Jökull - 01.12.1990, Page 118

other words a pH depression of about 1 pH unit.
The melting of snow does probably proceed in
several steps, with a loss of mass from the system. For
example if snow is melted and the meltwater leaves
the system, the system has lost some mass and the
snow that is left has lost its excess amount of protons.
This can be simulated by the model. If, for example
the mass of the original snow is 1 kg and 5% of it
has melted causing the pH of the remaining partially
melted snow to increase by 0.1 pH units, the pH of
the melt can then be calculated. This initial melt is
extracted from the system, and a new generation of
melt produced by melting 5% of the remaining mass,
causing a 0.1 pH rise in the remaining partially melted
snow as before. The pH of this melt can then be
calculated etc. Even assuming such fractional melting,
meltwaters with pH of at least one unit below that of
the original snow are produced. It should be noted
that the 0.1 pH rise in the remaining partially melted
snow is arbitrarily selected since no constraints from
natural systems are available at this stage.
These models simulate melting caused by the ad-
dition of heat but not mass. In other words it simulates
melting caused by the sun but not that caused by rain-
fall on snow with subsequent melting.
SUMMARY AND CONCLUDING RE-
MARKS
The dissolved solids in Icelandic precipitation are
dominated by the marine aerosol contribution with
the exception of that of sulfate and calcium where
some ”excess concentration" which might be partly
attributed to anthropogenic activities is present. Other
chemical constituents, often referred to as anthro-
pogenic, such as nitric oxides and derived species
have not yet been studied. The average pH of Ice-
landic precipitation is 5.4 which is slightly acid and
might be caused by some of the ”excess sulfate" con-
tent of the precipitation. The concentration of dis-
solved solids in the precipitation is greatest close to
the coast but decreases inland and with increased ele-
vation (Sigurðsson and Einarsson, 1988).
The snow on the Vatnajökull glacier is far away
from any anthropogenic aerosol source, thus the chem-
ical constituents are primarily of marine origin. The
concentration of chloride in snow close to the Gríms-
vötn area on the Vatnajökull glacier equals one drop of
seawater mixed with about 19000 drops of pure water.
The concentration of salts in the snow on the glacier
increases with increased elevation, contrary to what
has been documented for precipitation in Iceland and
other parts of the world (Sigurðsson and Einarsson,
1988; Herron and Langway, 1985). Thetotal saltcon-
tent of the 1987-1988 layer in cores 1 and 2 is two
to three times greater than the total salt content of the
1986- 1987 snow layer below. The average pH of the
samples in thetop layer( 1987-1988) incore 1 and2is
0.28 and 0.14 pH units lower respectively than in the
layer below (1986-1987). The increase in salt con-
centration with increased elevation and the downcore
changes in chemistry is attributed to chemical frac-
tionation caused by the partial melting of snow. The
snow under study has undergone partial melting, but
to a variable extent, depending on its location on the
glacier and on its depth in the glacier. During partial
melting the chemical constituents are preferentially
leached from the snow into the meltwater leaving be-
hind purified snow. However, some ions are more
readily released than others. The order of preferen-
tial release of ions from the partially melted snow
isH+ >Mg2+ >CL >Na+ >S02- >K+ >Ca2+.
The most readily released cations, with the same
charge, are the ones with the greatest effective hy-
drated diameter. The smaller the hydrated cation at a
given charge, the more strongly it adheres to the snow.
The preferential release of protons from the snow
causes the pH of the meltwater to be lower than the
pH of the remaining snow. This is reflected in the
relative high pH of partially melted snow and lower
pH of unmelted snow. The pH values of unmelted
1987- 1988 snow on Vatnajökull range from 5.40 to
5.55. This is slightly higher than the average pH (5.4)
of precipitation in Iceland and still about 0.2 pH units
lower than the pH of pure water saturated with air at
25 °C. The pH of the 1987-1988 snow on Vatnajökull
that has been partially melted reaches maximum at pH
6.1. The average pH of the samples from the 1987/88
layer in cores 1 and 2 respectively is 5.57 and 5.59,
but 5.85 and 5.73 in the partially melted 1986/87 layer
114 JÖKULL, No. 40, 1990