Investigating the influence of natural and artificial thermal maturation on sedimentary mercury (Hg)

Abstract

In recent decades, enrichment of mercury (Hg) in sedimentary deposits has been widely used as a proxy for volcanism from Large Igneous Provinces (LIPs). Mercury is naturally released into the atmosphere through volcanic exhalations and other processes, then dispersed and sequestered in sediments. Recent studies have delved into the impact of extremely high-temperature exposure on Hg in sediments, such as intrusion and contact metamorphism. However, the behaviour of sedimentary Hg exposed to more moderate warming, such as associated with thermal maturation and hydrocarbon formation in a sedimentary basin is still underexplored. We conducted a series of artificial maturation experiments on immature organic-rich marine mudrocks, specifically the Posidonienschiefer or Posidonia Shale (Lower Jurassic) in the Lower Saxony Basin, Germany. These pyrolysis experiments enabled us to investigate the changes in Hg concentration within rock residues and evolved organic fluids across varying maturation stages. Our findings reveal a progressive decline in Hg concentrations in sediments with increasing thermal maturity throughout the experiments (24 hours to 5 weeks at 325 ºC). Notably, the most significant Hg concentration loss occurs between the time-steps 5 days and 15 days. However, the substantial Hg loss from the pyrolysis experiments strongly differs from observations from naturally matured Posidonia Shale. In contrast to the slight decrease observed in the pyrolysis experiments, we recorded a trend of increasing Hg concentrations associated with higher maturity on three Posidonia Shale cores., We explore the mechanisms for these striking differences between the experimental and natural maturation and how these effects may have controlled Hg mobility during hydrocarbon formation and impacted the use of sedimentary Hg as a proxy for LIP-related volcanism.