# Temporal evolution and origin of radionuclide fallout and contaminants recorded in sediment from the Kerguelen Archipelago fjord system

Yangjunjie Xu-Yang, Olivier Evrard, Anthony Foucher, Pierre-Alexis

Chaboche, Irène Lefèvre, Nirina Taraconat, Judith Kobler, Christine Alewell, Silke Merchel, Peter Steier, et al.

► To cite this version:

Yangjunjie Xu-Yang, Olivier Evrard, Anthony Foucher, Pierre-Alexis Chaboche, Irène Lefèvre, et al. Temporal evolution and origin of radionuclide fallout and contaminants recorded in sediment from the Kerguelen Archipelago fjord system. Environmental Science and Technology, 2026, 10.1021/acs.est.5c13946. cea-05522268

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Submitted on 22 Feb 2026

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Temporal evolution and origin of radionuclide fallout and
contaminants recorded in sediment from the Kerguelen
archipelago fjord system

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Yangjunjie Xu-Yang¹*, Olivier Evrard¹, Anthony Foucher¹, Pierre-Alexis Chaboche¹, Irène Lefèvre¹, Nirina

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Taraconat², Judith Kobler³, Christine Alewell³, Silke Merchel⁴, Peter Steier⁴, Elisabeth Michel¹, Thomas

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Thiebault⁵, Emmanuel Chapron⁶, Léo Chassiot⁷, Sophie Ayrault¹

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¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE), CEA, CNRS, UVSQ, Université

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Paris-Saclay, Gif-sur-Yvette, France

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²Université Paris Cité, Institut de physique du globe de Paris, CNRS ; 75005, Paris, France

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³ Environmental Geosciences, Department of Environmental Science, University of Basel, Bernoullistrasse

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30, 4056 Basel, Switzerland

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⁴University of Vienna—Faculty of Physics, Isotope Physics, 1090 Vienna, Austria

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⁵EPHE, PSL Univ., Sorbonne Univ., CNRS, UMR 7619 METIS, 75005, Paris, France

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⁶Université Toulouse Jean-Jaurès, CNRS, GEODE, Toulouse, France

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⁷Département des Sciences de l'Environnement, Université du Québec à Trois-Rivières, Trois-Rivières,

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QC, Canada, G8Z 4M3

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*Corresponding author: Yangjunjie Xu-Yang, yangjunjie.xu@lsce.ipsl.fr

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2 Abstract (150-200 words)

3 Deposition of heavy metals and persistent organic pollutants (POPs) to fjord systems during ice melting was
4 investigated with a sediment core collected in Table Fjord from Kerguelen Island (49° 33.8 S-69° 13.9 E)
5 situated in the Southern Indian Ocean. Multiple radionuclides ($^{210}$Pb$_{ex}$, $^{137}$Cs, $^{240}$Pu/$^{239}$Pu) were used to
6 establish an accurate age-depth model and show the occurrence of French nuclear weapon test fallout in this
7 remote region for the first time. Environmental changes related to the retreat of the Cook Ice Cap since the
8 1960s were found to be one of the major factors dominating the dynamics of anthropogenic lead deposition
9 flux in the fjord through the release of long-range transported legacy anthropogenic lead. The released legacy
10 anthropogenic lead was likely transported across the proglacial Ampere Lake by a hypopycnal plume to the
11 fjord. Backward trajectories and lead stable isotopic signatures suggest the southern part of South Africa as
12 a major source of anthropogenic lead transported to Kerguelen Archipelago. In contrast, contamination by
13 arsenic, molybdenum, antimony and POPs was found to be more recent (since 2001). Fractionation of rare
14 earth elements was observed in the sediment due to the formation of proglacial Ampere Lake, which acts as
15 a sediment trap.

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18 Keywords (5-8):
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$^{137}$Cs, $^{240}$Pu/$^{239}$Pu, legacy contaminants, trace metals, persistent organic pollutants, Southern Ocean, fjord

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sediment core, climate change

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Synopsis (30 words):

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This study analyzes multiple radionuclides from French and global nuclear weapon tests to reconstruct the
history of recent and legacy contaminants in Table Fjord of Kerguelen Archipelago.

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# 1 Introduction

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The Southern Ocean (between 40° and 65°S) plays an important role in the autoregulation of the Earth
climate and is the major sink of CO₂ on Earth ¹,². It is known to be the largest high-nitrate low-chlorophyll
oceanic area ³. In this zone, iron (Fe) and other transition elements limit plankton growth and development
and thus, the associated CO₂ absorption ⁴⁻⁸. The Southern Ocean has undergone substantial climate changes
over the last several decades resulting in increasing stress for phytoplankton development ⁹,¹⁰. Anthropogenic
disturbance via trace metal inputs to this region also influences the development of phytoplankton, inducing
changes in global marine net primary productivity ¹¹,¹² and having a substantial influence on the global CO₂
sink ¹³,¹⁴.

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Fjord and adjacent coastal ocean ecosystems are hotspots of biogeochemical activity that accumulate and store organic carbon and nutrients, and represent significant organic carbon sinks ¹⁵,¹⁶. Fjords are particularly sensitive to climate change ¹⁵,¹⁷. Glaciers are exceptionally effective at eroding landscapes¹⁸,¹⁹ and generate highly reactive and quickly weathered sediments²⁰,²¹. This sediment therefore provides ideal archives for reconstructing high-resolution records of past glacier variability, which allows to investigate the climate and environmental changes ¹⁵,¹⁷,²². The Cook Ice Cap (CIC) of Kerguelen Island, situated in the Southern Indian Ocean (Figure 1), has experienced an extremely negative surface mass balance caused by the occurrence of lower precipitation and higher sea surface temperature since the 1950s ¹⁷,²³,²⁴, a trend that has accelerated

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since the 1990s ²⁵,²⁶. Future projections anticipate that the CIC will disappear by 2100, which could have
progressive and significant impacts on endemic and invasive fauna and flora ²⁴.

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Ongoing and future decreases of glacier cover may lead to the release of long-range transported legacy contaminants already deposited and stored in snow and ice ²⁷⁻²⁹. To elucidate this phenomenon in the Southern Indian Ocean region, in-depth investigations of long-range transported trace metals and persistent organic pollutants (POPs) in the fjord system of Kerguelen are required. To this end, the collection and dating of sediment archives using naturally-occurring and artificial radionuclides (²¹⁰Pbₐₓ, ¹³⁷Cs, ²³⁹Pu, ²⁴⁰Pu) may provide a powerful tool to establish environmental and climatic reconstructions and allow a better understanding of environmental changes, climate forcing, or contamination pressures across the globe ³⁰⁻³².

The documentation of these artificial radionuclides remains sparse in the Southern Hemisphere ³⁰⁻³² and ¹³⁷Cs will become increasingly difficult to measure due to its relatively short half-life (30 years) and its major input in the 1960s³³, since most of the inputs from the 1960s have already decayed. Furthermore, due to the discontinuous monitoring of the United States Atomic Energy Commission's Environmental Measurements Laboratory between 1954 and 1976, the contribution of French nuclear weapon tests (1966-1974) to the atmospheric ¹³⁷Cs has not been investigated in details. The emissions of ¹³⁷Cs originating from the French nuclear weapon tests, which peaked in 1968 in the Southern Hemisphere, may have been mistakenly attributed to global fallout ³⁰,³³,³⁴. Nevertheless, ²⁴⁰Pu/²³⁹Pu atomic ratios can help to distinguish between global [in 1964-1965, ²⁴⁰Pu/²³⁹Pu= 0.18³⁰] and French nuclear weapon tests fallout [1966-1974, ²⁴⁰Pu/²³⁹Pu= 0.04³⁰], because of the distinct signatures of these two sources. It is therefore particularly useful to combine the analysis of ¹³⁷Cs and ²⁴⁰Pu/²³⁹Pu in sediment profiles collected in this region to establish reliable age models.

