The Milankovitch Theory of orbital climate change postulates that changes in the caloric summer half-year insolation (or Northern Hemisphere summer insolation (NHSI) at ~65°N latitude) drive changes in the ice-sheets extent (i.e., global ice-volume) at Earth’s orbital periods (i.e., the sensu-stricto theory). These insolation-driven changes in turn, incite ancillary changes in other parts of the global climate systems via various forcing and feedback mechanisms (the sensu-lato hypothesis). In this theoretical framework the high-latitude glaciation processes took the center stage while the low-latitude global monsoon was essentially excluded. In the last two decades, large numbers of cave d¹⁸O records with precise radiometric chronologies have propelled speleothems to the forefront of paleoclimatology. Of particular interest are the speleothem records from North America that reveal a persistent orbital pacing of the North American climate at the precession band, which is nearly in phase with changes in the global ice-volume and atmospheric CO₂ but lags June insolation at 65°N by ~5000 years, in accordance with the sensu-stricto Milankovitch theory. Contrastingly, the low-latitude tropical speleothem records manifest an orbital-scale pattern of global monsoon, which is dominated by precession cycles with a nearly anti-phased relation between the two hemispheres. Importantly, the monsoon variations track summer (July/January) insolation without significant lags at the precession band. We thus suggest that precession-induced changes in summer insolation produce distinct climate variability in the ice-sheet proximal and tropical regions predominantly via the (delayed) ice-volume/CO₂ forcing/feedbacks and nearly-in-phase monsoon/CH₄ responses/feedbacks.

  As for global-scale millennial events that were superimposed on orbital-scale climate variations, the essence of these events—i.e., conventional ice age terminations and other smaller events (the so-called ‘low-amplitude versions of terminations’), is virtually similar. The time-series of millennial-scale variations after removing orbital insolation signals from the speleothem monsoon record and long-term trend in the Antarctic ice core temperature (δD) record characterize the millennial climate variances of both ice age termination and low-amplitude versions of termination events. Remarkably, the millennial-scale variations show significant obliquity and precession cycles that are in-phase with North Hemisphere June insolation, implying a critical role of changes in orbital insolation in triggering the ice age terminations. These observations, in turn, provide new insights into the classic ‘100 ka problem’.

  Indeed, a more comprehensive picture of orbital theory of climate is steadily emerging with the growth of new geological proxy data, particularly the low-latitude speleothem data from the vast global monsoon regime, providing critical complements to marine and ice-core data.

How to cite: Cheng, H.: Milankovitch Theory and Global Monsoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1422, https://doi.org/10.5194/egusphere-egu22-1422, 2022.

The mechanism of stochastic resonance (SR) in a bistable system was introduced [1] to explain the glacial-interglacial cycles in the Quaternary and is still regarded as a dynamical systems paradigm for those climate cycles. In the SR the stochastic forcing must satisfy a rather stringent condition; besides, glacial inceptions occur abruptly, as well as the glacial terminations. However, these conditions do not seem to be verified in the real climate system. Here it is shown that the alternative dynamical paradigm -that may be termed deterministic excitation (DE)- in which relaxation oscillations (ROs) are excited by the astronomical forcing in a purely deterministic framework, overcomes those limitations and may therefore provide a more plausible theoretical basis for the explanation of the glacial-interglacial variability.

In an excitable dynamical system a RO connects a basic state to an unstable excited state, which is then followed by a spontaneous, slow return to the original state. Such transition is self-sustained in a given parameter range of the autonomous system, otherwise it can be excited by an external deterministic time-dependent forcing (DE) or by noise (coherence resonance). Examples of DE in ocean dynamics are presented for the Kuroshio Extension in the North Pacific and for the Antarctic Circumpolar Current in the Southern Ocean.

