Response of the Elemental Chemistry of Carbonate Phases to Secular Change in Ocean Chemistry.

Abstract

Calcium carbonate is the dominant inorganic mineral precipitate of the global ocean and by no coincidence, it is also the most abundant mineral produced by life. Secular variation in the paleontologic, stratigraphic, petrographic, and geochemical character of limestones argues that the physicochemical conditions under which it has precipitated from seawater have not been constant over geologic time. A major aspect of this variation has been the modality in the primary mineralogy between times of dominantly low-Mg calcite and times of aragonite and high-Mg calcite. Because these minerals have different chemical characteristics, they respond differently during diagenesis and thus affect the evolution of important limestone attributes such as porosity and permeability. Seawater Mg/Ca likely controls this modality. To estimate seawater Mg/Ca from calcite Mg/Ca it has been customary to assume a linear relationship between these two quantities. However, experiments where biotic and abiotic calcites were precipitated in the laboratory suggest that this relationship is a power function. This power function, a “partition power function” , accounts for the observation that partition coefficients tend to decrease at high solution Mg/Ca because of interference with the growing calcite crystal surface by Mg. This method more accurately estimates paleoseawater Mg/Ca from ancient calcite phases. Partition power functions can be used to model the change in the Mg/Ca of calcite phases in response to secular variation in seawater Mg/Ca. The smaller range and average value of Mg/Ca in carbonate phases precipitated during times of low seawater Mg/Ca implies that during such times, there existed a reduced driving force for diagenesis because of the paucity of metastable high-Mg calcite. Calcite Mg/Ca has also been used as a thermometer of paleoceans. When applied to recent geologic history, this exercise is elementary because change in seawater Mg/Ca has been negligible. By combining a partition power function with a modern temperature calibration to produce “Mg-T partition functions,” secular variation in seawater Mg/Ca can be accounted for accurately for strata older than 5 Ma. In this way, reasonable paleotemperatures and seawater 18O values can be obtained while using a model of seawater Mg/Ca in accordance with geochemical proxy data.