Mid-Cretaceous Perturbations in Carbonate-Platform Sediment Production: Effects of Seawater Composition, Oceanic Anoxia, and Climate Change

Tethyan carbonate platforms experienced repeated episodes of crises and re-organisation during the mid Cretaceous. Demise of carbonate platforms occurred at the Early/Late Aptian transition and the Cenomanian/Turonian boundary and was linked to extinction events among important carbonate producers such as rudist bivalves, calcareous algae, and larger benthic foraminifers. Aragonite-dominated groups were the major victims, which is believed to be related to the very low Mg/Ca ratio of mid-Cretaceous seawater (molar Mg/C ratio similar to 1), favoring calcite as the dominant carbonate mineralogy. Platform demise was previously related to oceanic anoxic events, i.e. early Aptian OAE 1a, and OAE 2 at the Cenomanian/Turonian transition.

The evaluation of controlling factors of carbonate platform evolution requires a precise stratigraphy. We have studied carbonate platforms in the Mediterranean Tethys and Middle East, and established a high resolution stratigraphy of platform evolution based on carbon isotope and strontium isotope chemostratigraphy. Many platforms drowned before, during, or shortly after OAEs. Those platforms that survived went through a stage of microbial carbonate production during OAE 1a and OAE 2, but aragonite-dominated carbonate production resumed after OAE 1a in the Middle East, forming the prolific caprinid rudist buildups of the Shuaiba Formation. Extinction of the caprinids in the Middle East occurred during subaerial exposure that terminated platform aggradation at the Early/Late Aptian boundary, about two million years after OAE1a.

Our data suggest that the repeated demise of aragonite dominated carbonate production during the mid-Cretaceous period of an exceptionally low seawater Mg/Ca ratio is linked to changes in sea level and possibly climate. Mid-Cretaceous OAEs may have additionally stressed important groups of carbonate producers, but were not the major cause of platform drowning and extinction among major carbonate producers.