Are Early Triassic extinction events associated with mercury anomalies? A reassessment of the Smithian/Spathian boundary extinction

Abstract

High concentrations of mercury, possibly connected with widespread volcanism of the Siberian Traps, have previously been associated with the Smithian/Spathian (Early Triassic) boundary (SSB) in the Sverdrup Basin, Tethyan sections in India and China, as well as with a shallow-water record in western Spitsbergen. We confirm this Hg/TOC anomaly in the deeper water record at Wallenbergfjellet, central Spitsbergen. However, both paleontological age control and carbon isotopes indicate that the Hg anomaly occurred mainly within strata of middle Smithian age. Therefore, this Hg anomaly is unlikely to be directly and causally related to mechanisms contributing to the late Smithian global extinction of nektonic faunas. The TOC and trace element data suggest generally more oxygenated conditions during the Smithian compared to the Spathian, which is at odds with the hypothesis that oxygen depletion may have been a global kill mechanism for the SSB extinction. Further work is needed to assess if precise timing and paleogeographic distribution of anoxia shows any consistent pattern or not during the Smithian and Spathian. The very abrupt lower limb of the positive carbon isotope excursion (CIE) and the coarser grain size immediately below the boundary between the Lusitaniadalen Member and the Vendomdalen Member indicate a substantial stratigraphic gap of latest Smithian age, a previously neglected signal shared with many other boreal SSB sections. Ammonoid age control also indicates that the onset of the late Smithian gap in the high latitudes was earlier than in the Tropics. The gradual end of the positive CIE contrasts with the frequent spike shape observed in tropical shelf records and is definitively earliest Spathian in age. The middle Smithian Hg anomaly in the Boreal record is only visible in the Hg/TOC values, and is associated with a possible shift in organic matter type from terrestrial to marine in the case of Spitsbergen. This suggests that the middle Smithian Hg/TOC anomaly in Spitsbergen may not unequivocally originate from volcanism, and calls for additional caution before interpreting Hg spikes as a volcanic proxy.