Global environmental controls on Cretaceous reefal ecosystems

The hypothesis of global environmental control of reefal ecosystems is developed in order to explain the dramatic biotic changes in Cretaceous benthic communities. During a 30 m.y. period in the Early Cretaceous the dominant biota in the reefal ecosystem changed from corals, algae and rudists, to rudists with minor coral species. This change included the reduction of the binder niche, which in the Early Cretaceous was occupied by stromatoporoids and algal/cyanobacterial associations. The role of bafflers increased during the mid-Cretaceous as first the radiolitids, then the hippuritids, diversified and increased in abundance. The characteristic encrusters changed from green algae and bryozoans to red algae. The orbitolinid foraminifers were replaced by the alveolinids. otic changes are reflected in the feeding habit/substrate niche relationships within the communities and the biotic processes of competition for space and nutrients. These changes were closely linked with major, though subtle, global shifts in oceanographic current systems, in water mass chemistry, in nutrient levels, and in plate positions and oceanic volcanism associated with super plume activity. The development of widespread anoxic water masses influenced the composition of the oceans and the atmosphere itself. The feedback interactions among these processes may have had an indirect but unalterable affect on the composition and structure of Cretaceous reefs. Particularly, the frequency of sea level rise and flooding of the shelf with low oxygen waters increased by two times during the mid-Cretaceous. This increased frequency of stress upon the benthic communities may have crossed the threshold resulting in the biotic changes, such as the replacement of corals by rudists as the most abundant reefal group.