Abstract

The temperature and degree of carbonate mineral supersaturation (CO3−2 ion concentration) of seawater are the two most likely controlling variables on the compositions of recent marine carbonate cements. The relative importance of these variables is difficult to assess in nature because they have similar trends with depth (0–1500 m) and latitude in modern oceans. We carried out laboratory experiments to investigate the relative growth rates of calcite, Mg calcite, and aragonite in seawater as functions of both temperature (5, 25, and 37 °C) and of carbonate ion concentration (2.5 to 15 times supersaturated with respect to calcite).

The precipitation rates of aragonite relative to those of calcite increase strongly with increasing temperature and are not affected greatly by changes in saturation state. At 5 °C, calcite precipitation rates are nearly equivalent to those of aragonite, regardless of the degree of saturation. At both 25 and 37 °C, aragonite precipitation rates are much more rapid than those of calcite (up to a factor of 4), except at very low saturation states. Calcite compositions vary from less than 5 mol% MgCO3 at 5 °C to 14 mol% MgCO3 at 37 °C. Our results suggest that the well-documented shift toward precipitation of lower mol% Mg calcite and the decrease in abundance of aragonite cements with increasing oceanic depth and latitude can be attributed largely to lower temperatures.

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