Long-term mercury accumulation and climate reconstruction of an Australian alpine lake during the late Quaternary

Abstract

Mercury (Hg) is a volatile metal of international concern due to its toxicity, with a large atmospheric emission and transport capacity. The biogeochemical cycle of Hg is sensitive to changes in climate, yet our understanding of the specific impact of climatic factors on the Hg cycle remains limited. Here we use a multi-proxy framework, supported by AMS ¹⁴C dating, to interpret climatic events in South-Eastern Australia from ∼18,000 years to 6500 years before present from the sediments of Blue Lake in Australia's alpine region. By combining Hg analysis with Antarctic temperature records and iTRACE climate model outputs, carbon-to‑nitrogen ratios (C:N), macroscopic charcoal, and pollen analysis, we find Hg records within Blue Lake's sediments primarily reflect changes in the catchment as a result of a changing climate. The increase in Hg concentrations began with the onset of the Holocene, following a glacial period during which the region was predominantly rocky, relatively barren, and likely covered by ice and snow. The strong relationship between Hg and organic matter in our record indicates that soil development in the watershed post de-glaciation was a predominant driver of Hg concentration and deposition in Blue Lake. An increase in precipitation and temperature in the Holocene contributed to an increase in nutrients and organic matter, further increasing Hg concentration in Blue Lake. A primary challenge in modern Hg research, particularly in the context of climate change, involves distinguishing changes in Hg levels resulting from human activities from those driven by climatic variations. Our pre-anthropogenic data highlight the long-term interrelationships among climate dynamics, soil processes, and ecological transformations within lake catchments on the geochemical cycle of Hg. These connections should be factored into strategies aimed at mitigating Hg increases in lake sediments resulting from global warming.