Environmental Changes in the 

Tute Lake Basin, 

Siskiyou and Modoc Counties, California, 

from 3 to 2 Million Years Before Present 

U.S. GEOLOGICAL SURVEY BULLETIN 1933 
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Environmental Changes in the 

Tule Lake Basin, 

Siskiyou and Modoc Counties, California, 

from 3 to 2 Million Years Before Present 

By DAVID P. ADAM, J. PLATT BRADBURY, HUGH J. RIECK, 

and ANDREI M. SARNA-WOJCICKI 

U.S. GEOLOGICAL SURVEY BULLETIN 1933 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1990 

For sale by the 

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Library of Congress Cataloging-in-Publication Data 

Environmental changes in the Tule Lake basin, Siskiyou and Modoc Counties, 

California, from 3 to 2 million years before present I by David P. Adam ... 

[et al.]. 

p. cm.-(U.S. Geological Survey bulletin; 1933) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1933 

1. Paleoecology-California-Tulelake Region. 2. Paleobotany-California-Tulelake Region. 3.-Paleobotany-Piiocene. 4. Paleoclimatology-California-Tulelake Region. I. Adam, David P. II. Series. 

QE75.89 no. 1933 557.3 s-dc20 90-3026 

[QE720] [551.69794'21] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Site description 2 

Methods 3 

Results 3 

Lithology 3 

Dating 3 

Biostratigraphy 3 

Palynology 6 

Diatoms 7 

Discussion 11 

Summary 11 

References 12 

FIGURES 

1. Map showing Tulelake core site and vicinity 2 

2. Graph of percentages of pine, TCI' (Taxodiaceae, Cupressaceae, and Taxaceae ), 

and Artemisia pollen plotted against depth for entire Tulelake section 4 

3. Graph of selected variables plotted against depth and inferred age for the 3- to 

2-Ma part of the Tulelake section 5 

4. Graphs of percentages of pine pollen plotted against percentages of TCT pollen 

from Tulelake and Hodgdon Ranch, California 8 

5. Graphs of percentages of pine pollen plotted against percentages of TCI' pollen 

from Clear Lake, California 9 

6. Diagram of mean percentages of diatom groups in plankton and surfaee 

sediment of Upper Klamath Lake, Oregon 10 

TABLE 

1. Paleomagnetic horizons used for time control 6 

Contents Ill 

Environmental Changes in the Tule Lake Basin, 

Siskiyou and Modoc Counties, California, from 

3 to 2 Million Years Before Present 

By David P. Adam, J. Platt Bradbury, Hugh J. Rieck, and Andrei M. Sarna-Wojcicki 

Abstract 

Pollen and diatom analyses of a core from the town of 

Tulelake, Siskiyou County, California, for the period between 

3 and 2 Ma reveal a paleoclimatic and paleolimnologic 

sequence recording a long, warm time interval that lasted 

from about 2.9 to 2.6 Ma and had a short, cooler interval 

within it. During this warm interval, the regional vegetation 

surrounding ancient Tule Lake was a mixed coniferous 

forest, and Tule Lake was a warm monomictic lake. Approximate modern analogs for this Pliocene fossil record at 

Tulelake are found at least 2 degrees farther south. The 

Tulelake warm interval appears to have correlatives in the 

North Atlantic oxygen isotope record and in the pollen record 

of the Reuverian in the Netherlands. An interval beginning at 

about 2.4 Ma was characterized at Tule Lake by slow 

sedimentation, by changes in the relative amounts of algae in 

the lake, and by an increase in the maximum percentages of 

Artemisia pollen. 

INTRODUCTION 

The upper Klamath River drainage basin, in northernmost California and south-central Oregon, is physiographically part of the Basin and Range province. Several 

factors make it well suited for paleoclimatic and paleolimnologic studies: A number of modern and older· 

depositional basins within this region contain thick sedimentary sequences that span much of the Quaternary 

and perhaps reach well back into the late Cenozoic. The 

present climate is less arid than in the Great Basin 

proper, and modern lakes in many of these depositional 

basins provide habitats suitable for environmentally sensitive organisms such as ostracodes and diatoms as well 

as depositional environments suitable for the preservation of pollen. Numerous tephra layers, mostly from the 

volcanoes of the southern Cascades, can be used to 

correlate both between basins and with sequences in 

more distant areas. 

Manuscript approved for publication, Februaty 6, 1990. 

This paper describes results of diatom and pollen 

analyses of sediment deposited during the interval from 3 

to 2 Ma at what is now the town of Tulelake, Siskiyou 

County, California (fig. 1). The town lies within the bed 

of ancient Tule Lake, now largely reclaimed for agriculture. Diatoms and pollen grains are well preserved and 

abundant in sediments cored from the ancient lake bed, 

and their biostratigraphy documents limnological and 

environmental changes in this region for the last 3 million 

years. The interval between 3 and 2 Ma is of special 

interest because significant climatic changes during that 

interval have been inferred from the foraminiferal and 

oxygen isotope stratigraphy of marine cores (Loubere 

and Moss, 1986). Specifically, o

18 0 values of s3.5 per 

mil characterize the marine record earlier than about 2.4 

Ma, and these are interpreted to represent reduced 

extent of continental ice and consequently higher sea 

levels and generally warmer temperatures (Ruddiman 

and Raymo, 1988). Warmer climates in the future may 

be a result of the current increase in atmospheric C02 

released by the burning of fossil fuels. Late Pliocene 

warm climates may be relevant analogs for a C02 -

enhanced world, and hence it is important to document 

their characteristics. 

The biostratigraphy of the Tulelake core provides 

an opportunity for such documentation. This core, dated 

by paleomagnetic reversals and tephrochronology 

(Adam and others, 1989), includes a continuous lacustrine stratigraphic record between 3 and 2 Ma. Changes 

in the relative abundance of pine pollen versus pollen of 

the Taxodiaceae, Cupressaceae, and Taxaceae (TCf) 

group1 reflect changes in the composition of the regional 

forest through time; proportional shifts between planktonic diatoms (Aulacoseira so/ida and large species of 

1

Pollen grains of genera and species within the TCT group 

cannot be reliably distinguished in most cases, so they are lumped in a 

group for this study. Species that are particularly relevant to the 

interpretation of the Tulelake core include Juniperus occidentalis 

(western juniper) and Calocedrus decurrens (incense-cedar). 