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Kerguelen Archipelago, a part of the French Southern and Antarctic Lands, is a remote island in the Southern Indian Ocean, located far away from the main continents where most data on artificial radionuclides (¹³⁷Cs and ²⁴⁰Pu/²³⁹Pu) have been provided ³⁰⁻³³. It is one of the rare accessible land sites where artificial radionuclides can potentially be measured across this wide ocean. Detailed radionuclide measurements and the associated sediment age-depth model are hence essential to improve our understanding of the atmospheric
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## 2 Materials and Methods

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### 2.1 Sediment core samples

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The Kerguelen Archipelago is located in the Southern part of the Indian Ocean (49°S - 69°E; Figure 1).

The western part of the main island (7200 km²) is covered with the CIC, which culminates at 1049 m and forms the largest ice mass of the archipelago (Figure 1 A) with a surface area of 400 km² in 2020²⁴. A strong rainfall gradient is observed across the island, varying from 3200 mm in the west part to 700 mm at the Port-aux-Français weather station at the eastern end²⁵. The surface area occupied by the CIC decreased from 500 km² in 1963, to 448 km² in 1991 and further to 403 km² in 2003²⁵. The CIC retreat is associated with a major climatic drying and with a significant warming of the region²³,²⁴, which favoured the growth of proglacial lakes¹⁷,³⁵⁻³⁸. The climatic drying is caused by a poleward shift of the southern westerly wind (SWW) storm tracks induced by positive Southern Annular Mode (SAM) conditions since 1975²³, as shown in Figure 1 (F). The Kerguelen Island is situated in the high windspeed Southern Westerly Winds band, downwind of the southern part of South Africa, which is the major potential atmospheric pollution source in this region³⁹,
followed by the southern part of South America. Australia and New Zealand, which are almost never crossed
by the wind parcel nine days before arriving at the Kerguelen Island (Figure 1 D, E and F), have a very
limited contribution to the archipelago pollution.

Core MD09-3239 (36-m long giant piston core, section I, 49° 33.8 S-69° 13.9 E) investigated in the current
research was collected at the depocenter (180 m water depth) in the 15-km long and 2-km large Table Fjord
¹⁷, during a survey conducted in March 2009 on board of R/V Marion Dufresne from the French Polar
Institute (IPEV), as part of the MD172 expedition. The fjord is fed by meltwater from glaciers in the southern
CIC, including Ampere Glacier, who has significantly retreated in the past century ¹⁷,²². Glacial retreat led to
the formation of proglacial Ampere Lake in the early 1960s, which acts as a sediment trap between the glacier
front and the fjord ¹⁷,²⁵,³⁷. More detailed information on the core sampling and geological conditions of the
sampling site can be found in Chassiot et al. (2024)¹⁷. Core storage conditions and the sampling depth interval
are presented in Text S1.

<figure><img src="image_3.png" /><figcaption>Figure 1: (A) Kerguelen Island and sampling location of the sediment core in the Table Fjord; (B) zoomed region showing the location of proglacial lake Ampere, Cook Ice Cap and the core sampling site in Table Fjord; (C) Satellite photo of sampling site from Landsat 7; (D) total of flyover count of HYSPLIT backward</figcaption></figure>
trajectories between 01/01/2017 and 01/01/2022, 9 days length, one hour step with interval of 12 hours, 3000 m height for the Kerguelen sampling site. (E) average surface PM₂.₅ concentrations as estimated from Giovanni⁴⁰ (between 1981 and 2006, using 10-meter eastward and northward wind, using M2TMNXSLV product) showing that South Africa and South America are sources of emission situated upper wind of the site. Location of the sampling site is indicated by a blue or white star and a pink dot. (F) Wind speed and direction from Giovanni, showing the average wind direction and speed between 1980 and 2006, and showing that Kerguelen Island is situated in the high windspeed Southern Westerly Winds band.

## 2.2 Analytical methods

### 2.2.1. Radionuclide analysis

#### 2.2.1.1. Radionuclide activity concentration analysis for ¹³⁷Cs and ²¹⁰Pbₐₓ and sediment core dating

The ¹³⁷Cs and ²¹⁰Pbₐₓ activities were measured by gamma spectrometry at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE in Gif-sur-Yvette, France), using HPGe detectors (Mirion/HTDS). In all these samples, ¹³⁷Cs was detected by its characteristic emission peak at 662 keV. ²¹⁰Pbₐₓ was calculated by subtracting the supported activity concentration from the total ²¹⁰Pb activity concentration (detected at 46.5 keV) using two ²²⁶Ra daughters, ²¹⁴Pb (average count at 295.2 and 351.9 keV) and ²¹⁴Bi (609.3 keV). All measurements were corrected for background radioactivities determined every two months as well as for detector and geometry efficiencies. Activity concentrations measured by gamma spectrometry and presented in this study were decay-corrected to the sampling date (i.e., 7 March 2009). Details on gamma spectrometry analysis and quality assurance for activity concentration measurement can be found in Supporting information (Text S2 and Table S1).

Based on ²¹⁰Pbₐₓ activity concentrations, a CFCS age-depth model [Constant Flux and Constant Sedimentation ⁴¹] was computed using the “serac” R package ⁴². The ¹³⁷Cs and ²⁴⁰Pu/²³⁹Pu were measured in 11 layers of the sediment core to test the depth-age model established with ²¹⁰Pbₐₓ ¹⁷. The last detection of unsupported ²¹⁰Pb was dated to 1928 and for deeper sections, a linear interpolation was applied. This linear
interpolation relies on the mean sedimentation rate calculated in the upper radiometrically-dated section of the core (30-50 cm depth dated to 1927-1944 period), assuming that the sediment flux is constant during this period without significant human nor climate disturbance.

