TURRITELLINE GASTROPOD-DOMINATED ASSEMBLAGES POINT TO NON-UNIFORMITARIAN CARBONATE DEPOSITIONAL ENVIRONMENTS IN THE CRETACEOUS AND PALEOGENE

Modern marine carbonate sediments accumulate where carbonate-producing organisms are abundant and siliciclastic input is low. Such conditions occur today in two main environments: warm, low-nutrient (WLN; also known as tropical or chlorozoan carbonates) and cool, high-nutrient (CHN; also known as cool-water, temperate, non-tropical, foramol, or heterozoan carbonates). Few if any carbonates form today in warm, high-nutrient (WHN) conditions, principally because such environments do not currently exist on Earth. Modern WLN environments occur mainly in low latitudes where nutrients from upwelling or terrestrial runoff are low. Modern CHN environments occur mainly in middle to high latitudes where nutrients, mostly from upwelling, are abundant. WHN depositional environments may, however, have been widespread during the Cretaceous and Paleogene. During these times, upwelled nutrient-rich waters would, in general, have been warmer than they are today. Nutrients in these waters supported diverse carbonate-producing biotas not equivalent to either modern chlorozoan or heterozoan biotas.

These conclusions derive from analysis of a growing dataset of more than 40 turritelline gastropod-dominated fossil assemblages (TDAs), ranging from Lower Cretaceous to Pleistocene. TDAs today occur mainly in CHN conditions. They were widespread in both carbonate and siliciclastic facies in the Cretaceous and Paleogene, but (with only a single known exception) occur in the Neogene only in siliciclastic sediments. This change in environmental distribution can be explained by positing changes in: 1) modal environmental preferences of turritellines, from WHN to CHN, and 2) carbonate depositional environments, from WLN+WHN to WLN+CHN. Both changes were roughly coincident with the global change from “greenhouse” to “icehouse” conditions in the Oligocene.