LOWER TRIASSIC CARBONATE δ238U RECORD DEMONSTRATES EXPANDED OCEAN ANOXIA DURING SMITHIAN THERMAL MAXIMUM AND IMPROVED VENTILATION DURING SMITHIAN-SPATHIAN BOUNDARY COOLING EVENT

The Smithian-Spathian boundary (SSB) transition was recently identified as a major biocrisis on the road to marine ecosystem recovery following the end-Permian mass extinction. Marine anoxia is hypothesized to have played an important role in the SSB biocrisis. However, an understanding of the relationship between marine anoxia and the SSB biocrisis has been hampered by limited knowledge of the timing, duration, and extent of anoxia across the SSB. Here, we present a high-resolution carbonate U-isotope record from the Jiarong section of South China that spans the middle Smithian to early-middle Spathian interval. This record shows persistent negative δ²³⁸U values (averaging –0.56 ‰) in the late Smithian, followed by a rapid positive shift (from −0.78 ‰ to –0.10 ‰) across the SSB and then a more gradual shift back to lower δ²³⁸U values in the early to middle Spathian. U isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late Smithian and the early-middle Spathian, but that it contracted sharply at the SSB. This redox pattern shows an excellent correspondence to previously published tropical sea-surface temperature (SST) records, with peak ocean anoxia coinciding with the Smithian Thermal Maximum (STM) and diminished anoxia with a pronounced global cooling event at the SSB. Although paleontological records commonly do not distinguish between terminations during the late Smithian and SSB, we hypothesize that the SSB biocrisis, which was marked by sharp diversity losses among conodonts, ammonoids, and other marine invertebrates, was primarily associated with the STM, in which case ocean anoxia is likely to have been a major stressor of marine biotas.