A 4-dimensional nonlinear excitable spectral model of the wind-driven ocean circulation [2] is then used to briefly illustrate the main aspects of excitable climate dynamics, focusing on the occurrence of coherence resonance [3], on the DE of ROs under the action of an aperiodic forcing [4] and on the tipping points due to parameter drift [5]. Finally, a classical energy balance model is extended to obtain a minimal excitable model of the late Pleistocene ice ages [Pierini, in preparation]. The timing of the interglacials, determined by the DE caused by the variations of the Earth’s orbital eccentricity and axial tilt and precession, is found to be in significant agreement with proxy data. (Support from the IPSODES-P.N.R.A. project is acknowledged)

[1] Benzi R., Parisi G., Sutera A., Vulpiani A., 1982. Tellus 34, 10-16.

[2] Pierini S., 2011. J. Phys. Oceanogr. 41, 1585-1604.

[3] Pierini S., 2012. Phys. Rev. E 85, 027101.

[4] Pierini S., Ghil M., Chekroun M.D., 2016. J. Climate 29, 4185-4202.

[5] Pierini S., Ghil M., 2021. Sci. Rep. 11, 11126.

How to cite: Pierini, S.: On the functioning of the glacial-interglacial variability: deterministic excitation vs. stochastic resonance , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1039, https://doi.org/10.5194/egusphere-egu22-1039, 2022.

The stratigraphic records of shallow-marine environments are not commonly regarded as excellent climate archives because of their presumed temporal incompleteness. However, a recent study of lower Pleistocene strata in the Western Foreland Basin, Taiwan, reveals high-resolution records of past climate oscillations preserved within shallow-marine strata. Deriving such narratives is made possible because of the high accommodation and sedimentation rates in the basin, which enhanced the completeness of the stratigraphic record.

Here, we astrochronologically tune the Chinshui Shale and the lower part of the Cholan Formation of the Western Foreland Basin from approximately 3.5 to 2 Ma. These strata are calibrated to global deep-sea stable oxygen isotope (δ¹⁸O) records with established time scales detailing global climate change during the studied time period. The Chinshui Shale is mudstone-dominated and was deposited mostly in offshore settings, while the Cholan Formation comprises mainly heterolithic strata deposited in shallower settings (i.e., offshore transition, nearshore) of the paleo-Taiwan Strait. The data used herein are two borehole gamma-ray profiles through the Chinshui Shale and the Cholan Formation that have a proximal-distal relation to Taiwan. High gamma-ray values reflect clay-rich intervals and correlate to lower values of δ¹⁸O in the global reference records. Low gamma-ray values point to sand-rich packages and correlate with higher values of δ¹⁸O.

Preliminary results show that the alternating clay-rich to sand-rich deposits during the late Pliocene to early Pleistocene are orbitally paced. The results allow us to i) tune the upper Pliocene–lower Pleistocene Chinshui Shale and lower part of the Cholan Formation, ii) refine the magneto-biostratigraphic framework established for this time interval in the Western Foreland Basin of Taiwan, and iii) lay the groundwork for connecting climatic changes in Taiwan during this time period to the wider frame of global climate change. 

How to cite: Vaucher, R., Zeeden, C., Hsieh, A., Kaboth-Bahr, S., Lin, A. T., Horng, C.-S., and Dashtgard, S. E.: Towards an astrochronological tuned age model for the upper Pliocene–lower Pleistocene Western Foreland Basin of Taiwan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1435, https://doi.org/10.5194/egusphere-egu22-1435, 2022.

The early Eocene greenhouse climate is characterised by a series of ‘hyperthermal’ events, defined by transient negative excursions in marine carbonate carbon and oxygen isotopes. Proxy records of the larger magnitude hyperthermal events are consistent with massive carbon release to the ocean-atmosphere system and associated with global warming and ocean acidification. Such events therefore represent the best analogues for current anthropogenic climate change. However, the causes and nature of smaller early Eocene hyperthermals, particularly through the early Eocene Climatic Optimum (EECO), are less well understood. We know that hyperthermal events are paced by the 100 kyr (short) and 405 kyr (long) eccentricity cycles, indicating that Earth’s orbital parameters play a key role in driving carbon cycle perturbations, but the precise forcing mechanisms remain unclear. Additionally, few continuous records of the smaller, orbitally-paced hyperthermals exist and there have been no published high-resolution climate records from the Indian Ocean so far from this interval. High-resolution records across the full spectrum of hyperthermal events and from multiple ocean basins are needed to fully identify their cause(s). Here, we constrain the nature and magnitude of environmental change during hyperthermal events O-T in the Indian Ocean using a new, high-resolution benthic stable isotope record from IODP Expedition 369 Site U1514, Indian Ocean, from 50-51 Ma. Using spectral analysis techniques, we identify the dominant periodicities in the benthic stable isotope record and investigate the phasing between stable isotopes and other environmental records from Site U1514, including sedimentary Ca/Fe. We compare the Site U1514 stable isotope record with environmental records across this time interval from other sites to determine the synchronicity of climate and carbon cycle changes between different ocean basins, aiming to further examine the forcing mechanisms of these early Eocene hyperthermal events. 

