Arctic Ocean sea ice drift origin derived from artificial radionuclides

Arctic Ocean sea ice drift origin derived from artificial radionuclides P. Cámara-Mor a, ⁎, P. Masqué a,b , J. Garcia-Orellana a,b,c , J.K. Cochran c , J.L. Mas d , E. Chamizo e , C. Hanfland f a Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, E-08193. Bellaterra, Spain b Dpt. de Física, Universitat Autònoma de Barcelona, E-08193. Bellaterra, Spain c School of Marine & Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA d Dpto. de Física Aplicada, Universidad de Sevilla, 41012, Seville. Spain e Centro Nacional de Aceleradores (CNA), Avd. Thomas Alva Edison 7, Isla de la Cartuja, E-41092, Seville, Spain f Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany abstractarticle info Article history: Received 11 August 2009 Received in revised form 22 March 2010 Accepted 25 March 2010 Keywords: Arctic Ocean Sea-ice sediments 137 Cs 239,240 Pu 240 Pu/ 239 Pu atom ratio Sea ice origin Geotraces Since the 1950s, nuclear weapon testing and releases from the nuclear industry have introduced anthropogenic radionuclides into the sea, and in many instances their ultimate fate are the bottom sediments. The Arctic Ocean is one of the most polluted in this respect, because, in addition to global fallout, it is impacted by regional fallout from nuclear weapon testing, and indirectly by releases from nuclear reprocessing facilities and nuclear accidents. Sea-ice formed in the shallow continental shelves incorporate sediments with variable concentrations of anthropogenic radionuclides that are transported through the Arctic Ocean and are finally released in the melting areas. In this work, we present the results of anthropogenic radionuclide analyses of sea-ice sediments (SIS) collected on five cruises from different Arctic regions and combine them with a database including prior measurements of these radionuclides in SIS. The distribution of 137 Cs and 239,240 Pu activities and the 240 Pu/ 239 Pu atom ratio in SIS showed geographical differences, in agreement with the two main sea ice drift patterns derived from the mean field of sea-ice motion, the Transpolar Drift and Beaufort Gyre, with the Fram Strait as the main ablation area. A direct comparison of data measured in SIS samples against those reported for the potential source regions permits identification of the regions from which sea ice incorporates sediments. The 240 Pu/ 239 Pu atom ratio in SIS may be used to discern the origin of sea ice from the Kara–Laptev Sea and the Alaskan shelf. However, if the 240 Pu/ 239 Pu atom ratio is similar to global fallout, it does not provide a unique diagnostic indicator of the source area, and in such cases, the source of SIS can be constrained with a combination of the 137 Cs and 239,240 Pu activities. Therefore, these anthropogenic radionuclides can be used in many instances to determine the geographical source area of sea-ice. 1. Introduction The Arctic Ocean is often considered as a pristine area, but it cannot avoid the effect of industrialization and development. It is thus subject to inputs of contaminants such as heavy metals, persistent organic pollutants and anthropogenic radionuclides (MacDonald et al., 2005). Since the 1950s, anthropogenic radionuclides such as 137 Cs and the Pu isotopes (e.g. 239 Pu, 240 Pu) have been introduced and distributed worldwide, including the Arctic Ocean. During the past two decades numerous national and international programs have been carried out to study the distribution, sources, transport and behaviour of artificial radionuclides in the Arctic Ocean (e.g. Yablokov et al., 1993; JRNEG, 1994, 1996; AMAP, 1998). The main source of anthropogenic radionuclides in the Arctic Ocean has been global stratospheric fallout (JRNEG, 1996; Oughton et al., 2004). However, secondary sources have also been significant. Regional or tropospheric fallout resulted from nuclear weapons tests carried out by the former Soviet Union (FSU) at the Novaya Zemlya archipelago (85 atmospheric, 3 underwater, 2 surface water tests between 1955 and 1990) and at Semipalatinsk (86 atmospheric, 30 ground surface and 340 underground tests) (Salbu, 2001). Nuclear wastes from reprocessing facilities also contributed to the overall inventories of 137 CsandPuinArcticOcean,including discharges from Sellafield (UK) and, to a lesser extent, from La Hague (France) (Holm, 1994; Aarkrog, 2003). Also, the Ob and Yenisey rivers contribute terrestrial run-off which has received radionuclides from weapons testing at Semipalatinsk and discharges from nuclear facilities located near or on the rivers (Tomsk-7 and Mayak) (e.g. JRNEG, 1994, 1996, Oughton et al. 1999; Smith et al., 1995). For example, it has been documented the release of about 100 TBq of liquid waste from Mayak, including about 2 TBq of alpha emitters, to the Techa River during 1948– 1951 (Christensen et al., 1997; Vorobiova et al., 1999). Other reprocessing plants such as Krasnoyarsk-26 discharged about 30 to 100 TBq of 137 Cs into the Kara Sea between 1958 and 1993 (Vakulovsky et al., 1995). Finally, the FSU also dumped liquid and solid radioactive ⁎Corresponding author. Tel.: +34 93 581 11 91; fax: +34 93 581 21 55. E-mail address: [email protected] (P. Cámara-Mor). 