CL1.9

Glacial/Interglacial variability over the last 1.5 Myr.

Over the last 1.5 Myr the rhythm of glaciations changed from a 40 kyr world to a 100 kyr world, known as the Mid-Pleistocene Transition (MPT). This transition does not follow directly from Milankovitch theory. Against the background of the start of a new deep drilling project in Antarctica (Beyond-Epica) covering this period in a few years from now, we encourage the broader paleo community to show their latest results on the mechanisms underlying the transition. We invite presentations on changes in proxies over this time as well as model studies providing insight in the processes and drivers of the Earth climate system over the MPT.

Convener: Roderik van de Wal | Co-conveners: Margareta Hansson, Eric Wolff
vPICO presentations
| Wed, 28 Apr, 11:45–12:30 (CEST)

Session assets

Session materials

vPICO presentations: Wed, 28 Apr

Chairpersons: Margareta Hansson, Eric Wolff, Roderik van de Wal
11:45–11:50
11:50–11:52
|
EGU21-7850
|
ECS
Antoine Grisart, amaelle landais, barbara stenni, ilaria crotti, valérie masson delmotte, jean jouzel, fredéric prié, roxanne jacob, and elise fourré

The EPICA Dome C (EDC) ice core has been drilled from 1996 to 2004. Its study revealed a unique 800 ka long continuous climatic record including 9 deglaciations. Ice cores contain numerous proxies in the ice and in the air trapped in bubbles (chronological constraints, greenhouse gases concentration, local temperature proxies, mid to low latitude climate proxies). Here, we focus on information provided by the isotopic (and elemental) composition of water and oxygen archived in both ice and gas matrix. On one hand, the water isotopic composition brings information on past temperatures and water cycle re-organizations:   d18O or dD records past temperature, whereas the combination of d18O with dD or d17O provide information on the past water cycle organization through d-excess and 17O-excess linked to climatic conditions of the evaporative regions. On the other hand, the elemental composition of oxygen expressed in the O2/N2 ratio provides key information for orbital dating over the last 800 ka in complement with the isotopic composition of atmospheric oxygen (d18O of O2 or d18Oatm) which is related as well to the low latitude water cycle.

In this study, we present new high resolution records of water isotopes (d18O, d-excess and 17O-excess) as well as high resolution measurements of O2/N2 and d18Oatm over the last 9 deglaciations on the EDC ice core. We first use the high resolution records of O2/N2 and d18Oatm to improve absolute dating constrain over the glacial terminations and discuss the link between orbital forcing and climate variations recorded in the EDC ice core. In a second part, we use d-excess, 17O-excess and d18Oatm to constrain the relative chronology of high vs low latitude climatic events at sub-millennial scale over past deglaciations.

How to cite: Grisart, A., landais, A., stenni, B., crotti, I., masson delmotte, V., jouzel, J., prié, F., jacob, R., and fourré, E.: Sequence of events at high resolution during deglaciations over the last 800ka from the EDC ice core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7850, https://doi.org/10.5194/egusphere-egu21-7850, 2021.

11:52–11:54
|
EGU21-7899
|
ECS
Marion Peral, Thibaut Caley, Bruno Malaizé, Erin McClymont, Thomas Extier, Gulay Isguder, Dominique Blamart, Franck Bassinot, and Mathieu Daeron

The Mid-Pleistocene transition (MPT) took place between 1,200 Ma and 800 ka (still debated). During this transition, the Earth’s orbitally paced ice age cycles intensified, lengthened from ∼40 000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical while Earth’s orbital variations remained unchanged. Although orbital variations constitute the first order forcing on glacial-interglacial oscillations of the late Quaternary, they cannot explain alone the shifts in climatic periodicity and amplitude observed during the MPT. In order to explain the MPT, long-term evolution of internal mechanisms and feedbacks have been called upon, in relation with the global cooling trend initiated during the Cenozoic, the expansion of Antarctic and Greenland Ice Sheet and/or the long-term decline in greenhouse gases (particularly CO2). A key point is therefore to accurately reconstruction of oceanic temperatures to decipher the processes driving climate variations.

