Secular Variation in Abiotic Marine Carbonates: Constraints on Phanerozoic Atmospheric Carbon Dioxide Contents and Oceanic Mg/Ca Ratios

Abstract

Abiotic marine carbonates have been precipitated as ooids and synsedimentary cement within shallow marine carbonate sequences throughout the Phanerozoic. These components exhibit variable abundances and compositions that temporally correlate with estimates of global eustasy. With rising sea level, aragonite and acicular magnesium calcite precipitation gave way to the formation of equant calcite and possibly dolomite. Similarly, the abundance of ooids was limited by positions of global sea level and associated changes in ocean chemistry. Modern abiotic marine carbonates also exhibit variable abundances and compositions as a function of changing fluid chemistry with latitude and depth. Chemical data on ambient marine waters suggest that carbonate ion concentrations and rates of precipitation presently control the nature of marine cement and ooids. Using data from modern oceans to calculate the requisite atmospheric $$pCO_{2}$$ levels necessary to form this range of phases in ancient seas suggests that atmospheric $$pCO_{2}$$ values have ranged from about $$3 \times 10^{-4} to 40 \times 10^{-4} atm$$, increasing with marine transgression and decreasing with marine regression.