The global meridional overturning circulation (GMOC) plays an important role in transporting oceanic heat from one hemisphere to the other. At present, the AMOC descends in the North Atlantic and is responsible for transporting large amount of heat from the Southern Hemisphere (SH) to the Northern Hemisphere (NH). In the early Paleozoic, the continental configuration was nearly opposite to that of the present, with most of the landmass located in the SH and an ocean world of the NH. Here, we present simulation results to demonstrate that the GMOC in the Paleozoic was anticlockwise, with upwelling in the NH and descending in the SH, which is opposite to that of the present. The anticlockwise GMOC in the Paleozoic is mainly due to hemispheric asymmetry of wind stresses and freshwater input into the ocean. Stronger wind stress in the NH drives upwelling in the NH extratropics. Less freshwater input into the SH ocean causes saltier and heavier seawater, which is conducive to deep water formation in the SH ocean. These hemispheric asymmetries of wind stresses and freshwater are because of land-sea distribution in the Paleozoic. Two datasets are used, which show consistent results in general.

How to cite: Yuan, S., Hu, Y., Liu, Y., and Lunt, D.: The Global Meridional Overturning Circulation of the Paleozoic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10724, https://doi.org/10.5194/egusphere-egu23-10724, 2023.

How to cite: Ragon, C., Vérard, C., Kasparian, J., and Brunetti, M.: Steady states and bifurcation diagram for the Permian-Triassic paleogeography , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6695, https://doi.org/10.5194/egusphere-egu23-6695, 2023.

The El Niño–Southern Oscillation (ENSO), originating in the central and eastern equatorial Pacific, is a defining mode of interannual climate variability with profound impact on global climate and ecosystems. Although ongoing coordinated community efforts have offered insights into how ENSO will change in the future under anthropogenic warming, the geological history of ENSO remains intricate. In particular, there is a clear lack of systematic study on how ENSO has evolved in response to vast variations in land-sea distributions and climate mean states over geological timescales. To unravel this, we analyze a series of time-slice coupled climate simulations forced by changes of paleogeography, atmospheric CO₂ concentrations, and solar radiation in the past 250 million years (Myr). Our simulations for the first time demonstrate that ENSO is the leading mode of tropical Pacific sea surface temperature (SST) in the past 250 Myr. Further, the amplitude of ENSO is predominantly captured by the zonal advective feedback and thermocline feedback, both of which are primarily regulated by eastern equatorial Pacific climatological SST. These findings highlight the significance of climate mean states in interpretation of the amplitude of ENSO during the deep past, and provide enlightening implications for constraining future climate change.

How to cite: Li, X., Hu, S., Hu, Y., Guo, J., Lan, J., Lin, Q., and Yuan, S.: Variations in the amplitude of El Niño–Southern Oscillation in the past 250 million years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4227, https://doi.org/10.5194/egusphere-egu23-4227, 2023.

The Pangea era was an exceptional phase in Earth’s history. It was characterized by its hothouse climate state and the latest supercontinent. And the supercontinent is South-North oriented, nearly extending from South Pole to North Pole. Geological evidence shows that the climate of the supercontinent Pangea was not only hot but also dry. In this talk, I will show simulation results that the dry climate condition of Pangea was largely due to the broad landmass and its South-North orientation. Such a particular continental configuration resulted in much weaker precipitation in the tropics and extratropics, not only over land but also over ocean, compared with other hot periods in the Phanerozoic. Associated mechanisms of the slower hydrological cycle of the supercontinent Pangea will also be discussed. 

How to cite: Hu, Y.: The hot and dry climate of the supercontinent Pangea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3804, https://doi.org/10.5194/egusphere-egu23-3804, 2023.

The distribution of continents and oceans through deep-time has shaped the Earth’s changing climate and geography in a way that is vital for understanding processes ranging from the evolution and migration of living organisms to the distribution of economic mineral deposits, and the precise contribution of anthropogenic CO₂ to the present climate. Despite this, producing climate models of an evolving Earth across long periods of geological time have been challenging due to the immense computational resources required, and as a result, deep-time paleo-climate models have tended to focus on single ages. Here we demonstrate the use of an intermediate-complexity atmosphere-ocean Plasim-Genie tool to produce a suite of models that illustrate the evolving climate with evolving continents over hundreds of millions of years.

Using this approach, we explore the impact of modelling paleoclimate with a pure paleomagnetic plate reference frame versus a plate motion model that uses a mantle reference frame. These two plate reference frames may have latitudinal differences of up to 15 degrees even in the well-constrained timeframe of the last 100 Myr. We demonstrate with Plasim-Genie that these latitudinal differences result in significant discrepancies in climate in a range of key regions. Users of paleoclimate model data, particularly those studying biology and geography of the past should be aware of the plate reference frame used in generating the climate data. As the Earth’s magnetic field averages to align with the Earth’s spin axis, we suggest that a paleomagnetic reference frame is the preferred reference frame to use for paleoclimate modelling.

