Displays

Weathering of the Earth’s surface has commonly been invoked as a driver of global cooling through geologic time. During the Phanerozoic Eon (541–0 million years ago, Ma), for example, the periodic onset of icehouse conditions has variously been attributed to enhanced weathering fluxes associated with mountain building (e.g. the Himalayas) (1), reductions in the global extent of continental arc volcanoes (e.g. the present-day Andes) (2), and uplift of oceanic crust during arc-continent collisions (e.g. present-day Indonesia and New Guinea) (3). These processes, tied to the global plate tectonic cycle, are inextricably linked.  The resulting collinearity (i.e. independent variables are highly correlated) makes it difficult — using conventional statistical techniques — to isolate the contributions of individual geologic processes to global chemical weathering.   An example of this is the Late Cenozoic Ice Age (34–0 Ma) that corresponds both to uplift of the Tibetan Plateau and Himalaya, and a gradual reduction in the extent of the global continental arc system. 

We developed a machine learning framework to analyse the interdependencies between multiple global tectonic and volcanic processes (e.g., continental distribution, extent of volcanic arcs, mid-ocean ridges etc.) and seawater Sr composition (a proxy for weathering flux) over the past 400 million years. We developed a Bayesian network incorporating a novel algorithm that accounts for time lags for each of the predictor variables, and joint conditional dependence (i.e. how variables combine to influence the environmental outcome). Our approach overcomes problems traditionally encountered in geologic time series, such as collinearity and autocorrelation. Our results strongly indicate a first-order role for volcanism in driving chemical weathering fluxes since the mid-Palaeozoic. This is consistent with the strong empirical correlation previously observed between the strontium isotope composition of seawater and continental igneous rocks over the past billion years (4). Our study highlights how geologic processes operate together — not in isolation — to perturb the Earth system over ten to hundred million-year timescales.

References

(1). M. E. Raymo, W. F. Ruddiman, Tectonic forcing of late Cenozoic climate, Nature 359, 117 (1992).

(2). N. R. McKenzie, et al., Continental arc volcanism as the principal driver of icehouse greenhouse variability, Science 352, 444 (2016).

(3). F. A. Macdonald, N. L. Swanson-Hysell, Y. Park, L. Lisiecki, O. Jagoutz, Arc-continent collisions in the tropics set Earth’s climate state, Science 364, 181 (2019).

(4). C. P. Bataille et al., Continental igneous rock composition: A major control of past global chemical weathering, Science Advances 3, e1602183 (2017).  

How to cite: Gernon, T., Hincks, T., Merdith, A., Rohling, E., Palmer, M., Foster, G., Bataille, C., and Muller, D.: Tectonic forcing of global chemical weathering since the mid-Paleozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10623, https://doi.org/10.5194/egusphere-egu2020-10623, 2020.

Our understanding of the geological regulation of the carbon cycle has been deeply influenced by the contribution of Bob Berner with his well-known model GEOCARB. Here, we will present a fundamentally different carbon cycle model that explicitly accounts for the effect of the paleogeography using physically based climate simulations and using 22 continental configurations spanning the whole Phanerozoic (GEOCLIM, geoclimmodel.wordpress.com). We will show that several key features of the Phanerozoic climate can be simply explained by the modulation of the carbon cycle by continental drift with the notable exception of the Late Paleozoic Ice Age, which is explained by the intense weathering of the Hercynian mountain range. In particular, the continental drift may have strongly impacted the runoff intensity as well as the weathering flux during the transition from the hot Early Cambrian world to the colder Ordovician world. Another fascinating example is the large atmospheric CO₂ decrease simulated during the Triassic owing to the northward drift of Pangea exposing large continental area to humid sub-tropics and boosting continental weathering. Conversely, our model fails to reproduce the climatic trend of the last 100 Ma. This is due to the highly dispersed continental configurations of the last 100 Ma that optimize the consumption of CO₂ through continental weathering. This discrepancy may be reduced if we account for a larger influence of the Earth degassing flux on the atmospheric CO₂ evolution, which could come from the increase contribution of the pelagic component on the oceanic crust on the global carbonate flux and from the many sub-marine LIPs occurring during the Late Cretaceous.

How to cite: Godderis, Y. and Donnadieu, Y.: The contribution of numerical models to our understanding of the Phanerozoic CO2 history, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19675, https://doi.org/10.5194/egusphere-egu2020-19675, 2020.

The mid-Cretaceous was one of the warmest intervals of the past 140 million years (Myr) driven by atmospheric CO₂ levels around 1000 ppmv. In the near absence of proximal geological records from south of the Antarctic Circle, it remains disputed whether polar ice could exist under such environmental conditions. Here we present results from a unique sedimentary sequence recovered from the West Antarctic shelf. This by far southernmost Cretaceous record contains an intact ~3 m-long network of in-situ fossil roots. The roots are embedded in a mudstone matrix bearing diverse pollen and spores, indicative of a temperate lowland rainforest environment at a palaeolatitude of ~82°S during the Turonian–Santonian (93–83 Myr). A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric CO₂ contents of 1120–1680 ppmv and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo in high-CO₂ climate worlds.

How to cite: Klages, J. P., Ulrich, S., Torsten, B., Claus-Dieter, H., Karsten, G., Gerhard, K., Steven, B., Jürgen, T., Juliane, M., Thomas, F., Thorsten, B., Werner, E., Tina, V. D. F., Patric, S. P., Robert, L., Gerrit, L., Niezgodzki, I., Gabriele, U.-N., Maximilian, Z., and Cornelia, S. and the Science Team of Expedition PS104: Temperate rainforests near the South Pole during peak Cretaceous warmth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-242, https://doi.org/10.5194/egusphere-egu2020-242, 2020.

Reconstructing the environmental consequences of large carbon additions in the past has the potential to improve our understanding and prediction of how the Earth system will respond to human carbon emissions. However, uncertainties over the scale and timing of external carbon additions during past carbon emission events limit quantitative knowledge gained from the geological record. The metals Sr, Os, Li and Ca are essential proxies for changes in volcanic activity and terrestrial weathering rates, and thus for major causes of pre-industrial carbon emission and sequestration, because their isotopic compositions in old continental crust and Earth’s mantle differ significantly. So far, box models and equilibrium-state equations have been the only method to quantitatively relate weathering-derived and magmatic input fluxes to trace metal concentrations and isotopic ratios preserved in ancient sediments. This approach results most commonly in a first order estimate of emitted carbon or weathering changes, but it does not account for the effect of climate feedbacks on metal sources and sinks and associated variations in the residence time of these metals in the ocean. Particularly during fast carbon emissions (e.g. Cenozoic hyperthermals, Oceanic Anoxic Events), the processes which added isotopically traceable metals to the oceans also enchained environmental changes which would have affected metal cycles and residence times, resulting in significant alterations of the recorded isotopic excursion in marine sediments. To disentangle the signals of causes and consequences of environmental change recorded by trace metal isotopes, we simulated various coupled carbon and metal cycle perturbations in the 3D Earth system model of intermediate complexity cGENIE, now containing the first representation of isotope-enabled trace metal dynamics. Here, we present a resulting extended framework to reconstruct metal and carbon fluxes from the geological trace metal record during periods of environmental change.

