Sedimentary carbonates through Phanerozoic time

Abstract

Plate tectonic processes play a critical role in the origin and distribution of sedimentary carbonates through Phanerozoic time. The Phanerozoic age distribution of sedimentary properties like calcite/ dolomite ratio, inferred oöid and cement mineralogy, and survival rate of continental carbonates is cyclic. The cycles appear to be coupled to plate tectonic processes that give rise to global sea level change and changes in the properties of the ocean-atmosphere system. First-order changes in sea level are driven by the accretion of mid-ocean ridges: high accretion rate, high sea level; low accretion rate, low sea level. Although correlations between sea level and sedimentary carbonate properties are not strong, high sea level over an extended period of time appears to be correlated with low calcite/dolomite ratios, lack of inferred aragonite oöids and cements, and maxima in the survival rate of continental carbonates. The opposite is true for extended periods of low mid-ocean ridge accretion rates and global sea levels. The lack of strong correlations may reflect an insufficient data base and the possibility of lags between sea level change and change in carbonate properties. Furthermore, the survival rate of continental carbonates appears to be affected by differential cycling and, therefore, may not be directly related to accumulation rate. It appears that the environmental conditions for early dolomitization and calcite oöid and cement formation are best met during extended times of high sea level when atmospheric CO 2 levels are high and the saturation state of seawater with respect to carbonate minerals relatively low. During low sea levels, early dolomitization is less favored, and aragonite precipitates are more abundant because of low atmospheric CO 2 levels and enhanced seawater carbonate saturation states. Differential cycling has modified the Phanerozoic sedimentary carbonate mass-age distribution. Because of erosion of younger units within continental carbonate cycles, it may be difficult to derive an unequivocal record of the partitioning of carbonate between the deep-sea and shallow-water realms of deposition during the Phanerozoic. This difficulty must be considered in further quantification of geochemical models describing the geologic history of atmospheric CO 2 and climate change.