Coupled Precipitation-Vegetation Changes Affect Hg Accumulation and Isotope Composition in Loess Records

Geochemical records from loess-paleosol deposits offer valuable insights into past paleoclimate and paleoecological changes. Here we explore the potential of mercury (Hg) stable isotopes to study the changes of paleoprecipitation and paleovegetation and their causal effect on terrestrial Hg accumulation on the Chinese Loess Plateau (CLP). We first document how variations of Hg concentrations, Δ¹⁹⁹Hg and Δ²⁰⁰Hg isotope signatures in modern soils and loess are controlled by coupled precipitation-vegetation gradients on the CLP, and unaltered by postdepositional processes. Increased precipitation promotes vegetation growth and foliar gaseous elemental Hg uptake, lowering Δ¹⁹⁹Hg and Δ²⁰⁰Hg in modern soils and loess. Similar to modern soils and loess, we attribute Hg accumulation and isotope signatures in loess-paleosol deposits to changes in vegetation Hg uptake in response to variation in paleoprecipitation. Both modern and paleo loess and soils suggest faster Δ¹⁹⁹Hg changes and Hg accumulation to precipitation under high precipitation conditions (e.g., > 500 mm yr^-1) than under arid conditions, reflecting their potential as proxies for pronounced wet and dry climate changes in the CLP. Our results evidence enhanced sequestration of atmospheric Hg emissions in loess soils during interglacial wetter periods. Warming and changes in precipitation patterns in response to global climate change may therefore lead to enhanced Hg burial in soils.