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Vegetation and climate of the northwest coast of North America during the last 500 K.Y.: High-resolution pollen evidence from the northern California margin

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Pollen analyses of sediments from Holes 1019C, 1019E, 1020C, and 1020D as well as piston Core EW9504-17 provide continuous, chronostratigraphically controlled proxy vegetation and climate data for coastal northwest North America for the last ~500 k.y. Systematic changes in the representation of the diagnostic components of northern California plant assemblages clearly show orbital-scale variations. Interglacials are all marked by an abrupt increase in alder followed by expansion of lowland oak woodland and redwood forests. Glacials are dominated by montane forest and woodland assemblages. This sequence reflects large-scale climatic controls (e.g., orbital-scale variation in insolation and Northern Hemisphere ice sheets) in western North America over the last five glacial cycles. Regional climatic control (variations in sea-surface conditions) is implied by the differential development of xeric oak and mesic redwood communities.
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Lyle, M., Koizumi, I., Richter, C., and Moore, T.C., Jr. (Eds.), 2000
Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 167
217
1
Lyle, M., Koizumi, I., Richter, C., and Moore, T.C., Jr. (Eds.), 2000. Proc. ODP,
Sci. Results, 167: College Station TX (Ocean Drilling Program).
2
Lamont-Doherty Geological Observatory, Columbia Univeristy, Palisades NY
10987, USA. (Present address: Heusser and Heusser, Inc., 100 Clinton Road, Tuxedo
NY 10987, USA.) heusser@lamont.ldeo.columbia.edu
3
Center for Geophysical Investigation of the Shallow Subsurface, Boise State Uni-
versity, Boise ID 83725, USA.
4
College of Ocean and Atmospheric Science, Oregon State University, Corvallis
OR 97331, USA.
17. VEGETATION AND CLIMATE OF THE NORTHWEST COAST OF NORTH AMERICA
DURING THE LAST 500 K.Y.: HIGH-RESOLUTION POLLEN EVIDENCE
FROM THE NORTHERN CALIFORNIA MARGIN
1
L.E. Heusser,
2
M. Lyle,
3
and A. Mix
4
ABSTRACT
Pollen analyses of sediments from Holes 1019C, 1019E, 1020C, and 1020D as well as piston Core EW9504-17 provide
continuous, chronostratigraphically controlled proxy vegetation and climate data for coastal northwest North America for the
last ~500 k.y. Systematic changes in the representation of the diagnostic components of northern California plant assemblages
clearly show orbital-scale variations. Interglacials are all marked by an abrupt increase in alder followed by expansion of low-
land oak woodland and redwood forests. Glacials are dominated by montane forest and woodland assemblages. This sequence
reflects large-scale climatic controls (e.g., orbital-scale variation in insolation and Northern Hemisphere ice sheets) in western
North America over the last five glacial cycles. Regional climatic control (variations in sea-surface conditions) is implied by
the differential development of xeric oak and mesic redwood communities.
INTRODUCTION
It has been established that pollen deposited in marine sediments
on continental margins provides terrestrial vegetation and climate
records that are directly correlated with global chronostratigraphic
events (Dupont, 1992; Heusser and van de Geer, 1994; Van Campo et
al., 1982). Interpretations of past vegetation and climate of western
North America from pollen deposited offshore are based on compar-
isons of downcore pollen spectra with modern marine and terrestrial
pollen spectra (e.g., pollen assemblages from marine and terrestrial
sediment traps and surface samples), vegetation, and climate (Heusser
and Balsam, 1977; Heusser, 1988). Previous studies showed that Mi-
lankovitch-scale variations in vegetation of coastal Washington and
Southern California reconstructed from pollen deposited in the north-
east Pacific Ocean during the last glacial cycle were similar to those
inferred from onshore pollen data ascribed to the same time (Heusser
and Florer, 1973; Heusser, 1995). To extend these studies in space
and time, we present preliminary results from pollen analyses of sed-
iments deposited off Northern California during the last ~500 k.y.
Pollen from piston cores taken on the continental margin of west-
ern North America between 32°N and 43°N also showed systemati-
cally related short-term changes superposed on Milankovitch-scale
oscillations of the past 60 k.y. During oxygen isotope Stage 3, for ex-
ample, brief warming events in California and Oregon appear correl-
ative with interstadial events in waters offshore (Heusser, 1998).
Here we extend high-resolution pollen analyses of sediments depos-
ited on the northern California margin over the last ~140 k.y.
BACKGROUND
Natural coastal vegetation of California changes from xeric, open
oak woodland in the south to dense conifer rainforest in the north.
The complex mosaic of southern California scrub oak (Quercus), oak
woodland savanna, and oak-dominated foothill woodlands with iso-
lated groups of closed-cone pine and cypress (Pinus radiata and Cu-
pressus pygmaea) interfingers with chaparral and lowland sage
scrub. Conifer forests develop upslope in which open pine woodlands
with small, scattered stands of incense cedar (Libocedrus decurrens)
are succeeded at higher elevations by mid-montane parkland with
pine and incense cedar, upper montane juniper (Juniperus occidenta-
lis) woodland, and subalpine coniferous forests with lodgepole pine
(Pinus contorta) (Barbour and Billings, 1988; Barbour and Major,
1977; Franklin and Dyrness, 1973).
