70-year anthropogenic uranium imprints of nuclear activities in Baltic Sea sediments

doi:10.21203/rs.3.rs-383698/v1

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70-year anthropogenic uranium imprints of nuclear activities in Baltic Sea sediments

https://doi.org/10.21203/rs.3.rs-383698/v1

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Strongly stratified water structure and densely populated catchment make the Baltic Sea one of the most polluted seas. Understanding its circulation pattern and time scale is essential to predict the dynamics of hypoxia, eutrophication, and pollutants. Anthropogenic ²³⁶U and ²³³U have been demonstrated as excellent transient tracers in oceanic studies, but unclear input history and inadequate long-term monitoring records limit their application in the Baltic Sea. From two dated Baltic sediment cores, we obtained high-resolution records of anthropogenic uranium imprints originated from three major human nuclear activities throughout the Atomic Era. Using the novel ²³³U/²³⁶U signature, we distinguished and quantified ²³⁶U inputs from global fallout (43.3%-50.5%), Chernobyl accident (< 0.9%), and discharges of civil nuclear industry (48.6%-56.7%) to the Baltic Sea. We estimated the total release of ²³³U (7-15kg) from the atmospheric nuclear weapons testing, and pinpointed ²³³U peak signal in the mid-to-late 1950s as a potential time marker for the onset of the Anthropocene Epoch. This work also provides fundamental ²³⁶U data for Chernobyl accident and early discharges from civil nuclear facilities, prompting worldwide ²³³U-²³⁶U tracer studies. We anticipate our data to be a broader application in model-observation interdisciplinary research on water circulation and pollutant dynamics in the Baltic Sea.

Geochemistry

Environmental Chemistry

U-236

U-233

Baltic Sea

sediment

global fallout

Chernobyl accident

nuclear reprocessing plant

Anthropocene

This work supplements fundamental ²³⁶U and ²³³U source-term data for major human nuclear activities, facilitating regional and worldwide ²³³U-²³⁶U environmental tracer applications.

As the second-largest brackish water body worldwide, the Baltic Sea is a shallow enclosed marginal sea and characterized by a permanent halocline restricting the ventilation of bottom waters.^1,2 The renewal of deep waters mainly relies on the O₂-rich saline inflows from the North Sea.^1,3 Long residence time of saline water (~30 years),⁴ dramatic drop in the frequency of Baltic inflows since the 1980s, and the persistent eutrophication trigger the world's largest hypoxic area and make the Baltic Sea extremely sensitive to anthropogenic pollutants from its highly populated catchment.^5–7 Hence, investigation of the transit time and transport pathways of the saline waters in different sub-basins of the Baltic Sea is essential to predict the dynamics of hypoxia, eutrophication, and pollutants.

Our previous studies^8–11 indicate that the Baltic Sea receives radioactive discharges (e.g., ⁹⁹Tc, ¹³⁷Cs, ¹²⁹I, and ²³⁶U) from two European nuclear reprocessing plants at Sellafield and La Hague via Baltic inflows. Among these radionuclides, ²³⁶U (t_½ = 23.4 Myr) is a powerful oceanic tracer owing to its long half-life, high solubility, and long residence time in the ocean.^12,14,15 Throughout the Atomic Era, significant amounts of ²³⁶U have been released to the environment by human nuclear activities, such as global fallout of atmospheric nuclear weapons testing (900 - 2100 kg)^12,16–18 and deliberate discharges from Sellafield and La Hague (more than 263 kg)¹⁹. Coupled with ¹²⁹I, the point-like releases of reprocessing-derived ²³⁶U have been utilized to quantify timescales of regional ocean circulation, e.g. the Atlantic waters in the Arctic Ocean.^20–23

Reprocessing-derived ²³⁶U is a promising proxy to investigate the Baltic inflows, but it suffers from methodological difficulties in distinguishing it from ubiquitous global-fallout-derived ²³⁶U. Recently, a ²³³U-²³⁶U paired tracer system was proposed to identify emission sources of ²³⁶U.^8,14,24 This is based on the fact that ²³³U (t_½ = 0.16 Myr), another long-lived U isotope, was mostly released from nuclear weapons testing, while almost no ²³³U is produced in commercial nuclear power reactors or reprocessing plants.¹⁴ The representative ²³³U/²³⁶U atomic ratio of global-fallout signal was suggested to be (1.40 ± 0.15) × 10^-2.¹⁴ Discharge data of La Hague and reactor-modeling results indicate that 10^-8 - 10^-6 is the representative level of ²³³U/²³⁶U atomic ratio for the reactor-related releases.^14,25,26 Thereby, ²³³U/²³⁶U can be used as a robust fingerprint to quantify the global-fallout-derived and reactor-derived ²³⁶U. However, there is still a knowledge gap in the release history and global inventory of ²³³U, which is critical for its tracer application.

