Description: ABSTRACT FINAL ID: PP11A-1769 Cretaceous anoxic events may have been triggered by massive volcanic CO2 degassing as large igneous provinces (LIPs) were emplaced on the seafloor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seafloor, benthic fluxes of dissolved species across the seabed, and burial of particulates in marine sediments. The ocean is represented by twenty-seven boxes. To estimate horizontal and vertical fluxes between boxes, a coupled ocean–atmosphere general circulation model (AOGCM) is run to derive the circulation patterns of the global ocean under Late Cretaceous boundary conditions. The AOGCM modeling predicts a strong thermohaline circulation and intense ventilation in the Late Cretaceous oceans under high pCO2 values. With an appropriate choice of parameter values such as the continental input of phosphorus, the model produces ocean anoxia at low to mid latitudes and changes in marine δ13C that are consistent with geological data such as the well established δ13C curve. The spread of anoxia is supported by an increase in riverine phosphorus fluxes under high pCO2 and a decrease in phosphorus burial efficiency in marine sediments under low oxygen conditions in ambient bottom waters. Here, we suggest that an additional mechanism might contribute to anoxia, an increase in the C:P ratio of marine plankton which is induced by high pCO2 values. According to our AOGCM model results, an intensively ventilated Cretaceous ocean turns anoxic only if the C:P ratio of marine organic particles exported into the deep ocean is allowed to increase under high pCO2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redfield ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO2 emissions. The formation of C-enriched marine organic matter may also explain the frequent occurrence of global anoxia during other geological periods characterized by high pCO2 values.

Description: Stable isotope paleoaltimetry has been widely used to estimate Cenozoic surface elevation of major orogens. The influence of global climate change on stable isotope paleoaltimetry is uncertain, with proposals that warming could cause either overestimates or underestimates of past surface elevations. In this study we increase atmospheric pCO2 by two and four times in an isotope-tracking atmospheric general circulation model to investigate the effect of global warming on oxygen isotopic compositions of precipitation ({delta}18Op) over the continents. As in other climate models, the response in the GENESIS version 3 model to global warming is an amplification of upper troposphere temperatures through enhanced infrared absorption and a reduction in the surface to upper-level temperature gradient. Due to the temperature dependence of isotopic fractionation, vapor {delta}18O ({delta}18Ov) follows suit, leading to a reduction in the surface to upper troposphere {delta}18Ov gradient. In regions of subsidence, including the major orogens and deserts, downward mixing of 18O-enriched vapor from the troposphere to the near surface further reduces the lapse rate of {delta}18Ov. As a consequence of these effects, the isotopic composition of precipitation in high-elevation regions, including the Tibetan Plateau, Rocky Mountains, European Alps, and Andean Plateau, increases by 3{per thousand}-6{per thousand} relative to that at low elevations. Neglect of this climate effect on high-elevation {delta}18Op has likely led to underestimates of the surface elevation of Cenozoic orogens.