Mercury (Hg) is a global pollutant transported long distances via the atmosphere, reaching even polar regions. Elevated Hg levels have been observed in the Antarctic and Southern Ocean. Upon deposition into oceans, some Hg is re-emitted, while the remainder can be converted to methylmercury (MeHg), a highly toxic and bioavailable form that enters the food chain. In the photic zone, MeHg undergoes photodegradation, a significant sink for MeHg in the upper ocean. Factors influencing photodemethylation include light intensity, salinity, and dissolved organic matter, with sunlight being dominant. Studies in the Northern Hemisphere suggest sea ice cover alters MeHg photodegradation by affecting solar radiation penetration. This study aims to investigate this phenomenon in the Antarctic. Mercury isotopes have emerged as powerful tools for tracing Hg sources and pathways. Mass-dependent fractionation (MDF, δ²⁰²Hg) occurs in most Hg reactions, while mass-independent fractionation (MIF, Δ¹⁹⁹Hg and Δ²⁰¹Hg) occurs only in photochemical reactions. Photoreduction of inorganic Hg(II) and photodemethylation of MeHg enrich odd isotopes in the residual Hg(II) or MeHg. This odd-MIF can transfer to aquatic organisms, allowing for the study of photochemical processes preceding food web entry. Previous research used Hg MIF in biological samples (seabird eggs, seal tissues) to infer changes in photodemethylation, demonstrating a strong influence of sea ice. The Antarctic Peninsula, a rapidly warming region, experiences cryosphere and ecosystem changes, including glacier disintegration and reduced sea ice extent. These changes are expected to strongly affect the Hg cycle. This study uses Hg isotope compositions from a sediment core (HF4) heavily influenced by seal feces on the Fildes Peninsula to identify Hg sources and reconstruct past changes in photodemethylation in relation to climate change.

Numerous studies have established the presence and bioaccumulation of mercury in the Antarctic and Southern Ocean ecosystems. Research on mercury methylation and demethylation processes in marine environments has identified several key factors influencing these reactions, including sunlight, dissolved organic matter, and salinity. The use of mercury isotopes, particularly the mass-independent fractionation (MIF) of odd-mass isotopes (Δ¹⁹⁹Hg and Δ²⁰¹Hg), has proven valuable in tracing mercury sources and pathways, revealing the role of photochemical processes in influencing mercury speciation and bioavailability. Previous studies in the Arctic have demonstrated a clear relationship between sea ice extent and methylmercury photodegradation, highlighting the potential of mercury isotopes as indicators of past climate variability. However, the application of this approach to Antarctic environments is less extensive, creating a knowledge gap addressed by this study.

A 42.5 cm long sediment core (HF4) was collected from a terrestrial catchment on the west coast of the Fildes Peninsula, King George Island, Antarctica. This location is known to be strongly influenced by seal activity, particularly southern elephant seals. The core was sectioned, and the number of seal hairs was counted to quantify seal activity over time. The sediments were analyzed for mercury concentration using a thermal combustion method and a Hydra IIc Direct Mercury Analyzer. Mercury isotopic compositions (δ²⁰²Hg, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, Δ²⁰¹Hg, and Δ²⁰⁴Hg) were determined using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at the University of Toronto. Isobaric interference from ²⁰⁴Pb was corrected using ²⁰⁶Pb measurements. Mass-independent fractionation (MIF) was calculated using a previously described method to determine Δ¹⁹⁹Hg values. The chronology of the core was established using ¹³⁷Cs and ¹⁴C dating methods, with corrections applied to account for the marine reservoir effect. Statistical analyses, including Pearson's correlation, were conducted to examine relationships between Hg concentration, seal populations, and Hg isotope ratios. The study utilizes previously published data on sea ice concentration and the Southern Annular Mode (SAM) index to contextualize the findings regarding historical sea ice change.

Large amounts of seal hair were found in the upper sediment layers, indicating substantial seal feces input above 23 cm depth. Seal population changes over the past 1500 years were reconstructed based on seal hair counts. Hg content in the sediments showed a positive correlation with seal hair numbers, increasing significantly after ~470 CE. Before ~470 CE (Period I), δ²⁰²Hg and Δ¹⁹⁹Hg were low, similar to geogenic Hg. After ~470 CE (Period II), both δ²⁰²Hg and Δ¹⁹⁹Hg increased markedly, with significantly positive Δ¹⁹⁹Hg values. The positive Δ¹⁹⁹Hg values were consistent with those of seal feces in other Antarctic regions, suggesting a seal feces origin for Hg in Period II sediments. The variation in Δ¹⁹⁹Hg after ~750 CE was not solely attributable to changes in seal population. Instead, it was suggested that the Δ¹⁹⁹Hg variation reflected changes in Δ¹⁹⁹Hg in seal prey, linked to photochemical processes in seawater. The Δ¹⁹⁹Hg increase between ~1000 and ~1300 CE (Medieval Climate Anomaly, MCA) was linked to enhanced MeHg photodemethylation caused by decreased sea ice extent. Cold periods (~750–1000 CE and ~1300–1750 CE) showed lower Δ¹⁹⁹Hg values, suggesting reduced photodemethylation due to increased sea ice. The Δ¹⁹⁹Hg/Δ²⁰¹Hg slope of ~1.17 indicated a dominant contribution of MeHg photodegradation, and not Hg(II) photoreduction, to the Δ¹⁹⁹Hg values. The study compares results with other Antarctic studies, emphasizing regional differences in climate and environment influencing mercury cycling. Modern samples showed some inconsistencies potentially due to regional differences and individual variation.

The findings demonstrate the potential of mercury isotopes in seal feces-dominated sediments as a valuable proxy for reconstructing historical sea ice changes in the Antarctic. The strong correlation between Δ¹⁹⁹Hg variations and known periods of warmer and colder climate strongly supports the hypothesis that sea ice extent significantly influences the photodegradation of methylmercury in Antarctic waters. The study provides further evidence that sea ice plays a crucial role in modulating mercury biogeochemical cycling, complementing previous research conducted in Arctic environments. The observed differences in Δ¹⁹⁹Hg values between different periods are significant and consistent with expected changes in sea ice extent and associated photodemethylation processes. This adds to our understanding of the long-term impacts of climate change on mercury biogeochemistry in high-latitude regions.

This study successfully demonstrates the utility of mercury isotopes from seal feces-dominated sediments as a proxy for reconstructing historical changes in sea ice extent. The observed relationship between Δ¹⁹⁹Hg and known climatic periods emphasizes the significant influence of sea ice on methylmercury photodegradation in the Antarctic. Future research should focus on increasing temporal resolution and expanding geographical coverage to refine our understanding of this complex relationship. Further research is needed to understand the effects of changes in photodemethylation on Hg levels in organisms.

The resolution of the current sediment core might not be sufficient to fully capture the details of modern changes in sea ice and mercury cycling. The study relies on inferences about seal diet and assumes that mercury isotope signatures in seal feces accurately reflect the isotopic composition of methylmercury in their prey. Regional differences in environmental factors and individual variations in seal mercury metabolism may influence the interpretation of the results. More research is needed to fully quantify these factors and the exact relationship between sea ice, methylmercury photodegradation, and mercury levels in Antarctic organisms.