### 2.2.1.2. $^{240}$Pu/$^{239}$Pu atomic ratio analysis by ICP-MS and accelerator mass spectrometry
(AMS)

Seven selected sediment sections were analyzed for plutonium (Pu) isotope contents at Spiez laboratory (Switzerland) following procedures published elsewhere $^{43-45}$. In summary, radiochemical analysis was performed using aliquots of ashed material weighing up to 5 g. A known quantity of $^{242}$Pu (3 pg) was introduced as a radiochemical recovery tracer in order to measure the Pu isotope activity. After the samples were digested using borate fusion, the melt was dissolved in 4.5 M $HNO_3$. Following silicate filtering, TEVA extraction chromatography resin was used to separate and purify Pu radionuclides, as previously mentioned.$^{46}$. A sector-field ICP-MS (Element XR, Thermo Fisher Scientific) was used to conduct mass spectrometric analysis. In order to improve the signals for the plutonium isotope analysis, the relevant eluates were added to the ICP-MS Element XR using an Apex nebulizing system linked to an ACM desolvator (Elemental Scientific Incorporation). Tailing factors were used to adjust the uranium interferences for the abundance sensitivity. AMS was also used for $^{240}$Pu/$^{239}$Pu analysis to provide comparison. Details on Pu purification and instrumental conditions can be found in Supporting Information Text S3 and the validation of the analysis can be found in Table S2. About 1 g of each sample was ashed, spiked with $^{242}$Pu, and chemically processed to extract $Pu^{4+}$, which was retained on TEVA® resin, rinsed, and eluted. The plutonium fractions were coprecipitated with iron hydroxide, dried, and divided into two preparation conditions (80 °C and 600 °C). Samples, standards, and blanks were measured by AMS at VERA using $PuO^-$ extraction, helium stripping, and 3+ charge selection. Quality-control materials confirmed consistency, with most data agreeing within 1σ. Consistent results were obtained for analyses conducted under both 80 °C and 600 °C temperature conditions, which suggests that the 80°C temperature condition may be preferred in the future as it leads to faster and cheaper analyses. ColAMS$^{47}$ and IAEA-SRM384$^{48}$ were used to validate the analysis results and
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a comparison between measured and published values can be found in Table S2, which shows good agreement between measured values and published values.

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## 2.2.2. Elemental geochemistry analyses

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### 2.2.2.1. Sample digestion

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Geochemical measurements were performed on mineralized samples. Approximately 100 mg of sediment sample was dissolved by the addition of HF (4 ml, 47-51%) + HClO₄ (2 ml, 65-71%) and then HCl (3.75 mL, 34-37%) + HNO₃ (67%, 1.25 mL) (TraceMetal grade, Fisher Scientific) in Teflon vessel on hot-plates (Digiprep, SCP Science) as described in Briard et al. (2023).

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### 2.2.2.2. Elements and lead isotopic ratios analysis using TQ-ICP-MS

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Elemental concentrations were determined using a Thermo Scientific iCAPTM TQ-ICP-MS equipped with a Peltier cooled cyclonic spray chamber, glass concentric nebulizer (400 µL min⁻¹) and 2.5 mm internal diameter quartz torch at LSCE (Gif-Sur-Yvette, France). The instrument was operated in Kinetic Energy Discrimination (KED). In this mode, interference removal is achieved using helium in the collision reaction cell. Only ions retaining sufficient kinetic energy remaining will exit the cell and molecular interferences are efficiently eliminated. Ca, V, Cr, As, Se, Sr, Y, Mo, Ba and all rare earth elements (REEs) are also analyzed by O₂ mode with oxygen as the reaction cell gas and a mass shift of 16 amu to circumvent the most common atomic interferences for each of these elements. No significant difference is observed between results obtained with KED and O₂ mode for these elements except for Se which are better quantified in O₂ mode (¹⁵⁶Gd and ¹⁶⁰Gd produce doubly charged atomic interference on ⁷⁸Se and ⁸⁰Se in KED mode). In the case of
REEs, the difference between these two modes is also negligible implying that KED mode can efficiently eliminate the polyatomic interference of ¹⁴¹Pr¹⁶O⁺ or ¹³⁸Ba¹⁶O⁺, ¹⁵⁰Nd¹⁶O⁺, ¹⁵⁰Sm¹⁶O⁺, ¹⁵⁷Gd¹⁶O⁺, ¹⁵⁹Tb¹⁶O⁺ on the measurement of ¹⁵⁷Gd, ¹⁶⁶Er, ¹⁷³Yb, and ¹⁷⁵Lu in our case. Data presented in this paper are hence obtained with KED mode with exception of Se for which data obtained with O₂ mode is reported. Analytical uncertainties related to the instrumental analysis are described in Supporting Information Text S4.

Pb stable isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb) were analyzed in non-gas mode following the same procedures as those described by Briard et al., (2023). Briefly, samples were diluted to concentration with [Pb] = 2 ng ml⁻¹, isotope ratios were corrected with repeated measurements of the Pb reference material NIST SRM-981 ([Pb]=2 ng ml⁻¹) analyzed between every two samples (Supporting Information Text S5 and Text S6 for more details). The digestion blanks (also called procedural blanks) were around 0.004 ng mL⁻¹, while the analyzed solutions were at 1.4 ng ml⁻¹ after 10 times dilution of the digestion solution. A blank correction was therefore not conducted.

### 2.2.2.3. Validation of elements and lead isotopic ratios analysis

Five certified reference materials (CRM: IAEA lake sediment SL-1) samples were prepared during the sample digestion in order to check the measured elemental concentration. The median recovery rate was calculated by dividing the measured concentration by certified concentration and is listed in Table S3 in the Supporting Information. Three SL-1 (~100 mg) were mineralized with samples and measured for lead isotopic ratios. The measured and published isotope ratios ⁴⁹ are listed in Table S4 in the Supporting Information.

### 2.2.3. Analysis and determination of persistent organic pollutants (POPs) concentrations in the sediment core

In total, 16 PAHs (EPA's 15 priority PAHs plus naphtalene listed in Supporting Information Table S5), 2 pyrethroids (cypermethrin and deltamethrin), 4 organochlorines (PeCB, HCB, Lindane and 44'-DDE), 7 PCBs (28, 52, 101, 118, 138, 153 and 180) and 6 PBDE (28, 47, 100, 99, 154, 153) were analysed. The POPs analysis followed the established method described elsewhere ⁵⁰,⁵¹. Briefly, pressurized liquid extraction of
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POPs is conducted with 2g of dry sediment, using ASE-200 (Dionex). The extraction was performed in two steps with hexane/dichloromethane (1:1) and hexane/acetone (1:1) at 100°C prior a purification step onto Florisil cartridges. Extracts were then analyzed by gas chromatography (Agilent GC system 78980) coupled to a triple quadrupole mass spectrometer (Agilent MS/MS 7000 D). Further details on the preparation, analytical and validation procedures can be found in Supporting Information. The limit of quantification (LoQ) for each POP is listed in Supporting Information Table S5, further details on the preparation, analytical and validation procedures can be found in Text S7.

## 2.3 Compositional data analysis

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Compositional covariance biplot can be used to investigate relative compositional differences between samples. A detailed explanation of compositional data analysis and its application in geoscience can be found in van den Boogaart and Tolosana-Delgado⁵². Compositional biplot is made with the R package "compositions"⁵²,⁵³, which consists of a principal component analysis (PCA) projection of centered log-ratio (clr) transformed data⁵⁴. PCA is an interpretation of the singular value decomposition of a data matrix, which reduces dimensionality by transforming correlated variables into two orthogonal components capturing maximum variance⁵²,⁵³. The clr transformation is critical for the rigorous statistical analysis of geochemical datasets⁵⁴. The outcome of this approach is a simple visual analysis of the similarities and differences of the elemental content through the sediment core, displaying the relative variations more readily and allowing us to group elements showing the same behavior.

