The Relationship Between D-O Variability in Greenland Ice Core Deuterium Excess, Vapor Sourcing, and Source Region Humidity.

Greenland ice core deuterium excess (dxs = d2H-8xd18O) is thought to provide information on changes to evaporation site relative humidity, a concept largely derived from isotope theory. However, due to a lack of evidence from comprehensive isotope-enabled models, the main drivers of Greenland dxs variability during Dansgaard-Oeschger (D-O) events and glacial-interglacial transitions remain incompletely understood. For example, changes in vapor source location during D-O events can cause changes to ice core dxs regardless of changes to the humidity of oceanic evaporation, but vapor source changes are poorly understood. In this study, we use an isotope-enabled general circulation model, NCARs Community Atmosphere Model (iCAM), to examine the simulated spatial variability of nearby evaporation dxs and humidity under D-O sea ice and SST forcing. We implement water tags to distinctly identify changes in vapor sourcing for Greenland ice cores under D-O forcing. Our findings are twofold: first, there is evidence that Greenland ice core site precipitation dxs is recording information on vapor source region humidity characteristics; second, changes to vapor sourcing during D-O events induce changes in precipitation dxs that are driven simply by vapor coming from different parts of the Atlantic Ocean because of changes in SST and sea ice cover. The magnitude of spatial variability in evaporation dxs over the primary vapor source region is larger than D-O variability in evaporation dxs, as simulated by the model. Combined, these findings support the traditional perspective of individual dxs ice core measurements as a proxy for source region humidity characteristics, but also indicate that millennial-scale variability in ice core dxs is driven by changes to the general atmospheric circulation and patterns of vapor convergence over Greenland.