1A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations

We estimate tropical Atlantic upper ocean temperatures using oxygen isotope and Mg/Ca ratios in well-preserved planktonic foraminifera extracted from Albian through Santonian black shales recovered during Ocean Drilling Program Leg 207 (North Atlantic Demerara Rise). Based on a range of plausible assumptions regarding seawater composition at the time, the data support temperatures between 33 ° and 42°C. In our low resolution dataset spanning ~84-100 Ma, a local temperature maximum occurs in the late Turonian and a possible minimum occurs in the mid- to early late Cenomanian. The relation between single species foraminiferal G18O and Mg/Ca suggests that the ratio of magnesium to calcium in the Turonian-Coniacian ocean may have been lower than in the Albian-Cenomanian ocean, perhaps coincident with an ocean 87Sr/86Sr minimum. The carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples. The G13C values of phytane, combined with foraminiferal G13C and inferred temperatures were used to estimate atmospheric carbon dioxide concentrations through this interval. Estimates of atmospheric CO2 concentrations range between 600 and 2400 ppmv. Within the uncertainty in the various proxies, there is onl