Climatic Controls on a Holocene Mercury Stable Isotope Sediment Record of Lake Titicaca

Mercury (Hg) records in sediment archives inform past patterns of Hg deposition and the anthropogenic contribution to global Hg cycling. Natural climate variations complicate the interpretation of past Hg accumulation rates (HgARs), warranting additional research. Here, we investigated Hg stable isotopes in a ca. 8k year-long sediment core of Lake Titicaca and combined isotopic data with organic biomarkers and biogeochemical measurements. A wet period in the early Holocene (8000-7300 BP) induced strong watershed erosion, leading to a high HgAR (20.2 +/- 6.9 mu g m(-2) year(-1)), which exceeded the 20th century HgAR (8.4 +/- 1.0 mu g m(-2) year(-1)). Geogenic Hg input dominated during the early Holocene (f(geog) = 79%) and played a minor role during the mid- to late Holocene (4500 BP to present; f(geog) = 20%) when atmospheric Hg deposition dominated. Sediment Delta Hg-200 values and the absence of terrestrial lignin biomarkers suggest that direct lake uptake of atmospheric Hg(0), and subsequent algal scavenging of lake Hg, represented an important atmospheric deposition pathway (42%) during the mid- to late Holocene. During wet episodes of the late Holocene (2400 BP to present), atmospheric Hg(II) deposition was the dominant source of lake sediment Hg (up to 82%). Sediment Delta Hg-199 values suggest that photochemical reduction and re-emission of Hg(0) occurred from the lake surface. Hg stable isotopes show promise as proxies for understanding the history of Hg sources and transformations and help to disentangle anthropogenic and climate factors influencing HgAR observed in sediment archives.

Automated summary

This study investigated the Hg stable isotopes in a sediment core of Lake Titicaca, revealing the changes in Hg deposition during the early and mid-late Holocene. The findings showed that high Hg deposition rates were induced by watershed erosion during the early Holocene, while atmospheric deposition dominated during the mid-late Holocene. Moreover, photochemical reduction and re-emission from the lake surface played an important role in Hg deposition.