View figure 1.
Locations of sites discussed in text. The shaded areas represent elevations above 6000 ft (1829 m). Woodland and forest vegetation generally occurs above this elevation, steppe and desert below.
The regional vegetation of the eastern Great Basin is characterized by a general elevational progression of zones following the gradient from hot-dry environments on the lowest valley floors to cool-moist environments at high elevations. Sparse shadscale and greasewood steppe associations (such as that at the Pit of Death site) occur in the hottest and driest locales, giving way to more dense and slightly more diverse steppe communities on the upper bajadas. Sagebrush intermingles with these xeric elements on the upper bajadas and lower mountain slopes, and then continues as an understory element in higher elevation woodland and forest associations. Utah juniper (Juniperus osteosperma) is generally the lowest occurring tree in the region, and it forms pygmy-conifer woodlands with pinyon pine (Pinus monophylla) on the lower to middle mountain slopes. Ponderosa pine (Pinus ponderosa), fir/Douglas fir (Abies/Pseudotsuga) , and aspen (Populus tremuloides) forest assemblages occur with increasing elevation. The arid mountain ranges of westernmost Utah generally support limber pine / bristlecone (Pinus flexilis / P. longaeva) subalpine forests at high elevations, whereas the more massive Wasatch Range captures more precipitation and hosts spruce (Picea) forests at high elevations. The Wasatch Range also hosts oak- (Quercus-) dominated mountain brush communities that are not present in the more arid Great Basin ranges. Regional studies of the modern pollen rain (e.g. Davis, 1984; Thompson, 1992) indicate that the major vegetation assemblages of the Great Basin are reflected in the pollen rain.
Stratigraphy and sedimentology. Core sediments were logged in the field, and the stratigraphy and sedimentology was studied in greater detail in the laboratory by Oviatt and Kelsey (Black Rock) and Oviatt and Bracht (Pit of Death). These researchers have analyzed samples for carbonate, sand, and mud contents at approximately one-foot (30 cm) intervals throughout both cores. X-ray analysis is on-going for carbonate minerals from representative samples from both cores.
Paleomagnetism. Oriented paleomagnetic samples were taken by carving pedestals into the cores and slipping plastic boxes (2.5 x 2.5 x 1.5 cm) over the pedestals. In general, samples were collected at 1 m (3.3 ft) intervals throughout the length of both cores. For paleomagnetic studies, 307 and 139 samples were analyzed from the Black Rock and Pit of Death cores respectively. The samples were stepwise demagnetized by either alternating field (AF) or thermal methods. AF demagnetization was generally carried out at successive peak fields of 0, 10, 20, 25, 40, and 60 mT, and in some cases, additional intermediate steps were included at peak fields of 5, 15, 30, and 50 mT. Thermal demagnetization was carried out at successive steps of 20, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, and 600°C. Magnetic susceptibility was measured at each thermal demagnetization level to monitor for thermal alteration. Remanence measurements were made on a 2G Enterprises cryogenic magnetometer. AF demagnetization was carried out with a Schonstedt AF demagnetizer and thermal demagnetization was carried out with a Schonstedt thermal demagnetizing unit. Magnetic susceptibility was measured with a Bartington Instruments magnetic susceptibility meter.
Characteristic remanence directions were determined from stably magnetized samples by linear regression fits to the demagnetization data. Three types of demagnetization behavior are evident and are designated as types A, B, and C. Type A behavior is displayed by demagnetization data that decay to the origin of vector component plots with little deviation from linearity (Figure 2a, d). Type B behavior is displayed by samples that yield more scattered demagnetization data, nevertheless, the direction of magnetization can be unambiguously determined (Figure 2b, e). Type C behavior is displayed by samples for which no stable direction of remanence, or even an indication of polarity, can be determined (Figure 2c, f). In general, the data quality from the Pit of Death core are of substantially higher quality than those from the Black Rock core. Of the 139 samples analyzed from the Pit of Death core, 81%, 16%, and 3% of the samples displayed behavior of types A, B, and C, respectively. Of the 307 samples analyzed from the Black Rock core, 17%, 54%, and 29% of the samples displayed behavior of types A, B, and C,respectively. The Black Rock and Pit of Death cores were azimuthally unoriented. The paleomagnetic polarity was therefore determined solely from inclination data. All ages of paleomagnetic datums follow those of Cande and Kent (1992).
View figure 2a. sample BRU108, depth = 331'0"
View figure 2b. sample BRU145, depth = 447'11"
View figure 2c. sample BRU10, depth = 55'11"
View figure 2d. sample POD7, depth = 28'3"
View figure 2e. sample POD38, depth = 88'10"
View figure 2f. sample POD67, depth = 324'9"
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Palynology. Sediment samples from the Black Rock and Pit of Death cores were processed with chemical reagents (HCl, HF, heavy liquids) to remove unwanted mineral materials. The sample residues were analyzed under 400X to 1000X magnification, and a minimum of 300 terrestrial pollen grains were counted from each sample (except for samples above ~150 ft in the Black Rock core, where pollen concentrations were so low that only 200 grain counts were possible). Of the 159 samples processed from the Black Rock site, 142 contained sufficient pollen for analysis. Ten samples were processed from the Pit of Death core, and all were barren of pollen.
Plant macrofossil analysis. Seeds and leaf fragments and Ruppia and other plants are present throughout most of the Black Rock core. The occurrences of these plant macrofossils has been recorded during the sediment descriptions and by examination of slides prepared for ostracode analysis (see below).
Ostracode Analysis. Sediment samples for ostracode analysis were split into two fractions, a larger fraction for isotopic analysis of ostracode shells (~15 g) and a smaller fraction for ostracode counts (~5 g). The samples were subjected to a freeze/thaw process to disaggregate clay particles and then washed with hot water over a 100 mm mesh screen. Approximately 920 samples have been prepared from the Black Rock core, and 10 from the Pit of Death Core.
Diatom Analysis. Preliminary analysis of diatoms from the two cores were conducted using water mounts of unprocessed sediment smears. Twenty-four samples have been analyzed from the Black Rock core and 10 from the Pit of Death core.
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