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

摘要

Temperature-dependent aragonite and calcite formation by scleractinian corals were examined in low molar (m) Mg/Ca seawater, the experimental conditions replicating the fluctuating mMg/Ca levels prevailing throughout the Phanerozoic Eon. Incubation and skeletal growth monitoring of juveniles of the scleractinian coral Acropora solitaryensis for 4 months from the planula stage, in seawater with mMg/Ca ratios of 5.2, 1.0, and 0.5, and temperatures of 19-28 degrees C, indicated that polymorphism of present-day scleractinian corals in low mMg/Ca seawater is also influenced by seawater temperature. However, corals produced more aragonite than formed in inorganic CaCO3 precipitation experiments under the same conditions, except at 19 degrees C. Although the aragonite content reflected the results of the latter (abiotic) experiments at 19 degrees C, it is suggested that aragonitic scleractinian corals controlled skeletal formation biologically under low mMg/Ca conditions at higher temperature, growth rates being faster at 25 degrees C and slower at 19 degrees C for all mMg/Ca ratios. Compared with growth rates under the present-day-equivalent seawater Mg/Ca level of 5.2, juvenile growth decreased by 62.8% +/- 14.7% and 56.7% +/- 6.7% under mMg/Ca levels of 1.0 and 0.5, respectively; the results suggest that growth of aragonitic scleractinian corals is suppressed throughout varying seasonal temperatures under low mMg/Ca conditions. This supports previous findings from variable temperature perspectives that scleractinian corals grow more slowly in low mMg/Ca (Cretaceous) seawater, interpreted as a possible explanation for the hiatus in scleractinian reef building in the Cretaceous Period.