On polar ice sheets, water vapor interacts with surface snow, and through the exchange of water molecules, imprints an isotopic climate signal into the ice sheet. This exchange is not well understood due to sparse observations in the atmosphere. There are currently no published vertical profiles of water isotopes above ice sheets that span the planetary boundary layer and portions of the free troposphere. Here, we present a novel dataset of water-vapor isotopes ( δ18O , δD, dxs) and meteorological variables taken by fixed-wing uncrewed aircraft on the northeast Greenland Ice Sheet (GIS). During June-July (2022), we collected 105 profiles of water-vapor isotopes and meteorological variables up to 1500 m above ground level. Concurrently, surface snow samples were collected at 12-hour intervals, allowing connection to surface-snow processes. We pair observations with modeling output from a regional climate model as well as an atmospheric transport and water-isotope distillation model. Climate model output of mean temperature and specific humidity agrees well with observations, with a mean difference of +0.095 °C and -0.043 g/kg (-2.91 %), respectively. We find evidence that along an air parcel pathway, the distillation model is not removing enough water prior to onsite arrival. Below the mean temperature inversion (~200m), water-isotope observations indicate a kinetic fractionating process, likely the result of mixing sublimated vapor from the ice sheet surface along with an unknown fraction of katabatic wind vapor. Modeled dxs does not agree well with observations, a result that requires substantial future analysis of kinetic fractionation processes along the entire moisture pathway.

Using samples from the South Pole Ice Core (SPC14), we present new bubble number-density (BND) measurements and a modeled temperature history reconstruction for the South Pole site back through ~18.5 ka. Additionally, we show that 3D micro-CT sample imagery can accurately quantify BND, enabling more rapid and efficient future analyses. Using sampling and imaging techniques previously established for analyses of the WAIS Divide ice core (Spencer et al., 2006; Fegyveresi et al., 2016), we measured BND as well as other bubble characteristics from just below pore close-off depth starting at ~160 m, down to ~1200 m, at 20-meter intervals (53 total samples), with typical values ranging between 800 and 900 bubbles cm-3 over this interval. These values are higher than any previously recorded for ice-core BND, indicative of both colder average temperatures, and higher average accumulation rates at South Pole. Below ~1100 m, we noted significant bubble loss owing to the onset of clathrate-hydrate formation. Using micro-CT technology, we also tested the use of 3D imagery to accurately measure and evaluate BND as a supplement and future alternative to painstaking thin-section measurements. We imaged a secondary set of ice-core samples at 100-meter intervals starting at 200 m, and across the sample total depth range. Once corrected for cut- and micro-bubbles, our results show comparable values and thus similar trends to the thin-section data. For our temperature model, we determined an accumulation record using both measured annual layer thicknesses as well as estimated d15N-derived firn-column thicknesses estimates. Our temperature reconstruction was calculated using the model developed by Spencer et al. (2006), and using a South Pole site-specific bubble-to-grain ratio (G) of 1.6. the reconstruction reveals a warming across the glacial-interglacial transition of ~7°C, with a relatively stable trend through the Holocene (< 0.4°C warming). These results are in close agreement with those reported by other independent paleothermometers (i.e. isotope- and firn-derived reconstructions). Results of our temperature reconstruction also reveal that using 3D micro-CT imagery in place of traditional thin-section techniques produces comparable results, but with even greater accuracy, and lower measures of uncertainty.