How to cite: Kirby, N., Batenburg, S., Leng, M., Dunkley Jones, T., and Edgar, K.: Orbital forcing of early Eocene hyperthermal events: A new benthic foraminiferal record from the Indian Ocean, 50-51 Ma, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4667, https://doi.org/10.5194/egusphere-egu22-4667, 2022.

Many paleoclimate records show that the end of interglacials of the late Pleistocene was marked by abrupt cooling events and increased millennial variability. Strong abrupt cooling occurring when climate was still in a warm interglacial condition is puzzling and its cause remains uncertain. In this study, we performed transient climate simulations for all the eleven interglacial (sub)stages of the past 800,000 years with the model LOVECLIM1.3 (Yin et al., 2021). Our results show that there exists a threshold in the astronomically induced insolation below which abrupt changes at the end of interglacials occur. When the summer insolation in the Northern Hemisphere (NH) high latitudes decreases to a critical value, it triggers a strong, abrupt weakening of the Atlantic meridional overturning circulation (AMOC) and a strong cooling in the NH followed by high-amplitude variability. The mechanism involves sea ice feedbacks in the Northern Nordic Sea and the Labrador Sea. Similar abrupt oscillations happen in the simulated temperature, precipitation and vegetation from low to high latitudes. Our simulated results are supported by observations from many marine and terrestrial records, including for example the planktic d18O record from the Iberian Margin, the Greenland ice core record and the Chinese speleothem records. Our study shows that the astronomically-induced slow variation of insolation could trigger abrupt climate changes. The insolation threshold occurred at the end of each interglacial of the past 800,000 years, suggesting its fundamental role in terminating the warm climate conditions of the interglacials. Our results show that the next insolation threshold will occur in 50,000 years, suggesting an exceptionally long interglacial ahead, which is in line with what has been suggested by previous modelling studies. 

Reference:  Yin Q.Z., Wu Z.P., Berger A., Goosse H., Hodell D., 2021. Insolation triggered abrupt weakening of Atlantic circulation at the end of interglacials. Science, 373, 1035-1040, DOI: 10.1126/science.abg1737

How to cite: Yin, Q., Wu, Z., Berger, A., Goosse, H., and Hodell, D.: Astronomical forcing as a trigger of abrupt climate changes at the end of interglacials , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6073, https://doi.org/10.5194/egusphere-egu22-6073, 2022.

Some of the large climatic changes of the past originate in the variations of the Earth’s orbit and of its spin axis resulting from the gravitational pull of the planets and the Moon. These variations can be traced over several millions of years back in time (Ma) in the geological sedimentary records (e.g. Milankovitch cycles). Over the last decades, the Earth’s orbital and spin solutions have been used to establish a geological timescale based on the astronomical solutions. Nevertheless, extending this procedure through the Mesozoic Era (66-252 Ma) and beyond is difficult, as the solar system motion is chaotic. It will thus not be possible to retrieve the precise orbital motion of the planets beyond 60 Ma from their present state.

