TRACKING NEOPROTEROZOIC-CAMBRIAN MARINE ENVIRONMENTAL AND CLIMATE CHANGE THROUGH THE CARBONATE GEOLOGICAL RECORD

The Proterozoic-Cambrian boundary is traditionally defined as a sharp transition between recognisable Phanerozoic and ‘strange’ Neoproterozoic marine environments and seawater conditions. Marine carbonates, which precipitated from these ancient oceans, record the history of this interval of environmental change through their sedimentology and geochemical composition. A range of recent studies on these carbonates suggest a complex and protracted environmental evolution across the Precambrian-Cambrian boundary. This talk will focus on the carbonates which characterise the Cryogenian glaciations and their aftermath, which are commonly composed of dolomite. Authigenic dolomite is present through the Sturtian glacial stratigraphy in many locations globally, and has characteristics of precipitation from syn-glacial seawater. ‘Cap’ carbonates following glaciations, are in many ways, similar to dolomite marine hardgrounds which have been documented to occur throughout the Neoproterozoic stratigraphy in the Flinders Ranges, South Australia. In the context of global carbonate sediments from the Neoproterozoic, these Cryogenian dolomites are not so unusual. Instead, we suggest that these dolomite units are a normal product of seawater conditions which allow significant marine (e.g. fabric-preserving) dolomite precipitation, including low oxygen and high Mg²⁺ seawater conditions. To gain a global perspective on these observations, they are compared to compilations of the occurrence of fabric preserving dolomite, as well as redox geochemical data. A collection of carbonate petrographic data from specific depositional fabrics suggests a decline in marine dolomitisation starting in the Cryogenian but continuing though the Cambrian in some settings. Marine carbonate paleoredox data also shows a prolonged interval of highly variable redox conditions through the Neoproterozoic to Cambrian. These insights highlight that Neoproterozoic-Cambrian interval was characterised by protracted but substantial environmental change in marine settings, that had a profound impact on the style of carbonate precipitation.