CONSTRAINTS ON EARLY TRIASSIC CARBON CYCLE DYNAMICS FROM PAIRED ORGANIC AND INORGANIC CARBON ISOTOPE RECORDS

Large δ¹³C excursions (CIEs), anomalous carbonate precipitates, and evidence for marine euxinia in the Late Permian and Early Triassic are reminiscent of Neoproterozoic geochemistry. Middle Triassic diversification coincided with the waning of anoxia and stabilization of the global carbon cycle, suggesting that environment-ecosystem linkages were important to biological recovery. However, the Earth system behavior responsible for these phenomena remains poorly constrained. Here we examine the extent to which the Early Triassic CIEs record changes in δ¹³C of marine dissolved inorganic carbon (DIC). Biostratigraphically correlated strata across the Paleo-Tethys and Panthalassic oceans display multiple coeval δ¹³C shifts, suggesting the record is reflective of large perturbations in the exogenic carbon cycle. However, diagenetic processes and oxidation of a large dissolved organic carbon (DOC) pool are other proposed drivers of large δ¹³C excursions, including the negative excursion in δ¹³C at the end-Permian extinction horizon. If paired δ¹³C_carb and δ¹³C_org measurements are coupled and other evidence for diagenetic control of the δ¹³C record is absent, the record is likely driven by changes in marine DIC composition. If not, the perturbations may be related to non-steady state carbon cycle behavior and the buildup of a large DOC reservoir, as suggested for the Neoproterozoic.

Here we present one of the first Early Triassic δ¹³C_org records from south China and apply it as a test of carbon cycle coupling through this interval. Regression modeling demonstrates a clear correlation between δ¹³C_org and δ¹³C_carb across multiple sections and spanning a paleoenvironmental gradient. A simple explanation is that a substantial fraction of the preserved C_org formed from the contemporaneous DIC pool, implying that the observed excursions reflect variation in the δ¹³C of the exogenic carbon reservoir. Our results do not rule out a large DOC pool during Late Permian time, but require that it was nearly entirely oxizided by the earliest Triassic.