Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C₃₇ alkenones expressed as U^K'₃₇ is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the U^K'₃₇ proxy for assessment of Cretaceous paleoclimates. Herein, we report a C₄₀ alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C_40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C₄₀ alkadienone (tetraconta-16E, 23E-dien-3-one; C_40:2 Et) allowed calculation of the U^K’₄₀ unsaturation index, comparable to U^K’₃₇. Stratigraphic variations in the δ¹³C of C_40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO₂. The U^K’₄₀ profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO₂ during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.