# A peat core Hg stable isotope reconstruction of Holocene atmospheric Hg deposition at Amsterdam Island (37.8oS)

Chuxian Li, Maxime Enrico, Oliver Magand, Beatriz F Araujo, Gaël Le Roux, Stefan Osterwalder, Aurélien Dommergue, Yann Bertrand, Jérôme Brioude, François de Vleeschouwer, et al.

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Chuxian Li, Maxime Enrico, Oliver Magand, Beatriz F Araujo, Gaël Le Roux, et al. A peat core Hg stable isotope reconstruction of Holocene atmospheric Hg deposition at Amsterdam Island (37.8oS). Geochimica et Cosmochimica Acta, 2022, 341, pp.62 - 74. 10.1016/j.gca.2022.11.024. hal-03906056

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# A peat core Hg stable isotope reconstruction of Holocene atmospheric Hg deposition at Amsterdam Island (37.8°S)

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Chuxian Li<sup>a,b,c,*</sup>, Maxime Enrico<sup>a,b,d,1</sup>, Oliver Magand<sup>e</sup>, Beatriz F. Araujo<sup>b</sup>, Gaël Le Roux<sup>a</sup>, Stefan Osterwalder<sup>f</sup>, Aurélien Dommergue<sup>e</sup>, Yann Bertrand<sup>e</sup>, Jérôme Brioude<sup>g</sup>, François De Vleeschouwer<sup>h</sup>, Jeroen E. Sonke<sup>b</sup>

## ARTICLE INFO

**Article history:**

Received 16 November 2021

Accepted 22 November 2022

Available online 23 November 2022

Associate editor: Dominik Weiss

**Keywords:**

Hg stable isotopes

Southern Hemisphere

Peat

Rain

Hg deposition

## ABSTRACT

Mercury (Hg) stable isotopes have been broadly used to investigate the sources, transformation and deposition of atmospheric Hg during the industrial era thanks to the multiple isotope signatures deriving from mass-dependent (represented by δ²⁰²Hg) and mass-independent fractionation (represented by Δ⁵⁵⁵Hg) in the environment. Less is known about the impact of past climate change on atmospheric Hg deposition and cycling, and whether Hg isotopes covary with past climate. Here, we investigate Hg concentration and Hg isotope signatures in a 6600-year-old ombrotrophic peat record from Amsterdam Island (AMS, 37.8°S), and in modern AMS rainfall and gaseous elemental Hg (Hg⁰) samples. Results show that Holocene atmospheric Hg deposition and plant Hg uptake covary with dust deposition, and are both lower under a high humidity regime associated with enhanced Southern Westerly Winds. Modern AMS gaseous Hg⁰ and rainfall Hg¹¹ isotope signatures are similar to those in the Northern Hemisphere (NH). Holocene peat Δ¹⁹⁹Hg and Δ²⁰⁰Hg are significantly correlated (R² = 0.67, P < 0.001, n = 58), consistently oscillating between the modern Hg⁰ and rainfall Hg¹¹ end-members. Peat Δ²⁰⁰Hg and Δ¹⁹⁹Hg provide evidence of plant uptake of Hg⁰ as the dominant pathway of Hg deposition to AMS peatland, with some exceptions during humid periods. In contrast to NH archives generally documenting a modern increase in Δ¹⁹⁹Hg, recent peat layers (post-1900CE) from AMS show the lowest Δ¹⁹⁹Hg in the peat profile (−0.42 ± 0.27 ‰, 1σ, n = 8). This likely reflects a significant change in the post-depositional process on deposited anthropogenic Hg in 20th century (i.e. dark abiotic reduction), enabling more negative Δ¹⁹⁹Hg to be observed in AMS peat. We further find that the oscillations of Hg isotopes are consistent with established Holocene climate variability from dust proxies. We suggest peat Hg isotope records might be a valid rainfall indicator.

© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Mercury (Hg) is a global pollutant and its organic form, methylmercury, is a potent neurotoxin that harms humans (Roman et al., 2011), fish (Gentès et al., 2015) and wildlife

(Scheuhammer et al., 2007). Hg released from anthropogenic Hg emissions is currently estimated to be 2400 Mg yr⁻<fnref n="1" /> (e.g., coal combustion and mining, Streets et al., 2017), which is seven times higher than primary natural emissions 340 Mg yr⁻<fnref n="1" /> (i.e. from volcanic eruptions and degassing of the Earth's crust, Li et al., 2020a). Hg is emitted to the atmosphere mainly in its gaseous elemental form (GEM or Hg⁰, ~90%, Horowitz et al., 2017). Hg⁰ deposition over the Earth's surface predominantly occurs by vegetation uptake of Hg⁰ (dry deposition, Zhou et al., 2021a), and Hg⁰ gas exchange with aqueous water bodies including oceans (Jiskra

<footnote><sup>a</sup>Laboratoire Ecologie Fonctionnelle et Environnement, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France</footnote>

<footnote><sup>b</sup>Laboratoire Géosciences Environnement Toulouse, Université de Toulouse, CNRS, IRD, UPS, Toulouse, France</footnote>

<footnote><sup>c</sup>Department of Forest Ecology & Management, Swedish University of Agricultural Sciences, 90136 Umeå, Sweden</footnote>

<footnote><sup>d</sup>Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, TOTAL, LFCR, IPREM, 64000 Pau, France</footnote>

<footnote><sup>e</sup>Institut des Géosciences de l'Environnement, Université Grenoble Alpes, CNRS, IRD, Grenoble INP, 38400 Grenoble, France</footnote>

<footnote><sup>f</sup>Institute of Agricultural Sciences, ETH Zurich, 8092 Zurich, Switzerland</footnote>

<footnote><sup>g</sup>Laboratoire de l'Atmosphère et des Cyclones, UMR8105, CNRS, Université de La Réunion, 97744 Saint-Denis, France</footnote>

<footnote><sup>h</sup>Instituto Franco-Argentino para el Estudio del Clima y sus Impactos (UMI 3351 IFAECI/CNRS-CONICET-UBA-IRD), Universidad de Buenos Aires, Argentina</footnote>

<footnote>* Corresponding author at: Laboratoire Ecologie Fonctionnelle et Environnement, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France.</footnote>

<footnote>E-mail address: chuxian.li@slu.se (C. Li).</footnote>

<footnote n="1">¹ These authors contributed equally.</footnote>
et al., 2021). Hg⁰ can also be oxidized to gaseous and Hg'' forms that are scavenged by cloud droplets or bind to aerosols, forming particulate-bound Hg. The different cloud, aerosol and gaseous Hg'' forms can be rapidly removed from the atmosphere by rainfall, gravitational and dry deposition respectively (Fitzgerald and Lamborg, 2003). Atmospheric Hg deposition to terrestrial surfaces varies with plant species and vegetation cover (Jiskra et al., 2018), wind-blown dust fertilization (Vandal et al., 1993) and climate (e.g., solar radiation, temperature, wind speed and rainfall, Lindberg et al., 2002; Jitaru et al., 2009; Jiskra et al., 2018; Jiskra et al., 2021; Vandal et al., 1993; Sprovieri et al., 2017).

Vegetation uptake of Hg⁰ accounts for 60–90% of terrestrial Hg deposition and annually sequesters 30–70% of the modern global atmospheric Hg pool (Outridge et al., 2018; Zhou et al., 2021a; Obrist et al., 2021). The vegetation Hg pool in the Northern Hemisphere (NH) is approximately twice that in the Southern Hemisphere (SH), due to a twice larger land cover in the NH compared to the SH under similar annual climate conditions in both hemispheres (e.g., annual cloud cover, rainfall, temperature, Li et al., 2020a). Annual Hg wet deposition rate (µg m⁻² yr⁻¹) is positively correlated with precipitation rate (mm yr⁻¹) in the NH, while this relation is less obvious in the SH (Sprovieri et al., 2017), in part due to sparser observational data. In addition, while great efforts have been put in making modern Hg observations and link trends to climate change factors (e.g., Jiskra et al., 2018; Zhou et al., 2021a; Obrist et al., 2017; Lepak et al., 2020; Wang et al., 2020), much less is known about the long-term, Holocene Hg dynamics and its interaction with past climate (Vandal et al., 1993; Jitaru et al., 2009; Sial et al., 2016; Gleason et al., 2017; Jiskra et al., 2022).

