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Abstract. Above polar ice sheets, atmospheric water vapor exchangeoccurs across the planetary boundary layer (PBL) and is an importantmechanism in a number of processes that affect the surface mass balance ofthe ice sheets. Yet, this exchange is not well understood and hassubstantial implications for modeling and remote sensing of the polarhydrologic cycle. Efforts to characterize the exchange face substantiallogistical challenges including the remoteness of ice sheet field camps,extreme weather conditions, low humidity and temperature that limit theeffectiveness of instruments, and dangers associated with flying mannedaircraft at low altitudes. Here, we present an unmanned aerial vehicle (UAV)sampling platform for operation in extreme polar environments that iscapable of sampling atmospheric water vapor for subsequent measurement ofwater isotopes. This system was deployed to the East Greenland Ice-coreProject (EastGRIP) camp in northeast Greenland during summer 2019. Foursampling flight missions were completed. With a suite of atmosphericmeasurements aboard the UAV (temperature, humidity, pressure, GPS) wedetermine the height of the PBL using online algorithms, allowing forstrategic decision-making by the pilot to sample water isotopes above andbelow the PBL. Water isotope data were measured by a Picarro L2130-iinstrument using flasks of atmospheric air collected within the nose cone ofthe UAV. The internal repeatability for δD and δ18O was2.8 ‰ and 0.45 ‰, respectively,which we also compared to independent EastGRIP tower-isotope data. Based onthese results, we demonstrate the efficacy of this new UAV-isotope platformand present improvements to be utilized in future polar field campaigns. Thesystem is also designed to be readily adaptable to other fields of study,such as measurement of carbon cycle gases or remote sensing of groundconditions. 

Abstract. The South Pole Ice Core (SPICEcore) was drilled in 2014–2016 to provide adetailed multi-proxy archive of paleoclimate conditions in East Antarcticaduring the Holocene and late Pleistocene. Interpretation of these recordsrequires an accurate depth–age relationship. Here, we present the SPICEcore (SP19) timescale for the age of the ice of SPICEcore. SP19 is synchronized to theWD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) icecore using stratigraphic matching of 251 volcanic events. These eventsindicate an age of 54 302±519 BP (years before 1950) at thebottom of SPICEcore. Annual layers identified in sodium and magnesium ionsto 11 341 BP were used to interpolate between stratigraphic volcanic tiepoints, yielding an annually resolved chronology through the Holocene.Estimated timescale uncertainty during the Holocene is less than 18 yearsrelative to WD2014, with the exception of the interval between 1800 to 3100BP when uncertainty estimates reach ±25 years due to widely spacedvolcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rateof 7.4 cm yr−1 (water equivalent). The time variability of accumulation rateis consistent with expectations for steady-state ice flow through the modernspatial pattern of accumulation rate. Time variations in nitrateconcentration, nitrate seasonal amplitude and δ15N of N2 in turn are as expected for the accumulation rate variations. The highlyvariable yet well-constrained Holocene accumulation history at the site canhelp improve scientific understanding of deposition-sensitive climateproxies such as δ15N of N2 and photolyzed chemicalcompounds. 

Variability in sea ice is a critical climate feedback, yet the seasonal behavior of Southern Hemisphere sea ice and climate across multiple timescales remains unclear. Here, we develop a seasonally resolved Holocene sea salt record using major ion measurements of the South Pole Ice Core (SPC14). We combine the SPC14 data with the GEOS‐Chem chemical transport model to demonstrate that the primary sea salt source switches seasonally from open water (summer) to sea ice (winter), with wintertime variations disproportionately responsible for the centennial to millennial scale structure in the record. We interpret increasing SPC14 and circum‐Antarctic Holocene sea salt concentrations, particularly between 8 and 10 ka, as reflecting a period of winter sea ice expansion. Between 5 and 6 ka, an anomalous drop in South Atlantic sector sea salt indicates a temporary sea ice reduction that may be coupled with Northern Hemisphere cooling and associated ocean circulation changes.

 

Data from the South Pole ice core (SPC14) are used to constrain climate conditions and ice‐flow‐induced layer thinning for the last 54,000 years. Empirical constraints are obtained from the SPC14 ice and gas timescales, used to calculate annual‐layer thickness and the gas‐ice age difference (Δage), and from high‐resolution measurements of water isotopes, used to calculate the water‐isotope diffusion length. Both Δage and diffusion length depend on firn properties and therefore contain information about past temperature and snow‐accumulation rate. A statistical inverse approach is used to obtain an ensemble of reconstructions of temperature, accumulation‐rate, and thinning of annual layers in the ice sheet at the SPC14 site. The traditional water‐isotope/temperature relationship is not used as a constraint; the results therefore provide an independent calibration of that relationship. The temperature reconstruction yields a glacial‐interglacial temperature change of 6.7 ± 1.0°C at the South Pole. The sensitivity ofδ¹⁸O to temperature is 0.99 ± 0.03 ‰°C⁻¹, significantly greater than the spatial slope of 0.8‰°C⁻¹that has been used previously to determine temperature changes from East Antarctic ice core records. The reconstructions of accumulation rate and ice thinning show millennial‐scale variations in the thinning function as well as decreased thinning at depth compared to the results of a 1‐D ice flow model, suggesting influence of bedrock topography on ice flow.

 

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.