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Article 129
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Potential releases of I, U and Pu isotopes from the Fukushima Dai-ichi nuclear power plants to the ocean during 2013 to 2015 Núria Casacuberta, Marcus Christl, Ken O. Buesseler, YikSze Lau, Christof Vockenhuber, Maxi Castrillejo, Hans-Arno Synal, and Pere Masque Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03057 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017
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Environmental Science & Technology
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Potential releases of 129I, 236U and Pu isotopes from the Fukushima Dai-ichi
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nuclear power plants to the ocean during 2013 to 2015
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Núria Casacuberta*,1,2, Marcus Christl1, Ken O. Buesseler3, YikSze Lau4, Christof Vockenhuber1, Maxi Castrillejo5, Hans-Arno Synal1, Pere Masqué5,6,7.
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1
Laboratory of Ion Beam Physics, ETH Zürich, Switzerland
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Institute of Biogeochemistry and Pollutant Dynamics, Environmental Physics, ETH Zürich,
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Switzerland. 3
Woods Hole Oceanographic Institution, MA, USA.
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Lancaster Environmental Center. University of Lancaster, England.
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Institut de Ciència i Tecnologia Ambientals & Departament de Física, Universitat Autònoma de
Barcelona, Bellaterra, Spain. 6
School of Science, Centre for Marine Ecosystems Research, Edith Cowan University, Joondalup, WA,
Australia. Oceans Institute & School of Physics, The University of Western Australia, Crawley, WA. Australia.
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* Corresponding author: N. Casacuberta. ETH-Zürich, Laboratory of Ion Beam Physics,
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HPK G23. Otto-Stern-Weg 5, CH-8093 Zürich, Switzerland.
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E-mail:
[email protected]. Phone: +41446337576. Fax: +41446331067
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Abstract/TOC
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After the Fukushima Dai-ichi Nuclear accident, many efforts were put into the
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determination of the presence of 137Cs, 134Cs, 131I and other gamma-emitting
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radionuclides in the ocean, but minor work was done regarding the monitoring of less
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volatile radionuclides, pure beta-ray emitters or simply radionuclides with very long
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half-lives. In this study we document the temporal evolution of 129I, 236U and Pu
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isotopes (239Pu and 240Pu) in seawater sampled during four different cruises performed
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2, 3 and 4 years after the accident, and compare the results to 137Cs collected at the same
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stations and depths. Our results show that concentrations of 129I are systematically
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above the nuclear weapon test levels at stations located close to the FDNPP, with a
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maximum value of 790 x107 at·kg-1, that exceeds all previously reported 129I
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concentrations in the Pacific Ocean. Yet, the total amount of 129I released after the
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accident in the time 2011-2015 was calculated from the 129I/137Cs ratio of the ongoing
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137
Cs releases and estimated to be about 100 g (which adds to the 1 kg released during
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the accident in 2011). No clear evidence of Fukushima-derived 236U and Pu-isotopes has
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been found in this study, although further monitoring is encouraged to elucidate the
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origin of the highest 240Pu/239Pu atom ratio of 0.293±0.028 we found close to FDNPP.
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1. INTRODUCTION
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The Tohoku earthquake that took place on March 11 2011 near the coast off Japan was
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the initial trigger of the nuclear accident at the Fukushima Dai-ichi Nuclear Power
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Plants (FDNPPs). The loss of the cooling functions for three reactor units due to the
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damage of the power supply systems caused one of the largest anthropogenic
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radionuclide releases to the Pacific Ocean1, 2, increasing the current North Pacific
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inventory of 137Cs by about 20%.3 In the following years, major efforts have been
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devoted to investigating the impact of the accident and to quantify the total radionuclide
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releases to the marine environment.1-4 In addition to the studies conducted offshore, the
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operating company of the FDNPPs, TEPCO, has monitored the area close to the power
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plants and produced daily to monthly reports on radionuclides in the ocean and seafloor
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around the FDNPPs.5, 6 To better constrain radionuclide releases to the Pacific Ocean an
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increasing number of sampling locations inside and outside the FDNPP domain have
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been set up over the past 5 years, while at the same time detection limits of 137Cs were
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improved by a factor of >20 (Figure S1). TEPCO’s monitoring program focuses on the
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main gamma-ray emitters; 131I (T1/2=8.02 d), 134Cs (T1/2=2.06 a) and 137Cs (T1/2=30.17
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a), complemented on a much lower sampling frequency basis by 3H (T1/2=12.32 a), 90Sr
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(T1/2=28.79 a) and total beta activity. Reported concentrations of 137Cs and 134Cs on the
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premises and the surrounding area of FDNPPs have decreased several orders of
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magnitude since the accident (Figure S1), and apart from some reported Cs leakage
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events5, much smaller amounts of 137Cs and 134Cs were released into the Pacific Ocean
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after 2011.2, 3, 6 The decrease on Cs releases is mostly due to physical barriers, such as
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the installation of silt fences near the water intakes of the reactor units in 2011. The
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construction of decontamination systems such as the Multi-nuclide Removal Facility
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(Advanced Liquid Processing System; ALPS) in 2014 further reduced radionuclide
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concentration in wastewaters 2, 5, as well as the construction of a landside impermeable
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wall (frozen soil wall, or ice-wall) in 2016.
