Global Patterns in Opal Deposition from Late Cretaceous Late Miocene

Abstract

Biogenic silica accumulation rates were calculated at Deep Sea Drilling Project (DSDP) sites from Legs 1-44, and were plotted on global reconstructions at seven 10-m.y. intervals. These data were then contoured. Because of the correlation between opaline sediments and upwelling, these maps may be used to trace paleocirculation patterns through time. They demonstrate a gradual transition from the Cretaceous circum-global equatorial current to the modern circum-Antarctic divergence, as Tethys closed and Australia and South America separated from Antarctica. Modern bottom-water circulation tends to concentrate opaline sediments in the Pacific and carbonate sediments in the Atlantic. Basinal opal fluxes were measured for each interval and, in combination with carbonate compensation depth (CCD) histories, reveal four lagoonal versus estuarine exchange reversals from the Late Cretaceous to the present. These reversals can be directly related to the development of new bottom-water sources with changing climatic and tectonic conditions.