Introduction 1 
Stephanodiscus) and benthic diatoms (Fragilaria species) 

can be interpreted in terms of past changes in lake depth, 

open-water turbulence, and seasonal thermal structure. 

These parameters are potentially under climatic control 

and therefore may provide terrestrial paleoclimatic insights that can be compared with data from marine 

records for the same time interval. 

SITE DESCRIPTION 

The study area is a part of the Modoc Plateau 

(Hinds, 1952; Macdonald, 1966), physiographically transitional between the Great Basin to the east and the 

Cascade Range to the west (McKee and others, 1983). 

The region is underlain by Tertiary and Quaternary 

volcanic rocks broken by prominent normal faults that 

strike mostly north and north-northwest. The Tule Lake 

and Klamath basins are bounded by normal fault scarps 

to the west, by the Devils Garden and Medicine Lake 

122' 

'

Upper 

Klamath 

Lake 

Klamath\ 

OREG River \ 42 , CALiF--- - -~ 

41' ,... 20 40 Pit River 

KILOMETERS 

121' 

Figure 1. Core was drilled in town of Tulelake, shown 

by dot. Line pattern indicates historic Jakes; singleheaded arrows indicate flow direction; double-headed 

arrow indicates region of bidirectional flow between 

Lost and Klamath Rivers; heavy line across Klamath 

River indicates probable location of sill over which 

ancient lake system first spilled into Klamath drainage. 

Dot pattern indicates portions of Medicine Lake highland and Mt. Shasta region with elevations in excess of 

1,525 m. From Adam and others (1989). 

volcanic fields to the southeast and south, by normal 

scarps to the east, and by irregular highlands to the north. 

Volcanism and tectonism are both active processes in the 

region today; the Glass Mountain obsidian flow on the 

northeast flank of the Medicine Lake highland was 

erupted about 1 ka (Heiken, 1981 ), and Holocene lava 

flows are offset by the Gillems Bluff fault, which forms 

the western margin of the Tule Lake basin (DonnellyNolan and Champion, 1987). 

The Tule Lake basin, along with the Klamath basin 

to the north, was a part of the larger pluvial Lake Modoc 

of Dicken (1980). The maximum lake level, as determined from the elevations of lake deposits exposed 

around the edge of the lake basin, was about 1,292 m 

according to Dicken. This level is about 50 m higher than 

the present lake outlet and about 64 m higher than the 

lowest part of the Tule Lake basin. Much of Tule Lake 

was drained for agriculture by the 1920's (Turner, 1988); 

before that time, historic lake levels were as high as 

approximately 11 m above the present lake level 

(Cleghorn, 1959). 

Dicken (1980), although rather vague about the 

genesis of Lake Modoc, implied that it behaved like other 

Great Basin pluvial lakes-, waxing and waning in response 

to climatic changes. However, the basin that once contained Lake Modoc is not now topographically closed at 

the high elevations the lake is known to have reached; 

given the present topography, lake water would drain 

into the Klamath River before rising to the elevations at 

which many lake deposits are now exposed. In addition, 

there is no evidence in the upper Klamath River gorge of 

ancient processes-such as low-elevation glaciation or 

major landsliding-that could have confined the lake at 

its maximum level. 2 Therefore, we believe that tectonic 

and volcanic factors, as well as climatic factors, contributed to the formation of Lake Modoc. Very likely the 

lake was formed when a closed basin was created either 

(1) by faulting or (2) by blocking of an original southward 

drainage to an ancestral Pit River system by the eruption 

of the Medicine Lake volcanic field (see fig. 1). Once the 

closed basin was formed, it filled until the impounded 

lake overflowed to the west, cutting the present gorge of 

the Klamath River. Exposed benches of lake sediment 

stranded around the margins of the basin suggest that the 

lake eroded its outlet rather slowly, allowing time for the 

deposition of considerable amounts of diatomite at high 

lake levels. The diatoms found in these higher elevation 

deposits indicate that the time of maximum lake extent 

may be much older than the poorly defined Pleistocene 

age implied by Dicken (1980); the deposits we have 

examined all appear to be at least 3 m.y. old. 

2rrbe major landslide on the north side of Secret Spring Mountain has blocked the Klamath River at times, but at too low an 

elevation to have affected the Klamath Lake and Tule Lake basins. 

2 Environmental Changes In the Tule Lake Basin from 3 to 2 Million Years Before Present 
METHODS 

We selected a drill site at the town of Tulelake, in 

northeastern Siskiyou County, California (lat 4r57.27' 

N., long 121°28.83' W., elevation 1,229 m; fig. 1), near 

the Tulelake municipal water well. The water-well log 

records shallow-water sediments (lakebottom mud, pumice, sand, and shale) from the surface to a depth of 491 m; 

these are underlain by interbedded volcanic and sedimentary rocks that extend to a depth of at least 816 m 

(Hotchkiss, 1968). A 334-m-long sediment record (fig. 2) 

was cored at a location about 250 m south of the 

municipal water well. For the section described in this 

paper, 2.875-in wireline equipment was used down to a 

depth of 234.75 m; the interval below that depth was 

sampled using 2.25-in wireline equipment. The stratigraphic intervals from which core was recovered are 

shown in figure 3. Overall recovery within this interval 

was about 75.8 percent, with poorer recovery between 

231.70 and 279.57 m (56.4 percent) than above or below 

that interval (87 .4 and 86.2 percent, respectively). Drilling was terminated at a depth of 334 m when the drill 

stem became too heavy to lift safely with our equipment. 

An additional 155 m of sediment lies beneath the cored 

section above the first occurrence of "rock" noted in the 

log of the municipal water well (Hotchkiss, 1968). 

