High amplitude redox changes in the late Early Triassic of South China and the Smithian-Spathian extinction

The Early Triassic was a time of remarkably high temperatures, large carbon cycle perturbations and episodes of widespread ocean anoxia. The sediments in the Nanpanjiang Basin of South China provide superb opportunities to examine the sedimentary response to these extreme conditions especially during the crisis interval at the Smithian-Spathian (S-S) boundary. We have investigated a deep water section at Jiarong and a shallower water section at Mingtang. These contain a range of facies including black shales, micritic limestone units and rudaceous carbonate event beds that include flat pebble conglomerates and breccia debrites that bear similarities to the hybrid event beds seen in clastic turbidite successions.Redox proxies (pyrite framboids and trace metals) reveal that widespread anoxia in the late Smithian persisted into the Novispathodus pingdingshanensis Zone of the early Spathian before a sharp transition to highly oxygenated "griotte facies" (red marine strata) in the Icriospathodus collinsoni Zone that records an "oxic rebound". Benthic faunas are locally common but of low diversity and dominated by thin-shelled bivalves and ostracodes with small foraminifers and exceptionally rare fish remains. Bioturbation was intense only in the early-middle Spathian (I. collinsoni conodont zone) Griotte facies. Anoxia and extremely high temperatures probably played a role in severely restricting the abundance of fish and the small sizes of marine invertebrates at this time. The presence of ooids and seafloor fan cements in our study sections indicates highly saturated conditions rather than acidification at the end of the Smithian.