Preprints
https://doi.org/10.5194/tc-2022-49
https://doi.org/10.5194/tc-2022-49
 
10 Mar 2022
10 Mar 2022
Status: this preprint is currently under review for the journal TC.

Gas isotope thermometry in the South Pole and Dome Fuji Ice Cores provides evidence for seasonal rectification of ice core gas records

Jacob Davies Morgan1, Christo Buizert2, Tyler Jeffrey Fudge3, Kenji Kawamura4,5,6, Jeffrey Peck Severinghaus1, and Cathy M. Trudinger7 Jacob Davies Morgan et al.
  • 1Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA
  • 2College of Earth Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
  • 3Department of Earth and Space Science, University of Washington, Seattle, WA 98195, USA
  • 4National Institute of Polar Research, Tokyo 190-8518, Japan
  • 5Department of Polar Science, The Graduate University of Advanced Studies (SOKENDAI), Tokyo 190-8518, Japan
  • 6Japan Agency for Marine Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan
  • 7Climate Science Centre, CSIRO Oceans and Atmosphere, Aspendale, Victoria 3195, Australia

Abstract. Gas isotope thermometry using the isotopes of molecular nitrogen and argon has been used extensively to reconstruct past surface temperature change from Greenland ice cores. The gas isotope ratios δ15N and δ40Ar in the ice core are each set by the amount of gravitational and thermal fractionation in the firn. The gravitational component of fractionation is proportional to the firn thickness and the thermal component is proportional to the temperature difference between the top and bottom of the firn column, which can be related to surface temperature change. Compared to Greenland, Antarctic climate change is typically more gradual and smaller in magnitude, which results in smaller thermal fractionation signals that are harder to detect. This has hampered application of gas isotope thermometry to Antarctic ice cores.

Here, we present an analytical method for measuring δ15N and δ40Ar with a precision of 0.002 ‰ per atomic mass unit, a two-fold improvement on previous work. This allows us to reconstruct changes in firn thickness and temperature difference at South Pole between 30 and 5 kyr BP. We find that variability in firn thickness is controlled in part by changes in snow accumulation rate, which is, in turn, influenced strongly by the along-flowline topography upstream of the ice core site. Variability in our firn temperature difference record cannot be explained by annual-mean processes. We therefore propose that the ice core gas isotopes contain a seasonal bias due to rectification of seasonal signals in the upper firn. The strength of the rectification also appears to be linked to fluctuations in the upstream topography. As further evidence for the existence of rectification, we present new data from the Dome Fuji ice core that are also consistent with a seasonal bias throughout the Holocene.

Our findings have important implications for the interpretation of ice core gas records. For example, we show that the effects of upstream topography on ice core records can be significant at flank sites like South Pole—they are responsible for some of the largest signals in our record. Presumably upstream signals impact other flank-flow ice cores such as EDML, Vostok, and EGRIP similarly. Future work is required to confirm the existence of seasonal rectification in polar firn, determine how spatially and temporally widespread rectifier effects are, and to incorporate the relevant physics into firn air models.

Jacob Davies Morgan et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2022-49', Anonymous Referee #1, 06 Apr 2022
  • RC2: 'Comment on tc-2022-49', Anonymous Referee #2, 13 Apr 2022

Jacob Davies Morgan et al.

Jacob Davies Morgan et al.

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Short summary
The composition of air bubbles in Antarctic ice cores records information about past changes in properties of the snowpack. We find that, near the South Pole, thicker snowpack in the past is often due to steeper surface topography, where faster winds erode the snow and deposit it in flatter areas. The slope and winds seem to also cause a seasonal bias in the composition of air bubbles in the ice core. These findings will improve interpretation of other ice cores from places with steep slopes.