T198 Extreme Environmental Conditions and Biotic Responses during the Permian-Triassic Boundary Crisis and Early Triassic Recovery II

Tuesday, 21 October 2014
8:00 AM - 12:00 PM
215/216

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

Paper number: T198-13
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

Abstract

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 δ18Ocarb 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 δ13Ccarb further indicates that δ13Ccarb records syndepositional cementation of platform-derived sediment rather than cementation by early porewater diagenesis and organic matter oxidation. The stratigraphic variability in δ13Ccarb 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. δ13Ccarb 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 δ13Corg and δ15N and previously published conodont δ18Ophosvalues suggest substantial expansion and retraction of the oxygen minimum zone (OMZ) as a response to global warming and cooling.

Spectral analysis of δ13Ccarb 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 δ13Ccarb 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.