0048-9697/$ – see front matter doi:10.1016/j.scitotenv.2010.03.041 wastes into the Barents and Kara Seas between 1960 and 1991. Overall, the total amount of radioactive wastes dumped in the Arctic Ocean was estimated by the IAEA (1998) to be of approximately 37 PBq. Nuclear accidents have also contributed artificial radionuclides into the Arctic environment, such as those occurred in Kyshtym in 1957 and in Tromsk7in1993(Kabakchi et al., 1995; Waters et al., 1999). Once radionuclides are introduced into the sea they can be scavenged from seawater by particulate matter and be eventually deposited in sediments in the bottom floor (e.g. Livingston and Bowen, 1979; Baxter et al., 1995; Aarkrog, 2003). In the Arctic Ocean, several studies have focussed on studying the distributions of 137 Cs and 239,240 Pu and the 240 Pu/ 239 Pu atom ratio (which is a useful indicator of Pu origin —Masqué et al., 2003—) in bottom sediments from the central Arctic Basin (Huh et al., 1997; Cooper et al., 2000) and from the Fram Strait (Masqué et al., 2003). However, most studies have paid attention to the continental shelves, particularly along the Siberian shelves, as they are the most affected areas by introduction of artificial radionuclides (e.g. Baskaran et al., 1996, 2000; Cochran et al., 2000; Smith et al., 2000). The Arctic continental shelves, specially in the Siberian area, are one of the main sources of sea ice (Fig. 1). During sea ice formation in these shallow areas, sediments and suspended particles are mainly incorporated by suspension freezing into the ice. As a result, sea ice can contain a significant amount of sediments (sea-ice sediments, SIS) ranging from a few grams to tens of kilograms per cubic meter (Nürnberg et al., 1994). Although aeolian deposition onto the ice is also possible, field evidence for such a process is very sparse and its deposition rate on the central Arctic ice cover appears to be several orders of magnitude less than the other contributions to SIS loads (Pfirman et al., 1989, 1990; Nürnberg et al., 1994). Thus in general it is assumed that “dirty ice”(ice with high concentrations of sediment) has been formed on shallow shelves. The particulate matter and the associated chemical species contained in sea ice are transported from continental shelf areas to the central Arctic basin in association with the physical circulation. The mean sea-ice drift patterns are controlled by the Transpolar Drift (TPD) over the Eurasian Basin and the anticyclonic Beaufort Gyre in the Canada Basin (Thorndike, 1986). Sea ice formed over the western Siberian shelves is carried by the TPD with a transit time of 2–4 years to the Fram Strait (Thorndike and Colony, 1982). Sea ice which rotates in the Beaufort Gyre may circulate there for ∼5 to 15 years, and is generally thought to be formed in the Beaufort, Chukchi and East Siberian Seas, although it is also possible to find sea ice from the TPD in the Beaufort Gyre (Thorndike, 1986). As sea ice reaches ablation areas such as the Fram Strait and, to a lesser degree, also along the central Arctic Ocean or in the Canadian archipelago, it melts and releases the entrained particulate matter to the surface water (Pfirman et al., 1997; Rigor et al., 2002). Hence, sea ice has been identified as playing a potentially important role in the redistribution and transport of particulate matter and chemical species in the Arctic Ocean (e.g. Nürnberg et al., 1994; Landa et al., 1998; Masqué et al., 2003). In this work we use new and published data on concentrations of 137 Cs, 239,240 Pu and the 240 Pu/ 239 Pu atom ratio in SIS in the Arctic Fig. 1. Arctic Ocean topography and sea ice drift pattern. 3350 P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 Ocean to assess the possible origin of the sea ice which forms the ice pack. 2. Materials and methods 2.1. Sampling A total of 63 SIS samples were collected during several cruises of R/ V Polarstern (Table 1). Samples were recovered from annual or multiyear sea ice and icebergs from the central Arctic Ocean, the Nansen Basin and the Fram Strait. Approximately 10 to 200 g of SIS were collected from the upper surface of the ice floes, from ridges and from cryoconites holes (small holes produced by aggregation of particles in the surface ice by absorption of solar energy) by scraping with stainsteel shovels or/an ice-hammer in order to obtain blocs of turbid sea ice. Once onboard, sea-ice samples were thawed and SIS were isolated from the supernatant liquid by careful decantation. Afterwards, SIS samples were kept frozen and stored in plastic bags until their analysis in the laboratory. Before radionuclide analysis, all samples were dried in an oven at 60 °C and ground to a powder. 2.2. Radiometric analysis 2.2.1. 137 Cs The activity of 137 Cs (T 1/2 =30.1 y) was determined by gamma spectrometry using a Ge detector at the Universitat Autònoma de Barcelona (UAB, Spain)and at StonyBrook University.Dry samples were hermetically sealed in containers with well known geometries (plastic vials at UAB and aluminium cans at Stony Brook University). In both cases, 137 Cs was determined through its gamma emission at 661.6 keV. Counting times were typically about 2 days, with uncertainties b10%. All 137 Cs activities were decay-corrected to first of September 2007 in order to make all data comparable. 2.2.2. Plutonium isotopes Concentrations of 239 Pu (T 1/2 =24110 y) and 240 Pu (T 1/2 =6560 y) and the 240 Pu/ 239 Pu atom ratios in SIS samples collected during ARK XIV/1a and ARK XVII/2 cruises were measured by using a Finningan ELEMENT magnetic-sector inductively coupled plasma mass spectrometer (MS-ICPMS) at Woods Hole Oceanographic Institution, Massachusetts. These samples were analysed following the procedure described by Masqué et al. (2003) based upon the methods developed by Buesseler (1986) and Kenna (2002). Briefly, a known amount of 242 Pu was added as an internal yield tracer to 2–25 g of sample which were incinerated at 550 °C for 24 h. The ashes were digested with 8 N HNO 3 . Pu was separated by ion exchange, and samples underwent several steps in order to obtain an adequate separation from other transuranic nuclides. Pu was eluted from the final ion exchange column with 10% HNO 3 –1% HF and the solution was evaporated to 1 mL. In the case of SIS samples collected during ARK XVIII/1, ARK XIX/4 and ARK XXII/2 cruises, concentrations of 239 Pu, 240 Pu and the 240 Pu/ 239 Pu atom ratios were determined by accelerator mass spectrometry (AMS) at the Centro Nacional de Aceleradores (CNA, Sevilla). Details