Radiocarbon content, sedimentation rates, and a time scale for Core CL-73-4 from Clear Lake, California

Radiocarbon dating of disseminated organic matter from 10 horizons in Clear Lake core CL-73-4 produced apparent ages ranging from 4,230 to 36,650 years before present (B.P.). Old carbon from lake sediments and springs beneath the lake adds about 4,200 years to the apparent age of each Holocene sample. A significant component of younger carbon, which cannot be completely removed by cleaning in sodium hydroxide solution, makes the dates older than 20,000 years unacceptable. The younger dates are corrected for the old carbon effect, calibrated to the dendrochronologic time scale, and then used to derive a sedimentation rate for the Holocene part of the core. Sediment accumulation is expressed as the mass in kilograms per square centimeter of noncombustible overburden above a given level in the core in order to compensate for variations in degree of sediment compaction and organic content. The Holocene sedimentation rate, when applied to the entire core, yields an estimated core-bottom age of 133 ka. This independent evidence is consistent with the correlation of the high oak-pollen zone just above the base of the core with the last interglaciation. When the oak pollen maxima at the top and bottom of the core are equated with the Holocene and the last interglacial, the larger intervening fluctuations in the oak curve show a marked similarity to the climatic record preserved in deep ocean sediments. The major fluctuations of the oak pollen curve are correlated with their counterparts in the deep sea record, and the Clear Lake time scale is further refined by adjusting the age of the apparent Stage 5/4 boundary to 73 ka. The revised time scale indicates that sedimentation rates during the last glacial and interglacial were slightly higher and lower, respectively, than during the Holocene. According to the revised time scale, interstadial events in the Clear Lake pollen record appear synchronous with prominent radiocarbon-dated interstadials in other areas, as well as with high sea stands dated by uranium-series disequilibrium methods. (See also W89-10137) (Author 's abstract)