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 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 2 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.

Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery

“…The apparent duration of the SSB unconformity in Qiakong, Laren and Shanggang is of 64 ± 104, 149 ± 143, and 127 ± 137 kyr, respectively, and excludes large scale plate tectonic controls that operate at a much longer time scale. The consistent biochronological age of this worldwide gap (Galfetti et al, 2007a,b;Hammer et al, 2019) is also at variance with the timing of regional block faulting tectonics within the Nanpanjiang Basin (e.g. Sun et al, 2015).…”

“…With the exception of NE Nevada, the present ammonoid/conodont biochronological frame for the SSB in the Luolou Fm. is the most highly resolved one, and is diagnostic of the low latitude record (Hammer et al, 2019). In Qiakong and Lilong, the SSB is bracketed by conodont UAZ5 and UAZ6 and falls in the separation interval between GXZ and TAZ.…”

“…The four analyzed sections yield a spike-shaped positive CIE peaking either slightly below or at the boundary between the black shales (Unit IVb) and the nodular limestone (Unit Va). The particular shape of his CIE is known from almost all low latitude SSB sections and its variable amplitude suggests truncation of the CIE peak, thus pointing to a global hiatus (Hammer et al, 2019). In Qiakong, where sedimentation rates are higher and the record more complete, the CIE spans the upper half of the black shales and peaks at a δ 13 C value of +4.5 .…”

“…Exact timing established for the SLIP activity (Burgess et al, 2017;Augland et al, 2019), however, precludes invoking volcanic drivers younger than 250.5 Ma, incompatible with our timeline for an end-Smithian CIE starting at about 249.6 Ma (Figures 4-6 and Supplementary Table S2), and with our middle Smithian ages (samples SHA303 and 309). Weak mercury anomalies of Smithian age in the Boreal realm were also tentatively proposed as evidence for additional volcanism, however, Hammer et al (2019) showed that these are not of late Smithian but of middle Smithian age and that their volcanic origin is largely uncertain.…”

“…Unlike the PTBME anomaly, Smithian weak Hg/TOC anomalies do not appear to be synchronous but are scattered throughout middle and late Smithian times. Therefore, clear evidence for substantial middle Smithian volcanism that would lead to a thermal maximum is still wanted (see section "discussion" in Hammer et al, 2019).…”