The actively deforming Clear Lake basin has been shaped primarily by shear and tensional stresses within the broad San Andreas fault system; and it has been modified both by eruption and subsidence of the Clear Lake Volcanics and by depositional processes. Within the San Andreas fault system, shear has been dominantly right-lateral N35°W to N45°W, and maximum tension dominantly east-west. However, the current local maximum tension deduced from focal-plane solutions is N70°W, representing a secular change of unknown, possibly short duration, that could modify tensional basin-forming processes. The Collayomi fault zone (N45°W) partially bounds the basin on the southwest. The seismically active Konocti Bay fault zone (N25°W) across the basin merges into the less well documented N40°W Clover Valley fault zone that includes the faults in Clover Valley and near Lucerne and shorter faults near Clearlake Highlands and Lower Lake. The northeast margin of the main lake and both margins of the Oaks arm are largely fault-controlled; the Highlands arm is partly fault-controlled. East of the basin, sediments now assigned to the Cache Formation—ranging in age from more than 1.8 to about 1.6 m.y. old—were deposited in an earlier basin that was controlled mainly by N35-45°W (Bartlett Springs) and N20°E (Cross Spring) fault zones. The earliest stage of the present Clear Lake basin is dated by the 0.6-m.y.-old rhyolite of Thurston Creek, which is found at or near the base of lacustrine-fluvial deposits. The extensive rhyolite’s thickness and distribution suggest that its eruption triggered subsidence that initiated or accelerated basin formation. Limited data suggest that a dominantly volcanic subsidence feature within the larger Clear Lake Basin could have had dimensions of about 13 by 15 km and either an elliptical shape with major axis oriented west-northwest, or a rectilinear shape. Volcanism occurring between 0.6 and 0.3 m.y. ago partly filled the southern part of the Clear Lake basin with flows, pyroclastic materials, and clastic deposits. Deposition beneath Clear Lake (0.4 mm/yr for the past 0.45 m.y.) has kept pace with subsidence and tilting down to the northeast. Projected maximum sedimentary and volcanic thicknesses could total more than 1 km beneath Clear Lake, suggesting a possible overall subsidence rate of 1.7 mm/yr for the past 0.6 m.y.

Clear Lake, California, lies in a volcano-tectonic depression that received nearly continuous lacustrine deposition for the past 500,000 yr and probably longer. The lake has been shallow (<30 m) and eutrophic throughout its history. Sediments beneath the floor of the lake are fine grained (chiefly >7.0φ) and contain fossils of a large lacustrine biota, as well as a pollen record of land plants that lived in the basin. The sediments also contain tephra units of local and regional extent. The ages of the sediments in Clear Lake are determined from radiocarbon dates on the sediments, from correlation of regionally distrbuted tephra units, and from inferred correlation of oak-pollen spectra with the marine oxygen-isotope record. From the chronology of events recorded in the cores from Clear Lake, the late Quaternary history of the lake can be deciphered and the sediments correlated with other basins in northern California. Comparison of cores from Clear Lake with strata of the Kelseyville Formation, exposed south of the main basin, suggests a general northward migration of lacustrine sedimentation, which in turn suggests a northward tilt of the basin. Migration of the lake was a response to volcanism and tectonism. Volcanic rocks erupted from Mt. Konocti, on the southern margin of the lake, and displaced the shoreline to the west and north. Clear Lake is bounded by faults that are part of the San Andreas fault system. These faults strongly influenced the position, depth, and longevity of Clear Lake. Movement on these boundary faults deepened the Highlands and Oaks arms of the lake about 10 ka. Tectonic movement accompanying faulting was probably also largely responsible for hiatuses in the deposits beneath Clear Lake about 17 and 350 ka that have been inferred from the subbottom stratigraphy of the lake. Climate change, although responsible for large variation in the composition of the terrestrial flora of the Clear Lake drainage basin, has not influenced the areal extent, depth, or position of the lake.

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.

Clear Lake core CL-73-4 records fluctuating abundances of oak pollen during the last glacial/interglacial cycle that correlate remarkably well with fluctuations in extensive pollen records from Grande Pile in France and Tenaghi Phillipon in Macedonia, as well as with the oxygen-isotope records from deep-sea cores. The record correlates less closely with other extensive records, including those for Lake Biwa, Japan, and Sabana de Bogotá, Colombia. Correlation of the record with the early Weichselian climatic sequence of northwestern Europe is excellent; both sequences show a series of five cryomer/thermomer fluctuations between the end of the last interglaciation (Eemian/Konocti, which is correlated here with the end of marine oxygen-isotope Stage 5e) and the onset of full continental glaciation at the end of Stage 5a. The fluctuations correlate both in their relative durations and in their relative amplitudes. The Clear Lake record also correlates with various North American sequences. The Sangamon interval of the mid-continent area correlates with the entire Konocti thermomer and early Pomo cryomer interval, and correlations with the glacial sequences of the Sierra Nevada and Rocky Mountains suggest that some Tahoe, Mono Basin, and Bull Lake moraines may be of Sangamon age. The proposed correlations of the Clear Lake record with other sequences have not been proved. The overall impression, however, is one of remarkable consistency, and it is likely that further work will provide more evidence in support of the sequence of five cryomer/thermomer cycles between the end of the last interglacial period and the onset of full glacial conditions about 70,000 years ago. This sequence is much more complicated than has been generally recognized, although parts of it have been known for many years. The sequence, which has now been found in several widely separated areas, should no longer be ignored.

