CENOZOIC TRENDS IN SHALLOW-WATER CARBONATE PRODUCTION: INSIGHTS FROM CHEMISTRY, CLIMATE AND BIOLOGY

A paradox of Cenozoic carbonates is that coral diversity recovered during the Paleocene-Eocene, yet coralline algae and larger benthic foraminifers (LBF) mostly dominated shelf production, with the apex of LBF during the Eocene. Major coral-reef accretion was limited prior to the Oligocene, but active through the Neogene. A related paradox is that both zooxanthellate corals (Z-corals) and LBF host symbiotic algae and thrive in warm, clear, nutrient-poor waters. These paradoxes suggest several mechanisms: 1) Low Paleogene Mg/Ca ratios likely limited coral (i.e., aragonite) hypercalcification to shelf regions of relatively high salinity and thus high carbonate saturation. Coralline algae and most LBF produce variable-Mg calcite, which is energetically less expensive in low Mg/Ca waters. 2) Dinoflagellate RuBisCo (an enzyme essential to carbon fixation) is inefficient in discriminating between CO₂ and O₂, which can result in oxidative stress in high light, especially at higher pCO₂. The diatom symbionts of many LBF have more efficient RuBisCo. 3) Ecological differences, though both groups utilize photosynthate (simple sugars) produced by their algal symbionts for respiration and to enhance calcification, while food intake provides essential proteins and nucleic acids. Paleogene LBF produced substantial carbonate ramp deposits when deep-ocean waters were warmest and the thermocline was weakest, though LBF thrived in suitable habitats through most of the Cenozoic. In contrast, Z-corals produced substantial buildups when high latitudes cooled and thermoclines strengthened. Strong equator-to-pole thermal gradients promote strong wind systems that drive surface currents and waves; a strong thermocline provides a strong density gradient along which internal waves can propagate. With higher metabolic requirements and accretion potential than LBF under optimum conditions, Z-corals thrive and hypercalcify in clear, actively moving waters that supply plankton for food and enhance removal of excess oxygen radicals produced during photosynthesis. Recognizing ecological similarities and differences, as well as differences in accretion potential and loci of production, can provide insight into these and other paradoxes associated with Cenozoic carbonate systems.