Vertical δ13Corg gradients record changes in planktonic microbial community composition during the end-Permian mass extinction

The global carbon cycle experienced a major perturbation during the Permian-Triassic transition, reflected inter alia in a large increase in the vertical (shallow-to-deep) gradient of carbonate delta C-13 (Delta C-13(carb)) in Early Triassic seas. However, the effect of this perturbation on the vertical gradient of organic matter delta C-13 in seawater (Delta C-13(org)) is less known to date. Here, we examine geographic and secular variations in Delta C-13(org) in Late Permian-Early Triassic seas of the South China craton and their implications for contemporaneous changes in microbial community composition and paleoredox conditions. Vertical delta C-13(org) gradients document the existence of a narrow and relatively shallow oxygen-minimum zone (OMZ) within the open-ocean Northern Deepwater Basin (NOW) and a deeper and thicker OMZ within the semi-restricted Nanpanjiang Basin (NPJB). For the pre-extinction interval, Delta C-13(org) gradients between the shallow and intermediate watermasses of the NOB and between the shallow and deep watermasses of the NPJB record contributions to the sediment of organic matter from bacteria living below the ocean-surface layer. These bacteria were mainly anaerobic chemoautotrophs that inhabited suboxic to anoxic waters of an OMZ and that produced more strongly C-13-depleted organic matter than surface-water eukaryotic algae, thus increasing Delta C-13(org). For the post-extinction interval, a reversed Delta C-13(org) in the NOB is attributed to blooms of green sulfur bacteria or other anaerobic photoautotrophic bacteria that were resident in a deep chlorophyll maximum layer and that had smaller carbon isotopic fractionation than surface-water algae. Although quantitatively important in intermediate- to deep-water areas during the EarlyTriassic, such bacteria were absent in coeval shallow-water sections, reflecting water-depth controls on microbial community composition. A concurrent flatting of the Delta C-13(org) gradient in the NPJB was caused by a return of oxic conditions and/or a decline of primary productivity in the surface ocean. (C) 2014 Elsevier B.V. All rights reserved.