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Antarctica - Part 3

Smithsonian Contributions to the Earth Sciences, Number 23

Catalog of Antarctic Meteorites, 1977-1978

Nature of the Field Occurrence

During field seasons 1976-77 and 1977-78, two meteorites were found that
should be considered isolated occurrences. These are (1) the two Mt.
Baldr chondrites, which, based on noble gas content, have now been
paired (Weber and Schultz, 1978), and (2) an iron found by a member of a
University of Maine field party, Steven Kite, in glacial moraine at the
seaward end of Victoria Valley, near Purgatory Peak (Yanai et al.,
1978). A third, possibly isolated meteorite was found on bare ice near
the upper (southern) end of Manhaul Glacier only a few kilometers from
the main concentration site at Allan Hills. This specimen is included
tentatively as an isolated occurrence because the site does not appear
to be connected dynamically to the ice flow and ablation system that
apparently produced the Allen Hills concentration. It was, however,
found nearby.
A relatively high frequency of discovery for isolated meteorites should,
I suppose, be expected in the Antarctic environment because some ice
surfaces may have been in stagnant-flow conditions for very long periods
of time and weathering of rocks proceeds very slowly, so that
concentrations could build up over time. The Mt. Baldr (Cassidy et al.,
1977) and Manhaul Glacier (Yanai et al., 1978) occurrences are therefore
understandable as individuals whose survival was prolonged by fall onto
a static surface and by retardation of the weathering process. The
Purgatory Peak occurrence (Yanai et al., 1978) was slightly different in
that the iron probably was transported by ice and survived because of
the slow rate of weathering in the Antarctic environment.
The Allan Hills concentration is another matter. Through field season
1977-78 we had recovered about 350 specimens, representing perhaps 20 to
50 falls (Yanai et al., 1978). There is evidence of ice flowing into the
site and wasting by ablation, which suggests that meteorites found in
the ablation zone could have been transported over great distances after
fall.
The meteorites occur in a zone where ice moving off the Antarctic
Plateau apparently passes over a rocky ridge, thinning appreciably in
the process. The ice surface along this presumed ridge forms a monocline
whose lower limb is on the downstream side. Several kilometers further
downstream, the ice meets the absolute barrier of the Allan Hills.
Meteorites were found on both the upper and lower limbs of the monocline
within an area of about 100 square kilometers. They were relatively more
abundant on the lower limb. It seems likely that specimens first appear
on the surface of the ice at the upper limb and then are carried down to
the lower limb, where they are stored for great lengths of time. Smaller
specimens were often found at the northern border of the ice patch on
the lower limb, where a veneer of snow exists. It may be that these
smaller specimens have been blown across the ice by the strong katabatic
winds coming from the south and that once they reach snow their progress
stops, so that they are concentrated along the snow boundary.
The lower limb of the monocline is also penetrated here and there by
rocky outcrops. These produce much moraine debris, and meteorites can be
found mixed with this morainal material. The ice at these points
apparently has virtually zero thickness.
To summarize our field recoveries over two field seasons, it can be
stated that three meteorites were found that were not parts of high
residual concentrations, and 353 specimens, representing possibly 20-50
meteorites, were found in one residual concentration. The sporadic finds
seem to occur in areas of active ablation having no source of new ice,
while the high concentrations seem to require constant replacement of
ablated ice, as well as active ablation.


Best wishes,

Bernd

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