Methods
Approximately 0.5 cc of dry sample was placed in a 15 ml graduated test
tube and covered with about 5 ml concentrated HNO3. After cessation of
CaCO3 reaction, the test tubes were placed in 100o C hot water bath for 30
minutes to dissolve remaining carbonates and related minerals and to
oxidize organic matter. When cool, the acid was diluted with distilled
water and dissolved substances in the supernatent liquid were removed by
decantation following centrifugation. Rinsing, centrifugation and
decantation were repeated until all dissolved materials were removed from
the processed material. The processed sediment was then allowed to settle
in the tubes overnight, and the volume of water-saturated sediment was
determined in the graduated tube. The water-saturated sediment was then
suspended in 10x its water-saturated volume. Three drops of the diluted
suspension were placed in a 50 ml settling dish that contained an 18 mm
diameter coverslip. Water was added to the settling dish to resuspend the
sediment-laden drops so the sediment and diatoms could settle evenly over
the coverslip and floor of the settling dish (e.g. Battarbee, 1974).
After 3 hours, the water was removed from the settling dish by micro- capillary wicking and the coverslips were allowed to dry. Coverslips were mounted in hyrax, a refractive mounting medium (n = 1.65). Diatoms were enumerated at 1000x magnification along transects measured by stage micrometer until at least 30 mm of transect were examined or until at least 300 diatoms were counted. Because the water-saturated sediment volumes were all proportionally diluted (10x) and because the same volume (3 drops) of suspended sediment was placed in each settling dish, the diatoms encountered per mm of transect in each sample gives an approximation of diatom concentration in the sample.
The technique can only be approximate for the following reasons: (1) The original samples contain variable amounts of soluble minerals (mostly carbonates) and digestion therefore variably concentrates diatoms. (2) Samples rich in glacial flour, silt, clay, or diatoms behave differently during suspension and delivery to the settling dish so that different amounts of residue may be deposited onto the coverslip. (3) Measured volumes in graduated 15 ml test tubes and by drops from disposable pipettes have inherent inaccuracies. (4) Diatoms are variably fragmented and enumeration techniques discriminate against small fragments. Nevertheless, the variation in numbers of diatoms in the samples is greater than the variability in lithologies and quantification techniques. The results can be considered consistently semi-quantitative and provide a useful measure of diatom concentration.
Diatoms were identified as closely as possible to descriptions of taxa in standard texts or placed in an open nomenclature. However, the diatom flora of the western United States is poorly known, and some taxa from Quaternary deposits of distinctive paleolimnological environments are probably new to science. In some cases such taxa are recorded by "cf." or "aff." on the original worksheets; in other cases they are lumped together with the nearest taxon of similar morphological appearance. Typically diatom preservation is variable and often quite poor in Owens Lake sediment because of breakage and corrosion. The raw counts presented in Appendices 1 and 2 represent whole valves and (or) large (> 40%) fragments of valves. Table 1 provides the provisional species names represented by the coded abbreviations on the spreadsheets.
Overall, Figures 1-18 document rapid fluctuations of numbers of individual diatom taxa counted, even over short stratigraphic intervals. Diatom concentration, calculated as the number of all diatom valves encountered per mm of microscope transect (Fig. 19) also varies widely between adjacent samples, particularly in zones where saline diatoms are abundant. Throughout its history, Owens Lake was extraordinarily sensitive to hydrologic and climatic change, sometimes oscillating between a large, through-flowing, freshwater lake and a saline playa over short periods of time. Stratigraphic and paleolimnologic continuity is better developed during periods when the lake was fresh and overflowing than when it was shallow and saline. For example, the most closely spaced samples (e.g. Core 1; 9.5 - 19.0 m) are about 20 cm (~250 yr) apart (Fig. 20). Within this interval, the concentration of Asterionella formosa, a freshwater planktonic diatom documents cycles of limnological change during the full glacial (15.9 - 25.3 ka) at millennial to sub-millennial scales. The greater stratigraphic continuity during fresh, high-water stages probably reflects more stable limnologic environments that favor deposition rather than short cycles of deposition and erosion characteristic of low and saline lake stages.
It is clear that at least parts of the Owens Lake cores have the potential for high resolution studies of climate change if detailed chronologies can be established.
The diatoms of the Owens Lake core can be organized into ecological groups that have more or less coherent stratigraphic distributions. Five groups have been established (Figs. 21-22).
Peaks of freshwater planktonic diatoms logically correlate with increased flow of the Owens River and probably reflect moister, glacial climates during which Sierran glaciers would expand. Saline planktonic and benthic diatoms most likely correlate with arid, interglacial climates and reduced flow of the Owens River.
Species of Fragilaria and other freshwater benthic diatoms have a similar stratigraphic distribution, although benthic Fragilaria species dominate below 200 m (Fig. 22). Large concentrations of Fragilaria species indicate periods of time when Owens Lake was shallow, but fresh; probably a through-flowing marsh system. It is reasonable to suppose that tectonic rejuvenation of the basin was required to reestablish a lake morphometry suitable for planktonic diatoms, although geomorphic alteration of the Owens sill could also be involved. Perhaps the stratigraphic distribution of Fragilaria species documents periods of tectonic quiescence that allowed the Owens basin to fill with sediment and develop persistent shallow but fresh marsh environments.
The full paleolimnological interpretation of the diatom stratigraphy of the Owens Lake cores will require integration of ancillary stratigraphic data from pollen, ostracodes, geochemistry and mineralogy coupled with a detailed absolute chronology.
U.S. Department of Interior, U.S. Geological Survey
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