The Mg/Ca ratio of seawater, controlled primarily by rates of seafloor spreading, has governed the mineralogy of nonskeletal marine carbonate precipitates during the geologic past (Hardie, 1996; Lowenstein et al., 2002). The requisite Mg/Ca ratios are < 1.2 for low-Mg calcite (< 4 mole % Mg substituting for Ca ); 1.2-5.5 for high-Mg calcite, and > 2 for aragonite. Biologically simple organisms that secrete carbonate skeletons have generally flourished as reef-builders and producers of shallow-water sediment only when the Mg/Ca ratio of seawater has favored their skeletal mineralogy (Stanley & Hardie, 1998, 1999). High-Mg calcite and aragonite are the predominant mineralogies of such organisms in modern seas (Mg/Ca ratio=5.2).

In 3 chemically distinct artificial seawaters, we grew 3 species of coralline algae that today secrete high-Mg calcite. The molar concentrations of Ca2+ plus Mg2+ (combined) and of other chemical constituents were the same in all 3 treatments, but the Mg/Ca mole ratios were 1, 2.5, and 5.8, respectively. We switched specimens between the second and third treatments after growth, to control the experiments. All 3 species incorporated into their skeletons a percentage of magnesium proportional to the ambient Mg/Ca ratio and close to the percentage found in nonskeletal calcite precipitated from water of the same composition. The species may employ organic templates to specify precipitation of calcite, but the calcite’s magnesium content is clearly governed by the ambient Mg/Ca ratio; photosynthesis apparently induces calcification from seawater without biotic pumping of Ca2+. In the treatment that mimicked Late Cretaceous seawater (Mg/Ca=1), all specimens died within four days, but there was a progressive decline in the mole % Mg in calcite impregnating the walls of cell rows representing daily growth. Before dying, each species produced low-Mg calcite, the imputed nonskeletal carbonate precipitate from Late Cretaceous tropical seas.