The Episodic History of Cretaceous Carbonate Platforms: An Aptian Case Study

Peter Skelton
Open University, United Kingdom ([email protected])

Late Aptian rudist faunas differed markedly from those of the Early Aptian, mainly because of the extinction of almost all the predominantly aragonitic caprinids and the radiation of forms with a thickened calcitic outer shell layer, such as the polyconitids and the first radiolitids.
However, the taxonomic turnover occurred not in a single event, but through the Early Aptian, in association with the regionally progressive demise of carbonate platforms. The inception of this phase in the earliest Aptian was accompanied by a widely documented negative δ¹³C excursion, which has been interpreted as marking a methane-induced rise in atmospheric CO₂, with associated climatic warming.
Nevertheless, some characteristic Early Aptian rudists persisted in lower palaeolatitude platforms until the end of the Early Aptian, hinting that subsequent global cooling may account for their geographical retreat to eventual extinction. Such an interpretation would be consistent with (1) oxygen isotope data, (2) likely draw-down of atmospheric CO₂ accompanying the increasing ratio of organic-, to carbonate-carbon burial, and (3) geological evidence for yet cooler conditions in the Late Aptian. Moreover, if the previous maintenance of high rates of calcification on the platforms, despite elevated levels of atmospheric CO₂, had depended upon the thermal expulsion of aqueous CO₂ from the waters overlying them (‘the kettle effect’), cooling could be expected to have had a deleterious effect on the system.
This factor might also explain why rudist taxa that occupied the platform margins – which would have been most susceptible to flushing by cooler open water masses – suffered the greatest extinction. These observations and inferences suggest an intriguing linkage between the episodic history of carbonate platforms and their biota, perturbations of the global carbon cycle, and climate in the unstable greenhouse world of the Cretaceous.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008