Chapter 4 Chemostratigraphy and the Neoproterozoic glaciations

Abstract

Although the pre-glacial Proterozoic isotopic record is poorly constrained, it is apparent that the chemical and isotopic composition of the oceans began to change during the early to mid-Neoproterozoic and experienced considerable fluctuations alongside climatic instability during much of the subsequent Cryogenian and Ediacaran periods. The earliest known large negative δ 13 C excursion appears to post-date 811 Ma and fluctuations became progressively more extreme, culminating in the late-Ediacaran ‘Shuram–Wonoka’ anomaly. The negative excursions are commonly associated with pre-glacial and post-glacial times, while extremely high δ 13 C values are characteristic of strata between glaciations. The precise causal mechanism for these excursions is subject to debate. Seawater 87 Sr/ 86 Sr rose during the Neoproterozoic, with abrupt increases following deglaciation consistent with enhanced weathering rates. Reported marine sulphate and pyrite δ 34 S data exhibit marked variation through this interval, although the changes are not always consistent within or between sedimentary successions of equivalent age. Iron-speciation studies indicate that much of this variation was caused by fluctuating and low sulphate concentrations in seawater, which at times led to the build-up of ferruginous conditions in the ocean. The application of chemostratigraphy to understanding and correlating the Neoproterozoic glaciations evokes considerable controversy, and many questions persist regarding…