New constraints on the gas age-ice age difference along the EPICA ice cores, 0–50 kyr

Gas is trapped in polar ice sheets at ∼50–120m below the surface and is therefore younger than the surrounding ice. Firn densification models are used to evaluate this ice age-gas age difference (∆age) in the past. However, such models are not well tested on low accumulation and cold sites of the East Antarctic plateau, especially for 5 periods with different climatic conditions. Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We use the structured Be peak, occurring 41 kyr before present (BP) and 10 due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the ∆age and ∆depth during this event. It allows us to evaluate the model and to link together climatic archives from EDC and EDML to NorthGRIP (Greenland). Our results reveal an overestimate of the ∆age by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and tempera15 ture scenarios do not entirely resolve this discrepancy. Our finding suggests that the phase relationship between CO2 and EDC temperature inferred at the start of the last deglaciation (lag of CO2 by 800±600 yr) is overestimated and that the CO2 increase could well have been in phase or slightly leading the temperature increase at EDC.