Measurement and interpretation of bubble number-density evolution
through the upper 1200 meters of the SPC14 South Pole Ice Core
Abstract
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.