How to cite: Adloff, M., Greene, S. E., Monteiro, F. M., and Ridgwell, A.: A new framework to quantify carbon cycle perturbations using trace metal isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5962, https://doi.org/10.5194/egusphere-egu2020-5962, 2020.

When it comes to paleoclimate data-model integration, temperature is arguably the most important parameter. Although a range of temperature proxies has been developed over the decades, many of the available methods suffer from large calibration uncertainties, in particular when applied on deep-time intervals. Clumped isotope thermometry is based on thermodynamic principles and therefore can provide accurate temperature constraints for the deeper geological record. Recent analytical developments allow now the analysis of relatively small sample sizes and the application in paleoceanogaphic studies becomes more feasible. I will present new clumped isotope based temperature estimates for the Atlantic deep-sea across the Cenozoic. I will also show that the analysis of small samples now allows us to even resolve seasonal sea surface temperature estimates from high-resolution archives. Deep-sea temperatures as well as seasonally resolved surface temperature estimates are particularly useful for data-model comparison.

How to cite: Ziegler, M.: Cenozoic climate evolution revealed by clumped isotope thermometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20336, https://doi.org/10.5194/egusphere-egu2020-20336, 2020.

A compilation of benthic δ¹⁸O from the whole Atlantic and the Southern Ocean (Atlantic sector), shows two major jumps in the interbasinal gradient of d¹⁸O (Δδ¹⁸O) during the Eocene and the Oligocene: One at ~40 Ma and the second concomitant with the isotopic event of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago. From previously published circulation models, we show that the first Δδ¹⁸O jump reflects the thermal isolation of Antarctica associated with the proto-Antarctic circumpolar current (ACC). The second marks the onset of interhemispheric northern-sourced circulation cell, similar to the modern Atlantic meridional overturning circulation (AMOC). The onset of AMOC-like circulation probably slightly preceded (100-300 ky) the EOT, as we show by the high resolution profiles of δ¹⁸O and δ¹³C previously published from DSDP/ODP sites in the Southern Ocean and South Atlantic. We suggest that while the shallow proto-ACC supplied the energy for deep ocean convection in the Southern Hemisphere, the onset of the interhemispheric northern circulation cell was due to the significant EOT intensification of deepwater formation in the North Atlantic driven by the Nordic anti-estuarine circulation. This onset of the interhemispheric northern-sourced circulation cell could have prompted the EOT global cooling.

How to cite: Abelson, M. and Erez, J.: Was the onset of interhemispheric AMOC slightly prior to Antarctic glaciation at the Eocene-Oligocene transition?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2204, https://doi.org/10.5194/egusphere-egu2020-2204, 2020.

     Reliable temperature reconstructions of the ocean are often difficult to obtain due to the limitations of widely used proxies. The application of clumped isotope thermometry to coccolith calcite, which is geographical and chronological ubiquitously distributed, and whose production is limited to the photic zone, may provide ocean’s temperature information when and where other proxies have been shown inaccurate or not applicable.

     To evaluate the potential of coccolith clumped isotopes in paleoceanography we compare the temperatures derived from the fine fraction (<11µm), a pure mixed coccolith fraction (2-10 µm), and to a fraction of carbonate fragments from unidentified sources (<2 µm), with coeval alkenone sea surface temperatures (SST) from ODP Site 982 in the North Atlantic covering the last 16 Ma. The similarity in magnitudes and trends from the <11 and 2-10 µm size fractions, and trace element analysis of the <2 µm size fraction, suggest that for this site and time interval, exclusion of small unrecognizable fragments is not necessary to obtain reliable temperatures. The warmer values of alkenone SSTs compared to coccolith clumped isotope-derived temperatures cannot be explained by diagenetic processes, but may be related to temperature overestimations by alkenone calibrations, which assume a warm season and/or shallow production of coccolithophores in the study area.           

     Vital effects in coccolith clumped isotopes potentially associated to carbon limitation may also help to explain the differences in cooling magnitudes compared to the alkenone record. To further investigate vital effects in clumped isotopes, we compare calcification temperatures of three pure coccolith size fractions (3-5, 5-8, and 8-10 µm), and relate them to vital effects observed in their δ¹³C and δ ¹⁸O. The analysis of the fine fraction of Holocene sediments (<10 or <8 µm) showing a range of temperature and CO₂ concentrations also provide information on the potential effects of carbon availability in coccolith clumped isotopes, and suggests calcification of coccolithophores may occur in deeper habitats than those considered by alkenone calibrations. Our study shows clumped isotope thermometry applied to coccolith calcite as a promising alternative proxy for calcification temperature of coccolithophores.

How to cite: Mejia, L. M., Fernandez, A., Zhang, H., Guitian, J., Bernasconi, S., and Stoll, H.: Reconstructing ocean temperatures using coccolith clumped isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18410, https://doi.org/10.5194/egusphere-egu2020-18410, 2020.

Any type of non-buoyant material in the ocean is transported by currents during its sinking journey. This transport can be far from negligible for typical (plankton) particles with a low sinking velocity. To estimate the lateral transport, the material can be modelled as a set of Lagrangian particles advected by currents that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are often strongly eddying, providing flow fields with a horizontal resolution of  10km on a daily basis. However, many long term climate modelling studies (e.g. in palaeoclimate) rely on low resolution models that cannot capture mesoscale features. The lower model resolution could influence data-model comparisons using Lagrangian techniques, but this is not properly evaluated yet through a direct comparison.

In this study, we simulate the transport of sinking Lagrangian particles using low (1°; non-eddying)  and high (0.1°; eddying) horizontal resolution OGCMs of the present-day ocean, and evaluate the effect of the two resolutions on particle transport. We find major differences between the transport in the non-eddying versus the eddying OGCM (in terms of the divergence of particle trajectories and their mean trajectory). Addition of stochastic noise to the particle trajectory parameterizes the effect of eddies well in some regions (e.g. in the North Pacific gyre).

We recommend to apply sinking Lagrangian particles only in velocity fields with eddying OGCMs, which basically excludes all paleo-simulations. We are currently simulating the equilibrium Eocene (38Ma) climate using an eddying OGCM, to be able to apply these Lagrangian techniques in an eddying ocean of the past. We expect this to lead towards a better agreement between the OGCM and sedimentary fossil microplankton.

How to cite: Nooteboom, P., Delandmeter, P., Bijl, P., van Sebille, E., Dijkstra, H., and von der Heydt, A.: Resolution-dependent variations of sinking particle trajectories in general circulation models: Implications for data-model comparison in past climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3350, https://doi.org/10.5194/egusphere-egu2020-3350, 2020.

