Measurement and interpretation of bubble number-density evolution through the upper 1200 meters of the SPC14 South Pole Ice Core (SPICEcore)

Using new samples from the South Pole Ice Core (SPC14), we present results of bubble number-density analyses and use them to reconstruct a temperature history for the South Pole site back through the LGM ( 26 ka). Additionally, we show that 3D micro-CT sample imagery can quantify bubble number-density accurately, enabling additional future analyses.

Applying techniques previously established for analyses of the WAIS Divide core, and ice-core samples recently prepared at the National Science Foundation Ice Core Facility, we measured bubble number-density as well as other bubble characteristics from the approximate pore close-off depth at 100 m, down to 1200 m, at 20-m intervals. Some bubbles were noted at depths below 1200 meters; however, no samples were prepared at these depths because of bubble loss owing to onset of clathrate-hydrate formation due to high burial pressures.

The South Pole ice core project (SPICEcore) was recently completed near the U.S. Amundsen-Scott South Pole Station and has resulted in 1750 total meters of ice that reveal 55,000 years of atmospheric, isotopic, and glaciological history at the site. The most current dating of the SPIC14 core based on volcanic ECM sulfate matching to the WAIS Divide core, together with annually-resolved chemistry and visual stratigraphy, reveals an age at 1200 meters of approximately 26 ka, and provides an accumulation-rate history after simple correction for ice-flow thinning. Bubble number-density of a sample depends on temperature and accumulation rate while the sample was in the firn, allowing estimation of a history of past temperatures along the flow-line to South Pole. Our resulting temperature reconstruction agrees closely with independent estimates based on stable isotopic ratios of ice.

Additionally, using micro-CT technology at CRREL (Bruker SkyScan 1173), we tested the use of 3D imagery to accurately measure bubble number-density as both a supplement and possible future alternative to thin-section measurements. We measured samples from equivalent depths (20-m intervals) across the total depth range. Once corrected for cut- and micro-bubbles, our results show comparable values and thus similar trends to the thin-section data. This result motivates future bubble characterization studies using 3D micro-CT image analyses for the SPC14 core.