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Anthropogenic U in the North Sea – a closer look into a source region Marcus Christl, Núria Casacuberta, Johannes Lachner, Jürgen Herrmann, and Hans-Arno Synal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03168 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017
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Environmental Science & Technology
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Anthropogenic 236U in the North Sea – a closer look
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into a source region
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Marcus Christl1*, Núria Casacuberta1,2, Johannes Lachner1†, Jürgen Herrmann3, Hans-Arno-
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Synal1
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Laboratory of Ion Beam Physics, ETH Zurich, 8093 Zurich, Switzerland
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Environmental Physics, ETH Zurich, 8092 Zurich, Switzerland
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Bundesamt für Seeschifffahrt und Hydrographie, 22589 Hamburg, Germany
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†
present address: Faculty of Physics, University of Vienna, Währinger Straße 17, A-1090
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Vienna, Austria
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*
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Laboratory of Ion Beam Physics, Otto Stern Weg 5, 8093 Zurich, Switzerland
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(
[email protected])
author to whom correspondence should be addressed: Marcus Christl, ETH Zurich,
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KEYWORDS: Anthropogenic radionuclides, Uranium-236, North Sea, nuclear reprocessing.
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ABSTRACT In this study we present new sea water data of 236U and 238U sampled in the North
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Sea in 2010. The North Sea has been and is still receiving a considerable input of anthropogenic
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radionuclides from nuclear reprocessing facilities located in La Hague (France) and Sellafield
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(Great Britain). It therefore represents an important source region for oceanographic tracer
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studies using the transient signal of anthropogenic
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236
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covers the transition regions of the North Sea to the Atlantic Ocean, to the Baltic Sea, and
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upstream the Elbe River. It is discussed in the context of available 236U data from literature. Our
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results show that both,
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Sea can be explained by simple binary mixing models implying that 236U behaves conservatively
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in seawater. We further show that the input of
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might be a maximum input of 12 g/yr via the Baltic Sea. The results of the mixing models
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suggest that the source of this still unidentified
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water input.
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U. A proper knowledge of the sources of
U is an essential prerequisite for such tracer studies. The
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U concentrations and
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236
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U data set presented in this study
U/238U ratios in surface waters of the North
U by the Elbe River is negligible while there
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U contamination could be supplied by fresh
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Introduction
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Anthropogenic 236U (T½ = 23.5 Myr) is mainly produced in nuclear reactors by thermal
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neutron capture on 235U ((n, γ)–reaction) or in nuclear explosions by the interaction of fast
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neutrons with 238U (via a (n, 3n)-reaction).1, 2 About 1000 kg of 236U have been produced
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globally during the period of atmospheric nuclear bomb tests, reaching a peak in atmospheric
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deposition in the early/mid 1960ies.3, 4 Another about 100 kg 236U have been discharged locally
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into the North East Atlantic Ocean by nuclear reprocessing facilities with peak discharges
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occurring between 1980 and 2000.5
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In recent years, several studies investigated the potential of anthropogenic 236U as a new tracer
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in oceanography 6-16 and strongly promoted its use (also in combination with other anthropogenic
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tracers such as 129I) to assess water mass flow and mixing patterns and/or to estimate timescales
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of water mass movements.5, 7, 11 In this context, the North Atlantic and Arctic Oceans have been
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identified as key study areas because of their sensitive response to climate change and their
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important role in thermohaline circulation.11, 17-19 The surface waters in the North Atlantic and
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Arctic Oceans may contain high levels of anthropogenic radionuclides such as 3H, 90Sr, 99Tc,
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137
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reprocessing facilities La Hague (LH), France and Sellafield (SF), Great Britain (Figure 1). The
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temporally variable and point-like release of different radioactive contaminants from LH and SF
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into the ocean has been exploited in several studies to investigate the timescales of water mass
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dispersion into adjacent ocean basins.20-26 Recent studies confirm that elevated concentrations of
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236
Cs, 129I, 236U, and transuranic actinides20 originating from the two Northwest European nuclear
U (above global fallout levels) found in the North Sea, in the Atlantic surface waters of the
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Arctic Ocean, and in the overflow waters of Denmark Strait can be explained by the addition of
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236
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U released from the European nuclear reprocessing facilities.11, 14, 17
A strong argument for adding 236U to the existing suite of powerful ocean tracers is based on
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its conservative nature and the fact that the behavior of naturally occurring U-isotopes in the
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ocean is well understood.27 Natural U is present in open ocean sea water at concentrations of
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about 3.3 ppb (µg/l) and its conservative behavior is well-known.28 As a consequence, a strong
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correlation of natural U with salinity is observed in oxygenated ocean water.29, 30 The addition of
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anthropogenic 236U into the surface ocean by global fallout or nuclear reprocessing does/did not
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significantly change the 238U content of sea water. This simple input scenario for 236U implies
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that the isotopic ratio 236U/238U and 236U concentrations in ocean water are linearly correlated.
