A 22,000-Year Sediment Record from Burial Lake, Alaska, Shows a Rapid Twofold Increase in Mercury Concentration in Response to Early Holocene Climate Change

Arctic permafrost soils have recently been identified as the largest mercury (Hg) reservoir on Earth. Today, rapid warming in the high latitudes may be altering the Arctic Hg cycle by accelerating permafrost thaw, leading to changes including deepening of the active layer, increasing organic matter decay, and increasing seasonal groundwater flow. However, few studies have investigated how the Hg cycle has responded to past changes in climate, and there is a lack of Arctic records that span the late glacial to early Holocene when climate conditions changed abruptly. We propose that the geochemical and physical changes in the sediment record of Burial Lake (68.43ºN, 159.17ºW; 460 m ASL), which document climatic and environmental changes in northwestern Alaska after the Last Glacial Maximum (LGM), can be used as an analog to investigate how today’s rapid warming affects Hg mobilization from permafrost soils to surficial waters. Warming in the Northern Hemisphere between ~15.0 and 8.0 ka resulted in rapid changes in northwest Alaska, including the submergence of the Bering Land Bridge that reconnected the Pacific and Arctic Oceans (~11.0 ka), in addition to changes in the hydroclimate. Our results indicate that the Hg concentration was relatively low and stable in the Burial Lake record during the transition from the LGM to the late glacial (20.0 and 16.0 ka) with a mean concentration of 64±7 μg/kg. Mercury concentrations begin to increase after 16.0 ka. Then, coinciding with a rapid temperature increase at the beginning of the Bølling Allerød (14.7 to 12.9 ka), Hg concentrations increased by ~20% and showed higher variability as temperatures fluctuated until the end of the Younger Dryas (12.9 to 11.7 ka). At 11.0 ka, the Hg concentration increased rapidly. It peaked at 140 µg/kg, with a mean Hg concentration of 119 μg/kg between 11.0 to 8.8 ka, coinciding with evidence of a rapid increase in regional precipitation and flooding of the Bering Land Bridge. From 8.8 to 0.1 ka, the mean Hg concentration decreased to 107 μg/kg and then increased rapidly over the last 100 years to a maximum concentration of 196 μg/kg occurring during the 1990s. Throughout the majority of the Burial Lake sediment record, the Hg concentration is most strongly correlated with total organic carbon content and geochemical proxies sensitive to changes in redox conditions. We interpret this finding as an indication that a large fraction of Hg is mobilized from the lake catchment along with dissolved organic matter (DOM), iron (Fe), and manganese (Mn) that are mobilized as a result of saturation and deepening of the active layer during periods of warmer, but most importantly, wetter climate. The Hg record from Burial Lake suggests that as the climate warmed after the LGM, organic-rich permafrost soils and Hg accumulated in the catchment. The sudden increase in Hg mobilization from permafrost soils was then initiated at the onset of the Holocene due to the rapid increase in precipitation that coincided with the flooding of the Bering Land Bridge.