# A 14,000-Year Sediment Record of Mercury Accumulation and Isotopic Signatures From Lake Malaya Chabyda (Siberia)

Lara Hughes-Allen, Frédéric Bouchard, Boris K. Biskaborn, Sahara Cardelli, Dmitry A. Subetto, Laure Laffont, Jeroen E. Sonke

► To cite this version:

Lara Hughes-Allen, Frédéric Bouchard, Boris K. Biskaborn, Sahara Cardelli, Dmitry A. Subetto, et al. A 14,000-Year Sediment Record of Mercury Accumulation and Isotopic Signatures From Lake Malaya Chabyda (Siberia). Journal of Geophysical Research: Biogeosciences, 2024, 129, 10.1029/2023JG00786310.22541/essoar.169833397.78074302/v1. insu-04831465

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# JGR Biogeosciences

## RESEARCH ARTICLE

10.1029/2023JG007863

**Key Points:**

* Permafrost soils in Eurasia hold significant amounts of organic carbon (OC) and mercury from past and present periods

* Climate warming and permafrost thawing may release OC and mercury to rivers, lakes and the atmosphere

* Lake sediment records reveal that gradual climate warming has led to increased OC and mercury burial in northern lakes, with recent pre-industrial enhanced Hg burial possibly linked to increased lake productivity

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

L. Hughes-Allen and B. K. Biskaborn,
lara.hughes.allen@gmail.com;
Boris.Biskaborn@awi.de

**Citation:**

Hughes-Allen, L., Bouchard, F., Biskaborn, B. K., Cardelli, S., Subetto, D. A., Laffont, L., & Sonke, J. E. (2024). A 14,000-year sediment record of mercury accumulation and isotopic signatures from Lake Malaya Chabyda (Siberia). *Journal of Geophysical Research: Biogeosciences*, *129*, e2023JG007863. https://doi.org/10.1029/2023JG007863

Received 18 OCT 2023

Accepted 5 SEP 2024

© 2024. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

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## A 14,000-Year Sediment Record of Mercury Accumulation and Isotopic Signatures From Lake Malaya Chabyda (Siberia)

Lara Hughes-Allen¹,² ID, Frédéric Bouchard¹,³ ID, Boris K. Biskaborn², Sahara Cardelli⁴, Dmitry A. Subetto⁵,⁶ ID, Laure Laffont⁷, and Jeroen E. Sonke⁷ ID

¹CNRS, GEOPS, Université Paris-Saclay, Orsay, France, ²Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany, ³Department of Applied Geomatics, Université de Sherbrooke, Sherbrooke, QC, Canada, ⁴Department of Physics and Earth Sciences, University of Ferrara, Ferrara, Italy, ⁵Herzen State Pedagogical University of Russia, Saint Petersburg, Russia, ⁶Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences, Barnaul, Russia, ⁷Géosciences Environnement Toulouse, CNRS/IRD/Université Paul Sabatier, Toulouse, France

**Abstract** Eurasian permafrost soils contain large amounts of organic carbon (OC) and mercury (Hg), sequestered by vegetation during past and present interglacial periods. Lake sediment archives may help understand past OC and Hg dynamics and how they interact with climate-related variables. We investigated Hg accumulation, OC dynamics, and Hg and OC stable isotopes in a 14,000-year sediment record from Lake Malaya Chabyda (Central Yakutia, Russia). Sediment Hg was correlated to OC (p value < 0.01), with lower OC and Hg accumulation rates (OCAR, HgAR) during the cold Younger Dryas (YD, 12,900–11,700 cal BP), when the lake level was low. Elevated sediment $\Delta^{200}\text{Hg}$ (0.05‰ ± 0.11‰), representing dominant Hg¹¹ deposition, and low $\delta^{13}\text{C}$, indicates low lake primary productivity during the YD. During the early Holocene, $\Delta^{200}\text{Hg}$ and $\Delta^{199}\text{Hg}$ decreased, while $\delta^{13}\text{C}$, $\delta^{202}\text{Hg}$, OCAR, and HgAR increased, suggesting enhanced algal primary productivity in deeper, more turbid waters. From 4,100 cal BP to present, Hg/OC ratios and HgAR increased at constant OCAR, indicating an additional Hg source to the lake. Analysis of Hg isotopes suggests direct Hg⁰ uptake into lake waters, potentially driven by primary production and efficient Hg burial. Our observations suggest that the gradual climate warming since the Last Glacial Termination and into the early Holocene led to enhanced OC and Hg burial in northern lakes and watersheds. Late Holocene enhanced Hg burial, but not OC, is possibly related to a renewed increase in lake primary productivity. Continued global warming may lead to further Hg sequestration in northern aquatic ecosystems.

**Plain Language Summary** The frozen soils in Eurasia contain a lot of organic carbon (OC) and mercury. As the climate gets warmer and the soil thaws, carbon and mercury might be released into the air and water. To understand these dynamics, a study was conducted on sediment archives from a lake in Russia that go back 14,000 years. The study found that the amount of mercury in the sediment was connected to OC. During a very cold period called the Younger Dryas, when the lake level was low, both mercury and carbon in the sediment were low. The lake had poor conditions for life during this time. As the climate warmed, carbon and mercury inputs increased in the sediment, especially during the early Holocene period. This suggests that more aquatic algae and plant growth traps carbon and mercury. From 4,100 years ago until now, the ratio of mercury to OC kept increasing, indicating more mercury being deposited into the lake. The type of mercury in the sediment changed, showing that more of it was directly taken up by the water. This happened because of more plant growth and better trapping of mercury in the sediment.

