Displays

Late Paleozoic deglaciation is the only deep-time analogue of an icehouse-to-greenhouse transition in a vegetated world, but the detailed processes of this climatic upheaval are still under debate due to the absence of higher precision and accuracy in global correlations. The astronomical calibration of sedimentary cycles (3–4 m) in a carbonate succession from Naqing in South China to the 405 kyr eccentricity cycle reveals short eccentricity (135 kyr and 96.1 kyr), main obliquity (31.6 kyr), and precession (21.5 kyr and 19.3 kyr) for the early Cisuralian (Early Permian). 405-kyr-eccentricity-forced teleconnections are established between Paleo-Tethyan deep-marine carbonate cyclicity and U-Pb zircon ages-calibrated cyclothems from Euramerica in the Pangean paleotropics, providing a refined chronostratigraphy for the Asselian and Sakmarian stages on global scale. Geological record indicates a (s₄ − s₃) − (g₄ − g₃) resonance likely transited into (s₄ − s₃) − 2(g₄ − g₃) resonance at ~296.8 Ma, which confirms the chaotic dynamical behaviour of the Solar System during the Cisuralian. The synchronized proxies from marine records (magnetic susceptibility, gamma ray, carbon and oxygen isotope) and terrestrial climate indicators (paleosols, evaporates and tillites) across continents and latitudes demonstrate that long-term glacial, glacioeustatic, and climatic events were in pace with eccentricity and obliquity modulation cycles superimposed on secular global warming, reinforcing solid linkage between climate changes at low and high latitudes regardless of the ice sheet volume. Quasi-periodic alignments of the maxima (minima) of eccentricity and obliquity amplitude decelerated (accelerated) the trajectory of the CO₂-forced deglaciation. Intermittent nondeposition of the Cisuralian cyclothems on the North American Midcontinent correspond to the enhanced none-astronomical-related noise in the sedimentary record from South China, both of which were likely attributed to weaker or less apparent influence of astronomical forcing on the climate changes without an ice-sheet amplifier. Our study provides a better temporal resolution and understanding of the late Paleozoic deglaciation.

How to cite: Fang, Q. and Wu, H.: Astronomically paced climate changes during the demise of the penultimate icehouse, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-158, https://doi.org/10.5194/egusphere-egu2020-158, 2020.

At present Earth’s climate is warming and the frequency of large wildfires appears to be increasing (Westerling and Bryant, 2008). Long term trends in climate and the effect on wildfire are understudied and examining the geological record can aid current understanding of natural variability of wildfire over longer time scales. The Early Jurassic is a period of overall global warmth, and therefore serves as a suitable modern-day analogue to understand changes in the Earth System. The Early Jurassic was characterized by major climatic and environmental perturbations which can be seen preserved at high resolution on orbital timescales. Recent research has indicated from Quaternary deposits that wildfires respond to orbital forcings (Daniau et al., 2013). This study tests whether wildfire activity corresponds to changes over Milankovitch timescales in the deep past.

        A high-resolution astrochronology exists for the Upper Pliensbachian in the Llanbedr (Mochras Farm) borehole (NW Wales). Ruhl et al. (2016) show that elemental concentration recorded by hand-held X-ray fluorescence (XRF), changes mainly at periodicities of ~21,000 year, ~100,000 year and ~400,000 year, and which can be related to visually described sedimentary bundles.

        We have quantified the abundance of fossil charcoal at a high resolution (10-15 cm) to test the hypothesis that these well-preserved climatic cycles influenced fire activity throughout this globally warm period. Our results suggest that variations in charcoal abundance are coupled to Milankovitch forcings over periods of ~21,000 and ~400,000 years. Supplementary to the charcoal record, a high-resolution clay minerology dataset has been generated, which indicates the presence of the 400ky cycle. Decreased hydrology on land, corresponds to increased charcoal production. We suggest that these changes in fire relate to changes in seasonality and monsoonal activity that drove changes in vegetation that are linked to variations in the orbital forcing.

How to cite: Hollaar, T., Baker, S., Deconinck, J.-F., Mander, L., Ruhl, M., Hesselbo, S., and Belcher, C.: Orbital pacing of large fluctuations in wildfire activity during the Pliensbachian, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5383, https://doi.org/10.5194/egusphere-egu2020-5383, 2020.

The Indonesian Throughflow (ITF) operates as an important link in global thermohaline circulation and is often considered a modulator of global past climate changes, with effects as far as Africa or the Atlantic Ocean. Yet, to what extent ITF variability accounted for oceanographic change along the west Australian coast remains controversial. A tectonically reduced ITF has been invoked to explain the short, but intense Pliocene glaciation Marine Isotope Stage (MIS) M2 (3.3 Ma). The hypothesis hinges on a reduced equator-to-pole heat transfer in the Indian Ocean, in response to low connectivity with the Indo-Pacific warm pool. To clarify links between regional oceanographic change and global climate, we present a two-site multiproxy reconstruction from the Perth (U1459) and the Carnarvon (U1463) Basin. These sites provide the opportunity to unravel the Pliocene history of the Leeuwin Current (LC). We use the LC as a proxy for ITF connectivity, as the ITF is the source for the warm, low-salinity, nutrient-deficient LC. A U1459-U1463 comparison thus allows for investigating the possible relationship between mid-Pliocene glaciations and ITF heat flux. We show that the LC was active throughout the Pliocene, albeit with fluctuations in intensity and scope. We identify three main factors that controlled LC strength. First, a tectonic ITF reorganization caused an abrupt and permanent LC reduction at 3.7 Ma, coeval with the remarkably intense Pliocene glacial MIS Gi4. On shorter timescales, eustatic sea level and direct orbital forcing of wind patterns hampered or promoted the LC. At 3.3 Ma, LC intensity plunged in response to a eustatic ITF restriction. MIS M2 caused the latitudinal U1463–U1459 planktonic oxygen isotope gradient to steepen from 1.2 to 2.0‰ and the TEX₈₆ sea surface temperatures gradient to increase from 3 to 6°C. Yet, comparison with Exmouth Plateau Site 763 shows that the LC did not shut down completely during MIS M2: The ITF heat flux dwindled but did not cease. Weakened ITF connectivity led to a significant drop in Indian Ocean poleward heat transport and thus constitutes a positive feedback mechanism that contributed to the relative intensity of MIS M2 and the thermal isolation of Antarctica. This positive feedback mechanism is ultimately driven by orbital-scale changes in relative sea level in the ITF region.

How to cite: De Vleeschouwer, D., Füllberg, A., Smith, R., Auer, G., Petrick, B., Castañeda, I., and Christensen, B.: Pliocene ocean and climate dynamics in the eastern Indian Ocean and their implications for the global climate state., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2209, https://doi.org/10.5194/egusphere-egu2020-2209, 2020.