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### 2.3.1. Compositional data analysis and perturbation vectors calculation

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Compositional perturbation can be used to study compositional differences between two systems⁵⁵,⁵⁶. In the current study, perturbation is applied to study the rare earth elements (REEs) fractionation between sediment samples and a baseline (considered as REEs compositional mean of the 5 deepest sediment layers of the core analyzed in this study). A detailed explication of perturbation calculation and its application in geoscience can be found in van den Boogaart and Tolosana-Delgado⁵². This technique is particularly well adapted to situations where elemental ratios are more relevant than absolute concentrations. The suitable
sample space of any compositional vector x, representing a D-part subset of a whole x=[x₁, ..., x_D], is the simplex S^D. Let x = [x₁, ..., x_D] and y = [y₁, ..., y_D] denote two compositional vectors in S^D. Then, z, corresponding to the perturbation of x by y, in S^D is given by: z = x ⊕ y = c [x₁ y₁, ..., x_D y_D], with c the closure-to-unity operation defined as:

$$c(x) = \left[ \frac{x_1}{\sum_{i=1}^{D} x_i}, \dots, \frac{x_D}{\sum_{i=1}^{D} x_i} \right] \qquad (1)$$

The neutral element of the perturbation is e=c [1,...,1] =[1/D, ..., 1/D], and x = x ⊕ e, while the perturbation vector expression compositional changes from y to x, noted x ⊖ y, is equal to x ⊕ y⁻¹, with y⁻¹ = c[y₁⁻¹, ..., y_D⁻¹].

### 2.3.2. Computing compositional mean

The center or compositional mean of a dataset X with N samples and D elements is a composition x̄, which can be calculated as follows:

$$\bar{x} = \frac{1}{N} \odot \bigoplus_{n=1}^{N} x_n = \mathrm{clr}^{-1} \frac{1}{N} \sum_{n=1}^{N} \mathrm{clr}(x_n) \qquad (2)$$

Where x_n is the nth samples in the nth row of dataset X. “⊙” is power transformation (1/N ⊙ x = [x₁^1, ..., x_D^1]). " ⊕_n=1 x_n" means “sum up” all the compositions in the dataset (⊕_n=1 x_n = x₁ ⊕ x₂ ⊕ ... ⊕ x_N). This calculation can be easily operated using "compositions" package with line “mean (acomp(X))”⁵².

In order to study the rare earth compositional difference between a sediment sample recently deposited and sediment samples deposited earlier (the 5 deepest layer samples in the core), the REEs compositional mean (x̄) is calculated for the 5 deepest samples, which show sediment devoid of influence from human disturbance or climate change. The perturbation vector of each depth (x) by average composition of the five deepest samples (x̄) is calculated as x ⊖ x̄.
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## 2.4 Calculation of the French nuclear weapon test contribution to artificial radionuclides

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# 3 Results and Discussion

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## 3.1 Artificial radionuclides

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### 3.1.1. Radionuclide profiles of the sediment core collected in the Table Fjord situated in Kerguelen archipelago

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The ²¹⁰Pbₐₓ activities (Figure 2 and Table 1) showed a decrease with two distinct trends separating the layers between 0-30 cm (2009-1944) and those between 30-50 cm (1944-1927). An accumulation rate of 4.65 mm year⁻¹ can be calculated for the period between 1944 and 2009, while an accumulation rate of 11.58 mm year⁻¹ is found between 1927 and 1944. The changes in the accumulation rate can be explained by the formation of proglacial Ampere Lake, which acts as a sediment trap and which reduces the sediment flux arriving in the fjord system ¹⁷. ²⁴⁰Pu/²³⁹Pu values measured by ICP-MS and AMS are comparable and confirm the same trend (Figure 2). In the remainder of the text, discussions will be based on values measured by AMS. ²⁴⁰Pu/²³⁹Pu values in Kerguelen sediment core samples were found to vary between 0.124±0.011 (9-10 cm (1987-1990)) and 0.215±0.008 (24-25 cm (1955-1957)). The first detection of ¹³⁷Cs was found at 24-
1 25 cm depth, while the maximum activity concentration (4.7±0.3 Bq.kg⁻¹) was recorded at 19-20 cm depth.

2 A similar trend was observed for <sup>239+240</sup>Pu activity concentrations derived from ICP-MS analyses.

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<figure><img src="image_5.png" /><figcaption>Figure 2: Short-lived radionuclide data, and age-depth model established for the Kerguelen Table Fjord sediment core. Left: $^{210}$Pb$_{ex}$ activity concentration measured by gamma spectroscopy. Middle: $^{137}$Cs (black dots) and $^{239+240}$Pu (green dots) activity concentrations (measured by gamma spectroscopy and derived from</figcaption></figure>
ICP-MS measurement, respectively), and $^{240}$Pu/$^{239}$Pu ratios measured by ICP-MS (red dots) and AMS (blue dots, weight-averaged values under temperature conditions of 80°C and 600°C, see Supporting Information Text S3). Right: $^{210}$Pb$_{ex}$ depth-age model based on CFCS model $^{17,42}$. The $^{240}$Pu/$^{239}$Pu signature associated with the French nuclear weapon tests (0.04±0.01$^{30}$) is depicted by the green dotted vertical line and the green vertical rectangle for uncertainties, while the global signature (0.18±0.01$^{30}$) is represented by the red dotted vertical line and the pink vertical rectangle for uncertainties. Much of the energy yielded by the Pacific Proving Ground test (1946-1958) was generated between 1954-1958 (depicted by the red zone on the figure), while most energy of the French Tests (1964-1974) was yielded between 1966 and 1971 as well as in 1974. Accordingly, the period between 1966 and 1974 is marked in blue. Furthermore, 1962 is the year with the highest energy yielded in human history due to extensive USA and URSS nuclear weapon tests, and this year is indicated in purple. Energy of nuclear weapon was taken from Dicen et al. (2024)$^{33}$. The first fallout period (1956-1959) detected globally, the maximum global fallout period in the Southern Hemisphere (1964-1965) and onset of the French fallout period in the Southern Hemisphere (1966), are represented by dotted horizon lines.

Table 1: Properties of the 11 layers identified in the sediment core collected in the Table Fjord in Kerguelen archipelago. [%French presents the contribution of French nuclear tests to $^{239+240}$Pu]. "<DL" stands for "under the detection limits" and "NA" stands for "not available" (Pu was not analysed because $^{137}$Cs was under the detection limits). The $^{239+240}$Pu activity concentrations are provided for information.

