ALBERT

Description: A new model of the global atmosphere-ocean-continent-mantle system was set-up to investigate the triggering of the Oceanic Anoxic Event OAE2 through volcanic degassing processes at large igneous provinces (LIPs). The model simulates the changes in oceanic dissolved oxygen, phosphate, and carbon and the evolution of atmospheric pCO2 values under mid-Cretaceous boundary conditions. It considers the effects of pCO2 on element ratios in marine plankton (C : P) and includes new parameterizations for phosphorus and carbon burial at the seafloor based on modern observations. Independent isotopic and chemical time-series of ocean and atmosphere change over OAE2 are applied to evaluate the model results. The model results support the hypothesis that OAE2 was triggered by massive CO2 emissions at LIPs. According to the model, the phosphorus weathering flux into the ocean and the C : P ratio in marine plankton were enhanced by the rise in surface temperature and atmosphere pCO2 caused by mantle degassing. Marine export production and oxygen consumption in intermediate and deep water masses increased in response to the expansion of the dissolved phosphate inventory of the ocean and the change in plankton element ratios. The spread of anoxic conditions in bottom waters -induced by enhanced carbon export and respiration- was further amplified by the oxygen-dependent burial of phosphorus in marine sediments in a positive feedback loop. The modeling implies that enhanced CO2 emissions favor the spread of low-oxygen conditions also in modern oceans.

Description: Until recently it was assumed that the major modern ice sheets on Antarctica became established around the Eocene-Oligocene boundary about 34 Ma ago. But new evidence (e.g. Miller et al., 2008) indicates that continental ice may have been present much earlier, some of it probably even since the greenhouse times of the Late Cretaceous. Deep sea drilling data suggest changes in sea-level during the Late Cretaceous that could have been caused by the melting and freezing of vast ice sheets on Antarctica. Using a GCM approach to test the whether it would be possible to generate the described high-amplitude sealevel falls is one additional way to test this vigorously discussed issue. As shown above, our numerical approach indicates the possibility of a substantial Antarctic glaciation by changing the physical boundary conditions, eccentricity, pCO2, and elevation within reasonable Late Cretaceous ranges. Our simulations suggest that simulated snowfall and consecutive ice formation on Antarctica might yield sufficient volumes to account for the documented rapid, low-amplitude Cretaceous sea-level fluctuations. Based on cautious assumptions and possible errors the model results show that ice build-up could take place in realistic time spans and in accordance with the proxy records. Thus, the possibility of an Antarctic ice shield build-up large enough to drive sea level fluctuations on the order of tens of meters within 20,000-220,000 years is supported. The initial snow accumulation and following growth of Antarctic ice-sheets in the Cretaceous can be attributed to changes in southern hemisphere summer insolation due to reduced orbital eccentricity. Alternatively and/or additionally, declining atmospheric CO2 values caused further cooling