Long-term cooling, pCO₂ decline, and the establishment of permanent, polar ice sheets in the Neogene has frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric CO₂. However, geological records of erosion rates are potentially subject to averaging biases and the magnitude of the increase in weathering fluxes, and even its existence, remain debated. Moreover, a weathering increase scaled to the hypothesized erosional increase would have removed nearly all carbon from the atmosphere, leading to proposals of compensatory carbon fluxes in order to preserve carbon cycle mass balance. In contrast, increasing land surface reactivity—resulting from greater fresh mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views. We develop a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable ⁷Li/⁶Li and cosmogenic ¹⁰Be/⁹Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric CO₂, increase seawater ⁷Li/⁶Li, and retain constant seawater ¹⁰Be/⁹Be since 16 Ma. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux derived from silicate weathering—decreased, sustained by an increase in erosion. Thus, long-term cooling during the Neogene reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without increases in the silicate weathering flux. These changes in land surface reactivity through time suggest that the Earth system’s response to carbon cycle perturbations is not constant and that today’s Earth can more efficiently remove excess carbon than during analogous perturbations observed in the geologic record. 

How to cite: Caves Rugenstein, J., Ibarra, D., and von Blanckenburg, F.: Neogene changes in land surface reactivity and implications for Earth system sensitivity to carbon cycle perturbations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20554, https://doi.org/10.5194/egusphere-egu2020-20554, 2020.

Reconstructing Cenozoic terrestrial paleoclimate is frequently limited by temporal resolution and suitable quantitative tools to reliably assess changes in temperature and aridity. The dynamics of ocean temperatures¹ and chemistry², varying pCO₂³, and faunal assemblages are known to a certain extent, however, terrestrial data on temperatures, which are mostly indirectly derived from fossil assemblages and palynologycal data⁴ are rare. This study contributes to the understanding of the dynamics and variability of terrestrial temperatures during one of the most extreme Neogene climate changes, the Middle Miocene Climate Transition (MCT). The comparison of pCO₂ forecasts for the coming century and reconstructed Mid-Miocene pCO₂ levels suggest that the Mid-Miocene is an important time interval for ascertaining suitable model projections of the future anthropogenic impact on climate. In order to establish an appropriate understanding and modeling of the natural variability of the European/Mediterranean climate system, quantitative climate information of the European continental Mid-Miocene is mandatory. This would facilitate the identification of main drivers of climate evolution in an area which is exposed to the present climate change and its subsequent natural hazards.

This study presents a profound and well-dated terrestrial clumped isotope (Δ₄₇) paleosoil carbonate dataset from Spain that ranges from 13.0 to 15.1 Ma (100 kyr resolution) and hence covers an interval that was previously classified as the MCT. The Δ₄₇ data is supported by stable carbon and oxygen isotope analyses that are in agreement with previously published continental and oceanic records. A distinct decline in apparent Δ₄₇-based temperatures between 13.7 and 14.1 Ma reveals a substantial drop in continental temperatures and indicates changes in seasonality of pedogenic carbonate formation. The major cooling thereby coincides with a change in Milanković periodicities and can be linked to oceanic isotope records⁵. While the transition into the MCT shows a high temperature variability indicating varying environmental conditions, calculated oxygen isotopic values of the soil water point to a rather stable moisture source across the MCT in Southern Europe.

1: Super, J. R., Thomas, E., Pagani, M., et al. (2018) North Atlantic temperature and pCO₂ coupling in the early-middle Miocene. Geology, 46(6), 519-522.

2: Pearson, P. N., and Palmer, M. R. (1999) Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science, 284(5421), 1824-1826.

3: Pagani, M., Freeman, K. H., and Arthur, M. A. (1999) Late Miocene atmospheric CO₂ concentrations and the expansion of C4 grasses. Science, 285(5429), 876-879.

4: Lewis, A. R., Marchant, D. R., Ashworth, A. C., et al. (2008) Mid-Miocene cooling and the extinction of tundra in continental Antarctica. Proceedings of the National academy of Sciences.

5: Holbourn, A., Kuhnt, W., Clemens, S., et al. (2013) Middle to late Miocene stepwise climate cooling: Evidence from a high resolution deep water isotope curve spanning 8 million years. Paleoceanography, 28(4), 688-699.

How to cite: Löffler, N., Mulch, A., Krijgsman, W., Krsnik, E., and Fiebig, J.: The continental Middle Miocene Climatic Transition in Southern Europe as derived from clumped isotope analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10864, https://doi.org/10.5194/egusphere-egu2020-10864, 2020.

The South Asian Monsoon (SAM) is one of the most important climatic features of the Asian continent. Proxy-based reconstructions from continuous records in the Indian Ocean suggest a settlement of modern-like monsoon during the Miocene, with a modern winds distribution and strength potentially reached by ~13 Ma. Concurrent with the SAM intensification, a major reorganization of surface ocean currents occurred in the Indian Ocean. The timing of monsoon strengthening overlaps with changes in Indian Ocean and Indonesian Gateway configurations, Himalayas uplift, global cooling, as well as East Antarctic Ice Sheet expansion. Thus, the respective influence of each factor on SAM evolution and Indian Ocean paleoceanography is still poorly understood owing to the modification of multiple forcing mechanisms.

Here we will use a set of experiments with the IPSL-CM5A2 Earth System Model under early to late Miocene configurations in order to tease apart the effects of paleogeography changes, ice-sheet growth and CO₂levels on the Indian Ocean region during the Miocene. We will focus on the impact of increasing SAM winds and precipitation on the oceanographic conditions in the Indian Ocean including not only physical parameters but also biogeochemical ones.

How to cite: Sarr, A.-C., Donnadieu, Y., Bolton, C., and Suchéras-Marx, B.: A modeling study of physical and biogeochemical changes occurring in the tropical Indian Ocean during Miocene times. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9756, https://doi.org/10.5194/egusphere-egu2020-9756, 2020.

Alkenone sea surface temperature records recently observed suggest a substantial long-term and large-magnitude ocean surface cooling during the Late Miocene. At the same time, starting about seven million years ago, both hemispheres on Earth witnessed synchronous cooling and large areas of the continents experienced drying and enhanced seasonality. Coinciding with this climatic shift were significant changes in ecology, including the rise of C₄-photosynthesizing terrestrial plants and the emergence of so-called "vital effects" in oceanic coccolithophores. These changes are collectively hypothesized to be induced by declining atmospheric CO₂. However, the sparse proxy data available for this time interval limits our understanding of the link between these changes and atmospheric greenhouse gas fluctuations and has let people to propose a "climate-CO₂ decoupling".
In this study, the alkenone based pCO₂ proxy is used to reconstruct atmospheric CO₂ for the time interval between 4.5 and 8.5 Ma. Estimations are based on the carbon isotopic fractionation during photosynthesis (εp) and a new statistical multilinear regression model based on an analysis of culture and sediment data. Past coccolithophore growth rates are reconstructed using foraminiferal isotopic-based proxies, related to water column structure which favour or limit nutrient supply to the photic zone. A thorough sensitivity analysis of modern and past  εp values and its influencing factors in the Southern Ocean yield to a new, high resolution pCO₂ record. Estimated pCO₂ concentrations synchronously decline with the observed long-term cooling (5°C) from 6.8 to 5.9 Ma, periodically decreasing to sufficiently low values of <200 ppm, potentially inducing ephemeral Northern Hemisphere glaciation. CO₂ concentrations during the Late Miocene Cooling Event are thus successfully reproduced in this study and allow a reasonable interpretation of past conditions as has not yet been previously achieved in the relevant literature. 

