Constraints on Early Triassic carbon cycle dynamics from paired organic and inorganic carbon isotope records

摘要

Large δ~(13)C excursions, anomalous carbonate precipitates, low diversity assemblages of small fossils, and evidence for marine euxinia in uppermost Permian and Lower Triassic strata bear more similarity to Neoproterozoic carbonates than to the remainders of the Permian and Triassic systems. Middle Triassic diversification of marine ecosystems 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 large δ~(13)C excursions remains poorly constrained. Here we present a continuous Early Triassic δ~(13)Corg record from south China and use it to test the extent to which Early Triassic excursions in δ~(13)Ccarb record changes in the δ~(13)C of marine dissolved inorganic carbon (DIC). Regression analysis demonstrates a significant positive correlation between δ~(13)Corg and δ~(13)Ccarb across multiple sections that span a paleoenvironmental gradient. Such a correlation is incompatible with diagenetic alteration because no likely mechanism will alter both δ~(13)Corg and δ~(13)Ccarb records in parallel and therefore strongly indicates a primary depositional origin. A simple explanation for this correlation is that a substantial portion of the preserved Corg was derived from the contemporaneous DIC pool, implying that the observed excursions reflect variation in the δ~(13)C of the exogenic carbon reservoir (ocean, atmosphere, biomass). These findings support existing evidence that large δ~(13)C excursions are primary and therefore strengthen the case that large-scale changes to the carbon cycle were mechanistically linked to the low diversity and small size of Early Triassic fossils. Associated sedimentary and biogeochemical phenomena further suggest that similar associations in Neoproterozoic and Cambrian strata may reflect the same underlying controls.