Slopes of seasonal and annual δD-δ18O lines of precipitation and ice cores and their climatological significance

The meteoric water line was one of the earliest described characteristics in hydrogen and oxygen isotopic variations of precipitation. However, spatial and temporal variations in this slope are less studied. The slope of the δD-δ18O relationship is coupled with how d-excess covaries with δD or δ18O, and thus provides an integrated tool to infer hydrologic processes from the evaporation source to the condensation site. We present a study of the δD-δ18O relationship on seasonal and annual timescales for event-based precipitation samples and samples from a 15-meter ice core (Owen) from Summit, Greenland. Seasonally, precipitation δD-δ18O slopes are less than eight (summer = 7.71; winter = 7.77), while the annual slope is greater than eight (8.27). We suggest that intra-season slopes result from Rayleigh distillation, which, under prevailing Summit conditions, produces a slope less than eight. The summer line has a greater intercept (or higher d-excess) than the winter line. This separation causes the annual δD-δ18O slope to be greater than seasonal ones. Higher proportions of sublimated moisture cause larger separations, and thus higher annual slopes. We attribute high summer d-excess to the contribution of vapor sublimated from lower altitudes on the Greenland Ice Sheet, because this sublimated vapor has high d-excess values. Intra-seasonal distributions of precipitation amount also influence the annual δD-δ18O slope. We generate indices to quantify the sublimation proportion (SPI) and precipitation distribution (PDI), and find that annual slope measurements from the Owen ice core are statistically dependent on these indexes with the expected signs of sensitivity. Both sublimation and the annual precipitation distribution represent important characteristics of climate conditions, and we consider it a significant advance in our ability to interpret ice cores if their variations can be reconstructed.