How to cite: Tanner, T., Guitián, J., Hernández-Almeida, I., and Stoll, H.: Atmospheric CO2 during the Late Miocene Cooling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13972, https://doi.org/10.5194/egusphere-egu2020-13972, 2020.

In this work, we present different aspects of the mineral dust cycle dynamics and the linkage to loess deposits during the Last Glacial Maximum (LGM) in Europe. To this aim, we simulate the LGM dust cycle at high resolution using a regional climate-dust model. The simulated dust deposition rates are found to be comparable with the mass accumulation rates of the loess deposits determined from Loess sites across Europe. In contrast to the present-day prevailing westerlies, easterly wind directions (36 %) and cyclonic regimes (22 %) were dominant circulation patterns over central Europe during the LGM. This supports the hypothesis that recurring east sector winds, dynamically linked with a high-pressure system over the Eurasian ice sheet (EIS), are an important component for the dust transport from the EIS margins towards the central Europe loess belt. Our simulations reveal the occurrence of highest dust emission rates in Europe during summer and autumn, with the highest emission rates located near the southernmost EIS margins corresponding to the present-day German-Polish border region. Coherent with the persistent easterlies, westwards running dust plumes resulted in high deposition rates in western Poland, northern Czechia, the Netherlands, the southern North Sea region and on the North German Plain including adjacent regions in central Germany. Further, a detailed analysis of the characteristics of LGM cyclones shows that they were associated with higher wind speeds and less precipitation than their present-day counterparts. These findings highlight the importance of rapid and cyclic depositions by cyclones for the LGM dust cycle. The agreement between the simulated deposition rates and the mass accumulation rates of the loess deposits corroborates the proposed LGM dust cycle hypothesis for Europe.

How to cite: Ludwig, P., Schaffernicht, E. J., Shao, Y., and Pinto, J. G.: The linkage of dust cycle dynamics and loess during the Last Glacial Maximum in Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13946, https://doi.org/10.5194/egusphere-egu2020-13946, 2020.

Understanding the effect warming has on ice sheets is vital for accurate projections of climate change. A better understanding of how the Antarctic ice sheets have changed size and shape in the past would allow us to improve our predictions of how they may adapt in the future; this is of particular relevance in predicting future global sea level changes. This research makes use of previous reconstructions of the ice sheets, ice core data and Bayesian methods to create a model of the Antarctic ice sheet at the Last Glacial Maximum (LGM). We do this by finding the relationship between the ice sheet shape and water isotope values. 

We developed a prior model which describes the variation between a set of ice sheet reconstructions at the LGM. A set of ice sheet shapes formed using this model was determined by a consultation with experts and run through the general circulation model HadCM3, providing us with paired data sets of ice sheet shapes and water isotope estimates. The relationship between ice sheet shape and water isotopes is explored using a Gaussian process emulator of HadCM3, building a statistical distribution describing the shape of the ice sheets given the isotope values outputted by the climate model. We then use MCMC to sample from the posterior distribution of the ice sheet shape and attempt to find a shape that creates isotopic values matching as closely as possible to the observations collected from ice cores. This allows us to quantify the uncertainty in the shape and incorporate expert beliefs about the Antarctic ice sheet during this time period. Our results suggests that there may have been a thicker West Antarctic ice sheet at the LGM than previously estimated.

How to cite: Turner, F., Wilkinson, R., Buck, C., Jones, J. M., and Sime, L.: Antarctic Uncertainty: Learning more about past ice sheet shapes with Bayesian methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5526, https://doi.org/10.5194/egusphere-egu2020-5526, 2020.

As a significant terrestrial carbon reservoir, peatland has great potential to affect the global carbon cycle and global climate. However, our understanding of broad-scale mechanisms that control the long-term global peatland expansion and carbon accumulation rates is still limited. Here we present a new data synthesis of global coal deposit location, thickness changes, carbon concentration, and distribution area changes, along with new carbon pool estimates of global peatlands from Devonian through geological time. By identifying orbital cycles in coal seams, we show that the long-term rate of carbon accumulation (LORCA) in peatland calculated from published data is controlled by pO₂, pCO₂, temperature, precipitation, and total solar irradiance. We use this relationship and latitudinal temperature gradients to reconstruct the equations between LORCA with latitude on different geological time. The results suggest that there are three main sets of high carbon pool and high carbon accumulation rate of global peatlands in Late Paleozoic, Early-Middle Jurassic, and Late-Cretaceous to Early Cenozoic under low tectonic activity and high terrestrial plant diversity background. In addition, we measure the shortest distances between all coal locations and coastlines based on the new Scotese’s paleo-Atlas for the past 400 million years, in order to exhibit the extent of peatland expansion into inland. The result shows the Early-Middle Jurassic period has the longest average distance, which is probably due to the high sea level that minimizes the development of peat swamps on coastal areas and facilitated the moisture to move into deeper inland under the Jurassic Greenhouse climate condition. This study highlights that combining comprehensive coal-related database with paleoclimate, tectonics, and evolution of land plants provides insights into the mechanisms of the long-term behavior of the peatland expansion and carbon reservoir through deep time.

Keywords: Greenhouse climate, Peatland expansion, Carbon pool, Cabon accumulation rates, Coal

How to cite: Zhang, Z., Wang, C., Lv, D., and Wang, T.: Greenhouse climate forces expansion of peatlands into inland areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3988, https://doi.org/10.5194/egusphere-egu2020-3988, 2020.

The Neoproterozoic glaciations, referred to as snowball Earth periods, describe the most extreme transition from a very cold climate to a state of strong greenhouse effect. Atmospheric CO₂ concentrations are rising during the snowball, due to the shutdown of oceanic and terrestrial carbon sinks, until a tipping point is reached and a rapid deglaciation sets in. Subsequently, a warm and completely ice-free climate under very high CO₂ concentrations develops. We show first results of simulations using a coupled atmosphere-ocean general circulation model covering the initiation, as well as the melting of the Marinoan snowball Earth (645 – 635 My ago) and the greenhouse climate in its aftermath. CO₂ concentrations are decreased to initiate a global glaciation and then increased again in order to melt the snowball Earth. As soon as a certain CO₂ threshold is reached, sea-ice melts rapidly, reaching a completely ice-free ocean after only one hundred years, in our model without land glaciers. The ocean becomes strongly stratified, because at the surface the freshwater from the sea-ice melt is warming up quickly, whereas the deeper ocean remains cold and salty. Ocean surface currents return to their pre-snowball behavior soon after the melt, but destratification is slow. The largest mixed layer depths of up to 500 m are reached in the mid latitudes of the winter hemisphere. We compare the climate before and after the snowball state and estimate the time needed for destratification.

How to cite: Ramme, L. and Marotzke, J.: Ocean dynamics and climate during a Neoproterozoic snowball Earth and its aftermath as simulated in a coupled Earth system model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10320, https://doi.org/10.5194/egusphere-egu2020-10320, 2020.