concerning the chemical procedure and the AMS measurement technique can be found in Chamizo et al. (2008a,b). Briefly, about 2 g of dried SIS were spiked with 10 pg of 242 Pu as a yield monitor, ashed at 600 °C for 6 h and acid digested with HNO 3 (65%), H 2 O 2 (30%) and HF (40%). The supernatant was separated from the residue after centrifuging the solution at 4000 rpm. Prior to ion exchange purification, the solution was prepared to contain Pu only in the IV oxidation state by adding 0.18 g of NaNO 3 . Ion exchange separation was performed with TEVA-columns. The Pu fraction was isolated from the other actinides by washing the column with 6 M HCl to remove Th and 8 M HNO 3 to remove U. Pu was eluted with 0.002 M HF and 0.02 M HNO 3 and the eluate was evaporated to 3 mL and transferred to plastic vials for storage prior to measurement by AMS-CNA. All Pu concentrations and isotope ratios were blank corrected. The measured atom concentrations of 239 Pu and 240 Pu have been converted into activities and added to be expressed as 239,240 Pu in order to facilitate comparison with previous works. 3. Results The specific activities of artificial radionuclides ( 137 Cs and 239,240 Pu) and the 240 Pu/ 239 Pu atom ratios in sea-ice sediments are summarized in Table 2. The 137 Cs activities showed a large variability, ranging from 1.8 to 4·10 3 Bq kg −1 , although only 5% of the samples contained 137 Cs activities higher than 38 Bq kg −1 . Outliers have been identified by box plot analysis, based upon inter-quartile differences. Median values were used since the data do not follow a normal distribution. Excluding these samples, the average 137 Cs activity was 9.1±7.4 Bq kg −1 (n=51). The highest 137 Cs specificactivity, 4·10 3 Bq kg −1 , was determined in sediments collected from an iceberg sampled from the waters near Franz Josef Land. This value is even greater than concentrations of 137 Cs reported by Cota et al. (2006) in sea-ice sediments from the Canadian Archipelago (∼2.5·10 3 Bq kg −1 ). The 239,240 Pu specific activities also showed considerable variability, varying up to four orders of magnitude from 0.018 to 31.8 Bq kg −1 , although most of the samples (95%) had 240,239 Pu activities b1.4 Bq kg −1 . Excluding the samples with concentrations N1.4 Bq kg −1 , activities of 240,239 Pu in SIS averaged 0.32± 0.25 Bq kg −1 (n=45). The samples with 240,239 Pu concentrations N1.4 Bq kg −1 were collected throughout the Nansen Basin, although mainly in the western part of the Fram Strait. In general, high 240,239 Pu concentrations are also characterised by high 137 Cs concentrations (N45 Bq kg −1 )(Table 2). The 240 Pu/ 239 Pu atom ratios ranged from 0.118 to 0.253 (Table 2). Most of the 240 Pu/ 239 Pu atom ratios in SIS samples were consistent with the atom ratio characteristic of global fallout (0.183±0.009) measured in sediment samples collected at 70ºN by Efurd et al. (2005). 25% of the samples had 240 Pu/ 239 Pu atom ratios lower than 0.174, and they had been mainly collected in the Fram Strait and north of Franz Josef Land. Another 25% of the samples had 240 Pu/ 239 Pu atom ratios N0.195, ranging up to 0.253, and in most cases were also collected in the Fram Strait. Samples with relatively low 240 Pu/ 239 Pu atom ratios (b0.174) also generally had relatively high concentrations of 240,239 Pu (N1Bqkg −1 ) and 137 Cs (up to 45 Bq kg −1 ), except for samples 15, 228, 270-2 and PS70/3, which were collected in the Eurasian Basin. The samples with low 240 Pu/ 239 Pu atom ratios and high 137 Cs concentrations were collected in the Fram Strait and the Nansen Basin during summer in 2001 and 2002. 4. Discussion In order to study the geographical distributions of 137 Cs and 239,240 Pu concentrations and 240 Pu/ 239 Pu atom ratios in SIS along the Arctic Ocean it is necessary to consider our data in the context of previous results such as those by Meese et al. (1997), Landa et al. Table 1 Data on sampling cruises on board RV Polarstern, year, areas of study and number of SIS samples collected. Name cruise Year Expedition area Number of samples ARK XIV/1a July 1998 Central Arctic Basin 12 ARK XV II/2 August-September 2001 Nansen Basin 13 ARK XVIII/1 July-August 2002 Fram Strait 10 ARK XIX/4 Augost-September 2003 Fram Strait 12 ARK XXII/2 July-October 2007 Along Transporlar Drift 16 3351P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 Table 2 Specific activities (±1σ)of 137 Cs, 239,240 Pu and 240 Pu/ 239 Pu atom ratios in sea-ice sediments collected from the Arctic Ocean. n.m.: not measured. Code Date Lat N Long E 137 Cs (Bq kg −1 ) 239,240 Pu (Bq kg −1 ) 240 Pu/ 239 Pu ARK XIV/2 —Central Arctic Basin 5 08-jul-98 86.658 7.690 13.3±0.7 0.26±0.02 0.20±0.03 8 10-jul-98 88.073 −89.887 9.9±0.5 0.06±0.03 n.m 11 12-jul-98 87.572 −115.094 13.2±0.9 0.141±0.008 0.19±0.02 12 12-jul-98 87.578 −116.606 15.5±0.9 0.183±0.011 0.19±0.02 14 12-jul-98 87.520 −118.825 14.3±0.8 0.09±0.02 n.m 15 13-jul-98 86.992 −143.379 8.3±0.6 0.096±0.009 0.17±0.04 17 13-jul-98 86.457 −147.240 8.0±0.9 0.29±0.03 0.19±0.04 18 14-jul-98 86.373 −148.478 12.9±0.7 0.109±0.006 0.18±0.02 23 18-jul-98 85.673 −176.937 n.m 0.032±0.011 n.m 25 18-jul-98 85.653 −177.862 n.m 0.16±0.05 0.17±0.11 27 21-jul-98 83.560 144.983 n.m 0.084±0.014 0.18±0.06 29 23-jul-98 81.473 145.065 n.m 0.169±0.011 0.17±0.11 ARK XV II/2 —Nansen Basin 217 05-aug-01 83.950 24.250 3.7±0.3 0.136±0.007 0.18±0.02 218 06-aug-01 85.633 17.313 4.15±0.44 0.083±0.010 n.m 220 08-aug-01 84.667 5.100 14.4±1.1 0.97±0.04 0.184±0.016 222 10-aug-01 84.133 0.017 25.2±1.6 1.23±0.02 0.186±0.008 223 11-aug-01 83.636 −2.975 11.9±0.7 0.474±0.011 0.189±0.009 228 16-aug-01 83.791 −2.199 4.2±0.4 0.211±0.013 0.118 ±0.019 270-2 27-sep-01 84.289 28.258 14.2±0.8 0.54±0.08 0.17±0.05 270-3 27-sep-01 83.873 28.258 4.3±0.3 0.246±0.005 0.179±0.008 270-4 27-sep-01 83.853 27.855 13.8±1.4 0.94±0.05 0.20±0.02 Iceberg-1 21-aug-01 85.050 11.040 n.m 0.66±0.03 0.18±0.02 