Manganous water column in the Tethys Ocean during the Permian-Triassic transition

Abstract

Ocean anoxia was one of the key killing mechanisms responsible for the end-Permian mass extinction (~252 Ma). However, the temporal evolution and the triggering mechanisms of the end-Permian anoxia are controversial, with the current view being that the water column deoxygenation was a spatially and temporally heterogeneous event. Here, we use cerium-anomalies, uranium contents and rare earth element and yttrium (REY) compositions measured on the carbonate fraction of samples from two marine sections in Armenia and South China to constrain the evolution of end-Permian marine anoxia. In particular, we attempt to identify intervals of manganous redox conditions (i.e., Mn2+-rich water column). These are characterized by the reduction of Mn(IV)-oxides at a redox potential lower than of nitrate and higher than of Fe(III)-oxide reduction. Most of our samples yield REY patterns with seawater characteristics suggesting the dominance of hydrogenous REYs. While decreasing U concentrations in the C. yini Zone suggest an overall intensification of global marine anoxia in the latest Permian, Ce-anomalies shift from negative to positive above the C. yini Zone, implying a deterioration in the oxygen content of the local water columns. Highly positive Ce-anomalies (Ce/Ce* > 1.3) are firstly reported for the P-T interval, indicating enrichment of Ce in the water column and the establishment of manganous redox conditions at both locations. This change coincided globally with climatic warming, the onset of the marine extinction interval as well as locally with the disappearance of sponge spicules and radiolarians (South China) and the appearance of microbialites (Armenia).