DOLOMITE AT THE DAWN OF ANIMAL LIFE

Field and petrographic observations of carbonate rocks show that both the abundance and petrographic fabric of dolomite have changed throughout geologic time. Specifically, previous work – typically qualitative or limited in temporal or spatial scope – has suggested that both the amount and fabric of dolomites changed from more abundant and fabric-retentive to less abundant and fabric-destructive at the Precambrian-Cambrian boundary. These changes may reflect major shifts in the elemental chemistry of ancient oceans, and are thus important to constrain. Here, we present a global record of the abundance and fabric of dolomite through time, with a focus on the Ediacaran through Cambrian periods. Our work combines the high temporal resolution and near-perfect North American coverage of Macrostrat with the machine-reading infrastructure and large database of published literature of xDD (formerly geoDeepDive). We built a text mining and natural language processing-based application to rapidly “read” scientific papers to determine the age, location, mineralogy, and petrographic characteristics of carbonate rocks described in the literature. We focus on dolomites and related facies (e.g. stromatolites, evaporites) to test hypotheses of dolomite formation through time. Our results suggest that the abundance of dolomite relative to all carbonate rocks was constant from the Cryogenian through Devonian periods. However, the probability of finding stromatolites in a dolomitic formation decreases by the mid-Cambrian, which may indicate a shift away from microbial mediation as an important mechanism through which dolomite formed. Our results challenge the classical viewpoint that dolomite was more common in the Precambrian than in the Phanerozoic, and suggest the mechanisms by which dolomite formed may have changed around 540 Ma.