Carbonates before skeletons: A database approach

Carbonate minerals have precipitated from seawater for at least the last 3.8 billion years, but changes in the physical and chemical properties of carbonate rocks demonstrate that the nature, loci, and causes of this precipitation have varied markedly through time. Biomineralization is perhaps the most obvious time-bounded driver. However, other changes in the sedimentological character of Archean and Proterozoic carbonates offer an under-considered record of fluctuations in Earth’s chemical, biological, and physical systems before and during the rise of animal life. Geobiologists and geochemists have successfully used large geochemical datasets to track changes in Earth’s surface chemistry through time, but inconsistencies between proxies make the recognition of clear spatial-temporal patterns challenging. In order to place new constraints on the environmental history of Archean and Proterozoic oceans, especially with regard to redox state, we present for the first time a high-resolution database of global Precambrian and Cambrian carbonate sedimentation. Our initial datasbase, named Microstrat, comprises carbonate-bearing successions dating from c. 3.4 billion to ~500 million years old; these include more than 130 carbonate-bearing formations, digitized at the highest stratigraphic resolution possible and classified by environment of deposition. Lithofacies details are recorded at the finest scale available, including a range of microbial and other depositional fabrics, mineralogy, environment of deposition, age and location. We interrogate the Archean, Proterozoic, and Cambrian carbonate records, testing both long-standing and novel hypotheses about changes in Earth’s carbonate system through time. Changes in carbonate sedimentology illuminate global changes in bioturbation and microbial community structure; the depth-dependent timing of marine oxygenation and the transition from anaerobic to aerobic respiration at and within the seafloor; and temporal changes in patterns of dolomitization. Shifts in both the loci of carbonate precipitation and in markers of animal activity are consistent with time- and depth-dependent trends in ocean oxygenation. We also note changes through time in the morphologies of stromatolites and thrombolites, which we suggest reflect secular shifts in carbonate precipitation, clastic sediment transport, and eukaryotic evolution. Observations from the database and petrographic studies confirm that the nature and extent of Proter