How to cite: Leonard, J., Zahirovic, S., Salles, T., and Mallard, C.: Exploring the role of Mantle and Paleomagnetic Reference Frames with Intermediate Complexity Climate Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10673, https://doi.org/10.5194/egusphere-egu23-10673, 2023.

Polar amplification is the phenomenon that external radiative forcing produces a larger change in surface temperature at high latitudes than the global average, which is one of the most robust in climate changes with historical and future increases in atmospheric CO₂. Also, it is known that, polar amplifications occurred during past warm periods due to atmospheric CO₂ concentrations and orbital parameters different from those in the present day. The Cretaceous is known as one of the warmest periods in the Phanerozoic (Foster et al., 2017). The Cretaceous proxy data indicate remarkable temperature amplifications in the high-latitude and polar region (e.g., Jenkyns et al. 2004), resulting in small equator-to-polar temperature difference. Many previous studies using global climate models have invested the relationship between the polar temperatures and the greenhouse gasses, geography, vegetation, and cloud property in order to elucidate the mechanism (e.g., Otto-Bliesner and Upchurch 1997; Upchurch et al., 2015; Niezgodzki et al., 2017). On the other hand, it is not well understood that differences in polar amplifications between present day and Cretaceous with changes in atmospheric CO₂ concentration and orbital parameters. In this study, we systematically investigated the responses of the polar temperatures in the present-day Cretaceous to changes in atmospheric CO₂ concentration and the orbital parameters using an atmospheric-ocean-vegetation fully coupled model MIROC4m-LPJ. Our Cretaceous simulations succeeded in reproducing the polar temperature amplification at that time by considering variations in atmospheric CO₂ concentration and orbital parameters. Furthermore, it was clarified that, due to the differences in geographical conditions between the modern and the Cretaceous, the temperature of the polar regions responded more sensitively to external radiative forcing such as changes in atmospheric CO₂ concentration and orbital parameters in the Cretaceous than in the present-day.

How to cite: Higuchi, T., Abe-Ouchi, A., Chan, W.-L., and O'ishi, R.: Differences in polar amplifications between present day and Cretaceous with changes in atmospheric CO2 concentration and orbital parameters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11277, https://doi.org/10.5194/egusphere-egu23-11277, 2023.

One of the prominent features of the Cretaceous/Paleocene (K-Pg) mass extinction is the highly selective extinction of marine phytoplankton. It is generally thought that the disturbances in the marine biogeochemical processes were caused by the asteroid impact in the Yucatan and/or the Deccan volcanism. However, which one is dominant remains debated. Here, we use Community Earth System Model (CESM1.2.2) to explore their influences on the latest Cretaceous marine biosphere. It is found that the asteroid impact led to a decrease by ~32% in the calcareous algae but an increase of ~95% in the diatom, consistent with the divergent trends of the abundance of calcium carbonate and biogenic silica archived in marine sediments. The rapid decline of the calcareous algae was because of lowered temperature and decreased light due to the asteroid impact, whereas the increase of diatoms was induced by the input of impact-generated debris and enhanced vertical mixing of the surface ocean, both of which increased nutrient supply. The counteraction between calcareous algae and diatom, to some extent, ensured the resilience of the bulk ocean biogeochemical cycle to the asteroid impact in the latest Cretaceous, with the total biomass increasing by ~2.7%. In comparison, the long-term forcing  (CO₂-induced warming) due to Deccan volcanism reduced 10-20% of all types of phytoplanktons. The trend and magnitude of this change are significantly different from that triggered by the asteroid impact, suggesting that asteroid impact was more likely the primary driver of the selective phytoplankton extinction at the K-Pg boundary.

How to cite: Chen, Y., Zhang, J., Liu, Y., and Hu, Y.: Distinct Responses of Marine Phytoplanktons to the Asteroid Impact and Volcanism at the Cretaceous-Paleogene Boundary (K-Pg), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2778, https://doi.org/10.5194/egusphere-egu23-2778, 2023.

The Atlantic meridional overturning circulation (AMOC) is a major component of global ocean circulation and through the distribution of heat, salt, and nutrients exerts a fundamental influence on global and regional climates. However, there is limited understanding of AMOC stability or its sensitivity, under acute climatic warmth that is marked for Earth’s future. To tackle this important gap in our understanding, the climatic warmth of the Eocene (~34-56 Ma) offers a unique opportunity and setting to investigate existence, structure, stability, and operation of AMOC. These fundamental gaps in our knowledge and understanding limit the ability to ground-truth ocean model simulations of past warm climates, and thus also diminish our confidence in the capabilities of these models to predict ongoing changes to our oceans.