In the present work, we studied the marine sediment core MD96-2048 taken from south Indian Ocean (26*10’482’’ S, 34*01’148’’ E) in the region of the Agulhas current. We compared 5 paleothermometers: alkenone, TEX86, foraminiferal- transfer function, Mg/Ca and clumped isotope. Among these approaches, carbonate clumped-isotope thermometry (∆47) only depends on crystallization temperature, and the ∆47 relationship with planktonic foraminifer calcification temperature is well defined. Since Mg/Ca is not only controlled by temperature but is also affected by salinity and pH. The classical d18O in planktic is dependent on SST and d18Osw, which is regionally correlated with the salinity in the present-day ocean. Assuming that the present-day d18Osw-salinity relation was the same during the MPT, we are able to separate changes in d18Osw from temperature effects and reconstruct past salinity. Combining d18O, Mg/Ca and ∆47 on planktonic foraminifera allow in theory to reconstruct SST, SSS and pH.

Here, we measured d18O, Mg/Ca and ∆47 on the shallow-dwelling planktonic species Globigerinioides ruber ss. at the maximal of glacial and interglacial periods over the last 1.2 Ma. Our set of data makes it possible to estimate the long-term evolution of SST, salinity and pH (and thus have an insight into the atmospheric CO2 concentration) across the MPT. Frist, strong differences are observed between the 5 derived-SST: the alkenone and TEX86 recorded the higher temperatures than the other SST proxies. Alkenone derived-SST do not show glacial-interglacial variations within the MPT. The Mg/Ca and transfer function derived-SST show a good agreement each other, while the clumped-isotope derived-SST are systematically colder than the other derived-SST. Then, our ∆47-SST, salinity and pH results clearly show that amplitude of glacial-interglacial variations was insignificant between 1.2 and 0.8 Ma (within the MPT) and increased after the MPT. Finally, we also discussed the potential to use this unique combination of proxies to reconstruct changes of atmospheric CO2 concentration.

How to cite: Peral, M., Caley, T., Malaizé, B., McClymont, E., Extier, T., Isguder, G., Blamart, D., Bassinot, F., and Daeron, M.: Sea surface temperatures, salinity and pH reconstructions over the last 1,2 Ma in South Indian Ocean using the unique combination of Mg/Ca, d18O and ∆47 in planktonic foraminifera, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7899, https://doi.org/10.5194/egusphere-egu21-7899, 2021.

11:54–11:56
|
EGU21-12463
|
ECS
Jonathan Lambert, Kelly Gibson, Braddock Linsley, Samantha Bova, Yair Rosenthal, and Mina Surprenant

Pacific-wide measurements of nitrate and its isotopic composition have furthered our understanding of modern subsurface circulation and have revealed basin-scale connections between oceanographic and nitrogen cycle processes. From the Eastern Tropical Pacific (ETP), the isotopic signature of denitrification is spread zonally and meridionally via subsurface currents. From the Pacific sector of the Southern Ocean, Subantarctic Mode Water (SAMW) penetrates to the low latitudes, delivering nitrate (and likely its isotopic signature) to equatorial surface waters via upwelling. These two regional processes combine to inform much of the thermocline nitrogen dynamics of the Pacific. Here, we compare a new 1.4-Myr bulk sediment

How to cite: Lambert, J., Gibson, K., Linsley, B., Bova, S., Rosenthal, Y., and Surprenant, M.: Equatorial Pacific bulk 𝛿15N supports a secular increase in subantarctic zone nitrate utilization after the mid-Pleistocene Transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12463, https://doi.org/10.5194/egusphere-egu21-12463, 2021.

11:56–11:58
|
EGU21-3335
|
ECS
Parker Liautaud and Peter Huybers

Foregoing studies have found that sea-level transitioned to becoming approximately twice as sensitive to CO2 radiative forcing between the early and late Pleistocene (Chalk et al., 2017; Dyez et al., 2018). In this study we analyze the relationships among sea-level, orbital variations, and CO2 observations in a time-dependent, zonally-averaged energy balance model having a simple ice sheet. Probability distributions for model parameters are inferred using a hierarchical Bayesian method representing model and data uncertainties, including those arising from uncertain geological age models. We find that well-established nonlinearities in the climate system can explain sea-level becoming 2.5x (2.1x - 4.5x) more sensitive to radiative forcing between 2 and 0 Ma. Denial-of-mechanism experiments show that the increase in sensitivity is diminished by 36% (31% - 39%) if omitting geometric effects associated with thickening of a larger ice sheet, by 81% (73% - 92%) if omitting the ice-albedo feedback, and by more than 96% (93% - 98%) if omitting both. We also show that prescribing a fixed sea-level age model leads to different inferences of ice-sheet dimension, planetary albedo, and lags in the response to radiative forcing than if using a more complete approach in which sea-level ages are jointly inferred with model physics. Consistency of the model ice-sheet with geologic constraints on the southern terminus of the Laurentide ice sheet can be obtained by prescribing lower basal shear stress during the early Pleistocene, but such more-expansive ice sheets imply lower CO2 levels than would an ice-sheet having the same aspect ratio as in the late Pleistocene, exacerbating disagreements with 𝛿11B-derived CO2 estimates. These results raise a number of possibilities, including that (1) geologic evidence for expansive early-Pleistocene ice sheets represents only intermittent and spatially-limited ice-margin advances, (2) 𝛿11B-derived CO2 reconstructions are biased high, or (3) that another component of the global energy balance system, such as the average ice albedo or a process not included in our model, also changed through the middle Pleistocene. Future work will seek to better constrain early-Pleistocene CO2 levels by way of a more complete incorporation of proxy uncertainties and biases into the Bayesian analysis.

