CRETACEOUS VERSUS MODERN CARBONATE SEDIMENT MINERALOGY: EVIDENCE FROM EXPERIMENTS WITH HALIMEDA

The magnesium/calcium molar ratio (Mg/Ca) of seawater has oscillated throughout earth history, favoring nonskeletal precipitation of low-Mg calcite (<4% mol% Mg substituting for Ca) when it is below unity, high-Mg calcite when it is between unity and 2, and both high-Mg calcite and aragonite when it is above 2. In addition, throughout the Phanerozoic major reef builders and sediment producers have for the most part belonged to taxa whose skeletal mineralogy is favored by the Mg/Ca ratio of ambient seawater. The aragonitic codiacean green alga Halimeda exemplifies this pattern, contributing about 25% of the skeletal debris in shallow tropical perireefal settings of the modern ocean, in which the Mg/Ca ratio is 5.2. We grew Halimeda incrassata in artificial seawaters of differing Mg/Ca ratios, acclimating them to treatment with ambient ratios of 2.5 and 1.5 through stepwise reduction from the ambient ratio from 5.2. Carbonate skeletal material of specimens grown in seawater of modern seawater composition, on average, included 9% calcite. This percentage rose to 14 percent with ambient Mg/Ca = 2.5, and ranged as high as 87% in“Cretaceous” seawater (Mg/Ca = 1.5). As for a wide variety of other taxa studied previously, the mol% Mg in the skeletal calcite was positively correlated with the ambient Mg/Ca ratio, ranging from 17% (mean for the modern seawater treatment) to 6.5% (mean for the “Cretaceous” seawater treatment). Reflecting the fact that calcification releases carbon dioxide that is employed in photosynthesis, specimens grown within the aragonite Mg/Ca domain not only calcified more rapidly but also exhibited faster organic growth with elevation of either the ambient Mg/Ca ratio or the absolute concentration of Ca. Our experiments produced rapid organic growth and carbonate production by Halimeda in “Cretaceous” seawater, followed by a cessation of growth. Given the opportunity to adjust over geologic time, either phenotypically or through evolution, as the Mg/Ca ratio of seawater declined during Late Jurassic and Early Cretaceous time, codiacean algae probably contributed a considerable volume of calcitic material to late Mesozoic carbonate sediments.