Moreover, the ²³⁶U budget is still an open question in the Baltic Sea due to the unclear input function and limited observation data. Many studies have reconstructed the temporal distribution of ²³⁶U in marine waters from biogenic materials (like corals and shells) and further estimated the ²³⁶U inputs of global fallout and nuclear reprocessing plants into the ocean (Table S1).^{14,18,19,27,28} Unfortunately, such efforts do not apply to the Baltic Sea because of the absence of corals and the lack of routine sampling of shells. Hence, anoxic sediments from the deep basins of the Baltic Sea become an alternative promise considering the advantages of single sampling, flexible time scale, and high sample accessibility.

In this work, we resolved 70-year temporal evolution of ²³⁶U and ²³³U in the Baltic Sea from two dated sediment cores collected in the Gotland Basin, the largest Baltic sub-basin, and Landsort Deep, the deepest Baltic sub-basins, respectively (Figure 1). The historical ²³⁶U inputs from various sources were quantified with an endmember mixing model. The ²³⁶U budget in the Baltic Sea and the global release of ²³³U from atmospheric nuclear weapons testing were further estimated based on the new ²³⁶U-²³³U dataset.

A sediment core 7-MUC4 (L = 30 cm, Ф = 10 cm) was collected at site GB7-4 (57°16.98'N, 20°07.23'E, 241 m water depth) in the central Gotland Basin during a cruise with RV Elisabeth Mann Borgese in December 2018 (EMB201). The core covers a period from the 1940s until 2018. The composite core 11-10MUC2/36-MUC3 is compiled from two sediment cores collected at the same site LD1 (58°38.36'N, 18°16.04'E, 437 m water depth) in the Landsort Deep. Core 11-10MUC2 (L = 49 cm, Ф = 10 cm), collected during the EMB201 cruise, covers the period from ~1957 until sampling in 2018. To extend this sediment record to ~1948, six additional samples from core 36-MUC3 (L = 37.5 cm, Ф = 10 cm), previously obtained during a cruise with the RV Meteor at site LD1 in November 2011 (M86/1a), were added resulting in a composite profile. All sediment cores were collected by a multicorer device, sliced in 0.5-cm or 1-cm resolution on-board, and then freeze-dried and homogenized by an agate ball mill for geochemical and radiochemical analyses.

To investigate the behaviors of natural U in the Gotland Basin and Landsort Deep, water column samples collected by Pump-CTD and sediment porewater samples extracted by rhizons were analyzed for U concentrations.^29,30 For dissolved U, water column samples after filtration with 0.45 µm syringe filters (SFCA) and porewater samples were immediately acidified to 2 vol% of HNO₃. For particulate U, 2 L of the water sample was filtered through 0.4 µm polycarbonate filters (Millipore) and the residue was rinsed with 50 mL of ultrapure water to remove remaining salt. To determine total sulfide, porewater and filtrated seawater samples were added to 2 mL reaction tubes containing 20 µL of 20 vol% Zn acetate.

Further details of the analytical methods adopted in this work are provided in the Supporting Information. After total digestion with an acid mixture (HNO₃ : HClO₄ : HF = 1 : 2 : 2, v/v), the sedimentary Al, Mn, U, and stable Pb isotope ratios in Baltic Sea sediment cores were determined by inductively coupled plasma mass spectrometry (ICP-MS, iCAP Q, Thermo Fisher Scientific) or inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 7400 Duo, Thermo Fisher Scientific). The Hg contents were measured by a direct mercury analyzer (DMA-80; Milestone). The total organic carbon (TOC) contents were determined by the differences between total carbon and total inorganic carbon contents. All element and TOC contents are salt-corrected because of salt contents of up to 30 wt% in some freeze-dried samples.

