HIGH PALEOLATITUDE CENOMANIAN PLANKTONIC FORAMINIFERA FROM IODP EXPEDITION 369 SITES DRILLED ON MENTELLE BASIN, SE INDIAN OCEAN

Cenomanian planktonic foraminifera from southern high latitudes have been poorly documented because few austral deep-sea sites have penetrated the mid-Cretaceous sediment record and, among those, the drilled sequences were incomplete because of poor core recovery and/or presence of unconformities. During 2017, International Ocean Discovery Program (IODP) Expedition 369 recovered biostratigraphically complete late Albian-Cenomanian black claystone and nannofossil-rich claystone at Sites U1513, U1514 and U1516 in the Mentelle Basin, which is located in offshore SW Australia. These sites all were positioned at ~60°S paleolatitude in the SE Indian Ocean during the mid-Cretaceous. Foraminifera in the mid- to upper Cenomanian cores are moderately to well preserved and are quite rare relative to the sediment volume. The percentage of planktic specimens relative to benthics generally ranges from 70-90%. The planktic assemblages are dominated by small-sized species of Microhedbergella with very rare occurrences of Whiteinella spp. and Schackoina cenomana. Biserial and planispiral planktonic species are surprisingly rare and sporadic in their distribution at the Mentelle Basin sites; these taxa are generally considered as opportunistic and are relatively common at other austral locations during the mid-Cretaceous. Low latitude planktic species that have been traditionally used as biostratigraphic markers occur in very few samples and, where present, there is typically only 1 specimen. Along similar lines, lower Cenomanian foraminiferal assemblages are dominated by agglutinated species, and in some samples calcareous foraminifera are completely absent. The lower Cenomanian sequences show strong apparent changes in the carbonate compensation depth with the percentage of planktic specimens relative to benthics varying from 0-50%. Further study of the Expedition 369 Cenomanian core samples, combined with foraminiferal stable isotopes, will reveal much more about the paleoceanographic factors that influenced the planktic foraminiferal population dynamics and species composition.