Assessment of Hg Speciation Changes in the Sedimentary Rock Record From Thermal Desorption Characteristics

Abstract

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.

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

“…Our findings further highlight that it is important to note that the use of normalized Hg as a volcanic proxy must be used with caution as Hg does not linearly scale with TOC due to a variety of (syn-)depositional processes (e.g., Fendley et al, 2024;Frieling et al, 2023Frieling et al, , 2024Grasby et al, 2019;Schuster et al, 2018). In addition, we must consider similar processes acting on relatively low-TOC samples; maturation in such settings could easily lead to values that (far) exceed the Hg/TOC shale value.…”