New evidence for millennial-scale interactions between Hg cycling and hydroclimate from Lake Bosumtwi, Ghana

Abstract. Changing hydrology impacts the biogeochemical cycling of elements such as mercury (Hg), whose transport and transformation in the environment appear linked to hydroclimate on diverse timescales. Key questions remain about how these processes manifest over different timescales and their potential environmental consequences. For example, millennial-scale Hg-hydroclimate interactions in the terrestrial realm are poorly understood, as few sedimentary records have sufficient length and/or resolution to record abrupt and long-lasting changes in Hg cycling, and the relative roles of depositional processes on these changes. Here, we present a high-resolution sedimentary Hg record from tropical Lake Bosumtwi (Ghana, West Africa) since ~96 ka. A coupled response is observed between Hg flux and shifts in sediment composition, the latter reflecting changes in lake level. Specifically, we find that the amplitude and frequency of Hg peaks increase as the lake level rises, suggesting that Hg burial was enhanced in response to an insolation-driven increase in precipitation at ~73 ka. A more transient, threefold increase in Hg concentration and accumulation rate is also recorded between ~13 and 4 ka, coinciding with a period of distinctly higher rainfall across North Africa known as the African Humid Period. Two mechanisms, likely working in tandem, could explain this correspondence: (1) an increase in wet deposition of Hg by precipitation and (2) efficient sequestration of organic-hosted Hg. Taken together, our results reaffirm that changes in hydroclimate, directly and/or indirectly, can be linked to millennial-scale changes in tropical Hg cycling, and that these signals can be recorded in lake sediments.