There is considerable evidence that changes in seawater chemistry over the course of geologic time have profoundly influenced biomineralization by marine organisms.(1-3) This evidence, some of it based on experiments with living organisms, has revealed that many algae and invertebrates have less control over the chemical composition of their skeletons (are more at the mercy of seawater chemistry) than has traditionally been recognized. Nearly all of these organisms produce skeletons that consist largely of CaCO3. There is also much evidence that future changes in atmospheric pCO2 will affect calcifying organisms and that feedbacks entailing these organisms will exert a strong influence on the level of atmospheric pCO2.
Lime sediments, which consist of CaCO3 with a varying percentage of Mg substituting for Ca, are the precursors of limestone. Abiotic lime sediments form by precipitation in shallow tropical seas, where relatively high temperatures reduce the solubility of CO2 in seawater. These inorganic precipitates consist largely of three materials: (1) cement that forms in cavities within organic reefs, (2) ooids, which are roughly spherical, sand-sized grains that form by accretion of CaCO3 around nuclei of skeletal material on wave-agitated shallow sea floors (today these grains form magnificent dunes in the Bahamas), and (3) minute crystals that precipitate from seawater, although certain bacteria also induce the precipitation of such crystals by expelling Ca2+ and leaking inorganic carbon from their cells.(4) The mineralogy of these three entities has oscillated over the course of geologic time between aragonite (an orthorhombic polymorph of CaCO3) and calcite (a hexagonal polymorph of CaCO3), giving rise to the labels aragonite seas and calcite seas to describe marine conditions that existed over substantial intervals of geologic time.(5, 6) (Actually, as will be explained below, calcite with a high Mg content can form along with aragonite in aragonite seas.) Since the beginning of the Cambrian Period, when for the first time in Earth’s history numerous kinds of organisms produced skeletons of CaCO3, there have been three intervals of aragonite seas and two of calcite seas (labeled Aragonite I, Aragonite II, etc. in Figure 1).
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Stanley S. M., 2008. Effects of Global Seawater Chemistry on Biomineralization: Past, Present, and Future. Chemical Reviews 108(11): 4483-4498. Article (subscription required).
