Timing and Characterization of the 8.2 ka Event With Atmospheric Gases (O2, CH4, N2, Ar)

Atmospheric gas records from ice cores can complement traditional single-location paleo records, because the atmosphere acts as a nearly-instantaneous integrator of innumerable gas fluxes over a wide (though ill- defined) spatial area. The 8.2 ka event left a clear signature in atmospheric methane and the O-18 of O2 (δ18Oatm), thought to be sensitive to low-latitude terrestrial rainfall (Severinghaus et al., this meeting). The local temperature at Greenland summit (GISP2) is also recorded in the 15N/14N of N2 and 40Ar/36Ar, via thermal fractionation of these gases. Taken together, the suite of gases affords a comparison of the precise relative timing of the event in geographically distant regions. The abrupt cooling in Greenland (3.3±1.1°C from 15N) that marked the onset of the 8.2 ka event was synchronous with a 15% drop in methane sources to within ±4 years (1σ). The total duration of the event was ~150 years in 15N, with the most intense cold period lasting ~60 years. Photosynthesis is ubiquitous in high-rainfall areas, making O2 a more broadly representative indicator than methane, which is produced in more restricted settings (high- productivity anoxic wetlands). The δ18Oatm record is smoothed by the 1000-year turnover time of O2 in the atmosphere, making timing indistinct, but supports the hypothesis of a widespread low- latitude (Asian and N. African monsoon) terrestrial rainfall change associated with the 8.2 ka event. This finding underscores the potential risk of future climate change to rainfall-based agricultural societies in these regions.