Recent work has suggested that the stable isotope composition of Hg has potential to quantify Hg deposition pathways in terrestrial (e.g., peat soils, Enrico et al., 2016) and aquatic environments (e.g., sea water, Jiskra et al., 2021). Jiskra et al., (2022) further reveals a strong link between lake sediment Hg stable isotopes and Holocene wet climate in the Southern tropical Andes, demonstrating Hg isotope signatures as a new proxy to understand paleoclimate. Hg has seven stable isotopes which can undergo mass dependent fractionation (MDF) and mass independent fractionation (MIF) in the environment (Sonke, 2011; Blum et al., 2014). MDF (represented by δ²⁰²Hg) is common and occurs during all physical, chemical and biotic Hg transformations (Estrade et al., 2009; Kritee et al., 2013; Zheng et al., 2019; Demers et al., 2013; Enrico et al., 2016). MIF of odd Hg isotopes (represented by Δ¹⁹⁹Hg and Δ²⁰¹Hg) occurs mainly during aqueous photochemical Hg reduction by the magnetic isotope effect and to a much lesser extent by nuclear volume fractionation (Zheng and Hintelmann, 2010; Motta et al., 2020). Hg photoreduction has been shown to produce MIF in terrestrial foliage (Yuan et al., 2019), but no significant odd MIF is observed in biological Hg transformations (Kritee et al., 2007).

MIF of even Hg isotopes (reported as Δ²⁰⁰Hg and Δ²⁰⁴Hg) is found to be relatively conservative at the Earth's surface. Currently, even though the mechanisms leading to even MIF still remain unclear, some studies suggest it as a result of Hg⁰ photo-oxidation in the presence of aerosols above the tropopause (Chen et al., 2012), or Hg'' photoreduction on aerosols involving magnetic halogen nuclei and a molecular magnetic isotope effect (Fu et al., 2021). In general, background Hg⁰ is characterized by slightly negative Δ²⁰⁰Hg (−0.06 ± 0.02 ‰, 1σ, n = 71) and positive Δ²⁰⁴Hg (0.07 ± 0.05 ‰, 1σ, n = 32, Sherman et al., 2010; Gratz et al., 2010; Demers et al., 2013; Enrico et al., 2016; Fu et al., 2016a, b; Obrist et al., 2017). Different from Hg⁰, rainfall Hg'' is characterized by positive Δ²⁰⁰Hg (0.16 ± 0.07 ‰, 1σ, n = 55) and negative Δ²⁰⁴Hg (−0.16 ± 0.17 ‰, 1σ, n = 17, Gratz et al., 2010; Demers et al., 2013; Donovan et al., 2013; Enrico et al., 2016; Obrist et al., 2017; Motta

et al., 2019; Washburn et al., 2021; Fu et al., 2021). The different even Hg MIF signatures in Hg⁰ and rainfall allow the quantification of the two atmospheric Hg deposition pathways (Enrico et al., 2016; Jiskra et al., 2021).

Peat, formed by decomposed organic matter, sequesters Hg mainly by vegetation Hg⁰ uptake and by rainfall Hg input (Enrico et al., 2016). Studies have demonstrated that Hg stable isotopes in peat cores can serve as proxies to quantify atmospheric Hg dry and wet deposition (Enrico et al., 2016), to track anthropogenic Hg pollution (Shi et al., 2011), to reconstruct past atmospheric Hg⁰ concentrations (Enrico et al., 2017), to understand MIF mechanisms (e.g., magnetic isotope effect, Ghosh et al., 2008), and to identify Hg sources during springtime atmospheric mercury depletion events (Douglas and Blum, 2019). We hypothesize that variations of even-Hg MIF during the Holocene epoch reflects variable vegetation Hg⁰ uptake and Hg'' deposition and relates to climate variables such as precipitation regimes. In this study, we use a 6600 years old peat record from Amsterdam Island (AMS) to investigate this potential use of Hg stable isotopes as a climate proxy. This site was already characterized for dust flux, which was previously found to be closely associated to climate variability (Li et al., 2020b). We combine new observations of Hg stable isotope characteristics of the modern Southern Hemisphere (SH) atmospheric Hg pool (i.e. Hg⁰ and rainfall) to constrain Hg deposition mechanisms over the AMS peatland. We discuss the possible biases related to Hg cycling in peatland ecosystems, including non-photochemical and photochemical post-depositional Hg transformations. The objectives of this study are to: (i) investigate for the first time SH isotopic characterization of the different atmospheric Hg species (i.e. gaseous Hg⁰ and rainfall Hg''), (ii) explore the first SH mid-latitude peat Hg stable isotope variability in the Holocene and trace peat Hg cycling (i.e. deposition and potential mobility), and (iii) reconstruct Holocene rainfall based on Hg isotope signatures and compare the reconstructed rainfall profile to dust-flux-based climate dynamics.

## 2. Methods and materials

### 2.1. Study site, peat sampling and age information

AMS is a remote volcanic island located at the Southern Indian Ocean halfway between South Africa and Australia (37.8°S, 77.5°E, Fig. 1A). AMS rose up from the Ocean before 700 kyr BP. The island is ca. 9.2 km long and 7.4 km wide, which has an elliptic surface area of 55 km² (Frenot and Valleix, 1990) (Fig. 1B). Most of the coastline is characterized by vertical cliffs (Doucet et al., 2004). Mild oceanic conditions prevail with annual temperature of 14 °C and annual precipitation of 1100 mm (Lebouvier and Frenot, 2007). AMS is at the northern margin of the SWW, characterized by winds dominantly from west and northwest with an average speed of 7.4 m s⁻¹ (Frenot and Valleix, 1990). Air flow from continental regions is favored in austral late winter and early spring (Moody et al., 1991; Angot et al., 2014). More rainfall occurs in austral winter with an average 23 days per month of rain when the SWW seasonally shift equatorward, while in austral summer the rainfall frequency drops to 18 days per month when the SWW move poleward (Moody et al., 1991). AMS has peat deposits, with a large portion located in the caldeira at a height of 771 m a.s.l.. The 5 m long peat sequence used in this study was sampled at the caldeira peatland (Fig. 1B), which is dominated by brown moss species and *Sphagnum*. Peat core collection and age reconstruction are detailed in Li et al., (2017) and Li et al., (2020b). The last 150 years are established using a ²¹⁰Pb-based Constant Rate of Supply model coupled with four post-bomb dates (Li et al.,
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (A) Location of Amsterdam Island (AMS, red diamond); (B) Sampling sites of peat core (red star, at the caldeira), rainfall and gaseous Hg⁰ for isotope analysis (red triangle, at Pointe Benedicte). Black triangle at the top right represents the scientific base at AMS with 30 m above sea level. Pictures C, D and E show sampling conditions of gaseous Hg⁰, rainfall and peat, respectively. Photo credit to O. Magand, I. Jouvie (C, D) and F. De Vleeschouwer (E). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

2020b). This age model has been validated by $^{241}$Am and $^{137}$Cs chronomarkers (Li et al., 2017). The chronology of the deeper cores is reconstructed using $^{14}$C age-depth modeling (Li et al., 2020b). This island is largely free from human disturbance. The only potential local source of pollution on this small island is from the 30-member scientific base at "Martin-de-Viviès" at a height of 29 m a.s.l. We expect negligible Hg pollution from the base to our peat sampling site since it is located to the southwest of the base under eastward wind flow.

## 2.2. Hg⁰ sampling for isotope measurements

AMS atmospheric gaseous Hg⁰ concentrations are thought to represent the SH background level (Angot et al., 2014; Slemr et al., 2015, 2020; Sprovieri et al., 2016). Hg⁰ was collected for Hg concentration (Magand and Dommergue, 2021) and isotope analysis at the AMS Pointe Benedicte atmospheric research facility (55 m a.s.l.) located 2 km west from Martin-de-Viviès base (Fig. 1C). Hg⁰ for isotope measurement was sampled continuously at 1.0 l min⁻¹ on iodated activated carbon traps for periods of two weeks, following the published procedures (Fu et al., 2016a; Obrist et al., 2017). Twenty-five Hg⁰ samples were collected from December 2017 to November 2018.

