Development of a high-productivity and anoxic-euxinic condition during the late Guadalupian in the Lower Yangtze region: Implications for the mid-Capitanian extinction event

Abstract

Widespread marine anoxia is considered one of the most important causes for the mid-Capitanian mass extinction. However, the mechanisms and subsequent sulfidic process for the mid-Capitanian mass extinction are still debated. Here, we focus on a comprehensive study including marine productivity and redox conditions of the Lower Yangtze area across the mid-Capitanian mass extinction based on multiple geochemical indicators. Our results show that the redox conditions and marine productivity have gone through three stages. The pre-extinction early-middle Capitanian is characterized by anoxic conditions with intermittent euxinic episodes and moderate-high productivity, whereas the middle Capitanian is dominated by stronger euxinic conditions and higher productivity. Subsequently, an oxic shallow water environment and low productivity are recognized by significant decreases in all of these geochemical indicators across the mid-Capitanian. The appearance and development of the anoxic-euxinic conditions was probably caused by high productivity related to enhanced upwelling, which led to the intensified heterotrophic respiration and subsequent oxygen-depletion of the water column. More sluggish oceanic circulation that resulted from the global warming associated with the eruption of the Emeishan Large Igneous Province and upward movement of the chemocline due to great regression also further exacerbated the euxinic conditions during the middle Capitanian. The development of these euxinic conditions may have played a major role in triggering the super-anoxic ocean and is also coincident with the mid-Capitanian biotic crisis, which suggests the possible causal link between them. With comprehensive analysis of profiles across the globe, sulfidic models showing that the sulfidic water first occurred in the deep water during the early-middle Capitanian and shoaled to shallow water in the middle-late Capitanian could be established, despite the fact that the temporally and spatially heterogeneous sulfidic processes in the late Guadalupian are still debated.