Aragonitic scleractinian corals in the Cretaceous calcitic sea

Abstract

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. INTRODUCTIONVariation 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 CaCO 3 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 CO 2 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 evolv...

Temperature dependence of aragonite and calcite skeleton formation by a scleractinian coral in low mMg/Ca seawater

“…However, at the same time seawater temperatures were above average during this time (Veizer and Prokoph, 2015). In our experiments aragonite forms the main polymorph in the coral skeleton at warm water (>19 °C) conditions of mMg/Ca >0.5, and thus supports the continued existence and appearance of aragonitic corals throughout the Cretaceous calcite sea (Janiszewska et al, 2017). This is further supported when considering that spawning of corals takes place at higher temperatures (mostly >25 °C) for both subtropical and highlatitude species (Hayashibara et al, 1993;Nozawa, 2012); this is likely to result in the initial crystallization aragonite nuclei for shallow-water coral skeletons.…”

From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones

“…If the primary origin of such a calcitic shell layer is confirmed by more in-depth diagenetic studies, this finding may have major implications for cephalopod shell evolution, particularly in the context of the influence of the low Mg/Ca geochemistry of Paleozoic "calcitic seas" on shell mineralogy. Nonetheless, there is no evidence of the presence of a calcitic layer in the cephalopod shells studied here, which were formed in the Cretaceous when the ocean Mg/Ca ratio was near the lowest in the Phanerozoic [49].…”

Jurassic shift from abiotic to biotic control on marine ecological success

“…In most major clades, however, changes in mineralogy are rare: trilobites, echinoderms, ostracods are exclusively calcitic, and brachiopods and stenolaemate bryozoans are generally calcitic, with the exception of trimerellid brachiopods 52 and a few genera of Stenolaemata with minor skeletal aragonite content 115 . Ammonoids, nautiloids and scleractinian corals are, with rare exceptions in the latter two groups [121][122][123] , aragonitic. The box at the bottom shows whether the models were generated using ordinary least squares (OLS), or generalised least squares with autoregressive errors of the first order (ф).…”