New dating experiment on EPICA Dome C (EDC) ice core over the last 800 kyrs using the Bayesian tool Paleochrono and new records of elemental and isotopic composition in the air trapped in the EDC ice core.

To understand the causal relationship between forcing (orbital parameters, greenhouse gas concentration…) and the climate change, dating climate archives is crucial. Ice cores are unique archives because they provide a direct record of greenhouse gas concentration. However, dating ice cores is particular since they require two chronologies: one for the ice and one for the younger air trapped in bubbles inside the core. The coherent AICC2012 chronology was established for five ice cores: EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), North Greenland Ice core Project (NGRIP), Vostok (VK) and TALos Dome Ice CorE (TALDICE). A sedimentation model was used to reconstruct past variations of three parameters: accumulation of snow at surface, ice layer thinning in depth and Lock-In-Depth (LID), the depth where air is trapped. Ice and gas ages along the core are estimated from these parameters. Then, a Bayesian tool optimised the age scale by constraining the chronology to respect chronological observations (orbital tuning, stratigraphic links between cores, tephra layers…) and by fitting the three parameters to background scenarios (accumulation deduced from ice isotopes, LID from δ¹⁵N, …). The AICC2012 chronology is associated with an uncertainty which arises up to 6 kyrs due to the discontinuity of the ice core composition records and to the poor knowledge when it comes to choose an optimised target for orbital tuning.

Since AICC2012, many new data have been obtained to improve the ice core chronology and it is the right period to produce an updated coherent chronology which could also be extended to other ice cores. Here, we present a first step toward the construction of the next coherent ice core chronology by including new dating constraints from recent data on the EDC ice core: 1) air isotopes (δ¹⁸O_atm , δO₂ /N₂) and air content used as orbital dating constraints, 2) the δ¹⁵N signal used to estimate the background scenario for LID. In addition, we make use of the East Asian stalagmite δ¹⁸O_calcite signal as an alternative synchronisation target for the δ¹⁸O_atm (Extier et al. 2018).

This new dating experiment on EDC ice core aims to lower uncertainty of the chronology while providing a critical look on former hypotheses considered to establish AICC2012. For example, δ¹⁵N record was discontinuous at the time and it has been reconstructed based on its correlation with δD. Now that we have a continuous δ¹⁵N signal, we can evaluate the relevance of this reconstruction. Following this work, we will use new tie point constraints resulting from volcanic synchronisation which has recently been undertaken between Greenland and Antarctica (Svensson et al. 2020) and the ice cores Dome Fuji and WAIS Divide will be further studied to be included in the chronology.