## 1. Introduction

Increasing temperatures and other disturbances such as deforestation for agriculture in the Arctic and sub-Arctic are causing changes in carbon, and nutrient cycling. This in turn impacts pollutant dynamics, including heavy metals such as mercury (Hg), which are bound to permafrost soil and lake sediment organic carbon (OC). Hg can be released to the atmosphere, hydrological systems, and food chain (Dastoor et al., 2022; Outridge et al., 2007), but also sequestered in soils and sediments. Hg is a naturally occurring element, whose emission and release sources include volcanism, weathering and erosion of continental rock, and biomass burning (i.e., forest fires) (Pirrone et al., 2010). However, humans have been increasing emissions of Hg to the atmosphere as early as
thousands of years ago through the practice of burning forests to create agricultural lands (Driscoll et al., 2013) and mining activities (Streets et al., 2017). Natural archive studies utilizing sediment and peat cores show that Hg deposition to remote locations has increased substantially since the start of the industrial revolution around 1850 CE (Cooke et al., 2011; Li et al., 2020).

After emission, mercury can exist in the atmosphere as gaseous elemental (GEM; Hg⁰), gaseous oxidized (GOM; HgII), and oxidized particle bound (PBM; HgII). Hg⁰ has an atmospheric residence time up to 1 year and can be transported at substantial distances before deposition, due to its low solubility and high volatility (Driscoll et al., 2013; Kaulfus et al., 2017). Even remote regions in Siberia far from Hg sources can therefore experience substantial amounts of Hg deposition and accumulation (Biskaborn et al., 2021, 2023). Mercury emitted as HgII will usually be deposited locally, but Hg⁰ can be oxidized to HgII in the atmosphere, which can result in the deposition of both Hg⁰ and HgII species far from point sources (Dastoor et al., 2022; Driscoll et al., 2013). Hg inputs to ecosystems can occur by dry deposition of HgII, via HgII scavenging by rainfall (Driscoll et al., 2013) and via direct Hg⁰ uptake by vegetation, soil, and water bodies (Jiskra et al., 2021; Zhou et al., 2021).

Non-vascular plants such as lichen or mosses, which are abundant in permafrost landscapes, take up particularly high levels of atmospheric Hg⁰ compared to vascular plants (Olson et al., 2019). This has resulted in the substantial accumulation of Hg in permafrost soils (Obrist et al., 2009). In certain conditions, such as the anoxic water common to waterbodies found in permafrost landscapes, Hg can be transformed by bacteria to methylmercury (CH₃Hg), which is a hazardous and toxic form of organomercury. Organomercury compounds are easily transferred across biological membranes and bioaccumulated and there is thus concern that increasing concentrations of Hg and CH₃Hg in permafrost landscapes due to climate warming might have negative impacts on local ecosystems and communities (Ha et al., 2017). Several studies indicate the importance of surface permafrost soils in potential Hg release in a warming climate (Burke et al., 2018; Lim et al., 2019; Obrist et al., 2009; Schaefer et al., 2020; Schuster et al., 2011), but there is currently limited knowledge about the timing of Hg release from permafrost and its fate. Studying past permafrost Hg dynamics, via sedimentary archives of Hg accumulation, as a function of climate and environmental change may help understand future Hg trajectories upon permafrost thawing.

Combining Hg concentration with lake sedimentation rates can yield a reliable record of past Hg accumulation rates (HgAR). Changes in HgAR depend on the physical and biological characteristics of a particular lake and its associated catchment in addition to changes in the global atmospheric Hg pool (Cooke et al., 2019). Mercury can enter a lake through direct atmospheric deposition onto the lake surface as well as the secondary import of particulate Hg and associated with organic matter from permafrost thaw in adjacent drainage basins (Sanei et al., 2014). Organic matter, both of watershed and in-lake origin, is the main carrier of Hg sedimentation to sediments. Sources and potential past biogeochemical transformations within lake watersheds can also be investigated using Hg stable isotope analysis of paleolimnological records (Jiskra et al., 2022; Lee et al., 2021; Sun et al., 2022).

In this study, we present the results of Hg accumulation estimation and isotope analysis of a nearly 7 m-long sediment core from Lake Malaya Chabyda within the Central Yakutia region of the Russian Federation. This Hg analysis is based on a published robust age-depth model and biogeochemical analysis on the same sediment core (Hughes-Allen et al., 2021). The specific goals of this study are to: (a) characterize HgAR trends over the last 14 ka in the context of past climate variations and related changes in lake conditions; (b) compare HgAR trends to OC accumulation rate (OCAR) trends; (c) characterize trends in Hg isotope composition and link them to Hg biogeochemistry, climate and lake and watershed ecological factors.

## 1.1. Study Site

Lake Malaya Chabyda (Озеро Малая Чабыда) (61.9569°N, 129.4091°E) is a dune lake found 15 km southwest of the City of Yakutsk (Central Yakutia, Eastern Siberia) (Figure 1). This lake lies at 188 m a.s.l, has an area of 0.24 km², and a max depth of three m (Kumke et al., 2004). The lake catchment area spans 10 km², and the catchment area to lake surface area ratio is 42. Adjacent to Lake Malaya Chabyda within the same catchment is Lake Ulakhan Chabyda, approximately 3 km to the northwest (Figure 1) (Tarasov et al., 1996). Lake Ulakhan Chabyda has an area of 2.1 km², an average depth of 0.5 m and a maximum depth of 2.0 m. While there are no surface inflows into Lake Ulakhan Chabyda (Pestryakova et al., 2012), this lake discharges into Lake Malaya Chabyda during times of high water (i.e., after spring melting). Lake Malaya Chabyda and Lake Ulakhan
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Map showing the general location of the study site (yellow star) within the Russian Federation. (b) Global permafrost context for the study site (outlined by the red box and indicated by the yellow star). (c) Location of the coring site relative to Lake Malaya Chabyda, Lake Ulakhan Chabyda, the watershed for lakes Ulakhan Chabyda and Malaya Chabyda, and the City of Yakutsk.</figcaption></figure>

Chabyda are formed within the central Yakutian Depression on the former Lena River erosion-accumulation plain. This plain consists of Quaternary loams atop Cambrian limestones (Pestryakova et al., 2012). Both lakes have developed on massifs of spear-shaped dunes, anchored in place by vegetation growth since the onset of the Holocene (~12,000 cal BP).