## 2.3. Rain sampling for Hg isotope measurements

AMS rain samples dedicated to Hg isotope measurements were collected at the AMS Pointe Benedicte atmospheric research facility (55 m a.s.l.), in an open area away from any possible contamination sources (engines / generators / fires / dust sources) from December 2017 to 2019. The collection system, consisting of four acid-washed polypropylene buckets of 20 l each, was deployed according to the forecasted rainfall events and only when major precipitation was predicted or ongoing. Buckets were deployed and fixed on top of a large polypropylene box to avoid any splash from ground that would be susceptible to contaminating the collected samples. Buckets were covered with pre-cleaned mosquito nets to collect rainfall but preventing any entry of insects or coarse particles. Buckets were put outside between 6 and 24 h depending on rainfall intensity and the filling rate of the buckets. A minimum volume of 1 l per bucket was collected to for a decent Hg isotope analysis. Once collected, rain samples were transferred to PETG 2 l bottles, and acidified with ultra-clean bi-distilled hydrochloric acid (0.036 mol/L). PETG 2 l bottles were closed as tight as possible to avoid any gaseous Hg contamination during storage and transport until analysis. Bottles were packed in double-ziploc bags in a dry, dark and clean place until transport to France. Buckets and associated materials were carefully acid-cleaned and stored
between each rainfall sampling event. Note that rainfall collected using an Eigenbrodt collector for Hg wet deposition analysis is close to the base (Fig. 1D), instead of at Pointe Benedicte. We included Hg isotope signatures in rainfall samples from Maido at Reunion Island (MAI, 21.1°S, 55.4°E, 2160 m a.s.l.) and Tierra Del Fuego at the Ushuaia, Argentina (TDF, 54.8°S, 68.3°E, 272 m a.s.l.) for comparison. The methodology for rain sampling at MAI and TDF was identical to AMS.

## 2.4. Hg extraction and purification

Detailed information on peat sub-sampling and preparation have been shown in Li et al., (2020b). Briefly, cores were shipped by boat to France in +4 °C fridges. Cores were frozen before being sliced at roughly 1 cm resolution. Freeze-dried peat was analyzed for total Hg concentration (THg) on a Milestone Direct Mercury Analyzer (DMA80). Details of analytical performance on THg are summarized in Li et al., (2020a). Variable amounts of peat samples were used for extracting Hg for isotope measurement based on peat Hg concentration and optimum quantity of Hg required (10 ng) for a decent analysis. Hg from 58 peat samples was extracted using combustion and acid trapping methods (Sun et al., 2013). Briefly, we combusted peat samples in a double tube furnace at 1000 °C and trapped the released gaseous Hg⁰ with a 40% (v/v) inverse aqua regia solution (2:1 HNO₃:HCl). The solution was then determined for Hg concentration by cold vapor atomic fluorescence spectroscopy (CV-AFS). The extraction yields were in the range 83–116%. We observed a matrix effect on seven peat samples leading to lower than 50% Hg recoveries during Hg isotope analysis (samples AMS 1; 62; 134; 181; 189; 200; 244). We therefore further purified the Hg in 40% (v/v) inverse aqua regia solutions after oven combustion. The purification system consisted of a 250 ml glass vessel where we added 10 ml sample solution, 40 ml 10% SnCl₂ and 150 ml Milli-Q water. The sample solution was purged by bubbling Hg free argon (300 ml min⁻¹) for 1 h into a 15 ml falcon tube with again 40% (v/v) inverse aqua regia. The purification yields were 93 ± 17% (n = 11, including one NIST 1632-d Coal and three international standard NIST 3133).

## 2.5. Hg stable isotope measurements and analytical uncertainty

Following extraction, the Hg stable isotope compositions of 58 peat samples, 21 rain samples and 23 Hg⁰ samples were determined from 20% (v/v, diluted from 40%) inverse aqua regia solutions using cold-vapor multi-collector inductively coupled mass spectrometry (CV-MC-ICP-MS, Thermo-Finnigan Neptune, Midipyrenees Observatory, Toulouse, France). Sample isotopic ratios were corrected for mass bias by sample-standard bracketing using NIST 3133 (Blum and Bergquist, 2007). Results are reported as δ-values in per mil (‰) by referencing to NIST 3133, representing to Hg mass dependent fractionation (1).

$$ \delta^{XXX} \text{Hg} = \left\{ \left[ \left( \frac{^{XXX}\text{Hg}}{^{198}\text{Hg}} \right)_{\text{sample}} / \left( \frac{^{XXX}\text{Hg}}{^{198}\text{Hg}} \right)_{\text{NIST3133}} \right] - 1 \right\} * 1000 \quad (1) $$

MIF is calculated based on the deviations of δ-values from the theoretical MDF (2).

$$ \Delta^{XXX} \text{Hg} = \delta^{XXX} \text{Hg} - \beta * \delta^{202} \text{Hg} \quad (2) $$

where XXX represents for 199, 200, 201 and 204. Symbol β is 0.2520, 0.5024, 0.7520 and 1.493 for ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg and ²⁰⁴Hg, respectively.

The quality control of Hg isotope measurements is assessed by analyzing ETH-Fluka, UM-Almaden and procedural standards

(Coal, NIST 1632d, n = 4; Lichen, BCR482, n = 9). ETH-Fluka displayed δ²⁰²Hg and Δ¹⁹⁹Hg of -1.43 ± 0.15 ‰ (2σ, n = 15) and 0.05 ± 0.13 ‰ (2σ, n = 15), respectively. UM-Almaden showed δ²⁰²Hg and Δ¹⁹⁹Hg of -0.52 ± 0.13 ‰ (2σ, n = 7) and -0.03 ± 0.09 (2σ, n = 7), respectively. Hg isotopic signatures in procedural standards are reported in the Supplementary Information Table S1 for δ²⁰²Hg (maximum 2σ = 0.15 ‰), Δ¹⁹⁹Hg (maximum 2σ = 0.09 ‰), Δ²⁰⁰Hg (maximum 2σ = 0.10 ‰), Δ²⁰¹Hg (maximum 2σ = 0.12 ‰) and Δ²⁰⁴Hg (maximum 2σ = 0.26 ‰). In this study, maximum 2σ of Hg isotopes from procedural standards is used as 2σ of each sample. When it comes to summarizing a dataset (e.g., δ²⁰²Hg in all AMS Hg⁰), mean and 1σ of the dataset are reported (e.g., δ²⁰²Hg = 0.51 ± 0.16 ‰, 1σ, n = 23).

## 2.6. Hg isotope mass balance

Δ²⁰⁰Hg in modern atmospheric end-members (Hg⁰ and rainfall Hg¹¹) is thought to be conservative since no significant even MIF is measured during Hg transformations at the Earth's surface (e.g., Blum et al., 2014; Zheng et al., 2019). Although the even-Hg isotope mechanism and photo-induced redox reaction has not been identified, previous research suggested an origin at the tropopause (Chen et al., 2012). We also argue that Δ²⁰⁰Hg in both Hg⁰ and rainfall Hg¹¹ end-members is relatively constant from mid to late Holocene, which is based on the absence of dramatic climate events during the last 6600 years (e.g., glacial periods) that could greatly alter the chemical reactions in the upper atmosphere leading to significantly abnormal Δ²⁰⁰Hg production in the stratosphere (Chen et al., 2012; Fu et al., 2021). We therefore use Δ²⁰⁰Hg in modern AMS Hg⁰ and AMS rainfall Hg¹¹ to quantify the atmospheric Hg deposition pathways to peat, following the equations of (3) and (4) below.

$$ \Delta^{XXX} \text{Hg}_{\text{peat}} = \alpha * \Delta^{XXX} \text{Hg}_{\text{Hg0}} + \theta * \Delta^{XXX} \text{Hg}_{\text{Hg11}} \quad (3) $$

$$ \alpha + \theta = 1 \quad (4) $$

$$ \Delta^{199} \text{Hg}_{\text{peat-reconstructed}} = \Delta^{200} \text{Hg}_{\text{peat}} * \gamma \quad (5) $$

Symbols α and θ represent the proportion of Hg⁰ and rainfall Hg¹¹ deposition, respectively. Only the Δ²⁰⁰Hg of AMS rainfall with mean air pressure of trajectories above the marine boundary layers are considered. This is necessary because the site of rainfall sampling for Hg isotope analysis is at 55 m. a.s.l., while the peat sampling site is at 771 m a.s.l., which is above the marine boundary layer. We reconstructed the initially deposited Δ¹⁹⁹Hg to peatland, i.e. Δ¹⁹⁹Hg<sub>peat-reconstructed</sub>, using conservative Δ²⁰⁰Hg in peat following equation (5). Parameter γ is the Δ¹⁹⁹Hg/Δ²⁰⁰Hg slope in combined AMS Hg⁰ and rainfall Hg¹¹ data.

