We took 8 samples of the oolites from depths between 3.72 and 5.12m from OL-92-3, and 12 samples of silty clay from depths between 5.23 and 7.11m from OL-92-3, and from depths between 7.21 and 31.13m from OL-92-1. Correlation of beds between OL-92-1 and OL-92-3 based on lithology and on measured depth was straightforward and unambiguous. Each sample consisted of about 50 cc of wet sediment and represented about 3 cm of section. In addition, we took two samples of silty clay sediment from considerably deeper in the core, beyond the limit of radiocarbon dating in order to asses the 14C background and contamination limits for dating this material. These samples were taken at depths of 36.1m (ca. 50 kyrs) and 55.1m (ca. 80 kyrs).
Oolite samples were cleaned, weighed, and acidified with 10% HCl in vacuum. The resulting CO2 was dried by passing through an oxygen flow- combustion train, using hot platinum as a catalyst and trapping water and SO2. The CO2 gas volumes for oolites and humates were measured and admitted to graphitizers designed for AMS target preparation (Vogel et al., 1985). These small-volume units were charged with 400 Torr of CO2 and 900 Torr of H2. Heating overnight in the presence of iron and zinc catalysts at 675°C produced sufficient graphite to be pressed into targets for the Lawrence Livermore accelerator. All reactions were carried to completion to eliminate isotope fractionation due to processing. Replicate targets were made of 4 oolite samples and one humate sample evaluate contamination during target preparation and to evaluate precision.
For the two "infinitely old" control samples, targets were prepared of
the "total organic", "humate" and "humin" fractions to determine the
viability of the CuO-Cu-Ag procedure described above. For the total-
organic fraction, a decalcified sediment-sample was combusted. The humate
fraction was the NaOH extractable fraction from another aliquot, separated
out as above, and the humin fraction was the solid residue after NaOH
extraction. Graphite targets were prepared as above. Targets were
analyzed at the Center for Accelerator Mass Spectroscopy (CAMS) of the
Lawrence Livermore Laboratory by John Southon.
Results
Results for the samples from the top 31m are given in Table 1 and Figure 1.
Results from the two deeper samples are shown in Table 2. Dates are
based on the Libby half-life (5568 yr), using an assumed d13C of -5 o/oo
for the oolites and -15 o/oo for the organic fractions. No attempt has
been made to calibrate the results to absolute years (e.g. Bard, et al.,
1990). Counting errors on individual samples are generally on the order
of ±2% or less. Replicates on the 5 samples for which duplicate targets
were prepared agreed within counting error for two of the oolite samples
(4.8m and 5.02 m) and between 5 and 6% for the other two oolite samples
and the single humate (3.82 m, 4.29m and 31.13m). Results on the
"infinite" samples indicate a probable limiting age of between 30 and 35
kyrs (Table 2). For the 36m sample, three replicate analyses of the total-
organic fraction yielded dates from 25 to 27 kyr, while the corresponding
humate yielded 30 kyr, and the humin 32 kyr. Similarly, the total
organics for the 55m sample yielded dates from 35.0 to 35.7 kyrs, the
humate 38, and the humin 37. Targets made from infinitely old calcite
yielded dates of 43.5 kyr while that of infinitely old coal yielded 35
kyr (unpublished data of U.S.G.S. Reston radiocarbon laboratory),
indicating contamination levels introduced during sample processing of
carbonates and organic matter and the ideal dating limits. The results
from the 55m sample are at this limit, whereas those at 36m are
considerable younger. Thus, we conclude that the practical limit for
dating humates from the Owens core is about 30 kyr, about the same as
encountered in radiocarbon analyses of organic material extracted from
other lake sediments (i.e., Robinson et al, 1988; Thompson et al, 1990).
This somewhat low limit is probably due to the relative ease of
contamination of the plastic and porous-wet sediment by the circulating
drilling mud that entrains and slurries younger sediments from the side
wall of the hole in the upper parts of the section.
Results on samples younger than the limit of 30 kyrs are relatively coherent, and represent two linear trends with an apparent hiatus between 5100 and 8300 yrs bp (Table 1, Fig. 1). The twelve dates on the oolites, which span only 1.25m of section (3.72 to 5.02 m) define a linear trend which projects to a zero age at -1.5m depth. This depth is essentially that of the base of the historic salt layer (-1.3 m), a result which adds considerable confidence to the dates. The magnitude of the reservoir effect on the dates (low initial 14C/C ratios in lake water), based on the detailed analysis of Benson (1993) for nearby Walker Lake, should be on the order of 200 years and certainly less than 500 yrs.
The dates on the humate extracts likewise result in a linear trend, except for the three samples from -12.97 to -15.32 m. Reexamination of this interval in the core indicated clearly that this section is slumped. The character of this slumped sediment in texture, mottling, color, and bedding is identical to that found above in the section between 8 to 9m where the dates would plot exactly on the trend. The deepest samples at 23.27m and at 31.13m all give dates of 30 to 32 kyrs bp, essentially at the practical limit as indicated by the deeper control samples. Thus, we are left with a coherent trend in the humates beginning at the base of the oolites at 8200 yrs to a date of 25,400 yrs at 23.27 m. The apparent sedimentation rate of the oolites is 70 cm/kyr and that of the silty clay is 83 cm/kyr. The contact between the oolites and the silty clays is abrupt, the bedding apparently conformable, and there is no evidence of pedogenesis at the top of the silty clay. We interpret the offset between the two trends as a disconformity, possibly caused by a period of complete desiccation of the lake followed by wind deflation of pre-existing sediments, or, alternatively, a sublacustrine slumping away of part of the section. The linear trend of the humate dates projects to zero age at +2 m. Thus, because the true zero-age of the section is at -1.3 meters (base of the salt), the amount of missing section represented by the disconformity is about 3.3m.
Deeper in the core, there is no evidence of a significant change in
sedimentological conditions as might be expected for the Pleistocene/
Holocene transition, between 10,000 to 14,000 yrs. Rather the significant
change in lake conditions seems to have occurred only after 8200 yrs.
Tables
The samples were analyzed at the Center for Accelerator Mass Spectroscopy (CAMS) of the Lawrence Livermore Laboratory by John Southon. Column headings used in the table are listed below:depth = depth in meters material = oolites or humate CAMS = Sample identification number used by CAMS Lab = laboratory that prepared the sample, one of B = Institute of Arctic and Alpine Research R = USGS, Reston 14C age = 14C age in years before present error = estimate of uncertainty in the 14C age- AMS Radiocarbon dates on various organic fractions from two sediment samples from drill hole OL-92, Owens Lake, taken from depths below radiocarbon dating limit (Extractions and preparation of graphite targets carried out at Institute of Arctic and Alpine Research 14C laboratory. Samples were analyzed at the Center for Accelerator Mass Spectroscopy (CAMS) of the Lawrence Livermore Laboratory by John Southon.) Under "material", total refers to total organics in the sample.
Depth 36m, approximate age 50 kyr before present:
material CAMS Lab 14C age±error total 6923 B 27470±110 total 6924 B 26580±110 total 7562 B 25370±290 humate 6928 B 29550±190 humin 6926 B 31760±180Depth 55m, approximate age 80 kyr before present:material CAMS Lab 14C age±error total 6921 B 35030±150 total 7563 B 35190±990 total 6922 B 36730±130 humate 6927 B 38430±170 humin 6925 B 36730±240
U.S. Department of Interior, U.S. Geological Survey
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