Here, we present the first reconstruction of the early-middle Eocene AMOC using a meridional transect of Atlantic and Southern Ocean drill sites. Across sites, detailed chemostratigraphic correlations provide a common, high resolution age model spanning a 500 kyr interval (46.7-47.2 Ma). During this interval, high-resolution (~10 ka) carbon (δ¹³C), oxygen (δ¹⁸O), and neodymium (εNd) proxies were used to determine ocean ventilation state, temperature and salinity, and deep-water mass flow pathways. We find an early-middle Eocene AMOC, which consisted of bipolar deep-water formation forming two major cells, a southern and a northern cell. We will discuss characteristics of these water mass cells and their origin and operation using δ¹³C, δ¹⁸O, and εNd isotopic signatures. Evidence of deep-water mass formation in the North Atlantic is supported by sedimentological evidence from Hohbein et al. (2012) and Boyle et al., (2017), suggesting deep Nordic seas overflows at ~49 Ma and deep-water current flow forming contourite drifts on the Newfoundland Ridges at 47.8 Ma respectively.

Ocean circulation modelling of intervals of past extreme warmth, such as DeepMIP, provide understanding into potential ocean structures that could have existed during the early-middle Eocene. The most common feature of model predictions is a global meridional overturning circulation with strong deep convection in the Southern Ocean and no deep convection in the North Atlantic (Zhang et al., 2022). This study provides compelling evidence to bolster the Southern Ocean findings, yet a major data-modelling discrepancy exists within the North Atlantic, where most current model simulations don’t reproduce the proxy derived deep northern cell. This could point to non-CO₂ boundary conditions, such as North Atlantic bathymetry and gateways, as a cause of this discrepancy. Further proxy and modelling work is warranted to resolve the temporal extent of deep-water convection in the North Atlantic during the Eocene.

References

Boyle et al., 2017. Marine Geology, 385, 185–203.

Hohbein et al., 2012, Geology, 40, 3, 255–258.

Zhang et al., 2022, Paleoceanography and Paleoclimatology, 37, 3, 1–22.

How to cite: McIntyre, A., Sexton, P., and Anand, P.: Bipolar deep-water formation during the climatic warmth of the early-middle Eocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6249, https://doi.org/10.5194/egusphere-egu23-6249, 2023.

The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep-Time Model Intercomparison Project (DeepMIP). These simulations target the Early Eocene Climatic Optimum (EECO), a geological time period with high CO₂ concentrations, analogous to the upper range of end-of-century CO₂ projections. Preindustrial and early Eocene simulations at a range of CO₂  levels (1x, 3x and 6x preindustrial values) are used to quantify the MHT changes in response to both CO₂ and non-CO₂ related forcings. We found that atmospheric poleward heat transport increases with CO₂, while the effect of non-CO₂ boundary conditions (e.g., paleogeography, land ice, vegetation) is causing more poleward atmospheric heat transport on the Northern and less on the Southern Hemisphere. The changes in paleogeography increase the heat transport via transient eddies at the mid-latitudes in the Eocene. The Hadley cells have an asymmetric response to both the CO₂ and non-CO₂ constraints. The poleward latent heat transport of monsoon systems increases with rising CO₂ concentrations, but this effect is offset by the Eocene topography. Our results show that the changes in the monsoon systems’ latent heat transport is a robust feature of CO₂ warming, which is in line with the currently observed precipitation increase of present day monsoon systems.

How to cite: Kelemen, F. D., Steinig, S., de Boer, A., Zhu, J., Chan, W.-L., Niezgodzki, I., Hutchinson, D. K., Knorr, G., Abe-Ouchi, A., and Ahrens, B.: Meridional Heat Transport in the DeepMIP Eocene ensemble: non-CO2 and CO2 effects, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6749, https://doi.org/10.5194/egusphere-egu23-6749, 2023.

Elevation-dependent temperature change is a phenomenon found in mountain regions with complex terrain, mostly in the Himalayas and the high terrain of the Tibetan Plateau, where regions in high elevation feature high rates of warming than the region in lower elevation. This pattern referred to as elevation-dependent warming. However, does elevation-dependent warming exist in Eocene hothouse without Tibetan Plateau as well and lower altitude mountain ranges?

The Eocene era is considered a replication of the future climate with high atmospheric carbon dioxide. We utilized CESM1.2 as part of the DeepMIP simulations to analyze elevation-dependent temperature change in different mountain ranges in the Eocene and explained the findings using a linear surface energy balance decomposition. The results feature a land-sea contrast with amplification over land and elevation-dependent temperature changes in all mountain ranges with distinct seasonality and pattern. The results suggest that radiative feedback processes have a strong contribution to elevation-dependent warming in the warming climate. Our modeling results provide relevant information for mountain climate change in a past hot climate. Further, the analysis opens new mystery and perspectives related to elevation-dependent warming.

Keywords: Eocene, paleoclimate modeling, elevation-dependent warming, CO2

Related Publication:

Kad P, Blau M T, Ha K-J, and Zhu J (2022) Elevation-dependent temperature response in early Eocene using paleoclimate model experiment. Environmental Research Letters, 17(11), 114038. (https://doi.org/10.1088/1748-9326/ac9c74)

How to cite: Blau, M. T., Kad, P., Ha, K.-J., and Zhu, J.: How does Eocene warming affected elevation-dependent warming?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10201, https://doi.org/10.5194/egusphere-egu23-10201, 2023.