# 3. Results

## 3.1. Southern Hemisphere Hg⁰ and rainfall Hg¹¹ isotope signatures

Isotopic compositions of Hg⁰ samples from AMS and rainfall Hg¹¹ samples from AMS, MAI and TDF, are shown in Fig. 2. All of the AMS Hg0 samples are characterized by positive δ²⁰²Hg (0.51 ± 0.16 ‰, 1σ, n = 23) and negative Δ¹⁹⁹Hg (-0.28 ± 0.05 ‰, 1σ, n = 23, Fig. 2A; Supplementary Information Table S2). Slightly negative Δ²⁰⁰Hg was measured in AMS Hg⁰ (-0.06 ± 0.02 ‰, 1σ, n = 23, Fig. 2B). In contrast, Δ²⁰⁰Hg in SH rain samples was slightly positive with 0.08 ± 0.08 ‰ (1σ, n = 21). Compared to AMS Hg⁰, SH rainfall samples show lower, near-zero δ²⁰²Hg at AMS (0.20 ± 0.36 ‰, 1σ, n = 11), MAI (0.32 ± 0.53 ‰, 1σ, n = 3) and TDF (0.03 ± 0.41 ‰, 1σ, n = 7, Fig. 2A; Supplementary Information Table S2).
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (A) $\delta^{202}\text{Hg}$ vs $\Delta^{199}\text{Hg}$ and (B) $\delta^{202}\text{Hg}$ vs $\Delta^{200}\text{Hg}$ in AMS peat (squares), AMS $\text{Hg}^0$ (blue full triangles) and rainfall from AMS (red full dots), from Maido Observatory (MAI, La Reunion island; orange dots) and from Tierra Del Fuego (TDF, light orange dots). Peat Hg isotope signatures prior to 1450CE, between 1450CE and 1900CE, and post-1900CE, are shown in open squares, grey squares and dark squares, respectively. Red open dots represent the rainfall Hg isotope compositions from the remote areas of Northern Hemisphere (NH, refs see text), while blue open triangles represent those in the NH $\text{Hg}^0$ samples (refs see text). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Table S3). SH rainfall $\Delta^{199}\text{Hg}$ averages $0.21 \pm 0.42 \, \text{‰}$ (1σ, n = 21), with a wide range at TDF from -1.08 ‰ to 0.72 ‰.

## 3.2. Peat Hg accumulation rates and isotopic signatures

AMS peat Hg accumulation rates (HgAR) and Hg isotope variability are presented according to the period divisions on AMS dust flux established using change point analysis in Li et al., (2020b). The whole HgAR profile is thus divided into six sections: 4600BCE-4200BCE, 4200BCE-3000BCE, 3000BCE-1600BCE, 1600BCE-700BCE, 700BCE-600CE, and 600CE-present. The last period is further divided into three subsections: 600CE-1450CE, 1450CE-1900CE and post-1900CE based on the onset of large-scale Spanish mining and industrial activity (Amos et al., 2015). HgAR are calculated from peat Hg concentration (ng g⁻¹), density (g cm⁻³) and vertical accumulation rate (cm yr⁻¹), which have been detailed in Li et al., (2020a). Briefly, HgAR increase from $2.7 \pm 1.7 \ \mu\text{g m}^{-2} \text{yr}^{-1}$ (1σ, n = 84) during the natural background period of 4600BCE-1450CE, to $8.8 \pm 1.8 \ \mu\text{g m}^{-2} \text{yr}^{-1}$ (1σ, n = 8) during pre-industrial times of 1450CE-1880CE (Fig. 3A). HgAR profile shows maximum values during the 20th century of $14.5 \pm 2.9 \ \mu\text{g m}^{-2} \text{yr}^{-1}$ (1σ, n = 5), followed by a decrease to $4.5 \pm 1.0 \ \mu\text{g m}^{-2} \text{yr}^{-1}$ (1σ, n = 2) since the 21st century (Fig. 3A). The trend of post-1450CE HgAR profile echoes the historical anthropogenic Hg emission to the air in the combined sectors of South America, Africa and Oceania (Streets et al., 2019).

Peat $\delta^{202}\text{Hg}$ prior to 1450CE is characterized by negative values (Fig. 3B), ranging from -1.12 ‰ to -0.04 ‰ (average $\delta^{202}\text{Hg} = -0.69 \pm 0.25 \, \text{‰}$, 1σ, n = 41, Fig. 2A; 2B; Supplementary Information Table S4). Peat $\delta^{202}\text{Hg}$ shows a general decreasing trend from

4600BCE to 1600BCE in a range of -0.21 ‰ to -1.12 ‰. Afterwards, $\delta^{202}\text{Hg}$ increases up to -0.04 ± 0.15 ‰ (2σ) at 700BCE. Peat $\delta^{202}\text{Hg}$ remains relatively constant between 700BCE and 1450CE (-0.74 ± 0.14 ‰, 1σ, n = 19, Fig. 3B). Peat $\Delta^{199}\text{Hg}$ varies from -0.38 ‰ to 0.61 ‰ and peat $\Delta^{200}\text{Hg}$ ranges from -0.09 ‰ to 0.08 ‰ prior to 1450CE. Profiles of peat $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ vary from 4600BCE to 1450CE (R² = 0.68, P < 0.001, n = 41), fluctuating at centennial time scales (Fig. 3C; 3D).

Peat $\delta^{202}\text{Hg}$, $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ values during the pre-industrial period are -0.64 ± 0.25 ‰, -0.03 ± 0.23 ‰ and -0.02 ± 0.05 ‰ (mean ± 1σ, n = 10, 1450CE-1900CE, Fig. 2A; 2B), respectively, which are not significantly different from the 4600BCE-1450CE period. Again, $\Delta^{199}\text{Hg}$ is positively and significantly correlated to $\Delta^{200}\text{Hg}$ during the 1450CE-1900CE period, indicating similar atmospheric deposition origins, or similar anthropogenic Hg emission sources. Since the industrialization (post 1900CE), AMS peat $\delta^{202}\text{Hg}$ remains relatively stable with an average value of -0.59 ± 0.15 ‰ (1σ, n = 9) and is insignificantly different from the natural background period from 4600BCE to 1450CE (P > 0.05, two-sided t-test). Peat $\Delta^{199}\text{Hg}$ falls gradually from 0.40 ± 0.09 ‰ (2σ) at 1793CE to the lowest value at 1924CE (-0.73 ± 0.09 ‰, 2σ), and subsequently increases to 0.09 ‰ at 2014CE. Five $\Delta^{199}\text{Hg}$ values from 1900CE to 1975CE (-0.47 ‰ to -0.73 ‰) are more negative than modern $\text{Hg}^0$ $\Delta^{199}\text{Hg}$ signatures of NH remote areas (-0.18 ‰ ± 0.11 ‰, n = 157, Fig. 2A).

# 4. Discussion

## 4.1. Atmospheric Hg isotopic signatures

Both $\delta^{202}\text{Hg}$ and $\Delta^{199}\text{Hg}$ of AMS $\text{Hg}^0$ are similar to those in the NH remote areas ($\delta^{202}\text{Hg} = 0.39 \pm 0.67 \, \text{‰}$, 1σ, n = 154; $\Delta^{199}\text{Hg} = -0.18 \pm 0.11 \, \text{‰}$, 1σ, n = 157, Fig. 2A, Enrico et al., 2016; Demers et al., 2013; Fu et al., 2021; Obrist et al., 2017; Gratz et al., 2010; Sherman et al., 2010; Fu et al., 2016b; Yu et al., 2020; Kurz et al., 2020; Rolison et al., 2013; Jiskra et al., 2019). In contrast, anthropogenic emissions drive NH urban air $\text{Hg}^0$ to lower $\delta^{202}\text{Hg}$ (-0.33 ± 0.63 ‰, 1σ, n = 224) and near-zero $\Delta^{199}\text{Hg}$ values (-0.11 ± 0.11 ‰, 1σ, n = 224, Rolison et al., 2013; Yu et al., 2016; Xu et al., 2017; Fu et al., 2018; Yamakawa et al., 2017, 2019; Demers et al., 2015). AMS $\text{Hg}^0$ is identical to NH remote $\text{Hg}^0$ ($\Delta^{200}\text{Hg} = -0.06 \pm 0.04 \, \text{‰}$, 1σ, n = 156, Fig. 2B, Enrico et al., 2016; Obrist et al., 2017; Demers et al., 2013; Fu et al., 2021; Gratz et al., 2010; Sherman et al., 2010; Fu et al., 2016b; Yu et al., 2020; Kurz et al., 2020; Rolison et al., 2013; Jiskra et al., 2019). No significant seasonal variations in AMS Hg isotopes are observed (Supplementary Information Table S2). AMS $\text{Hg}^0$ isotope signatures are similar to those in Antarctica ($\delta^{202}\text{Hg} = 0.58 \pm 0.21 \, \text{‰}$; $\Delta^{199}\text{Hg} = -0.30 \pm 0.10 \, \text{‰}$; $\Delta^{200}\text{Hg} = -0.08 \pm 0.02 \, \text{‰}$, mean ± 1σ, n = 21, Yu et al., 2021), and New Zealand ($\delta^{202}\text{Hg} = 0.02 \pm 0.27 \, \text{‰}$; $\Delta^{199}\text{Hg} = -0.30 \pm 0.05 \, \text{‰}$; $\Delta^{200}\text{Hg} = -0.07 \pm 0.05 \, \text{‰}$, mean ± 2σ, n = 4, Szponar et al., 2020). The similarities of $\text{Hg}^0$ isotope compositions between AMS, Antarctica, New Zealand and the NH suggest a relatively homogeneous atmospheric $\text{Hg}^0$ isotope pool in global remote areas.