Besides sedimentary U contents, the concentrations of dissolved and particulate U, O₂, and sulfide in the water column, as well as the concentrations of dissolved U and sulfide in the porewaters of the sediments, were analyzed. O₂ concentrations in the water column were directly determined using an SBE 911+ CTD (Sea-Bird) equipped with an SBE 43 oxygen sensor. Total sulfide in porewater and water column samples was determined by the Cline method.³¹ The concentrations of dissolved U in water column and porewater samples were measured by ICP-MS coupled to a seaFAST-pico system (Elemental Scientific) as previously applied for Mo and W.³⁰ Except for decomposing of polycarbonate filter by HClO₄ before adding HF during total acid digestion, the U concentrations of the suspended particulate matter from the water column at site LD 1 were analyzed in the same way as the sediment samples.

An optimized radiochemical procedure modified from our previous work was utilized for the determination of ²³⁶U/²³⁸U, ²³³U/²³⁸U, and ²³³U/²³⁶U atomic ratios in sediments by accelerator mass spectrometry (AMS), including acid digestion, co-precipitation, and extraction chromatography.^32,33 As strong acids are able to leach ²³⁶U from soil and sediments quantitatively,^16,34 acid digestion with Aqua regia (HCl: HNO₃=3:1, v/v) rather than total dissolution was adopted in our procedure to deal with gram amounts of sediment samples. About 93% and 84% of ²³⁸U in the Gotland Basin and Landsort Deep cores (obtained by total dissolution using HNO₃-HClO₄-HF) were leached by Aqua regia digestion, respectively. A leaching experiment on core 7-MUC4 indicates that a single Aqua regia digestion is enough to extract >99% of leachable ²³⁸U and ²³⁶U from the sediments (Table S2). To avoid the introduction of extra ²³⁶U and ²³³U from yield tracer, ²³⁸U in the Aqua regia leachate was used as an intrinsic chemical yield tracer, and our radiochemical procedure provided satisfactory chemical yields (80 - 100%) for ²³⁶U and ²³³U. Nine procedure blanks were prepared following the same analytical protocol along with the sediment samples. Special measures presented in the Supporting Information were implemented for the background control of ²³⁶U and ²³³U.³³ The procedure blanks contained less than 5.3 ng of ²³⁸U, 1.8 × 10⁶ atoms of ²³⁶U, and 9.6 × 10⁴ atoms of ²³³U, which is negligible compared with minimum amounts of ²³⁸U, ²³⁶U, and ²³³U in the sediment samples except for the lowermost layer. To locate the layer recording the Chernobyl accident signal, ¹³⁷Cs and ²⁴¹Am activities in the sediment were directly measured by gamma spectrometry.

The gamma measurements of ¹³⁷Cs and ²⁴¹Am and the radiochemical separation procedure for ²³⁶U and ²³³U were performed at the Department of Environmental Engineering in the Technical University of Denmark (DTU-ENV); the AMS measurements of ²³⁶U and ²³³U were carried out at the Vienna Environmental Research Accelerator (VERA) facility in the University of Vienna; and all the other analyses were completed at Leibniz Institute for Baltic Sea Research Warnemünde (IOW).

Dating of the Sediment Cores.

As previous studies in the Gotland Basin and Landsort Deep revealed severe limitations in ²¹⁰Pb dating of sediment cores subjected by pronounced redox dynamics,^35–37 the age models for the obtained sediment cores in this work were developed by an event-stratigraphic approach (see Details for the Analytical Methods in Supporting Information and Figure S1-S7). Specific time markers were identified along the cores through geochemical and radiological analyses, including: Mn enrichments following the Major Baltic Inflows (1978, 1994, 2003, and 2014), Chernobyl-accident-derived ¹³⁷Cs and ²⁴¹Am peaks (1986), initial and maximum global-fallout-derived ²⁴¹Am (1954 and 1963), maximum Pb pollution identified by stable ^206/207Pb ratios (1970 - 1978), and onset of modern Hg pollution reflected by Hg/Al ratios (1950). Between these time markers, sedimentation rates vary between 0.25 - 1.44 cm/yr, providing nearly annual or biennial sample resolutions with conventional core slicing. The obtained age-depth relations enable to reconstruct the sedimentary chronology since the 1940s. All geochemical and radiochemical analysis data involved in this work can be found in Table S3-S6.

