Holocene climatic changes in Greenland: Different deuterium excess signals at Greenland Ice Core Project (GRIP) and NorthGRIP

Water stable isotope measurements (delta D and delta(18)O) have been conducted on the Holocene part of two deep Greenland ice cores ( Greenland Ice Core Project ( GRIP) and NorthGRIP), located similar to 320 km apart. These combined measurements provide the first two continuous Greenland Holocene deuterium excess profiles (d = delta D - 8 delta(18)O), a parameter strongly influenced by changes in moisture sources. We discuss here temporal and regional fluctuations of the deuterium excess within central to north Greenland, with a mean temporal resolution of similar to 4 years. Although GRIP and NorthGRIP exhibit similar annual mean surface temperatures and delta(18)O levels, a significant offset of modern deuterium excess is observed between the two sites. We attribute this offset to a different mix of modern moisture sources, pointing to regional-scale differences in moisture advection toward Greenland. The common long-term deuterium excess Holocene increasing trend is probably related to the increased relative contribution of low-latitude moisture to Greenland snowfall, in response to the change in the Earth obliquity, as symmetrically observed in Antarctica. Three abrupt declines punctuate the GRIP excess record (8.2, 4.5, and 0.35 ka BP), suggesting associated reorganizations of the northern high latitudes hydrological cycle. The 8.2 ka BP event is characterized by (1) a rapid cooling followed by a progressive warming and (2) a deuterium excess cooling restricted to GRIP, therefore totally different from rapid events during glacial times. By contrast, the NorthGRIP deuterium excess record is more stable. We propose that a slightly larger proportion of moisture supplied by local storm tracks to GRIP induces an isotopic compensation mechanism between simultaneous site and source temperature coolings, resulting in a rather temperature-insensitive delta(18)O profile, together with well-marked deuterium excess amplitudes. NorthGRIP delta(18)O seems less biased by isotopic processes and should provide a more reliable past temperature record.