Effects of secular variation in seawater Mg/Ca ratio (calcite–aragonite seas) on CaCO3 sediment production by the calcareous algae Halimeda, Penicillus and Udotea– evidence from recent experiments and the geological record

Abstract Independent lines of geological evidence suggest that fluctuations in the Mg/Ca ratio of seawater between 1.0 and 5.2 have caused the oceans to alternate between favouring the precipitation of the aragonite and high‐Mg calcite polymorphs of calcium carbonate (mMg/Ca > 2; aragonite seas) and the low‐Mg calcite polymorph (mMg/Ca < 2; calcite seas) throughout Phanerozoic time. The rise of aragonite‐secreting bryopsidalean algae as major producers of carbonate sediments in middle Palaeogene time, a role that they maintained through to the present, has been attributed to a transition from calcite‐to‐aragonite seas in early Cenozoic time. Recent experiments on the modern, carbonate‐sediment‐producing bryopsidales Halimeda, Penicillus and Udotea reveal that their rates of calcification, linear extension and primary production decline when reared in experimental calcite seawaters (mMg/Ca < 2). These normally aragonite‐secreting algae also began producing at least one‐quarter of their CaCO3 as calcite under calcite sea conditions, indicating that their biomineralogical control can be partially overridden by ambient seawater chemistry. The observation that primary production and linear extension declined along with calcification in the mineralogically unfavourable seawater suggests that photosynthesis within these algae is enhanced by calcification via liberation of CO2 and/or H+. Thus, the reduced fitness of these algae associated with their low rates of calcification in calcite seas may have been exacerbated by concomitant reductions in tissue mass and algal height.