Redox fluctuations during the Ediacaran-Cambrian transition, Nanhua Basin, South China: Insights from Cr isotope and REE+Y data

Abstract

Reconstructing fluctuations in marine and atmospheric redox conditions during the late Precambrian/Early Cambrian is essential to better our understanding of the link between oxygen and biological evolution. This study presents REE+Y and Cr isotopic data together with TOC data and redox-sensitive trace metal concentrations of marine black shales and cherts from the Cambrian Jiumenchong Formation and Ediacaran Liuchapo Formation from South China, in order to improve our understanding of the redox conditions in the Nanhua basin during this critical time period. The Liuchapo Formation records negative Ce anomalies and highly fractionated δ53Cr values of up to 1.06 ± 0.09‰, suggesting deposition under oxic conditions and implying high oxidative weathering rates during the end of the Ediacaran. The overlying Jiumenchong Formation is characterised by a stratigraphic upward decrease in δ53Cr values to typical igneous values as well as a decrease in negative Ce anomalies and an increase in positive Eu anomalies, suggesting an increase in detrital content due to more aeolian weathering and deposition under anoxic conditions at the beginning of the Cambrian. The latter is also confirmed by the trends in V/Cr and Th/U ratios, which further indicate an eventual return to (dys-)oxic seawater conditions during the Early Cambrian.