Astrogeo, a project funded by the European Research Council (ERC), will use the geological record as an input to break the horizon of predictability of 60 Ma resulting from the chaotic motion of the planets. This will be achieved by considering statistical methods and by using ancient geological data as an additional constraint in obtaining astronomical solutions. Astrogeo aims to provide a template orbital solution for the Earth that could be used for paleoclimate studies over any geological time. This will open a new era where the geological records will be used to retrieve the orbital evolution of the solar system. It will thus open a new observational window for retrieving not only the history of the Earth, but of the entire solar system. Here, we want to reach out to the broader cyclostratigraphic community to discuss suitable procedures and data sets to couple both theoretical solutions and geological observations. In particular, we are interested in examining high-quality data sets with clear and well-constrained (single or combined) expressions of the astronomical parameters of eccentricity, precession and obliquity.

How to cite: Sinnesael, M. and Laskar, J.: Integrating astronomical solutions and geological observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6423, https://doi.org/10.5194/egusphere-egu22-6423, 2022.

How the global carbon cycle and climate changes interact on orbital timescales under different boundary conditions remains elusive. Previous studies have found that changes in global ice-sheet volume and marine carbon cycle are synchronized at the eccentricity time scales with a slight lead of climate-cryosphere relative to carbon cycle throughout Oligo-Miocene (~34-6 Ma). Here, we analyze the evolutive phase relationship between benthic foraminiferal oxygen (δ¹⁸O) and carbon isotope (δ¹³C) to reveal an unnoticed phenomenon that variations of oceanic carbon cycle could lead those of global ice-sheet volume on 405-kyr cycle during Miocene Climate Optimum (MCO, ~17-14 Ma), which was a profound warming interval partly ascribed to the carbon emission from the eruption of the Columbia River Basalts Group (CRBG). Eccentricity sensitivity analysis indicate a relatively constant response of ice sheet to orbital forcing during MCO. Combined the results of box model, we propose that volcanic CO₂ input accelerates the response of marine carbon cycle to orbital forcing. The enhanced greenhouses effect probably had strengthened the low-latitude hydrological cycle and chemical weathering and ultimately generated the δ¹³C-lead-δ¹⁸O scenario.

How to cite: Liu, F., Huang, E., Du, J., Ma, W., Ma, X., Lourens, L., and Tian, J.: Perturbations of volcanic CO2 emission to orbital paced climate-carbon cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6429, https://doi.org/10.5194/egusphere-egu22-6429, 2022.

The Silurian (443.8-419.2 million years ago) is a period of important biodiversity changes, dynamic climate change, including strong sea level fluctuations and the development of low-oxygen conditions in the ocean^1-2-3. To date the Silurian lacks in (cyclostratigraphic) age constraints and in understanding in the way astronomical cycles modulate the Silurian climate, which hinders our understanding of Silurian climate dynamics. To assess the role of astronomical cycles in the pacing of the Silurian climate, we study the imprint of astronomical cycles on the record of the Sommerode-1 core from Bornholm, Denmark (53.65-118.66m).The core contains a near continuous Telychian record including the SOCIE and Valgu carbon isotope excursions/events^4-5-6.  The core was scanned at University of Bremen/ MARUM (November 2021) using the Bruker M4 Tornado µXRF scanner, enabling for a high-resolution cyclostratigraphic and chemostratigraphic study of the Telychian.

XRF core measurements provided semi-quantitative element data, spaced at 0.5 mm, were converted into element concentrations (ppm) using a set of reference standards. A Principal Component Analysis simplified the variability in our dataset into 3 components. PC1 has high loadings for Al, Si, K, Ti, Fe and Co, and is interpreted as a detrital component. PC2 has high loadings for Ca and Mn, and is interpreted as an indicator of oxygenation conditions. PC3 has high loadings for S, indicative for the sulphides/dysoxic/anoxic conditions^-8-9.

Peaks for Mn at 69-85m and S at 85-104m, indicate that part of the core (69-85 m) was deposited under oxic conditions while another part of the core (85-104 m) was deposited under anoxic/dysoxic conditions. We note that the transition to oxic conditions at 90 m coincides with the Valgu isotopic event⁴ while the SOCIE⁴ (80-70 m) event occurs during oxic conditions. Spectral analysis (wavelet, MTM and Evolutive Harmonic Analysis (EHA)) on the 3 components reveals the imprints of long and short eccentricity, obliquity and precession. An EHA spectra of the detrital component was used to trace the long eccentricity in the depth domain which was used to infer changes in sedimentation rates. The sedimentation rates are used to convert the record from the depth to time domain. Astronomical cycles filtered from the record in the time domain show that astronomical cycles exert a great control on the depositional record.  Indicating the astronomical cycles modulated the Telychian climate which in term paced oxygenation conditions at the sea-floor.