In Northern California, redwood (Sequoia sempervirens) distin-
quishes the southern extension of the Pacific Northwest evergreen
coniferous forests, which are unique in size and longevity among
temperate forest regions of the world (Waring and Franklin, 1979).
Associated with redwood are western hemlock (Tsuga heterophylla),
spruce (Picea sitchensis), Douglas fir (Pseudotsuga menziesii), and
western red cedar (Thuja plicata). Inland, xerophytic lowland decid-
uous oak communities (Quercus garryana, Quercus lobata, and
Quercus douglasii) and oak/pine/grassland mosaics develop. At
higher elevations in the Coastal Range, montane forest formations
develop with fir (Abies), hemlock, Douglas fir, pine, and evergreen
oaks (Barbour and Major, 1977). Just north of ~4°N in Oregon, west-
ern hemlock and Sitka spruce dominate along the Pacific Ocean, with
hemlock on the coast and spruce more prominent in the interior.
Common forest associates include Douglas fir, western red cedar (T.
plicata), and alder (Alnus) in moist habitats. Above the narrow band
of lowland forest are montane and subalpine forests of fir (Abies con-
color and Abies amabilis), pine (Pinus ponderosa and Pinus lamber-
tiana), and mountain hemlock (Tsuga mertensiana).
The two contrasting vegetation types of the California coast
(southern California oak woodland and Pacific Northwest conifer
forests) reflect significant differences in mean annual temperature
and precipitation (first-order controls of vegetation distribution).
South of ~40°Ν–42°N, mean annual temperatures and precipitation
average 19°C and 30 cm, and upper montane temperature and precip-
itation are ~8°C and ~57 cm. To the north in Oregon, mean annual
lowland temperature and precipitation are ~12°C and ~300 cm, and
subalpine temperatures and precipitation average ~10°C and ~140
cm (Elford, 1974; Sternes, 1974). The south–north transition from
excess evaporation to excess precipitation roughly coincides with the
frequency and intensity of frontal storms south and north of the atmo-
spheric and oceanic polar fronts.
L.E. HEUSSER ET AL.
218
Close to the ocean, California and Oregon temperatures and effec-
tive precipitation are moderated by fog associated with upwelling
(Barbour, 1988; Barbour et al., 1980) and by seasonal variations in
sea-surface temperatures (SSTs) of the southward-flowing California
Current and the poleward-flowing seasonal Davidson Current. South
of ~42°N, northerly winds drive near-coastal persistent seasonal up-
welling; to the north, upwelling intensity is more variable. Off Cali-
fornia and Oregon, waters north of 40°Ν42°N are subarctic in type
with mean SSTs of ~12° to ~13°C; off Southern California, mean
SSTs of ~14° to ~15°C reflect the presence of subtropical waters
(Gardner et al., 1997).
During the past glacial cycle, changes in the California Current
system and in California maritime vegetation and climate inferred
from pollen data appear to be related. Near-synchronous high-
frequency variations in the abundance of oak in Southern California
and SST in the Santa Barbara Basin characterize oxygen isotope
Stage (OIS) 5 (Heusser, 1995). Similar variations in redwoods in
Northern California correspond to temporal and geographic changes
in offshore upwelling and in movement of the Polar Front during
OISs 31 (Doose et al., 1997; Gardner et al., 1997; Heusser, 1998;
Lyle et al., 1992; Sabin and Pisias, 1996; Sancetta et al., 1992). Here
we use three pollen records from cores taken on the northeast Pacific
continental margin to document millennial-scale variability of coastal
North American ecosystems from OISs 1 through 13.
METHODS
Pollen records are from cores taken at two Ocean Drilling Pro-
gram (ODP) drill sites (Site 1019: 41°40.696N, 124°55.981W, 989
meters below seafloor [mbsf]; Site 1020: 41°0.051′Ν, 126°26.065W,
3050 mbsf) and from piston Core EW9504-17PC (42°14.55N,
125°53.28W, 2671 mbsf; Fig. 1). The siliciclastic clays and silt from
the upper 60 m (meters composite depth [mcd]) of Holes 1020C,
1020D, 1019C, and 1019E were routinely sampled at 20-cm intervals
(Lyle, Koizumi, Richter, et al., 1997). Core EW9504-17PC, com-
posed predominantly of hemipelagic clay (Lund and Mix, 1998), was
sampled at 5-cm intervals. Standard processing procedures that in-
cluded the addition of known amounts of an exotic tracer to calculate
pollen concentration were preceded and succeeded by sieving
through 7-µm nylon screening. Taxonomic identification of pollen
was based on comparison with modern pollen reference collections
from western North America. Specific epithets are indicated for
grains that were clearly identified; otherwise, pollen and spores are
assigned to genera or higher rank. Other than the papillate grains of
Sequoia (redwood) and the large inaperturate grains assumed to rep-
resent P. menziesii, inaperturate pollen of other genera in the Taxodi-
aceae, Cupressaceae, and Taxaceae that cannot be satisfactorily sep-
arated using light microscopy (e.g., Juniperus, Torreya, Cupressus,
Libocedrus, Chamaecyparis, and Thuja) are here referred to as cedar
type. Other synthetic pollen groups include chaparral (sclerophylous
shrubs and other members of the Anacardiaceae, Rhamnaceae, and
Rosaceae) and herbs (Gramineae, Cyperaceae, and Compositae, in-
cluding Artemisia or sage). A mininum of 300 pollen grains were
identified in each sample from Core EW9504-17PC; the initial pollen
counts from Sites 1019 and 1020 presented here averaged ~110 and
~150 pollen grains, respectively. Pollen percentages were based on
the sum of terrestrial pollen (excluding fern spores), and pollen con-
centration was calculated on the number of pollen grains per gram dry
weight of sediment (gdws).