Rapid climatic and carbon cycle upheavals in the early Eocene have been of strong interest for the past decades. Multiple of these events represent global warming events (i.e. hyperthermals), including the Paleocene Eocene Thermal Maximum (PETM; 56 Ma), but global data coverage is still too limited to fully resolve the spatial patterns of climate, ecosystems and hydrology for the other hyperthermals. This is particularly due to the lack of continuous continental margin sequences suitable for high-resolution paleoclimate reconstructions.

We present high-resolution, multi-proxy records of climatic and environmental changes across multiple hyperthermals from Core RH-323, Northern Negev Desert, Israel. Lower Eocene sediments, dominantly orbitally paced alternations of marls and chalk, were deposited in the Tethys Ocean at a latitude of ~15º N, on the northward dipping slope of the presumably dry northern bound of the African continent. They provide a unique opportunity to capture tropical climate variability and continental-hydrological response to hyperthermals. We reconstruct regional variability of (sub)surface temperature, plankton ecology and continental hydrology by TEX₈₆ paleothermometry, bulk carbonate isotope ratios, magnetic susceptibility, bulk-XRF and dinoflagellate cyst (dinocyst) assemblages.

We identify various hyperthermal events, including the PETM, ETM2 and ETM3, based on combined chemo- and biostratigraphy. During the PETM, TEX₈₆-based temperatures indicate a warming of 3­–4 ºC, coinciding with a high abundance of representatives of the classic warm water dinocyst genus Apectodinium. The PETM is marked by a thin marl-rich interval at the onset, followed by a carbonate-rich interval during the body, suggesting very different hydrological forcing of siliciclastic input than recorded at mid-latitude sites. We interpret this to reflect strong seasonality (possibly monsoon like) with periods of intense precipitation followed by prolonged drought. Interestingly, subsequent smaller hyperthermals seem to predominantly coincide with increased siliciclastic content, thus representing episodes of increased (seasonal) precipitation.

How to cite: Fokkema, C., Agterhuis, T., Kelly, L., Mannucci, A., Theijse, B., Bialik, O., Bijl, P., Brinkhuis, H., Dickens, G., Galeotti, S., Peterse, F., and Sluijs, A.: Hydrological upheaval across multiple early Eocene hyperthermal events in the north African arid zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16165, https://doi.org/10.5194/egusphere-egu23-16165, 2023.

High latitude warmth during the Eocene greenhouse climate has posed many challenges for climate modelling studies. Recent improvements in both the proxy records and model simulations are bringing these closer together, particularly regarding the meridional temperature gradient. Yet, it remains difficult to understand the climatic conditions around the greenhouse-icehouse transition which involved the glaciation of Antarctica. How can we explain indications of ice near the Antarctic coast well before the transition, especially since Antarctic glaciation is thought to express strong hysteresis? How did Antarctica remain mostly ice-free and vegetated through large climatic swings during the Eocene? If Antarctic warmth was so resilient, which process was responsible for its eventual demise?

We consider a set of existing climate simulations for the middle-to-late Eocene (42-34Ma) using the CESM model (Baatsen et al. 2020)¹. The original set of simulations was expanded to include possible scenarios of orbital forcing, atmospheric composition, and the continental geometry. In addition, we look at the output from DeepMIP simulations for the early Eocene. Using these results, we make a detailed study of the Antarctic climate and find that most of the continent saw monsoonal conditions during the Eocene. Only a small corridor near the coast experienced perennially mild and wet conditions, explaining the presence of temperate to paratropical vegetation. Further inland, we see a rapid increase in temperature seasonality along with the appearance of a summer monsoon. Summertime warmth made most of the Antarctic continent a hostile place for any significant ice growth. Meanwhile, mountainous regions near the coast were suitable candidates for the formation of ice caps that may have grown substantially during cooler intervals.

Our simulations can explain seemingly contradictory indications from proxy records, as well as strong regional variations in the Antarctic climate. The monsoonal nature of this climate during the Eocene proves to be particularly resilient to the changes in external forcing considered here. Identifying the potential mechanism to break up the monsoonal regime and eventually lead to Antarctic glaciation remains the subject of ongoing work.

1. Baatsen, M., von der Heydt, A. S., Huber, M., Kliphuis, M. A., Bijl, P. K., Sluijs, A., & Dijkstra, H. A. (2020). The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5. Climate of the Past, 16(6), 2573–2597. doi: 10.5194/cp-16-2573-2020.

How to cite: Baatsen, M., von der Heydt, A., Bijl, P., Sluijs, A., and Dijkstra, H.: Resilience and implications of an Antarctic monsoon during the Eocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16108, https://doi.org/10.5194/egusphere-egu23-16108, 2023.