1.Melchin et al. (2005) The Silurian Period 525–558 –

2. Bond & Grasby (2017) Palaeogeogr., Palaeoclim., Palaeoecol. 478, 3–29. –

3. Saltzman (2005) Geology, 33, 7, 573-576. –

4. Hammarlund et al. (2019) Palaeogeogr., Palaeoclim., Palaeoecol. 526, 126–135. –

5. Schovsbo, et al. (2015). Geological Survey of Denmark and Greenland Bulletin, 33, 9–12.

6. Loydell, D. K., et al. (2017). Bulletin of the Geological Society of Denmark, 65, 135–160.

7. Algeo, T. J., & Maynard, J. B. (2004). Chemical Geology, 206(3–4), 289–318.

8. Ferriday, T., & Montenari, M. (2016). Stratigraphy & Timescales (Vol. 1).

9. Rothwell, R. G., & Croudace, I. W. (2015). Tracking Environmental Change Using Lake Sediments. (Vol. 2)

How to cite: Arts, M., De Vleeschouwer, D., Schovsbo, N. H., Thibault, N., Nielsen, A. T., and Da Silva, A.-C.: Astronomical modulation of oxygenation conditions during the Telychian (Silurian) recorded in the Sommerodde-1 core from Bornholm Denmark., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11342, https://doi.org/10.5194/egusphere-egu22-11342, 2022.

Pleistocene climate is primarily driven by changes of the Earth’s orbital parameters. However, the Mid-Pleistocene Transition (MPT) (~0.8-1.2Myr) which corresponds to a gradual change of interglacial-glacial cyclicity from weak 40kyr climatic cycles to the current strong 100kyr cycles, remains largely unexplained. So far, models only based on orbital forcing were not capable to reproduce this transition, discarding the hypothesis of an orbitally-driven transition. Internal Earth system climate causes were thus explored as primary drivers of the MPT, as a gradual decrease in atmospheric CO2 concentrations or the removal of the regolith beneath the northern hemisphere ice sheets. 
Here we present an improved version of the conceptual model of Parrenin and Paillard (2012) modelling ice volume variations over the past 2Myr. Our model switches between two states, a glaciation state and a deglaciation one, following a threshold mechanism related to the input parameters and the modelled ice volume itself. The modelled ice volume is compared to the ice volume reconstructions inferred from paleodata. 
 We reproduced the MPT using three different models. The “orbital” model which only use orbital forcing parameters as input. The “gradual” model, which is similar to the orbital model plus a continuous drop of a physical parameter in addition to orbital forcing parameters. The “abrupt” model, also similar to the orbital model plus a time-determined abrupt variation of a physical parameter in addition to orbital forcing parameters. 
For the first time, our conceptual model is able to simulate qualitatively the Mid-Pleistocene Transition with only changes in the orbital forcing parameters, reproducing the change in frequency and amplitude of the transition. Moreover, the hypothesis of a coupled influence of orbital forcing and a decreasing deglaciation threshold parameter is by far a better hypothesis than considering an abrupt change regarding our model results. In fact, the “gradual” model contains less parameters and a smaller data-model standard deviation of residuals than the “abrupt” model. Orbital forcing could thus have enabled the Mid-Pleistocene Transition. A combined influence with a decreasing parameter, such atmospheric  CO2 concentration, would have triggered this transition.

References 
Parrenin, F., & Paillard, D. (2012). Terminations VI and VIII (∼ 530 and∼ 720 kyr BP) tell us the importance of obliquity and precession in the triggering of deglaciations. Climate of the Past, 8(6), 2031-2037.

How to cite: Legrain, E., Parrenin, F., and Capron, E.: A new conceptual model to explain the mid-Pleistocene transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12044, https://doi.org/10.5194/egusphere-egu22-12044, 2022.