The nature and dynamics of Pliocene climate has been a focus of intense study for many years. This is because the Pliocene has a unique potential to inform science/society about how the Earth system responds to forcing of direct relevance to future climate change. We examine large-scale climate features derived from the second phase of the Pliocene Model Intercomparison Project. PlioMIP2 is composed of simulations derived from sixteen coupled atmosphere-ocean and Earth System Models of a variety of vintages (IPCC AR3/4 to 6). This represents one of the largest ensembles of models ever assembled to represent a particular interval in Earth history. Each model has been set up to include the very latest Pliocene boundary conditions provided by the U.S. Geological Survey Pliocene Research Interpretation and Synoptic Mapping Project (PRISM4). As well as examining large-scale features of the PlioMIP2 model ensemble we further examine trends in model sensitivity versus model age in order to ascertain if newer versions of models are becoming more sensitive to Pliocene boundary conditions. We examine this across the PlioMIP2 ensemble as a whole and within individual model families, and examine what this implies in terms of the potential for individual models, or families of models, to represent patterns of surface temperature change reconstructed from geological proxies.

How to cite: Haywood, A. and Tindall, J.: Are models becoming more sensitive to Pliocene boundary conditions?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19313, https://doi.org/10.5194/egusphere-egu2020-19313, 2020.

The warm Pliocene serves as an analogue for predicted warming over the next century. However, large uncertainties exist for atmospheric circulation and land surface conditions during the Pliocene. Dust transported by wind to locations of accumulation (terrestrial or marine) can provide a record of wind intensity and/or direction. Few dust flux records spanning the Plio-Pleistocene exist. As such, there is ample opportunity to use marine sediments to reconstruct changes in atmospheric conditions during a warmer-than-present world, as well as across the onset/intensification of Northern Hemisphere Glaciation (NHG). During this time, East Asia’s interior, the second largest source of mineral dust today, experienced aridification, occurring alongside a major reorganization of the subarctic North Pacific circulation which led to stratification of the surface ocean. Here, we present two North Pacific marine sediment records of extraterrestrial (ET) ³He-derived terrigenous dust flux proxies (⁴He_Terr and Th), along with a record of multiple paleoproductivity proxies (Ba_xs, Opal, and C³⁷_Total) for the period spanning ~2.5-4.5 Ma. Our results show that dust flux to the western North Pacific was relatively low and constant through the Pliocene up until ~2.7 Ma, with minor peaks during cooler phases from ~2.9-3.1 Ma. At ~2.7 Ma, concurrent with the intensification of NHG and formation of a permanent halocline cap in the subarctic North Pacific, dust fluxes increase dramatically. The central North Pacific record shows a less drastic shift in dust, but generally displays higher fluxes after ~3 Ma. Dust fluxes in East Asia and the North Pacific are consistent during this time interval, as are global dust fluxes from the North Atlantic, South Atlantic and North Pacific. Western North Pacific dust, SST, and paleoproductivity records point to northward-shifted and weakened Northern Hemisphere westerlies during the warm Pliocene, with evidence for strengthening and southward movement of the westerlies during glacials after ~2.7 Ma. Changes in both winds and dust production mechanisms are likely working in tandem to produce the coherent global dust signals.

How to cite: Abell, J. T., Winckler, G., Anderson, R. F., and Herbert, T.: Reconstructing the intensity and location of Northern Hemisphere westerlies during the Plio-Pleistocene using marine sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5897, https://doi.org/10.5194/egusphere-egu2020-5897, 2020.

Hydrological extremes related to the South American Summer Monsoon (SASM) are expected to become more frequent in the near future and might have devastating socioeconomic consequences for the densely populated region of eastern Brazil. Given the complexity in SASM behaviour in space and time, a dense coverage of monsoonal precipitation records, particular those spanning multiple glacial-interglacial cycles, are urgently needed to constrain this high spatial-temporal variability. This information is necessary to reduce the uncertainty associated with projections of SASM precipitation in response to rising anthropogenic greenhouse gas (GHG) emissions. Here we use elemental ratios from X-ray fluorescence scanning of two sediment cores retrieved off the eastern Brazil margin to reconstruct long-term rainfall changes in the hinterland. Our findings from core M125-55-7 (offshore the Doce River, 20°S) reveal that during the past ~320 kyr, precession-paced insolation forcing is the primary pacemaker of variations in SASM precipitation over the Doce basin. We also determined an anomalous interval of weak monsoonal response to insolation forcing during Marine Isotope Stage 6, which we attribute to enhanced wintertime precipitation due to exceptionally strong southeast trade winds created by a steep South Atlantic latitudinal temperature gradient. Moreover, our results suggest that albeit predominantly driven by insolation forcing, the intensity of SASM rainfall responds negatively to GHG forcing, most likely through indirect feedbacks. We propose that GHG forcing directly influences the magnitude of both the inter- and intrahemispheric latitudinal temperature gradients, which in turn modify the strength of atmospheric circulation and precipitation in the tropics. Thus, we suggest that SASM rainfall intensity over tropical eastern Brazil will likely be suppressed by rising CO₂ emissions in the future. Our preliminary analysis of core M125-73-3 (off the Contas River; 12°S) reveals regional differences in monsoonal precipitation between the more northerly Contas basin and the more southerly Doce basin. Most notably, unlike the insolation-paced continental rainfall variability recorded at site M125-55-7, SASM rainfall intensity over the Contas basin appears to be more sensitive to glacial-interglacial scale pacing over the past ~800 kyr. Taken together, our records reveal both the high spatial variability in SASM precipitation over eastern Brazil and the dominant influence of orbital forcing on monsoonal rainfall intensity.

How to cite: Hou, A. M. X., Bahr, A., Raddatz, J., Voigt, S., Albuquerque, A. L., Chiessi, C. M., and Friedrich, O.: Orbitally-paced South American Summer Monsoon variability during the mid- to late-Pleistocene , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9472, https://doi.org/10.5194/egusphere-egu2020-9472, 2020.

The response of the climate system to astronomical parameters is an important scientific issue, but the internal processes and feedbacks need to be better understood. This study investigates the differences of the climate response to the astronomical forcing between the Northern (NH) and Southern (SH) hemispheres based on a more than 90,000-year long transient simulation using the model LOVECLIM. Our results show that the response of sea ice and sea surface temperature (SST) to precession and obliquity are different between the two hemispheres. Precession plays a dominant role on the NH sea ice. This is mainly due to its response to the local summer insolation and also, to a less degree, the influence of the northward oceanic heat transports. However, obliquity plays a dominant role on the SH sea ice through its influence on insolation and the westerly winds. As far as the SST is concerned, it shows a strong precession signal at low latitudes in both hemispheres. For the SST in the mid and high latitudes, obliquity plays a dominant role in the SH whereas precession is more important in the NH. This is largely due to the different response to insolation and feedbacks related to the different land-ocean distribution in the two hemispheres. Near the Equator, besides the precessional signal, the SST also shows strong half-precessional signal, which can be explained by the unique characteristics of the insolation variations at the Equator.

How to cite: Wu, Z., Yin, Q., Guo, Z., and Berger, A.: Different response of sea surface temperature and sea ice to precession and obliquity between the two hemispheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4765, https://doi.org/10.5194/egusphere-egu2020-4765, 2020.