Record of Uranium Contents in the Sediment Cores.

The sedimentary U levels from the Gotland Basin and Landsort Deep (ave. 31.7 and 16.7 mg/kg, respectively) are much higher than other type-localities of sulfidic basins such as the Black Sea and Cariaco Basin,^15,38,39 suggesting that the Gotland Basin and Landsort Deep act as effective U sinks during the past ~70 years. Distinct U enrichments appear in both cores especially in the organic-rich sapropel intervals deposited during sulfidic water-column periods (Figure 2 and Figure S8), because soluble U(VI) can be reduced to insoluble U(IV) in reductive condition (e.g., if H₂S is present)⁴⁰ and the formed U(IV) is easily adsorbed on suspended particles and deposited on the seabed. Water-column and porewater profiles obtained in the Gotland Basin and Landsort Deep in 2018 clearly document two scavenging processes of U (see Supplementary Discussions in Supporting Information): i) the redox-driven removal of dissolved U from the sulfidic water column to the sediment as particulate forms;⁴¹ and ii) the diagenesis-driven removal of dissolved U in the sulfidic porewater environment mediated through reductants, facilitating the diffusion of dissolved U from bottom water to the porewater.^42–44 On the other hand, a less effective U burial is observed in the Mn carbonate-rich sections of both cores, which were formed during the hypoxic (O₂< 2 mL/L ≃ 90 µM)^2,7,45 water-column periods.³⁶ This difference is most likely related to the limited redox-driven scavenging in the non-sulfidic waters. However, the diagenesis-driven removal should be maintained below the sediment-water interface by the pronounced reducing or even sulfidic conditions, as Al-normalized U concentrations ((0.70 - 19.6) × 10^-4) still strongly exceed the lithogenic background as represented by the upper continental crust (0.33 × 10^-4)⁴⁶.

The aforementioned results indicate that most of natural sedimentary U originates from the dissolved U in the water column. In principle, this should also be valid for soluble ²³³U and ²³⁶U, such as the dissolved fraction of global-fallout-derived ²³³U and ²³⁶U and liquid discharges of reprocessing-derived ²³⁶U. Hence, the anoxic sediments could record the historical compositions of U isotopes in the water column, mostly deep waters where U is efficiently scavenged. It is noteworthy that the sediments may also directly receive atmospheric depositions of particulate ²³³U and ²³⁶U, such as the global fallout from nuclear weapons testing and the regional fallout from the Chernobyl accident. Permanently hypoxic and periodically sulfidic water-column conditions prevent bioturbation and facilitate the preservation of U in its immobilized tetravalent state,⁴² making these ²³³U and ²³⁶U inputs ideal time markers for dating purpose.

Deposition History and Total Release of ²³³U from Global Fallout.

Most measured ²³³U/²³⁸U and ²³⁶U/²³⁸U atomic ratios in both sediment cores are much higher than their natural background (10⁻¹⁴ - 10⁻¹¹ and 10⁻¹⁴ - 10⁻¹⁰, respectively),^14,47 suggesting anthropogenic ²³³U and ²³⁶U have dominated the Baltic Sea since the mid-1940s. Both cores exhibit very similar temporal distribution patterns for ²³³U/²³⁸U atomic ratios, consisting of a notable peak in the mid-to-late 1950s and a following gradual decrease until 2018 (Figure 3). Furthermore, we compared our two sedimentary records with the only two reported ²³³U records resolved from a peat core (Black forest, Germany) and a coral core (Kume Island, Japan)¹⁴ (Figure 3). The peat core only received global fallout signal, and the coral core was affected by both global fallout and close-in fallout from Pacific Proving Ground (PPG) (Table S1).¹⁴ With dating uncertainties, elevated ²³³U/²³⁸U atomic ratios appear in all available records in the mid-to-late 1950s (blue bar in Figure 2), which indicates a remarkable global atmospheric deposition of ²³³U.

