Limited mercury (Hg) partitioning into bitumen and efficient gaseous Hg reabsorption during early thermal maturation of organic-rich mudrocks

Recent studies have looked into the impact of exposure to extremely high temperature, such as typifies contact metamorphism, on mercury (Hg) distributions in sediments, and observed significant Hg loss with increasing temperature. By contrast, sediment cores of Lower Jurassic organic-rich Posidonienschiefer (also known as Posidonia Shale) from the Lower Saxony Basin, Germany seemingly showed Hg enrichment after maturation related to basin subsidence and burial under more typical geothermal gradients. To investigate the apparent differences in Hg behaviour, we conducted a series of artificial maturation experiments on immature Posidonienschiefer samples, analysing Hg concentrations within rock residues and bitumen generated during early maturation stages. Thermal desorption profiles were used to track Hg speciation changes in the matured sediment. Our results show a progressive decrease in Hg concentrations in sediments with increasing thermal maturity throughout the experiments, which dominantly relates to released gaseous Hg with only a fraction of the Hg being partitioned into the bitumen (≤1% of the total initial Hg). Further experiments showed that gaseous Hg in closed vessels was rapidly (≤1 h) and efficiently (≥95 %) reabsorbed into the sediment during cooling. We speculate that our experiments may simulate some of the processes that drive Hg mobilisation and recapture occurring in contact aureoles, such as the rapid release and recapture of gaseous Hg. However, the Hg speciation changes that occur in our experiments and during natural burial maturation clearly differ. Specifically, the changes in Hg speciation in the natural system with burial-related maturation suggest that under those conditions organic matter associated Hg may instead transition into more thermally stable phases.