Age models were constructed by correlating the CaCO
3
and C
org
records to the best radiocarbon-dated sections and oxygen isotope
records. Methods are reported in detail in Lyle et al. (Chap. 32, this
volume). Age control for Core EW9504-17PC was achieved by (1)
correlating the CaCO
3
and C
org
records from Lyle et al. (Chap. 32, this
volume) with Core W870913PC (time scale of Lund and Mix, 1998)
for 050 ka, (2) correlating the oxygen isotope record from 50 to 140
ka in the core (Lund and Mix, 1998) with the Martinson et al. (1987)
age model, (3) allowing minor shifts in the record to minimize sedi-
mentation rate changes without losing significant correlation, and (4)
comparing the oxygen isotope record on the final age model to the
Martinson et al. (1987) age model to see if the two age models re-
mained consistent.
To construct an age model for Site 1020, we correlated the dated
carbonate and organic carbon records from Core EW9504-17PC with
the carbonate and organic carbon records of nearby Site 1020 and
hence were able to transfer the age model from Core EW9504-17PC
to Site 1020 (Lyle et al., Chap. 32, this volume). Below 140 ka, our
age model for Site 1020 is based upon reconnaissance-scale oxygen
isotope stratigraphy and biostratigraphic datums.
Development of an age model for Site 1019 was more problematic
(Lyle et al., Chap. 32, this volume). The preliminary age model used
here is based on reconnaissance-scale oxygen isotope stratigraphy
supplemented by radiocarbon age control from 6 to 24 ka. We note
that these preliminary age models will be modified when more age
control (radiocarbon and stable isotope data) is available. Using the
age models described above, the average sampling interval in Core
EW9504-17PC was ~500 yr, the average sampling interval in the up-
per 57 mcd of Holes 1020C and 1020D was ~1700 yr, and the average
sampling interval in the upper 40 m of Holes 1019C and 1019E was
~800 yr (Lyle, Koizumi, Richter, et al., 1997).
RESULTS AND DISCUSSION
In the hemipelagic sediments from Core EW9504-17 and from
Sites 1019 and 1020, pollen concentration is high, with slightly more
pollen in sediments deposited closest to land (Fig. 2). Mean pollen
abundance in sediments deposited at Site 1019 (~59 km west of the
California coast) is 5500 grains/gdws compared with mean concen-
trations of 4000 grains/gdws in sediments deposited at Core
Figure 1. Map of the northeast Pacific continental margin showing core loca-
tions. Site 1019 is in the coastal upwelling system. Site 1020 and Core
W8709-17 are under the northern California Current, south of the North
Pacific Subpolar Front.
HIGH-RESOLUTION POLLEN EVIDENCE
219
EW9504-17 (~120 km offshore) and 3600 grains/gdws in sediments
deposited at Site 1020 (~167 km offshore). Rapid oscillations in pol-
len abundance that occur in pollen records from each of the three lo-
calities display high-amplitude peaks that show systematic variations
in Core EW9504-17 and Site 1019. Age plots (Fig. 2) show that al-
though the overall trends of pollen abundance in Core EW9504-17
broadly correspond to interglacialglacial climate change (e.g., max-
ima occur in warm intervals such as the Holocene and OIS 5, and
minima are associated with cold intervals such as OIS 2), correlation
between pollen concentration and climatic events is not consistent.
At Site 1019, maxima in total pollen abundance coincide with some
(OISs 5 and 7) but not all major warm intervals, whereas variations
in mass accumulation rates of redwood at Site 1020 (like that of ter-
rigenous minerals; see Hovan et al., Chap. 18, this volume) corre-
spond fairly closely to global δ
18
O variations. Pollen concentration
on the California continental margin reflects vegetation density, pol-
len sedimentation (including fluviomarine sedimentary processes),
and climatic processes (regional and global; Heusser and Balsam,
1977; Traverse, 1988; Fig. 2).
