Goldilocks and the Three Biogenic Carbonate Minerals: What Determines 'Just Right'?

The Cenozoic began with surviving subsets of most major players in shelf carbonate sedimentation. Calcite production (with varying amounts of Mg) by coralline red algae and bryozoans dominated early, with larger benthic foraminifers (LBF) diversifying and producing carbonate buildups in the late Paleocene and Eocene. Despite diversification, aragonitic corals were generally minor contributors, with local exceptions, until the Oligocene. The Miocene saw spurts of dominance by coralline algae and secondarily by LBF. Only in the late Miocene did the scleractinian corals become unequivocally dominant reef builders? Now we are seeing coral assemblages in decline on reefs worldwide. Can present global changes provide insights to the past? Can the past provide insights into the future? Corals historically thrived in clear water, but recent bleaching events have revealed that water can be too clear! Corals need some nutrients and plankton for food, but not enough to promote algal overgrowth and rapid bioerosion rates. The corals of the Florida reef tract provide clues to how “Goldilocks” these reef builders can be. The offshore reefs, where waters are clearest, have declined dramatically over the past several decades such that coral cover is now too low to sustain reef accretion. Only on patch reefs is coral cover sometimes in double-digit percentages; but these often turbid-water reefs were marginally accretionary to start with. Did atmospheric carbon dioxide drop sufficiently low in latest Oligocene to allow scleractinian corals to hypercalcify through much of their shallow-water range? With co-evolving zooxanthellae clades and diversification of shallow-water coralline and calcareous algae, Neogene coral reefs precipitated huge volumes of calcium carbonate despite low, possibly limiting, concentrations of dissolved inorganic carbon. Now as carbon dioxide levels rapidly rise, will reefs again reach a threshold where corals can no longer hypercalcify? These questions are examined from physiological, ecological and paleoceanographic perspectives.