Calcareous nannofossils from the Great Australian Bight (IODP Site U1512) as a record of the peak and early decline of the mid-Cretaceous super greenhouse

Abstract

Abstract International Ocean Discovery Program drilling in the Great Australian Bight (Site U1512) provides a record of Turonian to Santonian hemipelagic sedimentation deposited in an elongate epicontinental basin that opened westward to the Southern Ocean during the peak and the early decline of the Cretaceous hot greenhouse. Despite its relatively high paleolatitude (at about 60°S), the biostratigraphy of calcareous nannofossils from this succession is directly correlative to low-latitude zonation schemes throughout the Turonian to Santonian. These calcareous nannofossil assemblages are somewhat restricted in species composition, lacking some elements generally found in Late Cretaceous assemblages (e.g., holococcoliths). In addition, two biostratigraphically-important high latitude taxa (Thiersteinia ecclesiastica, and Zeugrhabdotus kerguelenesis) have biostratigraphic ranges notably shorter than elsewhere, indicating range truncation through local extirpation. Quantitative paleoecological analysis of the calcareous nannofossil assemblages indicates that the major change in the paleoenvironment was the evolution from conditions that were quite warm and had relatively elevated surface water fertility in the Turonian to cooler and possibly more oligotrophic conditions in the Coniacian–Santonian. The first cooling began at about 92 Ma, accelerated at about 91.2 Ma and then continued to gradually cool during the Coniacian–Santonian. This paleotemperature scenario is consistent with those documented from other localities globally.