The pollen records from Sites 1019 and 1020 as well as Core
EW9504-17 are composed of taxa that presently grow in northwest
America. Paleoecologic interpretations of these pollen spectra are
based on several assumptions: (1) the diagnostic components of pol-
len assemblages from marine cores, like those from terrestrial cores,
reflect in varying degrees the composition of vegetation formations
from which they are derived (Heusser, 1983, 1988; Heusser and Bal-
sam, 1977); (2) modern climatic tolerances of vegetation with pollen
spectra similar to fossil pollen spectra provide a reliable foundation
for reconstructing past vegetation and climate (Heusser et al., 1980;
Whitlock and Bartlein, 1997); and (3) changes in the Quaternary veg-
etation of California and southern Oregon reflect regional and global
climatic change (Axelrod, 1977; Huntley and Webb, 1988).
At all three localities, pollen spectra of the uppermost samples are
marked by a succession of alder, oak, and redwood peaks accompa-
nied by ferns and lesser amounts of western hemlock, spruce, and ce-
dar (Figs. 35). These distinctive assemblages of pollen representative
of natural north coast forest and oak woodland communities occur re-
peatedly downcore: between ~13 and ~15 m in Core EW9504-17 (Fig.
3); also, between ~38 and ~32 mcd and between ~46 and ~66 mcd at
Site 1019 (Fig. 4). In the record from Site 1020, the alder-oak-
redwood-fern assemblage is repeated five times (Fig. 5). Although the
basic composition is essentially the same in the three records, differ-
ences exist in the sequence and relative abundance of taxa. In Holes
1019C and 1019E, the redwood maxima at 33.48 mcd precedes that of
oak (32.68 mcd). At Site 1020, redwood maxima vary in amplitude
from ~35% at 44 mcd to ~10% at 2628 mcd, and alder maxima range
from ~22% at ~55 mcd to ~7% at 14 mcd. Except for the peak at ~14
mcd, oak peaks display less variation.
The peaks of alder that initiate abrupt expansions of coastal low-
land forest types reflect the pioneer role of alder in aggressively colo-
nizing areas disturbed by catastrophic events (flooding and infrequent
wildfires) or by reorganization of plant communities related directly
EW9504-17PC
1019C/E
1020C/D
EW9504-17PC 1020C/D
0
25
0
0
0
0
100 20
25 12
0
0
0
0
0
5
10
15
20
40
60
10
30
50
50
100
150
50
150
250
350
Depth (m)
Age (k.y.)
Figure 2. Depth plot of pollen concentration in number of pollen grains per gram dry weight of sediment at Sites 1019 and 1020 and piston Core EW9504-17
(left), and time series of pollen concentration at Site 1020 and in piston Core EW9504-17 (right). Depths of samples from Holes 1019C, 1019E, 1020C, and
1020D are meters composite depths (Lyle, Koizumi, Richter, et al., 1997). Age models are based on time scales developed by Lyle et al. (Chap. 32, this volume).
L.E. HEUSSER ET AL.
220
or indirectly to climate change. The apparent lack of systematic vari-
ation in charcoal fragments found in pollen samples suggests that
wildfires were probably not the primary cause of the high-amplitude
alder events. Alder habitats are wet, along streams and in marshy
places in redwood and mixed evergreen forests (Alnus oregona and A.
sinuata), and moist places in the north Coastal Range (A. tenuifolia;
Barbour and Billings, 1988; Munz, 1968). Increased alder in sedi-
ments deposited on the California margin thus may reflect expansion
and/or recolonization of alder habitats related to increased precipita-
tion and/or increased runoff from snowmelt during deglaciation
(Heusser and Shackleton, 1979). Alder expansion in areas of revege-
tation following disturbance is well documented in the Pacific North-
west (Barbour and Billings, 1988; Grigg and Whitlock, 1998; Heus-
ser, 1985).
Subsequent expansion of the two different north coast lowland
habitatsmesic redwood forests along the narrow band adjacent to
the Pacific Ocean and xeric oak woodland farther inlandmay be re-
lated to changes in temperature, precipitation, and other atmospheric
and oceanic processes. Higher temperatures and change in the
amount and seasonal distribution of rainfall that enhanced summer
drought (increased summer moisture stress related to decreased ef-
fective precipitation and increased summer temperatures, for exam-
ple) would favor development of lowland oak communities. Such
conditions would not be conducive to redwood expansion. Coastal
redwood requires much lower diurnal and annual temperature fluctu-
ations than oak and is now restricted to that part of the coast where
temperature extremes and summer moisture stress are modified by
prolonged cloudy periods and marine fog associated with offshore
upwelling (Zinke, 1977). We suggest that redwood expansion was
probably closely related to the development of maritime conditions
(changes in the distribution and intensity of upwelling in the Califor-
nia Current, for example), which moderated north coast climate. De-
tailed statistical analyses of radiolarian species and pollen taxa from
Core EW9504-17 show a very high degree of correlation between ra-
diolarians associated with coastal upwelling and redwood (N.G. Pi-
sias, pers. comm., 1998).
