High-frequency redox variation across the Ordovician–Silurian transition, South China

Abstract

Mass extinction and extensive black shale deposition were associated with widespread ocean anoxia during the Ordovician–Silurian transitional interval, yet uncertainty among trace-metal proxies has precluded an improved understanding of redox dynamics at this time of Earth history and their relationship with animal evolution and accumulation of organic-rich shale. Herein, we describe results of a geochemical investigation of the Qianqian #1 section of South China that preserves the stratigraphic record of the inner shelf of the Upper Yangtze Platform. Specifically, newly acquired total organic carbon (TOC), total sulfur (TS), iron (Fe) speciation, and major element and trace metal abundances are considered with existing data from the studied section. Our results reveal that redox trends suggested by Fe speciation data are generally consistent with those based on Corg/P ratios and redox-sensitive trace element (RSTE) enrichment. However, documented differences among redox histories based on Fe speciation, Corg/P ratios, and RSTE enrichment may reflect the influence of high-frequency redox fluctuations. Comparison of bimetal ratios (U/Th, V/Cr, Ni/Co, and V/(V + Ni)) to Fe speciation, Corg/P ratios, and RSTE enrichment suggest that commonly cited thresholds of bimetal ratios for various redox states are unreliable, though variations in U/Th, V/Cr, and Ni/Co may provide a more robust indication of relative redox changes. A documented strong correlation of changing redox conditions and fossil distributions suggests that largely dynamic and ferruginous, perhaps euxinic, conditions contributed to biotic crisis and recovery. A strong coupling of TOC and redox proxies suggests that protracted anoxia or euxinia is critical to the accumulation and preservation of organic matter. Our study demonstrates that redox evaluations that rely upon a single geochemical proxy or elemental ratio are likely to be uncertain. Rather, multiple geochemical proxies are required to explore relationships among redox states, animal evolution, and deposition and preservation of organic-rich shale.