How to cite: Liautaud, P. and Huybers, P.: Bayesian analysis of sea-level sensitivity to CO2 forcing across the mid-Pleistocene transition: possible implications for early-Pleistocene ice-sheet extent, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3335, https://doi.org/10.5194/egusphere-egu21-3335, 2021.

11:58–12:00
|
EGU21-3630
Nicholas Golledge

During the Pleistocene (approximately 2.6 Ma to present) glacial to interglacial climate variability evolved from dominantly 40 kyr cyclicity (Early Pleistocene) to 100 kyr cyclicity (Late Pleistocene to present). Three aspects of this period remain poorly understood: Why did the dominant frequency of climate oscillation change, given that no major changes in orbital forcing occurred? Why are the longer glacial cycles of the Late Pleistocene characterised by a more asymmetric form with abrupt terminations? And how can the Late Pleistocene climate be controlled by 100 kyr cyclicity when astronomical forcings of this frequency are so much weaker than those operating on shorter periods? Here we show that the decreasing frequency and increasing asymmetry that characterise Late Pleistocene ice age cycles both emerge naturally in dynamical systems in response to increasing system complexity, with collapse events (terminations) occuring only once a critical state has been reached. Using insights from network theory we propose that evolution to a state of criticality involves progressive coupling between climate system 'nodes', which ultimately allows any component of the climate system to trigger a globally synchronous termination. We propose that the climate state is synchronised at the 100 kyr frequency, rather than at shorter periods, because eccentricity-driven insolation variability controls mean temperature change globally, whereas shorter-period astronomical forcings only affect the spatial pattern of thermal forcing and thus do not favour global synchronisation. This dynamical systems framework extends and complements existing theories by accomodating the differing mechanistic interpretations of previous studies without conflict.

How to cite: Golledge, N.: Emergence of critical climate states during the Pleistocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3630, https://doi.org/10.5194/egusphere-egu21-3630, 2021.

12:00–12:02
|
EGU21-14370
Kerim Nisancioglu, Anne Morée, Tianyi Sun, Eivind Straume, and Geoffrey Gebbie

The dominant pacing of glacial‐interglacial cycles in deep‐ocean δ18O records changed substantially during the Mid‐Pleistocene Transition. The precessional cycle (∼23 ky) is absent during the Early Pleistocene, which we show can be explained by cancellation of the hemispherically anti‐phased precessional cycle in the Early Pleistocene interior ocean. Such cancellation develops due to mixing of North Atlantic and Southern Ocean δ18O signals at depth, and shows characteristic spatial patterns. We explore the cancellation potential for different North Atlantic and Southern Ocean deep‐water source δ18O values using a tracer transport ocean model. Cancellation of precession occurs for all signal strengths and is widespread for a signal strength typical for the Early Pleistocene. Early Pleistocene precessional power is therefore likely incompletely archived in deep‐sea δ18O records, concealing the true periodicity of the glacial cycles in the two hemispheres.

How to cite: Nisancioglu, K., Morée, A., Sun, T., Straume, E., and Gebbie, G.: Cancellation of the precessional cycle in δ18O records during the Early Pleistocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14370, https://doi.org/10.5194/egusphere-egu21-14370, 2021.

12:02–12:04
|
EGU21-13787
|
ECS
Matthew Drew and Lev Tarasov

Is the regolith hypothesis consistent with the physics of glacial removal of mechanically weak surface material? 

 

The  mid-Pleistocene transition (MPT) from small 40 kyr glacial cycles to large, abruptly terminating 100 kyr ones represents a major climate system reorganization for which a clear understanding is lacking. A leading mechanism for this transition is a stabilization of ice sheets due to a shift to higher friction substrate. The Pleistocene saw the removal of deformable regolith -- laying bare hard higher-friction bedrock that would help preserve regional ice during warm interstadials. This is the regolith hypothesis. 

