Effect of secular variation in oceanic Mg/Ca on calcareous biomineralization

The polymorph mineralogy of simple, hypercalcifying marine organisms has generally varied in synchroneity with the polymorph mineralogy of abiotic CaCO3 precipitates (ooids, marine cements) throughout the Phanerozoic Eon. This synchroneity is caused by secular variation in the Mg/Ca ratio of seawater (SW; mMg/Ca > 2 = aragonite + high-Mg calcite; mMg/Ca < 2 = calcite), determined primarily by the mixing rate of mid-ocean-ridge/large-igneous-province hydrothermal brines and river water, driven by the global rate of ocean crust production. Here, we present experiments evaluating the effect of seawater Mg/Ca on the biomineralization and growth of extant representatives of hypercalcifying taxa that have been subjected to fluctuations in oceanic Mg/Ca in the past. Codiacean algae (arag), scleractinian corals (arag), coccolithophores (low-high Mg-calc), coralline algae (high Mg-calc), various reef-dwelling animals (echinoids, crabs, shrimp, calcareous serpulid worms; high Mg- calc), and calcifying microbial mats (arag + high-Mg calc) were grown in artificial SW formulated over the range of mMg/Ca (1.0 to 5.2) that occurred throughout each taxon's history. Codiacean algae and scleractinian corals exhibited higher rates of calcification and growth in artificial SW favoring their aragonite mineralogy and, significantly, produced a portion of their CaCO3 as calcite in the artificial calcite SW. Coccolithophores (low-high Mg calc.) showed higher calcification and growth rates and produced low-Mg calcite in the calcite SW. Likewise, coralline algae and the reef-dwelling animals (high-Mg calc) varied skeletal Mg/Ca with seawater Mg/Ca. The calcifying microbial mats grew equally well in the calcite and aragonite SW and varied their mineral polymorph commensurate with the SW (mMg/Ca<2 = low- Mg calc; mMg/Ca>2 = arag + high-Mg calc), suggesting a nearly abiotic mode of calcification. The precipitation of low-Mg calcite + aragonite by codiacean algae and scleractinian corals (arag in modern seas), and of low-Mg calcite by the coccolithophores and reef-dwelling animals (high-Mg calc in modern seas), in the calcite SW indicates that while these organisms exert significant influence over their calcification, their biomineralogical control can be partially overridden by ambient Mg/Ca. These results suggest that modern aragonite or high-Mg calcite organisms secreted aragonite + low-Mg calcite or only low-Mg calcite, respectively, in calcite seas of the past. Generally, photosynthetic organisms were more strongly influenced by ambient Mg/Ca ratios than were heterotrophs, with respect to skeletal Mg fractionation. This suggests that autotrophs exert less control over their biomineralization than heterotrophs, a possible consequence of autotrophs inducing calcification by raising alkalinity via photosynthesis (CO2 removal) versus heterotrophs controlling calcification with more sophisticated organic templates and ion/proton pumps. The elevated calcification and growth rates for codiacean algae and corals in aragonite SW, and for coccolithophores in calcite SW, indicate that hypercalcifying marine organisms have a competitive advantage when their mineral polymorph is favored by ambient Mg/Ca. This supports the assertion that secular variation in oceanic Mg/Ca has influenced which hypercalcifying marine organisms were the major reef-builders and sediment-producers throughout Earth history.