Sediments from the Tulelake deposits are highly 

diatomaceous and largely unconsolidated. Samples for 

diatom analysis were taken from 49 stratigraphic levels 

(fig. 3) and then were disaggregated in distilled water. 

After settling, supernatant water was decanted and one 

drop of water-saturated diatomaceous sediment was resuspended in 50 mL of distilled water. Diatoms were 

settled onto coverslips (Battarbee, 1973). At least 300 

.diatom valves were counted from each sampled stratigraphic level to obtain percentages of taxa present. 

Pollen samples were processed using standard 

techniques for Quaternary materials and then were 

mounted in silicone oil for counting. At least 200 fossil 

pollen grains were counted to estimate the percentages 

of taxa present. Algae were counted outside the pollen 

sum. For samples in which algae were much more 

abundant than pollen, the counts for all algae were 

recorded when the most common algal type reached a 

count of 100, along with the number of pollen grains 

recorded up to that point. Estimates of the numbers of 

the various algae were then calculated using the formula 

Nest = Nobserved * Spoiien I Salgae , 

where Nest is the estimated total number of a particular 

alga that would have been counted if counting had 

continued until the pollen sum was reached, Nobserved is 

the number of that alga counted when the algal counts 

were recorded, spo/len is the total number of pollen grains 

counted, and Sargae is the number of pollen grains counted at the time the algal counts were recorded. This 

system produces rather wide confidence limits on the 

estimates of algal frequencies when algae are abundant, 

but also requires counting fewer algae (Adam, 1988). All 

percentages are expressed with respect to the pollen sum, 

spol/en· 

RESULTS 

Lithology 

The part of the core described here consists of 

interbedded lake sediment and tephra. The lake sediment includes (1) olive-gray algal muds and marly units 

that are primarily massive but sometimes laminated and 

(2) darker gray to black silts and clays. Tephra units are 

found throughout the core and range from less than 1 

mm to as much as 1.4 m in thickness. The interval 

between 220.77 and 234.4 m is characterized by a welldeveloped platy structure that is not found in the adjacent 

parts of the section. 

Dating 

The core has been dated primarily through paleomagnetic and tephra studies (Adam and others, 1989). 

The upper part of the section contains numerous volcanic 

ash layers and records many paleomagnetic reversals, 

whereas the lower part of the section contains no widespread tephra layers and only a sparse record of reversals 

(fig. 2). The time scale used here for the Pliocene record 

is based on linear interpolation between the six ages 

listed in table 1. The sedimentation rate established for 

the Gauss Normal-Polarity Chron (the interval from 2.92 

to 2.48 Ma) is in general agreement with rates observed 

higher in the section, but sedimentation rates varied 

greatly within the interval from 2.48 to 2.01 Ma (see table 

1 and the age scale on fig. 3). In particular, sedimentation 

was very slow near the base of the Matuyama ReversedPolarity Chron (in the interval from 2.48 to 2.12 Ma; 

229.68 to 219 m depth). The low sedimentation rate 

inferred from the paleomagnetic data is corroborated by 

the lithology of the section, which shows a platy fracture 

pattern between 234.4 and 220.77 m depth that is not 

present above or below that interval. The platy fracture 

may be a result of early soil development caused by 

alternate wetting and drying of the sediment surface; thus 

it suggests that the lake bottom was frequently exposed 

during the cited interval. 

Biostratigraphy 

Pollen is present in most of the section (fig. 2), and 

diatoms are abundant throughout. Other microfossils 

include sponge spicules, chrysophyte cysts, acid-resistant 

algae, and ostracodes; these other microfossils are not as 

abundant or as commonly preserved as the diatoms and 

pollen. Macrofossils include gastropods and fish remains. 

Results 3 
Depth 

(m) 

Artemisia 

Total Pinus TCf 

0 25 50 75 100 0 25 50 0 

0 

100 

200 

300 

0 25 50 75 100 0 25 50 0 

Figure 2. Percentages of Pinus (pine), TCT (raxodiaceae, Cupressaceae, and Taxaceae), and Artemisia 

pollen plotted against depth for entire Tulelake section. 

Tick marks to left of vertical axes show locations of 

analyzed samples. Lightly shaded rectangles indicate 

intervals where pollen was absent or too sparse to 

count. In •Age control• column, ages derived from 

+ artemisioids 

25 50 Paleomagnetic units 

=Blak:e(7) 

Brunhes 

Olduvai 

Matuyama 

Kaena(7) 

25 50 

Age control 

+-23.4 ka (T) 

+-50-75 ka (T) +- -114 ka (M) 

+-400 ka(T) 

+-620 ka (T) 

+-730 ka (M) 

+- -800 ka (M) 

+-900ka (M) 

+-970 ka (M) 

+- -1.12 Ma (M) 

+-1.67 Ma (M) 

+-1.87 Ma (M) 

+-2.48 Ma (M) 

+-2.92 Ma (M) 

+-3.01 Ma (M) 

paleomagnetic data are indicated by •M• and ages 

derived from tephra by •T• (from Adam and others, 

1989). Paleomagnetic units in black are normal polarity, those in white are reversed polarity. Depth scale 

(unlabeled) added to left of • Age control• column to 

facilitate comparison of ages with depths. 

4 Environmental Changes In the Tule Lake Basin from 3 to 2 Million Years Before Present 
:a 

: 

c 

if 

Cll 

Depth 

(m) 

280 

300 

320 

Age 

(Ma) 

2.5 

2.6 

2.7 

2.8 

2.9 

3.0 

se-~'i ~e,c,O 

CP~e, ~\~v.s 

50 

50 

"c" \~v.,S 

"c"·J. 