Oceanic anoxic events (OAEs) are abrupt events of widespread deposition of organic-rich sediments and extensive seafloor anoxia. Mechanisms usually invoked as drivers of oceanic anoxia are various and still debated today. They include a rise of the CO2 atmospheric level due to increased volcanic activity, a control by the paleogeography, changes in oceanic circulation or enhanced marine productivity. In order to assess the role of these mechanisms, we use an IPCC-class model, the IPSL-CM5A2 Earth System Model, which couples the atmosphere, land surface, and ocean components, this last one including sea ice, physical oceanography and marine biogeochemistry which allows to simulate oceanic oxygen.

We focus here on OAE2, which occurs during the Cretaceous at the Cenomanian-Turonian boundary (93.5 Ma), and is identified as a global event with evidence for seafloor anoxia in the Atlantic and Indian Oceans, the Southwest Tethys Sea and the Equatorial Pacific Ocean. Using a set of simulations from 115 to 70 Ma, we analyze the long-term paleogeographic control on oceanic circulation and consequences on oceanic oxygen concentration and anoxia spreading. Short-term controls such as an increase of pCO₂, nutrients, or orbital configurations are also studied with a second set of simulations with a Cenomano-Turonian (90 Ma) paleogeographic configuration. The different simulated maps of oxygen are used to study the evolution of marine productivity and oxygen minimum zones as well as the spreading of seafloor anoxia, in order to unravel the interlocking of the different mechanisms and their specific impact on anoxia through space and time.

How to cite: Donnadieu, Y., Laugie, M., Ladant, J.-B., Raisson, F., and Bopp, L.: Modeling the impact of oceanic circulation and marine productivity on Cretaceous seafloor anoxia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20746, https://doi.org/10.5194/egusphere-egu2020-20746, 2020.

Geological evidence suggests that Earth's past featured periods during which the planet was largely or even entirely covered by ice, a state termed "snowball Earth".  Model based studies confirm that one of Earth's equilibrium states is a fully glaciated planet (hard snowball) but it is not clear how this state could have been left once it had been established. We use simulations with the Max-Planck-Institute for Meteorology's Earth system model to investigate the conditions that enable the transition out of the snowball-state. We show that the high albedo of pure snow would have prevented deglatiation, even for extremely high atmospheric CO2 concentrations. Terminal deglaciation is only triggered for surface albedos corresponding to old, darkened snow or sea-ice. Here, increasing snowfall rates, resulting from the intensification of the hydrological cycle with rising CO2 concentrations, would have prohibited the gradual build-up of dust that leads to a darkening of the surface.  Only when assuming dust deposition fluxes at least similar to present-day fluxes, can the deglation be triggered for plausible atmospheric CO2 concentrations.

How to cite: de Vrese, P., Stacke, T., Brovkin, V., and Caves Rugenstein, J.: The simulated transition from a hard snowball Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19916, https://doi.org/10.5194/egusphere-egu2020-19916, 2020.

Ocean anoxic event 2 (OAE2) was a large perturbation in the Earth's ocean carbon system, occurring at approximately 93.5 Ma, and is characterised by widespread black shales deposition in sediment records. This record has been interpreted as evidence of large anoxia in the global ocean for a long period, resulting in large scale extinction of marine life. However, the exact causes of OAE2, and how it initially developed, are not fully understood. We modelled the period leading up to OAE2 using the HadCM3L global climate model with full ocean (HADOCC) and terrestrial carbon cycle (TRIFFID) modules. We compared our results to equivalent simulations using late Cretaceous (Maastrichtian) paleogeographies. This allowed us to analyse the effects of continental configuration on the development to the OAE. Our results show that restricted ocean circulation, caused by the paleobathymetry, is necessary for anoxic conditions to develop but is not sufficient alone. This suggests that continental configuration is highly important in determining the ability of the oceans to develop an OAE and may explain why they only occur during some times during Earth history.

How to cite: Manning, A., Valdes, P., Monteiro, F., and Williams, J.: Role of paleogeography in preconditioning the Late Cretaceous Oceanic Event (OAE2) in a full global circulation Earth System model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15254, https://doi.org/10.5194/egusphere-egu2020-15254, 2020.

The Mesozoic Era (~252-66 Ma) is a decisive period in Earth’s history. It is marked by a tectonic transition from the Pangea supercontinent towards a modern continental configuration as well as the ecological success of the dinosaurs and the evolution of mammals, flowering plants, stony corals and important groups of planktic calcifiers. The Mesozoic is generally considered as a greenhouse climate period, with especially high global temperatures during the Triassic and the Late Cretaceous. Here, we present novel modeling results on the evolution of global climatic conditions through the Mesozoic.

An ensemble of equilibrium climate states for 40 geological timeslices between 255 and 60 Ma is simulated with the CLIMBER-3α Earth System Model of Intermediate Complexity. The influence of changing paleogeography, sea level, vegetation cover, solar luminosity, orbital configuration and atmospheric CO₂ concentration is systematically tested based on constraints from published geological proxy reconstructions and previous modeling work.

Atmospheric pCO₂ is found to be the strongest driver of global mean temperatures, which are generally elevated above the present and reach ≥20°C in the Late Triassic to Early Jurassic and the mid-Cretaceous if a recently published pCO₂ proxy compilation is employed. The simulated seasonal latitudinal shift of high precipitation zones exhibits a maximum during the mid-Triassic to Early Jurassic and therefore supports the notion of a “Megamonsoon” during this time. Simulated humid and arid climate zones generally agree well with spatial distributions of geologic climate indicators like coal and evaporites, although some discrepancies exist. The same applies to the correlation of fossil stony coral reef distributions with regions where seawater temperatures would have been suitable for (modern) coral reefs. We will discuss which changes of Earth System parameters throughout the Mesozoic can best explain shifts in these distributions.

How to cite: Landwehrs, J., Feulner, G., Hofmann, M., Petri, S., Sames, B., and Wagreich, M.: Exploring Mesozoic Climates - Modeling and Evaluation of Proxy Distributions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9074, https://doi.org/10.5194/egusphere-egu2020-9074, 2020.

Antarctic bryozoans are important colonial marine invertebrates in terms of their origin, palaeoenvironment and climatic approaches. The changes of the bryozoan fossil records during the last 55 Ma years  are well-defined by their biodiversity, taxonomic composition and colony growth-forms. The late Early Eocene biota from the shallow-marine–estuarine clastic succession of the lower part (Telm1-2) of the La Meseta Formation of Seymour Island are represented by the prolific, spectacular in size, massive multilamellar colonies dominated by the cerioporids as well as diverse ascophorans  cheilostomes (Hara, 2001). The free-living lunilitiform, disc-shaped colonies, which occur in the middle part of the La Meseta Formation (Telm4-Telm5), are characteristic for the warm, shallow-self environment and bottom temperature, which ranges from 10 to 29°C. The presence of the bimineralic skeletons of this fauna (such as Lunulites, Otionellina, and Uharella) with the traces of aragonite is indicative for the temperate shelf environment, sandy and often shifting substrate. Lunulitids are inhabited by the circumpolar to warm-temperate waters, at the present day. Contrary to that, the bryozoans  from the upper part of the LM (Telm6-7) are  represented by the scarce lepraliomorphs accompanied by the crustaceans, brachiopods and gadiform fish remains. The individuality of the Eocene bryozoan assemblages are well-correlated with the EECO, MECO and EOT climatic events, based on the other marine macrofaunal marine fossil records (see also Ivany et al. 2008). The lower Pliocene bryofauna recently described  from the Cockburn Island Formation  is composed of the rich encrusting shallow-water, membraniporiform zoaria (Hara and Crame, in review, 2020). The biota of thePectenConglomerate are indicative of the interglacial conditions during the deposition of the Cockburn Island Formation. At the present day bryozoans with the preponderance of cheilostomes are the most significant marine benthic community, thriving successfully in cool-water Antarctic  conditions.

