Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO(2) and paleo productivity

A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (delta C-13(phytane)), a biomarker for photosynthetic algae. The delta C-13(phytane) record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (similar to 7 parts per thousand) compared to existing C/T proto-North Atlantic delta C-13(phytane) records (3-6 parts per thousand). Overall, the amplitude of the excursion of delta C-13(phytane) decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal delta C-13(phytane) gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and delta C-13(phytane)-records from Site 1260, we subsequently reconstructed pCO(2) levels across the OAE-2 interval. Accordingly, pCO(2) decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO(2). Whereas the onset of OAE-2 coincided with increased pCO(2), in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO(2) related to perturbations of the global carbon cycle.