SH rainfall $\delta^{202}\text{Hg}$ is significantly higher than that in NH rainfall from remote areas (-0.08 ± 0.32 ‰, 1σ, n = 55, P < 0.05, two sided t-test, Fig. 2A, Obrist et al., 2017; Enrico et al., 2016; Demers et al., 2013; Fu et al., 2021; Gratz et al., 2010; Washburn et al., 2021; Motta et al., 2019; Donovan et al., 2013) and NH areas impacted by industrial sources (-0.66 ± 0.88 ‰, 1σ, n = 64, P < 0.05, two sided t-test, Wang et al., 2015; Zhang et al., 2020; Sherman et al., 2015, 2012). SH rainfall $\Delta^{200}\text{Hg}$ is slightly lower than that in the NH remote rainfall samples (0.16 ± 0.07 ‰, 1σ, n = 55, Obrist et al., 2017; Enrico et al., 2016; Demers et al., 2013; Fu et al., 2021; Gratz et al., 2010; Washburn et al., 2021; Motta et al.,
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Profiles of Hg accumulation rate (A, µg/m²·(l⁻)·yr⁻¹), δ²⁰²Hg (B, ‰), Δ¹⁹⁹Hg (C, ‰), Δ²⁰⁰Hg (D, ‰), dust flux (E, g/m²·(l⁻)·yr⁻¹), Hg/REE, (F, mg/g, ratio of Hg concentration in ppb to Rare-Earth-Element (REE) concentration in ppm) in the AMS peat core. Lines + Shaded fields in (B), (C) and (D) represent mean + maximum 2σ of analytical uncertainties on multiple standard materials (Supplementary information Table S1). White field (dry, high dust, SWW shifted poleward) and cyan field (wet, low dust, SWW shifted equatorward) separate the time series mainly following the dust periods established in Li et al., (2020b) (4600BCE-4200BCE, 4200BCE-3000BCE, 3000BCE-1600BCE, 1600BCE-700BCE, 700BCE-600CE, 600CE-present). Episode of 700BCE-600CE shown in gradient lines is a dust flux transition period characterized by gradual poleward displacement of the SWW (i.e. drying period with increasing dust, Li et al., 2020b). Horizontal-lines separate 600CE-1450CE, 1450CE-1900CE and post-1900CE based on the onset of large-scale Spanish mining and industrial activity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

2019; Donovan et al., 2013). Slightly negative Δ²⁰⁰Hg is measured in five SH rainfall samples out of 21 (one at TDF with −0.1 ‰, and four at AMS from −0.05% to −0.01%, 2σ = 0.10%), which is in contrast to the broadly observed positive NH rainfall Δ²⁰⁰Hg. Back trajectory models show that AMS rainfall Δ²⁰⁰Hg is significantly negatively correlated with the mean air pressure of the trajectories (*P* < 0.05, R² = 0.38, Supplementary Information Text S1 and

Fig. S1), i.e. the higher the air mass altitude of a rain event, the higher the Δ²⁰⁰Hg. Similarly, there is a significantly positive relationship between TDF Δ²⁰⁰Hg and the mixing depth of the trajectories (*P* < 0.01, R² = 0.76, Supplementary Information Fig. S2), indicating negative TDF rain Δ²⁰⁰Hg to be mainly produced in the marine boundary layer. We suggest that negative Δ²⁰⁰Hg in rainfall Hg'' from marine boundary layer trajectories at TDF and
AMS is likely produced by in-situ Hg⁰ oxidation by marine halogens and retains the isotopic signatures of originated Hg⁰ (Douglas and Blum, 2019), e.g., with negative Δ²⁰⁰Hg signature.

The AMS peatland is located at a much higher elevation than the rainfall sampling point (771 m vs 55 m a.s.l.). We therefore select the Δ²⁰⁰Hg signatures from those rainfall events with mean air pressure below 960 hPa (>450 m a.s.l.) to be included in the AMS rainfall Hg'' end-member. There are five AMS rainfall samples with mean trajectory air pressure below 960 hPa, which have an average Δ²⁰⁰Hg of 0.12 ± 0.07 ‰ (1σ, n = 5). This value is similar to the mean Δ²⁰⁰Hg signatures at the high-altitude-located MAI Observatory in Reunion Island (0.11 ‰ ± 0.04 ‰, 1σ, n = 3, 2160 m a.s.l.), and also the NH mean rainfall Δ²⁰⁰Hg at remote sites (0.16 ‰ ± 0.07 ‰, 1σ, n = 55, Fig. 2B). We use the AMS mean rainfall Δ²⁰⁰Hg of 0.12 ‰ for the Hg'' deposition end-member to the peatland, and we propose a SH mean rainfall Δ²⁰⁰Hg of 0.08 ± 0.08 ‰ (1σ, n = 21) based on all three sites, AMS, TDF, MAI. The AMS Hg⁰ Δ²⁰⁰Hg signature of -0.06 ± 0.02 ‰ (n = 23) is used as the Hg⁰ deposition end-member to the peatland.

## 4.2. Hg cycling in peatlands

Peatlands receive Hg mainly through plant uptake of atmospheric Hg⁰ and rainfall Hg'' input. Plant uptake of atmospheric Hg⁰ is likely followed by oxidation by reactive oxygen species or enzymes (e.g., catalase, Du and Fang, 1983; Liu et al., 2021) and the intra-cellular product Hg'' sequestered to thiol-carrying organic compounds. Rainfall Hg'' is also likely to be bind to extra-cellular thiol functional groups (Skyllberg et al., 2000). Thiol absorption of Hg'' is not found to cause any significant MDF (Wang et al., 2020). A two-year litter decomposition experiment shows a positive but small shift in δ²⁰²Hg (-3.23 ± 0.12 ‰ to -2.93 ± 0.12 ‰, 2σ), an insignificant change in Δ¹⁹⁹Hg (-0.28 ± 0.07 ‰ to -0.34 ± 0.07 ‰, and a constant Δ²⁰⁰Hg (-0.04 ± 0.09 ‰, 1σ, n = 8) in the residual Hg'' of decompositing litter (Yuan et al., 2020). This suggests an insignificant change in Hg isotope composition during litter decomposition. Similar to NH vegetation, AMS peat δ²⁰²Hg (-0.66 ± 0.24 ‰, 1σ, n = 58) is lower than corresponding Hg⁰ (Demers et al., 2013; Jiskra et al., 2015; Enrico et al., 2016; Yu et al., 2016; Zheng et al., 2016). It suggests the preferential uptake of light Hg isotopes by peatland vegetation, possibly related to diffusion and/or intra-cellular oxidation by enzymatic processes (Rutter et al., 2011; Zhou et al., 2021a), regardless of the vegetation species and geological substrate (Zheng et al., 2016). The shift of δ²⁰²Hg from AMS Hg⁰ to AMS peat (1.2 ‰) is however smaller than in other studies (e.g., 2.5 ‰ in French peat bogs, Enrico et al., 2016; >2.5 ‰ in a various vegetation types summarized in Zhou et al., 2021a). This is partly due to the contribution of Hg'' deposition, with positive δ²⁰²Hg, to AMS peat (Fig. 2).