Iceberg-2 29-aug-01 86.333 37.767 1.79±0.17 0.021±0.002 0.18±0.04 Iceberg-5 17-sep-01 86.720 46.653 4001.5±77.6 31.9±0.8 0.165±0.009 Iceberg-6 23-sep-01 85.800 21.390 4.2±0.3 0.205±0.008 0.25±0.02 ARK XVIII/1 —Fram Strait 01-1 30-jul-02 75.123 −16.528 n.m 0.044±0.004 n.m 02-1 30-jul-02 75.009 −13.641 10.1±0.4 0.24±0.09 0.189±0.015 03-1 8-aug-02 79.200 2.672 6.1±0.7 0.223±0.017 0.19±0.03 05-2 12-aug-02 78.967 0.655 6.4±0.5 0.71±0.05 0.210±0.024 06-2 13-aug-02 78.782 −2.002 3.5±0.2 0.70±0.03 0.21±0.02 07-1 14-aug-02 78.943 −4.600 7.7±0.3 0.086±0.005 n.m 08-1 14-aug-02 78.756 −7.113 480.6±2.3 7.69±0.13 0.166±0.007 08-3 14-aug-02 78.756 −7.113 651.7±3.3 9.5±0.4 0.187±0.007 09-1 15-aug-02 78.909 −14.648 4.8±0.5 0.75±0.05 n.m 10-1 15-aug-02 78.844 −17.657 136.3±0.9 2.52±0.10 0.155±0.009 ARK XIX/4 —Fram Strait PS64 HELI02-2 15-aug-03 76.380 −4.654 3.9±0.7 0.144±0.008 0.187±0.017 PS64 HELI04-1 16-aug-03 76.750 −5.480 5.4±0.8 0.262±0.017 0.19±0.02 PS64 HELI04-2 16-aug-03 76.747 −5.459 4.2±0.4 0.254±0.012 0.193±0.013 PS64 HELI05-1 20-aug-03 77.150 −1.172 5.6±0.3 0.144±0.011 0.17±0.02 PS64 HELI06-1 20-aug-03 77.150 −1.201 5.6±0.3 0.186±0.015 n.m PS64 HELI07-1 25-aug-03 75.586 −8.048 4.5±0.6 0.124±0.010 0.16±0.03 PS64 HELI09-2a 29-aug-03 75.013 −20.101 22.4±0.2 0.141±0.012 n.m PS64 HELI11-1 5-sep-03 76.115 −8.745 4.3±0.6 0.44±0.02 0.175±0.012 PS64 HELI11-3 5-sep-03 76.216 −8.999 126.2±8.4 2.92±0.13 0.143±0.009 PS64-HELI12.2 6-sep-03 75.622 −19.735 23.1±0.2 n.m n.m PS64-HELI12.3 6-sep-03 75.261 −20.905 8.7±1.5 n.m n.m PS64 HELI14-2a 14-sep-03 73,357 −23.818 n.m 0.018±0.005 n.m ARK XXII/2 —Along transpolar drift PS70/1.1 6-aug-07 83.994 34.026 31.8±1.3 n.m n.m PS70/1.3 6-aug-07 83.994 34.026 29.9±2.8 0.78±0.03 0.157±0.006 PS70/2.C.2 6-aug-07 83.993 34.385 29.6±0.9 0.68±0.02 0.177±0.007 PS70/2.D.2 6-aug-07 83.993 34.385 19.2±0.5 0.66±0.02 n.m PS70/4.1 12-aug-07 83.605 60.3989 4.8±1.0 0.210±0.018 0.21±0.03 PS70/4.2 12-aug-07 83.605 60.399 9.7±1.4 0.210±0.011 0.188±0.014 PS70/5 14-aug-07 83.423 61.986 4.5±1.0 n.m 0.180±0.018 PS70/3 11-aug-07 85.145 60.815 11.2±1.9 n.m n.m PS70/6 7-sep-07 84.499 −138.389 15.1±6.0 n.m n.m PS70/7.1.2 8-sep-07 84.450 −147.572 2.5±0.3 0.229±0.014 0.22±0.02 PS70/7.2.2 8-sep-07 84.450 −147.572 5.2±0.7 0.246±0.017 n.m PS70/7.3.2 8-sep-07 84.450 −147.572 2.7±0.9 0.226±0.018 0.20±0.03 PS70/8.1 17-sep-07 84.261 108.746 16.6±3.8 n.m n.m PS70/8.2 17-sep-07 84.261 108.746 8.1±0.9 n.m n.m PS70/8.3 17-sep-07 84.261 108.746 7.2±1.5 0.233±0.012 0.224 ±0.017 PS70/9.2 17-sep-07 84.215 108.916 20.4±1.1 0.329±0.018 0.216±0.019 3352 P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 (1998); Cooper et al. (1998), Masqué et al. (2003; 2007) and Cota et al. (2006). The combined dataset shall be sufficiently detailed to permit identification of areas of sea ice origin, by comparing the 240 Pu/ 239 Pu atom ratios and 137 Cs, 239,240 Pu concentrations in SIS with those reported in bottom sediments from the Arctic continental shelves. The specific activities of anthropogenic radionuclides in SIS can be explained by multiple factors, including sediment source area, grainsize fractionation during sea-ice formation and addition of radionuclides to the ice from atmospheric deposition or scavenging from surrounding sea water during drift (Landa et al., 1998; Cooper et al., 2000; Baskaran, 2005). At the time of collection of the samples considered here (1998–2007), the atmospheric fluxes of 137 Cs and 240,239 Pu were negligible. Another process which might enhance the radionuclide concentration in SIS is the direct uptake from ice during seasonal ice melting. However, this process is also likely to be insignificant because chemical compound solutes are generally excluded from the ice during formation due to the segregation of ions from the crystal ice (Weeks and Ackley, 1986). Deposition of dust onto sea ice is generally regarded as unimportant (Pfirman et al., 1989, 1990). Hence radionuclides associated with SIS are likely to reflect dominantly the isotopic signature of sediments in source areas and thus might be used as a source signature or fingerprint of the area in which the ice incorporated its sediment. 4.1. Distribution of anthropogenic radionuclides concentrations in sea-ice sediments The specific activities of both 137 Cs and 239,240 Pu in SIS in the Arctic Ocean range over four orders of magnitude, from 1.8 to 4·10 3 Bq kg −1 for 137 Cs and from 0.021 to 31.8 Bq kg −1 for 239,240 Pu (Fig. 2). The highest activities of 137 Cs and 239,240 Pu were measured in the same SIS sample collected from an iceberg close to Franz Josef Land. High concentrations of 137 Cs were also measured at Resolute Bay (Canadian archipelago) (1785 and 2094 Bq kg −1 ), in two SIS samples collected from multi-year ice (Cota et al., 2006). However, most of the SIS samples contained less than 1.4 Bq kg −1 and less than 45 Bq kg −1 of 239,240 Pu and 137 Cs, respectively (Fig. 2). These values were identified as extreme-outliers values based upon box-quartile analysis, and thus they were used as criteria to identify samples with anomalously high activities. Despite the large variation in activities of both radionuclides, three main sectors could be identified within the Arctic Ocean based on 137 Cs and 239,240 Pu activities: the Eurasian Basin (n=41), the Canadian Basin (n=30) and the Fram Strait (n=31)(Fig. 2). The largest variability for both radionuclides was found in the Eurasian Basin, where 137 Cs and 239,240 Pu concentrations ranged from 1.8 to 4·10 3 Bq kg −1 and from 0.02 to 31.8 Bq kg −1 , respectively, and also in the Fram Strait, with 137 Cs and 239,240 Pu ranging from 2.2 to 651 Bq kg −1 and 0.09 to 9.5 Bq kg −1 , respectively. In contrast, activities in SIS in the Canadian Basin vary relatively less than in other sectors (0.10–1.82 Bq kg −1 for 239,240 Pu and 1.7–73 Bq kg −1 for 137 Cs). It is interesting to notice that these three sectors mirror the