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Core OL-92 from Owens Lake, southeast California

Introduction

George I. Smith
U.S. Geological Survey, Menlo Park, California
An important element of the investigations supported by the USGS through its Global Change and Climate History (GCH) Program is the record of past changes in precipitation in now-arid parts of the United States. More than a century of geologic investigations has shown that major changes in precipitation and runoff occurred throughout much of this region, as shown by the evidence of fluctuations in the levels of lakes in the Great Basin. Although these basins--sometimes termed "nature's rain gages"--clearly document major changes in climate during the past few hundred thousand years, there is an inadequate consensus about those lakes' ages or the quantitative meaning and meteorological significance of their fluctuations. The timing and intensities of these climatic changes pose important questions to earth and paleoclimatic scientists, among them: Milestone studies of these Pleistocene lake histories published prior to 1980 are summarized by Smith and Street-Perrott (1983), and subsequent studies are noted by Benson and others (1990). The late-Pleistocene and Holocene histories of some of the lakes discussed in these summaries have been determined in detail. Comparable information about earlier (ca. >150 ka ) lake histories is more difficult to extract from geologic records, however, because most of the sediments and other evidence of former lakes have been destroyed by erosion or buried by younger deposits. This older segment of the continental record is important because we do not have enough geologic perspective on past climates to reconstruct the prevailing air-mass circulation patterns, or to judge whether or not the climate exemplified by the Holocene Epoch should be considered "typical" of a major part of the Quaternary Period.

Records of many earlier Pleistocene-age lakes can be found in deposits beneath the surfaces of modern lakes or playas, but core drilling is required to obtain them. A drilling program such as this, lasting several years, was envisioned by several of us affiliated with a GCH workshop in the Spring of 1990. Owens Lake, a closed basin in southeast California, was determined to be one of the promising sites because:

  1. The Owens Lake area lies in a well known geologic and paleoclimatic setting. In the geological past, it was the first in a series of Pleistocene lakes that at times extended south and east to Indian Wells, Searles, Panamint, and Death Valleys, the floors of which are now dominated by playa lakes (see Smith, 1993, Fig. 1). The number of perennial lakes in that succession primarily reflected the amounts of precipitation falling in their collective basins, which included the high eastern slopes of the southern Sierra Nevada which drain into the Owens River, as well as the slopes of lower-elevation ranges that adjoin those lake's basins. Variations in wind, relative humidity, temperature, and other climatic variables that influence evaporation rates were also factors in determining lake sizes, but changes in them were less important than variations in precipitation (Smith, 1991). Published studies of exposed lacustrine outcrops, cores, and landforms have helped reconstruct the past histories of lakes in the downstream basins that were part of this formerly-extended drainage (Gilbert, 1875; Gale, 1914; Blackwelder, 1933; Smith, 1962; Hooke, 1972; Smith,1975; Smith, 1979; Smith and others, 1983; Smith, 1984); glacial, geomorphic, and botanical studies in these and adjoining areas provide additional criteria that help reconstruct past climates (Blackwelder, 1931; Sharp and Birman, 1963; Martin and Mehringer, 1965; Burke and Birkeland, 1983; Sharp, 1987). These and many other studies promised to provide constraints when interpreting the lacustrine record of Owens Lake because these areas were all part of the same climatic and hydrologic system, and their histories, or some modification of them, must end up in agreement.

  2. Owens Lake today lies in a well-known hydrologic setting. Its drainage area is one of the most thoroughly studied in the United States as a result of more than a century of measurements by scientists and engineers concerned with the water supply for the City of Los Angeles. The relation between modern precipitation and runoff, therefore, is well documented. For this reason, past relations between temperature, precipitation, evaporation, and runoff in the Owens River drainage can be estimated on the basis of a well-established foundation of numerical data.

  3. Geophysical studies have shown that more than 1.8 km of low-density sediments underlies Owens Lake's surface (Pakiser and others, 1964), meaning that a long record of valley-filling sediments of late Cenozoic age is likely to be preserved. As these geophysical studies also show the bedrock surface beneath this part of the basin to be the deepest and broadest in Owens Valley, it is also a likely site for that deposition to have been in a lake.

  4. Geological studies show that the central Sierra Nevadas have been within a few hundred meters of their present elevations during the past million years, approximately the maximum period of interest in this study; meteorological considerations suggest that this would have been sufficiently high to condense enough precipitation over the Sierras to support a perennial water body in Owens Lake. Uplift rates of the Sierran terrain near the north end of the Owens River drainage area elevated the range's crest to within about 1 km of its present altitude by 3 m.y. ago (Huber, 1981), so the crest elevation during a less-than-1-m.y.-long period was probably within 300 to 400 m of its present level; a reduction of 350 m in adiabatic uplift would reduce air-mass cooling by about 3°C. Throughout that period, therefore, atmospheric moisture moving east from the Pacific was carried by air masses that were adiabatically cooled to temperatures that were within 3°C of present condensation temperatures, causing nearly as much moisture to precipitate over the Sierran crest which would be distributed areally about as at present. This would have provided amounts of runoff that were likely to have supported a perennial water body in Owens Lake. This scenario is supported by the absence of salts below those deposited in the lake earlier this century, as indicated by the 278.5-m-deep core record obtained in 1953 from Owens Lake (Smith and Pratt, 1957, p. 5-14). Perennial-lake waters, therefore, appear to have covered the lake floor for the period represented by that core. Finally, Holocene climates in the American Southwest have been substantially drier than Pleistocene climates during the past 30 k.y or more (Baker, 1983), yet historical records (up to about 1912, when the Owens River was diverted to Los Angeles) consistently describe Owens Lake as a perennial body of water. A perennial lake would have accumulated a lacustrine record that was both continuous and unaffected by subaerial erosion.
The drilling project at Owens Lake commenced in April, 1991. This Open- File Report represents an effort to make available to other researchers our preliminary data collected during the first year of study following completion of the core-drilling phase. Nineteen data collections and preliminary interpretations are presented in the following sections. They are the work of fifteen first-authors and their numerous co-authors. Broadly, their topics include a field log of the core (1 contribution), sedimentological analyses (1), clay-mineral identification (1), geochemical analyses (5), dating and age estimates of the cored sediments (4), and identifications of fossil materials (7).

Supplemental data are also included on the depths of various sample sets used by these investigators, and the depths in the core assigned to the tops of each "run" and "slug" (see next section for explanation of these two terms); these will enable future researchers to locate the horizons in the cores from which we took samples, and therefore accurately place new data, based on their samples, into the same stratigraphic order. Details about the location of the core site, drilling equipment and methods, sampling and curating procedures, and lithologic-description criteria are presented in the following section.

References

U.S. Department of Interior, U.S. Geological Survey
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