Mo isotope and trace element patterns of Lower Cambrian black shales in South China: Multi-proxy constraints on the paleoenvironment

Abstract

The Precambrian/Cambrian boundary marks a critical interval in Earth history. Paleoenvironmental changes of the oceans have been suggested as a possible explanation for some significant events such as the marine biological evolution (“Cambrian explosion”). Here we report a remarkable variation range in trace element, iron speciation and molybdenum isotope signatures of black shales from the Early Cambrian Dingtai profile (Guizhou province) and three synsedimentary polymetallic Ni–Mo–(PGE–Au) sulfide ore deposits (Dazhuliushui and Maluhe in Guizhou province, and Sancha in Hunan province) on the Yangtze Platform, South China. Based on the (isotope) geochemical features, we classify the Dingtai profile into three intervals: interval 1 (0–1m) has extreme metal and TOC enrichment and heavy δ98/95Mo values. This unit has all features of a euxinic environment, and is then followed by interval 2 (1–11m) with variable δ98/95Mo composition. Elevated contents of redox-sensitive elements and their ratios, and relatively high FeHR/FeT ratios (“reactive iron”) suggest anoxic conditions. The overlying interval 3 (11–33m) reveals oxic to suboxic conditions with generally light δ98/95Mo values in the lowermost part and fluctuant δ98/95Mo values in the uppermost part, and relatively low redox-sensitive element contents. We propose a model of a partly closed ocean basin with periodic replenishment together with changing redox conditions for the remarkable variability of δ98/95Mo values. Light δ98/95Mo values indicate restricted seawater circulation dominated by local continental input, periodically replenished by fresh seawater with heavy δ98/95Mo.A global compilation of Mo isotope data from black shales indicates that the Mo isotopic variations have a consistent temporal trend, reflecting the oxygen evolution of the oceans. Relatively light δ98/95Mo values of the Lower Cambrian black shales compared to those of modern euxinic sediments are indicative of more widespread anoxic marine environments during the Early Cambrian.