Permafrost in this region remains continuous and thick, reaching depths exceeding 500 m, with the upper 30–50 m composed of late Pleistocene-age fluvial and aeolian sediments, known as Yedoma, containing significant ground ice (50%–90% by volume) (Figure 1) (Strauss et al., 2017; Ulrich et al., 2017). Central Yakutia primarily features a middle taiga landscape regime, abundant with larch, pine, and birch forests (Desyatkin, 2009). Grasslands prevail in unforested areas, including lands cleared for agriculture or ranching, as well as in the depressions of old
thaw lakes from the early Holocene, known as “alases.” Following the cold temperatures and low precipitation during the Younger Dryas period (12,900 to 11,700 years BP), Central Yakutia witnessed a gradual yet persistent rise in temperature and precipitation (Biskaborn et al., 2012; Müller et al., 2009; Nazarova et al., 2013). These changes led to extensive permafrost degradation (Biskaborn et al., 2012), resulting in the formation of alas de- pressions that are now widespread throughout Central Yakutia (Desyatkin, 2009).

## 2. Methods

### 2.1. Field Sampling

During a German-Russian Expedition in March 2013 between the North Eastern Federal State University in Yakutsk (NEFU) and the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), eight overlapping sediment cores were collected from Lake Malaya Chabyda, representing a composite sequence of approximately 6.6 m. Two parallel coring holes were used to obtain 1-m core sections that overlap by approx- imately 20 cm. Each individual core sample was collected using a Russian peat corer (70 mm core diameter) and supported by a UWITEC gravity coring system. The cores were stored in waterproof sealed, transparent PVC plastic tubes in cool and dark conditions and transported to Potsdam, Germany, where they were stored at 4°C in the cold rooms at AWI without experiencing any visible drying or surface oxidation during storage (Hughes- Allen et al., 2021).

### 2.2. Sediment Core Dating and Subsampling

The dating strategy, age-model used, and OC data and interpretation for this lake sediment core have already been published in Hughes-Allen et al., 2021. In 2018, seven samples were extracted from sediment cores and sent for radiocarbon dating. Bacon in R and the IntCal20 calibration curve were used to model the age-depth relationship, which showed no significant reservoir effect in the lake. The sediment mass accumulation rate (MAR) and OC accumulation rate (OCAR) were calculated by multiplying dry bulk density (DBD) by the sedimentation rate (SR) and adjusting the product of MAR and total organic carbon concentration (TOC), respectively, with uncertainties calculated from the 95% uncertainty ranges of SR.

Subsampling of the core for Hg laboratory analyses was completed in May 2022. One-cm-thick subsamples were taken at approximately 10 cm intervals using a stainless-steel spatula. Each subsample was separated into two parts, one with a known volume. Both parts of each subsample were placed in separate plastic containers and weighed. Samples were freeze-dried for 48 hr until completely dry and then weighed again. The samples with unknown volumes (larger of the two samples) were then transported to Géosciences Environnement Toulouse (GET) (Toulouse, France) where they were ground and homogenized using a Fritsch Pulverisette 2 mortar grinder for 3 minutes.

### 2.3. Total Mercury Analysis

Total mercury concentrations (THg) were determined using a Milestone Direct Mercury Analyzer (DMA)-80. This instrument uses thermal decomposition, amalgamation, and atomic absorption spectrophotometry principles to determine the total mercury concentration in each bulk sample. NIST1632d coal (92.8 ± 3.3 ng g⁻¹) and the in- house GEOTRACES Hg ICC MIO marine sediment (141 ± 6 ng g⁻¹) (Jiskra et al., 2021) were used as reference materials at the beginning of each sampling day and between every approximately eight samples. Two samples were run as duplicates. The average THg of NIST1632d coal was 87.6 ± 7.7 ng g⁻¹ (n = 12). The average THg of MIO sediment was 128.3 ng g⁻¹ ± 3.4 (n = 5). The average standard deviation of the repeat samples was 1%. DBD, SR, and THg were used to calculate Hg accumulation rate (HgAR).

### 2.4. Mercury Stable Isotope Analysis

Analysis of Hg stable isotopes was completed at the Observatoire Midi-Pyrénées (Toulouse, France) using cold- vapor generation multi-collector inductively coupled plasma mass spectrometry (CV-MC-ICP-MS, Thermo- Finnigan Neptune Plus) following previously published protocols (Jiskra et al., 2022; Sun et al., 2013). Sam- ples were pre-concentrated by combustion in pure O₂ in a double-stage tube furnace, with the first furnace ramping from room temperature to 1,000°C over 3 hr, and the 2nd pyrolysis furnace maintained at 1,000°C at all times. The thermally reduced sample Hg was trapped in an acidic solution at and O₂ carrier gas flow rate of
<table><caption>**Table 1**

Analytical Precision (1σ) of the Four Hg Standards Measured During Hg Isotopic Analysis</caption><thead><tr><th>Standard</th><th>n</th><th>δ<sup>202</sup>Hg</th><th>Δ<sup>199</sup>Hg</th><th>Δ<sup>200</sup>Hg</th><th>Δ<sup>201</sup>Hg</th><th>Δ<sup>204</sup>Hg</th></tr></thead><tbody><tr><td>NIST1944 sediment</td><td>2</td><td>-0.41 ± 0.06</td><td>0.02 ± 0.02</td><td>0.02 ± 0.01</td><td>-0.01 ± 0.03</td><td>-0.04</td></tr><tr><td>NIST1632D coal</td><td>1</td><td>-1.81</td><td>-0.07</td><td>-0.05</td><td>-0.07</td><td>-0.04 ± 0.01</td></tr><tr><td>UM-Almaden</td><td>6</td><td>-0.58 ± 0.07</td><td>-0.02 ± 0.05</td><td>0.01 ± 0.05</td><td>-0.02 ± 0.05</td><td>0.01 ± 0.11</td></tr><tr><td>ETH- Fluka</td><td>6</td><td>-1.46 ± 0.04</td><td>0.09 ± 0.04</td><td>0.01 ± 0.04</td><td>-0.00 ± 0.04</td><td>-0.01 ± 0.06</td></tr></tbody></table>