Uncertainties on the radiometric ages of Devonian stage boundaries are currently on the order of several millions of years. A cyclostratigraphic approach is the foremost way forward to improve the Devonian geological time scale. To do so requires well-preserved continuous records, as well as reliable paleoclimatic proxies.  The NY Route 199 section, from Kingston, in the Hudson Valley of eastern New York, is a road cut outcrop, which exposes most of the Schoharie Formation. It corresponds to the upper portion of the Emsian Stage (upper Lower Devonian, ~400 to ~394 Ma), with essentially continuous deposition. The lithology consists of a mixed siliciclastic-carbonate succession with overall increasing carbonate upsection, showing various degrees of bioturbation (traces includes primarily Zoophycos, Planolites and Chondrites); colors range from white to beige, brown or dark grey. The quality of most of the outcrop is so remarkable that the color variations by themselves permit recognition of Milankovitch cycles, with prominent bundles of light and dark beds. One type of cycle expression is represented by a succession of about six darker beds nested between lighter beds, which is interpreted as six precession cycles within a short eccentricity cycle (precession in the Devonian was ~17 kyr).

Samples were collected every 2 cm through 38 m of the section for magnetic susceptibility measurements. On top of these measurements, we provide elemental geochemistry, carbon isotopes and hysteresis measurements (every 50 cm) to constrain the depositional setting and the diagenesis. Hysteresis measurements show that despite being remagnetized (throughout the Appalachians, these Paleozoic rock sequences are all remagnetized during the Variscan-Alleghenian Orogeny), the magnetic susceptibility reflects depositional information. The geochemistry and carbon isotopes give insight into the occurrence of oxic/reducing conditions and detrital inputs. Milankovitch cycles are visible on the outcrop and in the magnetic susceptibility record, allowing a precise floating timescale framework to be constructed for this interval.

How to cite: Da Silva, A.-C., Bartholomew, A., Brett, C., Hilgen, F., Ver Straeten, C., and Dekkers, M.: Exceptionally preserved Milankovitch cycles in Lower Devonian argillaceous limestone of the Hudson Valley, New York State (USA) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7565, https://doi.org/10.5194/egusphere-egu2020-7565, 2020.

The Late Paleozoic Ice Age (LPIA), one of the best known and prolonged glaciation events in Earth's history, resulted in the widespread deposition of glacial sediments over Gondwana (Crowell, 1999). Some of the most important LPIA deposits of the multiple glacial-deglacial episodes (Isbell et al., 2003) were preserved in the Itararé Group of the Paraná Basin (Brazil). This unit presents continental and marine glacially-influenced deposits formed by advances and retreats of glaciers and consists in an opportunity to better understand the mechanisms forcing climate shifts during the LPIA. In low latitudes, the deposition of the Carboniferous cyclothems was controlled by long- and short-eccentricity (Davydov et al., 2010). In high latitudes, orbital-scale climate cycles may also be preserved in the sedimentary succession. We aim to recognize whether or not orbital and millennial-scale climate cycles are preserved in the sedimentary succession of a core drilled in the southeastern border of the Paraná Basin. Here, we present the first cyclostratigraphic study based on X-ray fluorescence records from a 27 m-long interval of LPIA rhythmites of the Rio do Sul Formation (top of the Itararé Group). The sedimentary succession is composed of lithological couplets of fine-grained siliciclastic sediments, locally displaying subtle plane-bedding. Such rhythmites are characterized by abrupt contacts between couplets and normal grading internally. TiO₂ and Fe₂O₃ vary in phase and display well-defined cyclicities in the stratigraphic domain. The TiO₂ series presents millennial and orbital scale periodicities. Variations in the concentrations of the analyzed terrigenous components are likely indicative of glacial-interglacial changes, reflected by advances and retreats of glaciers under drier and wetter climate conditions, respectively. Here we show that these high latitude glacial-interglacial cycles were probably paced by short-eccentricity, as previously suggested for Carboniferous cyclothems in low latitude deposits, and highlight the importance of millennial-scale climate cycles forcing high latitudes glacial-related deposits, similar to patterns seen in Pleistocene records.

References:

Crowell, J. C. (1999). Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate 375 System. Geologic Society of America Memoir 192, pp. 1–112.

Davydov, V. I., Crowley, J. L., Schmitz, M. D., & Poletaev, V. I. (2010). High-precision U-Pb zircon age calibration of the global Carboniferous time scale and Milankovitch band cyclicity in the Donets Basin, eastern Ukraine. Geochemistry, Geophysics, Geosystems, 11.

Isbell, J. L., Miller, M. F., Wolfe, K. L., & Lenaker, P. A. (2003). Timing of late Paleozoic glaciation in Gondwana: Was glaciation responsible for the development of Northern Hemisphere cyclothems? In Geologic Society of America Special Paper 370, pp. 5–24.

How to cite: Kochhann, M., Cagliari, J., Kochhann, K., and Franco, D.: Climate variability during the Late Paleozoic Ice Age in the southwestern Gondwana: records of orbital and millennial-scale cycles in the Carboniferous rhythmite of the Paraná Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11886, https://doi.org/10.5194/egusphere-egu2020-11886, 2020.

The cumulative work of geoscientists over the past decades has shown that the Silurian Period which was once thought as warm and climatically stable time interval is in fact punctuated by numerous paleoenvironmental perturbations or events. These Silurian events follow a similar pattern where a minor extinction event precedes a substantial carbon isotope excursion. Many theories have been brought forward to explain these events ranging from glaciations, to changes in precipitations patterns, ocean currents and ocean anoxia. Constraints on the duration and timing of these extinction events and subsequent positive carbon isotope excursions are weak, which hampers a full understanding of the processes at play.

The data from the Altajme core from Gotland, Sweden provides us with a unique opportunity to look at two of these climatic perturbations during the Silurian. The Altajme core spans both the Sheinwoodian Ireviken event and the Homerian Mulde event. The Altajme core dataset includes a litholog, high-resolution δ13C data, correlated bentonites with U-Pb dates and a high-resolution XRF core scan: important data required for and integrated stratigraphic study. The U-Pb-dated bentonites give us age constraints. The δ13C data in combination with the high resolution XRF scan gives us insights into the changes in the ocean before during and after the events, while the XRF is also used to build cyclostratigraphic age constraints for the events and for the whole core. This stratigraphic study will provide us with a palaeoclimatological insights to explain these two events and provide us with a cyclostratigraphy based age model for the Middle Silurian.

How to cite: Arts, M., Cramer, B., Calner, M., Rasmussen, C., Bancroft, A., Oborny, S., Hartke, E., Biebesheimer, E., and Da Silva, A.-C.: Cyclo and chemostratigraphic characteristics of the Middle Silurian in Gotland, Sweden, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9556, https://doi.org/10.5194/egusphere-egu2020-9556, 2020.