,o?J.e, 

~?>'"!:' 

,.,,o. i~'"""'"?J.~ r....1\~'(f> ·~~ ,. ~"~ 

50 0.0 1.0 2.0 3.0 0 3000 

50 0.0 1.0 2.0 3.0 0 3000 

Figure 3. Selected variables plotted against depth and inferred age for the 3- to 

2-Ma part of the Tulelake section. Tick marks to left of vertical axes show locations 

of analyzed samples. TCT indicates Taxodiaceae, Cupressaceae, and Taxaceae 

group. Pollen percentages expressed as percent of total pollen; TCT:Pinus ratios 

dimensionless. Curve for acid-resistant algae (those algae that survived pollen 

extraction process) derived from pollen counts and expressed as percent of total 

pollen; diatom percentages expressed as percent of total diatoms. Smoothed 

·~0.1\0. 

~'(o.'h\ 

50 

50 

~O.c,O 

~v. 

~\'6,0. 

.'<o.so 

s~\ 

50 

50 

s . c,O. 

of>\Sc,\\ ob.o.~\ "X-0.~ \\0.'0 

C.\~'Q \0\~ ;J C'l" Environmental interpretation 

50 0 50 Lake Climate 

Fragilaria Becoming drier 

Cyclotella Cool and wet 

Aulacoseira Moderate 

precipitation 

Very dry 

Fragilaria, 

Pediastrum Somewhat drier 

Shallow lake, 

marshy at times 

Aulacoseira Moderate 

Shallow, precipitation, 

turbulent, warm warm winters 

monomictic lake 

--

S tephanodiscus Moderate 

Seasonally precipitation, 

stratified cold winters 

dimictic lake 

50 0 50 

Depth 

(m) 

200'1 

220 

240 

260 

280 

300 

3ro1 

curve (heavy line) shown for TCT:Pinuscalculated as weighted three-level moving 

average, with both TCT:Pinusand depth values smoothed. Shaded areas in ·eore 

recovery• column show intervals from which core was recovered, and white 

spaces show intervals from which no core was recovered; horizontal lines within 

shaded areas represent boundaries between core segments. Shaded rectangle in 

•environmental interpretation• column indicates transitional interval. 
Palynology 

The general character of the pollen record from the 

Pliocene part of the section is markedly different from 

that in the younger part of the section. The two most 

important pollen types in the Pliocene section are pine 

and TCT, which are plotted against age and depth in 

figure 3 and with respect to each other in figure 4. Within 

the Pliocene, pine and TCT pollen show a well-developed 

inverse relation (fig. 4A), and the highest TCT frequencies commonly exceed 50 percent. By contrast, in the 

younger part of the section the highest TCT frequencies 

are significantly lower (fig. 4B), and other pollen types, 

particularly Arlemisia (sagebrush), become more important (fig. 2). 

Significant glacial activity in the North Atlantic 

ocean began about 2.37 Ma, not far above the Gauss/ 

Matuyama boundary (Shackleton and others, 1984). The 

pollen record in the Tulelake core does not show major 

changes at that time. Pollen samples within the Matuyama (dots, fig. 4A) are somewhat lower in maximum TCT 

frequency and somewhat higher in minimum pine frequency, but they display the same basic pattern as that 

displayed by the samples from the Gauss (open hexagons, fig. 4A). 

The strong inverse relationship between pine and 

TCT pollen in the Pliocene sediments of the Tulelake 

core (fig. 4A) is distinctly different from the pattern 

recorded in the section of the core that spans the past 1 

m.y. (fig. 4B), but is generally similar to the pattern 

observed in Holocene deposits at Hodgdon Ranch, lat 

38° N ., on the lower western slope of the Sierra Nevada in 

Yosemite National Park (fig. 4C; Adam, 1967), and to 

selected parts of the upper Pleistocene pollen record 

from Clear Lake, Lake County, California, lat 39° N. (fig. 

5; Adam, 1988). A reasonable assumption is that regional environmental conditions at ancient Tule Lake (lat 42° 

N.) between about 3 and 2 Ma were more similar to 

conditions on the lower western slope of the Sierra 

Nevada today and to the regional environment around 

Clear Lake during the cooler parts of the last glacial cycle 

(roughly the last 125,000 yr) than they were to modern 

conditions near the town of Tulelake. 

At Hodgdon Ranch (elevation 1,400 m), increases 

in TCT pollen at the expense of pine pollen were 

interpreted by Adam (1967) to represent warming during 

middle Holocene time; the increase in TCT pollen was 

attributed to an upward migration of the ecotone between a lower elevation forest dominated by Ca/ocedrus 

decurrens (incense-cedar) and a higher elevation pine 

forest. The Hodgdon Ranch section is not entirely satisfactory as a modern analog for the Tulelake Pliocene 

section, because it is from valley-bottom alluvial fill in a 

fairly small meadow, whereas both the diatom evidence 

and the regional geomorphology at the Tulelake site 

indicate that the Pliocene sediments there were depositTable 1. Paleomagnetic horizons used for time control 

[Ages follow Mankinen and Dalrymple (1979)] 

Depth Interval Age Sedimentation 

Boundary thickness rate (m) (m) (Ma) (mm yr1

) 

Reunion 2 Subchron (top) 200.00 1.97 

4.00 0.100 

Reunion 2 Subchron (base) 204.00 2.01 

8.50 .283 

Reunion 1 Subchron (top) 212.50 2.04 

6.50 .081 

Reunion 1 Subchron (base) 219.00 2.12 

10.68 .036 

Matuyama/Gauss boundary 229.68 2.48 

81.52 .185 

Kaena Subchron (top) 311.20 2.92 

20.20 1.185 

Bottom of core 331.40 1

3.03 

1

Extrapolated value. 

ed in a large basin much· like the modern one, so that 

surrounding forests must have been at least several 

kilometers away, across a stretch of open water. 

A better geomorphic analog is represented by the 

Clear Lake section, which records regional climatic history near the middle of a large lake during the past 

130,000 years (Adam, 1988). The pollen rain at Clear 

Lake (elevation 400 m) has been dominated by oak 

pollen throughout the Holocene; conifer pollen is dominant only at higher elevations. During the cooler parts of 

the last glacial cycle, however, the oaks in the Clear Lake 

basin were largely or entirely displaced by a mixed 

coniferous forest. The pattern displayed by the full set of 

observations of pine and TCT percentages (fig. SA) does 

not match the Tulelake Pliocene pattern displayed in 

figure 4A, but there is considerable overlap in the two 

distributions. When the samples from the warm intervals 

(thermomers) dominated by oak (Quercus) pollen at 

Clear Lake are excluded, the comparison is much better 

(figs. SB, C). The cooler intervals (cryomers) at Clear 

Lake during the past glacial cycle appear to provide a 

reasonably good match for the regional conditions that 

prevailed at ancient TuleLake during the late Pliocene 

(figs. 4A, SD). 

We assume that the same general biogeographic 

model used at Hodgdon Ranch and Clear Lake can be 

used for the Pliocene section at Tulelake. According to 

this model, the belt of highest percentages of TCT pollen 

is found at elevations below the belt of highest pine 

pollen percentages, so increases in the ratio of TCT to 

pine pollen correspond to upslope migrations of the 

ecotone between the two belts and, by inference, to 

warming and (or) drying conditions. 

6 Environmental Changes in the Tule Lake Basin from 3 to 2 Million Years Before Present 
If this model is correct, then the ratio of TCT to 

pine pollen may be used as a proxy measure of relative 

warmth and (or) dryness. The smoothed curve for TCT: 

Pinus in figure 3 shows a weighted three-level moving 

average of the observations. Using the model described 

above, we infer from the pollen data a period of relative 

warmth that began about 2.8 Ma and persisted until 

shortly after 2.6 Ma. The magnitude of the inferred 

temperature oscillations is not known. 

An alternative interpretation would attribute the 

TCT pollen in th~ Tulelake core primarily to Juniperus 

occidentalis (western juniper), which currently forms 

extensive open woodlands to the south of Tulelake 

(Kuchler, 1977). However, Artemisia (sagebrush) is a 

major associate of western juniper in the juniper woodlands, and there was no apparent increase in Artemisia 

pollen to accompany the higher TCT frequencies during 

the Pliocene (fig. 2). Also, because Tule Lake was 

probably a fairly large lake, the pollen record probably 

represents the vegetation over a large area, and it seems 

unlikely that a relatively sparse juniper woodland could 

produce enough pollen to supplant pine as a dominant 

pollen type over a wide area, given the prodigious 

amounts of pollen produced by most pines and the ability 

of pine pollen to disperse widely by air transport. For 

these reasons, we prefer to interpret the TCT pollen 

record of the Tulelake Pliocene section as representing 

primarily Calocedrus. 

One of the most striking features of the Pliocene 

pollen record in the Tulelake core is the extent to which 

it is dominated by pine and TCT pollen. The vegetation 

surrounding the lake must have been coniferous forest, 

which contrasts sharply with the sagebrush steppe that 

Kuchler (1977) mapped as the present natural vegetation 

of the lower parts of the Tule Lake basin. The climatic 

rain shadow of the southern Cascade Range makes the 

modern climate at Tulelake quite arid; only at elevations 

well above the valley floor is there enough precipitation 

to sustain a vigorous conifer forest. It is likely that aridity 

was not as pronounced during the late Pliocene in the 

Tulelake area as it is today. Volcanic, tectonic, and 

climatic causes probably all contributed to a more humid 

climate, but we lack the data to determine to what extent 

each factor operated. 

Diatoms 

The dominant diatoms in the Tulelake core consist 

of species of Fragilaria, Aulacoseira, and Stephanodiscus 

(fig. 3). At some levels, especially near the base of the 

core, Cyclotella elgeri and related forms are common. 

Three major diatom zones characterize the Tulelake paleolimnological record between 3.0 and about 2.4 

Ma: a basal zone, 3.0 to 2.9 Ma, dominated by Stephanodiscus; an intermediate zone, 2.9 to 2.65 Ma, characterized by Aulacoseira so/ida (=Melosira so/ida Eulenstein ); 

and an upper zone, 2.65 to 2.4 Ma, dominated by 

Fragilaria (fig. 3). After 2.4 Ma, Aulacoseira so/ida again 

dominated, but the trend was interrupted by significant 

percentages of Cyclotella bodanica between· about 2.08 

and 2.03 Ma (215 to 210 m depth). Aulacoseira so/ida 

returned to dominance by 2.02 Ma, but at the end of the 

Pliocene, Fragilaria species became dominant once more 

(fig. 3). 

The large, heavily silicified Stephanodiscus species 

at the base of the Tulelake core are unidentified but are 

closely related to S. asteroides Gasse, S. carconensis 

Grunow, and S. subtransylvanicus Gasse. The taxonomy 

of these species is unresolved, and its resolution will 

require detailed scanning electron microscope work because the forms vary morphologically, perhaps in relation 

to nutrient supplies (for example, Theriot and Stoermer, 

1984; Hakansson, 1986). for the purposes of this paper, 

these forms are referred to simply as Stephanodiscus, and 

the assumption is made that because they are morphologically similar, they have similar nutrient requirements 

and paleolimnological significance (for example, Kilham 

and others, 1986). 

Aulacoseira so/ida and Stephanodiscus (identified 

as S. carconensis) live today in Lake Biwa, Japan (Mori, 

1975; Mori and Rorie, 1975, 1984). Their paleolimnological significance can be tentatively inferred from their 

occurrence in that lacustrine system, although in recent 

years S. carconensis has become rare in Lake Biwa, 

perhaps as a result of pollution (Nakanishi, 1984). The 

Lake Biwa Stephanodiscus blooms between January and 

March, whereas Aulacoseira ( = Melosira) so/ida thrives 

in November, December, and April (Negoro, 1960). 