The Hg-thiol complexes can be eventually transformed to immobile nanoparticulate mercury sulfides within the plant (Manceau et al., 2015), or possibly reduced by photochemical, microbial and/or non-photo abiotic processes, leading to partial Hg⁰ evasion. Hg evasion from vegetation and organic soils is a topic of active research and debate (Agnan et al., 2016; Cooke et al., 2020; Bishop et al., 2020; Zhou et al., 2021a) due to varying magnitudes of evasion flux found in different studies. For example, Hg exchange studies indicate important net Hg⁰ emission from Scandinavian peat at 64°N based on continuous flux measurement during snow-free period (May-Oct) (Osterwalder et al., 2017), moderate Hg re-emission from Scandinavian organic forest soils at 64°N based on Hg isotopic tracing approach (Jiskra et al., 2015), or temporally limited Hg⁰ emission from the Alaskan permafrost tundra at 68°N based on both flux and isotope measurements (Obrist et al., 2017). Limited Hg⁰ formation can also be found in the peat catotelm - a section permanently below the peat

water table- due to strong binding of Hg'' to organic matter (Poulin et al., 2019). Other studies provide evidence that vegetation type (Rydberg et al., 2010), temperature as well as insolation (Fahnestock et al., 2019), microbial activity (Fritsche et al., 2008), and soil characteristics (Zhou et al., 2021b) influence net Hg⁰ exchange.

The magnitude of Hg⁰ re-emission and associated reduction processes are likely to be site-specific and closely associated with local ecology and climate. Such processes can be traced using Hg MDF and/or MIF of odd-mass isotopes. Jiskra et al., (2015) indicated for boreal organic soils how trends in Δ¹⁹⁹Hg/δ²⁰²Hg and Δ¹⁹⁹Hg/Δ²⁰¹Hg ratios reflect O- and S-ligand assisted photochemical, dark abiotic, and microbial reduction. Enrico et al., (2016) observed small negative Δ¹⁹⁹Hg shifts, relative to atmospheric deposition Δ¹⁹⁹Hg, in ombrotrophic peat cores from the French Pyrenees, suggesting minor photochemical re-emission of Hg at the peat surface. The AMS peat Δ¹⁹⁹Hg/Δ²⁰¹Hg slope is 1.29 ± 0.10 (1σ, Fig. 4A), which is similar to the slope of 1.19 in the French peat cores (Enrico et al., 2017). Both these two peat Δ¹⁹⁹Hg/Δ²⁰¹Hg slopes are significantly higher than the SH and NH atmospheric Hg slopes of 1.0 (R² = 0.92, p < 0.001, Fig. 4A) and 0.92 (Supplementary Information Fig. S3), respectively, which indicate a magnetic isotope effect similar to Hg'' photoreduction reactions in the presence of natural organic matter (Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio of 1.0 ± 0.01, 1σ, Blum and Bergquist, 2007).

Peat Δ¹⁹⁹Hg during pre-1900CE periods shows negligible to a small positive shift from the theoretical Δ¹⁹⁹Hg based on peat Δ²⁰⁰Hg and conservative mixing of atmospheric end-members (Fig. 4B). The positive shift of Δ¹⁹⁹Hg is compatible with enhanced photochemical MIF, via the (+)magnetic isotope effect (MIE) mechanism operating during aqueous Hg'' photoreduction in the presence of organic oxygen-containing ligands (Fig. 4B). We find that the magnitude of Δ¹⁹⁹Hg shift is positively correlated with Δ¹⁹⁹Hg itself (R² = 0.82, P < 0.001, Supplementary Information Fig. S4), indicating that during enhanced Hg'' wet deposition periods with higher Δ¹⁹⁹Hg, the (+)MIE is dominant. This is consistent with observed (+)MIE during Hg photoreduction in rainfall (Fu et al., 2021), suggesting that the Hg''-DOC complexes in rainfall (Yang et al., 2019) lead to (+)MIE during and after deposition during pre-1900CE periods.

From pre-1900CE to 20th century (1906CE-1988CE), AMS peat Δ¹⁹⁹Hg shows a negative shift by -0.25 ‰, which is opposite to the ones found in NH lake sediments (+0.23 ‰ shift, Kurz et al., 2019; +0.08 ‰ to +0.30 ‰, Lepak et al., 2020), in SH lake sediment (+0.23 ‰ to +0.34 ‰, Cooke et al., 2013; +0.23 ‰, Jiskra et al., 2022) and in NH peat records (+0.13 ‰ to +0.30 ‰, Enrico et al., 2017). The opposite shift in Δ¹⁹⁹Hg between AMS peat and other global archives suggests a significant change in the post-depositional process on deposited anthropogenic Hg in 20th century, enabling negative Δ¹⁹⁹Hg and (-)MIF to be observed in AMS peat. Negatively-shifted Δ¹⁹⁹Hg in association with the corresponding Δ¹⁹⁹Hg/Δ²⁰¹Hg slope of 1.55 ± 0.44 (1σ) is not in line with the process of foliar Hg'' photoreduction, which produces a Δ¹⁹⁹Hg/Δ²⁰¹Hg slope of 0.79 ± 0.06 (Yuan et al., 2019). Rather, negatively-shifted Δ¹⁹⁹Hg in 20th century is likely driven by nuclear volume fractionation, possibly via dark abiotic Hg reduction (negative Δ¹⁹⁹Hg in residual Hg'' and Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio of 1.6, Zheng and Hintelmann, 2010). Similar findings are also made in Yuan et al., (2020), who show a Δ¹⁹⁹Hg/Δ²⁰¹Hg slope of 1.36 ± 0.28 with negative Δ¹⁹⁹Hg in a forest organic soil profile (1603-2002CE) and suggests dark abiotic reduction of Hg'' as the most probable mechanism. Even though different Hg reduction mechanisms with variable odd-MIF in our study may suggest minor post-depositional odd-MIF in the AMS peat core, peat Δ¹⁹⁹Hg and Δ²⁰⁰Hg are overall significantly correlated along the Hg⁰ and Hg'' deposition end-member mixing line (R² = 0.67, P < 0.001, n = 58, supplementary
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (A) $\Delta^{199}\text{Hg}$ vs $\Delta^{201}\text{Hg}$ in the whole peat core (grey full squares), in peat in 20th century (1906CE-1988CE, pink open squares), AMS $\text{Hg}^0$ (blue open triangles) and SH rainfall $\text{Hg}^{\text{II}}$ (blue open circles). Lines represent York linear regression in $\text{Hg}^0$ and rainfall (blue), the whole peat core (grey) and 20th century peat samples (red). (B) $\Delta^{199}\text{Hg}$ vs $\Delta^{200}\text{Hg}$ in AMS rainfall $\text{Hg}^{\text{II}}$ (grey open circles), $\text{Hg}^0$ (grey open triangles) and peat (pink squares: 20th century, open squares: 1450CE-1900CE, full triangles: 600CE-1900CE, open triangles: 700BCE-600CE, full dots: 1600BCE-700BCE, open dots: 3000BCE-1600BCE, full diamonds: 4200BCE-3000BCE, open diamonds: 4600BCE-4200BCE). Blue line represents a York linear regression of AMS $\text{Hg}^0$ and rainfall $\text{Hg}^{\text{II}}$ end-members with considering $2\sigma$ analytical uncertainties. Upward orange dash arrow stands for the trajectory of residual $\text{Hg}^{\text{II}}$ under aqueous photoreduction of $\text{Hg}^{\text{II}}$ in presence of dissolved organic matter (DOC) with (+) Magnetic Isotope Fractionation (MIE), while downward orange dash arrow represents dark reduction by natural organic matter under (–) nuclear volume effect. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Fig. S5; Fig. 4b). This suggests that peat $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ variability is mostly driven by the Hg deposition sources, i.e. variable $\text{Hg}^0$ plant uptake as well as $\text{Hg}^{\text{II}}$ wet and dry deposition.

### 4.3. Peat $\Delta^{200}\text{Hg}$ reconstruction of $\text{Hg}^0$ uptake and $\text{Hg}^{\text{II}}$ deposition

Most AMS peat Hg MIF resembles $\text{Hg}^0$ MIF signatures (Fig. 2A; 2B). This indicates that vegetation uptake of $\text{Hg}^0$ is an important source of Hg to AMS peat. We reconstructed peat dry HgAR ($\text{Hg}^0$AR, i.e. plant $\text{Hg}^0$ uptake, Fig. 5A) and wet HgAR ($\text{Hg}^{\text{II}}$AR, Fig. 5B), by making use of total peat HgAR, and the $\Delta^{200}\text{Hg}$ mass balance with equations (4) and (5). We find that, on average, plant $\text{Hg}^0$ uptake dominated Holocene Hg sequestration in AMS peat (>80%). Vegetation $\text{Hg}^0$ uptake as dominant Hg pathway to AMS peat is consistent with the findings in Pyrenean peatlands (Enrico et al., 2016).