Large uncertainties exist on the numerical ages of the stages in the Early Cretaceous which hamper from an accurate reconstruction of the past climate. Recent radio-astrochronologic data suggest to move the ages of the Tithonian to the Hauterivian stages by 3 to 5 Myr toward younger ages (Lena et al., 2019; Aguirre-Urreta et al., 2019). As the numerical ages in the Cenomanian are constrained with radio-astrochronology, this means that the duration of the Barremian to the Albian stages is overestimated. The duration of the Barremian Stage was estimated by bed counting on the assumption of a control by precession and eccentricity cycles (e.g., Bodin et al., 2006). The alternations and bundling can vanish leading to uncertainties in the duration estimates. Here, we provide an astrochronology from the eccentricity cycles based on spectral analyses performed on both magnetic susceptibility and calcium carbonate content series. Two sections are studied here in the Subbetic Domain (SE Spain). They are composed of marl-limestone alternations which reflect humid-arid cycles orbitally-driven. Detailed ammonite and calcareous nannofossil controls allow correlations with other sections in the basin and in the Tethyan Realm. The short and long-eccentricity cycles are identified throughout the Late Hauterivian to the earliest Aptian. The interval around the Hauterivian-Barremian boundary was recovered in a section previously studied for astrochronology and shows that the eccentricity cycles can be correlated to the sections studied here, validating the interpretations. From the record of the 405-kyr eccentricity cycle, the duration of the Barremian Stage is proposed at 4.25 ± 0.13 Myr. Anchoring this duration on previously obtained radio-astrochronology at the end of the Hauterivian, the Barremian Stage started at 125.91 ± 0.06 Ma and ended at 121.67 ± 0.11 Ma. The age of the latest Barremian agrees well with the age of the base of magnetochron M0r calculated from a synthesis of radiometric ages (Olierook et al., 2019). The Faraoni, Mid-Barremian and Taxy episodes show a pacing of 2.34 Myr, suggesting a strong orbital control on the expansion of oceanic anoxic conditions in the Tethys.

References:

Aguirre-Urreta, B., et al., 2019. Gondwana Res., 70, 104–132. https://doi.org/10.1016/j.gr.2019.01.006.

Bodin, S., et al., 2006. Palaeo-3, 235, 245–264. https://doi.org/10.1016/j.palaeo.2005.09.030.

Lena, L., et al., 2019. Solid Earth, 10, 1–14. https://doi.org/10.5194/se-10-1-2019.

Olierook, H.K.H., et al., 2019. Earth-Sci. Rev., 197, 102906. https://doi.org/10.1016/j.earscirev.2019.102906.

How to cite: Martinez, M., Aguado, R., Company, M., Sandoval, J., and O'Dogherty, L.: Astrochronology of the Barremian Stage: implications for the dynamics of the anoxic events in the Early Cretaceous, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8923, https://doi.org/10.5194/egusphere-egu2020-8923, 2020.

Fifty-one years of scientific ocean drilling through the International Ocean Discovery Program (IODP) and its predecessors generated a treasure trove of Cenozoic climate and carbon cycle dynamics. Yet, it remains unclear how climate system and carbon cycle interacted under changing geologic boundary conditions. Here, we present the carbon isotope (d¹³C) megasplice, documenting deep-ocean d¹³C evolution since 35 million years ago (Ma). We juxtapose the d¹³C megasplice with its d¹⁸O counterpart and determine their phase-difference on ~100-kyr eccentricity time-scales. This analysis uncovers that 2.4-Myr eccentricity modulates the in-phase relationship between d¹³C and d¹⁸O during the Oligo-Miocene (34-6 Ma), potentially related to changes in continental weathering. At 6 Ma, a striking switch from in-phase to anti-phase behaviour occurs, signalling a threshold in the climate system. We hypothesize that Arctic glaciation and the emergence of bipolar ice sheets enabled eccentricity to exert a major influence on the size of continental carbon reservoirs. Our results suggest that a reverse change in climate - carbon cycle interaction should be anticipated if CO₂ levels rise further and we return to a world of unipolar ice sheets.

How to cite: De Vleeschouwer, D., Drury, A. J., Vahlenkamp, M., Liebrand, D., Rochholz, F., and Pälike, H.: Thirty-five million years of changing climate – carbon cycle dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21479, https://doi.org/10.5194/egusphere-egu2020-21479, 2020.

Variations in solar insolation exert a fundamental control on the high-latitude climate–cryosphere system. Controversy, however, exists about the relative importance of orbital eccentricity versus axial tilt (obliquity) in driving pre-Quaternary Antarctic ice sheet variability. This problem is particularly acute during the late Oligocene-to-early Miocene interval (Oligo-Miocene, ~27-21 Ma), because several benthic foraminiferal oxygen isotopes (δ¹⁸O) records show strong pacing by obliquity, while others primarily show eccentricity pacing. The differences in orbital pacing are impossible to reconcile with the globally congruent imprint of ice volume on benthic δ¹⁸O on orbital time scales. Here we present a new astronomically tuned δ¹⁸O record generated at Integrated Ocean Drilling Program (IODP) Site U1406 (north-western Atlantic Ocean), a key area in modern-day thermohaline circulation. Clear imprints of both obliquity and eccentricity on the δ¹⁸O record are observed at Site U1406 throughout the study interval, irrespective of changes in sedimentation rate. The eccentricity variations at Site U1406 are remarkably similar to those seen in all other δ¹⁸O records, suggesting that eccentricity exerts a strong control on the high-latitude climate–cryosphere system via the modulation of the precession cycle. In contrast, the δ¹⁸O sensitivity to obliquity is globally variable, suggesting the influence of temperature in different bottom-water masses.

How to cite: van Peer, T., Taylor, V., Liebrand, D., Brzelinski, S., Möbius, I., Bornemann, A., Friedrich, O., Bohaty, S., Xuan, C., Lippert, P., and Wilson, P.: Eccentricity-paced ice sheet variability and obliquity-driven bottom-water changes during the Oligocene-Miocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15256, https://doi.org/10.5194/egusphere-egu2020-15256, 2020.

The early Pliocene, with atmospheric CO₂ concentrations at levels similar to today, is seen as a case study for Earth’s future climate evolution. During this period the progressive closing of the Central American Seaway led to increased poleward heat and salt transport within the Atlantic with North Atlantic Deep Water (NADW) becoming warmer and saltier and resulting in an enhanced Atlantic Meridional Overturning Circulation (AMOC). In order to evaluate how stable the Pliocene AMOC really was, we are producing surface and deep-water records for IODP Site U1313 (41°N, 33°W, 3412m) for the interval from 3.3 to 4.1 Ma. This site is ideally located to monitor past AMOC changes with North Atlantic Drift waters at the surface and NADW, exported by the deep western boundary current, in the deep. Surface water conditions are reconstructed based on the stable isotope data of planktonic foraminifer species Globigerinoides ruber (white) or Globigerinoides extremus with centennial-scale resolution and on sea-surface temperatures (U^k_37' alkenone thermometer) with an average 4 ky resolution. Stable isotope records of the benthic foraminifer genus Cibicidoides reveal changes in the deep water.

Besides the interglacial/glacial cycles, higher frequency oscillations are recorded in both the planktonic and benthic foraminifer stable isotope records. Varying surface water conditions, especially during Late Pliocene interglacial periods, are reflected in the Globigerinoides isotope data and appear to be linked to salinity changes since they are not recorded in the sea-surface temperature data. The high-frequency oscillations in the planktonic isotope records are related to precession (insolation) forcing, especially its harmonics in the 5.5 ky and 11 ky ranges. The benthic δ¹³C values indicate nearly continuous NADW presence and confirm a strong AMOC throughout the studied interval, also during most of the glacial periods. Excluding the pronounced M2 glacial, glacial stage Gi 6 had a stronger impact on the AMOC, as revealed by cooler, less ventilated surface waters and a less ventilated NADW, than Gi 2 and Gi 4. Overall, the AMOC was strong throughout, but experienced high frequency oscillations at a level similar to the middle Pleistocene interglacial periods.

