The redox transformation in the Yangtze Sea across the Ordovician-Silurian transition: Evidence from zinc isotopes in organic-rich shales

Abstract

Across the Ordovician-Silurian transition, the Yangtze Sea exhibited a complex redox pattern influenced by various environmental factors. Consequently, investigating this phenomenon is crucial for understanding the paleoenvironmental changes and their implications. Numerous and continuously evolving geochemical proxies provide insights into the redox conditions of paleo-oceans, among which the zinc isotope composition (δ⁶⁶Zn) in organic rich shales has been recently discovered as a potential alternative proxy for local marine redox conditions. In this study, the core JY143, drilled in the middle Yangtze shelf, was selected as the research subject. The variation in δ⁶⁶Zn values can be categorized into three distinct stages. The substantial influx of isotopically light Zn from igneous materials and its interaction with seawater may contribute to an overall lighter Zn isotope composition in seawater (δ⁶⁶Zn_sw). Under conditions characterized by high paleoproductivity and elevated organic burial rates, biological absorption of light Zn is primarily responsible for the low δ⁶⁶Zn values observed in organic-rich shales, indicating that organic-rich shales serve as an isotopically light Zn sink in the Yangtze Sea across the transition. Notably, the δ⁶⁶Zn_sw values observed during stage 1 are significantly lower than those recorded in stages 2 and 3, as well as below lithogenic source, possibly because during the early diagenetic stage, an excess of isotopically light Zn relative to ZnS in pore water was remobilized and subsequently released into deep seawater under oxidizing conditions. In summary, the transformation from an oxic environment to an anoxic (and even euxinic) environment resulted in a marked increase in δ⁶⁶Zn_sw values. This study presents new evidence from Zn isotopes regarding the oxic-anoxic (and even euxinic) transformation in the Yangtze Sea during the transition, while further underscoring the potential utility of δ⁶⁶Zn in organic-rich shales as a redox proxy for local paleo-ocean environments.