80 ml min⁻¹ (40%, v/v inverse aqua regia; 2.1 N HNO₃, 0.8 N HCl) (Jiskra et al., 2022; Sun et al., 2013). Mass-dependent fractionation (MDF) is expressed as small delta (δ<sup>xxx</sup>Hg) per mil (‰) deviation from the NIST 3133 Hg bracketing standard (Equation 1).

$$ \Delta^{xxx}Hg = \left( \frac{^{xxx/198}Hg_{\text{sample}}}{^{xxx/198}Hg_{\text{NIST3133}}} - 1 \right) \times 10^3 \quad (1) $$

where “xxx” is the mass number of the measured Hg isotopes: 199, 200, 201, and 204. Mass independent fractionation (MIF) is expressed as capital delta (Δ<sup>xxx</sup>Hg). Δ<sup>xxx</sup>Hg is the deviation of the measured isotope ratio from the isotope ratio predicted using the kinetic MDF law (Equation 2).

$$ \Delta^{xxx}Hg = \delta^{xxx}Hg - \beta_{xxx} \times \delta^{202}Hg \quad (2) $$

where β<sub>xxx</sub> is the scaling factor for each isotope based on the kinetic mass-dependent law (¹⁹⁹Hg = 0.2520, ²⁰⁰Hg = 0.5024, ²⁰¹Hg = 0.7520, and ²⁰⁴Hg = 1.493) (Blum & Bergquist, 2007). The analytical precision of four Hg standards measured during the Hg isotopic analysis are given in Table 1.

We use a Δ<sup>200</sup>Hg mass balance to estimate the fractions, f, of atmospheric Hg⁰ and Hg¹¹ to the lake sediments, based on Δ<sup>200</sup>Hg⁰ and Δ<sup>200</sup>Hg¹¹ end-members of -0.05‰ and 0.13‰ from Jiskra et al., 2021, and the notion that f_Hg⁰ + f_Hg¹¹ = 1:

$$ f_Hg^0 = \frac{(\Delta^{200}Hg^{\text{sed}} - \Delta^{200}Hg^{\text{II}})}{(\Delta^{200}Hg^0 - \Delta^{200}Hg^{\text{II}})} \quad (3) $$

We also quantify the shift in sediment Δ<sup>199</sup>Hg relative to the expected Δ<sup>199</sup>Hg, based on the Δ<sup>200</sup>Hg-derived f_Hg⁰ and f_Hg¹¹ fractions, and the atmospheric Δ<sup>199</sup>Hg⁰ and Δ<sup>199</sup>Hg¹¹ end-members of -0.20‰ and 0.37‰ from Jiskra et al., 2021:

$$ \Delta^{199}Hg^{\text{shift}} = \Delta^{199}Hg^{\text{sed}} - \left( f_{\text{Hg}^0} \times \Delta^{199}Hg^0 + f_{\text{Hg}^{\text{II}}} \times \Delta^{199}Hg^{\text{II}} \right) \quad (4) $$

## 2.5. Inorganic Chemistry and Biogeochemical Analyses

The data related to trace and major element composition, total carbon content (TC), total OC content (TOC), total inorganic carbon content (TIC), total nitrogen content (TN), and δ¹³C analyses are adopted from Hughes-Allen et al. (2021).

## 3. Results and Discussion

The sediment core is separated into three distinct units based on the stratigraphy and biogeochemical analyses detailed in Hughes-Allen et al., 2021, and summarized here in brief: The biogeochemical analyses included TOC (wt%), TN (wt%), δ¹³C, and grain size analysis. The sediments in the lowest unit (Unit 1) were deposited between 14,100 and 12,300 cal BP and this unit extends from the bottom of the core (663 cm depth) to 584 cm depth (Figure 2). The sediments in Unit 2 were deposited between 12,300 and 9,000 cal BP and this unit extends from 584 cm depth to 376 cm depth. Unit 2 is the only unit which exhibits any visible layering, in the form of mm-to cm-scale light-colored laminations. The sediments in Unit 3 were deposited since 9,000 cal BP (Figure 2). The most recent sediments, including the industrial period since ~1,850, at the top of the core, were not preserved during extraction of the sediment core.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Relevant mercury, carbon, and mercury isotope profiles for the Lake Malaya Chabyda sediment core. The horizontal black lines represent the boundaries between the three units.</figcaption></figure>

### 3.1. Hg Accumulation and Deposition

Incorporation and preservation of Hg in lake sediments are dependent on many variables including atmospheric circulation of Hg, climate variables (e.g., precipitation rates), lake catchment properties (e.g., soil erosion rates, vegetation cover, and mineralogical characteristics), and lake properties (e.g., rates of primary productivity, lake morphology, and inflow and outflow characteristics) (Dastoor et al., 2022; Deison et al., 2012; He et al., 2007; Jiskra et al., 2022; Muir et al., 2009; Sanei et al., 2014). Lake sediments accumulate Hg from both direct deposition onto the lake's surface and from Hg deposited in the lake's watershed. Hg deposited in the lake's watershed can then be carried to the lake through runoff from the surrounding catchment area. Some Hg deposited on the lake and to the catchment will be lost to Hg re-emission (Jiskra et al., 2022; Sanei et al., 2014; Semkin et al., 2005). Re-emission and Hg loss can occur through biotic, abiotic, and photo-reduction of Hg¹¹ and Hg⁰ evasion to the atmosphere. Depending on catchment conditions, some Hg is sequestered in soils over millennia, thereby escaping or delaying transport to the lake (Semkin et al., 2005). Results from the Lake Malaya Chabyda Hg analyses can clarify possible correlations between Hg variables and changes in lake and watershed conditions related to climatic and landscape trends.