All the pollen records contain lengthy intervals dominated by
seemingly uniform assemblages composed of pine, herbs, and cedar,
along with spruce, hemlock, and mountain hemlock (T. mertensiana,
a subalpine species; Barbour and Billings, 1988). To some extent, the
apparent uniformity reflects limitations of pollen analysis; that is, the
presence of genera and families that cannot be discriminated into
more ecologically specific taxonomic taxa. The composition of these
conifer-dominated assemblages alternates between coastal forest
taxa like those now growing north of the present distribution of red-
wood (in northern Oregon, Washington, and British Columbia) and
forests or open pine woodlands not unlike those that now occur at
higher elevations (Barbour and Major, 1977). Oscillations in herbs
that occur throughout indicate variable development of vegetative
cover.
Similar sequences of temperate lowland conifer forest and oak
woodland assemblages alternating with those of montane forest/
woodland occur in upper Quaternary pollen records from sites in
Northern California, western Oregon, and Washington as well as in
0
40
Herbs
0
80
Cedar/pine
0
80
Ferns
0
30
Spruce
0
25
Hemlock
0
40
Redwood
0
25
Oak
0
50
0
4
8
12
16
Alder
Depth (m)
Figure 3. Depth plots of percentages of selected pollen types from piston Core EW9504-17. The relative abundance of sage (Artemisia) is shown as a solid black
line in the herbs plot; other herb percentages are shown by a dotted line. The percentage of cedar is a solid black line; pine percentages are plotted with a dotted
line. Note that scales vary for different taxa.
HIGH-RESOLUTION POLLEN EVIDENCE
221
Quaternary pollen data from other marine cores taken off Northern
California, Oregon, and Washington (Adam and West, 1983; Grigg
and Whitlock, 1998; Heusser, 1985, 1998; Heusser and Shackleton,
1979). Regional differences in vegetation are readily apparent in the
composition of the warm temperate pollen assemblages (i.e., the
prominence of redwood and/or oak in the south and of western hem-
lock and spruce in the north) and in the composition of the more ho-
mogeneous pollen assemblages in the intervening cooler intervals,
which are characterized by the greater prominence of juniper and ce-
dar types in cores from the south. The similarity between the system-
atic downcore variations in marine and continental records from the
same geographic area implies that marine pollen records, like those
on land, capture systematic variations in regional vegetation and cli-
mate.
When pollen data are plotted against age (Figs. 68), it is evident
that in Core EW9504-17, alder, oak, redwood, and fern maxima cor-
respond to benthic δ
18
O minima and that downcore variations in pol-
len stratigraphies of Sites 1019 and 1020 also reflect orbital-scale
global climate fluctuations. Small differences in the relative abun-
dance of the mesophytic, temperate taxa between OISs 1, 5, 7a, 7c,
and 9 suggest that the development and composition of interglacial
vegetation in Northern California was not always identical in each of
the last four interglacials. Because of the preliminary nature of age
models for Sites 1019 and 1020 (Lyle et al., Chap. 32, this volume),
we focus on the well-dated EW9504-17 time series.
The double beat of OIS 7 δ
18
O at Site 1020 (Fig. 8) is mirrored in
muted alder, oak, and redwood peaks that are more robust (as are
ferns) in the preceding interglacial (OIS 9). The OIS 6/5e transition
in Core EW9504-17 and at Site 1019 is marked by an abrupt rise in
alder that is rapidly succeeded in OIS 5e by oak and redwood peaks
in Core EW9504-17. At Site 1019, the redwood maximum precedes
that of oak. Multiple oscillations in these taxa occur in OIS 5c. These
large-scale patterns also occur in pollen assemblages in the upper
~130 k.y. of the lower resolution Site 1020 pollen record (Fig. 8). It
is worth noting that in all three records, pollen assemblages from OIS
5e are not exact replicates of those in OIS 1. During the last intergla-
cial, oak was more abundant than in the Holocene; the converse is
true for alder and redwood in Core EW9504-17 and at Site 1020.
Pine, dominant in OIS 5d and 5b, became increasingly important
during the last full glacial, as did sage and other herbs. In OIS 3, brief
pine events occurred at ~14, ~16, ~35, ~38, and ~42 k.y. in Core
EW9504-17 (Fig. 6). Two well-defined events coincide with major
episodes of North Atlantic ice rafting (Heinrich Events H1 and H4).
The low amplitude of redwood and oak oscillations during OIS 3,
which partly reflects overrepresentation of pine (a common feature of
pollen dispersal and sedimentation; Traverse, 1988), precludes corre-
lation with interstadial events elsewhere.
In Holes 1020C and 1020D, the rhythmic pattern of downcore
variation in the alder, oak, redwood, and fern maxima (Fig. 5) is evi-
dent through OIS 13 (M. Lyle, pers. comm., 1998). Between ~43 and
~45 mcd (OIS 11), the robust redwood acme is preceded by a sub-
stantial peak in alder and ferns. Oak percentages are comparable to
those of previous interglacials (excluding OIS 5). The high-ampli-
tude pulse of alder between ~51 and ~55 mcd (OIS 13) leads a lesser
080
Cedar/pine
0
100
Ferns
035
Spruce
030
Hemlock
050
Redwood
025
Oak
050
Herbs
0
20
40
60
0 100
Alder
Depth (m)
Figure 4. Depth plots of percentages of selected pollen types from the upper 66 m of Holes 1019C and 1019E. The relative abundance of sage (Artemisia) is
shown as a solid black line in the herbs plot; other herb percentages are shown by a dotted line. The percentage of cedar is a solid black line; pine percentages
are plotted with a dotted line. Note that scales vary for different taxa.