The early Eocene is a warm period suitable for studying the influence of high atmospheric CO₂ concentrations on climate, the terrestrial biosphere, and their interaction. Here, we simulated the terrestrial biosphere of the early Eocene with a dynamic global vegetation model (LPJ-GUESS), driven by climate input from the Deep-Time Model Intercomparison Project (DeepMIP). We find a strong expansion of tropical and temperate forests to higher latitudes, which is more pronounced when assuming CO₂ concentrations at the high end of plausible values. The modeled vegetation distribution is compared to a recently compiled extensive early Eocene paleobotanical global dataset. Simulated and reconstructed vegetation show good agreement, which improves with higher CO₂ concentrations. In contrast to earlier vegetation modeling studies our simulations are able to accurately simulate the expansion of tropical forests under CO₂ concentrations larger than 4-times pre-industrial CO₂. Given the good agreement between modeled vegetation and paleobotanical data our simulations allow us to gain a more comprehensive understanding of the early Eocene terrestrial biosphere, including the global carbon cycle and the role of wildfires. In addition, our comparison of modeled vegetation and paleobotanical data is an important test of the performance of the climate and vegetation models used, which are also used to simulate future impacts of climate change.

How to cite: Brugger, J., Thompson, N., Utescher, T., Salzmann, U., and Hickler, T.: Simulating the terrestrial biosphere in the high CO2 world of the early Eocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8997, https://doi.org/10.5194/egusphere-egu23-8997, 2023.

Paleogeographic evolution is a major forcing of climate on long (multi-million years) time scales, which in turn can drive important vegetation cover variations. While recent studies have shown the important role played by abiotic factors on species diversification and on the establishment of modern biodiversity gradients, and although climate models are designed to simulate increasingly realistic vegetation cover, it remains difficult to estimate the accuracy of the results obtained. Indeed, the fossil record available to estimate the evolution of the paleovegetation cover remains heterogenous and fragmentary in some regions.  

Here, we present climate and vegetation cover response to paleogeography evolution throughout the Cenozoic as simulated with IPSL-CM5A2 Earth System Model. In a second time, we wish to explore means of using temperature and precipitation values extracted from such models simulations, in order to constrain birth-death diversification models. While the use of regionally-averaged abiotic parameters seems a potentially considerable improvement, as opposed to current methods based on global climate curve temperature, this methodology presents some technical challenges that remain to be tackled.

How to cite: Tardif, D., Sepulchre, P., Condamine, F., and Thomas, C.: Exploring Cenozoic vegetation cover and paleobiodiversity evolution induced by paleogeography and climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15740, https://doi.org/10.5194/egusphere-egu23-15740, 2023.

At the Eocene-Oligocene Transition (~34 Ma), ephemeral ice sheets grew into a large continental-scale Antarctic ice sheet. During the late Eocene there is evidence for short-lived, continental-scale glaciations in the benthic oxygen isotope record, as well as geomorphic evidence pointing towards large-scale glaciations on Antarctica. Here, the modelled mean oxygen isotope ratio of these early Cenozoic Antarctic ice sheets is presented. Since benthic oxygen isotopes are a proxy for both the deep sea temperature and the ice volume stored on land, it is possible to estimate the benthic oxygen isotope change once the mean oxygen isotope content of the ice sheet is known.

The modelled ice sheet oxygen isotope ratio of the late Eocene Antarctic ice sheets are strongly dependent on the size of the modelled continental-scale ice sheet, which in turn is determined by the bedrock. The ice sheet volume expansion during the Priabonian Oxygen Isotope Maximum (at around 37.2 Ma during the late Eocene) results in a modelled benthic oxygen isotope change between 0.3‰ and 0.55‰, sufficient to explain the observed excursions in the benthic oxygen isotope records. At the Eocene-Oligocene Transition, the modelled benthic oxygen isotope change due to ice sheet growth is found to be between 0.65‰ and 0.75‰. The remainder 0.45‰ to 0.55‰ of the observed benthic oxygen isotope change should therefore have been caused by oceanic cooling.

How to cite: Van Breedam, J., Huybrechts, P., and Crucifix, M.: Modelling the early Cenozoic Antarctic ice sheet oxygen isotope ratio and implications for the benthic δ18O change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6599, https://doi.org/10.5194/egusphere-egu23-6599, 2023.

Atmospheric concentrations of carbon dioxide (pCO₂) play a critical role in a number of earth system components, including the biosphere, geosphere, and atmosphere. Understanding how pCO₂ has changed over geologic time may provide critical context for predicting future climate scenarios. However, constraining past pCO₂ values remain a challenge in paleoclimate studies due to difficulties in constraining proxy calculation parameters. Marine-based pCO₂ proxies rely on at least one temperature parameter (i.e., via Henry’s Law to convert dissolved CO₂ concentrations from the ocean into atmospheric pCO₂), which is difficult to constrain in deep-time. Here, we highlight the importance of temperature and propose a new method of model-derived temperatures based on the paleo-location of the sample site. This methodology can be applied to any pCO₂ proxy with temperature input; here we use the example of the pCO₂ proxy based on the stable carbon isotopic composition of phytane, the diagenetic product of chlorophyll-a, due to its spatial and temporal ubiquity over the past ca. 465 million years.