References,

Hara, U., 2001. Bryozoans from the Eocene of Seymour Island, Antarctic Peninsula, Palaeontogia Polonica , 60:33-155.

Hara, U., Mors, T., Hagstrom, and Reguero M. A., 2018. Eocene bryozoans  assemblages from the La Meseta Formation of Seymour Island. Geological Quarterly, 62: 705-728. 

Hara., U., and Crame, J.A. 2019. Paleobiodiversity of the Lower Pliocene bryozoan  benthic community and its response to interglacial conditions.  Geological Review (in review).

Ivany L.C., Lohmann, K. C. Hasiuk, F., Blake D.B., Glass A., Aronson R.B., and Moody R.M. 2008. Eocene climate record of the high southern latitude continental shelf: Seymour Island, Antarctica. Geological Society of America Bulletin, v. 120, no. 5-6: 659-678.

How to cite: Hara, U.: Cenozoic bryozoan biota and their response to climatic changes in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22540, https://doi.org/10.5194/egusphere-egu2020-22540, 2020.

The Middle Eocene represents the last ice-free period of the Cenozoic. Vegetation proxy data (wood, leaves, palynomorphs) discovered in the Antarctica peninsula and neighbouring islands or hosted in sedimentary sequences deposited on the continental margin reveal the presence of paratropical rain forests which thrived along the Antarctica coast during the Early Eocene. During the Middle and Late Eocene these flora have been progressively replaced by temperate Nothofagus-dominated rainforests (Contreras et al., 2013). Jacques et al. (2012) proposed, using a physiognomic approach (CLAMP), that a warm temperate and wet climate (with a marked summer rainy season) prevails until the middle Eocene (43±2 Ma) on the tip of the Antarctica Peninsula.

            To better constrain the climate in Antarctica and understand processes governing the polar climate during the Middle Eocene, we performed a set of experiments using the IPCC-like Earth System Model (IPSL-CM5A2-VLR) forced with a Middle Eocene (~40 Ma) paleogeography reconstruction and a 4 times pre-industrial atmospheric CO₂ level (1120ppm). To highlight the importance of the seasonality, we launched 6 orbital configurations exploring end-members situations. To complete the procedure, simulated sea surface temperatures and sea ice extents were then employed as boundary conditions to force the Atmospheric General circulation model LMDz6 (run at higher spatial resolution) coupled with a soil and vegetation model ORCHIDEE to simulate the corresponding vegetation over Antarctica. The 6 end-members Earth's orbital configuration allows exploring the full climatic spectrum which would have been recorded by proxy data. Simulated changes in atmospheric circulation will be discussed and the simulated climate and vegetation will be confronted to paleoclimatic indicators and vegetation data.

How to cite: Fluteau, F., Tardif, D., Le Hir, G., Donnadieu, Y., Sepulchre, P., Ladant, J.-B., Poblete, F., and Dupont-Nivet, G.: The climate in Antarctica during the Middle Eocene: a modelling perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18748, https://doi.org/10.5194/egusphere-egu2020-18748, 2020.

Simulating the proxy-derived surface warming and reduced meridional temperature gradient of the early Eocene greenhouse climate still represents a challenge for most atmosphere-ocean general circulation models. A profound understanding of uncertainties associated with the respective model results is thereby essential to reliably identify any similarities or misfits to the proxy record. Besides incomplete knowledge of past greenhouse gas concentrations and other boundary conditions, structural and parametric uncertainties are the main factors that determine our confidence in paleoclimate simulation results.

The recent publication of coordinated model experiments that apply identical paleogeographic boundary conditions for key time periods of the early Eocene (DeepMIP) allows a systematic analysis of inter-model differences and therefore of structural uncertainties in the simulated surface warming. Here we additionally explore the parametric uncertainty of the early Eocene climatic optimum (EECO) surface warming within one DeepMIP model. For this we performed perturbed parameter ensemble (PPE) simulations with HadCM3B at different atmospheric CO₂ concentrations following the DeepMIP protocol. Twenty-one parameter sets based on changes in six atmospheric parameters, a sea-ice parameter and the ocean background diffusivity were branched off from the respective DeepMIP control simulations and integrated for a further 1500 model years. The selected parameter sets are based on previous results demonstrating their ability to simulate a pre-industrial global-mean surface temperature within ±2 °C of the standard configuration.

Preliminary results indicate a large spread of the simulated low-latitude surface warming in the PPE and therefore significant changes of the large-scale meridional temperature gradient for the EECO. Some ensemble members develop numerical instabilities at CO₂ concentrations of 840 ppmv and above, most likely in consequence of high temperatures in the tropical troposphere. We further compare the magnitude of the parametric uncertainty of the HadCM3B perturbation experiments with the structural differences found in the DeepMIP multi-model ensemble and explore the sensitivity of the results to the strength of the applied greenhouse gas forcing. Model skill of the PPE members is tested against the most recent DeepMIP compilations of marine and terrestrial proxy temperatures.

How to cite: Steinig, S., Bragg, F. J., Irvine, P. J., Lunt, D. J., and Valdes, P. J.: Estimating structural and parametric uncertainties in the simulated early Eocene surface warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8370, https://doi.org/10.5194/egusphere-egu2020-8370, 2020.

Changes in ocean gateway configuration are known to induce basin-scale rearrangements in ocean characteristics throughout the Cenozoic. However, there is large uncertainty in the relative timing of the subsidence histories of ocean gateways in the northern high latitudes. By using a fully coupled General Circulation Model we investigate the salinity and temperature changes in response to the subsidence of two key ocean gateways in the northern high latitudes during early to middle Miocene. Deepening of the Greenland-Scotland Ridge causes a salinity increase and warming in the Nordic Seas and the Arctic Ocean. While warming this realm, deep water formation takes place at lower temperatures due to a shift of the convection sites to north off Iceland. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface causes a cooling in the southern high latitudes. These characteristic impacts in response to the Greenland-Scotland Ridge deepening are independent of the Fram Strait state. Subsidence of the Fram Strait for a deep Greenland-Scotland Ridge causes less pronounced warming and salinity increase in the Nordic Seas. A stronger salinity increase is detected in the Arctic while temperatures remain unaltered, which further increases the density of the North Atlantic Deep Water. This causes an enhanced contribution of North Atlantic Deep Water to the abyssal ocean and on the expense of the colder southern source water component. These relative changes largely counteract each other and cause little warming in the upwelling regions of the Southern Ocean.