How to cite: Voelker, A. H. L., Sierro, F. J., Naafs, B. D. A., Andersen, N., and Kuhnert, H.: Early to Late Pliocene climate change in the mid-latitude North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10667, https://doi.org/10.5194/egusphere-egu2020-10667, 2020.

The Sahel region is one of the most vulnerable regions on Earth to anthropogenically-driven climate change, but also one of the least equipped to deal with the consequences. Predictions of precipitation levels over the forthcoming centuries diverge, not only in magnitude, but also in the sign of change. One key aspect of this uncertainty comes from the role of Atlantic Ocean sea surface temperatures (SST), which are known to exert a strong control over precipitation in the Sahel and are implicated in both the major drought of the late 20^th century and extreme droughts associated with the Heinrich events of the last glacial. To better understand how Sahelian hydroclimate may respond to SST variability in a warmer world, we turn to the Pliocene epoch, when atmospheric CO₂ levels were comparable to present.

We studied sediments from Ocean Drilling Project Site 659, which is situated in the subtropical North Atlantic beneath the major modern summer Saharan dust plume. Our new dust accumulation rates and X-ray fluorescence core scan data indicate that there were major shifts between highly arid conditions and humid intervals with vegetated or “Green Sahara” conditions over much of northern Africa, driven by both solar insolation and glacial-interglacial variability. We also report three unusually long Plio-Pliocene humid intervals (each lasting ca. 100 kyr) characterised by very low dust emissions, that we term “Green Sahara Megaperiods (GSMPs)”. All three of these GSMPs occur at times when insolation variability was weak, resulting in values close to the long-term mean. This observation strongly suggests that factors other than insolation drove the sustained humidity of GSMPs. We present paired alkenone SST estimates and multi-species planktonic foramaniferal isotope records from 3.5–2.3 Myr ago to explore the extent to which the GSMPs were accompanied by intervals of extended warmth in the surface waters of the North Atlantic Ocean.

How to cite: Wilson, P., Jewell, A., Crocker, A., Buchanan, S., Mitsunaga, B., Westerhold, T., Röhl, U., Russell, J., and Herbert, T.: Green Sahara Megaperiods during the Pliocene: What was the role of North Atlantic Ocean temperature?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20327, https://doi.org/10.5194/egusphere-egu2020-20327, 2020.

The Piacenzian stage of the Pliocene (2.6 to 3.6 Ma) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher (by ~2-3 ^oC) than today. Quantifying CO₂ levels during the mid-Piacenzian Warm Period (mPWP) provides a means, therefore, to deepen our understanding of Earth System behaviour in a warm climate state. Here we present a new high-resolution record of atmospheric CO₂ using the δ¹¹B-pH proxy from 3.35 to 3.15 million years ago (Ma) at a temporal resolution of 1 sample per 3-6 thousand years. Our study interval covers both the coolest marine isotope stage of the mPWP, M2 (~3.3 Ma) and the transition into its warmest phase including interglacial KM5c (centered on ~3.205 Ma) which has a similar orbital configuration to present. We find that CO₂ ranged from ca. 390 ppm to ca. 330 ppm, with CO₂ during the KM5c interglacial being ca. 370 ppm. Our findings corroborate the idea that changes in atmospheric CO₂ levels played a distinct role in climate variability during the mPWP. They also facilitate ongoing data-model comparisons and suggest that, at present rates of human emissions, there will be more CO₂ in Earth’s atmosphere by 2025 than at any time for at least the last 3.3 million years.  

How to cite: de la Vega, E., Chalk, T. B., Wilson, P. A., Bysani, R., and Foster, G. L.: Atmospheric CO2 during the Mid-Piacenzian Warm Period and the M2 glaciation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11480, https://doi.org/10.5194/egusphere-egu2020-11480, 2020.

Models from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the mid-Pliocene Warm Period (mPWP) was a warmer and wetter world than today. However, there is not strong model agreement as to how tropical precipitation was different in the mPWP. Although PlioMIP2 models agree that there was more precipitation associated with the African Monsoon and the Asian Monsoon, away from these regions models do not show a consistent and robust change in precipitation between the mPWP and the preindustrial.

Here we use the HadGEM2 model to explore changes in tropical precipitation between the mPWP and the preindustrial, particularly those associated with the position and strength of the Intertropical Convergence Zone (ITCZ). Reasons for these changes within HadGEM2 will be discussed. We will also expand our discussion of the ITCZ to the PlioMIP2 ensemble in order to show the differing factors that could influence ITCZ characteristics in a warmer world.

How to cite: Tindall, J. and Haywood, A.: Modelling Tropical Precipitation in the mid-Pliocene Warm Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19088, https://doi.org/10.5194/egusphere-egu2020-19088, 2020.

Today in the North Pacific only intermediate water forms because of a strong halocline, but Pacific Meridional Overturning Circulation (PMOC) may have existed in the past. The mid-Pliocene warm period (3.264-3.025 Ma) is a time of sustained warmth where atmospheric carbon dioxide concentrations were similar to today and the northern hemisphere was relatively ice free – making it a pseudo-analogue for future climate change. North Pacific sedimentological and climate modeling evidence suggests a PMOC formed during this time.  To determine the spatial extent of a PMOC during the mid-Pliocene warm period, we constructed a depth transect of sites between 2400 to 3400 m water depth on Shatsky Rise by measuring stable isotopes of Cibicidoides wuellerstorfi. We compare these new results with previously published records and calculate anomalies using the OC3 water column and core-top data products. The δ¹³C spatial pattern is consistent with a modest PMOC of intermediate depth (core ~2000 m) extending to the equator during the mid-Pliocene warm period. Ventilation of the North Pacific by a PMOC has broad implications for deep ocean carbon storage as the North Pacific contains the oldest, carbon-rich waters today. Future work will include minor and trace element analyses to determine the temperature and carbon characteristics of the PMOC water mass and comparisons with PlioMIP modeling outputs.

How to cite: Ford, H. L., Burls, N., and Hodell, D.: Pacific Meridional Overturning Circulation during the Mid-Pliocene Warm Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21860, https://doi.org/10.5194/egusphere-egu2020-21860, 2020.

The South Asian or Indian Summer Monsoon (ISM) brings seasonal winds and rains to the Indian subcontinent and affects billions of people.  It is likely that the global monsoon will strengthen in a 1.5 °C warming scenario (IPCC special report (2018)), however our ability to predict ISM behaviour in the future is restricted due to lack of understanding of its behaviour under varying climatic conditions before instrumental records began.  Thus, reconstructing the palaeo-monsoon using proxies gives insight into past and potentially future controls on the ISM.  We present new data covering the interval ~5 to ~2 million years ago (Ma), during the Pliocene and early Pleistocene when the long-term Cenozoic cooling trend culminated in intense northern hemisphere glaciations from 2.7 Ma.  At this time, global temperatures are suggested to have been 2-3 °C warmer than today and atmospheric CO₂ was over 400 ppm (similar to today). 

