Climate cooling at the onset of the early Aptian Oceanic Anoxic Event (OAE1a)

We present millennial-scale resolution stable isotope data from a marly subtropical shelf succession (La Bédoule, SE France) with minimal diagenetic overprint, which track the onset of Oceanic Anoxic Event 1a (OAE1a). Our 2 kyr-resolution δ13C record is characterized by a continuous succession of stepwise increases (overall amplitude: >2‰). In contrast, the δ18O record exhibits orbital periodicities corresponding to short eccentricity, obliquity and precession, and reveals a prominent cooling event within the initial positive δ13C shift of OAE1a. This cooling event bears similarity to glacio-eustatic sealevel falls and cooling events in the early phase of major carbon isotope shifts in the middle Cenomanian, late Cenomanian and middle Miocene. Using climate simulations with the coupled global climate model GENESIS v.3.0, we test the hypothesis that these events are related to ice-sheet buildup in Antarctica in the initial phase of positive δ13C excursions. We explore the physical potential of snow accumulation and ice-buildup in Antarctica under middle Cretaceous boundary conditions and various elevated atmospheric CO2 concentrations and changing orbital configurations (eccentricity). We suggest that snow/ice accumulation occurred during eccentricity minima, mainly due to a strong albedo feedback, when pCO2 and elevation allowed a perennial snow cover.