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However, the correlation breaks down if 236U is added to sea water from other sources that
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influence both, 236U and 238U concentrations. The dominant source of natural U for the oceans is
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river runoff27 and typical natural U concentrations between 0.4 and 2 µg/L are reported for rivers
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in Western Europe.31 It is still an open question how much anthropogenic 236U is transported into
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the oceans by rivers because river data is lacking. Elevated levels of 236U/238U have been found
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in Austrian river waters (all draining into the Black Sea) and were attributed to global fallout
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and/or to fallout from the Chernobyl accident.32
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To apply 236U/238U as a reliable transient oceanic tracer the sources of 236U and therefore the
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input function have to be properly known. Significant effort has already been devoted to assess
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the sources and the distribution of 236U in the North Atlantic and Arctic Ocean in order to
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develop the application of anthropogenic 236U as new transient tracer.11, 13, 14, 17 It has been shown
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that releases from nuclear reprocessing (SF plus LH) raised 236U/238U levels in the North Sea by
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more than one order of magnitude above global fallout (GF) levels of about 1 x 10-9.13, 14
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Furthermore, a first (model based) reconstruction of the combined 236U input from the nuclear
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reprocessing facilities LH and SF has been presented (black line in Figure 2).5 According to the
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model results the 236U input from 1965 to 2012 from LH amounts to less than 25kg (and 62±31
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kg from SF). This reconstruction still has some uncertainties because a substantial data basis
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only exists for 236U releases from LH, while it is extremely scarce for SF. Therefore, the
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236
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function for the SF branch waters (red and blue lines in Figure 2, adapted from ref. 5).
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Nevertheless, the above results show that water masses labeled with 236U in the North Sea region
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after ~1980 by one of the reprocessing facilities (LH and/or SF) are clearly distinguishable from
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Atlantic waters that carry the GF signal only. It is, however, not possible to track a single
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reprocessing signal on its way through the Arctic Ocean because the characteristic 236U/238U ratio
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is modified while waters disperse and mix with surrounding ocean water carrying different
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236
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as an extension of the 236U/238U tracer method.11 In this study, we focus on the tracer application
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of 236U/238U and on possible 236U sources in the North Sea region.
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U/238U input function for the LH-branch waters is considered more reliable than the input
U/238U ratios. To overcome this problem, the combination of 236U with 129I has been suggested
The model based input function5 of 236U still needs to be confirmed by historical 236U data.
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Additionally, other potential sources of 236U in the Northeast Atlantic Ocean need to be
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identified or excluded. For example, a recent study presenting first data from the Baltic Sea
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suggests the presence of an additional yet unidentified source of 236U.33 Furthermore, it is still
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unknown if and in case how much 236U is entering the North Sea from rivers. For example, the
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Elbe River is draining the waters from large abandoned U mining areas in the Western Ore
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Mountains (Eastern Germany) and also hosts 5 nuclear reactors along its course. However, it is
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not expected that nuclear reactors contribute significantly to the 236U budget in the North Sea as
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no accidental releases of 236U from nuclear reactors in Western Europe are documented. In
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contrast, the input of natural U (including some 236U) from former U-mining areas can be
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significant.
