HIGH AMPLITUDE REDOX CHANGES IN THE LATE EARLY TRIASSIC OF SOUTH CHINA AND THE SMITHIAN/SPATHIAN EXTINCTION

Diverse Smithian-Spathian (S-S) sediments in the Nanpanjiang Basin of South China provide superb opportunities to examine changes in the oceanic redox conditions and their correlated responses in marine ecosystems during this critical time interval. We investigate a deep water section at Jiarong (Guizhou Province) as well as a shallow water section at Mingtang (Guizhou Province) to constrain these changes. A range of event beds are seen, including flat pebble conglomerates and breccia debrites that bear similarities to the hybrid event beds seen in clastic turbidite successions.

Analyses of pyrite framboids diametres show that widespread anoxia in the late Smithian persisted into the Ns. pingdingshanensis zone of early Spathian. The transition from the black shale facies to red beds records well ventilated conditions indicating a form of “oxic rebound” from the euxinic waters of the S-S interval. This may have been triggered by climatic cooling and oxygen increase driven by organic carbon/pyrite burials. The balance between euxinic and super-oxic conditions was a delicate one in the early Spathian. Anoxia and δ¹³C oscillations favour a model in which the late Smithian switch from a negative to a positive trend in δ¹³C is best explained as increased burial of organic matter.

Marine communities in the S-S interval show a monotonous pandemic combination: benthic faunas are locally common but show very low diversity and dominated by thin-shelled bivalves and ostracodes with rarer echinoderm grains, foraminifer and very rare microgastropods. Calcifiers such as, brachiopods, bryozoans, calcareous algae/tubiphytes, corals, and sponges are absent. Deep water environments were rather inhabitable in the Smithian, evidenced by rare fossils at Jiarong. Bioturbation in shallow water environments only returned in the early-middle Spathian (Icriospathodus collinsoni conodont Zone). Anoxia is postulated to play a role in the S-S crisis possibly exacerbated by extremely high temperatures. The latter is evidenced by the small size of the fauna and ichnofauna and suggested by the extreme rarity of fish. Acidification – a popular extinction mechanism for the crisis – is not in accord with evidence for carbonate oversaturated conditions such as, ooids and seafloor fan cement that are seen in the studied S-S sections.