Precipitation and ice core δD-δ¹⁸O line slopes and their climatological significance

Abstract. The meteoric water line, defined by the correlation of hydrogen (δD) and oxygen (δ¹⁸O) values, is one of the earliest described characteristics of precipitation isotopic variations. However, spatial and temporal variations in the slope of this line are less studied. The slope of the δD-δ¹⁸O relationship is coupled with how d-excess covaries with δD or δ¹⁸O, and may provide an integrated tool for inferring hydrologic processes from the evaporation to condensation site. We present a study of δD-δ¹⁸O relationships on seasonal and annual timescales for event-based precipitation and a 15-meter ice core (Owen) at 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 intra-season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than eight. The summer line has a greater intercept (higher d-excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d-excess to contributions of vapor sublimated from the Greenland Ice Sheet. Higher sublimated moisture proportions in summer cause larger separations between seasonal δD-δ¹⁸O lines, and thus higher annual slopes. Intra-seasonal distributions of precipitation amount also influence annual slopes because slopes are weighed by the number of storms each season. We generate indices to quantify sublimation proportion (SPI) and precipitation distribution (PDI), and find that annual Owen core slope measurements are significantly related to these indices, demonstrating that sublimation and precipitation distribution represent important climate conditions recorded in ice cores.