How to cite: Hossain, A., Knorr, G., Lohmann, G., Stärz, M., and Jokat, W.: Thermohaline Fingerprints of the Greenland-Scotland Ridge and Fram Strait Subsidence Histories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13450, https://doi.org/10.5194/egusphere-egu2020-13450, 2020.

Fossils of planktic foraminifera are found in marine sediments and are widely used as a proxy for past ocean conditions. The habitat of these unicellular marine zooplankton ranges from tropical to polar regions and is mostly located in the upper mixed layer of the ocean. The foraminifera form a calcium carbonate ’shell’ around their cell during their lifespan. When they die, foraminifera lose their ability to control their buoyancy and their shells sink to the ocean floor. It is often assumed that the proxies which are derived from the shells in sediment cores represent ocean conditions above the location of deposition. However, foraminifera are transported by ocean currents, both during and after their lifespan. Hence, the paleoclimatic conditions recorded from their shells may originate far from the core site, generating large footprints in foraminifera-based paleoclimatic proxies. 

In this project, we quantify the influence of the transport by ocean currents on the proxy signal of foraminifera found at core sites in the Uruguayan margin of the Punta del Este basin. This is a region where two western boundary currents meet: The southward flowing Brazil current and the northward flowing Malvinas current. We use a high resolution (0.1° horizontally) ocean general circulation model to track virtual sinking particles and the local oceanic conditions along their pathways. These model results are compared to proxy- and species analysis from the core sites. We found that offsets in modelled proxy signals due to transport in the Uruguayan margin are strongly linked to the relative position of the core site to the Brazil-Malvinas confluence. These offsets are most pronounced in the tails of the temperature distributions where they can reach up to +/- 7°C at sites located in the confluence zone. Species analysis from core tops taken slightly north of this region show more cold water species than reflected by the modelled temperature distributions, suggesting biological activity and nutrient availability not taken into account in the model play an important additional role in the relative abundances of species.
Our model simulations have provided both a first order insight into the potential proxy-signal offsets in highly dynamic ocean regions and show that understanding of the interplay between transportation effects and the biological activity of foraminifera is crucial for the interpretation of these proxies.

How to cite: Kruijt, A., Mair, A., Nooteboom, P., von der Heydt, A. S., Ziegler, M., and Aze, T.: Transport of planktic foraminifera by ocean currents in the Uruguayan margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10818, https://doi.org/10.5194/egusphere-egu2020-10818, 2020.

Geological evidence indicates considerable Antarctic ice volume variations during the early to mid-Miocene. Hitherto, ice modelling studies have mostly used equilibrium simulations to explain this variability. In these simulations, the gain in precipitation due to increased temperatures has to outweigh the loss caused by increased ice melt, to obtain simultaneous ice sheet growth and CO₂ level rise. Here, conceptualising ice dynamical model results, we find that this is not a necessary condition for the transiently evolving Miocene Antarctic ice sheet. Instead, ice volume increase when CO₂ levels are rising can also be explained as a consequence of disequilibrium between the transiently changing ice volume and forcing climate. This disequilibrium permits a continuation of ice sheet growth after a gradual CO₂ decline. When the CO₂ level is increased again, the ice sheet is still adapting to a relatively large equilibrium volume. Lowering the periodicity of the forcing leads to a larger disequilibrium, and consequently larger CO₂-ice volume phase differences. Furthermore, amplified forcing variability increases ice volume variations, because the growth and decay rates depend on the forcing. It also leads to a reduced average ice volume, which is induced by the growth rate generally being smaller than the decay rate. We therefore submit that retrieval of high resolution proxy-CO₂ records covering the Miocene would be very beneficial to constrain ice modelling studies.

How to cite: Stap, L., Knorr, G., and Lohmann, G.: Concurrent Miocene Antarctic ice sheet growth and CO2 increase caused by disequilibrium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5974, https://doi.org/10.5194/egusphere-egu2020-5974, 2020.

The North American Great Plains are characterized by a sharp aridity gradient at around the 100^th meridian with a more humid climate to the east and a more arid climate to the west. This aridity gradient shapes the region's agriculture and economy, and recent work suggests that arid conditions on the Great Plains may expand eastward with global warming. The abundant Neogene sediments of the Ogallala Formation in the Great Plains present an opportunity to reconstruct regional hydroclimate conditions at a time when pCO₂ and global temperatures were much higher than today, providing insight into the aridity and ecosystem response to warming. We present new paleosol carbonate δ¹³C and δ¹⁸O data (n=366) across 37 sites spanning the Great Plains and compile previously published measurements (n=381) to evaluate the long-term hydroclimatic and ecosystem changes in the region during the late Neogene. This study combines a spatial and temporal analysis of carbon and oxygen isotope data with reactive-transport modeling of oxygen isotopes constrained by climate model output, providing critical constraints on the paleoenvironmental and paleoclimatological evolution of the Neogene Great Plains. Carbonate δ¹⁸O demonstrate remarkable similarity between the spatial pattern of paleo-precipitation δ¹⁸O and modern precipitation δ¹⁸O. Today, modern precipitation δ¹⁸O over the Great Plains is set by the mixing between moist, high-δ¹⁸O moisture delivered by the Great Plains Low-Level Jet and drier, low-δ¹⁸O westerly air masses. Thus, in the absence of countervailing processes, we interpret this similarity between paleo and modern δ¹⁸O to indicate that the proportional mixing between these two air masses has been minimally influenced by changes in global climate and that any changes in the position of the 100^th meridian aridity gradient has not been forced by dynamical changes in these two synoptic systems. In contrast, prior to the widespread appearance of C₄ plants in the landscape of the Great Plains, paleosol carbonate δ¹³C show a robust east-to-west gradient, with higher values to the west. We interpret this gradient as reflective of lower primary productivity and hence soil respiration to the west. Close comparison with modern primary productivity data indicates that primary productivity has declined and shifted eastward since the late Neogene, likely reflecting declining precipitation and/or a reduction in CO₂ fertilization during the late Neogene. Finally, δ¹³C increases across the Miocene-Pliocene boundary, which, consistent with previous studies, we interpret as a shift from a C₃ to a C₄ dominated landscape. We conclude that, to first order, the modern aridity gradient and the hydrologic processes that drive it are not strongly sensitive to changes in global climate and any shifts in this aridity gradient in response to rising CO₂ will be towards the west, rather than towards the east.

How to cite: Manser, L., Kukla, T., and Caves Rugenstein, J. K.: Long-term stability of large-scale hydroclimate processes in the North American Great Plains revealed by a Neogene stable isotope study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11141, https://doi.org/10.5194/egusphere-egu2020-11141, 2020.

For several decades, the comparison of climate data with results from water isotope-enabled Atmosphere General Circulation Models (AGCMs) significantly helped to a better understanding of the processes ruling the water cycle, which is one of the main drivers of the climate variability. For the modern period, the use of AGCMs nudged with weather forecasts reanalyses is a powerful way to obtain model outputs under the same weather conditions than at the sampling time of the observations.