This study focuses on sediments from Site U1443 ( 5°N, 90°E), drilled during International Ocean Discovery Program (IODP) Expedition 353 in the Bay of Bengal (BoB) for the Pliocene – early Pleistocene.  We present X-ray fluorescence (XRF)-derived bulk sediment geochemical data and suggest that erosional flux (terrigenous elements/total counts) as well as productivity (Br/Cl) varied in response to runoff strength, precipitation, and wind stress at the study site to reconstruct ISM variability.  Additionally, new nannofossil assemblage and morphometric data, collected using the automated system SYRACO, are used to reconstruct BoB stratification and productivity and thereby assess ISM dynamics.  A new benthic oxygen isotope-based age model will allow us to place the Site U1443 records into the context of existing climate and monsoon records and evaluate ISM response due to external and internal climate forcing factors.

How to cite: Gray, E., Anand, P., Bolton, C., Murayama, M., and Badger, M.: Reconstructing Past Indian Summer Monsoon Productivity and Stratification During the Late Pliocene and Early Pleistocene , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-399, https://doi.org/10.5194/egusphere-egu2020-399, 2020.

A range of future climate scenarios are projected for high atmospheric CO₂ concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205 +/- 0.01 Ma) when atmospheric CO₂ concentrations were higher than pre-industrial, but similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO₂. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial, by ~2.3 ºC for the combined proxy data (foraminifera Mg/Ca and alkenones), or by ~3.2ºC (alkenones only). Compared to the pre-industrial, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values.  Our results demonstrate that even under low CO₂ emission scenarios, surface ocean warming may be expected to exceed model projections, and will be accentuated in the higher latitudes.

How to cite: McClymont, E., Ford, H., Ho, S. L., Tindall, J., Haywood, A., Alonso Garcia, M., Bailey, I., Berke, M., Littler, K., Patterson, M., Petrick, B., Peterse, F., Ravelo, C., Risebrobakken, B., De Schepper, S., Swann, G., Thirumalai, K., Tierney, J., van der Weijst, C., and White, S.: Lessons from a high CO2 world: an ocean view from ~3 million years ago, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2427, https://doi.org/10.5194/egusphere-egu2020-2427, 2020.

It remains unclear how El Niño–Southern Oscillation (ENSO)—the prominent interannual anomalous climate mode—varied during the full glacial cycles. We study the evolution of ENSO of the last 300,000 years using continuous fully-coupled climate model simulations. How the slow time‐varying changes in insolation, greenhouse gases concentration, and continental ice sheets could influence the behaviours of El Niño are taken into account. The simulated ENSO variance and the tropical eastern Pacific annual cycle (AC) amplitude change in phase, and both have pronounced precession-band variance (~21,000 years) rather than the obliquity-band (~40,000 years). The precession‐modulated slow (orbital time scales) ENSO evolution is determined linearly by the change of the coupled ocean‐atmosphere instability, notably the Ekman upwelling feedback and thermocline feedback. In contrast, the greenhouse gases and ice sheet forcings (~100,000‐year cycles with sawtooth shapes) are opposed to each other as they influence ENSO variability through changes in AC amplitude via a common nonlinear frequency entrainment mechanism. The relatively long simulations which involve pronounced glacial‐interglacial forcing effects gives us more confidence in understanding ENSO forcing mechanisms, so they may shed light on ENSO dynamics and how ENSO will change in the future.

How to cite: Lu, Z.: Prominent precession-band variance in El Niño–Southern Oscillation Intensity over the last 300,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11315, https://doi.org/10.5194/egusphere-egu2020-11315, 2020.

Seasonality is a dominant factor in the Earth’s climate system, but proxy reconstructions on this time scale are sparse. Corals provide an excellent archive to reconstruct environmental conditions on seasonal time scale using geochemical proxies. Here, we use subfossil (~6.2-7.1 ka BP) Siderastrea siderea and Pseudodiploria labyrinthiformis corals from a pristine Mid-Holocene reef, located in Panamá, southwestern Caribbean. Mid-Holocene insolation seasonality in the Northern Hemisphere was stronger than at present. We investigate the resulting changes in SST and hydrological seasonality using coral Sr/Ca, δ¹⁸O and δ¹³C. To evaluate, if the coral heads can be utilised for geochemical analyses, they have been screened for diagenetic alteration (2D-XRD, thin section analysis). Obtained modern coral Sr/Ca-SST based annual cycle corresponds well with in situ measured SST. Fossil coral Sr/Ca-SST based cycles exceed the modern one by up to 50%. Fossil coral δ¹⁸O seasonal amplitudes are higher than the modern one by up to 30% and show a reduction in the mean gradient between wet and dry period, attributable to the northward shift of the Intertropical Convergence Zone. Increased SST and δ¹⁸O seasonality are consistent with model simulated SSTs (Kiel Climate Model) and model-based calculated pseudocoral δ¹⁸O, but the model underestimates the seasonality increase in the Mid-Holocene.

How to cite: Skiba, V., Struck, U., Reuning, L., Garbe-Schönberg, D., Frank, N., Leinfelder, R., O'Dea, A., and Zinke, J.: Coral reconstructed Mid-Holocene seasonality in the southwestern Caribbean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1092, https://doi.org/10.5194/egusphere-egu2020-1092, 2020.

Enhanced winter precipitation over the Mediterranean Sea at times of minimum precession and maximum obliquity could provide freshwater required to form orbitally-paced sedimentary cycles across the Mediterranean Sea floor, offering an alternative to monsoonal runoff. We investigate the sources of the enhanced winter precipitation by applying a moisture tracking model (WAM-2layers) on the results of idealized orbital extreme experiments with a state-of-the-art climate model (EC-Earth).

Tracking the moisture sources of the enhanced winter precipitation over the Mediterranean Sea shows that the source differs during the winter half year. In fall, the majority of the precession-induced precipitation increase originates from the Mediterranean itself. However, in late winter, the increase can be attributed to enhanced moisture advection from the Atlantic. This agrees with changes in evaporation and air-sea temperature differences over the Mediterranean. The obliquity-induced precipitation increase shows much less differences, with an equal contribution of local and Atlantic sources.

The mechanism behind the Atlantic source of moisture is not related to storm track activity, but to a weakened Azores High and slightly higher surface pressure over North Africa. The resulting anomalous circulation patterns generate enhanced Atlantic moisture transport towards the Mediterranean. Our combined climate and moisture tracking modelling approach thus provides an alternative mechanism for Atlantic sources of orbitally-paced Mediterranean precipitation changes.

The results of this study have been published in:

Bosmans, J. H. C., van der Ent, R. J., Haarsma, R. J., Drijfhout, S. S. and Hilgen, F. J.: Identifying sources of changed precipitation in paleoclimate studies through moisture tracking: A case study for orbital extremes over the Mediterranean Sea, Paleoceanogr. Paleoclimatology, accepted, doi:10.1029/2019PA003655, 2020.