Both $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ display higher values at 4200BCE-3000BCE ($\Delta^{199}\text{Hg} = 0.38 \pm 0.14 \text{ ‰}$, $\Delta^{200}\text{Hg} = 0.06 \pm 0.01 \text{ ‰}$, $1\sigma$, n = 3) and 1600BCE-700BCE ($\Delta^{199}\text{Hg} = 0.10 \pm 0.34 \text{ ‰}$, $\Delta^{200}\text{Hg} = 0.00 \pm 0.05 \text{ ‰}$, $1\sigma$, n = 6), than during other pre-1450CE periods ($\Delta^{199}\text{Hg} = -0.08 \pm 0.23 \text{ ‰}$, $\Delta^{200}\text{Hg} = -0.04 \pm 0.03 \text{ ‰}$, $1\sigma$, n = 41, Fig. 3C; 3D). These two periods with high values of $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ lie closer to the AMS rainfall Hg isotope end-member, and likely correspond to periods of enhanced rainfall, also inferred

from Rare-Earth-Element (REE)-based dust proxies (Li et al., 2020b). Both $\Delta^{199}\text{Hg}$ and $\Delta^{200}\text{Hg}$ are significantly correlated with dust flux prior to 1450CE ($R^2 = 0.19 \Delta^{199}\text{Hg}$-dust = 0.25, P < 0.001; $R^2 = 0.19 \Delta^{200}\text{Hg}$-dust = 0.29, P < 0.001, 2-tailed test of significance). Higher peat $\text{Hg}^0$AR is observed during the dry, high dust periods (3000BCE-1600BCE and 700BCE-600CE) than during other pre-1450CE wetter, low dust periods (Fig. 5A).

### 4.4. Role of dust deposition and climate on peat Hg accumulation

Prior to 1450CE, when Hg deposition to the peatlands was dominated by natural origins (Amos et al., 2015), AMS HgAR are significantly correlated with REE-based dust flux (Fig. 3E, P < 0.001, $R^2 = 0.64$, 2-tailed test of significance). Peat Hg/REE varies from 23 to 273 mg g$^{-1}$, which is much higher than the Hg/REE ratio of 0.38 mg g$^{-1}$ in the crustal dust (Crust$_{\text{Hg}}$ = 56 ng g$^{-1}$, and Crust$_{\text{REE}}$ = 148 µg g$^{-1}$, Rudnick et al., 2003). This indicates that a direct Hg contribution from natural atmospheric mineral dust to peat is negligible. Low ash content in AMS peat profile (<3% in the whole profile, Supplementary Information Table S5) further suggests that Hg in mineral dust is unlikely a significant Hg source to peat. Hg/REE increases at 3000BCE-1600BCE and 700BCE-600CE (Fig. 3F; Supplementary Information Table S5), where enhanced dust flux and enhanced $\text{Hg}^0$AR are observed (Fig. 5). This suggests an indirect influence of dust on bog vegetation $\text{Hg}^0$ uptake, via changes in regional or even hemispheric marine productivity and its influence on atmospheric $\text{Hg}^0$ levels, peat primary productivity, or peat bog ecology.

High dust, high $\text{Hg}^0$AR periods of 3000BCE-1600BCE and 700BCE-600CE correspond to poleward-shifted SWW, which is the “austral summer-like” climate pattern with warmer Southern Ocean surface (Metzl et al., 2006) and stronger solar radiation. The poleward-shifted SWW during these periods can potentially enhance the upwelling of Hg-rich deep waters in the Southern Ocean by intensifying the Antarctic Circumpolar Current (Bentley et al., 2009). High $\text{Hg}^0$AR periods of 3000BCE-1600BCE and 700BCE-600CE coincide with higher marine productivity in Southern subtropical waters inferred from the foraminiferal productivity index in two marine cores from the same sector as AMS (H214 core in Samson et al., 2005; MD97-2121 in Marr et al., 2013; Bostock et al., 2019). During modern austral summer, the oceanic region located southwest of AMS is highly productive (Angot et al., 2014). It seems to be reasonable to speculate that high $\text{Hg}^0$AR at 3000BCE-1600BCE and 700BCE-600CE can be attributed to elevated atmospheric $\text{Hg}^0$ levels from warmer or more productive SH ocean regions, which are major sources for SH atmospheric $\text{Hg}^0$ (Vandal et al., 1993; Bieser et al., 2020). The reduction of surface oceanic $\text{Hg}^{\text{II}}$ to $\text{Hg}^0$ can be mediated by solar radiation (Soerensen et al., 2010), marine biota (Fitzgerald et al., 1984), and organic compounds (Soerensen et al., 2013). Despite these deductions, modern qualified $\text{Hg}^0$ observational data at AMS (2012–2021 time period) shows no seasonality with no statistically distinguishable $\text{Hg}^0$ concentrations in austral winter ($1.07 \text{ ng m}^{-3}$) and summer ($1.05 \text{ ng m}^{-3}$) (Magand and Dommergue, 2021), arguing against a strong link between marine $\text{Hg}^0$ emission and peat $\text{Hg}^0$AR.

A more likely explanation for enhanced peat $\text{Hg}^0$AR at 3000BCE-1600BCE and 700BCE-600CE is an increase in peat primary productivity. Higher atmospheric dust deposition can enhance the fertilization of the peatland through more nutrient inputs and then promote peat vegetation primary productivity (Kylander et al., 2018). High primary productivity will increase annual peat $\text{Hg}^0$ sequestration per surface area (in µg m$^{-2}$) due to the additional biomass production. The net peat accumulation rate (g m$^{-2}$ yr$^{-1}$) at AMS, i.e. a combination of synchronous changes in the primary productivity and decomposition rate, is not significantly different
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Temporal profile of (A) reconstructed Hg dry deposition (Hg⁰AR, µg/m²·(⁻|⁻) yr⁻¹, mean + 1σ) and (B) reconstructed Hg wet deposition (Hg''AR, µg/m²·(⁻|⁻) yr⁻¹, mean + 1σ). High dust periods of 3000BCE-1600BCE and 700BCE-600CE are indicated in Fig. 3.</figcaption></figure>

at 3000BCE-1600BCE and 700BCE-600CE from the corresponding previous periods (*P* > 0.05 for both, two sided *t*-test), based on the age model derived from 20 radiocarbon-dated sections (Li et al., 2020b). We therefore assume a synchronous higher primary productivity (discussed above) and higher decomposition rate at 3000BCE-1600BCE and 700BCE-600CE. The assumption of higher decomposition can be supported by a relatively dry climate during these two intervals inferred from dust proxies, where more dust input at AMS results from less wind and less humidity. Relatively dry climate lowers the peatland water table, which enhances the dominant aerobic decay in the peatland (i.e. high decomposition, Clymo, 1984; Young et al., 2019). Increased peat Hg accumulation and high decomposition likely occur synchronously during high dust and humidity regime, leading to enhanced net peat Hg⁰AR. In summary, variability in AMS peat Hg deposition prior to 1450CE appears to be mainly driven by changes in SWW-driven peatland vegetation primary productivity, and less likely by changes in marine Hg⁰ evasion and SH Hg⁰ levels.

## 4.5. Qualitative and quantitative rainfall reconstruction from an isotopic perspective

A recent SH Holocene Hg isotope record in lake Titicaca sediment shows a strong link between deposited Δ²⁰⁰Hg and paleo-rainfall, suggesting temporal variation of Δ²⁰⁰Hg in archives could reflect a change in rainfall rates prior to industrialization (Jiskra et al., 2022). AMS Δ²⁰⁰Hg variability follows the trend of dust flux profile, a qualitative proxy for SWW (Fig. 6A; 6B). This highlights the potential of using Δ²⁰⁰Hg as a qualitative rainfall proxy. Above we used a binary isotope mixing model based on distinct Δ²⁰⁰Hg signatures in both atmospheric end-members (i.e. Hg⁰ and rainfall Hg'' equations (3) and (4)), to estimate past Hg⁰AR and Hg''AR. Here we take this approach a step further to try and estimate Holocene rainfall rate, by making use of the relationship between modern rainfall and Hg'' wet deposition. Modern Hg'' wet deposition has been enhanced by anthropogenic Hg emission, which should be considered in any rainfall reconstruction model. To present different possibilities of using Hg''AR and Δ²⁰⁰Hg signatures to estimate the Holocene rainfall, we propose two models under different assumptions (Model 1 & Model 2).

