Geology Reference
In-Depth Information
than continue on the previous bearings. If the find is in an
area certain to be revisited and time is short, the specimen
may be flagged and left for later recovery. When few or no
meteorites are encountered, reconnaissance transects will
explore as much of the icefield as possible, to eliminate
(as far as possible) the possibility that a concentration
was missed. When more meteorites are encountered,
spacing within the transects may be narrowed to better
define the scope of the concentration; and when abun-
dant meteorites are encountered, the reconnaissance
team may choose to conduct fully systematic recoveries
using overlapping, highly controlled transects (as
described in the next section). Typically, this happens
when the scale of the concentration appears too small to
warrant a future visit by a larger team, and enough time
remains in the season to complete the work. Alternatively,
the reconnaissance team may choose to move on to a new
site as soon as possible and leave the rest of the recoveries
to a larger, better-equipped team. Reconnaissance at any
scale is considered complete when the value of possible
future visits to the site by ANSMET is known.
During early recovery efforts, single transects may be
used as sampling tools to prioritize among several search
areas. Because each field party member is capable of
independent mobility (everyone has their own snowmo-
bile), it is not unusual for an ANSMET party to split into
temporary subparties to cover immediate needs (such as
distinct GPS surveying and sample recovery groups when
a large number of specimens has been found in a con-
fined area). The location and path of at least one team
member's snowmobile is continuously recorded to estab-
lish the geographical location of the transects and pro-
vide a record of the field team's progress (Figure 2.2).
This record of traverses, available with the advent of
high-resolution satellite imagery and GPS, has dramati-
cally improved ANSMET's ability to track our own
progress both during and between field seasons.
2.4. METEORITE RECOVERY TECHNIQUES
2.4.1. Minimizing Biases
The recovery of scientific samples always involves sam-
pling biases related to the techniques used to acquire the
samples and the choices made by the scientists during
sampling. Throughout its history, ANSMET has chosen
to use a simple and inexpensive but very effective mete-
orite detection system: the human vision system. For
areas where the background of terrestrial rock is very low
or absent, the innate human ability to rapidly differentiate
a scene into key elements and recognize those that are
unique or out of place allows field party members to scan
enormous areas of blue ice quickly and immediately
notice any rocks upon its surface. This ability is limited
only by the seeing conditions and the resolution of the
human eye, which typically allows a dark, centimeter-sized
meteorite to be resolved at distances of up to 100 meters
on the light-colored ice [
Harvey
, 2003]. Given that ANSMET
searches typically involve much shorter distances, we rou-
tinely recover meteorite specimens much smaller than
this; catalogs of Antarctic specimens contain many rare
types recovered in the subcentimeter size range.
Meteorite recovery tasks become more difficult and
the risk of biases rises when terrestrial rocks become
abundant, such as on icefields adjacent to nunataks and
moraines, or in the moraines themselves. In the earliest
years of ANSMET fieldwork, moraine searches were
avoided because many regions free of terrestrial rock
were available for searching and the difficulty of distin-
guishing terrestrial rocks from meteorite could be easily
avoided. ANSMET has tried several different meteorite
detection strategies and techniques in such environments,
and we have found none more effective than simply trust-
ing the human eye-brain combo to identify the rocks that
“don't belong” after a period of familiarization with local
2.3.1.4. Systematic searching procedures.
Systematic
searching is among the most basic of ANSMET field
activities. It involves the methodical recovery of meteor-
ites from a stranding surface where a meteorite
concentration is known to exist and the potential for
large numbers of recoveries is high. Typically, systematic
searching field teams consist of eight individuals, but
there can be more or fewer depending on factors such as
logistical availability and the area of ice to be searched.
Systematic search teams are normally only sent to sites
that have been explored in some detail by prior recon-
naissance teams, allowing priorities for a given season to
be set in advance and logistical demands to be well
constrained.
ANSMET search strategies typically follow the transect
sampling model in use by natural scientists for hundreds
of years [e.g.,
Anderson et al.
, 2002;
Barabesi et al.
, 2002;
Chen et al.
, 2002;
Hammond et al.
, 2002]. During these
transects the field team forms a line, each member a few
tens of meters to several tens of meters apart. The team
then proceeds to cross the meteorite stranding surface in
a direction perpendicular to this line. After each pass is
completed, the team changes direction and a new transect
is started, covering new ground and exploring new areas
of exposed ice (Figure 2.2). The orientation and pattern
of the traverses are adapted to local geographical fea-
tures, hazards, and weather conditions (wind, Sun angle,
and snow cover) to maximize the coverage and efficiency
of the search. The spacing, amount of overlap, and
method of travel (foot or snowmobile) may also vary
depending on frequency of meteorite encounter and
density of terrestrial rock.
