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Measurement and interpretation of bubble number-density evolution through the upper 1200 meters of the SPC14 South Pole Ice Core
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  • John Fegyveresi,
  • Richard Alley,
  • Joan Fitzpatrick,
  • Donald Voigt,
  • Zoe Courville,
  • Christo Buizert,
  • Bradley Markle,
  • Eric Steig
John Fegyveresi
Northern Arizona University

Corresponding Author:john.fegyveresi@nau.edu

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Richard Alley
Pennsylvania State University
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Joan Fitzpatrick
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Donald Voigt
Pennsylvania State University Main Campus
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Zoe Courville
Cold Regions Research and Engineering Laboratory
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Christo Buizert
Oregon State University
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Bradley Markle
University of Colorado, Boulder
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Eric Steig
University of Washington
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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.