δ13C evidence that high primary productivity delayed recovery from end-Permian mass extinction

Euxinia was widespread during and after the end-Permian mass extinction and is commonly cited as an explanation for delayed biotic recovery during Early Triassic time. This anoxic, sulfidic episode has been ascribed to both low- and high-productivity states in the marine water column, leaving the causes of euxinia and the mechanisms underlying delayed recovery poorly understood. Here we use isotopic analysis to examine the changing chemical structure of the water column through the recovery interval and thereby better constrain paleoproductivity. The δ13C of limestones from 5 stratigraphic sections in south China displays a negative gradient of approximately 4‰ from shallow-to-deep water facies within the Lower Triassic. This intense gradient declines within Spathian and lowermost Middle Triassic strata, coincident with accelerated biotic recovery and carbon cycle stabilization. Model simulations show that high nutrient levels and a vigorous biological pump are required to sustain such a large gradient in δ13C, indicating that Early Triassic ocean anoxia and delayed recovery of benthic animal ecosystems resulted from too much productivity rather than too little.

Research Highlights
► Lower Triassic δ13C data from a carbonate platform show a 2–4‰ depth gradient.
► The δ13C gradient diminished to near zero by the mid-Anisian (Middle Triassic).
► Earth system model shows high primary productivity likely drove the δ13C gradient.
► Modeling shows high primary productivity also explains Lower Triassic marine anoxia.
► High primary productivity helped to delay recovery from end-Permian extinction.