main patterns described from the mean field of sea ice motion: the TPD dominates in the Eurasian Basin, the Beaufort Gyre dominates in the Canadian Basin and the Fram Strait is the principal ablation area. Within the Eurasian Basin, the highest variability of both radionuclides was found in the Siberian sector, in the Nansen and Amundsen Basins (from 0.4 to 4·10 3 Bq kg −1 for 137 Cs and from 0.018 to 31.9 Bq kg −1 for 239,240 Pu). This variability is explained by the fact that this area is a potential convergence region of sea ice originated mainly in the Russian shelves, where comparable 137 Cs and 239,240 Pu activities have been reported in bottom sediments (AMAP, 2002). Also, Pavlov et al. (2004), based on a sea ice drift model, identified the Kara Sea and the Novaya Zemlya archipelago as the most likely origin for sea ice surrounding Franz Joseph Land and Svalbard; this supports the comparison of 137 Cs and 239,240 Pu activities reported in SIS and surface sediments from the vicinity of the Novaya Zemlya archipelago (AMAP, 2002; Smith et al., 1995, 2000). Fig. 2. Box analysis of 137 Cs concentrations (a) and 239,240 Pu atom ratios (b) in sea-ice sediments in the Arctic Ocean (including data from this study and from Meese et al. (1997), Landa et al. (1998), Cooper et al. (1998), Baskaran (2005), Masqué et al. (2003, 2007) and Cota et al. (2006). The median, first and third percentile, 95% percentile (vertical line), outliers (filled points), maximum and minimum values (stars) are indicated. 3353P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 The Laptev Sea is an area where the highest median 137 Cs and 239,240 Pu activities in SIS were measured: 26 Bq kg −1 and 0.515 Bq kg −1 , respectively. Also, variability in concentrations is rather limited, ranging from 19.5 to 54.8 Bq kg −1 for 137 Cs and from 0.45 to 1.012 Bq kg −1 for 239,240 Pu, and compare well with reported concentrations in bottom sediments of the Laptev Sea shelf: from 0.01 to 2 Bq kg −1 for 239,240 Pu (AMAP, 2002) and from 0.86 to 16 Bq kg −1 for 137 Cs (Johnson-Pyrtle and Scott, 2001). The only other possible source area of these sea-ice sediments is the Kara Sea, reaching the area through the Vilkitsky Strait. Indeed, SIS collected from a floe in the Vilkitsky Strait had activities as high as 54.8 Bq kg −1 for 137 Cs and 1.02 Bq kg −1 for 239,240 Pu (Landa et al., 1998). Both values are comparable to those measured in bottom sediment from the Kara Sea and hence suggest that this particular ice floe incorporated the sediments in the Kara Sea. In the Canadian Basin, a region dominated by the Beaufort Gyre, three subsectors could also be identified; the Alaskan coast (n=9), the Canada and Makarov Basins (identified as Beaufort in Fig. 2) (n=8) and the central Arctic Ocean (n=13) (Fig. 2). Despite the fact that 137 Cs and 239,240 Pu activities vary similarly, slight differences exist in relation to the median activities. For 137 Cs, the median activity in the Canada and Makarov Basins is approximately 3 times greater than that in the Alaskan continental shelves (4.9 Bq kg −1 ). In contrast, the median 239,240 Pu concentration in the Canada and Makarov Basins) is more than 2 times lower than in the Alaska continental shelves (0.28 Bq kg −1 ). These results suggest that the dominant source of sea ice is different for each of these regions. From comparison of 137 Cs activities in shelf-bottom sediments with those in SIS it can be inferred that sea ice in the Canada and Makarov Basins is more likely to be formed in the Siberian shelves, while sea ice in the Alaska continental shelves is formed along the Chukchi Sea and the Canadian archipelago. Median activities of 137 Cs and 239,240 Pu in SIS from the central Arctic Basin were 12.4 and 0.16 Bq kg −1 , respectively, which compare well with concentrations reported in SIS from the Alaska continental shelves and the Canadian and Makarov Basins, suggesting a mixture between both areas. This is in agreement with the findings of Pfirman et al. (1997), who stated that sea ice in the central Arctic Basin is formed by ice from diverse sources, identifying Alaska, the Canadian archipelago and the East Siberian Seas as main sources of sea ice. The distribution of sea ice in the Fram Strait is governed by the oceanographic circulation, and is formed by a mixture of sea ice floes with distinct origins because the diverse trajectories for sea ice drift merge here. This fact is reflected in a large degree of variability in 137 Cs and 239,240 Pu activities in the Fram Strait (Fig. 2). The highest range of activities of both radionuclides, 10–652 Bq kg −1 for 137 Cs and 0.31– 9.5 Bq kg −1 for 239,240 Pu, were measured along the permanent ice covered region on the western side. The East Greenland Current flows through this area, and sea ice could have been formed in the eastern Arctic continental shelves and have been driven by the TPD (Wadhams, 1983). This is in agreement with the fact that reported 137 Cs and 239,240 Pu activities in bottom sediments from the Siberian continental shelves are higher than those in the western Arctic Ocean (e.g. Baskaran et al., 1995; 2000; Huh et al., 1997; Meese et al., 1997; Smith et al., 2000; Johnson-Pyrtle and Scott, 2001; Oughton et al., 2004). 