<table><thead><tr><th>Depth</th><th>Min age</th><th>Max age</th><th><sup>210</sup>Pb<sub>ex</sub><br>(Bq.kg<sup>-1</sup>)</th><th><sup>137</sup>Cs<br>(Bq.kg<sup>-1</sup>)</th><th><sup>239+240</sup>Pu<br>(Bq.kg<sup>-1</sup>)</th><th><sup>240</sup>Pu/<sup>239</sup>Pu<br>(AMS)</th><th>%French<br>(AMS)</th><th>Sedimentation rate<br>(mm/year)</th></tr></thead><tbody><tr><td>5-7 cm</td><td>1993.9</td><td>1998.2</td><td>406±8</td><td>2.0±0.3</td><td>0.3193</td><td>0.152±0.011</td><td>20±10%</td><td>4.6±0.2</td></tr><tr><td>9-10 cm</td><td>1987.4</td><td>1989.6</td><td>338±8</td><td>2.8±0.4</td><td>0.3375</td><td>0.124±0.011</td><td>40±9%</td><td>4.6±0.2</td></tr><tr><td>13-14 cm</td><td>1978.8</td><td>1981.0</td><td>265±5</td><td>3.5±0.3</td><td>0.3757</td><td>0.149±0.009</td><td>22±9%</td><td>4.6±0.2</td></tr><tr><td>17-18 cm</td><td>1970.2</td><td>1972.3</td><td>205±4</td><td>4.2±0.2</td><td>0.5528</td><td>0.130±0.006</td><td>36±7%</td><td>4.6±0.2</td></tr><tr><td>19-20 cm</td><td>1965.9</td><td>1968.0</td><td>169±5</td><td>4.7±0.3</td><td>0.5780</td><td>0.149±0.008</td><td>22±8%</td><td>4.6±0.2</td></tr><tr><td>21-23 cm</td><td>1959.4</td><td>1963.7</td><td>154±4</td><td>4.6±0.3</td><td>0.5938</td><td>0.182±0.01</td><td>0%</td><td>4.6±0.2</td></tr></tbody></table>
<table><tr><td>24-25 cm</td><td>1955.1</td><td>1957.2</td><td>113±5</td><td>2.1±0.4</td><td>0.5017</td><td>0.215±0.008</td><td>0%</td><td>4.6±0.2</td></tr><tr><td>27-28 cm</td><td>1948.6</td><td>1950.7</td><td>101±5</td><td>&lt;DL</td><td>NA</td><td>NA</td><td>NA</td><td>4.6±0.2</td></tr><tr><td>30-31 cm</td><td>1943.4</td><td>1944.3</td><td>81±4</td><td>&lt;DL</td><td>NA</td><td>NA</td><td>NA</td><td>11.6±0.2</td></tr><tr><td>38-39 cm</td><td>1936.5</td><td>1937.4</td><td>65±4</td><td>&lt;DL</td><td>NA</td><td>NA</td><td>NA</td><td>11.6±0.2</td></tr><tr><td>49-50 cm</td><td>1927.0</td><td>1927.9</td><td>48±3</td><td>&lt;DL</td><td>NA</td><td>NA</td><td>NA</td><td>11.6±0.2</td></tr></table>

The first detection of <sup>137</sup>Cs in the core is dated between 1955 and 1957, coinciding with the highest <sup>240</sup>Pu/<sup>239</sup>Pu atomic ratio observed, likely due to the 67 nuclear weapons tests conducted by the USA in the Marshall Islands between 1946 and 1958 (referred to as Pacific Proving Ground tests with most of energy yielded between 1954-1958<sup>33</sup>, and corresponding to the red zone shown in Figure 2), for which <sup>240</sup>Pu/<sup>239</sup>Pu ratios exceeded 0.20<sup>60,61</sup>. One <sup>137</sup>Cs maximum activity concentration peak was then detected in the core (in two sediment samples dated to 1959-1964 and 1966-1968, respectively). In the sediment sample dated to 1959-1964, the high <sup>137</sup>Cs activity concentration can be attributed to the extensive nuclear weapon tests conducted in 1962 by the USA and USSR<sup>33</sup>, which resulted in a maximum fallout of <sup>137</sup>Cs in 1964-1965 in the Southern Hemisphere<sup>62,63</sup> (dotted horizon line in Figure 2). Accordingly, a 100% contribution of the global fallout was calculated for plutonium in the sediment layer attributed to 1959-1964 (Table 1), with a high <sup>240</sup>Pu/<sup>239</sup>Pu atomic ratio (0.182±0.01). From 1966 to 1974, France conducted 41 atmospheric nuclear tests in the atolls of Mururoa and Fangataufa in French Polynesia, with most of energy yielded in 1969, 1970, 1971 and 1974<sup>33</sup> (blue zone in Figure 2 corresponding to the period 1966-1974). French tests in 1966 and 1967 contributed 22±8% of the plutonium fallout in the corresponding sediment layer (1966-1968) and led to prolonged high levels of <sup>137</sup>Cs activities (4.7 Bq kg<sup>-1</sup>) detected in the core section dated back to this period (1966-1968)<sup>30</sup>. The intensive French nuclear weapon tests conducted between 1969 and 1971 (with fallout peaks between 1969 and 1971 in the Southern Hemisphere) contributed to relatively high level of <sup>137</sup>Cs activities in the section dated back to 1970-1972, with 36±7% of the <sup>239+240</sup>Pu detected during this period attributed to the intensive French tests conducted in 1969-1971 (Table 1). Similar trends and contributions from French Polynesia nuclear weapon tests were also observed in other studies, with those conducted in South America including in Lake Chungará (18°S)<sup>30</sup> and ice core records obtained in Antarctica at 75°S<sup>64</sup>.
In South America, the surface layers of soils of many regions located between 20 and 40°S still show nowadays $^{240}$Pu/$^{239}$Pu values lower than the global fallout value due to the significant contribution of French nuclear tests (conducted in French Polynesia, $^{240}$Pu/$^{239}$Pu = 0.04$^{30}$) to atmospheric fallout observed in these regions $^{30,65}$. This is probably also the case for the surface core section of the sediment core analyzed in the current study, showing $^{240}$Pu/$^{239}$Pu values (0.152±0.011 for section 1994-1998) lower than the global fallout signature, with 20±10% of $^{239+240}$Pu estimated to have been contributed by French nuclear weapon tests. The highest French nuclear weapon tests contribution (40±9%) was detected in the core section attributed to 1987-1990, followed by the section dating back to 1970-1972 (with 36±7% estimated to be contributed by French nuclear weapon tests). To the best of our knowledge, this is the first time that such a significant contribution of Pu attributed to the French nuclear tests is found in sediment collected in the Southern Ocean region, located far east and south compared to the corresponding locations in South America where it was already detected. This demonstrates the wide spatial area exposed to fallout associated with French nuclear tests conducted in Polynesia, which should be taken into account when dating archives collected in this part of the world. In the near surface layer located between 5-7 cm (1994-1998), $^{240}$Pu/$^{239}$Pu values approached global levels probably due to the accelerated retreat of CIC$^{25}$ leading to the release of legacy $^{239+240}$Pu deposited in CIC during nuclear weapon tests conducted by USA and USSR in 1962.

This study highlights the possibility of using radionuclides for dating sediment core from a fjord system and offers a rare successful example of its application. The remobilization of $^{137}$Cs and $^{239}$Pu, $^{240}$Pu from particles of fjord sediment has not been well documented to date. The bulk of cation ions in seawater were predicted to exchange with $^{137}$Cs and $^{239+240}$Pu, which may cause $^{137}$Cs and $^{239+240}$Pu desorption from particles into saltwater. However, it has been shown that the strong adsorption of $^{137}$Cs onto the phyllosilicate fraction of soils, suspended particulates, and sediments in freshwater systems is nearly irreversible, and only 1-7% of $^{137}$Cs would be desorbed by seawater within 7 days after riverine particles were delivered to the sea $^{66}$. The diffusional migration of $^{137}$Cs in the sediments was shown to be relatively low in estuarine regions or river-dominated coastal environments $^{67,68}$. Some studies showed that the exchangeable fraction of $^{239+240}$Pu in sediment transported in sea water was estimated to be lower than 2.6% of the total $^{239+240}$Pu $^{69}$. The $^{240}$Pu/$^{239}$Pu
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ratio remains constant without fractionation during chemical and physical processes that may occur in the environment, making it a powerful source tracer.⁷⁰ Our observation and conclusions are therefore consistent with the aforementioned studies.

