Description: Paleoelevation reconstructions of the North American Cordillera in- ferred from the oxygen (delta 18O) and hydrogen (delta D) isotope ratios of terrestrial paleoclimate proxy materials (soils, ashes, lake sediments) suggest rapid north-to- south migration of topography in the early Cenozoic (pre-49 Ma to 28 Ma). The validation of this reconstruction relies on an accurate understanding of the delta 18Op and the associated regional climate change in response to the uplift of the western North America. Here we study this response using a global climate model (GCM) with explicit delta 18Op diagnostics (ECHAM5-wiso) focusing on the isotopic effects of different types of precipitation, vapor mixing, recycling and moisture source and compare the response against estimates made using a Rayleigh distillation models of moist adiabatic condensation (RDM). Four experiments are performed with Eocene topography inferred from terrestrial stable isotope paleoaltimetry records to investigate how southward propagation of topography affects regional climate (temperature, precipitation and circulation pattern) and dela 18Op over North America. Our experiments predict delta 18Op patterns that are broadly consistent with maps of temporally binned proxy delta 18O and generally support an early Cenozoic north-to-south propagation of high topogra- phy in the North American Cordillera. They do not support the commonly made assumption that isotopic fractionation occurs primarily through rainout following Rayleigh distillation nor the application of modern empirical delta 18Op lapse rates to past environments. In our GCM simulations, precipitation processes and climate changes that are not captured by RDMs substantially affect delta 18Op. These processes include shifts in local precipitation type between convective and large-scale rain and between rain and snow; intensification of low-level vapor recycling particularly on leeward slopes; develop- ment of air mass mixing and changes in wind direction and moisture source. Each of these processes can have significant (〉2‰) influences on delta 18Op that are comparable in magnitude to surface uplift of hundreds or even thousands of meters. In many regions, these processes fortuitously compensate each other, explaining the apparent agreement between ECHAM5-wiso and proxy delta 18O and, more broadly, between RDM estimates and observed delta 18O-elevation relationships. In some regions, compensation is incomplete, and as a result, ECHAM5-wiso delta 18Op does not agree with estimates from the RDM. In these regions, including the interior of the northern cordillera and the eastern flank of the southern Cordillera, moderate adjustments of paleoelevations may be in order.

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.

Description: Stable isotope records of precipitation d18O (d18Oprec) have been used as paleoclimate and paleoelevation archives of orogens. However, interpretation of these records is limited by knowledge of how d18Oprec responds to changes in global and regional climate during mountain-building events. In this study the influence of atmospheric CO2 levels, the extent of the Antarctic ice sheet, changes in Andean surface elevation, and the presence of the South American inland seaway on climate and d18Oprec in South America are quantified using the GENESIS v3 atmospheric general circulation model with isotope-tracking capabilities. Results are presented in the context of Cenozoic South American climate and d18Oprec changes. More specifically, we find: (1) Precipitation rates in the Andes are sensitive to Andean surface elevation, the seaway and, to a lesser extent, CO2 levels. Increasing Andean elevations and the presence of a seaway both cause large increases in precipitation, but in different parts of the Andes. The growth of the Antarctic ice sheet is found to have a small influence on South American precipitation. (2) The stable isotopic composition of precipitation is sensitive to all of the parameters investigated. An increase in d18Oprec of up to 8‰ is found in simulations with higher atmospheric CO2. In agreement with previous studies, d18Oprec decreases with increasing Andean elevation by an amount greater than that predicted by the modern adiabatic lapse rate. Furthermore, the presence of an inland seaway causes a decrease in d18Oprec of 1–8‰ in the northern and central Andes. The amount of depletion is dependent on the isotopic composition of the seaway. Simulations without the Antarctic ice sheet result in d18Oprec that is 0–3‰ lower than the modern. Finally, time-specific simulations for the Miocene and Eocene show that d18Oprec has decreased during the Cenozoic and that local geographical gradients of d18Oprec have increased, particularly in regions of high modern elevation. We demonstrate that in addition to Andean uplift and associated climate change, CO2 levels and an inland seaway are likely to have influenced d18Ocarb records from South America. Consideration of these global and paleogeographic changes is necessary when interpreting paleoclimate or paleoelevation from stable isotope records of d18Oprec.