MERCURY STABLE ISOTOPES AS A NOVEL PROXY FOR PHOTIC ZONE EUXINIA

Photic zone euxinia (PZE) occurs when anoxic, sulfide rich waters expand into the photic zone. Although this condition is rare today, PZE is thought to have been widespread in some ancient marine ecosystems with potentially profound effects on marine and terrestrial ecosystems. PZE has been invoked as an impediment to the evolution of complex life in the Precambrian, and as a kill mechanism during Phanerozoic mass extinctions.

Despite these important implications, developing convincing evidence of PZE from ancient sedimentary rocks has proven difficult. The current approach relies on the detection of organic biomarkers diagnostic of anoxygenic phototrophs (e.g., green and purple sulfur bacteria) in sedimentary records. However, the interpretations of these biomarkers are sometimes problematic due to non-unique synthetic pathways of some biomarkers and possible contamination with allochthonous pigments.

Mercury stable isotope compositions in marine sedimentary rocks may serve as a new inorganic proxy of PZE. The basis of this novel proxy is that Hg undergoes unique mass independent isotope fractionation (MIF) during aqueous photoreduction of Hg(II) when complexed by reduced sulfur ligands. Prior work has shown that photoreduction of Hg(II) bound to oxygen-containing ligands (e.g., carboxyl) produces positive MIF (enrichment of ¹⁹⁹Hg and ²⁰¹Hg) in the residual Hg(II), whereas photoreduction of Hg(II) bound to reduced sulfur ligands (e.g., thiol) produces negative MIF in the residual Hg(II).¹

To test this proxy in ancient sediments, we measured Hg isotope compositions in Late Mesoproterozoic (~1.1 Ga) shale deposits of shallow epicontinental seas (Atar and El Mreiti Groups, Taoudeni Basin) with a dynamic water column redox chemistry ranging from oxic to euxinic conditions. We found zero to slightly positive MIF for Hg isotopes in sediments deposited under oxic water columns. In contrast, sediments deposited under euxinic water columns showed significantly negative MIF. The contrasting Hg isotope compositions in oxic vs. euxinic conditions is best explained by the photoreduction of Hg(II) bound to non-sulfur vs. reduced sulfur ligands, suggesting that Hg stable isotopes in ancient sedimentary rocks are a promising tracer of PZE.

1. W. Zheng, H. Hintelmann, J. Phys. Chem. A, 114, 4246 (2010).