Laboratory experiments were conducted to subject the calcareous alga Halimeda, a prolific producer of aragonite in the modern ocean, to a variety of ambient Mg/Ca ratios. These experiments were based on three predictions: (1) that Halimeda would calcify at a lower rate when the ambient Mg/Ca ratio was reduced from its high level in modern seawater (5.2); (2) that this alga would grow more slowly under these conditions because its calcification yields as a byproduct CO2, which is available for photosynthesis; and (3) that the calcite content of Halimeda's skeleton would increase as a percentage of total CaCO3. As predicted, reduction of the Mg/Ca ratio of ambient artificial seawater from its modern level to estimated early Oligocene and early Eocene levels (2.5 and 1.5, respectively) resulted in progressively lower rates of calcification, production of organic matter, and linear growth for Halimeda incrassata colonies. The ambient Mg/Ca ratio exerted a stronger influence over the rate of calcification than [Ca2+], which in the geologic past has risen in seawater whenever the ambient Mg/Ca ratio has declined. Thus, although elevation of [Ca2+] at such times would have promoted the calcification and growth of Halimeda, the concomitant decline of the Mg/Ca ratio would have had a stronger negative effect. In addition, the percentage of skeletal material consisting of calcite rose from 8% in modern-day seawater to 16% in artificial early Oligocene seawater and 46% in artificial early Eocene seawater. Magnesium was incorporated into this calcite as a function of the ambient Mg/Ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite.

Halimeda exerts less biological control over its biomineralization than the related genera Penicillus and Udotea, which in previous experiments produced only 22–25% of skeletal CaCO3 as calcite in seawater having a very low Mg/Ca ratio (1.0). Nonetheless, it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low-Mg CaCO3 as calcite in ancient seas, such as those of the Cretaceous, whose Mg/Ca ratios were close to unity.

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