Here we present new isotopic simulations results from ECHAM6-wiso [1] nudged with the last reanalyses dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA5 [2], at different spatial resolutions over the period 1979-2018. Model results are evaluated against isotopic data compilations, including GNIP (Global Network of Isotopes in Precipitation [3]), speleothems [4], ice cores datasets and water vapor measurements. To quantify the impact of these reanalyses on our simulations, we also performed nudged simulations with the previous model version ECHAM5-wiso [5] by using ERA5 data and its predecessor ERA-Interim [6].

These new simulation products could be a useful contribution to the isotopic data community for the interpretation of their water isotope records and for the exploration of the mechanisms controlling the variability of the surrounding water isotopic composition.

[1] Cauquoin et al., Clim. Past, 15, 1913–1937, https://doi.org/10.5194/cp-15-1913-2019, 2019.

[2] Copernicus Climate Change Service (C3S), 2017.

[3] IAEA, the GNIP Database, available at: https://nucleus.iaea.org/wiser.

[4] Comas-Bru et al., Clim. Past, 15, 1557–1579, https://doi.org/10.5194/cp-15-1557-2019, 2019.

[5] Werner et al., Geosci. Model Dev., 9, 647–670, https://doi.org/10.5194/gmd-9-647-2016, 2016.

[6] Dee et al., Q. J. R. Meteorol. Soc., 137, 553–597, https://doi.org/10.1002/qj.828, 2011.

How to cite: Cauquoin, A. and Werner, M.: High-resolution isotopic simulations from ECHAM6-wiso nudged with ERA5 reanalyses: new products for isotopic model-data comparisons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12319, https://doi.org/10.5194/egusphere-egu2020-12319, 2020.

It has been well demonstrated that the variations of orbital parameters, known as Milankovitch theory, are one of the most important drivers of the Earth’s climate system. However, the way how the changes in orbital forcing imprint the glacial-interglacial cycles recorded in paleo-proxies, such as stable water isotopes in ice cores and speleothems, is still unclear. One way to progress in this question is to make direct comparisons of isotopic data with simulation results from isotope-enabled General Circulation Models (GCMs). We use here such a model, the Japanese atmospheric GCM MIROC5-iso[1], to perform simulations under different idealized paleoclimate conditions. For that, corresponding orbital parameters and greenhouse gases concentrations are set. Prescribed sea surface temperature and sea ice coverage boundary conditions from the fully coupled atmosphere-ocean GCM MIROC (MIROC-AOGCM) experiments are used, after an adaptation to the MIROC5-iso grid. Because earlier version of MIROC-AOGCM has been widely used for paleoclimate modeling purposes, the climatological mean states of MIROC5-iso under preindustrial conditions are evaluated against simulation results from different versions of MIROC-AOGCM (MIROC4m, which is a slightly updated version of MIROC3.2(med), and MIROC5 [2]). In addition, several interglacial periods and idealized paleoclimate experiments will be investigated and implications for the interpretation of water isotope response to the changes in orbital forcing will be discussed.

[1] Okazaki and Yoshimura, J. Geophys. Res. Atmos, 124, 8972–8993, 2019.

[2] Watanabe et al., J. Climate, 23, 6312–6335, 2010.

How to cite: Kino, K., Okazaki, A., Cauquoin, A., and Yosnimura, K.: Investigation of the response of water isotope records to the changes in orbital forcing with the isotope-enabled AGCM MIROC5-iso, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12420, https://doi.org/10.5194/egusphere-egu2020-12420, 2020.

The hydrological response to radiative forcing is less understood than the thermal one: many climate models have difficulties in simulating seasonal rainfall and its variability. Indeed, future precipitation projections are much more uncertain than those of temperature. However, confident projections of precipitation are of crucial importance, particularly for highly populated regions where agriculture strongly relies on seasonal rainfall, such as South and Central Asia.

Instrumental data from Eurasia show a negative correlation between temperature and precipitation on short timescales (10^-3 to 10⁰ years). However, on longer timescales (10¹ to 10³ years), proxy data covering the Holocene show a positive correlation between temperature and precipitation. Climate models in contrast simulate a negative correlation on all timescales. To extend previous estimates to longer time scales, we focus on the last Glacial period, characterized by colder temperature than the Holocene as well as pronounced millennial-scale climate fluctuations in the Northern Hemisphere.

We reconstruct temperature and precipitation from four high resolution pollen records at mid-latitudes in the Northern Hemisphere. The estimates are compared with climate simulations. The chosen proxy sites cover the East and West coasts of both the Eurasian and North American continent. We employ four different statistical reconstruction methods to assess validity and biases of each method. The differences between reconstructed and simulated temperature-precipitation relationships as well as the zonal structure of orbital- and millennial-scale variations are examined. In particular, we explore the thermodynamic and dynamic contributions to the inferred relationships between temperature and precipitation.

How to cite: Sommani, A., Weitzel, N., and Rehfeld, K.: Northern Hemisphere temperature to precipitation relationships during the last Glacial from pollen records and climate simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7293, https://doi.org/10.5194/egusphere-egu2020-7293, 2020.

The project PalMod II is the second phase of Germany’s national paleoclimate modelling initiative (www.palmod.de) whose aim is to model the transient climate evolution from the last interglacial to the anthropocene with state of the art earth system models. The second phase more precisely wants to perform simulations for the last glacial inception, the marine isotope stage 3, and the last deglaciation. It further plans to compile paleo-observational proxy data over the full glacial cycle from about 130,000 years before present until today. Models of differing complexity (fully-coupled earth system models and models of intermediate complexity) will be used to assess the scientific questions posed in PalMod II. Model output will be combined with the compiled paleo-proxy data for validation purposes. The sheer data amount in excess of several petabytes and different data handling practices of the participating communities require dedicated management of the data workflow both in- and outside of the immediate PalMod community.

The PalMod II data management takes place in close collaboration between data management specialists and the scientists. The objectives include the standardisation of each simulation and proxy dataset, the facilitation of data sharing and data reuse between work packages, the access channels for external collaborators, and the long-term preservation of the data. The data management follows the concept of the "Active Data Management Plan", which foresees a continuous development of the data management plan (DMP), starting with an initial basic version. The DMP covers the entire life cycle of the research data generated in the project, from generation and analysis to data publication and archiving. This includes aspects such as data formats, metadata standards and data usage licenses. Ownership and responsibilities for simulation and paleo data sets as well as the input data during and after the end of the project will also be considered.

This contribution will present the initial DMP for PalMod II. It will describe the amount of data produced in the project, highlight how the above mentioned aspects will be dealt with, and present how the project aims to ensure the Findability, Accessibility, Interoperability, and Reusability, i.e. the FAIR data principles, of simulation output, post-processed model data, and paleo-proxy data from PalMod II.

* This contribution presents results of the full PalMod II initiative, the authors present them on behalf of the initiative.

How to cite: Bothe, O. and Peters, K.: The initial Data Management Plan for PalMod II - FAIR simulation and paleo data from the Last Interglacial to the Anthropocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13189, https://doi.org/10.5194/egusphere-egu2020-13189, 2020.