Aragonitic scleractinian corals in the Cretaceous calcitic sea

Changes in seawater chemistry have affected the evolution of calcifying marine organisms, including their skeletal polymorph (calcite versus aragonite), which is believed to have been strongly influenced by the Mg/Ca ratio at the time these animals first emerged. However, we show that micrabaciids, a scleractinian coral clade that first appeared in the fossil record of the Cretaceous, when the ocean Mg/ Ca ratio was near the lowest in the Phanerozoic (thus a priori favoring calcitic mineralogy), formed skeletons composed exclusively of aragonite. Exceptionally preserved aragonitic coralla of Micrabacia from the Late Cretaceous Ripley Formation (southeastern USA) have skeletal microstructures identical to their modern representatives. In addition, skeletons of Micrabacia from Cretaceous chalk deposits of eastern Poland are clearly diagenetically altered in a manner consistent with originally aragonitic mineralogy. These deposits have also preserved fossils of the scleractinian Coelosmilia, the skeleton of which is interpreted as originally calcitic. These findings show that if changes in seawater Mg/Ca ratio influenced the mineralogy of scleractinian corals, the outcome was taxon specific. The aragonitic mineralogy, unique skeletal microstructures and ultrastructures, and low Mg/Ca ratios in both fossil and living micrabaciids indicate that their biomineralization process is strongly controlled and has withstood major fluctuations in seawater chemistry during the past 70 m.y. INTRODUCTION Variation in seawater chemistry is thought to have played an important role in the evolution of skeleton-forming marine organisms and in the composition of their skeletons. The Mg/Ca ratio of seawater in particular has varied dramatically throughout the Phanerozoic (Stanley and Hardie, 1998; Porter, 2010). Abiotic precipitation experiments with seawater analogs have shown that precipitation of different CaCO3 polymorphs is mainly controlled by the Mg/Ca ratio and temperature (Morse et al., 1997). At ~25 °C, present-day seawater ionic strength, and atmospheric CO2 concentrations, the Mg/Ca ratio separates a regime of low-Mg calcite precipitation (Mg/Ca < 2) from a regime of aragonite/high-Mg calcite precipitation (Mg/Ca > 2) (Hardie, 1996). Based on the composition of early diagenetic carbonate cements and oolites (Sandberg, 1983), fluid-inclusion data (e.g., Lowenstein et al., 2001; Horita et al., 2002), and modeling of Mg/ Ca fluctuations linking the chemical composition of the oceans with rates of oceanic crust formation (Hardie, 1996), the Phanerozoic has thus been divided into periods of calcitic and aragonitic seas, respectively (Fig. 1). It was suggested that the carbonate polymorph of hypercalcifying marine organisms was dictated by the seawater Mg/Ca ratio: during periods with high Mg/Ca ratio, aragonite-forming organisms dominated, whereas during periods with low Mg/Ca ratio, calcite-secreting organisms flourished (Stanley and Hardie, 1998). Furthermore, it was proposed that the skeletal mineralogy of newly evolved, CaCO3 biomineralizing organisms was determined by the Mg/Ca ratio at the time of their origin (Porter, 2007, 2010). For nonhypercalcifiers, groups of organisms that appeared during aragonitic sea conditions would continue to produce aragonitic structures even in subsequent times of calcitic seas, but their calcification rate would be reduced, and vice versa (Stanley and Hardie, 1998; Porter, 2010). Balthasar and Cusack (2015) demonstrated that temperature plays a key role in controlling abiotic calcite versus aragonite precipitation. Calcite precipitation is inhibited under aragonitic sea conditions at temperatures >20 °C, whereas during calcitic sea conditions, coprecipitation of aragonite and calcite can occur at temperatures >20 °C. According to this work, a pure calcitic sea condition (<1% of aragonite in CaCO3 abiotic precipitates) would be rare in the geological past and constrained to temperatures <20 °C. During the same periods, in warm-water settings, abiotic precipitates could contain a significant fraction of aragonite (>50%). The modern seawater Mg/Ca ratio is ~5.2 mol/mol and all adult scleractinian corals, including deepand/or cold-water species, form aragonitic skeletons (Cairns, 1995, p. 30). *E-mail: [email protected] GEOLOGY, April 2017; v. 45; no. 4; p. 1–4 | Data Repository item 2017094 | doi:10.1130/G38593.1 | Published online XX Month 2017 © 2017 eological Society of A erica. For permission to copy, contact [email protected]. 0 1 2 3 4 5 6 se aw at er M g/ C a [m ol /m ol ] 0 1 2 3 4 5 6 Time [Ma] ARAGONITE and HIGH-Mg CALCITE