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In conclusion, peaks of ¹³⁷Cs activities associated with French, USA and USSR nuclear weapon tests can be merged into one single wide peak of ¹³⁷Cs activity concentration in the sediment cores collected in this part of the Southern Hemisphere, which complicates the interpretation of ¹³⁷Cs activities in the core, their attribution to given nuclear tests and their use for establishing an age model. The occurrence of this wide ¹³⁷Cs peak requires the additional measurement of ²⁴⁰Pu/²³⁹Pu atomic ratios. This allows to correctly attribute fallout radionuclides in successive sediment layers to different nuclear weapon tests conducted by different countries and therefore, to validate the first age-depth model established with ²¹⁰Pb<sub>ex</sub>. Importantly, the use of the 0.04 ± 0.01 signature for characterizing French nuclear tests appears to be the most relevant estimate currently available. Nevertheless, this value and the overall contribution of French nuclear weapon tests should be interpreted with caution given the absence of a well-defined composite isotopic signature (information on the isotopic signatures of the different tests remains, classified nowadays) and the uncertainties regarding the spatial and temporal extent of fallout in the Southern Hemisphere from the French nuclear test program conducted in Polynesia between 1966-1974.

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## 3.2 Elemental composition shows a progressive enrichment of Pb, As, Mo and Sb since 1940s

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The compositional data were explored using the compositional covariance biplot (Figure 3), which is a PCA projection of clr transformed data⁵²,⁵⁴ (clr transformed data is presented in Supporting Information Figure S2). The first two principal components explain 81% of the total variance (71% and 10% respectively), a value relatively high for a set of 30 elements. The links between two arrow heads approximates the standard deviation of their corresponding log-ratios between the two involved elements. If the link is very long, the log ratio of these two elements involved is highly variant in sediment samples. Several groups of elements [Se], [Pb, As] [Cu, Zn] and [Mo, Sb] are presented by relatively long arrows. Long links are observed between [Mo, Sb] and elements close to Al, between [As, Pb] and elements close to
Ti, and finally between [Cu, Zn] and Se, indicating the most important log-ratios [log(Mo/Al), log(Pb/Ti) and log(Cu/Se) for example] variations across the sediment. The proximity of Sb and Mo and long arrows presenting these elements tending toward sediment samples corresponding to the 2004-2006 period indicate remarkable high log-ratios of Sb and Mo to other elements in this sediment (Figure 3 and Supporting Information Figure S2), both of Sb and Mo are strongly related to car braking emissions and anthropogenic coal combustion <sup>39,71</sup>. High log-ratios of Pb and As to other elements can also be observed in sediment samples corresponding to 1993-2001 period. A progressive increase in clr(Pb), clr(Mo), clr(As), clr(Sb) and clr(Se) between 1850 and 2007 can be observed in Figure 5 and Figure S2.

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<figure><img src="image_6.png" /></figure>

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Figure 3: Compositional covariance biplot [Principal Component Analysis (PCA) of clr transformed elemental concentration data] showing the appearance of metal contaminants (As, Pb, Sb, Mo) in the

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sediment core collected from Table Fjord in Kerguelen Archipelago and the progressive enrichment in these
elements since the 1940s.

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These results confirm the emergence of Sb and Mo contamination in remote regions during the recent
period, as shown by records of anthropogenic contamination all around the world, including in Antarctic ice
cores 72–74, in the Tibetan plateau ice cap 75 and in Alpine snow and ice cores 76,77. While many of the trace
metal contaminants show declining concentrations in the atmosphere and sediment worldwide, human
impacts on the Sb emissions, a toxic metal considered to be an ‘emergent’ contaminant, were shown now to
exceed those of Pb – a ‘historical’ contaminant – at global levels 78. The enrichment in other trace elements
has also been observed in Antarctica 79 and Arctic 80 ice cores.

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The Se is mainly a biogenic element, with 65% of atmospheric Se being biogenic and the rest – 35% –
being anthropogenic ⁸¹,⁸². The biogenic Se can be emitted via biomethylation and subsequent volatilization
from marine and terrestrial biosphere ⁸³⁻⁸⁵. Volatile methylated forms of Se are quickly oxidized under
atmospheric conditions and undergo gas-particle conversion⁸³. Both gas and particle phases of Se can be
transferred from the atmosphere to the land by wet and dry deposition ⁸¹. The distinct emission sources and
deposition mechanisms of selenium, in contrast to anthropogenic elements (As, Pb, Mo and Sb), may help
to elucidate why the selenium vector is separated from these elements in Figure 3. However, the behavior of
Se in the sediment and fjord system has not been well studied. All these reasons limit the conclusions that
may be drawn from the results associated with this element.

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In dry sediment samples, the median concentration and concentration range of As (4.5 mg.kg⁻¹, ranging
from 2.3 to 7.6 mg.kg⁻¹), Cd (0.18 mg.kg⁻¹, ranging from 0.13 to 0.29 mg.kg⁻¹), Cr (25 mg.kg⁻¹, ranging from
19 to 46 mg.kg⁻¹), Pb (5.6 mg.kg⁻¹, ranging from 3.0 to 14.0 mg.kg⁻¹), Ni (18 mg.kg⁻¹, ranging from 15 to 31
mg.kg⁻¹) and Ag (0.14 mg.kg⁻¹, ranging from 0.09 to 0.17 mg.kg⁻¹) were lower than minimal-effect thresholds
(8.2, 1.2, 81, 46.7, 21 and 1.0 mg.kg⁻¹ for As, Cd, Cr, Pb, Ni and Ag respectively)⁸⁶ that are rarely associated
with ecological effects and established for marine and estuarine sediment, implying that the ecotoxicological
risk remains low or negligible. Median concentration and concentration range of Zn (120 mg.kg⁻¹, ranging
from 112 to 293 mg.kg⁻¹) and Cu (32 mg.kg⁻¹, ranging from 21 to 52 mg.kg⁻¹) can slightly exceed the
1 thresholds (150 and 34 mg.kg⁻¹ for Zn and Cu respectively) in some layers, which may lead to some
2 ecotoxicological effects. The median concentration and concentration range of Sb (0.14 mg.kg⁻¹, ranging
3 from 0.05 to 0.59 mg.kg⁻¹), Mo (2.5 mg.kg⁻¹, ranging from 1.4-5.7 mg.kg⁻¹), Se (0.35 mg.kg⁻¹, ranging from
4 0.11 to 0.57 mg.kg⁻¹) and all other analyzed elements are listed in the Supporting Information (Table S6).
5 No established threshold could be found in the literature for these elements under marine environmental
6 settings. Their ecological impact therefore remains not well known in marine and fjord environments.
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8 3.3 Rare earth elements (REEs) fractionations as an indicator of weathering condition changes

9 The recent CIC glacial retreat and its significant acceleration since 1990s ²⁵ led to the formation of
10 proglacial Ampere lake, which can preferentially trap larger particles, leaving only the finer suspended
11 fraction of the sediment for downstream export ⁸⁷, resulting in REEs fractionations (Figure 4). A slight
12 fractionation can also occur in sediment between the light rare earth elements (LREEs: La and Ce) and the
13 middle and heavy rare earth elements (MHREEs: Pr to Lu) when sediment is in contact in the aquatic
14 environment with different sources of water with contrasted salinity levels and organic matter sources ⁸⁸⁻⁹¹.
15 The dissolution of MHREEs is negatively correlated to water salinity ⁹²,⁹³. The contact with low salinity
16 water in the Ampere lake before reaching the fjord system may also have increased the MHREEs solubility
17 in the water and induced a depletion of MHREEs in the sediment compared with sediment deposited earlier.
18 This depletion of MHREEs is also reflected by an enrichment in La ⁹³.
<figure><img src="image_7.png" /><figcaption>Figure 4: Rare earth elements (REEs) perturbation vector calculated as composition of sediment samples $\ominus$ average composition of samples formed between 1849-1883 (the 5 deepest layers). The neutral perturbation 1/14=0.0714 is marked by the blue dotted line (the zone below this line corresponds to a depletion while the zone over this line indicates an enrichment compared to the 1849-1883 period). This figure shows the fraction of light REEs (LREEs: La to Ce) and middle and heavy REEs (MHREEs: Eu to Lu) between recently deposited core sections (red and dark red, after 1980) and earlier deposited core sections. The recently deposited core sections are depleted in MHREEs, implying a fractionation during transport and sedimentation.</figcaption></figure>

