Are sulfides the primary host of sedimentary Hg? A case study from the Lower Jurassic of the Surat Basin (Australia)

Several mass extinction events and major paleoenvironmental perturbations during the Phanerozoic have been linked to massive volcanic eruptions, especially those associated with large igneous province (LIP) emplacement. Because magmatism during LIPs can be accompanied by the release and widespread dispersal of mercury (Hg), Hg concentration in sedimentary rocks has proven to be a useful proxy for paleovolcanism. Organic matter is often the primary host of Hg in sedimentary rocks, so total organic carbon (TOC) is commonly used to normalize Hg and thus robustly assess anomalies. Nevertheless, the singular use of anomalies in Hg/TOC to assess volcanic activity is questionable. Sulfides or clay minerals also act as hosts for Hg, with sulfides in particular having a strong affinity for Hg in reducing environments. At the same time, the source of organic matter has a potentially very strong influence on Hg abundance because the Hg binding potential of different organic matter types varies. Here, we investigate the influence of organic matter content and type on Hg concentrations and investigate how Hg is hosted in shallow marine and nearshore sedimentary rocks of the Lower Jurassic Precipice Sandstone and Evergreen Formation from the Surat Basin, Australia. Our results demonstrate that sulfides were likely the main host of Hg in these rocks, despite the general absence of reducing conditions in these rocks and the significant variability in organic matter sources. Our results suggest that researchers should routinely collect sulfide or at least total sulfur concentration data when testing for volcanic Hg enrichment in ancient sedimentary rocks. •Sulfide important host for mercury, even in an oxic environment.•Collection of Hg/TS or Hg/sulfide recommended for paleovolcanism analysis.•Hg/TOC data potentially misleading for inferring volcanic activity.