THg was lowest in Unit 1 (Figure 2a, bottom unit, average 11.6 ng g⁻¹), and gradually increased through Units 2 and 3 to a maximum of 60 ng g⁻¹ in the top unit (Unit 3 average = 43 ng g⁻¹). The THg concentration values observed in this sediment core are comparable to values reported in other studies on sediment cores from Arctic and sub-arctic lakes, which range from 10 to 130 ng g⁻¹ (He et al., 2007; Jiang et al., 2011; Lockhart et al., 1998;
<caption>**Table 2**</caption>

Spearman Correlation Coefficient ($r_s$) and p Values Between Total Mercury Concentrations, Total Organic Carbon Concentration, and Selected Elements and Element Ratios From Hughes-Allen et al. (2021)

<table><thead><tr><th></th><th></th><th>TOC (wt%)</th><th>MAR (g cm<sup>-2</sup> y<sup>-1</sup>)</th><th>Rb (CLR)</th><th>Zr (CLR)</th><th>Si (CLR)</th><th>Ti (CLR)</th><th>Fe (CLR)</th><th>Mn (CLR)</th><th>Br (CLR)</th><th>Fe/Ti (ALR)</th><th>Mn/Fe (ALR)</th></tr></thead><tbody><tr><td rowspan="2">THg</td><td>r<sub>s</sub></td><td><strong>0.68</strong></td><td><strong>-0.63</strong></td><td>-0.27</td><td>-0.16</td><td>0.01</td><td><strong>0.50</strong></td><td>-0.23</td><td><strong>-0.5</strong></td><td><strong>-0.54</strong></td><td><strong>-0.41</strong></td><td><strong>-0.38</strong></td></tr><tr><td>p value</td><td><strong>&lt;0.01</strong></td><td><strong>&lt;0.01</strong></td><td>0.02</td><td>0.17</td><td>0.93</td><td><strong>&lt;0.01</strong></td><td>0.05</td><td><strong>&lt;0.01</strong></td><td><strong>&lt;0.01</strong></td><td><strong>&lt;0.01</strong></td><td><strong>&lt;0.01</strong></td></tr><tr><td rowspan="2">TOC</td><td>r<sub>s</sub></td><td><strong>-0.51</strong></td><td>-0.34</td><td>-0.38</td><td><strong>-0.53</strong></td><td>0.22</td><td>0.05</td><td>-0.34</td><td>-0.29</td><td>-0.05</td><td>-0.37</td></tr><tr><td>p value</td><td><strong>&lt;0.01</strong></td><td>0.005</td><td>0.002</td><td><strong>&lt;0.01</strong></td><td>0.07</td><td>0.70</td><td>0.004</td><td>0.02</td><td>0.66</td><td>0.002</td></tr></tbody></table>

Note. Statistically significant relationships are in bold.

Muir et al., 2009). Because the top core with recent anthropogenically impacted layers was partly missing, a precise age cannot be attributed to the highest, upper-level Hg concentrations. Many studies have demonstrated a positive relationship between THg and algal-derived OC, often referred to as the “algal scavenging hypothesis,” indicating a link between aquatic primary productivity and Hg deposition (Burke et al., 2018; Deison et al., 2012; Outridge et al., 2007; Sanei et al., 2014). Algal scavenging is the absorption and adsorption by algae of Hg in the water column and its subsequent sequestration to lake sediments during sedimentation (Burke et al., 2018; Deison et al., 2012; Grigal, 2002; Kainz & Lucotte, 2006). In the Lake Malaya Chabyda sediment core, there is a statistical correlation ($r_s = 0.68; p < 0.01$) between THg and TOC (wt%) in Units 1 and 2, into the mid-Holocene (Table 2; Figure 3). Consequently, the HgAR profile closely follows the OCAR profile until a divergence between the two profiles in the second part of the Holocene (from approximately 4,000 cal BP; Figures 2d and 2e). After approximately 4,000 cal BP, there is an increase in Hg variables (THg, THg/TOC (wt%)), and HgAR that is not apparent in OCAR, indicating a possible shift in catchment input and/or net atmospheric deposition of Hg. At the same time, $\delta^{13}$C shifts to more negative values by -2 per mil.

Proxy analyses (i.e., TOC (wt%) to TN (wt%) ratio and TOC/TN<sub>atomic</sub> to $\delta^{13}$C ratio) indicate that carbon input during deposition of Unit 1 was dominated by terrestrial vegetation from the lake catchment (Unit 1 TOC (wt%) average = 12%) (Hughes-Allen et al., 2021) (Figures 2f and 2g). Climatic conditions during this time were cold and dry, with severe winters and prolonged ice cover on lakes (Biskaborn et al., 2012, 2016; Kostrova et al., 2021). During this time, primary productivity in the lake was likely low and the lake received higher mineral detrital input compared to Unit 2 and Unit 3 as indicated by analysis of detrital elements Rb, Zr, Si, and Ti (Hughes-Allen et al., 2021). It is probable that low primary productivity within the lake and high detrital input contributed to low levels of Hg scavenging and subsequent deposition, resulting in relatively low THg concentrations in Unit 1. HgAR in Unit 1 decreases from a moderately high value (~8.0 HgAR $\mu g m^{-2} y^{-1}$) at the bottom of the core to a low point (~3.0 HgAR $\mu g m^{-2} y^{-1}$) at the bottom of Unit 2, following the same general trend as OCAR. The THg/TOC (wt%) ratio is lowest for Units 1 and 2 (Figure 2).