A previous study proposed two potential sources for global-fallout-derived ²³³U.¹⁴ The first one is the nuclear weapon tests directly using ²³³U as fissile material and releasing unfissioned ²³³U into the atmosphere.¹⁴ To the best of our knowledge, one experimental device using a composite pit of ²³⁹Pu and ²³³U was donated by the United States in 1955 at the Nevada Test Site (Operation Teapot "MET" test, 22kt).⁴⁸ The former Soviet Union's two-stage hydrogen bomb tested in 1955 at the Semipalatinsk Test Site ("RDS-37", 3Mt) also utilized ²³⁵U and ²³³U as fuel in the fissile core.⁴⁹ Considering a much higher yield, the "RDS-37" test likely ejected more ²³³U debris into the stratosphere and thereby resulted in a global fallout.⁵⁰ The second source is the thermonuclear weapon tests using enriched U (so-called oralloy) as tamper material and producing ²³³U via the fast neutron reaction ²³⁵U(n,3n)²³³U.¹⁴ According to the disclosed information, the first thermonuclear weapon test with an oralloy tamper is the Operation Castle "Nectar" test conducted by the United States at the PPG in 1954.¹⁴ In addition, several thermonuclear weapon tests of the Operation Hardtack I with Mt yields implemented at the PPG in 1958 were also suspected to use oralloy tampers, as indicated by elevated ²³⁶U/²³⁹Pu atomic ratios found in other corals at Enewetak Atoll.¹⁴

The universe ²³³U signal in the three locations across Europe to Asia is likely attributed to the detonation of two ²³³U devices in 1955, especially the former Soviet Union's "RDS-37" test. The Baltic Sea sediment cores have much higher maximum ²³³U/²³⁶U atomic ratios ((6.42 -15.3) × 10^-2) than the peat and coral cores, suggesting that the Baltic Sea is closer to the test site that is most likely the Semipalatinsk Test Site for "RDS-37" test. Unfortunately, the resolutions in the peat core and the sediment cores are rather poor in the late 1950s and early 1960s to provide a further evidence on whether the United States' Operation Hardtack I tests released significant amounts of ²³³U or not.

Based on the areal cumulative inventories of ²³³U in sediment and peat cores, the representative areal inventory of ²³³U from the direct deposition of global fallout should be at the level of 10¹⁰ - 10¹¹atom/m² in the Northern European region (see Supplementary Discussions in Supporting Information). The total release of ²³³U from the nuclear weapons testing is roughly estimated to be 7 - 15 kg, assuming an areal ²³³U inventory of (5 - 12) × 10¹⁰ atom/m² in the Northern Hemisphere and a one-third level in the Southern Hemisphere according to the investigations on the global-fallout-derived ⁹⁰Sr.⁵⁰ This estimation is at the same order of the calculation results (12 - 29 kg) based on the estimated global-fallout-derived ²³⁶U inventory (900 - 2100 kg) and representative ²³³U/²³⁶U atomic ratios for global fallout ((1.40 ± 0.15) × 10^-2).

Quantification of ²³⁶U from Different Nuclear Activities.

The Gotland Basin and Landsort Deep sediments have similar areal cumulative inventories of ²³⁶U, which are estimated to be (3.53 ± 0.19) × 10¹³ atom/m² and (3.16 ± 0.13) × 10¹³ atom/m², respectively. Except for a pronounced peak in ~1986 only observed in the Landsort Deep sediment core, the depth profiles of ²³⁶U/²³⁸U atomic ratios in both sediment cores are also very consistent, including an increase before the late 1970s, a relatively constant level during the 1980s, and a gentle decline after the early 1990s (Figure 3).

As the reducing bottom water conditions are favorable for the sedimentation and preservation of U isotopes, a total inventory of 607 - 679 g²³⁶U can be roughly estimated in the Baltic sediments assuming a hypoxic area of ~ 49,000 km² (avg. during 1961-2000)² and similar areal cumulative ²³⁶U inventory in the hypoxic area to the Gotland Basin and Landsort Deep. However, the deposition of ²³⁶U in the oxygenated area of the Baltic Sea was not considered in this estimation due to lack of observation data.

