Triple isotopic composition of oxygen in surface snow and water vapor at NEEM (Greenland)

The isotopic composition of water in polar ice cores is commonly used to reconstruct the climatic conditions both at the site of precipitation and at the site of oceanic source region. Theoretical studies have suggested that the variability of the parameter 17Oexcess resulting from the triple isotopic composition of oxygen in water should be driven by the relative humidity of the oceanic source region. Our new 17Oexcess measurements in surface water vapor and snow at the Greenland NEEM camp demonstrate the potential of 17Oexcess as a marker of source relative humidity. Using concomitant measurements of water vapor and precipitation, we first confirm the theoretical relationship between the fractionation coefficients at vapor-solid equilibrium α17eq_vap_sol and α18eq_vap_sol associated respectively with δ17O and δ18O: ln (α17eq_vap_sol)/ln (α18eq_vap_sol) = 0.528. Our data reveal 17Oexcess seasonal variations in Greenland firn over the years 2003-2005. Their magnitude is of the order of 25 ppm, with a minimum in July leading d-excess maximum level reached ∼3 months later. This is in agreement with our understanding of 17Oexcess and d-excess variations in polar regions with minima in 17Oexces in phase with maxima in source relative humidity and maxima in d-excess largely influenced by source temperature increase. In a large northern Atlantic sector, relative humidity reaches its seasonal maximum in summer, earlier than the sea surface temperature maximum which takes place in fall.