Carbon input, based on $\delta^{13}$C and TOC/TN ratios, shifts to lacustrine dominated after 12,300 cal BP (bottom of Unit 2), indicating an overall increase in primary productivity within the lake. TOC (wt%) averages of Unit 2 and Unit 3 are consistent with lacustrine dominated organic matter (Unit 2 TOC average = 31 wt%; Unit 3 TOC average = 34 wt%), rather than terrestrial vegetation like in Unit 1. Increases in aquatic primary productivity likely led to amplification of algal scavenging and the observed overall higher values of THg in Unit 2 and Unit 3 compared to Unit 1 (Burke et al., 2018; Deison et al., 2012; Sanei et al., 2014). The THg/TOC ratio increases steadily throughout Unit 2, remains constant in Unit 3 until approximately 4,000 cal BP, after which the ratio increases substantially and remains high to the top of the core (Figure 2c). HgAR also increases from the bottom of Unit 2 (~13,000 cal BP) until approximately 7,000 cal BP (Unit 3) (maximum value ~12.0 HgAR $\mu g m^{-2} y^{-1}$). An increase in primary productivity and OCAR is likely responsible for this trend. After ~7,000 cal BP, there is a decrease in HgAR until 4,100 cal BP where HgAR increases and stays above 8.0 $\mu g m^{-2} y^{-1}$ until the top of the core. Between 7,000 and 4,100 cal BP, Siberia experienced a shift from a relatively warm and wet climate to a cooler and drier climate (Anderson et al., 2002; Harding et al., 2020). These climatic changes could have led to changes in vegetation patterns, fire frequency, permafrost extent, and watershed erosion rates (Bezrukova et al., 2017; Biskaborn et al., 2012, 2016; Kostrova et al., 2021), all of which can affect the cycling and accumulation of mercury in the environment. The HgAR decrease between 7,000 and 4,100 cal BP is driven by a decrease in mass SR, but not by Hg or OC concentration, nor by OC dynamics, since concentrations and Hg/TOC
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Relationship between total mercury concentrations (ng g⁻¹) and total organic carbon concentration (wt%).</figcaption></figure>

ratio remain constant. Specifically, the decrease in precipitation and increase in aridity could have led to less soil erosion and a decrease in detrital matter sedimentation in the lake.

There is substantial variation in the rate of Hg delivery over time as reflected in the HgAR profile for Lake Malaya Chabyda. HgAR varies from 1.6 to 11 µg m⁻² yr⁻¹, with a full core mean of 6.9 ± 2.3 µg m⁻² yr⁻¹ (Figure 2d). These values are similar to late Holocene HgAR of 4.8 µg m⁻² yr⁻¹ in a Svalbard sediment core (Jiang et al., 2011) but much higher than Holocene pre-1450 AD HgAR of <1 µg m⁻² yr⁻¹ observed in a ¹⁴C dated sediment core from the Canadian High Arctic at 70.5°N (Cooke et al., 2011). HgARs have been shown to scale linearly with catchment-to-lake area ratio (Cooke et al., 2019). Proxy analyses (e.g., δ¹³C) suggest that Lake Malaya Chabyda could have undergone periods of substantial deepening and shallowing, which were likely accompanied by changes in the lake surface area and therefore the watershed-to-lake area ratio (Figure 2f) (Hughes-Allen et al., 2021). δ¹³C values in lakes have been found to vary inversely with water depth and/or more directly with primary productivity (Meyers, 2003). Therefore, it is possible that Lake Malaya Chabyda experienced lake deepening in the first half of Unit 2, followed by a decrease in lake depth in the second half of Unit 2. Primary productivity could also have increased in Lake Malaya Chabyda throughout Unit 2 (see Hughes-Allen et al., 2021 for a full explanation). In the following we explore if Hg stable isotope signatures provide additional information on these environmental changes, and on Hg and OC cycling in the watershed.

## 3.2. Understanding Changes in Atmospheric Hg Sources Using Δ²⁰⁰Hg

The Hg isotope trends observed throughout the core can represent changes in Hg source inputs to the Lake Malaya Chabyda in addition to changes in the processes related to in-lake and in-watershed Hg transformations (Jiskra et al., 2021; Sun et al., 2022; Yin et al., 2016). Δ²⁰⁰Hg in particular can provide information on the relative rates of Hg¹¹ deposition via dust or rainfall versus Hg deposition via Hg⁰ uptake by vegetation, or in-lake Hg⁰ uptake via gas exchange. Figure 4 summarizes Malaya Chabyda sediment data in δ²⁰²Hg versus Δ²⁰⁰Hg space relative to the Hg isotope signatures of atmospheric and

watershed inputs in boreal and Arctic ecosystems. The Hg¹¹ wet deposition endmember for the northern hemisphere has a positive Δ²⁰⁰Hg of 0.14 ‰ (quartile range, QR, 0.09‰–0.20‰), while the Hg⁰ dry deposition endmember has a negative Δ²⁰⁰Hg of –0.05 (QR, –0.08‰ to –0.03 ‰) (Jiskra et al., 2022). Wet deposition refers to the process of Hg¹¹ being carried down from the atmosphere to the surface by precipitation such as rain, snow, and fog. Wet deposition can be an important source of Hg to aquatic ecosystems as it can directly deposit into rivers, lakes, and oceans, where it can accumulate in sediments and bioaccumulate in aquatic organisms. Dry deposition, on the other hand, refers to the process of Hg⁰ and Hg¹¹ being deposited from the atmosphere to the surface without the involvement of precipitation. This process occurs when Hg-containing particles and gases in the atmosphere settle onto the surface of the Earth through gravitational settling or turbulent mixing. Dry deposition can be influenced by various factors such as wind speed, particle size, and surface characteristics, but also vegetation uptake and can be an important source of Hg to terrestrial ecosystems (Cooke et al., 2019; Lee et al., 2021).

