Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 51

Since Icelandic glaciers are classified as temperate glaciers with
soaked zones and possibly as the lowest part of the percolation zone,
the isotopic changes in ice-water systems will be given some atten-
tion here.
5. 2 Post depositional clianges in the firn of temperate
glacier ice.
Numerous studies on the isotope content of snowpack and ice
profiles in the percolation and soaked zones have been carried out
during the last fifteen years. In all cases, the investigators have
observed that the 8-variations of the precipitation, still preserved
in the snowpack at the end of the winter period, are usually rapidly
disturbed during the following melting season. In some cases almost
complete homogenization occurs. At the same time, the remaining
firn becomes enriched in the heavier isotopes deuterium and O18.
Since the summer precipitation on glaciers such as the glaciers
of the Alps and the glaciers in North America contains much more
deuterium and O18 than does precipitation deposited during the
winter, authors who have studied glaciers in these regions have
suggested that isotopic homogenization and heavy isotope enrich-
ment of the remaining firn is due mainly to the freezing of rain
and meltwater and the capture of snow in crevasses (Epstein et al.
1959, Sharp et al. 1960, Deutsch et al. 1966, Macpherson et al 1967,
West et al. 1970). Sharp et al. (1960) also mentioned that a part
of the heavy isotope enrichment may he ascrihed to freewater con-
tent in the snow layers. Freewater, which is thought to be in the
neighbourhood of 10% by weight, is considered to be mainly sum-
mer rain and therefore considerably richer in the heavier isotopes
than the winter snow. None of the above mentioned authors con-
sidered isotopic exchange between ice and water important for the
enrichment process.
Studies on Icelandic glaciers, however, have shown that isotopic
exchange is the most important factor governing the homogeniza-
tion process and the enrichment of heavier isotopes during the
firnification processes (Ámason 1969 a and 1970).
Fig. 9 shows deuterium concentration against depth from three
pits and shallow boreholes on the Vatnajökull glacier in Iceland.
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