Sea Chemistry Changes and Biotic Consequences among Mesozoic Reef Ecosystems: Insight and Paradox

In reef evolution it is unclear how important secular changes in calcite and aragonite were as principal drivers. Icehouse intervals favored high-Mg calcite/aragonite while greenhouse times were low-Mg calcite. Events of extinction, recovery and adaptive radiation relate to ecology, climate, CO2 and sea chemistry changes. The Permo-Triassic extinction during an aragonite sea, witnessed the disappearance of calcitic corals. The Early Triassic reef gap is associated with anoxia, high CO2 and ocean acidity and the absence of corals during an aragonite phase. The appearance of aragonitic Scleractinia during the mid-Triassic recovery, reflects a calcification response by anemone-like “naked” corals. Scleractinians formed communities with chambered demosponges and algae but ironically did not build reefs until near the end of the Triassic—a time of transition to calcite seas. The T/J reef collapse coincided with secular change to calcite and a greenhouse supercycle. The warm Jurassic calcite seas contain diverse scleractinians that were neither as fast-growing nor as efficient as Cenozoic counterparts. Cretaceous coral diversity increased but reef production was assumed by more calcitic rudistids. Ironically some corals secreted calcite. The end-Cretaceous to early Paleogene reef dynamics of extinction were complex, involving nutrients, CO2 and temperature flux. Despite warm conditions, coral diversity dropped during the Paleocene. Mesozoic coral diversity tracks calcium ion concentrations until the end-Cretaceous extinction after which reef production by corals acquired a negative relationship with calcium. Corals did not build extensive reefs until much later in the Cenozoic—in the transition to another aragonite phase. Mesozoic reef provide both insight and paradox with respect to calcite/aragonite sea dynamics. Some paradoxes are resolved by biomineralization preferences, ecological factors and CO2 saturation. A correlation is apparent in Ca/Mg levels and reefbuilders. Mesozoic reefs however are not the best keys to the Neogene nor “Anthropogene”.