Sedimentary analysis of cores from Bear Lake emphasizes textural characteristics
that can be observed continuously, rather than by sample-dependent analytical
techniques. The textural observations emphasize characteristics such
as layering, bioturbation features, and distribution of grain sizes and
grain types (Smoot, 1998; Smoot and Benson, 1998; Kemp and others, 2001;
Last, 2001). The sedimentary analysis not only constrains the variability
of the depositional environment, but also provides important information
about the nature of the materials analyzed biologically and chemically.
The origin or significance of the sedimentary features observed may be
inferred by comparison to features from other lakes or from basic mechanical,
chemical, or biological principles (Lomando and Schreiber, 1994; Smoot
and Benson, 1998). More often, the sedimentary features need additional
constraints from chemical or biological analyses in order to distinguish
among several possible origins.
Three Kullenburg piston cores (Glew and others, 2001) were collected along an
east-to-west transect in 1996. The cores were cut into meter-long segments
and each segment was split in half lengthwise for sampling and archival
purposes. The archive half was cleaned with a straight razor, described
with a 10-power hand lens, logged, and photographed with 35-millimeter
slide film in 14-centimeter (14 cm) overlapping segments. The photographs
for each core segment were scanned at 3,000 dpi (dots per inch), stored
on compact discs (CDs), and then combined into a composite core photograph
using Adobe Photoshop v.4.2. The composite photographs and log descriptions
were used to produce detailed graphical core descriptions. A 1-cm-thick
slab was cut from each archive core segment for x-radiography in a Picker
Minishot 1 using 17-inch Agfa Structurix D4DW film. The x-radiographs
were described and logged with a 10-power hand lens to produce the sections
presented here.
Smear slides were collected at 10-cm intervals and at visible changes in sediment
character. A subsequent set of smear slides was collected where changes
in fabric are visible in the x-radiographs. The smear slides were examined
using a Zeiss binocular petrographic scope with 10-power occular and
40-power objective lens. Several smear slides were counted along a ten-micron
grid for variability in diatoms and grain types. Three hundred diatoms
were counted on each slide, whereas grain types were estimated by using
a shadow diagram (Terry and Chilingar, 1955). A few intervals were washed
in a wet sieve to examine sand-sized material using a Nikon Zoom binocular
scope at 40-power resolution. No attempt was made to identify genera
or species of shells observed in this manner. Polished thin sections
were made at select intervals from chips imbedded with Spurr resin as
described in Kemp and others (2001). These were examined by petrographic
microscope and Scanning Electron Microscope.
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Posted August 2005 |
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