Lithium isotope composition of Ediacaran dolostones from the Nuccaleena and Doushantuo formations

Abstract

The end of the Cryogenian glaciations undoubtedly affected the chemistry of the Neoproterozoic oceans, with potential consequences for the evolution of life; the duration and extent of this influence are poorly constrained. Lithium (Li) isotopes in carbonates can be used to investigate past weathering events and riverine input into the oceans. Here, we report the Li isotope (δ7Li) composition of the Ediacaran cap dolostones (Marinoan) from the Nuccaleena Formation, South Australia and the Doushantuo Formation, South China to investigate changes in weathering during the aftermath of the Marinoan glaciation. The origin of dolomite formation is still hotly debated, and the inability to precipitate dolomite at ambient temperatures (the ‘dolomite problem’) has not yet been resolved. The dominant hypothesis for the presence of marine dolomite is that it is of secondary origin, owing to diagenetic replacement of calcium carbonate, but a second hypothesis is that it forms as a result of primary marine deposition; both are plausible hypotheses. Using carbon isotopes (δ13C) and ratios of manganese and strontium (Mn/Sr), we suggest that diagenesis may not have significantly altered the δ7Li composition of the primary dolomite, but this cannot be discounted without further diagenetic modelling. As a result, we cannot infer the δ7Li composition of the Ediacaran oceans, as further work must be done to address the impact of diagenesis on the δ7Li values. Nevertheless, this dataset contributes to a Li isotope chemostratigraphic record of the Proterozoic, which is a key element to understanding the emergence of complex life.