PALEOENVIRONMENTAL CONTROLS ON SPATHIAN MARINE REPTILE FAUNA AND THE TIMING OF RECOVERY

The Majiashan section in Chaohu, South China archives a complete Olenekian succession, which yields abundant, well-preserved and diverse Early Triassic marine vertebrate fossils. Detailed petrographic study coupled with stable isotopic analysis of microdrilled components and overlap in microdrilled and bulk rock δ¹³C values indicate complete diagenetic resetting of δ¹⁸O_carb but excellent preservation of bulk rock δ¹³C from the late-Smithian to late Spathian interval (200 m thick). Lack of correlation between carbonate percentage and δ¹³C_carb further indicates that δ¹³C_carb records syndepositional cementation of platform-derived sediment rather than cementation by early porewater diagenesis and organic matter oxidation. The stratigraphic variability in δ¹³C_carb defines a large-magnitude shift from -2‰ to +4‰ at the Smithian-Spathian boundary (SSB) following the widely recognized negative excursion in the late Smithian. δ¹³C_carb values stabilize at +2 to +3‰ through the early Spathian decreasing in a stepwise manner to -2‰ in the late Spathian. This 5 ‰ shift in seawater δ¹³C in the slope setting of Majiashan over a ~3 Ma period cannot be explained primarily by changes in productivity. Rather correlation of inferred seawater δ¹³C with changes in facies patterns, organic matter δ¹³C_org and δ¹⁵N and previously published conodont δ¹⁸O_phosvalues suggest substantial expansion and retraction of the oxygen minimum zone (OMZ) as a response to global warming and cooling.

Spectral analysis of δ¹³C_carb combined with cyclostratigraphy defines eight 400 ky eccentricity cycles in the Spathian. These orbital-scale cycles provide a precise chronology for the timing of oceanic anoxic events and marine ecosystem recovery. Recent studies reveal that the global marine environment in end-Smithian was anoxic, hot and stressful for marine ecology. For the Chaohu Fauna, fish records and late Spathian δ¹³C_carb perturbation, which is near analogous to that of the late Smithian, indicate that anoxia and warming were re-established ~1.6 Ma after the end-Smithian global warming event. The first occurrence of the marine predator Chaohusaurus in the mid-Spathian, however, indicates that the marine ecosystem had recovered prior to the return to anoxic conditions — a finding that is much earlier than previously believed.