Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition

摘要

Shifts in the Mg/Ca ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from sea-water on time scales of 10~8 years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient Mg/Ca ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO_3). We grew three species of these algae in artificial seawaters having three different Mg/Ca ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient Mg/Ca ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO_2 for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low Mg/Ca ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO_3), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas.