Core OL-92 from Owens Lake, southeast California

Continental-marine correlation of Late Pleistocene climate change: Census of palynomorphs from core OL-92, Owens Lake, California

Ronald J. Litwin
Norman O. Frederiksen: U.S. Geological Survey, Reston, VA
David P. Adam: U.S. Geological Survey, Menlo Park, CA
Victoria A.S. Andrle
Thomas P. Sheehan: U.S. Geological Survey, Reston, VA

Introduction

The initial palynomorph census obtained from core OL-92 indicates the presence of a nearly continuous record of pollen assemblages through approximately 320 meters of lake sediments from Owens Lake, Inyo County, southeastern California. The sediments recovered range in age from present day to approximately 0.8 Ma, i.e., the entire Brunhes polarity chron and the uppermost portion of the Matuyama polarity chron (Smith, this volume; Glen et al., this volume). Preliminary palynological results suggest a high correlation of the terrestrial fossil record to d18O isotope curves derived from marine core analyses. Core OL-92 therefore has potential to become an important baseline record for correlating climate change in terrestrial and marine environments through the Late Cenozoic.

This study had four purposes:

to assess the quality and continuity of the palynomorph record in core OL-92,
to determine the core's potential for providing a high resolution signal of climate change for the southern California Basin and Range through the Late Cenozoic, on the basis of its fossil pollen,
to correlate this terrestrial climate change record with high resolution marine cores through the same temporal interval, and
to create a statistically significant pollen database for comparison with coeval Late Cenozoic long terrestrial records across the western U.S..

Preliminary conclusions are discussed here; detailed analyses of the pollen assemblages from these core samples will be presented at a later date.

Location

The field location and a detailed core log of OL-92 are noted in Smith (this volume). The core was sited at approximately 36° 22'50"N, 117° 57' 40" W, in Owens Valley, at the southwestern end of Owens Lake (Figure 1A).

Provenance

The palynological record preserved in the core sediments represents a regional vegetational record of the Owens Valley watershed, derived in large part from the eastern flank of the southern Sierra Nevada (John Muir Wilderness Area and Inyo National Forests) and the western flank of the Inyo Mountains (including part of the Inyo National Forest)(Figure 1A). Drainages from these two areas combine in the valley to form the south- flowing Owens River that presently terminates in Owens Lake at the southern end of the valley. To the southwest of the core site Ash Creek drains approximately 7400 ft. of vertical relief above lake level on the eastern flank of Muah Mountain. To the northwest of the core siteCottonwood Creek drains nearly 9300 ft. of vertical relief above lake level on the eastern flank of Cirque Peak and the southern flank of Wonoga Peak.

Age calibration

The pollen record of Owens Lake core OL-92 is especially valuable for climate change research because it can be calibrated for age by several independent means. First, coring at 309.1-308.5 m depth penetrated a tephra marker bed, the Bishop Ash (primary ashfall). This tephra unit is geographically widespread from California to Kansas (Izett et al., 1988; Figure 1B) and has been dated radiometrically at an absolute age of 0.78±0.02 Ma (Izett and Obradovich, 1991), on the basis of 40Ar-39Ar laser fusion analysis of the sanidine component of the tuff. Second, the Brunhes-Matuyama polarity chron boundary likely was penetrated just below the tuff, in our sample depth interval 309.1-312.33 m (see discussion, below). Izett and Obradovich (1991) back-calculated the Brunhes-Matuyama chron boundary at approximately 0.79 Ma, on the basis of inferred deposition rates for other lake cores in the western region that also recorded the tuff and polarity datums. They noted that their age estimate corroborated ages that were previously derived independently by oxygen isotope analyses (Shackleton et al., 1990) and astronomical calculations (Johnson, 1982). Third, Bischoff et al. (this volume) calculated age/depth relationships for the core on the basis of 14C isotope ages (upper part of core) and on 40Ar-39Ar laser fusion analyses of the Bishop Ash (Izett and Obradovich, 1991) in the lower part of the core.