To clarify the sources of ²³⁶U to the Baltic Sea, we compared the temporal distribution of ²³³U/²³⁶U atomic ratios in the sediment, peat, and core cores (Figure 3).¹⁴ It should be noted that the ²³³U/²³⁶U atomic ratios in various records have different enlightenments (see Supplementary Discussions in Supporting Information). Before the mid-1960s, an earlier intensive deposition of ²³³U than ²³⁶U leads to higher ²³³U/²³⁶U atomic ratios than the modern integrated global fallout signal ((1.40 ± 0.15) × 10^-2) in the Baltic sediment cores, which is consistent with the peat core record. In the mid-to-late 1960s, ²³³U/²³⁶U atomic ratios approach to 1.4 × 10^-2 in all settings (green bar in Figure 3). The record of the peat core indicates that more than 90% of ²³³U and 80% of ²³⁶U have deposited by the year of 1970 due to the partial nuclear test ban treaty in 1963. Therefore, the main source term of ²³⁶U in the Baltic sediments before the late 1960s should be identical to that in the peat core, i.e., global fallouts. Since 1970, ²³³U/²³⁶U atomic ratios in the peat and coral cores remain constant at 1.4 × 10^-2, suggesting that the ratio of ²³³U and ²³⁶U on the earth's surface environment reach equilibrium. While,²³³U/²³⁶U atomic ratios start to fall below 1.4 × 10^-2 in the Baltic sediments, reflecting the appearance of reactor-derived ²³⁶U input(s) (e.g., from reprocessing plants and other local facilities) to the Baltic Sea. The remarkable drop of ²³³U/²³⁶U atomic ratios in ~1986 in the Landsort Deep composite core is attributed to the reactor-derived ²³⁶U from the Chernobyl accident (26^th April, 1986), which is supported by the pronounced ¹³⁷Cs signal in the same sediment interval (Figure 3).

Using ²³³U/²³⁶U as signature, a two-endmember mixing model is adopted to calculate the proportions of global-fallout-derived and reactor-derived²³⁶U(/²³⁸U) in two sediment cores (see Calculations in SI Appendix), and the results are shown in Figure 4. The global-fallout-derived and reactor-derived ²³⁶U account for (43.3 ± 2.8)% and (56.7 ± 8.0)% of total ²³⁶U in the Gotland Basin sediment core, respectively. While the global-fallout-derived, reactor-derived (excluding Chernobyl accident), and Chernobyl-derived ²³⁶U contribute (50.5 ± 2.1)%, (48.6 ± 5.3)%, and (0.9 ± 0.2)% of total ²³⁶U to the Landsort Deep composite core, respectively. In general, global fallout and discharges of civil nuclear industry (i.e. reprocessing plants and local nuclear facilities) have comparable ²³⁶U inputs to the Baltic Sea, whereas contribution from the Chernobyl accident is negligible.

Input History of ²³⁶U to the Baltic Sea.

The potential ²³⁶U inputs to the Baltic Sea include: (i) global-fallout-related inputs, comprising the direct atmospheric deposition, and indirect inputs from the North Sea and North Atlantic via Baltic inflows and from the catchment via river run-off; and (ii) reactor-related inputs, including the discharges from nuclear reprocessing plants followed by transport to the Baltic through oceanic currents, releases from local nuclear facilities, and regional fallout from the Chernobyl accidents (Figure 1).

i) Global fallout inputs. Global-fallout-derived ²³⁶U/²³⁸U atomic ratios in Baltic sediment cores represent an integration of directly deposited particulate fallout and scavenged soluble global-fallout-derived ²³⁶U from the water column. As more than 95% of ²³⁶U fallout have deposited by 1980 according to the record of the peat core, the ²³⁶U in the Baltic sediments mainly originates from the scavenging processes since 1980 and sedimentary global-fallout-derived ²³⁶U/²³⁸U atomic ratios should mainly reveal the historical levels in the deep waters (where U is scavenged). The decreasing trends in the Baltic sediments since 1980 is consistent with the reported records of coral cores from the Caribbean Sea, Japan Sea, and the Northwest Pacific Ocean (Figure S9).^18,27,28 We calculated the effective half-life of global-fallout-derived ²³⁶U based on the observation data (see Details for the Calculations in Supporting Information), which is estimated to be 29 - 54 years in these seas with different hydrographic and physio-chemical conditions. The effective half-life of ²³⁶U is much shorter than its radioactive half-life (23.4 Myr), and this should be considered in its future tracer studies.

