CARBON ISOTOPE EVIDENCE THAT HIGH PRIMARY PRODUCTIVITY DELAYED RECOVERY FROM END-PERMIAN MASS EXTINCTION

Ocean anoxia and euxinia were widespread during and after the end-Permian mass extinction and are commonly cited as explanations 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. To examine the chemical structure of the water column through the recovery interval and thereby better constrain paleoproductivity, we measured the carbon isotopic composition of carbonates from an exceptionally preserved carbonate platform in the Nanpanjiang Basin of south China. δ¹³C of limestones from 5 stratigraphic sections in south China displays a gradient of approximately 4‰ from shallow to deep water facies within the Lower Triassic. The limestones are systematically enriched in the platform interior relative to coeval slope and basin margin deposits by 2-4‰ at the peaks of correlative positive and negative δ¹³C excursions. This intense gradient subsequently collapses to less than 1‰ in 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, suggesting that Early Triassic ocean anoxia and delayed recovery of benthic animal ecosystems resulted from too much productivity rather than too little.