Model 1 is free from the effects of anthropogenic Hg input and peat Hg re-emission rates by making use of the Δ²⁰⁰Hg compositions in atmospheric end-members, deposited Hg''AR, and deposited Hg⁰AR (Supplementary Information Text S2 and equations S1 to S6 with detailed assumptions). Briefly, it assumes that (i) rainfall Hg'' concentration and atmospheric Hg⁰ are proportional at all times, (ii) peat Hg re-emission does not discriminate between the original deposition source (Hg'' vs Hg⁰), and (iii) the peat vegetation uptake rate (cm s⁻¹) of gaseous Hg⁰ was constant over time. In other words, the variations in Hg''/Hg⁰ accumulation (and thus peat Δ²⁰⁰Hg) are exclusively the result of variable rainfall rates. The rainfall profile estimated from Model 1 follows the trends of dust flux and peat Δ²⁰⁰Hg (Fig. 6A; 6C, details please see Supplementary Information Text S2).

Model 2 is based on the relationship between Hg'' wet deposition and rainfall in modern AMS observations, which shows that AMS daily rainfall volume from 2013 to 2014 is significantly correlated with the associated Hg wet deposition rate (ng m⁻² d⁻¹, *P* < 0.01, R² = 0.53, Supplementary Information Fig. S6). We extend this daily relationship to the annual scale by multiplying with a factor of 365 (Supplementary Information Text S3 and equation S7). Modern Hg wet deposition (post 1990CE), is however strongly influenced by the x3 anthropogenic enrichment of SH atmospheric Hg relative to pre-1450CE and x2.5 relative to 1450CE-1880CE, respectively (Li et al., 2020a). To translate Holocene peat Hg''AR into rainfall rate, we therefore correct reconstructed peat Hg''AR by a factor of 3 and 2.5 for pre-1450CE Hg''AR and 1450CE-1900CE Hg''AR, respectively (Supplementary Information equations S8 and S9). No correction for Hg re-emission from peat is made in Model 2, i.e. we assume that reconstructed Hg''AR is equal to Hg'' wet deposition. This assumption implies that either peat Hg re-emission was small, or that it was significant, but proportional to the Hg deposition rate. This assumption is based on (i) the broadly consistent anthropogenic Hg enrichment factors between sediment and peat in both hemispheres (13 vs 16 in the NH and 4 vs 6 in the SH, Li et al., 2020a), and (ii) the reconstructed 2001CE-2014CE rainfall of 840 mm yr⁻¹ (median, 570–1060 mm yr⁻¹, interquartile range), is in good agreement with observed rainfall at AMS with 833 mm yr⁻¹ in 2013 and 864 mm yr⁻¹ in 2014 (Sprovieri et al., 2017). The reconstructed Holocene rainfall rates broadly follow the trend of the qualitative SWW strength profile
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Profiles of (A) dust flux (reversed scale in Y axis, µg/m²·(−|−|) yr⁻¹, mean + 1σ), (B) Δ²⁰⁰Hg (‰, mean + 1σ), (C) Δ²⁰⁰Hg-based reconstructed rainfall rate using Model 1 under Monte Carlo simulation (mm yr⁻¹, median + interquartile), and (D) Δ²⁰⁰Hg-based reconstructed rainfall rate using Model 2 under Monte Carlo simulation (mm yr⁻¹, median + interquartile). The strength of the Southern Westerly Winds is shown in “+” and “−”, which is indicated by the dust flux variation (Li et al., 2020b).</figcaption></figure>

inferred from the dust flux (Fig. 6A; 6D). The discrepancy of reconstructed rainfall rates between Model 2 and dust proxies at 1600BCE-700BCE highlights the need for further investigations on the model methods (e.g., potential Hg re-emission) and/or cross-validation using other climate indicators (e.g., plant macrofossil, testate amoebae).

Both models capture the broad trend of SWW dynamics inferred from the dust deposition proxy (Fig. 6), even though model results show different quantitative rainfall rates for the same periods. These differences in the rainfall estimates can be attributed to their distinct assumptions and methods (e.g., re-emission rates, adjustment on anthropogenic Hg input and constant uptake rate of gaseous Hg⁰). For example, the high rainfall rate estimate of 5700 mm yr⁻¹ for 4200BCE-3000BCE in Model 2 is a consequence of the assumed constant uptake rate of gaseous Hg⁰ (see Supplementary Information Text S3). Although neither of these two models is perfect, they each have their strengths in estimating rainfall from peat Hg isotopes. Model 1 allows a variable uptake rate of gaseous Hg⁰, while Model 2 is independent from re-emission rates and anthropogenic enrichment factors. Overall, our findings highlight the potential of using Hg stable isotopes to reconstruct the paleo-rainfall patterns as well as to trace Hg sources and post-depositional processes.

## 4.6. Implications

The Amsterdam Island peatland is an ideal location to document the temporal changes in SH atmospheric Hg deposition and its relation to climate due to its remote location and two main

ways of Hg sequestration in peat via vegetation uptake of Hg⁰ and rainfall supply of Hg¹¹. Coherent changes in Amsterdam Island peat Hg accumulation rate, dust deposition rate, odd MIF and even MIF reflect changes in Holocene SWW dynamics. Poleward shifted SWW lead to drier, warmer climate conditions at the Amsterdam Island, where higher continental dust deposition provides nutrients that stimulate peatland primary production and concomitant bog vegetation Hg⁰ uptake. MDF and odd MIF of Hg isotopes enable us to evaluate Hg cycling upon its deposition to the peatland (e.g., potential mobility), while even MIF allows us to reconstruct the two main Hg deposition pathways (i.e. dry and wet deposition). Even MIF further shows its potential as a qualitative and semi-quantitative paleo rainfall proxy. To establish Hg isotopes as a robust climate indicator or post-depositional process tracer in peatland, more investigations are still needed on the mechanisms of Hg cycling in the atmospheric sources (e.g., MDF and MIF under Hg oxidation within and above marine boundary layers) and in the peatland with not only one but multiple cores (e.g., MDF and MIF during uptake of Hg⁰ among different plant species). Presently, we suggest that Hg stable isotopes in peat records can at least be used as a qualitative climate proxy identifying dry and wet periods. A better understanding of peat Hg cycling and associated isotope fractionation, together with a better characterization of rainfall isotope signatures, might allow for quantitative reconstructions.

## Data availability

The data supporting the findings of this study are provided in the article and Supplementary Material.
Declaration of Competing Interest

The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.

Acknowledgements

We deeply thank all overwintering staff at AMS and the French Polar Institute Paul-Emile Victor (IPEV) staff and scientists who helped with the setup and maintenance of the experiment at AMS in the framework of the GMOStral-1028 IPEV program as well as the support they provided during the 1066 PARAD field campaign. We acknowledge OPAR (Observatoire de Physique de l'Atmosphère à La Réunion), funded by CNRS-INSU and Université de La Réunion and managed by OSU-R (Observatoire des Sciences de l'Univers à La Réunion, UMS 3365). We thank the NOAA Air Resources Laboratory (ARL) for providing the HYSPLIT transport and dispersion model and the READY website (https://www ready.noaa.gov). We are deeply grateful for Andrea Coronato and Ramiro López from CADIC-CONICET (Ushuaia), Kayne Dipilato and Pablo DéAntueno for their incredible efforts on rainfall sampling and coordination in Tierra del Fuego. We appreciate the great efforts from Jérôme Chmeleff and Laure Laffont for their maintenance on the analytical machines MC-ICPMS and DMA-80, respectively. Amsterdam Island Hg0 data, accessible in GMOS-FR national data portal (https://gmos.aeris-data.fr/) were collected via instruments coordinated by the IGE-PTICHA technical platform dedicated to atmospheric chemistry field instrumentation and maintained by the French national center for Atmospheric data and services AERIS, which is acknowledged by the authors. Amsterdam Island Hg0 data are funded by the European Union 7th Framework Programme project Global Mercury Observation System (GMOS 2010-2015, Nr. 26511 to O.M.), the GMOStral-1028 IPEV program since 2012 (to O.M.), and the French national LEFECCHAT CNRS/INSU program (TOPMMODEL project, Nr. AO2017-984931 to O.M.). We thank the China Scholarship Council for supporting C. Li's PhD work (Nr. 201506990002). Field work was partially funded by the French Polar Institute (IPEV, Brest, France) through the IPEV Programmes 1066 "PARAD" to F.D.V., J.E.S. and O.M. acknowledge funding from the H2020 ERA-PLANET iGSP and iCUPE programs (Nr. 689443). We thank three anonymous reviewers for their time and very helpful comments.

**Appendix A. Supplementary material**

All primary research data generated from this study are pro-
vided in the Supplementary Materials. Supplementary material to
this article can be found online at https://doi.org/10.1016/j.gca
2022.11.024.

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