The resolved global-fallout-derived ²³⁶U/²³⁸U atomic ratios in the Baltic Sea are about one order of magnitude higher than other aforementioned seas (Figure S9). Assuming a ²³⁸U concentration of 1.2 μg/kg in the bottom waters of the Gotland Basin and Landsort Deep according to their bottom average salinity,⁵¹ the global-fallout-derived ²³⁶U concentration is estimated to be ~1.8 × 10⁷atoms/kg in the 2010s. This is comparable with the observation results of the surface water in the central Baltic Sea in 2015 ((1.6 - 1.8) × 10⁷atoms/kg)⁸, and about two times higher than the representative level in the open sea as well as the simulated level in the North Sea (~8 × 10⁶atoms/kg)²³. A potential reason is that the long residence time of water in the Baltic Sea (~30 years)⁴ is favor to the preservation of "aged" waters with higher global-fallout-derived ²³⁶U concentrations e.g. from the North Sea and/or river runoff.

ii) Chernobyl accident input. The notable ²³⁶U/²³⁸U peak in Landsort Deep composite core with no significant change in the ²³³U/²³⁸U atomic ratios indicates a pure ²³⁶U input in ~1986 (Figure 3). In contrast, no ²³⁶U/²³⁸U peak is observed in the Gotland Basin sediment core in ~1986, even though clear Chernobyl ¹³⁷Cs and ²⁴¹Am signals show up (Figure 3). One possible explanation is that the Landsort Deep is closer to the regions that was highly contaminated by the Chernobyl fallout, including the Bothnian Sea and the central Sweden.⁵² Another reason is that Chernobyl-derived ²³⁶U might deposit to the Baltic sediments as "hot particles". Different from the ¹³⁷Cs activities that decline gradually after 1986, the ²³⁶U/²³⁸U atomic ratios in the Landsort Deep composite core quickly decrease to the pre-Chernobyl level, as also seen in the ²⁴¹Am peak of the Gotland Basin sediment core. Considering that Am is highly particle-reactive and less mobile than Cs, the input of the Chernobyl-derived ²³⁶U was likely a pulse atmospheric deposition as particulate-associated forms as well.

iii) Civil nuclear industry inputs. Excluding the Chernobyl-derived ²³⁶U input, the temporal distribution of reactor-derived²³⁶U/²³⁸U atomic ratios in the Baltic Sea sediment cores mainly reveal the input history of reactor-derived ²³⁶U from civil nuclear industry (Figure 4), including the liquid discharges from the two major European nuclear reprocessing plants and local nuclear facilities ⁸.

As dominating regional sources, the reprocessing plants at Sellafield and La Hague were estimated to release >237 kg and ~25 kg of ²³⁶U,¹⁹ and the reprocessing-derived ²³⁶U are transported to the North Sea via the Scottish Coastal Current and English Channel Current.⁵³ As previous model calculations indicated that 1% of the reprocessing discharges can reach the Baltic Sea,⁵² the total inputs of reprocessing-derived ²³⁶U to the Baltic Sea can be roughly estimated as 2.6 kg. The decreasing of reactor-derived ²³⁶U/²³⁸U atomic ratios in the Baltic sediments since the mid-1980s is also consistent with the declined discharges of reprocessing-derived ²³⁶U discharges in the recent decades.¹⁹ For comparison, the total deposition of global-fallout-derived ²³⁶U on the Baltic Sea is estimated to be 1.5 kg (ignoring the indirect inputs).⁸ The above rough estimations indicate that the inputs of reprocessing-derived and global-fallout-derived ²³⁶U seem to be at the same magnitude, which agree well with our sediment observation data.