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Less effort has been devoted to study Fukushima’s so-called forgotten radionuclides.7, 8
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Some of the emitted nuclides such as 89Sr, 90Sr, 103Ru, 106Ru, 236U and Pu-isotopes are
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more refractory, and thus their releases were smaller and would pose a significantly
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lower environmental impact and radiological risk to humans. Additionally, more
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complex analytical techniques are required for some of the emitted radionuclides (e.g.
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3
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Finally, most of the ‘forgotten’ radionuclides are very long-lived (e.g., 36Cl (T1/2=301
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ka), 99Tc (T1/2= 211 ka), 129I (T1/2=15.7 Ma), 236U (T1/2=23 Ma), 239Pu (T1/2= 24 ka) and
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240
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activities, and thus are less radiologically relevant.7 The monitoring of the forgotten
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radionuclides is however important, since they can be used as tracers of oceanographic
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processes.1 In addition to this, the use of isotopic ratios is a powerful tool to identify
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sources of contamination (nuclear forensics), and this is especially true when half-lives
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of isotopes are long enough so that different sources of contamination can be identified
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back in time. For example, the 240Pu/239Pu atom ratio can be used as a fingerprint to
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identify their sources.9 The average 240Pu/239Pu atom ratio accumulated from global
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fallout is 0.178±0.023.10 Lower ratios indicate weapon-grade Pu sources, while higher
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ratios would reveal reactor-grade Pu, such as Chernobyl (0.38±0.07)9 and Fukushima
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(0.320-0.356 for reactors and 0.394-0.468 for spent fuel pools).11 In a similar way, the
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137
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open ocean.12 in 2011, waters released from the FDNPP had a 137Cs/90Sr activity ratio of
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3812, which was significantly higher than the modern average global fallout ratio of
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1.6.13 In 2013, the 137Cs/90Sr ratio was 3.5, while concentrations of both 90Sr and 137Cs
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remained high, indicating either new radionuclide releases from FDNPP or a change of
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the initial signature of the FDNPP source e.g. caused by changes in the waste water
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treatment.8
H, 14C, 35S, Pu-isotopes), which preclude obtaining large datasets in a timely manner.
Pu (T1/2= 6.6 ka)), implying that they are present in the environment at lower specific
Cs/90Sr activity ratio was used to identify Fukushima-derived radionuclides in the
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Today, more than 5 years after the accident, the shorter-lived radionuclides (e.g. 131I,
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134
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available, and the concentrations of all isotopes have decreased significantly (e.g. 90Sr).
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As a consequence, long-lived radionuclides that were released in small quantities7,such
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as 129I, 236U, 239Pu and 240Pu may play an increasingly important role for future studies.
