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

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. Plain Language Summary: This study examines how mercury (Hg) behaves in organic‐rich sediments that undergo thermal maturation, which is crucial for using Hg concentrations as a proxy for paleo‐volcanic activity. We analyzed three drill cores from the Lower Saxony Basin in Germany, each representing different stages of thermal maturity (immature, mature, and post‐mature). We measured Hg concentrations, organic content, and sulfur levels in these sediments. We found that mature and post‐mature sediments show more than double the Hg concentrations compared to immature sediments. This increase is likely due to the loss of organic matter and rock mass during maturation, which concentrates the remaining Hg. The relatively low temperatures (70–260°C) during maturation kept Hg from moving significantly, resulting in higher concentrations in mature sediments. There might be some redistribution of Hg within mature sediments, seen as enrichment in certain layers. Overall, the study highlights that thermal maturation effects must be accounted for when using Hg as a proxy for paleo‐volcanic activity. Key Points: Hg concentrations in thermally matured organic‐rich deposits have increased >2‐foldHg mobilization and recapture may occur during thermal maturation, and may result in stratigraphic increases in Hg concentrationsThe thermal history of sediments must be considered when using Hg as a proxy for volcanism [ABSTRACT FROM AUTHOR]