 

The removal of regolith by Pleistocene ice sheets remains poorly constrained. To date, only models with a forced change in area of regolith cover or 1D flow line models with simplistic sediment transport have been used to probe the role of regolith in the MPT. It is therefore unclear if the appropriate amount of regolith removal can occur within the time-frame of the MPT.

 

To properly test the hypothesis, at least three components are required: capable model, observational constraint, and a probe of uncertainties. A capable model must explicitly represent relevant processes in a fully coupled self-consistent manner. We have therefore configured a state of the art 3D glacial systems model (GSM). The GSM incorporates a state-of-the-art fully coupled sediment production/transport model, subglacial hydrology, visco-elastic glacial isostatic adjustment, 3D thermomechanically coupled hybrid shallow ice/shallow shelf ice dynamics, and internal climate solution from an energy balance model. The model generates sediment by quarrying and abrasion, and both subglacial and englacial sediment transport. The subglacial hydrology model employs a linked-cavity system with a flux based switch to tunnel drainage, giving dynamic effective pressure needed for realistic sediment and sliding processes. The coupled model is driven only by prescribed atmospheric CO2 and orbitally derived insolation.

 

The required observational constraints include present-day regolith distribution and inferred Pleistocene ice volume from proxy records.

 

The final component is  a large ensemble of full Pleistocene simulations that probe both initial regolith distribution uncertainties and model parametric uncertainties. We present the results of such an ensemble, examining both rates of computed regolith removal and changes in ice volume cycling across the MPT interval.

How to cite: Drew, M. and Tarasov, L.: Self-consistency of the regolith hypothesis for the mid-Pleistocene Transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13787, https://doi.org/10.5194/egusphere-egu21-13787, 2021.

12:04–12:06
|
EGU21-13981
Peter U. Clark, Jeremy Shakun, Yair Rosenthal, Peter Köhler, Dan Schrag, Dave Pollard, Zhengyu Liu, and Pat Bartlein

The Middle Pleistocene Transition (MPT) has been characterized as the transition in temperature and sea level from low-amplitude, 41-kyr variability to high-amplitude, quasi-100-kyr variability in the absence of any orbital forcing between 1.2 and 0.7 Ma. The regolith hypothesis is one of a class of hypotheses developed to explain the MPT in sea level, which has been largely inferred from d18Obenthic records. Here we use a global array of 130 sea-surface temperature (SST) records based on Mg/Ca, alkenone, and faunal proxies to reconstruct global and regional SST change over the last 4.5 Myr. Average global temperature cooled by ~6.5oC since ~3.5 Ma, with the MPT represented by a significant increase in the rate of cooling between ~1.4 and 0.8 Ma, and a change from dominant 41-kyr to dominant quasi-100-kyr frequencies at ~1.2 Ma that are well correlated with CO2 over the last 800 ka (r2=0.6). Temperature terminations after 1.2 Ma correspond to skipped obliquity beats and, for the last 800 ka, large increases in CO2. We use our global SST reconstruction to remove the temperature signal from the Ahn17 d18Obenthic stack to derive d18Oseawater. Accounting for the influence of changing temperature on the isotopic composition of ice sheets, we use the d18Oseawater record to reconstruct global sea level for the last 4.5 Myr. These results suggest sea-level minima equivalent to or lower than the LGM sea-level low stand (130 m) throughout the Pleistocene. Since inception of Northern Hemisphere glaciation ~3 Ma, sea level varied linearly with obliquity until ~1.2 Ma, when sea-level began to vary nonlinearly with obliquity, with the largest terminations occurring at the same time as temperature terminations that correspond to increasing obliquity and CO2. These results suggest that the MPT is largely a temperature phenomenon likely associated with CO2. The regolith hypothesis other hypotheses developed to explain a transition from low- to high-amplitude sea level variability during the MPT are no longer required, with the MPT change in sea-level response to obliquity likely due to modulation by CO2.

How to cite: Clark, P. U., Shakun, J., Rosenthal, Y., Köhler, P., Schrag, D., Pollard, D., Liu, Z., and Bartlein, P.: Requiem for the Regolith Hypothesis: Sea-Level and Temperature Reconstructions Provide a New Template for the Middle Pleistocene Transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13981, https://doi.org/10.5194/egusphere-egu21-13981, 2021.

12:06–12:30