USGS Open-File Report 94-023
Pliocene Pollen Data Set Dynamics: Tulelake, California, And Lost Chicken Mine, Alaska
Pollen data sets of Pliocene age from continental sites are relatively
rare. Pollen work with a paleoecological slant has focused primarily on
deposits of Holocene and Upper Quaternary age, which are common and
readily sampled in many regions. Stratigraphic palynologists, in
contrast, have focused on pre-Pliocene deposits; most of the pollen grains
encountered in Pliocene deposits cannot be distinguished from modern
forms, and the simple presence or absence of particular forms is not as
informative as it is for older deposits.
- David P. Adam
- U.S. Geological Survey, Menlo Park, CA 94025
Although many or most of the pollen types important in Pliocene deposits
represent plants still extant, the associations formed by those plants in
the past may well have been different than those of today. Even if
Pliocene pollen assemblages have modern analogs, those analogs often
represent geographical ranges far removed from the Pliocene sites. For
example, poorly-preserved pollen samples from the Pliocene Santa Clara
Formation near San Francisco contain up to 50% spruce (Picea) pollen;
spruce does not occur in the area today (Adam et al., 1983).
Pollen records from the Pliocene often span rather long time intervals;
the longer the interval under study, the more work is required to achieve
fine time resolution. Time series are thus often severely under sampled
compared to Holocene studies, with sampling intervals measured in
millennia rather than centuries or decades. Pollen zones then tend to be
characterized not only by the frequencies of the various pollen types
within them, but also by the nature of the variability within and between
Because pollen assemblages from Pliocene deposits often suggest climatic
conditions significantly different from those of today, it is important to
understand not only the nature of the assemblages, but their underlying
dynamic structure. Assuming that the sequence of pollen samples through
time is known, the dynamics can be investigated by plotting the samples in
stratigraphic order in some appropriate phase space, chosen so as to
clarify the patterns present in the data. This approach to past
vegetation dynamics is illustrated below using data from Tulelake,
California, and Lost Chicken mine, Alaska.
The Tulelake record is from a 334-m core that spans the past 3 Myr (Adam
et al., 1989, 1990). Initial inspection of the pollen diagram suggested
that the behavior of pine pollen vs. TCT (Taxodiaceae, Cupressaceae, and
Taxaceae) pollen varied through time in a significant way. A plot of pine
vs. TCT pollen for the entire section (1A) was not helpful, but a plot of
the Pliocene part of the core (1B) indicated a well-developed pattern of
variability between the two types. When stratigraphically adjacent
samples are connected with lines (Figure 1C), the nature of the underlying
dynamics of the data set emerge. Nearly all of the connecting lines have
a negative slope, indicating that when pine increases, TCT decreases and
vice versa. Only rarely do both types increase or decrease together.
Comparison of the pattern of pine vs. TCT pollen discussed above with
modern data from central California suggested that the late Pliocene
climate at Tulelake had much in common with the present climate at middle
elevations along the western slope of the Sierra Nevada near Yosemite, and
with the climate that prevailed at lower elevations in the northern
California Coast Ranges during the cooler parts of deep-sea oxygen isotope
Stage 5 (Adam et al., 1990). Comparison of the various sites was greatly
facilitated by the phase-plot technique illustrated here.
- Figure 1. Plots of pine pollen percentage vs. TCT (Taxodiaceae,
Cupressaceae, and Taxaceae) pollen from Tulelake, Siskiyou County,
- This figure is available as a
TIFF (line-art) image. The three parts
- A) data for entire section;
- B) data for samples with ages from 2.0-3.1 Ma;
- C) same data as in B, but with stratigraphically adjacent samples connected by lines. Data are from Adam and Vagenas (1990).
Lost Chicken Mine
The Lost Chicken Mine is a working placer mine along the Taylor Highway in
east-central Alaska (see also Ager, this volume). It includes exposures
of plant- and mammal-bearing fluvial sediments and peats of upper Pliocene
age, as indicated by the Lost Chicken tephra layer, with a glass
isothermal fission-track age of 2.9 ± 0.2 Ma (John Westgate, oral
communication). Thirty-seven pollen samples were analyzed from various
stratigraphic units; age relationships between samples are sometimes clear
but sometimes inferred. The Pliocene samples apparently represent a warm
interval at or near the end of the Gauss Normal paleomagnetic chron; the
Holocene samples all predate the White River Ash, which has an age of 1200
years (Péwé, 1975).