The atmospheric moisture tracking through WAM-2layers revealed concrete information about the evaporative sources of enhanced/reduced precipitation. This method has not been previously applied in paleoclimate studies, but thus proved to be a powerful tool in attributing reasons for precipitation changes in addition to climate model experiments and classical meteorological analyses. New ideas for collaborations to apply this method in other (paleo)climate studies are welcome.

How to cite: van der Ent, R., Bosmans, J., Haarsma, R., Drijfhout, S., and Hilgen, F.: Identifying sources of changed precipitation in paleoclimate studies through moisture tracking: A case study for orbital extremes over the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2961, https://doi.org/10.5194/egusphere-egu2020-2961, 2020.

The ocean is the largest heat capacitor of the earth climate system and a main source of atmospheric moist static energy. Especially, upper ocean heat content changes in the tropics can be taken as the heat engine of global climate. Here we provide an orbital scale perspective on changes in OHC obtained from a transient simulation of the Community Earth System Model under orbital insolation and GHG forcings. Considering the vertical stratification of the upper ocean, we calculate OHC for the mixed layer and the upper thermocline layer according to the isotherm depths of 26℃ and 20℃ respectively. Generally, our simulated OHC are dominated by thickness changes rather than temperature changes of each layer. In details, there are three situations according to different forcings:

(1) Higher GHG induces positive mixed layer OHC anomalies inside the western Pacific warm pool but with neglected anomalies outside it. For the upper thermocline layer, there are negative OHC anomalies inside the warm pool and positive anomalies in the subtropical Pacific of two hemispheres. For the total OHC above 20℃ isotherm depth, positive anomalies mainly come from the mixed layer between 15ºS-15ºN and from the thermocline between 15º-30º. Lower obliquity induces similar spatial patterns of OHC anomalies as those of higher GHG, but total OHC anomalies are more contributed by upper thermocline anomalies.

(2) Lower precession results in positive mixed layer OHC anomalies in the core of warm pool (150ºE-150ºW, 20ºS-10ºN) and the subtropical northeastern Pacific, but with negative anomalies in other regions of the tropical Pacific. Upper thermocline layer OHC anomalies have similar patterns but with opposite signs relative to the mixed layer in regions between 15ºN-30ºS. As a combination, positive total OHC anomalies occupy large areas of 130ºE-120ºW from 30ºS to10ºN, while negative anomalies dominate the subtropical north Pacific, the western and eastern ends of the tropical Pacific.

If confirmed by paleoceanographic proxies, our simulated OHC results can be served as the first guide map of anomalous energetic storage & flows in the earth climate system under orbital forcings.

How to cite: Wang, Y., Jian, Z., Dang, H., Liu, Z., Jin, H., Zhang, S., Luo, L., and Wang, X.: Upper ocean heat content (OHC) changes in the tropical Pacific induced by orbital insolation and greenhouse gases (GHG), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6318, https://doi.org/10.5194/egusphere-egu2020-6318, 2020.

The climate's mean state reflects only part of the changing climate and how it affects everyday lives. Understanding the climate's variability is crucial to provide more reliable simulations and projections, but temporal and spatial variability patterns and how they are related to changes in the mean state remain unclear. Here, we examine changes in variability since the Last Glacial in response to the warming of the global climate by several degrees. The analysis uses simulations from climate models of different complexity: a two-dimensional energy balance model (TransEBM), an earth system model of intermediate complexity (LoveClim), and a general circulation model (HadCM3). We analyse the simulated variability with respect to the different processes and parameterizations included in the different models and compare the temporal and spatial patterns that emerge. Commonalities as well as differences between models and how they relate to the changing mean state show that fast, low complexity models can capture a range of features of a climate variable's development, but also where such reduced descriptions fall short. As such, the results offer implications for the complexity that is needed and sufficient in parameterizations of climatic processes. Furthermore, we envisage that a comparison to paleoclimate archives can provide limits on the temporal and spatial scales that dominate the variability of climate.

How to cite: Ziegler, E. and Rehfeld, K.: Evaluation of simulated climate variability since the Last Glacial using climate models of varying complexity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8615, https://doi.org/10.5194/egusphere-egu2020-8615, 2020.

Periodic bottom water oxygen deficiency in the Mediterranean Sea has led to the deposition of organic rich sediments during geological history, so called sapropels. Although a mechanism linking the formation of these deposits to orbital variability has been derived from the geological record, physics-based proof is limited to snapshot and short time-slice experiments with (Oceanic) General Circulation Models. Specifically, previous modelling studies have investigated atmospheric and oceanographic equilibrium states during orbital extremes (minimum and maximum precession).

In contrast, we use a conceptual box model that allows us to focus on the transient response of the Mediterranean Sea to orbital forcing and investigate the physical processes causing sapropel formation. The model is constrained by present day measurement data, while proxy data offers constraints on the timing of sapropels.

The results demonstrate that it is possible to describe the first order aspects of sapropel formation in a conceptual box model. A systematic model analysis approach provides new insights on features observed in the geological record, such as timing of sapropels, intra-sapropel intensity variations and interruptions. Moreover, given a scenario constrained by geological data, the model allows us to study the transient response of variables and processes that cannot be observed in the geological record. The results suggest that atmospheric temperature variability plays a key role in sapropel formation, and that the timing of the midpoint of a sapropel can shift significantly with a minor change in forcing due to nonlinearities in the system.

How to cite: Dirksen, J. P. and Meijer, P.: The mechanism of sapropel formation in the Mediterranean Sea: Insight from long duration box-model experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8717, https://doi.org/10.5194/egusphere-egu2020-8717, 2020.

Variations of the Earth’s orbital parameters are known to pace the ice volume variations of the last million year [1], even if the precise mechanisms remain unknown.
Several conceptual models have been used to try to better understand the connection between ice-sheet changes and the astronomical forcing. An often overlooked question is to decide which astronomical forcing can best explain the observed cycles.

A rather traditional practice was to use the insolation at a some specific day of the year, for instance at mid-july [2] or at the june solstice [3].
But it was also suggested that the integrated forcing above some given threshold could be a better alternative [4]. In a more recent paper, Tzedakis et al. [5] have shown that simple rules, based on the original Milankovitch forcing or caloric seasons, could also be used to explain the timing of ice ages.
Here we adapt and simplify the conceptual model of Parrenin and Paillard 2003 [6], to first reduce the set of parameters.
Like in the original conceptual model from [6], this simplified conceptual model is based on climate oscillations between two states: glaciation and deglaciation. It switches to one another when crossing a defined threshold. While the triggering of glaciations is only triggered by orbital parameters, the triggering of deglaciations is triggered by a combination of orbital parameters and ice volume.
Then, we apply the different possible forcings listed above and we try to adapt the model parameters to reproduce the ice volume record, at least in a qualitative way. This allows us to discuss which kind of astronomical forcing better explains the Quaternary ice ages, in the context of such simple threshold-based models.

