Oceanic water chemistry evolution and its implications for post-glacial black shale formation: Insights from the Cryogenian Datangpo Formation, South China

Abstract

The Cryogenian Nanhua Basin in South China comprises one of the best-preserved Neoproterozoic glacial-interglacial sedimentary fills. The Datangpo Formation (ca. 663–654 Ma) was deposited after the Sturtian glaciation and is characterised by extensive organic-rich black shales and interbedded manganese deposits. Trace element data suggest overall anoxic waters with varying degrees of euxinia. These anoxic and sulfidic conditions, as inferred by high ratios of FeHR/FeT and Fepy/FeT and significant enrichments of molybdenum, developed due to high organic carbon loading, resulting in the formation of the black shales. The high organic matter flux was likely the result of increased nutrient supply from continental weathering, upwelling nutrient-rich deep seawater, and seawater exchange due to elevated sea levels after deglaciation, which resulted in a temporary connection with the open sea. An episode of oxygenation during deposition of the manganese-bearing interval shortly after deglaciation is indicated by a negative excursion in the iron speciation data, and generally low enrichments of trace elements. Muted sulfate reduction rates, and thus inferred less euxinic conditions, occurred due to decreased organic matter loading during deposition of the upper-part of the black shales. The lower marine primary productivity was likely a result of suppressed bio-essential elements due to progressively weaker chemical weathering under cooler climatic conditions. Superheavy pyrite with δ34Spy up to +61‰ for the Datangpo Formation is consistent with a compilation of published data. We propose that multiple processes contributed to the superheavy pyrite formation, and suggest a new, more accurate way of recalibration of the sulfate reservoir in sedimentary basins with similar geological and/or geochemical settings. This dataset and model serves as a reference for better understanding the formation of post-glacial organic-rich black shales, including the temporal evolution of oceanic conditions after low-latitude glaciations.