L.E. HEUSSER ET AL.
222
rise in redwood, oak, and ferns. As in the younger part of the pollen
record, interglacial assemblages display individualistic variations.
At glacialinterglacial transitions (OISs 8/7, 6/5e, 2/1, and most
probably 10/9), the abrupt changes in west coast vegetation (identi-
fied by the rapid expansion of the pioneer alder) and global warming
(higher benthic δ
18
O) are nearly synchronous (Figs. 6, 8). The initial
increase in alder and the shift in δ
18
O at glacial terminations occurs in
exactly the same sample depths in Core EW9504-17 and at Site 1020.
Directly correlative pollen and δ
18
O data from two other northeast Pa-
cific cores (Core Y7211-1 taken at 43°15N, 126°22W; Site 893
taken at 34°17.25N, 120°02.19W; Fig. 1) showed similar relation-
ships at the OIS 6/5e transition (Heusser, 1995; Heusser and Shackle-
ton, 1979). These data imply that glacialinterglacial variations in
northwest North American climate and vegetation over the last 350
k.y. were (within constraints of sample resolution) apparently nearly
synchronous with orbital-scale global ice-volume variations. Directly
correlative terrestrial/marine records from piston cores taken in the
northwest Pacific (Morley and Heusser, 1997) and from Site 594 in
the southwest Pacific (Heusser and van de Geer, 1994) showed that
large-scale variations in Japanese and New Zealand ecosystems over
the last ~350 k.y. could be attributed to orbital forcing of global cli-
mate mechanisms.
To display pollen data in a form less affected by overrepresenta-
tion of pine, we use pollen ratios (Fig. 9). The redwood and western
hemlock/spruce ratio can be regarded as a temperature indicator of
mesophytic lowland forests because average July temperatures in ar-
eas now dominated by redwood and/or western hemlock are ~1° to
2°C higher than in areas dominated by spruce (Heusser, 1985; Heus-
ser and Shackleton, 1979; Zinke, 1977). The oak/pine ratio serves as
an indicator of temperature trends in the more arid northern Califor-
nia interior since temperatures in the lowland oak woodlands are sev-
eral degrees higher than temperatures in montane pine forests (Adam
and West, 1983). At ~42°N, maximum mean monthly temperatures
at 332-m elevation in the nearby mountains are 6.2°C higher than on
the coast (Barbour and Major, 1977). Although we describe these cli-
mate proxies as temperature indicators, we recognize that effective
precipitation is a major factor in vegetative composition and cannot
be effectively separated from temperature in our paleoclimatic prox-
ies.
The close correspondence between pollen ratio, insolation, and
δ
18
O curves implies that north coast environmental fluctuations were
broadly synchronous with changes in global climate over the last
~150 k.y. (Fig. 9). As suggested earlier, the lag in the response of red-
wood/western hemlock communities probably reflects the significant
role of sea-surface conditions in the development of north coast mar-
itime vegetation (Lyle et al., Chap. 32, this volume).
SUMMARY
Pollen records from the northern California and southern Oregon
margin of the northeast Pacific Ocean (Sites 1019 and 1020 and pis-
0 100
Cedar/pine
0
60
Ferns
0
25
Spruce
0
25
Hemlock
0
40
Redwood
0
20
Oak
0
35
Herbs
0
10
30
50
0
25
Alder
Depth (m)
Figure 5. Depth plots of percentages of selected pollen types from the upper 60 m of Holes 1020C and 1020D. The relative abundance of sage (Artemisia) is
shown as a solid black line in the herbs plot; other herb percentages are shown by a dotted line. The percentage of cedar is a solid black line; pine is plotted with
a dotted line. Note that scales vary for different taxa.
HIGH-RESOLUTION POLLEN EVIDENCE
223
ton Core EW9504-17) capture vegetational-environmental changes
on the North American northwest coast. Glacial vegetation is domi-
nated by montane conifer types accompanied by a significant amount
of herbs. Major climatic thresholds such as glacialinterglacial tran-
sitions are marked by high-amplitude peaks of the pioneer alder that
precede the expansion of interglacial lowland oak woodlands and
coastal redwood forests. The similarity in the structure of oak- and
redwood-based pollen ratios, insolation, and δ
18
O curves implies that
environmental fluctuations on the Pacific Northwest coast reflect
large-scale variations in global climate over the last ~150 k.y. Com-
parison of preliminary pollen and oxygen isotope data from Sites
1019 and 1020 indicates that similar relationships prevailed over the
last 500 k.y. We suggest that differences in the development of low-
land oak- and redwood-dominated vegetation on the northwest coast
of North America reflect fluctuations in large-scale climate controls
and fluctuations in regional maritime conditions offshore.
