Description: The transition from the Cretaceous “Supergreenhouse” to the Oligocene icehouse provides an opportunity to study changes in Earth system dynamics from a time when climate models suggest CO2 levels may have been as high as 3500 ppmv (parts per million by volume) and then declined to less than 560 ppmv. During the Supergreenhouse interval meridional temperature gradients were very low and oceanic deposition was punctuated by episodes of widespread anoxia, termed Oceanic Anoxic Events (OAEs) resulting in large scale burial of organic carbon reflected in positive delta 13C excursions. High CO2, greenhouse climate conditions are envisioned for the near future calling for action to get a better understanding of their potential impacts and dynamics. Climate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways, pointing towards a progressive strengthening of the Antarctic Circumpolar Current as the major cause for cooler deep ocean temperatures. Analogous arguments point to an important role for deep circulation in explaining Late Cretaceous climate evolution. The Agulhas Plateau is located in a key area for retrieving high-quality geochemical records to test competing models, e.g. to what extent and exactly when the opening of Drake Passage contributed to cooling of the deep ocean. The proposed drill sites on Agulhas Plateau and Transkei Basin are at high latitudes (65°S-58°S from 100 to 65 Ma) and within a gateway between the newly opening South Atlantic, Southern Ocean and southern Indian Ocean basins. Recovery of expanded and stratigraphically complete pelagic carbonate sequences from this region, and comparison with drilling results from Naturaliste Plateau (760-Full), will provide a wealth of new data to significantly advance the understanding of how Cretaceous temperatures, ocean circulation, and sedimentation patterns evolved as CO2 level rose and fell, and the breakup of Gondwana progressed.

Description: The transition from the Cretaceous “Supergreenhouse” to the Oligocene icehouse provides an opportunity to study changes in Earth system dynamics from a time when climate models suggest CO2 levels may have been as high as 3500 ppmv (parts per million by volume) and then declined to less than 560 ppmv. During the Supergreenhouse interval meridional temperature gradients were very low and oceanic deposition was punctuated by episodes of widespread anoxia, termed Oceanic Anoxic Events (OAEs) resulting in large scale burial of organic carbon reflected in positive delta 13C excursions. High CO2, greenhouse climate conditions are envisioned for the near future calling for action to get a better understanding of their potential impacts and dynamics. Climate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways, pointing towards a progressive strengthening of the Antarctic Circumpolar Current as the major cause for cooler deep ocean temperatures. Analogous arguments point to an important role for deep circulation in explaining Late Cretaceous climate evolution. The Agulhas Plateau is located in a key area for retrieving high-quality geochemical records to test competing models, e.g. to what extent and exactly when the opening of Drake Passage contributed to cooling of the deep ocean. The proposed drill sites on Agulhas Plateau and Transkei Basin are at high latitudes (65°S-58°S from 100 to 65 Ma) and within a gateway between the newly opening South Atlantic, Southern Ocean and southern Indian Ocean basins. Recovery of expanded and stratigraphically complete pelagic carbonate sequences from this region, and comparison with drilling results from Naturaliste Plateau (760-Full), will provide a wealth of new data to significantly advance the understanding of how Cretaceous temperatures, ocean circulation, and sedimentation patterns evolved as CO2 level rose and fell, and the breakup of Gondwana progressed.

Description: Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high ( 〉  800 ppmv) atmospheric CO2 concentrations. Although a post hoc intercomparison of Eocene ( ∼  50  Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 ×  CO2 simulations, and additional sensitivity studies, these form the first phase of DeepMIP – the Deep-time Model Intercomparison Project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, astronomical configuration, solar constant, land surface processes, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological data sets, which will be used to evaluate the simulations, will be developed.

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: We present results from an ensemble of eight climate models, each of which has carried out simulations of the early Eocene climate optimum (EECO, ∼ 50 million years ago). These simulations have been carried out in the framework of the Deep-Time Model Intercomparison Project (DeepMIP; http://www.deepmip.org, last access: 10 January 2021); thus, all models have been configured with the same paleogeographic and vegetation boundary conditions. The results indicate that these non-CO2 boundary conditions contribute between 3 and 5 ∘C to Eocene warmth. Compared with results from previous studies, the DeepMIP simulations generally show a reduced spread of the global mean surface temperature response across the ensemble for a given atmospheric CO2 concentration as well as an increased climate sensitivity on average. An energy balance analysis of the model ensemble indicates that global mean warming in the Eocene compared with the preindustrial period mostly arises from decreases in emissivity due to the elevated CO2 concentration (and associated water vapour and long-wave cloud feedbacks), whereas the reduction in the Eocene in terms of the meridional temperature gradient is primarily due to emissivity and albedo changes owing to the non-CO2 boundary conditions (i.e. the removal of the Antarctic ice sheet and changes in vegetation). Three of the models (the Community Earth System Model, CESM; the Geophysical Fluid Dynamics Laboratory, GFDL, model; and the Norwegian Earth System Model, NorESM) show results that are consistent with the proxies in terms of the global mean temperature, meridional SST gradient, and CO2, without prescribing changes to model parameters. In addition, many of the models agree well with the first-order spatial patterns in the SST proxies. However, at a more regional scale, the models lack skill. In particular, the modelled anomalies are substantially lower than those indicated by the proxies in the southwest Pacific; here, modelled continental surface air temperature anomalies are more consistent with surface air temperature proxies, implying a possible inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large-scale features and model–data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean circulation, hydrological cycle, and modes of variability, and encourage sensitivity studies to aspects such as paleogeography, orbital configuration, and aerosols.

Description: The Last Glacial Maximum (LGM, ∼ 21 000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models has been used to generate LGM simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here, we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4, most of which are PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3, most of which are PMIP3-CMIP5). We show that the global averages of the PMIP4 simulations span a larger range in terms of mean annual surface air temperature and mean annual precipitation compared to the PMIP3-CMIP5 simulations, with some PMIP4 simulations reaching a globally colder and drier state. However, the multi-model global cooling average is similar for the PMIP4 and PMIP3 ensembles, while the multi-model PMIP4 mean annual precipitation average is drier than the PMIP3 one. There are important differences in both atmospheric and oceanic circulations between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large. Therefore, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land–sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the paleoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. These results point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

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.