Zinc isotopic evidence for enhanced continental weathering and organic carbon burial in the Early Silurian

Abstract

The Late Ordovician and Early Silurian periods experienced glaciation and mass extinctions. However, debates exist regarding the factors that initiated the glaciation and the subsequent delay in biotic recovery. To uncover the relationships between paleoclimate, marine redox states, and biotic recovery in the Early Silurian, temporal variations in Zinc (Zn) isotopic compositions along with other multi-geochemical proxies were analyzed for a carbonate-dominated section in South China. A stratigraphic δ66Zn trend was found in studied section, indicating a widespread disturbance to the marine Zn cycle. A significant negative shift in δ66Zn by up to ∼0.7 ‰ was observed immediately following the Hirnantian Glaciation and HICE, which can be attributed to a combination of reduced burial efficiency of organic carbon, fast weathering of large igneous provinces and increased riverine Zn input. This was followed by a ∼ 0.4 ‰ rise in δ66Zn values, alongside a sharp decrease in δ13C values, indicating an increase in sulfide precipitation (e.g., ZnS, FeS) due to expansions of euxinia. Subsequently, a notable rise in δ66Zn values by up to ∼1.1 ‰ is interpreted as an increase in organic carbon burial, given the concurrent variation of δ66Zn and δ13C during this period. The findings suggest that the increase in organic carbon burial, a reduction in reverse weathering, or a combination of them contributed to the glaciation in Silurian. Furthermore, the weathering-induced high primary productivity led to anoxic water conditions in the latest Hirnantian and Rhuddanian. This prolonged marine anoxia in seawater thus caused the deposition of organic-rich sediments and hindered biotic recovery in the Rhuddanian.