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3.4 Release of accumulated historic anthropogenic Pb following environmental changes linked to ice melting into the Kerguelen Table Fjord system

<figure><img src="image_8.png" /></figure>

Figure 5: (A): Lead isotopic ratios $^{207}$Pb/$^{206}$Pb as a function of the year in the sediment core. Corresponding values found in Kerguelen rock are shown by the blue zone $^{94,95}$; those of gasoline (South African gasoline used in Johannesburg) are shown by the red arrow and are out of the range covered by the axis $^{96}$. (B): Temporal evolution of centred log-ratio Pb [clr(Pb), in black], Mo [clr(Mo) in purple], Sb [clr(Sb), in orange] and As [clr(As), in green]; (C): Temporal evolution of clr (Ti) and average air temperature of Kerguelen Island [from NOAA-CIRES Twentieth Century Reanalysis (V2c, 1000mb)] and time series of modelled specific mass balance of CIC surface$^{23}$. clr(Ti-Ka-Area) from Chassiot et al. (2024) $^{17}$ is marked in gray,
showing identical trend with clr(Ti) of current study. Blue long dash dotted horizon lines indicate peaks of
$^{207}\text{Pb}/^{206}\text{Pb}$ ratios outside of the isotopic composition range of Kerguelen rocks.

Between 1850 and 1900, the $^{207}\text{Pb}/^{206}\text{Pb}$ values found in the sediment remained rather stable and varied within the range of the Kerguelen bedrock signature $^{94,95}$, implying that Kerguelen rock erosion supplied the major source of Pb during that period (as shown by Figure 5 and Supporting Information Figure S3). This is consistent with the high value of clr(Ti) obtained during this period, reflecting the high load in Ti, which can be interpreted as an input of sediment originating from basaltic rocks that dominate the fjord watershed $^{17,22,94,97}$. Erosion and sediment load enriched in Ti to the fjord was facilitated by a proximal glacier position, prior to the formation of proglacial Ampere lake in the watershed $^{17,37}$.

Seven peaks were detected for both $^{207}\text{Pb}/^{206}\text{Pb}$ and clr(Pb) (Figure 5 A and B). The three deepest peaks were found at 49-50 cm (1927-1928); 30-31 cm (1943-1944); and 13-14 cm (1979-1981) depths. Lead isotopic ratios of these samples suggest a potential influence of South Africa's anthropogenic aerosols as an end-member and likely supplying a part of Pb source during these periods (Supporting Information Figure S3). Anthropogenic aerosol emissions from South Africa also supply a major source of lead in aerosols and atmospheric deposits samples collected in Kerguelen as shown by earlier research $^{98}$. This anthropogenic lead from South Africa may originate from industrial activities that took place in the early 1900s and the onset of the widespread use of leaded gasoline in the Southern Hemisphere in the 1940s $^{96,99}$. Other independent studies using model and field measurements also pointed out South Africa as the major source of anthropogenic Mo $^{39}$, anthropogenic Fe $^{11}$, anthropogenic Mn $^{100}$, natural dust $^{101,102}$ and pollen $^{103}$ to the Southern Indian Ocean including Kerguelen Archipelago.

As the depletion in Ti [reflected by low clr(Ti)] indicates the occurrence of environmental changes related to the retreat of the ice cap due to the formation of proglacial lake Ampere trapping large sediment particles $^{17}$ and given that the down-core clr(Pb) is negatively correlated to the clr(Ti) (Supporting Information Figure S4), the release of the anthropogenic legacy Pb into the fjord was likely facilitated by these environmental changes related to the ice retreat. Anthropogenic Pb can be associated with fine particles, which can be
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Other elements such as As, Mo and Sb showed different temporal trends compared to those observed for Pb. These elements show different atmospheric evolution histories associated with contrasted industry and mining activities developed during different periods as observed in other parts of the world <sup>104</sup>. Nevertheless, the recent increase in clr(Sb), clr(Mo) and clr(As) observed in the sediment core all indicate the occurrence of recent release increases after the 1970s. The absence of correlation between clr(Ti) and these elements [clr(Sb), clr(Mo) and clr(As)] (Figure 5) suggests that these elements are not legacy contaminants and that there has been no recent accumulation of these recently emitted elements in the CIC. Their atmospheric transport to Kerguelen Island and their deposition in the sediment core appear to be synchronous and recent (after 2000).

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## 3.5 Recent presence of persistent organic pollutants (POPs) in the sediment

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The POPs were only detected in 6 layers, which are listed in Table S7 in Supporting Information. Only 4 low mass PAHs (acenaphthene, naphthalene, fluorene and phenanthrene), 1 organochlorine (HCB), 2 PCB (28, 52) and all of the 6 analysed PBDE were detected in different layers, with the most recent layer (2005-2007) being the single one containing all these contaminants. In the most recent layer (2005-2007), $\Sigma_{15}$PAHs (13.4 ng g⁻¹) and PCB (0.32 ng g⁻¹) concentrations are comparable to those found in sediment cores collected in remote regions such as the Antarctic (PAH ranging from 1 to 205 ng g⁻¹ and PCB ranging from 0.32 to 0.83 ng g⁻¹) <sup>105</sup>. For each of the detected POPs, the concentration always remained below the minimal-effect thresholds that are rarely associated with ecological effects <sup>86</sup>, which implies that the ecotoxicological risk may be considered as negligible. The total absence of heavy PAHs with more than 3-ring homologues suggests a long-range transported source and the absence of pollution from the Marion Dufresne French ship (in service since 1995) during sampling and the negligible impact from the local scientific station located in downwind direction of the sampling site (at Port-aux-Français, Figure 1)<sup>106,107</sup>. The synchronous occurrence of PAHs with only 2&3-ring homologues in the three most recent layers (between 2000 and 2007) and of typical human-made PCB (2005-2007) and HCB (2003-2007) in the uppermost layer(s) suggests a recent
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## 4 Contributions and perspectives

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long-range transported common source from industrial, traffic <sup>107</sup> and agriculture (biomass, coal and petrol combustion <sup>106</sup>) activities for these POPs. The high enrichment in trace metals (Pb, As, Sb and Mo) observed in these layers (Figure 3) also supports this conclusion. This clearly established metal deposition chronology may be of help to reconstruct increasingly robust age models of ice archives in this remote archipelago.

