Ca and Sr isotope constraints on the formation of the Marinoan cap dolostones

Abstract

Neoproterozoic cap dolostones, which ubiquitously overlie Marinoan glacial diamictites, may record marine and climatic paleo-environmental conditions at the termination of the largest glacial epoch in Earth's history. Many geochemical indices have been used to interpret cap dolostone formation in the context of extreme climate change in the aftermath of the Marinoan glaciation. However, there are significant debates about whether these geochemical data represent global signals or regional sedimentary and/or diagenetic processes. Here we analyzed cap dolostones from three different continental cratons for their Ca-isotope, Sr-isotope and trace element compositions in order to obtain new insights into formation of the Marinoan cap dolostone and post-Marinoan paleo-environmental conditions (<∼635 Ma). In three globally separated sections from South China (Yangtze Gorges area), North China (Northwest Tarim) and northwest Namibia, a similarly large negative δ44Ca excursion (∼0.6‰), coupled to a positive 87Sr/86Sr excursion, is recorded in the lower part of these cap dolostone successions. In the context of a relatively short duration for Marinoan cap dolostone (∼104 year timescale), we propose that the preservation of both the large negative δ44Ca excursion and the positive 87Sr/86Sr excursion in three widely-separated stratigraphic sections was not caused by globally uniform changes in isotopic compositions of the whole marine Ca and Sr reservoir. Instead, these excursions may have been caused by the addition of terrestrial meltwater and later deglacial runoff, carrying large amounts of Ca and Sr sourced from continental weathering into shallow seawater. A combined diagenetic-mixing model is used to track the coupled δ44Ca and 87Sr/86Sr variations in the cap dolostones. Large inputs of terrestrial meltwater and deglacial runoff under high CO2 atmospheric conditions in the aftermath of Marinoan glaciation is likely to have supplied abundant Ca, Mg and bicarbonate to shallow shelf seawater, helping facilitate cap dolostone deposition and contributing to the recorded negative Ca isotope excursion.