Quantification of Surface Temperature Changes During Rapid Climatic Events 18-20 From air Isotopic Measurements in NorthGRIP ice Core and Precise Phasing With CH4 Variations

Although water stable isotope profiles from Greenland ice cores have evidenced the succession of glacial climate variability, their quantitative interpretation in terms of temperature changes remains uncertain due to possible changes in the seasonality of the precipitation. Here we use an alternative paleothermometry method based on the gravitational and thermal diffusion of permanent gases in the firn in response to abrupt temperature changes. The variety of measurements conducted on the air trapped in the ice enables to study the relative timing of fluctuations in local temperature (isotopic measurements of 15N/14N, d15N and 40Ar/36Ar, d40Ar), ice volume (18O/16O of atmospheric oxygen, d18Oatm) and wetland CH4 production. We have obtained high resolution profiles of these tracers measured along the Dansgaard-Oeschger events 18, 19 and 20 from the recently drilled NorthGRIP ice core. d15N combined to CH4 data on the whole profile confirm the in phase increase of both temperature and CH4 during DO events. d15N, d40Ar and firn densification associated to heat diffusion modeling enable us to estimate the associated temperature changes (to be compared to the estimate by Lang et al, GRIP event 19). Indeed, nitrogen combined to argon isotopic anomalies enable to extract the sole thermal effect from the total signal. Finally, the air d18Oatm shows a slow increasing trend due to the ice sheet growth. Gathering information on temperature changes and ice sheet evolution from the air isotopic measurements led to a better understanding of the relationship between water isotopes and surface temperature.