Experiments on the effect of secular variation in seawater magnesium/calcium (calcite and aragonite seas) on calcareous biomineralization

The polymorph mineralogy of simple, hypercalcifying marine organisms has varied in concert with abiotic CaCO3 precipitates throughout the Phanerozoic. This synchronicity is caused by secular variation in the Mg/Ca ratio of seawater (mMg/Ca > 2 = aragonite + high-Mg calcite; mMg/Ca < 2 = calcite), determined by the mixing rate of mid-ocean-ridge/large-igneous-province hydrothermal brines and river water, driven primarily by the rate of ocean crust production.; Here, I present experiments evaluating the effect of seawater Mg/Ca on the biomineralization and growth of extant representatives of geologically significant calcifying taxa which were subjected to the fluctuations in oceanic Mg/Ca responsible for the aragonite and calcite seas of the Phanerozoic. Sediment-producing codiacean algae (aragonite) and coccolithophores (low-high Mg-calcite); reef-building scleractinian corals (aragonite) and coralline algae (high Mg-calcite); and reef-dwelling animals (high Mg-calcite) were grown in artificial seawaters formulated over the range of mMg/Ca ratios (∼1.0--5.2) which occurred throughout these taxa's history.; Codiacean algae and scleractinian corals exhibited higher rates of calcification and growth in seawater favoring their aragonitic mineralogy (mMg/Ca > 2), producing a significant portion of their CaCO3 as calcite in the calcite seawater (mMg/Ca < 2). Coccolithophores showed higher calcification and growth rates, producing low-Mg calcite in the calcite seawater. The coralline algae and reef-dwelling animals also varied their skeletal Mg/Ca with seawater Mg/Ca.; The codiacean algae's and scleractinian corals' precipitation of both aragonite and calcite in calcite seawater and the coccolithophores', coralline algae's and reef-dwelling animals' precipitation of calcite with skeletal Mg/Ca varying with seawater Mg/Ca, indicate that while these organisms exert significant influence over their calcification, their biomineralogical control can be partially overridden by ambient Mg/Ca. This suggests that organisms which secrete aragonite or high-Mg calcite in modern aragonite seas, secreted an aragonite + low-Mg calcite mixture or only low-Mg calcite, respectively, in calcite seas.; The elevated calcification and growth rates for codiacean algae and corals in aragonite seawater, and for coccolithophores in calcite seawater, indicate that hypercalcifying organisms have a competitive advantage when their polymorph mineralogy is favored by ambient Mg/Ca.; These experiments support the empirical fossil evidence that oceanic Mg/Ca has greatly influenced the success and mineralogy of the sediment-producing and reef-building hypercalcifiers throughout the Phanerozoic.