ACKNOWLEDGMENTS
This research was supported by JOI/USSAC and by National Sci-
ence Foundation grants. We thank the Ocean Drilling Program and
the Leg 167 Scientific Party. Other individuals who have contributed
to the development of this manuscript include N. Pisias, C. Heusser,
and E. Stock. We thank L. Dupont and A. Mix for reviewing the
manuscript. Oregon State University curating facilities are supported
by NSF grant OCE94-02298.
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Date of initial receipt: 30 September 1998
Date of acceptance: 18 June 1999
Ms 167SR-206
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0
100
080
050
025
0
100
0
20
60
100
140
OIS 1
OIS 3
OIS 5
Alder/ferns Oak Redwood Cedar/pine
Herbs
Age (k.y.)
Figure 7. Time series of selected pollen types from Hole 1019C/E and Core EW9504-17 with benthic OISs 1 through 5 superimposed. This preliminary time
series is based on the initial time scale developed by Lyle et al. (Chap. 32, this volume). Percentages of ferns, pine, and herbs (excluding sage, which is shown
as a solid line in the herbs plot) are shown as dotted lines. OIS = oxygen isotope stage.
L.E. HEUSSER ET AL.
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0
35
0
100
0
30
0
20
0
30
35.5
0
50
150
250
350
OIS 5
OIS 7
OIS 9
Age (k.y.)
δ
18
O Alder/ferns Oak Redwood Cedar/pine Herbs
Figure 8. Time series of selected pollen types and preliminary stable isotope data from Holes 1020C and 1020D with benthic OISs 1 through 9 superimposed.
Percentages of ferns, pine, and herbs (excluding sage, which is shown as a solid line in the herbs plot) are shown as dotted lines. OIS = oxygen isotope stage.
0
35
0
5
3
5.5
-800-600
0
50
100
150
Age (k.y.)
Insolation (65°N) δ
18
O Oak/pine ratio
Conifer ratio
Figure 9. Comparison of July insolation with time series of δ
18
O and pollen ratios (oak/oak + pine and redwood + western hemlock/spruce) from piston Core
EW9504-17.
... The concentration of pollen in marine sediments reflects the abundance of vegetation present as determined by regional and global climate patterns (Heusser and Balsam, 1977;Traverse, 1988), as well as the sedimentary processes responsible for delivering it to the deposition site. Typically, higher pollen abundances occur in marine sediments lying closer to their continental source (Muller, 1959;Groot and Groot, 1966;Traverse and Ginsburg, 1966;Groot et al., 1967;Heusser and Balsam, 1977;Heusser, 1978b;Melia, 1984;Heusser et al., 2000) and opposite major stream drainage areas (Cross et al., 1966;Groot et al., 1967;Heusser and Balsam, 1977;Heusser, 1978a;Heusser et al., 2015). Pollen concentrations in core S3-15G obtained~135 km offshore range between 5162 and 14,473 grains/g dry weight sediment (gdws) with a mean of 8700 grains/gdws (Table 3). ...
... Pollen concentrations in core S3-15G obtained~135 km offshore range between 5162 and 14,473 grains/g dry weight sediment (gdws) with a mean of 8700 grains/gdws (Table 3). These pollen concentrations are higher than those found off northern California and Oregon by Heusser et al. (2000), in which the mean pollen abundances were 5500, 4000, and 3600 grains/gdws at distances of~59,~120, and~167 km offshore, respectively. The higher pollen concentrations in S3-15G may simply reflect more dense vegetation onshore opposite the deposition site (Cross et al., 1966). ...
... Instead, higher pollen abundances in the Holocene most likely reflect warming conditions compared to the Pleistocene. Heusser et al. (2000) also noted maxima in pollen abundance during the warm intervals (Oxygen Isotope Stage (OIS) 5 and the Holocene) and minima during the cold intervals (e.g., OIS 2) off the northern California margin. ...
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... [59] On the other hand, an increasing contribution of redwood pollen after 5.2 ka (Figure 7) might be taken as evidence of increasing coastal upwelling at Site 1019 during the later part of the Holocene [Heusser, 1998;Heusser et al., 2000]. Forests of coastal redwood are best developed where fog associated with cold upwelling waters moderates summer temperatures (mean July temperature of 17°C) and ameliorates drought. ...
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... [59] On the other hand, an increasing contribution of redwood pollen after 5.2 ka (Figure 7) might be taken as evidence of increasing coastal upwelling at Site 1019 during the later part of the Holocene [Heusser, 1998;Heusser et al., 2000]. Forests of coastal redwood are best developed where fog associated with cold upwelling waters moderates summer temperatures (mean July temperature of 17°C) and ameliorates drought. ...