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Their potential as ocean tracers rather than their radiological risk is the key motivation
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for this research. For example, the reported core activity inventories of these
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radionuclides at Units 1-3 (active units by the time of the accident) were 0.0027 PBq for
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Cs) have totally/partly decayed and their unique Fukushima signal is no longer
U, and 2.6 PBq and 3.3 PBq for 239Pu and 240Pu, respectively.1, 14 A core inventory of
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0.2 TBq (0.0002 PBq) of 129I was calculated from estimates of the 129I/131I isotopic ratio,
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which ranged from 27±8 to 32±9.15, 16 The activity inventories of these long-lived
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isotopes are smaller compared to, for example, the inventories of 6000 PBq of 131I or
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700 PBq of 137Cs. Yet the mass inventory of Units 1-3 were 40 kg, 1100 kg, 1140 kg
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and 390 kg for 129I, 236U, 239Pu and 240Pu, respectively, which is, in all cases, of the
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same order of magnitude than total amounts released during the nuclear weapon tests in
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the 1950’s and the 1960’s.9, 17, 18 The releases from FDNPP of even a tiny fraction of
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the total core inventory of these long lived radionuclides (as estimated here and by
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others) would therefore be enough to produce an identifiable Fukushima signal in the
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North Pacific Ocean. These long-lived radionuclides can now be analyzed at very high
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sensitivity levels using compact Accelerator Mass Spectrometry (AMS) systems 19-21,
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which allow for their determination in relatively small samples (e.g. 0.3 L for 129I, 1-2 L
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for 236U and 10 L for Pu-isotopes), overcoming the problem of large volume sample
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collection during ocean expeditions. In addition, Inductively Coupled Plasma Mass
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Spectrometry (ICP-MS) can still be used for the analysis of Pu-isotopes at somewhat
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lower background suppression but simpler preparation and short analysis time.22 Also
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the new ICP Triple Quad (ICP-QQQ) allows measuring the 236U/238U ratio in a 10-10
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range, which would be in the upper range of seawater concentrations.
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In this work we studied the distribution of 129I, 236U, 239Pu and 240Pu along the coast off
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Japan, sampled 2, 3 and 4 years after the nuclear accident. The aim of the study was to
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evaluate the concentrations and distributions of these nuclides during the years
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following the Fukushima accident, in order to constrain their releases to the ocean and
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to study the feasibility of using them and their isotopic ratios (e.g. 129I/137Cs,
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Pu/239Pu) as contamination sources and oceanographic tracers.
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2. MATERIALS AND METHODS
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2.1 Study area and sampling. Seawater samples were collected during four
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oceanographic expeditions performed in September 2013, May 2014, October 2014 and
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October 2015 on board the R/V Daisan Kaiyo Maru (2013) and R/V Shinsei Maru
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(2014 and 2015) in the coast off Japan (Figure 1). Samples were collected from coastal
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and offshore waters, including surface and shallow profiles, for 129I (n=86, all cruises),
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U (n=51, September 2013 and October 2014) and Pu-isotopes (n=27, October 2014).
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For the offshore samples, seawater throughout the water column was collected using an
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oceanographic rosette equipped with conductivity, temperature and pressure sensors,
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and 12 Niskin bottles of 10 L each. Surface seawater was collected at 1-2 m depth using
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a deck-mounted pump. For the coastal samples, surface seawater was collected for the
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analysis of 236U and 137Cs. In September 2013, 6 samples were taken at Nagahama and
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Nobiru beaches, Sendai Bay, about 100 km north of the FDNPPs (Figure 1b and 1c). In
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October 2014, 6 more samples were collected in Nakoso Beach, Yotsukura Beach and
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Acquamarine-Fukushima, in Iwaki prefecture, about 35 to 60 km south of the FDNPP
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(Figure 1b and 1c). Surface seawater samples were filtered using a 1µm pore size
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cartridge filter to remove suspended matter. Seawater samples collected using Niskin
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bottles and coastal samples taken at beaches were not filtered.
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2.2 Radiochemistry and measurement of seawater samples. Different seawater
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sample sizes and extractions methods were applied for the individual radionuclides and
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on each expedition. Between 250 mL and 500 mL of seawater was used for the
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determination of 129I. Woodward stable iodine carrier (1-1.5 mg) was added to all
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samples and iodine was pre-concentrated as AgI after its purification with BioRad® 1x8
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analytical grade resins.23, 24 The 129I/127I atom ratio was determined for each sample with
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the ETH Zurich 0.5 MV AMS system Tandy21, which allowed for the estimation of
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final 129I concentrations (reported as atoms·kg-1). The measured 129I/127I ratios were
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normalized to the ETH Zurich in-house standard D22 with a nominal ratio 129I/127I =
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(50.35±0.16) x10-12.20 A total of 7 lab blanks were prepared using milliQ water,
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obtaining an average 129I/127I ratio of (0.3±0.1) x10-12 (corresponding to