Materials and methods

This study was a reconnaisance-scale palynological census of the OL-92 interval 21.09-323.28 m depth. Fine resolution pollen analysis of the core interval 0-60 m is in progress by researchers at the University of Arizona (Woolfenden, this volume), and the pollen assemblage dataset through the 0-20 m depth interval is not included in this report. Both reconnaissance and fine scale sampling were done through the depth interval 20-60 m, to permit comparison of results of the two sampling scales. The sampling interval for the pollen assemblages presented in this report (138 samples) is noted in Figure 2 and Tables 2-3, and correlates directly with samples obtained for diatom analyses (Bradbury, this volume). For pollen extraction, approximately 2-5 grams of sediment were decalcified with 20% HCl and demineralized with 52% HF (in a fume hood) until the mineral fraction was disaggregated. Strew slides of neutral pH residue were made to identify incompletely disaggregated samples. Samples of silt-sand then were acidified in 2% HCl and centrifuged in an aqueous solution of zinc chloride (s.g. 2.0) for specific gravity separation. Undigested or clay-rich samples were retreated in HCl, HF, or were winnowed of clay before specific gravity separation. Clay removal was accomplished by centrifugation of mud in a dilute low-sudsing detergent (surfactant). (For specific procedures see Litwin and Andrle, 1992). The residual organic fraction was isolated by the specific gravity separation mentioned above. Palynomorphs also were recaptured from the clay-rich supernatant fraction by vacuum-filtering with an 8 _m nylon filter. These were rinsed in HCl and combined with the other palynomorph fraction. Melted glycerine jelly was added to the recombined palynomorph fraction for permanent microscope slide preparation.

Tabulation

Minimum census counts of 200 specimens were made from each sample where possible (total dataset of ~28,600 specimens). Forty two taxa and census categories (combinations of taxa) were identified. Pollen identifications mostly were made to generic level and in several cases to family level. Conifer pollen frequently was encountered as broken specimens in these preparations. To correct this we counted isolated sacci as half-specimens and did not count isolated corpi. The subtotal of broken grains was added to counts of complete conifer pollen grains for a final sum. All identifiable taxa were codedfor census (Table 1). Census figures are presented in Tables 2a and 2b. All counts below 208 m depth were normalized to 200 specimens (Table 2b) before preliminary climatic trends were interpreted. Original preparations from which these census data were compiled are reposited at the U.S. Geological Survey (Reston, VA).

Discussion

The initial pollen analyses indicate that the Owens Lake core OL-92 preserves a high quality, nearly continuous terrestrial vegetational record from the Owens Valley watershed that spans the past 0.8 Ma. A high rate of change was noted for individual taxon frequencies and for assemblage diversities. We attribute this in part to our relatively low sampling density, and in part to the presence of a high resolution climate signal. Figure 3A illustrates the curve for pines (genus Pinus, including all species) plotted against core depth (unit depth), with ages assigned on the basis of the age/depth values calculated by Bischoff et al. (this volume). We used Pinus relative frequency to test for response to changes in temperature and precipitation associated with glacial and interglacial events because it was the dominant element in our long terrestrial record, because the genus is known to be a prodigous producer of pollen, and because its relative frequency had a high rate of change. Relative frequencies below 50% and above 90% were chosen for initial comparison (minima M1-M8 and peak abundances P1-P7, Figure 3A) to known climatic shifts. The deep sea _18O isotope curve of Ruddiman et al. (1989) from Site 607 (North Atlantic Ocean) was chosen as a good approximate glacial/ interglacial record for comparison to the Pinus curve of Core OL-92. The age/depth values in Figure 3A then were transposed and the Pinus curve was replotted (Figure 3B) so that preliminary comparison could be made between the Pinus and _18O curves at the same scale (as a tentative estimate of unit time).

The initial results indicate that the pollen record for the pines shows a strong positive correlation with each of the marine oxygen-isotope Stages for this time interval. The stratigraphic interval examined for this report probably correlates with _18O isotope Stages 2 (base of 1?) to 22 (23?). The Pinus minima M8-M1 appear to correspond to isotope stages 2, 4, 6 (~M6 + M5), 8, 10, 14, and 22?, respectively. The Pinus peak abundances P7-P1 appear to coincide with isotope stages 3, 9, 11, 13 (~P4 + P3), 18?, and 21?, respectively. With the exception of P2, peak abundances of Pinus appeared to be coincident with interglacial intervals, and minima appeared to be coincident with glacial intervals. Additionally, there is preliminary evidence that the pollen curves at least in some cases have the capability to resolve substage equivalents within the isotopic record (Figure 3B, compare the isotopic and pollen excursions for isotope Stage 7). Both convergent and divergent intervals are present in the pollen curve, with respect to the isotopic trends. Presently we suspect that convergent trends ("in phase" intervals) may represent larger scale vegetational response to climatic change, and that divergent trends ("out of phase" intervals) may represent geographically localized terrestrial response, forest disequilibrium, or artifacts of our sampling interval and/or density. Some response lag may be present in parts of the pollen curve (Figure 3B); this may be real or an artifact. Vegetational leads or lags, with respect to glacial and interglacial trends, will be able to be established more conclusively once radiometric and paleomagnetic studies are finished, and calibration of core Ol-92 is finalized.