How to cite: Witkowski, C., Farnsworth, A., and Valdes, P.: Investigating the role of temperatures in proxy-based pCO2 estimates: An integrated model-proxy approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8237, https://doi.org/10.5194/egusphere-egu23-8237, 2023.

The South Asian summer monsoon (SASM) remarkably strengthened during the Middle Miocene (16-11 Ma), coincident with the rapid uplifts of the Iranian Plateau (IP) and the Himalaya (HM). Although the development of the SASM has long been linked to the topographic changes in the Tibetan Plateau (TP) region, the effects of the HM and IP uplift are still vigorously debated, and the underlying mechanisms remain unclear. Based on Middle Miocene paleogeography, we employ the fully coupled earth system model CESM to perform a set of topographic sensitivity experiments with altered altitudes of the IP and the HM. Our simulations reproduce the strengthening of the SASM in northwestern India and over the Arabian Sea, largely attributing to the thermal effect of the IP uplift. The elevated IP insulates the warm and moist airs from the westerlies in the south of the IP and produces a low-level cyclonic circulation around the IP, which leads to the convergence of the warm and moist air in the northwestern India and triggers positive feedback between the moist convection and the large-scale monsoon circulation, further enhancing the monsoonal precipitation. Whereas the HM uplift produces orographic precipitation without favorable circulation adjustment for the SASM. We thus interpret the intensification of the Middle Miocene SASM in the western part of the South Asia as a response to the IP uplift while the subtle SASM change in eastern India reflects the effects of the HM uplift.

How to cite: Zuo, M., Sun, Y., Zhao, Y., Ramstein, G., Ding, L., and Zhou, T.: South Asian summer monsoon enhanced by the uplift of Iranian Plateau in Middle Miocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3814, https://doi.org/10.5194/egusphere-egu23-3814, 2023.

The Miocene Climatic Optimum (MCO, ~17-15 Ma) was a relatively warm interval that interrupted the Cenozoic cooling trend and bears analogies with projected end-of-the-century climate. Proxy data and model simulations indicate temperatures were on average ~7 – 8°C higher than present day and atmospheric pCO₂ values were 500-600 ppmV. At high latitudes, upper ocean temperatures were ~10 – 15°C warmer than present day, but available tropical temperature records are limited. Importantly, to be able to use Miocene paleoclimate records to quantify key climate parameters like the polar amplification of climate change, accurate reconstructions of tropical surface oceans are required.

We present high resolution Early to Middle Miocene (~15 – 18 Ma) records of tropical sea surface temperature (SST) variability based on the lipid biomarker paleothermometer TEX₈₆ at Ocean Drilling Program (ODP) Site 959 in the eastern equatorial Atlantic Ocean and at ODP Site 1007 at Bahama Bank in the western tropical Atlantic Ocean. The age model for both sites is based on chemo- and biostratigraphy. Moreover, analyses of bulk carbonate oxygen- (δ¹⁸O) and stable carbon (δ¹³C) isotope ratios at ~2 – 4 kyr resolution at Site 959 facilitated orbital tuning to eccentricity, obliquity and precession. Bulk elemental composition records, total organic carbon concentrations and dinoflagellate cyst assemblages were also generated to assess paleoenvironmental change.

At both sites, warming associated with the onset of the MCO (~17 Ma) was identified as an average SST increase of ~2°C (using the TEX₈₆-H calibration). At ~16.8 Ma, bulk carbonate δ¹³C gradually increased by ~1‰ at both sites, indicating the onset of the Monterey carbon isotope excursion. Combined with available temperature information from the high latitudes and deep ocean, we assess meridional temperature gradients across the MCO.

At ODP Site 959, sediments are characterized by alternations of biogenic silica, carbonates, and terrigenous material (i.e., clays). Following the onset of the MCO (~17 – 16.5 Ma), high variability in the oceanographical setting is reflected in striking Ba_bio peaks, indicating productivity changes. These peaks coincide with lowest SSTs (~28°C) and increased dust supply (increased Fe and Ti) on precession and obliquity timescales. In the same interval, (inner-) neritic dinoflagellate species indicate increased water column stratification. Heterotrophic dinocyst groups vary on timescales coherent with the other geochemical records, relative to comparably stable background abundances of oceanic genera throughout the MCO. Combined, this suggests that a highly dynamic monsoon-driven upwelling regime was present at Site 959 during the MCO. Combined, these patterns imply a highly dynamic African monsoon-driven upwelling regime that intensified at the onset of the MCO.

How to cite: Wubben, E., Spiering, B., Veenstra, T., Bos, R., van Dijk, J., den Hollander, J., Wang, Z., Raffi, I., Witkowski, J., Hilgen, F., Sangiorgi, F., Peterse, F., and Sluijs, A.: Tropical Atlantic Ocean climate variability during the Miocene Climatic Optimum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15801, https://doi.org/10.5194/egusphere-egu23-15801, 2023.

