Sampling Recommendations
The results of grain-size analyses are sensitive to the manner in which
the original samples are collected, handled and stored. For example, it
is clear that the introduction of foreign particulate matter into the
sample through improper care or cleaning of equipment, or through
improper processing, can alter the texture. The growth of authigenic
minerals because of improper storage can have a similar effect and,
although physical disturbance may not generally have a direct effect on
subsequent analyses, the trends of those data may be altered if sediment
layering has been disrupted. In sum, analyses subsequent to improper
fieldwork or storage techniques may be meaningless.
To keep sampling artifacts and variation in data to a minimum, certain
precautions should be taken in the field during sampling:
- Records on sampling, including location field measurements,
should be painstakingly maintained. The appropriate field measurements
and any information peculiar to the sample should be supplied to the
laboratory along with the sample.
- Particularly when the top of a sediment section is of interest, care
must be exercised during sampling. Over-penetration by sampling device
and jarring or bumping during retrieval should be avoided. Surficial
samples for grain-size analyses are typically defined as having been
collected from 0-2 cm
below the sediment/water interface.
- If the samples are in cores with liners that need to be split, the
device chosen for cutting should not add plastic shards to the samples
since contamination from these shards affects the textural analysis.
Core halves should be stored in well-sealed D-tubes with a moist piece
of
floral "Oasis" or sponge to retard evaporation. If the cores are split,
it is recommended that one half should be held in reserve until all
analyses are complete, and data quality can be assured.
- The subsamples should be stored in containers that are appropriate
to the individual analyses. Inert air-tight containers such as plastic
jars, boxes, or bags are preferable to metal cans that could rust and
contaminate the sample, or cloth bags that allow evaporation and the
loss of fines.
- All sampling utensils and containers must be clean and free of
contamination. If splits of the samples are to be used for geochemical
analyses (for example, trace metals) (Jablonski
and others, 2002), the sampling gear should be plastic or Teflon-A
coated and the containers acid washed.
- Care must be taken to obtain a representative split when sampling in
the field. Be aware of lateral and vertical variability in grab samples.
Collect larger samples from gravelly sediments.
- Careful labeling is critical. Labels must be legible and permanent.
Test the permanency of markers on the sampling containers prior to
fieldwork.
- To prevent geochemical reactions, the growth of organics or
evaporation in a sample, refrigeration or freezing is usually necessary.
Such reactions may change the mineralogy and grain-size distributions,
complicate laboratory analyses, and increase costs. For example,
authigenic minerals such as gypsum may form individual crystals that may
grow as large as sand or even gravel-sized particles.
- Similarly, excessive evaporation must be avoided, especially
if the samples are marine and it is necessary to correct for salt
content.
All analyses should be performed within a reasonable time (less than 2
months). The longer a sample is stored, the greater the opportunity for
storage-related alteration.
- Illustrations of some of the sampling devices used at the
U.S.
Geological Survey, Woods Hole Coastal and Marine Science Center (WHCMSC) are shown on the
following pages.
|
OFR 2005-1001
Home /