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Divides the woodlands into 7 regional or elevational classes and then into 20 communities. Eight taxa can define each class: Quercus douglasii for the blue oak woodland, Q. agrifolia for the southern oak woodland, Q. chrysolepis for the mixed evergreen forest, Pinus ponderosa and P. jeffreyi for the mid-montane and eastside conifer forests, P. contorta var. murrayana for the upper montane conifer forest, and P. albicaulis and P. flexilis for the subalpine woodland. Greater floristic detail and associated environmental factors are given for each class. -P.J.Jarvis
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Pollen records from two sites in western Oregon provide information on late-glacial variations in vegetation and climate and on the extent and character of Younger Dryas cooling in the Pacific Northwest. A subalpine forest was present at Little Lake, central Coast Range, between 15,700 and 14,850 cal yr B.P. A warm period between 14,850 and 14,500 cal yr B.P. is suggested by an increase inPseudotsugapollen and charcoal. The recurrence of subalpine forest at 14,500 cal yr B.P. implies a return to cool conditions. Another warming trend is evidenced by the reestablishment ofPseudotsugaforest at 14,250 cal yr B.P. Increased haploxylonPinuspollen between 12,400 and 11,000 cal yr B.P. indicates cooler winters than before. After 11,000 cal yr B.P. warm dry conditions are implied by the expansion ofPseudotsuga.A subalpine parkland occupied Gordon Lake, western Cascade Range, until 14,500 cal yr B.P., when it was replaced during a warming trend by a montane forest. A rise inPinuspollen from 12,800 to 11,000 cal yr B.P. suggests increased summer aridity.Pseudotsugadominated the vegetation after 11,000 cal yr B.P. Other records from the Pacific Northwest show an expansion ofPinusfrom ca. 13,000 to 11,000 cal yr B.P. This expansion may be a response either to submillennial climate changes of Younger Dryas age or to millennial-scale climatic variations.
Article
Vegetation records spanning the past 21kyr in western North America display spatial patterns of change that reflect the influence of variations in the large-scale controls of climate. Among these controls are millennial-scale variations in the seasonal cycle of insolation and the size of the ice sheet, which affect regional climates directly through changes in temperature and net radiation, and indirectly by shifting atmospheric circulation. Longer vegetation records provide an opportunity to examine the regional response to different combinations of these large-scale controls, and whether non-climatic controls are important. But most of the longer North American records are of insufficient quality to allow a robust test, and the long European records are in regions where the vegetation response to climate is often difficult to separate from the response to ecological and anthropogenic controls. Here we present a 125-kyr record of vegetation and climate change for the forest/steppe border of the eastern Cascade Range, northwest America. Pollen data disclose alternations of forest and steppe that are consistent with variations in summer insolation and global ice-volume, and vegetational transitions correlate well with the marine isotope-stage boundaries. The close relationship between vegetation and climate beyond the Last Glacial Maximum provides evidence that climate variations are the primary cause of regional vegetation change on millennial timescales, and that non-climatic controls are secondary.
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A north-south transect of 17 cores was constructed along the eastern boundary of the California Current system from 33° to 42°N to investigate the changes in biogenic sedimentation over the past 30 kyr. Percentages and mass accumulation rates of CaCO3, Corg, and biogenic opal were assembled at 500 to 1000 years/sample to provide relatively high resolution. Time-space maps reveal a complex pattern of changes that do not follow a simple glacial-interglacial two-mode model. Biogenic sedimentation shows responses that are sometimes time-transgressive and sometimes coeval, and most of the responses show more consistency within a limited geographic area than any temporal consistency. Reconstructed conditions during late oxygen isotope stage 3 were more like early Holocene conditions than any other time during the last 30 kyr. Coastal upwelling and productivity during oxygen isotope stage 3 were relatively strong along the central California margin but were weak along the northern California margin. Precipitation increased during the last glacial interval in the central California region, and the waters of the southern California margin had relatively low productivity. Productivity on the southern Oregon margin was relatively low at the beginning of the last glacial interval, but by about 20 ka, productivity in this area significantly increased. This change suggests that the center of the divergence of the West Wind Drift shifted south at this time. The end of the last glacial interval was characterized by increased productivity in the southern California margin and increased upwelling along the central California margin but upwelling remained weak along the northern California margin. A sudden (
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Proxy records of northeast Asian precipitation (marine pollen) and northwest Pacific sea surface temperatures (marine fauna) contain concentrations of variance (frequency peaks) coincident with those of orbital variations (eccentricity, obliquity, and precession). Time series of these monitors of various components of the east Asian monsoon system are coherent with changing solar insolation and climate boundary conditions (ice volume, snow cover, albedo) at 41- and 23-kyr frequencies. Winter sea surface temperature (WSST) maxima are in phase with global ice volume minima at all three Milankovitch frequencies (100-, 41-, and 23-kyr periods). WSST maxima are also linearly related to summer insolation at 30°N, lagging radiation maxima by ~55° (~3.5kyr) across the precessional frequency band (23-kyr period). Our east Asian monsoon rainfall indicator (% Cryptomeria) reaches highest values immediately following summer insolation maxima during periods of low ice volume (interglacials). Across the eccentricity frequency band (100-kyr period), % Cryptomeria maxima are in phase with ice volume minima. At the higher frequency of orbital precession, monsoon rainfall lags both summer radiation maxima (~125° ~8kyr) and ice volume minima (~60° ~3.8kyr). Whereas much of the variation in northwest Pacific WSST can be attributed to summer radiation forcing and changes in global climate, mechanisms driving east Asian monsoon precipitation are more complex, involving other (possibly internal) factors.