Except for nuclear reprocessing plants, our recent research estimated that additional 200 ± 47 g of reactor-derived ²³⁶U was released to the Baltic Sea by an unknown source.⁸ Excluding the Chernobyl accident, the most possible candidates are the local nuclear facilities, including nuclear power plants in Sweden (Barsebäck, Ringhals, Oskarshamn, and Forsmark), Finland (Olkiluoto), Russia (Leningrad and Kaliningrad) and Germany (Greifswald), some research facilities (Risø and Studsvik) and the nuclear fuel fabrication facility (Westinghouse) in Sweden. The only documented ²³⁶U sources are 0.44 g of aquatic discharge and 0.016 g of airborne emission from Westinghouse during 2002-2017,²⁶ which are negligible compared with the above-mentioned additional reactor-derived ²³⁶U input. No significantly high levels of ²³⁶U have been reported in the surrounding environment of other nuclear facilities, except for the nuclear research company Studsvik, which has been in operation since the 1950s and reported to dump 64 tons of radioactive waste into the coastal area nearby during 1959-1961.⁵⁴ However, no detectable reactor-derived ²³⁶U signal is observed in the Baltic Sea sediments cores around 1960. Further investigation is still needed to uncover the source of additional reactor-derived ²³⁶U. We should be aware that these additional reactor-derived ²³⁶U only contribute a small fraction to the ²³⁶U budget of the Baltic Sea.⁸ For instance, even if 20% of the unknown ²³⁶U source (~ 40 g) is scavenged to the sediments, they are still minor compared with the present total sedimentary reactor-derived ²³⁶U (295 - 385 g).

Supporting Information.

Details for the Analytical Methods, including: dating for sediment cores; determination of Al, Hg, Mn, U contents, ^206/207Pb ratios and TOC in the sediments; determination of dissolved U, particulate U, and sulfide in seawater and porewater; determination of ²³⁶U/²³⁸U and ²³³U/²³⁸U and ²³³U/²³⁶U atomic ratios in the sediments; background control measures for ²³⁶U and ²³³U analyses; and determination of ¹³⁷Cs and ²⁴¹Am activities in the sediment. Details for the calculations, including: quantification of anthropogenic ²³⁶U from different sources and calculation of effective half-life of global-fallout-derived ²³⁶U. Supplementary discussions for scavenging processes of U, areal inventory of ²³³U from the direct deposition of global fallout, and interpretations behind the records of peat, coral, and sediment cores. Supplementary figures, including: the depth profiles in the sediment core 7-MUC4; the age-depth relation for the core 7-MUC4; instrumental time series of O₂/H₂S in the Landsort Deep and the depth profiles in the sediment core 36-MUC3; the combined age-depth relation for the core 36-MUC3; transfer of the age model from the core 36-MUC3 to the core 11-10MUC2; the resulting age-depth relation for the composite core 11-10MUC2/36-MUC3; comparison between instrumental time series of O₂/H₂S in the Landsort Deep and the depth profile of Mn contents in the composite core 11-10MUC2/36-MUC3; comparison of water-column time series and the sedimentary record, as well as water column and pore water profiles from the Landsort Deep; and contemporary comparison of global-fallout-derived (GF-derived) ²³⁶U/²³⁸U atomic ratios. Supplementary tables, including: overview of the resolved records of ²³⁶U and ²³³U in the published and present studies; results for a leaching experiment of the Baltic Sea sediments; measurement results of U, Mn, Al, TOC contents, and mass ratio of U/Al in the core 7-MUC4 and the core 11-10MUC2/36-MUC3; and measurement results of ²³⁶U/²³⁸U, ²³³U/²³⁸U, and ²³³U/²³⁶U atomic ratios, ¹³⁷Cs activities, ²⁴¹Am activities, and calculation results of global-fallout-derived and reactor-derived ²³⁶U/²³⁸U atomic ratios in the core 7-MUC4 and the core 11-10MUC2/36-MUC3.

Author Contributions

M.L. performed sample analysis and wrote the manuscript; J.Q. designed and coordinated the research; X.H. assisted with supervision. O.D. provided samples; O.D., P.S., K.H., R.G., and L.Z. assisted with sample analysis; All authors contributed to reviewing and editing.

Notes

The authors declare no competing interests.

Acknowledgment

We wish to thank all colleagues in DTU-ENV for the support on this research, the crews and captains of RVs Elisabeth Mann Borgese and Meteor for technical support during field work, and Anne Köhler for lab assistance at IOW. The work at IOW was funded by the Leibniz Association, through grant SAW-2017-IOW-2 649 (BaltRap).

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70-year anthropogenic uranium imprints of nuclear activities in Baltic Sea sediments

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