MARINE CARBONATES RECORD A PROTRACTED NEOPROTEROZOIC-CAMBRIAN ENVIRONMENTAL TRANSITION (Invited Presentation)

The “Cambrian explosion” of animal life traditionally defined the boundary between recognisable Phanerozoic ecosystems and the strange environments of the Neoproterozoic. Over many decades, researchers realised that the record of animal and other complex life originated much earlier than the Cambrian. Comprising complex reefs frameworks, sandy seabeds rich with Ediacaran biota and seawater host to many other unusual organisms; Neoproterozoic marine environments were diverse habitats. Similar to the fossil record, it is also now apparent that environmental conditions did not suddenly change from the Neoproterozoic into the Cambrian. Marine carbonates, which precipitated from these ancient oceans, record the history of this environmental change and suggest a much more complex and protracted environmental evolution through the Precambrian-Cambrian boundary. Paleo-redox data (new and published Ce anomaly proxy records) from marine carbonate precipitates show a prolonged interval of highly variable redox conditions from the Tonian through Cambrian, with no evidence for a sustained step change in oxygenation. Marine carbonate mineralogy reflects Mg-rich conditions in Neoproterozoic seawater, which only gradually transitions towards “calcite seas” starting in the late Ediacaran, with links to the advent of carbonate biomineralisation at this time. Recent compilations of the extent of fabric preserving dolomite (building on Hood and Wallace, 2018; Cantine et al., 2019) suggest a decline in marine dolomitisation starting in the Cryogenian but continuing though the Cambrian in some settings, with implications for the “Precambrian dolomite problem”. These insights collectively highlight that the terminal Precambrian and early Paleozoic were characterised by protracted but substantial environmental change.