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 Sa ndberg [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 wit h synchronous oscillations in the mineralogy of marine potash evaporites, c an 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 pr edicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbon ate mineralogy of hypercalcifying organisms (ones that have produced massiv e skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by th e Hardie model. Particular groups of corals, sponges, and algae appear to h ave been dominant reef builders only when favored by an appropriate Mg/Ca r atio in seawater. In early and middle Paleozoic calcite seas (Calcite I), r eefs 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 a cme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dom inant reef builders, along with high-Mg calcitic coralline algae. Major sed iment-producing algae exhibit temporal patterns similar to those of reef bu ilders. Calcitic receptaculitids flourished during Calcite I, whereas arago nitic dasycladaceans did not become dominant rock formers until Aragonite I T. During Calcite II, calcitic nannoplankton formed massive coccolith chalk s in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of sea water had reached a very low value and calcium concentration, a very high v alue. As the Mg/Ca ratio of seawater rose and calcium concentration fell du ring the Cenozoic Era, individual coccoliths, on average, became less massi ve 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 the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment pr oducer in reef tracts. The influence of seawater chemistry on skeletal secr etion appears to have been especially strong for morphologically simple tax a that exert relatively weak control over their own calcification. Such gro ups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcificati on, has given the Mg/Ca ratio of seawater strong control over the success o f individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of spo nges and cheilostome bryozoans throughout their history. We conclude that t hroughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biologic al composition of reef communities and bioclastic carbonate deposits. (C) 1 998 Elsevier Science B.V. All rights reserved.