[1] Variations in the Earth's Orbit: Pacemaker of the Ice Ages, Hays et al., 1976, Science


[2] Modeling the Climatic Response to Orbital Variations, Imbrie and Imbrie, 1980, Science


[3] The timing of Pleistocene glaciations from a simple multiple-state climate model, Paillard, 1998, Nature

[4] Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing, Huybers et al., 2006, Science

[5] A simple rule to determine which insolation cycles lead to interglacials, Tzedakis et al., 2017, Nature

[6] Amplitude and phase of glacial cycles from a conceptual model, Parrenin Paillard, 2003, EPSL.

How to cite: Leloup, G. and Paillard, D.: On the astronomical forcing of simple conceptual ice age models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10137, https://doi.org/10.5194/egusphere-egu2020-10137, 2020.

Cylcostratigraphy is used to investigate quasi-cyclic patterns in sediments. It often provides insight about time and climate. While most studies utilize proxies related to precipitation and temperature, reconstruction of wind and flow directions is more challenging. Due to this, the dynamic change of atmospheric circulations from geophysical data is not well established on orbital timescales. One key method for this purpose is the assessment of the anisotropy of the magnetic susceptibility. Nevertheless, the so derived data are of volume-integrated nature, i.e. a result of the combined mineral composition and structure of the entire investigated sample material. Accordingly, it would be most favorable to link and assess the volume integrated data with spatial sample features. X-ray micro computed imaging enables extensive and non-destructive sample material characterization in three dimensions, with special regards to mineralogical, textural, geometrical and topological material features. By combining volume specific magnetic anisotropy data with state of the art X-ray micro CT imaging data sets, we can derive spatially resolved information about (e.g.) grain sizes, grain shapes, sorting, layering patterns, preferential grain / pore/ layer orientations, secondary precipitates, pore sizes, pore shapes and many other parameters. With this, we greatly increase our understanding about the ancient depositional environment, which is important for investigating and characterizing the dynamic and quasi-cyclic wind and flow fields.

How to cite: Halisch, M., Zeeden, C., and Rolf, C.: Towards image based assessment and characterization of cyclic paleo-wind and flow fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19923, https://doi.org/10.5194/egusphere-egu2020-19923, 2020.

Lake Towuti is a tectonic lake on central Sulawesi, Indonesia. It is located within the Indo Pacific Warm Pool, a convection cell which has major impact on tropical climate and the ability to project its influence on a global scale (Chiang, 2009; De Deckker, 2016). Pre-site surveys using seismic methods and piston cores indicated that sediments in Lake Towuti provide best conditions to obtain a long-term paleoclimate record in this key region (Russel et al., 2014).  

During an ICDP-project in 2015, downhole logging equipment of the Leibniz Institute for Applied Geophysics was used at two drill-sites to record a series of chemical and physical parameters (spectral gamma ray, magnetic susceptibility, resistivity, sonic velocity, dipmeter, ultrasonic imaging of the borehole wall). Continuous lithological logs based on downhole logging data were constructed using cluster analysis. Although the spatial resolution of constructed logs is not as detailed as core descriptions, good correlation to core descriptions and differentiation between the upper lacustrine facies and the lower pre-lacustrine facies (Russell et al., 2016) show that cluster analysis is a powerful tool in giving an instant overview of in situ sediments and determining their physical properties.

Cyclostratigraphic methods in downhole logging can help developing a better understanding of sedimentation rates and thus improving age-depth models for lacustrine sediments (Molinie and Ogg, 1990; Hinnov, 2013; Baumgarten et al., 2015). In case of Lake Towuti, a magnetic susceptibility log from the upper lacustrine facies (0-98 meters below lake floor) was analysed to calculate changes in sediment influx. A careful pre-processing of the data is crucial to secure undisturbed amplitude spectra. This includes the identification and exclusion of event-layers (tephra and turbidite-like mass movement deposits) from the log. Also side effects of those layers to surrounding sediments were diminished from the record.

Sedimentation rates for certain parts were calculated and complement the preliminarily age model derived from ¹⁴C- (Russel et al., 2014) and tephra-dating (A. Deino, personal communication, December, 2018). Further refining of the model and omission of an interpretation of long cyclicities results in the most detailed age-depth model for Lake Towuti, and thus is a fundamental step towards our understanding of paleoclimate processes in this region.

How to cite: Ulfers, A., Hesse, K., and Wonik, T.: Paleoenvironmental indications and cyclostratigraphic studies of sediments from tropical Lake Towuti obtained from downhole logging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21806, https://doi.org/10.5194/egusphere-egu2020-21806, 2020.

The Milutin Milankovic Astronomical Model of Ice Ages revisited

As per the M.M.-Model, the 3 combined precessional effects have a cycle of ca. 21,000 years;

the cycle of the axis tilting from 21.8⁰ to 24.4⁰ runs ca. 41,000 years; the cycle from circle to ellipse, back to circle spans ca. 90,000 - 120,000 years. Science predicts the inception of a new ice age, fearing that the period the system achieved its best parameters is already behind us.

However, the data from other sources, from Plato and the Greek Sibyls to the New Testament and beyond, predict the imminence of a Golden Age, with optimal weather patterns, following a prophesied earthshaking and a few other astrophysical and geophysical woes. This is most consistent with what Milankovic’s true parameters would predict, once certain hidden variables are dealt with.

Besides the pull of sun, moon and planets, affecting the motions of the earth and insolation levels on a regular basis of solar system motions, we must factor in periodic entries of special "controller" comets whose purpose is to exercise potent "sucking" power, which helps re-calibrate the motions of the earth. Such comets do not cause impacts, but earth-shaking all the same, due to the reaction of the earth to such potent attraction. We have evidence of many comets entering the system (the "myths", Plato's Timaeus, Critias, Politicus, etc.) and geological evidence of the effects of such cometary activity.

Depending on the comet's size and its motion parameters, we get ekpyrosis and cataclysm, at global levels. Plate tectonic activity due to a major earthshaking `fatal attraction' will most definitely influence the axis' obliquity, once the `dust is settled'. If at the same time we get a minor impact that generates Flooding and enhanced volcanic activity, the results are more pronounced.

What is the periodicity of such "controller" comets which enter the inner solar system and change so drastically our motion parameters? The notion of `aeon' as per Heraclitus of Ephesus deserves our attention.

The Heraclitus aeon is a period of 10,800 years. Besides being twenty times the `age of Phoenix' calculated at 540 years, the aeon of 10,800 years is the most accurate unit for measuring the cycles of the Milankovic model. In fact,

10,800 x 2 = 21,600 our best approximation to the combined effects of all three precessional cycles.

10,800 x 4 = 43,200 (the cycle of axis tilting)

10,800 x 10 =108,000 the best calibration, so far, of the "eniautos" from circle to ellipse and back.

It is no `co-incidence' that makes the numbers fit so neatly. Not to mention the `ancient myths' which loved periods of 540 years; or 432,000; the combined effects of 25,920 and 108,000 etc.

Very soon we shall witness such a`controller comet', making the year 360 days long. It will provide the parameters for a new Golden Age for the survivors of the Floods and the Ekpyrosis. Golden Ages and years of 360 days with enhanced insolation come at a high price in the Drama Of Evolution.

How to cite: Otto, H.: The Milutin Milancovic Astronomical Model of Ice Ages Revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22130, https://doi.org/10.5194/egusphere-egu2020-22130, 2020.