Fluctuations in chemical weathering on the Yangtze Block during the Ediacaran-Cambrian transition: Implications for paleoclimatic conditions and the marine carbon cycle

The Ediacaran-Cambrian transition was characterized by profound climatic, tectonic, and oceanographic fluctuations, followed by the remarkable radiation of metazoans. To further understand the paleoclimatic and paleoenvironmental conditions during this interval, comprehensive mineralogical and geochemical analyses were performed on Ediacaran-Cambrian shales (in the Doushantuo through Jiumenchong to Bianmachong formations) at Daotuo, South China. Their major and trace element compositions and ratios indicate that all of the studied shales were derived from basically uniform source rocks that were similar to granodiorite. The chemical index of alteration (CIA) values in the Doushantuo and Bianmachong formations mostly range from 65 to 80, indicating that they experienced a moderate degree of chemical weathering and were thus deposited in a warm and humid climate. In contrast, a sharp drop in CIA values (~ 60-70) in the lower Jiumenchong Formation, together with decreased Al/Ti ratios that are indicative of increased eolian inputs, are suggestive of a relatively cold and arid climate for the Early Cambrian Yangtze Block. In this scenario, eolian fluxes could have supplied nutrients to the marine environment; increased upwelling, along with a rising thermocline and nutricline during the extensive marine transgression, could have returned abundant nutrients to the surface seawater, thus greatly stimulating primary productivity and intensifying anoxic conditions, which would ultimately have resulted in a high degree of organic burial in the sediments. Therefore, the negative correlation between the corrected CIA values and the total organic carbon (TOC) content may imply that climate-related changes played a role in the enrichment of organic matter during the Early Cambrian.