The Mid-Pleistocene Climatic Transition
3D pseudo-steady age modelling in the Dome C region, Antarctica
Understanding how ice flows in East Antarctica is of uppermost importance to determine the distribution of age with depth for deep ice cores, to determine potential future drilling sites or improve the ice-dynamic context of a chosen site to support the correct interpretation of ice-core climate proxies. One of the most important applications is to produce the depth vs age relationship, hence the first age model, for the Oldest Ice projects, such as the European Beyond EPICA (BE) project and the Australian Million Year Ice Core project (MYIC). Hence, this study will concentrate on the wider Dome C area. However, the methodological results of this project can also be applied to other deep drilling targets, as proposed under the IPICS umbrella (http://pastglobalchanges.org/science/end-aff/ipics/intro).
The approach of this PhD project will be twofold. First, the candidate will develop a 3D numerical model to simulate the age in an ice sheet on a regional scale, using a novel numerical approach. The model will be a so-called pseudo-steady model, where the geometry of the ice sheet and the velocity profiles are steady. Only a temporal factor can be applied to the surface accumulation rate. Then, the candidate will use existing isochronal horizons traced and dated by linking them to the EPICA DC ice core to constrain the 3D numerical model, involving innovative inverse approaches. We will invert for the thickness of a possible layer of stagnant ice, which has been suggested by preliminary studies (Lilien et al., 2021). For a faster inversion of the 3D model, it might be necessary to implement an analytical Jacobian of the forward model.
The outcomes of this PhD project will be: 1) A more reliable model of the age of ice in an ice sheet; 2) an improved estimate of the age and age resolution of basal ice in the Dome C region; 3) an improved mapping of the stagnant layer and basal melting rate at the base of the ice sheet; 4) an improved mapping of the surface accumulation rate in the Dome C region; 5) an improved mapping of the velocity profiles in the ice sheet.
Modelling the past movements of the dome at Dome C, Antarctica
Understanding how ice flows in East Antarctica is of uppermost importance to determine the distribution of age with depth for deep ice cores, to determine potential future drilling sites or improve the ice-dynamic context of a chosen site to support the correct interpretation of ice-core climate proxies. One of the most important application is to produce the depth vs age relationship, hence the first age model, for the Oldest Ice projects, such as the European Beyond EPICA (BE) project and the Australian Million Year Ice Core project (MYIC). Hence, this study will concentrate on the wider Dome C area. However, the methodological results of this project can also be applied to other deep drilling targets, as proposed under the IPICS umbrella (http://pastglobalchanges.org/science/end-aff/ipics/intro).
Dome positions and flow lines may have been affected by the evolution of the Antarctic ice sheet over glacial interglacial cycles. Here we will use the GRISLI ice sheet model) to study possible dome migrations in the past and their impact on ice flow lines and ice sheet stratigraphy. GRISLI is a 3D thermomechanical ice sheet model which allows for long (i.e. several glacial-interglacial cycles) transient ice sheet simulations in response to atmospheric, oceanic and sea level forcings. First, we will make use of the existing ensemble of simulations of the last 400 kyr from Quiquet et al. (2018) to assess the occurrence of dome migrations in the past. This ensemble of simulations covers the uncertainty in terms of mechanical parameters in the model for a given climate and sea level forcing. Second, as a complement to these simulations, we will explore the sensitivity of dome positions to past climate uncertainties. While Quiquet et al. (2018) used homogeneous atmospheric and oceanic perturbations in the past, we will test here spatially heterogeneous climatic perturbations. All new simulations will make use of the passive tracer transport model of Clarke and Marshall (2002) embedded in GRISLI (Lhomme et al., 2005) which will provide the modeled ice sheet stratigraphy (age-depth relationship). The coarse spatial resolution of the model (40 km) will not allow us to perform direct data-model comparisons but in turn we will be able to quantify the effect of regional ice sheet geometry changes (e.g. elevation changes and grounding line migration) on ice sheet stratigraphy and ice flow in the vicinity of the domes.
The outcomes of this project will be: 1) an improved reconstruction of the past movements of Dome C and the flow lines in this region; 2) an improved understanding of the relationship between ice sheets boundary conditions and domes movements.
Conceptual models of global climate for the Mid-Pleistocene transition
In the context of the current continuing greenhouse gas emissions in the atmosphere, it is essential to turn to the past to understand how the climate system behaves. A particularly interesting problem is the shift in Earth’s climate response to orbital forcing during the ‘Mid-Pleistocene Transition’ (MPT), around 1 Myr ago, when the dominant glacial/interglacial cyclicity changed from 40 kyr to 100 kyr. Several hypotheses have been formulated to explain this MPT, and one in particular relates to the long-term cycle of CO2 in the atmosphere.
In this PhD project, we will use so-called conceptual models to explain and understand the global climate evolution during the last 2 million years. These models do not have spatial representations derived from the fundamental laws of physics, but only contain a few variables which are supposed to represent the most important mathematical features of the global climate. Several formulations will be tested and the resulting models will be tuned onto observations, such as the global ice volume or atmospheric CO2 concentrations derived from paleoclimatic archives.
The outcomes of this project will be: 1) a better understanding of the main mathematical features of the global climate evolution; 2) a better understanding of the causes of the MPT; 3) a better understanding of the mechanisms responsible for the glacial-interglacial cycles, and in particular the role of atmospheric CO2 variations; 4) a synthetical curve of CO2 variations before the 800,000 years ice core era.