d13CORG AND d13CCARB ACROSS THE P-TR BOUNDARY OF A SHALLOW MARINE CARBONATE PLATFORM, NANPANJIANG BASIN, S. CHINA: IMPLICATIONS FOR METHANE POISONING

A distinct d13C excursion towards more negative values in carbonate and organic carbon is documented in a 60m Permian-Triassic section deposited as a shallow-marine isolated carbonate platform in the Nanpanjiang Basin, south China. This section spans from the Changxinian to Dienerian (identified by conodont biostratigraphy) and is characterized by a distinctly different marine boundary facies, changing from massive, skeletal lime-packstone in the Changxinian to distinctive calcimicrobial framestone (interpreted as photosynthetic calcified cyanobacteria) in the Griesbachian H. parvus zone. The excursions across the Permian-Triassic boundary in d13Corg (by 2.8‰ to -27.8‰) and d13Ccarb (by 4.2‰ to-0.9‰) occur immediately above the paleontologically-defined boundary interval at the change from skeletal packstone to microbial framestone and are associated with a drop in average TOC from 6.3% to 1.1%. This negative shift in both d13Corg and d13Ccarb together with low TOC content persists throughout the Griesbachian H. parvus zone. Average d13Corg values remain depleted in the overlying I. isarcica zone whereas d13Ccarb values return to more enriched values. At the base of the upper Griesbachian N. carinata zone both d13Corg and d13Ccarb show an abrupt enrichment, which continues into the Dienerian to -25.9‰ for d13Corg and +1.9‰ for d13Ccarb;TOC content increases to 1.9% in the Dienerian. These data document for the first time a corresponding negative shift of d13Corg, d13Ccarb, and TOC with the base of the Griesbachian H. parvus zone and the onset of growth of calcified microbial framestones following the extinction. Correlation of the d13C excursion with the GSSP at Meishan shows that the calcified microbial framestones and H. Parvus zone (~13 m thick) have exceptionally high sediment accumulation rates relative to the correlative zone at Meishan which is ~ 30 cm thick (beds 25-27c). These results have important implications for the interpretation of the causes of the P-Tr extinction. The carbon isotope excursion concurrent with a decrease in TOC does not support an incursion of anoxic deep-ocean waters into shallow-platform, photic environments. Alternatively, a significant input of 13C-depleted methane and its oxidation product CO2 could cause hypoxia even in a shallow marine environment and would promote the survival of photosynthetic organisms. This scenario is supported by the widespread occurrence of the calcimicrobial framestone and the abrupt 13C-depletion in d13Corg and d13Ccarb.