137 Cs and 239,240 Pu activities in the central-eastern side of the Fram Strait and in the Eastern Greenland continental shelves vary from 3.8 to 23 Bq kg −1 and from 0.018 to 0.71 Bq kg −1 , respectively. However, SIS samples from the Eastern Greenland area consisted mostly of sand (70%) (data not shown), in agreement with the description of bottom sediments by Berner and Wefer (1990), who reported that sand accounts for more than 50% of the sediments in the shelf areas of Greenland. Earlier sea-ice sediment studies carried out in the central Arctic Basin and in the Eurasian shelves (e.g. Nürnberg et al., 1994; Dethleff, 2005; Eicken et al., 2005) suggested that entrained materials consist of 60–90% fine-grained (b63 mm) silt and clay, with an essentially terrestrial origin. Therefore, the presence of either high or low concentrations of 137 Cs and 239,240 Pu associated with low fine-grained content must be driven by the transport of particulate matter for long distances across the Arctic Ocean and not from Greenland. Overall, and despite the large variability of 137 Cs and 239,240 Pu activities in SIS along the Arctic Ocean, a reasonable correlation exists between both radionuclides (R 2 =0.82, Fig. 3). As a general trend, most SIS samples contain 137 Cs and 239,240 Pu in similar proportions, with a median 239,240 Pu/ 137 Cs ratio of 32·10 −3 . This suggests that the ultimate source of most 239,240 Pu and 137 Cs in the SIS, and therefore in the bottom sediments mostly delivered by rivers of continental shelves, is the same around the Arctic Ocean. In fact, the median 239,240 Pu/ 137 Cs ratio is in good agreement with the global fallout value, ∼34·10 −3 in 2007, based upon decay correction of data in Beck and Krey (1983). This conclusion is reasonable because global fallout is regarded as the main source of both radionuclides to the Arctic Ocean (JRNEG, 1996, Oughton et al. 1999). However, it is necessary to use caution when using the ratio to attribute the source of 137 Cs and 239,240 Pu to global fallout. The 239,240 Pu/ 137 Cs ratio in bottom sediments depends on many factors: decay, scavenging of the radionuclides by particles during sedimentation, water column depth and proximity to river outflows, sediment characteristic and composition or radionuclide chemical features. In particular the distribution coefficients (K d ) for Pu and Cs in the marine environment are significantly different (1·10 5 and 2·10 3 , respectively; IAEA, 1985), as Pu is more particle reactive than 137 Cs. In spite of the good general correlation between 137 Cs and 239,240 Pu concentrations, slight differences were observed, suggesting that SIS were imprinted by secondary sources in addition to global fallout (Fig. 3). This is markedly the case for samples considered as anomalous based on the criteria of higher activities (N45 Bq kg −1 for 137 Cs and N1.4 Bq kg −1 for 239,240 Pu). 4.2. Distribution of 240 Pu/ 239 Pu atom ratios in sea-ice sediments The 240 Pu/ 239 Pu atom ratio can be used to identify local sources of Pu other than global fallout. Although the main source of Pu isotopes to the Arctic Ocean is global fallout from the nuclear weapons testing (Oughton et al., 2004; Skipperud et al., 2004), additional local and regional sources, including local fallout from tropospheric weapons testing, dumping of nuclear waste, marine and terrestrial transport from the reprocessing plants, and input from nuclear accidents that occurred in proximity to the Arctic Ocean (Tomsk-7) have all contributed to enhancing 239,240 Pu and 137 Cs activities and modifying the 240 Pu/ 239 Pu atom ratios in specific areas of the Siberian shelves (Cochran et al., 2000). In general, the 240 Pu/ 239 Pu atom ratio is relatively uniform in the bottom sediments along the continental shelves of the Arctic Ocean (Skipperud et al., 2004) and is comparable to the characteristic ratio of global fallout, (0.183±0.009). However, low 240 Pu/ 239 Pu atom ratios have been reported in bottom sediments from areas such as Kara Sea and Novaya Zemlya archipelago, ranging from 0.03 at Chernaya Bay to 0.16±0.03 at the Kara Gate (Smith et al., 2000; Oughton et al., 2004). On the other hand, ratios slightly higher than the global fallout (0.1939±0.0013), were measured in sediments at Point Barrow, Alaska (Kelley et al., 1999). Huh et al. (1997) suggested that 240 Pu/ 239 Pu atom ratios in sediment cores from the deep Arctic Basin correspond to a mixture of global fallout inputs, which decreases with increasing latitudes, and discharges from reprocessing plants in Russia and in the Atlantic area. Cooper et al. (2000) suggested that Pu associated with SIS can be considered as a source of Pu to the bottom sediments, although the total flux of Pu from SIS to the deep sea would be relatively small. Recent studies of 240 Pu/ 239 Pu atom ratios in bottom sediments in the Fram Strait have provided evidence of the long distance dispersion of Pu in the Arctic Ocean associated to sea-ice sediments (Masqué et al., 2003). This demonstrates that sea-ice is an efficient transport agent of 3354 P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 Pu associated with sediments and the fate of sea ice and associated radionuclides to sea-ice sediments is closely coupled. The 240 Pu/ 239 Pu atom ratios in SIS are generally comparable to that of global fallout (0.18, JRNEG, 1996; Krey et al., 1976). This is expected because most of the continental shelves have been affected by Pu derived from global fallout (Fig. 4). Deviations from it have been observed at the Fram Strait (0.14 –0.21) and the Eurasian Basin (0.12–0.22) due to the fact that the former area is the region where all drift pathways merge while the latter is influenced by the extensive source areas of the Siberian shelves (Nürnberg et al., 1994; Eicken et al., 1997). In the western Arctic Ocean, in contrast, 240 Pu/ 239 Pu atom ratios range from 0.17 to 0.19, showing clearly the signature of global fallout. However, the exceptions to this trend are two samples, 218-1 and 234-1, collected close to the North Pole. These samples likely originated in the vicinity of the Kara Sea, as inferred from their lower 240 Pu/ 239 Pu atom ratios. The ice floes carrying these SIS could reach Fig. 3. Relationship between all 239,240 Pu vs 137 Cs activities in sea-ice sediments from the Arctic Ocean (including data from this study and from Meese et al. (1997), Landa et al. (1998), Cooper et al. (1998), Masqué et al. (2003, 2007). Confidence intervals at 95% are shown (R 2 =0.82). Red points are considered as outliers and are not included into the regression. Fig. 4. Relationship of all available data of 137 Cs activities vs 240 Pu/ 239 Pu atom ratios in sea-ice sediments collected in the Arctic Ocean including data from this study and from Meese et al. (1997), Landa et al. (1998), Cooper et al. (1998) and Masqué et al. (2003, 2007). Samples are divided according to the mean sea ice drift patterns: Fram Strait (circles), Siberian (solid squares), Laptev Sea (reversed triangle), Polar-central Arctic (triangle), Beaufort (rhombus) and Alaska continental shelves (solid triangle). Five clusters are identified using a 240 Pu/ 239 Pu atom ratio of 0.183±0.009 and a 137 Cs activity of 20 Bq kg −1 as limits. These clusters are used to hypothesized source areas for sea-ice sediments (see text for details). 