Average Δ²⁰⁰Hg in Unit 1 is 0.05‰ ± 0.02‰ (1σ, n = 5, Figure 2i), suggesting approximately equal contributions of atmospheric Hg⁰ and Hg¹¹ deposition (Figure 2i). There is a significant positive increase in Δ²⁰⁰Hg to 0.10‰ ± 0.06‰ (1σ, n = 5) in Unit 2 between 13,000 cal BP and 11,500 cal BP (maximum value of 0.16‰ at 523 cm depth (~11,300 cal BP)) (Figure 2i). The positive Δ²⁰⁰Hg excursion suggests dominant (73% ± 21%) Hg¹¹ deposition to the lake and coincides with the timing of the YD, which occurred between 12,900 and 11,700 cal BP in Eastern Siberia (Anderson et al., 2002; Andreev et al., 1997; Kokorowski et al., 2008; Velichko et al., 1997). During the YD, Siberia experienced a period of abrupt cooling and drying. The climate was characterized by cold
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Comparison of Malaya Chabyda sediment core Hg isotope signatures to published data on atmospheric Hg⁰ and Hg¹¹ deposition, boreal river runoff and boreal vegetation and soils. The broad positioning of the sediment core in the center of atmospheric input and river runoff Hg inputs illustrates the likely contribution of these Hg sources to the lake sediments. Literature data: North American (N-America) boreal rivers (Woerndle et al., 2018); Alaska (AK) tundra vegetation (Obrist et al., 2017; Olson et al., 2019); Alaska soils (Obrist et al., 2017); Northern hemisphere Hg⁰ and Hg¹¹ complied by Jiskra et al. (2021); Yenisei river (Araujo et al., 2022).</figcaption></figure>

and arid conditions, with temperatures dropping by as much as 10–15°C accompanied by a substantial decrease in precipitation. The positive shift in Δ²⁰⁰Hg toward the Hg¹¹ endmember is therefore more likely to reflect enhanced dry Hg¹¹ deposition than Hg¹¹ wet deposition. Studies suggest that lake levels in eastern Siberia were low during the YD, indicating a decrease in moisture availability (Anderson et al., 2002; Biskaborn et al., 2023; Velichko et al., 1997). This may have been due to changes in atmospheric circulation patterns or a reduction in the amount of moisture transported from the oceans (Andreev et al., 2003; Cheng et al., 2020). One possible explanation for elevated Δ²⁰⁰Hg during the YD in Siberia is changes in atmospheric circulation patterns, precipitation regimes, and vegetation Hg⁰ uptake. Grassland ecosystems expanded and forests retreated during the YD due to the persistent cold and dry conditions (Andreev et al., 2003; Bezrukova et al., 2017; Biskaborn et al., 2012, 2016). These changes in vegetation cover could have affected the cycling of Hg in the environment by altering patterns of wet and dry deposition. For example, changes in wind patterns and precipitation regimes could have led to more frequent dust storms and dry deposition of Hg¹¹, leading to an increase in the sedimentary Δ²⁰⁰Hg signature. In addition, changes in vegetation cover, lower primary productivity, and therefore lower foliar Hg⁰ uptake would also drive lake Hg deposition to the higher Hg¹¹ endmember Δ²⁰⁰Hg signature.

Δ²⁰⁰Hg values decrease in Unit 2 after 11,500 cal BP to a low value of 0.0‰ at 10,500 cal BP. Δ²⁰⁰Hg values remain near 0.0‰ until the bottom of Unit 3 (9,100 cal BP), where they start to increase (Figure 2i). Between 11,000 and 9,500 cal BP, the climate in Siberia was transitioning from the cold and dry conditions of the YD period to the warmer and wetter conditions of the early Holocene (Anderson et al., 2002; Andreev et al., 1997; Kokorowski et al., 2008; Velichko et al., 1997). This period was characterized by a gradual warming trend with intermittent cooling events. The vegetation in Siberia during this time was shifting from steppe tundra to boreal forest (Bezrukova et al., 2017; Kostrova et al., 2021). Overall, this was a period of significant climatic and ecological change in Siberia (Courtin et al., 2021). Between 8,500 cal BP and 4,100 cal BP, Δ²⁰⁰Hg values return to mixed Hg⁰ and Hg¹¹ inputs with 0.07‰ ± 0.11‰ (Figure 2i). Siberia experienced a period of warming known as the Holocene Thermal Maximum between approximately 9,000 cal BP and 5,000 cal BP. This period was characterized by higher temperatures and increased humidity compared to the preceding period, which allowed for the expansion of forests in the region (Bezrukova et al., 2017; Kostrova et al., 2021; Müller et al., 2009; Nazarova et al., 2013). The overall climate was warmer and wetter than the previous period, which may have contributed to the stabilized Δ²⁰⁰Hg values in the sediment core. After 4,000 cal BP, Δ²⁰⁰Hg values decrease substantially until the top of the sediment core (Figure 2i). The shift toward negative Δ²⁰⁰Hg values and the Hg⁰ end member at the top of the core could indicate a higher relative proportion of Hg⁰ inputs (77% ± 14%), possibly from watershed Hg inputs resulting from vegetation Hg⁰ uptake. In that case, we would expect a concomitant decrease in δ²⁰²Hg because vegetation preferentially takes up light Hg⁰, which are transferred to soil and soil runoff. Figure 2j shows this is not the case, indicating that the Hg⁰ source might be direct lake water uptake of Hg⁰ (with low Δ²⁰⁰Hg) related to enhanced algal primary productivity.