The diatom succession of Lake Biwa has not been 

studied in relation to lake circulation and nutrient fluxes, 

but the temporal distribution of the dominant species fits 

with current ideas on the relationship between diatom 

abundance and lake nutrient dynamics. The growth of 

Stephanodiscus is known to be favored by high phosphorus:silicon ratios (Kilham and others, 1986). Phosphorus 

enters lake water most readily by dissolution from anoxic 

sediments and decomposition of organic remains during 

periods of stagnation. Thus, the growth of Stephanodiscus is likely to be favored in a lake that is stratified for 

part of the year-a dimictic lake that freezes in the 

winter, for example-giving the stagnant (anoxic) bottom 

waters time to acquire abundant phosphorus. Stephanodiscus will bloom when this phosphorus-rich water is 

brought to the surface, during spring, for example, when 

vertical mixing resumes. The nutrient requirements of 

Aulacoseira are not so well known, but Aulacoseira tends 

to be abundant in turbulent lakes, possibly because 

turbulence continually brings nutrient-rich bottom waters to the surface and because it keeps the diatoms in the 

photic zone. In any case, Aulacoseira will be favored in 

lakes that experience substantial vertical mixing throughout the open-water season. 

Results 7 
Lake Biwa is a warm monomictic lake. Typically, 

such lakes begin to circulate when falling autumn air 

temperatures cool the epilimnion and the thermocline 

deepens. As water from the hypolimnion is brought to 

the photic zone, nutrients (nitrogen, phosphorus, and 

silicon) become available for diatom growth. At Lake 

Biwa, the increased turbulence and availability of nutrients, especially silicon, appear to provide conditions 

favorable for Aulacoseira so/ida, which dominates the 

lake in November and December. Its heavy cells 

presumably extract silicon rapidly from the lake water 

during those months. Progressively deeper mixing prob80 A 

70 Tulelake 

0 ~0 3-2Ma 

!Z; 60 

t.Il 

u ~0 

fil 50 oco t8e p... 

z ~ 40 

• :~ t.Il 0 8 0 0 :3 oe o 

0 30 0 p... ct!l b 

E-< 20 ~·ec~o • 0 0. 

0 0 

00. cl) 0 10 c1ll ~ 0 

0 jl ·~ • 

10 20 30 40 50 60 70 80 90 

80 

... c 70 Hodgdon Ranch ... ... Holocene 

!Z; 60 ... ... t.Il 

u ... fil 50 ... ... p... ... ~ 

~ 40 ... ... ... ... :3 ...... 0 30 ... ... p... 

b ... E-< 20 

10 

10 20 30 40 50 60 70 80 90 

PINE POLLEN, IN PERCENT 

Figure 4. Pine pollen plotted against TCT (Taxodiaceae, 

Cupressaceae, and Taxaceae) pollen, both expressed as 

percent of total pollen. A, Tulelake, Siskiyou County, California, 3 to 2 Ma. Open hexagons represent samples within 

Gauss Normal Polarity Chron (>2.48 Ma), and dots repreably occurs in midwinter (January and February) and 

supplies additional nutrients, including phosphorus from 

the profundal zone of the lake. At that time, phosphorus:silicon ratios are likely to be high and thus to facilitate 

the bloom of Stephanodiscus that occurs between January and March. The observed increase in A. so/ida in 

April may result in part from nutrients, especially silicon, 

supplied by runoff during the early spring (Kotoda and 

Mizuyama, 1984; Koyama and others, 1984). 

In this Lake Biwa scenario, any climatic change 

toward warmer winters would permit more continuous 

vertical mixing and thus favor Au/acoseira so/ida. Climat80 

70 

60 

50 

+ 40 

30 

+ 

20 

+ + 10 

10 

80 

... 

70 

... ... 60 ... 

50 ... 

40 

30 

20 

10 

10 

B 

Tulelake 

1-0Ma 

+ 

20 

0 

... 

20 

+ 

+ 

+ 

+ + 

++ + -# + + 

+t+. + +rt- * 

+++++ *-7-t* + 

++11=~= •+* -f +} 

30 40 50 60 70 80 90 100 

D 

ConA 

~0 

... ~0 

o~ t8e 

... . ... ... •'8 ~ "'o "'Cl 0 

... oe o (). 

~ 

• ~·~~0 

0 0. 

0 0 

00. cl) 0 

c1ll ~ 0 

0 jl ·~ • 

30 40 50 60 70 

PINE POLLEN, IN PERCENT 

80 90 100 

sent samples within Matuyama Reversed Polarity Chron 

( < 2.48 Ma); 8, Tulelake, 1 Ma to present; C, Hodgdon 

Ranch, Yosemite National Park, California, Holocene 

(Adam, 1967); D, superimposition of Con A. 

8 Environmental Changes In the Tule Lake Basin from 3 to 2 Million Years Before Present 
ic change toward colder winters would produce a period 

of stagnation while the lake lay frozen and stratified; this 

period would allow bottom waters to become rich in 

phosphorus and enable the growth of Stephanodiscus 

when the lake circulated again in the spring. The stratigraphic record of Lake Biwa supports this hypothesis: A. 

so/ida predominates today and dominated during the 

warm climate phases of the Holocene, whereas Stephanodiscus dominated the colder late Pleistocene levels 

(Mori and Horie, 1984). Pollen studies of the Lake Biwa 

core (Fuji, 1976) suggest a 7 °C lowering of mean annual 

temperature about 25 ka when Stephanodiscus dominat80 A 

70 X Clear Lake, core 4 

Entire core ~ 60 

~ 

u 

~ 50 

~ 

~ ~ 40 

~ 0 30 ~ 

t> 

E-o 20 

10 

X X 

X 

10 20 30 40 50 60 70 80 90 

80 c 70 Clear Lake, core 4 

0 Stage 5 cryomers ~ 0 60 

~ 

u 0 ~ 

~ 

50 oo 

0 ~ ~ 0 oo ~ 40 0 0 :J 0 0 30 ooco ~ 

E-o 

u 0 

E-o 20 

10 

0 10 20 30 40 50 60 70 80 

PINE POLLEN, IN PERCENT 

Figure 5. Pine plotted against TCT (Taxodiaceae, Cupressaceae, and Taxaceae) pollen for Clear Lake, Lake County, 

California, core 4, (130 to 0 ka) (Adam, 1988). Variables 

expressed as percent of total pollen. A, all of core 4; 8, 

middle and late Porno cryomers, correlative with deep-sea 

90 

ed this record (see also Tsukada, 1986), and it is quite 

probable that Lake Biwa froze in the winter and became 

a dimictic lake at that time. 

