Marine primary productivity and redox conditions during the Permian-Triassic transition

The end-Permian mass extinction occurred during an interval of extreme global warming caused by enormous greenhouse gas emissions from Siberian Traps volcanism. A common concomitant effect of global warming is ocean deoxygenation which can be observed in geological and modern times. In the case of the end-Permian mass extinction, marine anoxia has long been postulated as one of the key killing mechanisms. However, causes for the Permian-Triassic (P-T) deoxygenation are under debate. Two frequently invoked scenarios are eutrophication and ocean stagnation.

We present geochemical data from two P-T carbonate sections across the Paleotethys Ocean. Productivity-related proxies (reactive P, TOC and trace elements) indicate high organic matter and P export to the sediments during the late Permian. A decrease in all these proxies during the C. yini conodont Zone suggests a decline of marine primary productivity at the study sites, approximately 30 kyr prior to the main marine extinction interval. Moreover, C/P ratios document a switch from intense P-recycling to efficient P-burial. Above the C. yini conodont Zone, Ce-anomalies (measured on the carbonate fraction of our samples) shift from negative to positive revealing deoxygenation of the local water columns.

Our proxy data imply that low productivity coincided with anoxic conditions at the study sites, hence not supporting a eutrophication scenario as a cause for the intensification of anoxia. Instead, we argue that ocean stagnation caused a stably stratified water column with reduced mixing, upwelling, overturning and ventilation. Regenerated P was trapped in the deeper, aphotic zones of the stagnant Paleotethys Ocean and was not available for photosynthesis.

We suggest that those settings of the Paleotethys Ocean represented by our study sections (deep slope and distal carbonate ramp) were characterized by high productivity and well-ventilated conditions during the relatively cool late Permian. Prior to the marine extinction interval, conditions switched to a low-productivity-anoxic state which persisted into the Early Triassic. This productivity collapse likely resulted in food shortage for higher trophic levels further stressing heterotrophic organisms before and during the extinction event.