Abstract

Periods of widespread abundance of calcified cyanobacteria in marine environments are here termed cyanobacterial calcification episodes (CCEs). Intense such episodes occurred in the Cambrian-early Ordovician, Late Devonian and Mid-Late Triassic. Mild episodes occurred during the Early Carboniferous and Jurassic-Early Cretaceous. Possible episodes during parts of the Silurian and Permian await confirmation. Cyanobacterial calcification is not obligate but is dependent upon environmental factors favouring carbonate precipitation. Cyanobacterial calcification episodes are therefore interpreted as reflecting periods of increased marine carbonate precipitation. This is supported by the general abundance of marine carbonate cements and oolites during cyanobacterial calcification episodes.

Cyanobacterial calcification episodes correspond with Phanerozoic periods for which elevated global temperatures have been inferred. They do not appear to correlate closely with inferred changes in aragonite/calcite facilitation (the Sandberg curve) and PCO2 through time. It is concluded that variations in the precipitation rates of abiotic and quasi-abiotic marine carbonates through time have been controlled primarily by temperature. The mineralogy of these precipitates appears instead to have been mainly influenced by PCO2 and the Mg/Ca ratio, also linked to sea-level through plate tectonic processes. Thus, the controls on rate and mineralogy of precipitation were independent of one another.

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