Paired mass-dependent and mass-independent Hg isotope excursions through non-euxinic intervals: Signals of post-depositional Hg isotope fractionation?

Abstract

Mercury (Hg) in sedimentary deposits has become a widely used proxy for reconstructing paleo-volcanic activity. However, a comprehensive understanding of Hg accumulation pattern and the isotope variation during early diagenesis is still lacking. A thorough diagenetic evaluation of sedimentary rocks is crucial prior to the interpretation of geochemical data, as diagenetic changes can lead to considerable differences between the original seawater signatures and the sedimentological records. We observed a Hg isotopic pattern characterized by a negative trend in δ²⁰²Hg with a concurrent gradual positive shift in Δ¹⁹⁹Hg throughout non-euxinic intervals in multiple marine sedimentary successions. We propose that the synchronous Hg mass dependent fractionation (MDF) and mass independent fractionation (MIF) were driven by diagenetic events. The release of organic-bound Hg and its subsequent diffusion downward to sites where it complexed with pyrite in early diagenetic stages may induce δ²⁰²Hg shift towards more negative values in the Hg precipitate, and in turn, contribute to the concurrent positive shift of Δ¹⁹⁹Hg observed in these sections.