Assessment of Hg speciation changes in the sedimentary rock record from thermal desorption characteristics

Sedimentary mercury (Hg) has become a widely used proxy for paleo-volcanic activity. However, scavenging and drawdown of Hg by organic-matter (OM) and sulfides are important non-volcanic factors determining variability in such records. Most studies, therefore, normalize total Hg (HgT) to a Hg “host-phase” proxy (e.g., HgT/TOC for OM, HgT/TS for sulfides), with the dominant host-phase determined based on the strongest observed (linear) correlations. This approach suffers from various non-linearities in Hg-host-phase behavior and does not account for succession-level, let alone sample-level, Hg speciation changes. Thermal desorption characteristics or “profiles” (TDPs) for many Hg species during pyrolysis analysis are well-established with applications including distinguishing between OM-bound Hg and different Hg sulfides and oxides in (sub-)recent sediments. We explore the use of TDPs for geological sediment (rock) samples and illustrate the presence of multiple release phases (Hg species)—correlated to geochemical host-phase—in (almost) all the 65 analyzed Tithonian (146–145 Ma) silt and mudrock samples. By quantifying the Hg in each release phase for every sample, we find TOC concentration may determine ∼60% of the variability in the first (lower temperature) Hg TDP release phase: a stark difference with the total Hg released from these samples, where ∼20% of variation is explained by TOC variability. TDPs provide insight on sample-level Hg speciation and demonstrate that, while the common assumption of single-phase Hg speciation in sedimentary rocks is problematic, differences in Hg speciation can be detected, quantified, and accounted for using commonly applied techniques—opening potential for routine assessment.