| Summary: | International audience The Dansgaard-Oeschger (DO) event 25 initiates the millennial-scale climatic variability of the Northern Hemisphere during the last glacial period. This first rapid event is identified in the NorthGRIP ice core (Greenland - 75.10 °N, 42.32 °W, elevation 2917 m. a. s.l., modern accumulation rate 17.5 cm w.e.yr-1) δ18Oice record as a sharp 2.3‰ increase (half of a “classical” DO interstadial magnitude) occurring at ~112 ka (EDC3 age scale). While DO event 25 remained so far poorly studied, the other DO events of the glacial period have revealed (1) large amplitudes of temperature change over Greenland (8-16°C) (2) synchronous rapid increases of methane (CH4) and Greenland temperature (3) seesaw behaviour with Antarctic temperatures increasing slowly 300-3400 yrs before the rapid Greenland abrupt temperature changes. Thus, investigating in detail the first DO event is of primary importance to understand the onset of the climatic variability of the glacial period. To provide a complete description of DO event 25, we present new results from the EPICA Dronning Maud Land (EDML, Antarctica - 75.00 °S, 0.07 °E, 2882 m a.s.l., 6.4 cm w.e.yr1) and the NorthGRIP ice cores. We use high-resolution measurements of the isotopic composition of air nitrogen (δ15N) as a temperature proxy in the gas phase to infer the amplitude of temperature changes over Greenland. These data moreover allow determining precisely the phasing between temperature and CH4 concentration in the air trapped in the ice. Finally, the combination of new high-resolution CH4 measurements on the NorthGRIP ice core and δ18Oatm records over this first rapid event permits to propose the first accurate common timescale between EDML and NorthGRIP over DO event 25 and to discuss the bipolar seesaw pattern at the onset of the last glacial period.
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