The general character of the Lake Biwa diatom 

record, with its alternation between an Aulacoseiradominated warm monomictic lake during warm periods 

and a Stephanodiscus-dominated dimictic lake during 

cooler periods, makes it a reasonable qualitative analog 

for the Tulelake diatom record, even though the exact 

climatic and limnologic characteristics of Lake Biwa 

during the late Quaternary may not match those of Tule 

Lake in the Pliocene. Although the dominant species of 

80 

70 

60 

50 

40 

30 

20 

10 

10 

80 

70 

60 

50 

40 

30 

20 

10 

0 10 

B 

x Clear Lake, core 4 

• •x~ Stage 2-4 equivalents x~xx~ 

···~··· X ~Xx 

x~x~ ~ X 

X X 

X X 

X 

20 30 40 50 60 70 80 90 

D 

Clear Lake, core 4 

All cryomers 

X 

20 30 40 50 60 70 80 90 

PINE PO~EN. IN PERCENT 

oxygen isotope stages 2-4 (solid dots represent the Halika 

thermomers, which are subunits within the middle Porno 

cryomer); C, Tsiwi cryomers, correlative with the cooler parts 

of deep-sea oxygen isotope substages Sa through 5d; D, 

superimposition of 8 and C. 

Results 9 
Quaternary diatoms at the two lakes are different, the 

genera are the same: Aulacoseira dominated the Holocene and Stephanodiscus the late Pleistocene at both 

Lake Biwa and Tule Lake. From the Lake Biwa analogy, 

we infer that a warm climate prevailed at Tule Lake 

between 2.9 and 2.65 Ma, whereas colder climates were 

dominant between 3.0 and 2.9 Ma. Within the colder 

interval, one distinct climatic reversal to warm, mild 

winters occurred between 2.96 and 2.94 Ma (fig. 3). 

The section of the Tulelake core that spans the 

period from 2.65 to 2.4 Ma is dominated by Fragilaria 

species (principally F. pinnata, F.~ cons flU ens and varieties, and F. brevistriata). These benthic diatoms live 

loosely attached to submerged aquatic vegetation or to 

sand grains in the shallow, sunlit areas of lakes. However, 

they can become opportunistically planktonic with sufficient turbulence and enter shallow, open-water environments far from shore. Plankton and modern surface 

sediment samples of Upper Klamath Lake, Oregon, 

about 56 km northwest of the Tule Lake basin, are 

commonly dominated by these Fragilaria species (fig. 6). 

Today, Upper Klamath Lake has a mean depth of 

about 2.5 m and an area of 31,000 ha (Sanville and 

others, 1974). This lake is in the same structural basin (a 

graben) as was ancient Tule Lake and in the past may 

have been part of the same lacustrine system (for exam-

. pie, Dicken, 1980). Therefore, present-day Upper Klamath Lake makes a reasonable analog for Fragilariadominated intervals in the Tulelake core. By this analogy, 

Tule Lake was a shallow, eutrophic, marsh-fringed freshwater lacustrine system between 2.65 and 2.4 Ma. 

Figure 6. Mean percentages of diatom groups in plankton 

and surface sediment of Upper Klamath Lake, Oregon 

(based on data from 3 sarnples). 

Fragilaria species were especially abundant between 2.4 and 2.3 Ma (inferred from a single sample at 

227m, fig. 3). The diatoms within this interval are poorly 

preserved and suggest unusually low lake levels and 

therefore dry conditions at this time. In some marine 

records (for example, Shackleton and others, 1984), high 

o180 values and low carbonate percentages between 2.4 

and 2.3 Ma suggest a brief period of increased continental ice volume. Because dry and cold conditions characterized some climates during the Wisconsin full glacial 

period in the northwestern United States (Barnosky and 

others, 1987), it is possible that some Pliocene glacial 

climates were also dry. Perhaps the Tulelake and marine 

records from 2.4 to 2.3 Ma correspond to and reflect 

global cooling. 

The reappearance of Aulacoseira so/ida after 2.3 

Ma indicates the return of higher lake levels and 

generally warm climatic conditions until near the close 

of the Pliocene. Cyc/otel/a bodanica first appears in the 

Tulelake core at about 217 m, or shortly before 2.0 Ma. 

Today this species characterizes temperate and north 

temperate lakes with reasonably abundant precipitation, 

cold winters, and warm summers. In Finland and northern Europe it is typically found in relatively deep, transparent, oligotrophic lakes (Huber-Pestalozzi, 1942; 

Molder and Tynni, 1968). The closely related species 

Cyc/otel/a comta lives in similar habitats in Minnesota 

(Brugam, 1983). The presence of Cyc/otel/a bodanica 

near the top of the Pliocene in the sediments of the 

Tulelake core may record a shift to cooler and moister 

climates that presaged the climate of some parts of the 

Pleistocene. 

As at 2.4 Ma, marine oxygen-isotope records also 

reflect glacial climates around 2.0 Ma (for example, 

Shackleton and others, 1984). In the Tulelake record, 

however, the inferred climatic changes are not the same 

as those at 2.4 Ma, demonstrating that the continental 

climatic response to glaciation can be variable. Other 

long paleolimnological records will be required to document this variability through space and time before it can 

be explained in climatic terms. 

At the generic level, changes in the diatom stratigraphy of the Tulelake core between 3.0 and 2.4 Ma are 

similar in character to changes at the Pleistocene-toHolocene transition both at Tulelake and at other sites, 

for example at Clear Lake, California (Bradbury, 1988). 

These similarities indicate that Pliocene terrestriallimnologic and climatic changes were not drastically different from Quaternary climatic changes. They were 

somewhat different, however, because the planktonic 

diatom species dominant. in ancient Tule Lake between 

3.0 and 2.4 Ma do not live there today but disappeared 

from this record by 1.4 Ma. 