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In this paper we present new 236U data from the North Sea sampled in 2010. The new data set
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covers three important hydrographic transitions: (i) to the open Atlantic Ocean in the North, (ii)
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to the Baltic Sea in the East, and (iii) to the Elbe River in the Southeast. The data set
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complements existing 236U data from the North Sea13, 14 and the Baltic Sea33 and therefore allows
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presenting a comprehensive picture of 236U in this region including a discussion of possible
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additional sources of 236U entering the North Sea via the Baltic Sea and/or the Elbe River. By
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combining our data with data from literature, we will show that both, the observed 236U
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concentrations, and 236U/238U ratios can be explained by simple binary mixing of water masses.
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Material and Methods
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Hydrography of the North Sea
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The North Sea is a shallow marginal sea that is connected to the North Atlantic Ocean via the
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narrow Dover Strait in the Southwest and via a broad opening in the North (Figure 1). Its
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complex hydrographic situation is mainly characterized by the inflow of saline Atlantic Waters,
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contributions of less saline waters entering from Baltic Sea via Kattegat and Skagerrak, and the
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inflow of fresh waters from rivers and net precipitation. The resulting distribution of salinity is
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temporally stable and, forced by the prevailing westerlies, a mean cyclonic circulation pattern
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builds up in the North Sea34 (Figure 1). In Fall/Winter 2009 exceptionally strong cyclonic
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circulation was observed resulting in an enhanced inflow of Atlantic waters via the English
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Channel (peak flow: 1 Sv = 106 m3/s) and an increased outflow of Baltic Sea waters in early
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201034.
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The state of the North Sea is intensely monitored by the riparian countries that are maintaining a
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dense observational network. In this context, annual sampling campaigns are carried out by the
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BSH (Germany) to document the standard oceanographic parameters but also to monitor
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chemical and radioactive pollutants (such as nutrients, organic compounds, metals, 3H, 137Cs,
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90
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period 2008-2011 are summarized in a comprehensive report on the state of the North Sea that
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also covers the time period of this study34.
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The radionuclide signal from the reprocessing facilities SF and LH enters the North Sea mainly
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via the Scottish Coastal Current and the English Channel Current, respectively (SCC, ECC in
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Figure 1). Model based reconstructions of the 236U/238U ratios in those currents together with the
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global fallout signal (as extracted from ref. 5) are presented in Figure 2.
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Sample location, preparation, and measurement of 236U
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During the North Sea summer survey carried out with R/V Pelagia (PE232) by the Federal
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Maritime and Hydrographic Agency (BSH) in August 2010 several large volume water samples
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were collected from 80 different stations (70l for 90Sr, 100l for transuranic actinides, 100-150l
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for 137Cs)35. Since it was initially not planned to analyze 236U/238U no separate water samples
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were collected for U-analysis. However, U extraction could be performed on fifty 8-24l samples
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of previously untreated surface water that was left over from 90Sr sample preparation (sample
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locations are shown in Figure 1). To investigate possible contributions of 236U and 238U
Sr, and several isotopes of the transuranic elements Pu, Am, and Cm). The results of the survey
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transported by the Elbe River two samples were collected in upstream direction (Stations “Stade”
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and “Medem” in Table S2).
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At BSH Hamburg, the unfiltered seawater samples were acidified with a few ml of conc. HNO3,
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about 10 pg of a 233U spike (IRMM058) and about 500 mg of a (previously U-cleaned) iron
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nitrate solution was added. After equilibration (typically over night) NH4OH was added and U
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was co-precipitated together with the Fe-hydroxide. U-separation and AMS target preparation
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was carried out at ETH Zurich using previously described methods.14, 17
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(AMS) at ETH Zurich using previously described methods.13 Measured 233U/238U and 236U/233U
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ratios were normalized to the ETH Zurich in house standard ZUTRI36 and blank corrected. 236U
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and 238U concentrations were calculated from the known amount of 233U spike. 236U and 238U
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concentrations, the 236U/238U ratio and the associated combined uncertainties are reported in
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Table S2. The typical 1σ uncertainty of the 236U and 238U data is between 2% and 5%. Full-
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process blanks prepared from spiked MilliQ water contained less than 1 fg (
