Biogeochemical contrasts between mid‐Cretaceous carbonate platforms and Cenozoic reefs

Abstract

AbstractCarbonate sediments of mid‐Cretaceous platforms on Allison and Resolution Guyots, Mid‐Pacific Mountains (ODP Leg 143, Sites 865, 866, 867 and 868) and those of upper Oligocene to Pliocene reefs of the Kita‐daito‐jima Borehole were studied. The mid‐Cretaceous platforms abound with abiotic (?) precipitates (ooids) and microbial carbonate grains/sediments (oncoids and ‘algal’ laminites), whereas the Cenozoic reefs consist mainly of coral and non‐geniculate coralline algae, major frame‐builders, benthic foraminifers and codiacean alga (Halimeda). There exists a remarkable difference in a mode of calcification between the mid‐Cretaceous platforms and Cenozoic reefs. The major reef‐builders of Cenozoic reefs precipitated carbonates within closed to semiclosed spaces within their bodies. In contrast, the mid‐Cretaceous platforms contain abundant grains/sediments formed by chemical (?) precipitations and biotic extracellular calcification. This contrasting feature reflects different modes of biogeochemical cycles between the mid‐Cretaceous and Cenozoic. Increased CO2 (degassed by active volcanism) and resultant high temperature and intensive weathering may have brought high concentration of Ca2+ and HCO3− into the mid‐Cretaceous sea, which enhanced abiotic and extracellular calcification. Inverse processes are true for the Cenozoic.