3355P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 the western Arctic as a result of exchange between TPD and Beaufort Gyre. Also, 240 Pu/ 239 Pu atom ratios slightly higher than global fallout were found in SIS in the Alaska coast (Landa et al., 1998). Those are comparable to 240 Pu/ 239 Pu atom ratios determined in the sediments of Point Barrow, suggesting that their origin was close to the site of sampling. 4.3. Linking radionuclide signatures of SIS to areas of sea ice origin As shown in Fig. 4 and noted above,most SIS samples have 240 Pu/ 239 Pu atom ratios comparable to that of stratospheric bomb fallout (0.183±0.009), irrespectively of their 137 Cs concentrations. In particular, samples with elevated 137 Cs concentrations are not necessarily characterized by non-global fallout Pu atom ratios, implying that 137 Cs alone cannot be used to identify an origin area of sea ice. The combination of both datasets may allow us to identify source areas of the SIS and thus of sea ice. Several clusters of radionuclide signature can be identified according to two criteria (Fig. 4): deviations of the 240 Pu/ 239 Pu atom ratios relative to the global fallout value (0.183±0.009) and 137 Cs activities higher or lower than 20 Bq kg −1 , as it has been reported that concentrations of 137 Cs originated from global fallout in most continental shelf sediments are b20 Bq kg −1 (AMAP, 2002). These clusters include: i) samples with 240 Pu/ 239 Pu atom ratios b0.174 and 137 Cs activities N20 Bq kg −1 ; ii) samples with 240 Pu/ 239 Pu atom ratios lower than 0.174 and 137 Cs activity b20 Bq kg −1 ; iii) samples with 240 Pu/ 239 Pu atom ratios within the range of global fallout and 137 Cs activities N20 Bq kg −1 ; iv) samples with 240 Pu/ 239 Pu atom ratio greater than global fallout but with low 137 Cs activities (b20 Bq kg −1 ) and v) samples with 240 Pu/ 239 Pu atom ratio comparable to the global fallout and 137 Cs activities b20 Bq kg −1 . 4.3.1. Kara Sea As shown in Fig. 4, cluster (i) (short dash-dots line), a group of SIS samples is characterised by 240 Pu/ 239 Pu atom ratios lower than the global fallout value (b0.174) and 137 Cs activities greater than 20 Bq kg −1 . Low 239 Pu/ 240 Pu atom ratios correspond to a mixture between global fallout and low-yield nuclear testing fallout or material released from nuclear reprocessing plants. Among the Arctic continental shelves, low 240 Pu/ 239 Pu atom ratios have been reported only in the Kara Sea and the Novaya Zemlya archipelago, as a result of local fallout, discharge from rivers and nuclear waste dumping (Smith et al., 2000; Skipperud et al., 2004). 137 Cs and 239,240 Pu activities in SIS are greater than those typical for continental shelf sediment affected by global fallout (20 Bq kg −1 for 137 Cs and 1 Bq kg −1 for 239,240 Pu; Smith et al., 2000). Previous studies have reported elevated activities of 137 Cs and 239,240 Pu (N45 Bq kg −1 and N1Bqkg −1 , respectively) in bottom sediments of the Kara Sea (AMAP, 1998; Baskaran et al., 1995; Smith et al., 2000), and thus this could be a likely origin area for SIS with low 239 Pu/ 240 Pu atom ratios and high 137 Cs and 239,240 Pu activities. The SIS samples showing these isotopic signatures were collected in the west side of the Fram Strait and in the Siberian Branch of the TPD, regions that are principally within the drift path for sea ice formed along the western Arctic shelves, particularly between the Laptev Sea and the Novaya Zemlya Archipelago (Pfirman et al., 2004; Wadhams, 1983). Another group of samples are characterized by low Pu atom ratios (b0.174) but also low 137 Cs specific activities (b20 Bq kg −1 ), cluster (ii) (Fig. 4,dash line). Given the low 240 Pu/ 239 Pu atom ratios of these samples, the most probable source area would also be the Kara Sea and its surrounding areas. The low 137 Cs specific activities are likely due to the other factors that affect specific activity (described in Section 4.1), and in any case are within the range of activities found in surface bottom sediments (2-33 137 Cs Bq kg -1 ,Livingston and Povinec, 2000). This reinforces the concept that the 240 Pu/ 239 Pu atom ratio is a better indicator of SIS source than the 137 Cs specific activity. 4.3.2. Laptev–Kara Sea A number of SIS samples display 240 Pu/ 239 Pu atom ratios typical of global fallout and high 137 Cs activities (N20 Bq kg −1 ), cluster (iii) (Fig. 4, dot line). The 240 Pu/ 239 Pu atom ratios suggest that any continental shelf area might be the origin of the sediments. High 137 Cs activities were measured in bottom sediments in the western Kara and Laptev Seas (AMAP, 1998; Baskaran et al., 1995; Smith et al., 2000, personal communication by A. Johnson-Pyrle in Meese et al., 1997), although elevated inventories of 137 Cs in bottom sediments could result from a combination of processes related to the sedimentation dynamics, Johnson-Pyrtle and Scott (2001) hypothesized that elevated 137 Cs inventories in bottom sediments may be the result of 137 Cs origin other than direct atmospheric fallout, indicating the Lena river as a secondary source. In the absence of any