Gravitational Enrichment of 84Kr/36Ar
Ratios in Polar Ice Caps: A Measure of Firn Thickness and Accumulation
Temperature
H. Craig
and
R. C. Wiens
*
Measurements of 84Kr/36Ar ratios in
Greenland ice show that gravitational separation in the firn
layer is responsible for the enrichments relative to atmospheric
ratios. The 84Kr/36Ar ratio is enriched by 12.8 per mil and is 24 times the 18O/16O enrichment
in trapped O2, as predicted for gravitational
fractionation. Because gravitational enrichment depends on firn
thickness, which in turn depends on annual mean temperature, noble gas
ratios provide a method for determining paleotemperatures and ancient
firn thicknesses in polar ice caps. The gravitational effects are
modulated by about 10 to 15 percent by atmospheric concentration
changes caused by temperature effects on oceanic gas solubilities. The
availability of five noble gases should make it possible to deconvolute
the solubility and gravitational enrichments for calibration of
18O paleotemperatures throughout the polar ice sheets.
Isotope Laboratory, Scripps Institution of Oceanography,
University of California at San Diego, La Jolla, CA 92093, USA.
*
Present address: Division of Geological and Planetary Science,
California Institute of Technology, Pasadena, CA 91125, USA.
