Terrestrial regulation of lacustrine Hg deposition during glacial‐interglacial cycles

Mercury (Hg) is a toxic trace metal. It is clear that its natural cycle has been highly disturbed by human activities, but there remains much to understand about how it operated before these perturbations. For example, the influences of glacial-interglacial climate changes on the geochemical cycle of environmental Hg remain poorly understood. While key Hg surface reservoirs are sensitive to millennial-scale climate variations, it is unclear whether these responses influence their long-term behavior. Here, we explore how the terrestrial Hg cycle responds to environmental changes over multiple glacial-interglacial cycles by analysis of a ∼1.36-million-year-long sedimentary Hg record from the ancient Lake Ohrid (SE Europe): with the objective of understanding which processes may impact the behavior of this cycle on millennial timescales. Our analysis reveals periodic behavior in Hg between 1,360 and 780 thousand years ago (ka), but a weaker link from ∼780 ka to present. This transition corresponds roughly to the Mid-Pleistocene transition (MPT), which is observed in climate and ice-volume proxies in both Northern and Southern Hemispheres. These data confirm that long lacustrine Hg records are modulated by, and can capture variability in, terrestrial reservoirs for Hg. We propose that the change in Hg behavior corresponds to a reduction in catchment vegetation and soil carbon, and, consequently, Hg reservoir capacity following the Mid-Pleistocene transition. Our findings demonstrate that climate-driven changes in terrestrial reservoir size and stability can significantly influence the long-term behavior of Hg, which could have major implications for our understanding of this cycle on a regional to global scale.