Modern-day Clear Lake is a turbulent, turbid, permanent, polymictic lake. It is also a highly productive fresh-water lake whose dominant solutes are Mg 2+ + Ca 2+ – HCO 3 − . The lake has a diverse and abundant limnetic community, yet has a depauperate benthic community. The benthic community structure appears to be ecologically simple as the result of turbulence-induced substrate instability coupled with unpredictable periods of anoxia induced by the oxygen-consuming organic matter. Ostracodes, which are ubiquitous, largely benthic, environmentally sensitive, and diverse organisms, are represented in Clear Lake only by the nektic species Cypria ophtalmica . The substrate conditions provide adequate reason for the absence of most ostracodes from the modern lake, but their absence in the fossil record suggests that the modern lacustrine environment existed in the past despite known climate changes. This seeming paradox can be explained by considering the influence of various types of climatic change on the lacustrine environment; certain types of climate-environmental changes would maintain a lacustrine environment unsuited to ostracodes. These hypothesized climate-lacustrine environmental changes would favor the modern (Holocene) and other oak-dominated periods to be the warmest and driest in Clear Lake history, whereas the pine-TCT (Taxodiaceae, Cupressaceae, and Taxaceae) periods would be cooler and wetter than today. The largely barren ostracode record, coupled with rare ostracode occurrences, would support a glacial-interglacial Clear Lake climatic history characterized primarily by changes in the annual precipitation-evaporation budget.

We have identified ash beds in sediment cores of Clear Lake, California, by the chemistry of their volcanic glasses and petrography. These identifications enable us to correlate between cores, and to correlate three ash beds to several localities outside the Clear Lake basin where they have been isotopically dated or their ages estimated by stratigraphically bracketing dates. The three dated ash beds are ash bed 1 (Olema ash bed), estimated to be between 55 and 75 ka, in two deep cores CL-80-1 and CL-73-4, and two ash beds in core CL-80-1, ash bed 6 (Loleta ash bed), estimated to be between 0.30 and 0.39 Ma, and ash bed 7, estimated to be about 0.4 Ma. Available age control from extrapolation of radiocarbon ages downward in the two cores, age constraints from correlations of ash beds, and etching of mafic minerals in ash beds at depths below about 118 m in core CL-80-1 suggest the following depositional histories for the two cores: in core CL-73-4, sedimentation appears to have been rapid (about 1 mm/yr) and continuous from about 120 ka to the present, corresponding to a depth interval from about 115 m to the present lake bottom. In the deeper core CL-80-1, sedimentation took place at a relatively moderate rate (0.4 mm/yr) from about 460 ka until sometime between about 300 and 140 ka, corresponding to a depth interval from about 168 to 118 m. Slow deposition or erosion took place sometime during the interval from about 300 to 140 ka, corresponding to an inferred hiatus at a depth of about 118 m. From 140 ka to the present, rapid sedimentation took place at about the same rate (about 0.8 mm/yr) as in core CL-73-4, corresponding to a depth interval from about 118 m to the present lake bottom. The age of sediments in Clear Lake is not well constrained within the depth interval of about 70 to 130 m in the two deep cores, and the duration of the putative hiatus at about 118 m in the core CL-80-1 may be shorter than we propose. The presence of a hiatus at about 118 m depth in this core, however, is suggested by etching of mafic minerals in tephra layers below this level but not above, indicating that a period of subaerial exposure, or exposure above the groundwater table, had occurred for sediments below this level.

Radiocarbon dating of disseminated organic matter from 10 horizons in Clear Lake core CL-73-4 produced apparent ages ranging from 4,230 to 32,650 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 yr 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. We correlate the major fluctuations of the oak pollen curve with their counterparts in the deep-sea record, and further refine the Clear Lake time scale 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.

The diagenesis of amino acids in sediments from Clear Lake core CL-80-1 is indicated by changes in amino acid concentrations, compositions, and stereochemistry. Concentrations of total amino acids decrease with depth, but the decrease is not systematic, possibly reflecting a nonuniformity in sedimentary and postdepositional processes affecting the amino acids. Ratios of neutral/acidic amino acids may indicate that the pH of interstitial water is slightly alkaline to slightly acidic and that the organic matter is well humified. Ratios of nonprotein/protein amino acids suggest that some changes in amino acids with depth result from microbial degradations. The extent of racemization of alanine increases with depth; the trends of these data may be explained, in part, by rapid sedimentation within the lake. Agreement between extents of alanine racemization for sediments from equivalent depths in two cores from the lake suggests that diagenetic temperatures are uniform within the sediments of the northern basin of Clear Lake.

A deep-sea core collected on the continental slope off northern California contains a pollen stratigraphy for the past 20,000 yr that can be correlated to the pollen stratigraphy from the upper section of Clear Lake core CL-73-4. The occurrence in one sequence of pollen, reflecting the local continental paleoclimates, and marine microfossils reflecting the local paleoceanography, allows a comparison of concurrent responses of the local ocean and adjacent continental area to global climate changes. The interpretation of the two data sets gives a complex progression of changes that are probably interrelated, such as upwelling that produced coastal fogs. The changes in climatic and oceanographic environmental conditions that occurred in response to the switch from global glacial to interglacial conditions was not a smooth progression of increasingly moderate regimes; rather, the changes appear to be a complicated series of states that suggests a disequilibrium mode lasting from about 15,000 to 5,000 yr ago.