Aulacoseira so/ida was present in Clear Lake, California (lat 39° N.), until about 100 ka. Lake Biwa (lat 35° 

N.), however, still containsAulacoseira so/ida. The Steph10 Environmental Changes In the TuleLake Basin from 3 to 2 Million Years Before Present 
anodiscus species there is also quite similar to the 

Tulelake and Clear Lake forms and invites the comparison of these respective limnologic environments. Lake 

Biwa has been a stable, deep lacustrine environment for 

more than 500,000 years (Ueno, 1975; Nishimura and 

Yokoyama, 1975), whereas the paleolimnologic records 

at both Tulelake (Adam and others, 1989) and Clear 

Lake (Sarna-Wojcicki and others, 1988) include hiatuses 

that probably resulted from desiccation and absence of 

lacustrine deposition (for example, Bradbury, 1988). 

Such desiccation events, whether resulting from climate 

change or from tectonism, caused the local extinction of 

planktonic diatoms. Repopulation of the lakes by the 

same species could occur only under climatic and (or) 

limnologic environments closely similar to previous conditions. The permanent disappearance of these species 

from the Clear Lake and Tulelake records probably 

indicates changes to inhospitable climatic and (or) limnologic conditions following desiccation events. In contrast, climatic and limnologic change at Lake Biwa was 

not so extreme as to permanently eliminate populations 

of Aulacoseira so/ida and Stephanodiscus (for example, 

Ueno, 1975), although these populations fluctuated in 

abundance, apparently under the effects of modest climate change on lake circulation (Mori, 1974 ). 

The abundance of Aulacoseira during the warm 

climatic interval at ancient TuleLake from about 2.9 to 

2.6 Ma implies a deep, warm monomictic lake. According to Hutchinson and LOffler (1956, fig. 1), modern 

warm monomictic lakes are not found farther than 40° 

from the equator, except near sea level. We see no reason 

to postulate a low elevation for the Tule Lake basin (lat 

42° N.) during the late Pliocene; rather, we infer that the 

Pliocene climate recorded by the Tulelake core has its 

best modern analogs at lower latitudes. Pliocene climatic 

conditions at ancient Tule Lake must have required 

substantially increased annual precipitation and mean 

annual temperatures warmer and more equable than 

present. By analogy with Lake Biwa, Japan (mean annual 

temperature about 14 °C), the Tulelake area (present 

mean annual temperature about 9 °C) may have been 

about 5 °C warmer in the late Pliocene than at present. 

Higher sea levels implied by oxygen isotope records at 2.8 

Ma (Shackleton, 1986), different circulation patterns in 

the North Pacific from those at present (Sancetta and 

Silvestri, 1986), and reduced rain shadow effects (lower 

elevations of the Coast Ranges) may have combined to 

create these climatic conditions. 

DISCUSSION 

Comparison of the pollen and diatom records for 

the Pliocene section of the Tulelake core indicate general 

agreement that a rel~tively warm interval occurred between about 2.9 and 2.6 Ma. The lake in the basin was 

fairly deep and monomictic, so winter temperatures must 

have been much milder than at present. The proposed 

modern and Pleistocene analogs for the fossil pollen 

spectra lie within a Mediterranean climate, several degrees of latitude south of Tulelake, and above the lowelevation band of high frequencies of oak (Quercus) 

pollen. The general absence of spruce (Picea) pollen in 

the pollen spectra is also consistent with a Mediterranean 

climate and indicates that summers were dry. 

Although the pollen and diatom records are in 

general agreement in recording a pronounced warm 

interval between about 2.9 and 2.6 Ma, some changes in 

the diatom record occurred tens of thousands of years 

earlier than changes of similar magnitude in the pollen 

record (fig. 3). These lags indicate that the limnological 

system and the regional vegetation responded differently 

to environmental changes. In particular, changes in basin 

morphology and sill depths may have produced changes 

in the heat storage capacity and chemical behavior of the 

lake that caused the lacustrine system to behave independently of climate. Tectonic, volcanic, or depositional 

processes could all have thus affected the lake. 

On the basis of o180 analyses of foraminifers of the 

Globorotalia inflata group, Loubere and Moss (1986, fig. 

2) proposed a warm interval between 3.2 and 2.4 Ma for 

DSDP Site 548 in the North Atlantic. This warm interval 

also affected the limnology of ancient Tule Lake and the 

composition of the regional forest in the surrounding 

area, as inferred from pollen and diatom stratigraphy in 

the Tulelake core. Although Loubere and Moss do not 

present a detailed time scale for their warm interval, the 

brief cool interval that appears at the top of their core 25 

may well correlate with the relatively cool interval at 

about 2.7 Ma that is found in the Tulelake record in both 

the TCT:Pinus and Aulacoseira so/ida curves (fig. 3). 

The Tulelake warm period lies within the Gauss 

Normal Polarity Chron (Adam and others, 1989) and can 

be correlated on that basis with the Reuverian deposits of 

the Netherlands. The Reuverian B deposits record a 

three-part warm interval consisting of two thermomers 

separated by a cryomer (Zagwijn, 1974; Sue and Zagwijn, 

1983, fig. 2). This sequence may be correlative with the 

similar sequence observed in the Tulelake record. 

SUMMARY 

The Pliocene sequence in a core from the town of 

Tulelake, California, records a three-part warm period 

that occurred between about 2.9 and 2.6 Ma. Changes in 

regional vegetation, as recorded in the pollen record, are 

offset from the lacustrine changes recorded by the diatom record by tens of thousands of years. The three-part 

nature of the warm interval is consistent with other 

records from both continental and marine environments. 

A marked decrease in sedimentation rate is observed in 

Summary 11 
the Tulelake record near the time of the onset of glacial 

sedimentation observed in the North Atlantic at about 

2.37 Ma. The pattern of variation of regional vegetation 

was not strongly affected at this time; however, significant 

changes, such as a sharp increase in the abundance of the 

diatom genus Fragilaria, did occur within the lacustrine 

environment. 

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12 Environmental Changes in the Tule Lake Basin from 3 to 2 Million Years Before Present 
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References 13 



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