LATE CRETACEOUS MARINE REPTILES AND COOLING AT THE SOUTH ATLANTIC COAST INFERRED THROUGH STABLE OXYGEN ISOTOPES OF INOCERAMUS FROM THE NAMIBE BASIN, ANGOLA

The opening of the South Atlantic Ocean enhanced global ocean circulation and contributed to the transition from warmer temperatures during the middle Cretaceous to cooler climates characterizing much of the Cenozoic. We present δ¹⁸O values derived from bivalve shells to elucidate nearshore temperature change in southern Angola during the Late Cretaceous development of the South Atlantic Ocean. Inoceramus and other bivalve shells were recovered from marine sediments at Bentiaba, Angola, that overlie non-marine redbeds deposited during the initial rifting of Africa and South America. The section is anchored by a radiometric age of 84.6 Ma on an intercalated basalt and the δ¹³C stratigraphy derived from shells is correlated to global carbon isotope events from the Late Cenomanian to Early Maastrichtian. The δ¹⁸O stratigraphy derived from shells indicate an overall increase from -4.5‰ in the Late Cenomanian to -1.2‰ in the Late Campanian, which is a similar trend observed in oxygen isotopes in foraminifera globally. Assuming a constant oceanic δ¹⁸O value, the change in oxygen isotopes reflects cooling of ~15° for the shallow marine environment at Bentiaba. Early to Late Campanian inoceramids yield the highest δ¹⁸O values, between -1‰ to -2‰, and are offset by about +1‰ from published records for benthic foraminifera and bathyal Inoceramus at Walvis Ridge. This offset in δ¹⁸O values indicate a temperature difference of ~5° between coastal and deeper water offshore Angola prior to the latest Campanian. The stratigraphic distribution of marine reptile fossils coincides with cooler temperatures at Bentiaba implied by more positive δ¹⁸O values derived from bivalves. A diverse marine reptile fauna has been recovered from Bench 19 that was deposited at the Campanian-Maastrichtian Boundary during a time of increased global ocean connectivity and circulation of cooler productive high latitudinal waters. This pattern aligns with the larger context of the mosasaur record, which indicates productivity driven evolution accompanied by an increase in size disparity, in diversity, and in niche differentiation.