ALBIAN-CENOMANIAN FORAMINIFERA AND THE WARM TO HOT CRETACEOUS GREENHOUSE CLIMATE TRANSITION AT SOUTHERN HIGH LATITUDES: IODP DRILLING RESULTS FROM THE SE INDIAN OCEAN

Albian–Cenomanian sediments from IODP Expedition 369 Sites U1513, U1514, and U1516, drilled on Mentelle Basin (MB) in the SE Indian Ocean (60°S paleolatitude), yield extraordinarily well-preserved foraminiferal assemblages that provide new insight to biotic, paleoceanographic, and paleoclimatic changes at southern high latitudes (SHL). Age control is provided by calcareous nannofossils as the co-occurring planktonic foraminiferal assemblages are very low in species diversity and age-diagnostic taxa are nearly absent. Foraminiferal-rich intervals are dominated by a single small-sized, long-ranging planktonic species. These intervals alternate with carbonate-poor intervals that are dominated by radiolarians and, occasionally, the zeolite mineral clinoptilolite, which has replaced the original radiolarian and calcisphere shells. Occurrence of clinoptilolite in carbonate sequences is considered as a proxy for enhanced biogenic silica productivity. Alternation between the carbonate-rich vs. radiolarian or clinoptilolite intervals is inferred to have been dependent on availability of primary biogenic silica and its reaction with interstitial clay minerals.

Stable isotope measurements generated from single species of planktonic and benthic foraminifera at all three MB sites show parallel trends, with little change throughout the Albian-Cenomanian until the onset of warming initiated during the latest Cenomanian. Early Albian benthic and planktonic δ¹⁸O values average -0.3‰ and -1.3‰, respectively at ~111 Ma and average -0.9‰ and -1.7‰, respectively, at 95 Ma, indicating a 2–3° warming of bottom and surface waters. Transition from the warm greenhouse of the Albian–Cenomanian to the hot greenhouse of the latest Cenomanian–late Santonian led to an increase in planktonic foraminiferal diversity and more consistent occurrence of age diagnostic species at the SHL sites. This was followed by a long-term cooling and reduction in species diversity from the late Santonian through the Maastrichtian.