Mercury cycling since 90 ka recorded in lacustrine sediments from SE Europe

Understanding the processes that drive mercury (Hg) cycling through the environment is essential to constrain the rate, magnitude, and legacy of natural and anthropogenic Hg emissions. The global Hg cycle involves transportation, re-mobilization and, eventually, deposition in soils, lacustrine and marine sediments; yet is also susceptible to environmental changes that govern (1) emission of Hg from deep Earth reservoirs to the surface environment; and/or (2) varying rates of Hg exchange between surface reservoirs. However, these processes remain poorly understood as they typically occur on timescales that extend beyond direct observations. Lake sediment cores offer the opportunity to explore if, how, and on what timescales, different environmental changes could perturb this cycle. We present two records of Hg deposition in two interconnected lakes both situated in the Central Balkans, ~10 km apart. Lakes Prespa and Ohrid are connected through karst networks, and are well known for their near-continuous, high-resolution records of environmental change over the last glacial cycle. Sediments in both lakes show high variability in Hg deposition between 90 and 0 ka. Variability is most pronounced in Lake Prespa; Hg accumulation rates range from 0.1 to 60 ng/cm2/year (1.3 - 9.8 ng/cm2/year in Lake Ohrid) and we see several prominent Hg concentration peaks. A distinct, three-fold, Hg enrichment in the Prespa core occurs between 21.3 - 17.5 ka, coeval with deglaciation of the Balkans after the last glacial maximum (LGM). We postulate this reflects a perturbation to Lake Prespa's water balance caused by an influx of Hg-rich glacial meltwater. However, a similar signal is not preserved in Lake Ohrid, potentially explained by sealing of the karst system by permafrost formed during the LGM that acted to limit subterraneous outflow. The variable Hg signals observed in both records from 90 to 21.3 ka appear connected to shifts in vegetation and catchment hydrology, and globally increasing Hg emissions resulting from anthropogenic activity become progressively more important between 17.5 ka and present-day. Our results highlight how natural processes influence regional Hg cycling.