In both plots (Figure 3A and Figure 3B) the pollen curve appears to exhibit two general signal frequencies. The shorter of these frequencies may correspond to a 41,000 year orbital cycle, and the longer of these to a 100,000 year orbital cycle. If this is true, the Pinus curve (in particular) may provide terrestrial evidence that the periodicity shift from the Matuyama chron (dominantly a 41,000 year frequency) to the Brunhes chron (dominantly a 100,000 year frequency) may not have occurred before the beginning of the Brunhes chron. This polarity shift was noted in marine records by Ruddiman et al. (1989). Additionally, the pollen records may suggest some overprinting of the shorter cycle on the longer through much of the Late Cenozoic.

Once the OL-92 pollen dataset is resolved more finely (i.e., with a closer sampling interval), it may permit a direct comparison to other exceptionally long terrestrial records such as those at Clear Lake (Adam et al., 1981; Adam, 1988a, 1988b) and at Tule Lake (Adam et al., 1989), and direct comparison to Pacific marine records, such as those by Heusser and Shackleton (1979), Adam (1988b), Gardner et al. (1988), and Heusser and Heusser (1990). Although these results are preliminary, they do suggest that the terrestrial vegetational record preserved in the sediments of Core Ol-92 shows marked response to climatic shifts, and has the ability to be correlated to the deep sea d18O isotopic record.

Current work

Our current studies focus on testing the response of our other census categories (and combinations of them) for the OL-92 record. Ultimately, we hope to correlate our pollen record with other long terrestrial records, such as the Tule Lake and Clear Lake records, and to test the practical limits of resolution in several depth intervals of core OL-92, in order to determine the resolution level of climatic response in the fossil floral record. We would like to determine the heterogeneity in resolution potential through core OL-92, as we do not expect glacial and interglacial resolution potentials necessarily to be equivalent. Results of these studies will be presented elsewhere, but initial results of testing this model to the Brunhes chron pollen record at Tule Lake tentatively suggest that we can resolve each of the equivalent isotope stages where sampling density and pollen recovery were sufficient.

Conclusions

Initial analyses of the long terrestrial pollen record of Owens Lake core OL-92 suggest the following conclusions. First, the potential for examining high resolution climate change through the Late Cenozoic appears to be excellent, as recorded in the fossil palynomorph record in sediments of Owens Lake and previously studied western U.S. long terrestrial records. This potential for the Owens Lake pollen record is enhanced by independent age controls (radiometric and paleomagnetic datums) that exist in parts of the cored interval and which permit a high quality age calibration for its fossil vegetational record. Second, correlation of the OL-92 pollen record with high resolution deep sea cores as far back as the Brunhes-Matuyama chron boundary appears to be feasible, and oxygen-isotope Stages 2(1?)-22(23?) tentatively were identified in this core. Fluctuations in floral diversity were noted through the sample interval, but probable causative factors were not established. We only note that diversity in any sample was in part inversely correlated with Pinus frequency, and that our studies on this topic are in progress. Lastly, we believe the palynomorph record at Owens Lake appears to have high potential for calibrating geographically separated Late Cenozoic long terrestrial records and for correlating marine and terrestrial Late Cenozoic climate change in the western U.S., through the Brunhes-Matuyama chron boundary.

Acknowledgements

This study is part of the U.S. Geological Survey's Global Change and Climate History Program. The authors acknowledge with gratitude the following: G.I. Smith (USGS, Menlo Park) for sample splits from core OL-92 and helpful discussion, J.L. Bischoff (USGS, Menlo Park) for sharing age/ depth calibration values, Thomas M. Cronin, Scott E. Ishman, and Jurate Landwehr (USGS, Reston) for helpful discussion, R.Z. Poore (USGS, Reston) and Thomas M.Cronin for critical review of this manuscript, and Nancy J. Durika (USGS, Reston) for preparation of tables and technical support.

References

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