Cretaceous Marine Nutrients, Greenhouse Carbonates, and the Abundance of Turritelline Gastropods

Modern marine carbonate sediments accumulate where carbonate‐producing organisms are abundant and siliciclastic input is low. Such accumulations occur today in two main environments and may be characterized as warm, low‐nutrient (WLN; also known as tropical or chlorozoan) carbonates or cool, high‐nutrient (CHN; also known as cool‐water, temperate, nontropical, foramol, or heterozoan) carbonates. Few carbonates form today in warm, high‐nutrient (WHN) conditions because such environments are very rare. 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. However, WHN depositional environments may 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 that are not equivalent to either modern chlorozoan or heterozoan biotas. These conclusions derive from analysis of a growing data set of more than 40 turritelline gastropod‐dominated fossil assemblages (TDAs), a term herein redefined, ranging in age from Lower Cretaceous to Pleistocene. Today TDAs 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 in the Pliocene of Chile) 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.