Reliable Orbital Dating in Deep Ice Core Provides Accurate Marine-Ice Sequences over Old Terminations

Past climate and environmental changes can be reconstructed from palaeoclimate archives, including marine sediment and polar ice cores. Understanding mechanisms associated with major climate changes requires an accurate and precise chronology for each archive and the synchronisation of these individual chronologies to a common multi-proxy timescale. Discrepancies between the individual chronologies can lead to misinterpretation of the phase relationships and the climate dynamics. For old terminations that occurred more than 700,000 years ago, when using original chronologies, ice core data suggest that the increase in atmospheric CO₂ concentration lags behind the sea level rise recorded in marine sediment cores. This finding strongly contradicts the established understanding of the climate mechanisms during deglaciations based on observations over the seven most recent terminations, suggesting a mismatch between the site-specific chronologies. Deep ice core age scales are generally developed based on orbital dating correlating gas orbital tracers with insolation variations. However, in the deepest and oldest section of an ice core, thermally enhanced gas diffusion and extensive annual layer thinning significantly mute the proxy records, hampering precise orbital dating. 

In this study, we evaluate the diffusion effect on the frequencies of the gas records critical for orbital dating and explore the incoherence within and between the AICC2023 ice core chronology and modelled LR04 age scale for the marine sediment cores for the period between 600,000 to 800,000 years ago utilising new high-resolution data (~700 years on average instead of >1000 years on average in the previous chronology reconstruction) from the deepest 200 metres of the EPICA Dome C (EDC) ice core. Spectral analyses of CH₄, δ¹⁸O of O₂, and δ(O₂/N₂) confirm that diffusion does not significantly affect the orbital-scale variability, which enables us to revise the existing depth-age relationship for EDC on its deepest section. Integrating chronological information from the ice core and a continuous high-resolution stable oxygen isotope record of benthic foraminifera using the statistical dating tool Paleochrono-1.1, we link the ice core chronology to marine sediment cores and propose an improved and coherent timescale to reconceive the CO₂ and sea level scenario over old terminations.