As significant data gaps remain regarding the magnitude and distribution of POPs contamination in the Southern Indian Ocean, it is difficult to conclude a this stage about its overall extent. Continued climate changes <sup>108</sup> during the last 30 years in the Southern Ocean and the industrial development of this region may have impacted the processes that release POPs and re-mobilize the legacy Pb stored in the ice in the sediment deposited since then. Further research including the additional analysis of surface sediment (deposited after 2006) may shed light into the evolution of the concentrations in these contaminants in the fjord.

Long-range transported legacy and recent contaminants released by human activities have been detected in a sediment core collected in the Kerguelen Archipelago fjord system. The age-depth model based on <sup>210</sup>Pb<sub>ex</sub> activities was validated and refined through the combined analysis of <sup>137</sup>Cs activities and <sup>240</sup>Pu/<sup>239</sup>Pu atomic ratios measured with depth in the core. This allowed a detailed analysis of the temporal evolution of pollutants released into the fjord system during the last century, in the current context of climate change and increasing human pressures. In the fjord system of Kerguelen Island, the shrinkage of glacier cover induced by climate change is leading to the release of long-range transported anthropogenic legacy lead deposited and stored in snow and ice that accumulated in the past. The delivered anthropogenic Pb may be transported by a hypopycnal flow from melting glacial ice to the fjord system. Lead stable isotope ratios, the analysis of backward trajectories and a critical literature review on atmospheric models all suggest South Africa to supply a major source of anthropogenic Pb and other trace elements transported to these remote regions of the Southern Indian Ocean. Contemporary atmospheric inputs for other trace elements such as As, Mo, and Sb (direct atmospheric deposit or via runoff) after the onset of ice melting have been observed since 1990
and were shown to follow a different temporal evolution than that of Pb. Persistent organic pollutants (POPs) were also detected in the uppermost layer of the core, suggesting the appearance of long-range transported organic contamination during the most recent period. The current ecological impact of these contaminants likely remains negligible or very low as their concentrations were minimal. This is – to the best of our knowledge – the first time that the fallout radionuclide contribution of French Nuclear Weapon tests conducted in French Polynesia is detected and quantified in a sediment core collected in an island of the Southern Indian Ocean. This investigation therefore constitutes an important milestone for quantifying $^{240}$Pu/$^{239}$Pu atomic ratios in sediment of islands across the Southern Indian Ocean, which can then be used to interpret changes in the sources of $^{137}$Cs along sediment cores, which may then support the refinement of age-depth models established using $^{210}$Pb$_{ex}$ activities alone. We highlighted in particular the importance of measuring the $^{240}$Pu/$^{239}$Pu in sediment sequences collected in the Southern Hemisphere to obtain reliable age models. Future research could benefit from the potential of differentiating radioactive fallout sources to establish more accurate age-depth models of sediment cores through the incorporation of other artificial radionuclides including $^{135}$Cs/$^{137}$Cs, U-isotopes, and $^{241}$Pu. The analysis of trace metals, POPs, and radionuclides in sediment cores collected in the fjord after 2006 would be of great interest to investigate the recent evolution of this contamination and elucidate the origin and fate of these potentially harmful substances in this remote region largely impacted by climate-induced changes.

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## Author Contributions

O.E. and E.C. designed the research. Experiments were coordinated by O.E., S.A., and A.F. in details, L.C. and E.C. coordinated sample collection. Y.X.-Y. and N.T. conducted the ICP-MS analysis, and L.C., L.C. conducted XRF analysis. I.L. conducted γ spectrometry analyses. J. K.W., S.M. and P. S. conducted plutonium analyses. T.T. conducted POPs analysis. Y.X.-Y. coordinated the interpretation of the results. All authors reviewed and edited the manuscript.

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## Supporting Information
Core storage conditions and the sampling depth interval (Text S1); quality assurance for activity concentration measurements with gamma spectrometry (Text S2); radiochemical separation and analysis of Pu using ICP-MS and AMS (Text S3); analytical uncertainty computation for TQ-ICP-MS-analyzed elements (Text S4); correction of <sup>204</sup>Pb signal from <sup>204</sup>Hg interference signal (Text S5); uncertainty computation for lead isotopic ratio (Text S6); extraction and purification for Persistent Organic Pollutants (POPs) analysis and validation of the method (Text S7); validation of <sup>137</sup>Cs, <sup>210</sup>Pb and <sup>226</sup>Ra activity concentrations measured at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) between 2022 and 2025 (Table S1); validation of plutonium isotopic ratios (<sup>239</sup>Pu/<sup>242</sup>Pu and <sup>240</sup>Pu/<sup>239</sup>Pu) (Table S2); recovery rate of elements in SL-1 georeference samples prepared with samples (Table S3); validation of lead isotopic ratios (<sup>206</sup>Pb/<sup>204</sup>Pb, <sup>207</sup>Pb/<sup>204</sup>Pb, <sup>208</sup>Pb/<sup>204</sup>Pb) (Table S4); list of investigated organic pollutants with the recovery ratio corrected by ILIS in %, RSD the relative standard deviation in % and LoQ the limit of quantification (Table S5); elemental concentrations measured in sediment in mg/kg (Table S6); concentration of POPs in sediment layers in which at least one pollutant has been detected (Table S7); duplicate AMS measurements and ICP-MS measurements of <sup>239</sup>Pu/<sup>242</sup>Pu (Figure S1); centered log-ratio (clr) of elements (Figure S2); scatter plot of <sup>208</sup>Pb/<sup>204</sup>Pb vs. <sup>206</sup>Pb/<sup>204</sup>Pb and <sup>208</sup>Pb/<sup>206</sup>Pb vs. <sup>207</sup>Pb/<sup>206</sup>Pb (Figure S3); Correlation between sediment elemental compositions and lead isotope ratio (Figure S4); list of references cited in Supporting Information

## ACKNOWLEDGMENTS

This work benefited of support from the French National Research Agency (ANR) in the framework of the AVATAR project (ANR-22-CE93-0001). We thank E. Douville for his help during TQ-ICP-MS analysis. F. Alliot and S. Traoré are gratefully acknowledged for the sample extraction of POPs for their quantification. The AMS measurement at VERA had been profited from trans-national access (TNA) funding by the European Union's Horizon 2020 research and innovation program under grant agreement No 824096 (RADIATE).

## ABBREVIATIONS

AMS: Accelerator Mass Spectrometry

C.E.: Common Era

CFCS: Constant Flux Constant Sedimentation model

CIC: Cook Ice Cap
1 Clr: Centered Log-Ratio

2 ICP-MS: Inductively Coupled Plasma Mass Spectrometry

3 KED: Kinetic Energy Discrimination

4 HCB: Hexachlorobenzene

5 LoQ: Limit of Quantification

6 LREEs: Light REEs, La to Ce)

7 LSCE: Laboratoire des Sciences du Climat et de l'Environnement

8 MHREEs: Middle and Heavy REEs (Eu to Lu)

9 PAHs: Polycyclic aromatic hydrocarbon

10 PBDE: Polybrominated diphenyl ethers

11 PCA: Principal Component Analysis

12 PCB: Polychlorinated biphenyl

13 POPs: Persistent Organic Pollutants

14 REEs: Rare Earth Elements

15 TQ-ICP-MS: triple quadrupole Inductively Coupled Plasma Mass Spectrometry

16 USA: the United States of America

17 USSR: Union of Soviet Socialist Republics

18 XRF: X-Ray Fluorescence

19

20 Data Availability Statement

21 All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supporting

22 Information and can be downloaded at https://doi.org/10.5281/zenodo.17248730.

23

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