Investigating the Behavior of Sedimentary Mercury (Hg) During Burial-Related Thermal Maturation

Abstract

Understanding the behavior of mercury (Hg) in organic-rich sediments as they undergo thermal maturation is important, for example, because enrichment of Hg in sedimentary deposits has become a widely used proxy for volcanism from Large Igneous Provinces (LIPs). In this study, we evaluate the effects of such processes on sedimentary Hg concentrations by investigating a common stratigraphic interval in three drill cores with different levels of thermal maturity (immature, mature and post-mature) in Toarcian sediments (Posidonienschiefer Formation) from the Lower Saxony Basin, Germany. We present Hg concentrations, bulk organic geochemistry, and total sulfur data. Mercury concentrations in the mature and post-mature sediments are increased >2-fold relative to the immature material, which is greater than any potential differences in original Hg concentrations in the studied successions prior to burial. Organic-carbon and host-rock mass loss during thermal maturation may have concentrated Hg in the mature sediments to some extent, provided Hg is considered effectively immobile. The increased Hg, TOC-normalized Hg, and TS-normalized Hg are most likely linked to the “closed system” behavior of Hg in sedimentary basins and the relatively low temperatures (70–260°C) during maturation that resulted in limited Hg mobility. More speculatively, a certain degree of redistribution of Hg within the mature sediments is suggested by its enrichment in distinct stratigraphic levels. Regardless of the exact mechanisms at play, the elevated Hg concentrations in mature sediments amplify both Hg/TOC and Hg/TS, implying that thermal effects must be considered when using normalized Hg as a proxy for far-field volcanic activity.