During the Neogene, major tectonics events occurred: uplift of mountain ranges (including the Tibetan Plateau and surrounding regions), opening and closing of seaways, and large variations of atmospheric CO₂ and sea level. What were the consequences of such changes on the dispersal of hominoid populations? We focus on the Miocene to Pliocene time interval (23 Ma-2.6 Ma). First, we analyze the spread of hominoids from their original geographic range, the tropical forests of Africa during the Early Miocene to the first expansion to Eurasia during the mid-Miocene Climate Optimum. Niche modelling combined with paleoclimatic simulations, provides means to circumvent the fragmentary nature of this record. We identify how the large climatic changes during Miocene Transition impact the potential habitats of hominoids and compare our findings to both the related fossil records and paleoenvironmental proxies.

Second, we analyze human distribution during the Mid to Late Pliocene (4-3 Ma), i.e. before the triggering of the large perennial Greenland ice sheet, and of huge amplitude northern hemisphere glacial interglacial cycles, while CO₂ evolved between 400 and 300 ppm. In Africa, tropical areas experienced drastic hydrological changes, mainly driven by precession cycles, which deeply modulated monsoon intensity and precipitation patterns, as illustrated by the paleoshore level record of Mega Lake Chad. To explore how orbital parameters strongly modify hydrological cycles over tropical Africa and, the associated dispersion of the genus Australopithecus, we simulated the response of climate, vegetation, and hydrological cycles of the mid to Late Pliocene conditions.

For both geological contexts, we provide and analyze a series of Earth System models (IPSL-CM5A2 LR-low resolution-) coupled simulations. Moreover, we associate these fully-coupled simulations with high resolution atmosphere-only model simulations, and a dynamic vegetation model (ORCHIDEE). Both models were used to estimate ecological niches with a spatial resolution of 50 km.

We describe the imprint of slow climate changes during the Miocene Climate Transition (MCT) as well as more rapid climate evolution during mid to Late Pliocene, associated with higher frequency oscillation of orbital parameters.

This study demonstrates how, for these periods, climate and especially hydrological variations were pivotal to the understanding of hominoid migrations. We compare our findings to fossil records and paleoenvironmental proxy reconstructions.

To conclude, we discuss the strengths and limitations of such approaches, which were made possible through a large trans-disciplinary effort between paleontologists, paleoanthropologists, paleoenvironmentalists, paleoclimatologists, and niche modelers.

How to cite: Ramstein, G., Gibert, C., Segalla, D., Fluteau, F., Banks, W., Barboni, D., Vignoles, A., Contoux, C., Boisserie, J.-R., Chavasseau, O., Guy, F., Otero, O., Sepulchre, P., Souron, A., and Colleoni, F.: Hominoid dispersal during Neogene, from tectonics to Milankovich forcings as major driving factors to explain the spread of population, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12549, https://doi.org/10.5194/egusphere-egu23-12549, 2023.

The world’s largest oxygen minimum zone (OMZ) resides in the eastern tropical Pacific where poorly ventilated “shadow zone”, created by stagnant tropical cyclonic gyre is complemented by intensive biological consumption of dissolved oxygen, thereby promoting oxygen deficiency in this region. The present-day continental configuration with the presence of the Isthmus of Panama prevents water mass exchange between the tropical Pacific and the Caribbean Sea, shaping the climate and marine biogeochemistry features in the eastern tropical Pacific. The tectonic closure of the American Seaway during the mid-Miocene to mid-Pliocene epoch (~16-3 Ma BP) is thought to be a key factor for the development of stagnating conditions in the eastern equatorial Pacific leading to emergence of tropical Pacific OMZs . 

This study aims at investigating the impact of the CAS opening on the large-scale ocean circulation and oxygen minimum zone in the tropical Pacific. To this end, we performed a series of sensitivity experiments with the global climate model KCM where a sill of the open CAS was set at different depths, ranging from shallow to deep levels. Our results confirm previous studies that Panamanian gateway closure during the Pliocene may have led to an intensification of the Atlantic Meridional Overturning Circulation (AMOC) due to a termination of fresh-water supply from the tropical Pacific to the North Atlantic. It was found that the CAS opening drives eastward subsurface flow in the northern tropical Pacific. This, in turn, facilitates stronger west-to-east oxygen supply and subsequent overall oxygen enrichment in the subsurface Pacific waters with strongest anomalies observed in the eastern tropical Pacific. In addition, the marine net primary production is slightly weakened in this region due to an export of nutrients to the Caribbean Sea through the open Panamanian gateway. This, in turn, leads to a weaker export of particulate organic carbon towards the ocean interior, and, therefore, to lower biological consumption of oxygen in this region.

How to cite: Khon, V., Hoogakker, B., Schneider, B., Segschneider, J., and Park, W.: Effect of tectonic closure of the American Seaway on oxygen minimum zone in the tropical Pacific from model simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7320, https://doi.org/10.5194/egusphere-egu23-7320, 2023.

