Temperature reconstruction from measured bubble number-density evolution in the South Pole Ice Core since the late-glacial (∼19.5 ka)

We present analyses of bubble number-density (BND) data from the South Pole Ice Core (SPC14) showing warming of ∼7.5°C from the Late Glacial (∼19.5 ka), then relatively stable temperatures during the Holocene (<0.5°C warming), in close agreement with results of independent paleothermometers. The BND data span from ∼160 m just below pore close-off, to ∼1200 m, where bubble loss by clathrate formation is significant. Measurements were made with standard bubble ‘thick’-section techniques and a new application of three-dimensional micro-computed tomography (CT) imagery; the nearly identical results recommend the faster, nondestructive micro-CT. The very high BND at South Pole, typically 800 and 900 bubbles cm⁻³, reflects the joint effects of the relatively low mean-annual temperature (−49°C) and high accumulation rate (∼7.5 cm w.e. a⁻¹). High BND is physically linked to small grain sizes at pore close-off, which in turn helps explain the near-absence of brittle-ice behavior at the site, contributing to the high quality of the recovered core with implications for siting of future ice cores. The accumulation history, derived from δ¹⁵N-N₂ firn-column thickness estimates, correlates with the temperature history but varies somewhat more than saturation vapor pressure, suggesting dynamic controls including upstream slope variability.