Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of