During the late Miocene and the Pliocene, changes in the Central American and Indonesian seaway geometry are very important for ocean circulation and global climate. Various modelling studies have examined the separate effects of these two tropical seaways, especially their link to the onset of the Northern Hemisphere ice sheets through changes in the Atlantic meridional overturning circulation and associated heat and moisture transport. Although the existence of dual tropical seaways is closer to reality, there are very scarce modelling studies exploring the co-effects of dual tropical seaway changes, especially on the Pacific ocean circulation. Here we provide the results of a modelling study on this issue. Our results show that the combined shallow opening of tropical seaways can generate an active Pacific meridional overturning circulation (that is absent in modern conditions) by which the meridional and zonal sea surface temperature gradient in the Pacific largely reduce. In contrast, a deeper opening of tropical seaways cannot produce these changes in the Pacific ocean circulation. This study provides insights into and a better understanding of the role of tropical seaways in shaping the Pacific climate and highlights the importance of the sill depth of each seaway.

How to cite: Tan, N., Zhang, Z., Guo, Z., Guo, C., Zhang, Z., He, Z., and Ramstein, G.: Recognizing the Role of Tropical Seaways in Modulating the Pacific Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10963, https://doi.org/10.5194/egusphere-egu23-10963, 2023.

The warm Pliocene epoch is used to estimate Earth's equilibrium climate sensitivity (ECS), which is the long-term temperature change after a sustained doubling of atmospheric CO₂ over pre-industrial levels. Using an emergent constraint on the relationship between mid-Pliocene Warm Period simulated temperatures and ECS, we estimate ECS to be 4.8 K, which is higher than previous studies on the Pliocene. This is partly due to using warmer SST reconstruction than before; a consequence of focusing modelling efforts on the mid-Pliocene warm period. Using the temperatures of a broader period within the Pliocene, we quantify ECS to be 3.1 K. Further uncertainties on proxy data and data-model disagreements are expected to affect ECS estimates. We find that CO₂ uncertainties are the main driver of variations in ECS estimates, followed by biases from seasonal temperatures. The bias from polar amplification is apparently small, but could be an overlooked source of error. We conclude that the Pliocene-based emergent constraint is nonetheless robust and is likely to improve further as geological reconstructions improve.

How to cite: Renoult, M., Sagoo, N., and Mauritsen, T.: High-biased climate sensitivity estimates from mid-Pliocene Warm Period temperatures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3438, https://doi.org/10.5194/egusphere-egu23-3438, 2023.

The Pliocene (~3 million years ago) is of great interest to the palaeoclimate community as a potential palaeoclimate analogue for future climate change. It was the most recent period of sustained warmth above pre-industrial levels, was recent enough to have similar-to-modern continental configuration, and had similar-to-modern atmospheric CO₂ concentrations around 400 ppm. If we are to use the Pliocene as a palaeoclimate analogue for our warmer future, it is important to consider the drivers of Pliocene climate change as well as its comparable large-scale climate features.  

We implement a novel, simple linear factorisation method to assess the relative influence of CO₂ forcing in Pliocene climate change compared to non-CO₂ forcings such as changes to ice sheets, orography and vegetation. Outputs of this method are termed “FCO₂” and reflect the relative influence of CO₂, where 1 represents wholly dominant CO₂ forcing and 0 represents wholly dominant non-CO₂ forcing.

The accuracy of the FCO₂ method is evidenced by comparison to an energy balance analysis using a method previous used in Pliocene climate research, and the energy balance analysis also adds nuance to the FCO₂ results and highlights feedbacks that arise from CO₂ forcing.

We apply the FCO₂ method to seven models from the PlioMIP2 ensemble (CCSM4-UoT, CESM2, COSMOS, HadCM3, IPSLCM5A2, MIROC4m and NorESM1-F) which are found to be representative of the ensemble in terms of the modelled climate sensitivity and global mean surface air temperature anomaly.

CO₂ forcing is found to be the most important driver of surface air temperature change in six of the seven models (global mean FCO₂ of ensemble = 0.56), and five of the seven models for sea surface temperature (global mean FCO₂ of ensemble = 0.56). CO₂ forcing is also the most important driver for precipitation change (global mean FCO₂ of ensemble = 0.51), but spatial patterns in precipitation change are predominantly driven by non-CO₂ forcings and the effects of these must not be overlooked.

CO₂ forcing being the most important driver of change in the climate variables considered here suggests that the Pliocene is a relevant analogue for our warmer future, but attention must also be paid to the significant effects of non-CO₂ forcing in the Pliocene which may be less analogous to the present and near-term future.

Our results also have implications for the interpretation of Pliocene proxy data and data-model comparison. For example, by assessing FCO₂ in regions with large data-model discord it could become possible to highlight where the implementation of boundary conditions is largely responsible for the discord and, hence, where model boundary conditions should be modified. Given the spatial and latitudinal patterns seen in the FCO₂ results, it may also be possible to suggest new sites from which additional proxy data would be most useful.

How to cite: Burton, L., Haywood, A., Tindall, J., Dolan, A., and Hill, D. and the PlioMIP2 participants: On the climatic influence of CO2 forcing in the Pliocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2017, https://doi.org/10.5194/egusphere-egu23-2017, 2023.