### 3.3. Processes and Trends Related to Odd Hg-Isotope Mass Independent Fractionation

Odd Hg isotope MIF signatures, for example, Δ¹⁹⁹Hg, are mainly produced during photo-reduction and photo-degradation of Hg¹¹ and monomethylmercury (MeHg¹¹), respectively. Similar to Δ²⁰⁰Hg, atmospheric deposition of Hg has contrasting odd-MIF in Hg¹¹ (median 0.37‰, QR 0.18 to 0.52), and in Hg⁰ (median -0.20‰, QR -0.25 to -0.13) (Jiskra et al., 2021). In this sediment core, Δ¹⁹⁹Hg values decrease throughout the core from a peak value of 0.70‰ at 623 cm depth (~13,000 cal BP) to -0.15‰ near the top of the core (Figure 2h). Δ¹⁹⁹Hg signatures therefore do not closely follow Δ²⁰⁰Hg and the relative proportion of atmospheric Hg deposition pathways. We estimate that post-depositional odd-MIF has shifted Δ¹⁹⁹Hg by 0.50‰ ± 0.12‰ in Unit 1 (SI). Assuming an aqueous Hg¹¹ MIF factor, E¹⁹⁹Hg of -2.75‰ ± 0.14‰ (Sun et al., 2019), this suggests that approximately 17% ± 4% of Hg in the lake was lost to the atmosphere by photochemical reduction and evasion.
The positive Δ¹⁹⁹Hg values between 14,100 cal BP and 11,000 cal BP might therefore be related to enhanced photoreduction of Hg¹¹ in Lake Malaya Chabyda. Based on other proxy analyses (Hughes-Allen et al., 2021), it is likely that during this time, Lake Malaya Chabyda was a nascent lake with relatively shallow waters. Limited primary productivity in the lake as shown by TOC (wt%), TOC/TN<sub>atomic</sub>, and δ¹³C records would have resulted in clear waters, which might have enhanced Hg¹¹ photoreduction. Δ¹⁹⁹Hg decreases relatively consistently throughout Unit 2, likely indicating increased lake development, more primary productivity in the lake compared to Unit 1 and a more opaque, turbid water column. Δ¹⁹⁹Hg values are relatively low and stable throughout Unit 3, suggesting more consistent lake conditions and depth.

### 3.4. Processes and Trends Related to Mass-Dependent Fractionation

δ²⁰²Hg is related to several processes such as changes in Hg atmospheric source, photoreduction, algal and/or vegetation uptake, remineralization, and diagenesis (Jiskra et al., 2021; Lee et al., 2021). Values of δ²⁰²Hg ranged from -1.75 to -0.5 (‰) in the sediment core (Figure 2j; Figure 4). Unit 1 and Unit 2 have similar average δ²⁰²Hg values (Unit 1 = -1.41; Unit 2 = -1.38), while Unit 3 has substantially higher δ²⁰²Hg values (Unit 3 = -0.83) (Figure 2). Negative δ²⁰²Hg values near -1.50 typically indicate soil Hg input (Lee et al., 2021), due to preferential foliar uptake of light Hg⁰ isotopes in watershed vegetation (Demers et al., 2013; Enrico et al., 2016). The positive shift in δ²⁰²Hg at 10,000 cal BP then possibly reflects diminished watershed Hg inputs and relatively more direct atmospheric Hg⁰ and Hg¹¹ deposition to the lake. Direct gaseous Hg⁰ uptake by waterbodies, such as the ocean (Jiskra et al., 2021), is not associated with the strong light Hg isotope enrichment observed during foliar Hg⁰ uptake (Zhou et al., 2021).

### 4. Conclusions

Analysis of the sediment record from Lake Malaya Chabyda at 62°N provides valuable insights into the accumulation rates and dynamics of mercury (Hg) over the past 14,000 years. The average Hg accumulation rates (HgAR) in the sediment were consistently high at 6.9 ± 2.3 µg m⁻² yr⁻¹, primarily driven by the high rates of OC net burial. Within this period, HgAR exhibited a three-fold variation due to multiple factors, including climate change, watershed erosion, and variations in both in-lake and watershed primary productivity. Strong correlation between sediment Hg concentration and TOC was observed, indicating that algal scavenging likely controls the Hg concentration in Lake Malaya Chabyda. Additionally, during the YD period (12,900 to 11,700 cal BP in Eastern Siberia), characterized by cold climatic conditions and minimal lake level, both HgAR and OCAR were lower, suggesting reduced primary productivity in the lake and watershed.

The elevated Δ²⁰⁰Hg values observed during the YD further support the dryer and colder conditions, indicating lower atmospheric Hg⁰ deposition and reduced primary productivity in the lake and its watershed. Similarly, increased sediment Δ¹⁹⁹Hg values during deglaciation and the YD suggest enhanced photo-reductive lake Hg re-emission, facilitated by clearer waters and lower lake primary productivity. During the early Holocene, we observed a decrease in Δ²⁰⁰Hg and Δ¹⁹⁹Hg values, coupled with an increase in δ¹³C, δ²⁰²Hg, OCAR, and HgAR, indicating enhanced algal primary productivity and subsequent OC and Hg sequestration. From 4,100 cal BP to the present day, Hg/OC ratios and HgAR continued to increase, while OCAR remained constant, suggesting an additional Hg source to the lake. The shift to negative Δ²⁰⁰Hg values during this period indicates a larger contribution of Hg⁰ to Hg deposition, potentially driven by increased primary production and efficient Hg burial.

In summary, our findings suggest that gradual climate warming since the Last Glacial Termination has led to enhanced OC and Hg burial in northern lakes and watersheds. The late Holocene period witnessed increased Hg burial, possibly attributed to renewed increases in lake primary productivity. Furthermore, the high watershed to lake area ratio of Lake Malaya Chabyda, along with interactions with nearby Lake Ulakhan Chabyda, likely influenced the deposition of Hg isotopes. In a continued climate warming scenario, it is likely that boreal and Arctic aquatic ecosystems will experience further Hg sequestration in the future.

### Data Availability Statement

Data are freely available in the Mendeley data repository at https://data.mendeley.com/preview/wbrxbydwds?&a=84d5816f-c60c-4447-9817-b1db9ece08c3. Data are also available in Supplementary Table 1.
Acknowledgments

This work was supported by public funds received in the framework of GEOSUDDINAMIS Data TERRA, a project (ANR-10-EQPX-20) of the program "Investissements d'Avenir" (PIA) managed by the French National Research Agency. Funding was also provided to the project PERmafrost and Greenhouse gas dynamics in Siberia (PEGS) of the PIA (ANR-17-MPGA-0014) through the Make Our Planet Great Again (MOPGA) initiative. We thank Jerome Chmeleff for expert management of the OMP mass spectrometry facilities in Toulouse, France. SMC acknowledges her PhD scholarship and a travel grant from the University of Ferrara, Italy.

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