez, A.J., 2018. Carbon burial and storage in tropical salt marshes under the influence of sea level rise. Sci. Total Environ. 630, 1628–1640. Sanchez, A.L., Wright, S.M., Smolders, E., Naylor, C., Stevens, P.A., Kennedy, V.H., Dodd, B.A., Singleton, D.L., Barnett, C.L., 1999. High plant uptake of radiocesium from organic soils due to Cs mobility and low soil K content. Environ. Sci. Technol. 33, 2752–2757. S´ anchez-Cabeza, J.A., Ruíz-Fern´ andez, A.C., 2012. 210 Pb sediment radiochronology: an integrated formulation and classification of dating models. Geochem. Cosmochim. Acta 82, 183–200. Sanders, C.J., Smoak, J.M., Waters, M.N., Sanders, L.M., Brandini, N., Patchineelam, S.R., 2012. Organic matter content and particle size modifications in mangrove sediments as responses to sea level rise. Mar. Environ. Res. 77, 150–155. https://doi.org/ 10.1016/j.marenvres.2012.02.004. Sun, X., Fan, D., Tian, Y., Zheng, S., 2018. Normalization of excess 210Pb with grain size in the sediment cores from the Yangtze River Estuary and adjacent areas: implications for sedimentary processes. Holocene 28 (4), 545–557. Vidmar, T., 2005. Efftran - a Monte Carlo efficiency transfer code for gamma-ray spectrometry. Nucl. Instrum. Methods 550, 603–608. Wilson, C.A., Hughes, Z.J., FitzGerald, D.M., Hopkinson, C.S., Valentine, V., Kolker, A.S., 2014. Saltmarsh pool and tidal creek morphodynamics: dynamic equilibrium of northern latitude saltmarshes? Geomorphology 213, 99–115. Wright, S.M., 2016. Predicted Caesium-137 Deposition from Atmospheric Nuclear Weapons Tests. NERC Environmental Information Data Centre. https://doi.org/ 10.5285/c3e530bf-af20-43fc-8b4b-92682233ff08. Yang, J., Gao, J., Liu, B., Zhang, W., 2014. Sediment deposits and organic carbon sequestration along mangrove coasts of the Leizhou Peninsula, southern China. Estuar. Coast Shelf Sci. 136, 3–10. Middelburg, J.J., Vlug, T., van der Nat, F.J.W.A., 1993. Organic matter mineralization in marine systems. Global Planet. Change 8, 47–58. Mighanetara, K., Braungardt, C.B., Rieuwerts, J.S., Azizi, F., 2009. Contaminant fluxes from point and diffuse sources from abandoned mines in the River Tamar catchment, UK. J. Geochem. Explor. 100, 116–124. Nature, English, 2006 (online). Available from: accessed. www.english-nature.org.uk. (Accessed 14 March 2016). Nellemann, C., Corcoran, E., Duarte, C.M., Valdes, L., DeYoung, C., Fonseca, L., Grimsditch, G., 2009. Blue carbon. A Rapid Response Assessment. United Nations Environment Programme, GRID-Arendal, Norway. Pedersen, J.B.T., Bartholdy, J., Christiansen, C., 2007. 137 Cs in the Danish Wadden Sea: contrast between tidal flats and salt marshes. J. Environ. Radioact. 97, 42–56. Ralph, I.S., 1956. The ecology of the Tamar Estuary VII. Observations of the interstitial salinity of intertidal muds in the estuarine habitat of Nereis Diversicolor. J. Mar. Biol. Assoc. U. K. 35, 81–104. Robbins, J.A., 1978. Geochemical and Geophysical applications of radioactive lead