Global cooling during an Oceanic Anoxic Event in the mid-Cretaceous greenhouse

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Mueller, Antje۱; Schouten, Stefan۱; Pross, Joerg۲; Sinninghe Damste, Jaap S.۱
Holding Date 07 October 2008

The mid-Cretaceous (~125-88 Ma) was a period which is thought to have 3-12 times higher-than-modern atmospheric carbon dioxide (CO2) concentrations and, consequently, a significantly warmer climate with higher sea surface temperatures (SSTs) and a much reduced equator-to-pole thermal gradient than today. Against this background, episodes (<1 Ma) of enhanced organic carbon (OC) burial rates in various marine settings occurred during so-called Oceanic Anoxic Events (OAEs), in which large parts of the ocean became devoid of oxygen. These OAEs led to massive perturbations of the global carbon cycle. The probably most extensive event was OAE 2, ~ 93.5 Myr ago, and is thought to have resulted in the enhanced sequestration of atmospheric CO2 and thus may have caused a substantial pCO2 drawdown in the atmosphere with potentially large climatic consequences. Recently, the re-assessment of pCO2 level estimates from the carbon isotopic composition of specific biomarkers suggest a ca. 25% drop from ca. 1300 ppmv in the pre-and post- OAE 2 phase to pCO2 levels around 1000 ppmv during the event. However, the climate consequences of this drop are not yet well understood. Here we present the first, continuous high-resolution SST record from a deep-marine sedimentary sequence across the OAE 2, recently cored in the NW Atlantic ocean at ODP site 1276 (~45°N), off Newfoundland. We find that base level SSTs, estimated using the organic palaeothermometer TEX86 were extremely high, substantially exceeding annual mean SSTs in this region today. Within the first phase of the OAE 2, this extreme warm thermal regime was punctuated by a remarkable cooling (up to 11°C), which is associated with the first positive carbon isotope shift maximum caused by the burial of OC. Based on chemostratigraphic correlation, this cooling is synchronous with the Late Cenomanian cooling of up to 6°C, recently noted in SST records at equatorial Atlantic sites and synchronous with a documented cool episode ("Plenus Cold Event"), evident from palaeontological findings in contemporaneous NW European shelf sea sections. This globally observed cooling phase in the early stage of the OAE 2 seems synchronous with the presumed drop in pCO2 levels as reconstructed from biomarker isotopes. Consequently, this significant perturbation of the extreme mid-Cretaceous global warmth, is likely not due to local factors and, in fact, acted as a direct response to the pCO2 drawdown, probably resulting from globally increased OC burial rates after the onset of OAE 2.