other direct source of 137 Cs in its drainage basin, it is likely that 137 Cs introduced into the Lena river comes from the erosion of global fallout and possibly from Chernobyl derived contamination. Several SIS samples with these characteristics were collected in the Laptev Sea and along the northwest of Franz Josep Land. The Franz Josef Land is within the Siberian Branch of the TPD. Prior studies have considered the Laptev Sea as the major source of sea ice to the Siberian Branch of the TPD, with the Kara Sea as a secondary source (Nürnberg et al., 1994; Pfirman et al., 1997). Back trajectory analysis of three samples (sample coded PS93-2351-2; Landa et al., 1998 and 212-2 and 218-1; Cooper et al., 2000) collected in the western part of the Arctic Ocean suggested that they originated from the Canadian archipelago and the Beaufort Sea (Tucker et al., 1999). However, the 137 Cs and 240,239 Pu activities and 240 Pu/ 239 Pu atom ratios of these samples argue against these areas as the origin, as no elevated 137 Cs and 239,240 Pu concentrations have been reported in North American continental shelves (Meese et al., 1997; Landa et al., 1998). It is more likely that these sediments originated and were incorporated into sea ice in the Laptev Sea, and reached the western basin by exchange between the TPD and the Beaufort Gyre. The most likely scenario for the transport of Laptev Sea-derived SIS into the Beaufort Gyre occurs when a positive Arctic Oscillation (AO), which favours strong advection of ice away from the Siberian shelves into the central Arctic, is followed by negative AO conditions, increasing the size of the Beaufort Gyre and capturing sea ice into the Beaufort Gyre. The three samples from the western part of the Arctic Ocean were collected in 1994 (Cooper et al., 1998). The years immediately preceding the sampling activities (1989 to 1995) were mostly years of a+AO (Mysak, 2001), supporting the hypothesis that they could have originated in the Laptev Sea. During these years, the TPD would be strengthened and pushed closer to Beaufort Sea, making the transport of Laptev Sea into the Beaufort Gyre more likely. 4.3.3. North American shelves Most of the samples collected in the western part of the Arctic Ocean close to the Alaska coast, as well as samples collected on the western side of the Fram Strait form another cluster with 240 Pu/ 239 Pu atom ratios greater than global fallout (0.19–0.25) and 137 Cs activities mostly below 10 Bq kg −1 , cluster (iv) (Fig. 4, dash-dot line). North American shelves are likely the origin area for such SIS: relatively elevated 240 Pu/ 239 Pu atom ratios were measured in bottom sediments from Point Barrow (Kelley et al., 1999), while average activities of 137 Cs in surface bottom sediments in the Bering and Chukchi Seas and in the East Chukchi Sea are low (4.2±2.8 Bq kg −1 ,Meese et al. (1997) and 2.9±0.7 Bq kg −1 ,Baskaran and Naidu (1995), respectively). The high 240 Pu/ 239 Pu atom ratios in samples from the western Fram Strait highlight the importance of long-distance transport SIS from the Beaufort Sea across the Arctic Ocean. These sea ice floes and their contained SIS would have originated in the shallow parts of the 3356 P. Cámara-Mor et al. / Science of the Total Environment 408 (2010) 3349–3358 Beaufort and Chukchi Seas, close to the shore, and drifted eastward, traversing the Arctic Ocean via the Beaufort Gyre and being captured by the Polar Branch of the TPD before finally reaching the Fram Strait. Indeed, this is supported by Pfirman et al. (1997), that concluded that North American shelves were the origin for sea ice that flows into western Arctic Ocean and the western part of the Fram Strait. 4.3.4. Unidentifiable source areas Approximately 50% of the analysed samples have 240 Pu/ 239 Pu atom ratios comparable to that of global fallout and 137 Cs specific activities lower than b20 Bq kg −1 , cluster (v), (Fig. 4, solid line). The anthropogenic radionuclide distributions in most of the continental shelves of the Arctic Ocean are typical of global fallout origin, and thus the isotopic signature is not distinct as to identify the source areas of SIS. Additional analyses, such as Fe oxide mineral grains (Darby, 2003), planktonic diatom species (Abelmann, 1992), or smectite and illite analysis (Wollenburg, 1993; Dethleff et al., 1993; Nürnberg et al., 1994) may help to constrain more accurately the origin of these samples. 5. Conclusions Based on a combined dataset of previously published and new analyses of 137 Cs and 239,240 Pu and the 240 Pu/ 239 Pu atom ratios in Arctic sea-ice sediments (SIS), we conclude that these anthropogenic radionuclides can be used in many instances to determine the geographical source area in which the sediments were incorporated into the ice. This information, in addition, can be used to elucidate the sea ice floes formation areas. The 240 Pu/ 239 Pu atom ratio, in combination with the 137 Cs or 239,240 Pu activity, is especially useful in this regard. SIS originating in the Laptev and Kara Seas have 240 Pu/ 239 Pu atom ratios lower than those imprinted by global fallout (b0.18), while SIS originating from the Alaskan shelf is characterised by 240 Pu/ 239 Pu atom ratios greater than global fallout. The specific activities of 137 Cs and 239,240 Pu are less diagnostic of sea-ice origin, because many processes in addition to source can affect their values; however, sediments of the Kara and Laptev Seas can have markedly elevated specific activities of 137 Cs that are imprinted on SIS originating in those areas. In approximately 50% of the samples analyzed, the isotopic signatures are not distinctive as to SIS origin and additional approaches are required to better resolve possible source areas. Acknowledgements This work was partially funded by the Ministerio de Eduación y Ciencia of Spain (POL2006-00449). 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