Responses of oceanic chemistry to climatic perturbations during the Ordovician-Silurian transition: Implications for geochemical proxies and organic accumulations

Abstract

The organic-rich sediments deposited in the Ordovician-Silurian transition (OST) on the Yangtze Platform not only represent significant shale gas sources, but also record information on the evolution of the paleo-environment. Much effort has been made to clarify the correlations among the climate, paleo-ocean, and organic accumulations, but debates remain. In this study, we further investigate the major and trace elements and total sulfur contents of the Datianba section, which has been studied for C–Fe–S geochemistry. Our new results, combined with the published Fe speciation, show that diagenesis may have a significant impact on Fe speciation. The incorporation of Fe into carbonate may lead to mixed signals of pore water and water columns, limiting the use of Fe speciation in carbonate-rich strata. In the pre- and post-Hirnantian periods, the high productivity caused by strong upwelling and chemical weathering, combined with euxinic water conditions, facilitated organic accumulation. However, productivity may play a more important role in the pre-Hirnantian period because it not only provides organic matter, but also reforms the redox conditions. Although euxinia and upwelling also developed in the early Hirnantian, the depleted micronutrient inventories caused by euxinia and the reduced chemical weathering inhibited productivity, and thus the organic accumulation. In Hirnantian, the poor preservation conditions (suboxic water columns and non-euxinic pore waters) led to low organic accumulations. This study stresses the importance of a multi-proxy method in paleo-environmental reconstruction and reveals climate-controlled oceanic evolution and organic accumulation.