The pollen counts were reduced to a condensed data set and subjected to a
detrended correspondence analysis (DCA; Gauch, 1982). The DCA reduced the
data set to four primary orthogonal axes that summarize the variability
within the data. Because each axis reflects patterns of variability that
apply to the entire data set rather than just to individual variables,
shifts in the behavior of the data with respect to these axes are likely
to reflect changes in the dynamics of the underlying climate system than
simple plots of one taxon versus another.
- Figure 2. Plot of Lost Chicken, Alaska, pollen samples against
Detrended Correspondence Analysis axis 1 and 2.
- This figure is available as a
TIFF (line-art) image.
Plot symbols represent various stratigraphic units, numbers are sample
numbers (some numbers are "missing"), and arrows indicate direction of
stratigraphic succession (older to younger).
The pollen samples are plotted against the first two DCA axes in 1, with
samples from the same stratigraphic unit in known stratigraphic order
connected by lines. The Holocene samples (29-33) are clearly set apart
from the other samples, which are all Pliocene in age. In addition,
inspection of the sequence of samples through time suggests that the
Pliocene samples represent two separate dynamic regimes: (1) Axis 1 and
Axis 2 scores positively correlated, and (2) Axis 1 and Axis 2 scores
Changes in the vegetational dynamics recorded in the Tulelake and Lost
Chicken data sets are reflected in the phase plots shown in figures 1 and
2 by changes in the slopes of the lines connecting stratigraphically
adjacent points. Particular dynamic regimes are represented by elongate
clouds of points in the phase space. These clouds are roughly linear in
the examples selected, but curvilinear clouds may also occur. The
character of a cloud representing a single regime should be independent of
the time intervals between samples; this property is useful in dealing
with Pliocene data sets that usually lack detailed time control. Shifts
in the location or orientation of the cloud are best identified through
inspection of the sequence of lines connecting the points in stratigraphic
order; the shifts show up as abrupt changes in slopes of the lines.
The method illustrated here is intended to augment rather than supplant
standard methods of interpretation. It provides a way to characterize
groups of samples that covary, and to contrast them with adjacent groups.
When samples are spaced relatively far apart compared to the frequencies
of the underlying climate signals, such characterizations may provide
useful insights into the behavior of the record.
- Adam, D. P., Adams, D. B., Forester, R. M., McLaughlin, R. J., Repenning, C. A., and Sorg, D. H., 1983, An animal- and plant-fossil assemblage from the Santa Clara Formation (Pliocene and Pleistocene), Saratoga, California, in Andersen, D. W., and Rymer, M. J., eds., Tectonics and sedimentation along faults of the San Andreas System: Los Angeles, Society of Economic Paleontologists and Mineralogists (Pacific Section), p. 105-110.
- Adam, D. P., Sarna-Wojcicki, A. M., Rieck, H. J., Bradbury, J. P., Dean, W. E., and Forester, R. M., 1989, Tulelake, California: the last 3 million years. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 72, p. 89-103.
- Adam, D.P., and Vagenas, G.L., 1990, Pollen data for a 3-m.y. core record from Tulelake, Siskiyou County, California: U.S. Geological Survey Open-File Report 90-65, 307 p.
- Adam, D. P., Bradbury, J. P., Rieck, H. J., and Sarna-Wojcicki, A. M., 1990, Environmental changes in the Tule Lake basin, Siskiyou County, California, from 3 to 2 million years before present: U.S. Geological Survey Bulletin 1933, 13 p.
- Gauch, H.G., Jr., 1982, Multivariate Analysis in Community Ecology: Cambridge, England, Cambridge University Press, 298 p.
- Péwé, T.L., 1975, Quaternary stratigraphic nomenclature in unglaciated central Alaska: U.S. Geological Survey, Professional Paper 862, 32 p.
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