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Clear Lake occupies a structural depression in the northern California Coast Ranges at an elevation of 404 m. Eight sediment cores were taken from the lake in 1973. This paper reports the palynology of cores CL-73-4 and CL-73-7. The former is 115 m long, and is interpreted to cover the entire last glacial cycle; the latter is 27.5 m long and covers at least the last 40,000 radiocarbon yr.

The pollen record of core CL-73-4 is dominated by three pollen types (oak, pine, and TCT [Taxodiaceae, Cupressaceae, and Taxaceae]) that together account for between 75 and 99 percent of the pollen in each sample. Core CL-73-7 is similarly dominated by these pollen types, but aquatic and riparian pollen types are also locally important.

The present vegetation around Clear Lake consists of oak woodland; mixed coniferous forest is found at higher elevations in the surrounding mountains. The present pollen rain into Clear Lake is dominated by oak pollen. During the cooler parts of the last glacial cycle, oak pollen influx to the sediments of Clear Lake was largely and at some times entirely replaced by coniferous pollen (mostly pine and TCT) in response to vertical migration of vegetation belts caused by climatic changes.

Pollen data were reduced using a Q-mode factor analysis. Five factors were defined that account for more than 98 percent of the variance. Three of the factors summarize aspects of the behavior of the regional forest vegetation around Clear Lake, and two summarize the behavior of the aquatic and swamp vegetation in the lake itself.

Zoning of the pollen diagrams was accomplished using an iterative program that minimized the total sums of squares of the factor loadings within zones. Twenty-one pollen zones are defined for core CL-73-4, four for core CL-73-7.

The pollen zones of core CL-73-4 are used to propose a series of informal climatic units that include the time interval from the penultimate glaciation to the present. The major units proposed, from oldest to youngest, are: (1) Tsabal cryomer, (2) Konocti thermomer, (3) Pomo cryomer, and (4) Tuleyome thermomer (Holocene).

The Pomo cryomer is divided into early, middle, and late phases. The early Pomo includes a series of five cold/warm oscillations that are designated the Tsiwi cryomers and the Boomli thermomers, numbered from Tsiwi 1 (oldest) to Boomli 5 (youngest). Middle Pomo time includes the Cigom 1 cryomer and the Halika thermomers, a series of three minor warm intervals. Late Pomo time includes the Cigom 2 cryomer and a transitional interval following it and preceding the Holocene.

The climatic oscillations of the Tsiwi cryomers and Boomli thermomers were often quite abrupt; both sudden warmings and sudden coolings occurred. The most severe of these changes was the cooling that occurred at the end of the Konocti thermomer, when oak pollen frequencies dropped from more than 60 percent to about 25 percent within a stratigraphic interval of only 23 cm. These sudden changes were climatic catastrophes for the ecosystems that experienced them.

The record in the sediments of algae with acid-resistant remains indicates that lake productivity was relatively high during warm intervals in the past, and that overall productivity increased as the lake became shallower and its thermal inertia decreased. The lake waters were probably transparent during the cooler parts of the last glacial cycle, but Clear Lake has probably not been as clear a lake during the Holocene.

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