Diffusion of stable isotopes in polar firn and ice: the isotope effect in firn diffusion.

Physics of Ice Core Records, Edited by T. Hondoh, Hokkaido University Press, Sapporo, 2000

S.J. Johnsen, H.B. Clausen
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen. .
K.M. Cuffey
Department of Geography, 501 MacCone Hall, University of California Berkeley, California 94720-4740, USA.
G. Hoffmann
Laboratoire des Sciences du Climat et de l'Environnement , Gif-Sur-Yvette, France.
J. Schwander
Physics Institute , University of Bern, Bern, Switzerland.
T. Creyts
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada.

ABSTRACT.
Ice core records are often affected by post-depositional processes that need to be better understood to prevent wrong interpretation of the data. Records of stable isotopes are affected by diffusion both in the firn and in the deeper ice. We present a quantitative theory for diffusion in firn that applies the measured tortuosity factors for O₂and CO₂ in firn to the diffusing water vapor. Because of different fractionation factors, the theory predicts stronger smoothing for δ¹⁸O than for δD, in excellent agreement with our data. This effect opens up the possibility for using detailed isotope records to estimate paleotemperatires in deeper strata. We show that this differential smoothing can create an artificial annual cycle in deuterium excess, which was not present at the time of deposition. It also increases the slope observed in high resolution data series between δD and δ¹⁸O variations. For the annual cycles, we observe that this slope can increase from 8 at the surface up to 11 in deeper firn. In the Holocene ice for the GRIP core, we observe much stronger smoothing than predicted from diffusion in solid ice; this suggests an anomalous diffusion process in glacier ice. Possible models for this excess diffusion are discussed i.a., in terms of the thickness of water films on grain boundaries and in veins.