Late-glacial to holocene changes in winds, upwelling, and seasonal production of the northern California current system

QUATERNARY

RESEARCH

38, 359-370 (1992)

Late-Glacial to Holocene Changes in Winds, Upwelling, and Seasonal Production of the Northern California Current System CONSTANCE SANCETTA,MITCHELLLYLE,ANDLINDAHEUSSER Lamont-Doherty

Geological Observatory, Palisades, New York 10964

RAINERZAHN GEOMAR,

Wischhofstrasse l-3, Gebiiude 4, D-2300 Kie114, Germany

AND

J. PLATTBRADBURY U.S. Geological Survey MS 919, Box 25046, Federal Center, Denver, Colorado 80225 Received February 26, 1991 A core 120 km off the coast of southern Oregon was examined for changes in lithology, diatoms, and pollen over the past 30,000 yr. Primary production during the late Pleistocene was about half that of the Holocene. Evidence from diatoms and pollen indicates that summer upwelling was much weaker, implying an absence of strong northerly winds. Early Pliocene diatoms found throughout the late Pleistocene section were probably derived from diatomites east of the Cascades and provide evidence for strong easterly winds over a dry continental interior. The findings verify predictions of a climate model based on glacial maximum conditions. There is no compelling evidence for a climatic reversal corresponding to the European Younger Dryas chron. During the early Holocene (9000-7000 yr B.P.) there may have been years when winds were insufftciently strong to support upwelling, so that warm stratified waters lay closer to the coast. 0 1992 Univenity of Washington.

INTRODUCTION

Oceanographic conditions near continental margins, especially eastern margins, can be revealing for paleoclimate studies because they monitor the regions of highest marine primary production, they provide evidence for linkages between continental and marine systems, and because they can monitor changes in upwelling timing and intensity and thus reveal information concerning paleowind direction and stress. Often the data produced are difIicult to interpret because of the possibility of sediment displacement and frequently poor preservation of microfossils. Highly variable sedimentation of continental detritus may complicate age models by negating the assumption of constant accumulation rates. In the North Pacific Ocean, calcite is often absent in Holocene sections and forms only a mi-

nor component in sediment of glacial maxima, which makes dating and isotope studies difficult. The Multitracers project was designed to elucidate how the production of biogenic sedimentary components responds to the annual oceanographic cycle, and how the preservation of individual biogenic components has been affected by local sedimentary conditions (Collier et al., 1989; Lyle et al., 1989). Investigators are also studying the burial fluxes of geochemical and microfossil tracers in order to determine how primary productivity off coastal Oregon and northern California has been affected by the waning of the great Pleistocene ice sheets. The project involves collection of sediment-trap time series on a zonal transect off the southern coast of Oregon, analysis of regional patterns in surface samples from box cores, and downcore studies

359 0033-5894192 $5.00 Copyright 0 1992 by the University of Washington. All rights of reproduction in any form reserved.

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of various microfossil and geochemical tracers. Here we apply the results of the sediment-trap studies, together with other regional data, to examine the record of the last 30,000 yr in a core near the base of the continental slope, within the distal influence of plumes and jets from the upwelling system. REGIONAL

SETTING

The eastern margin of the mid-latitude North Pacific, from Washington State to Baja California, is a well-known region of coastal upwelling. The upwelling is driven by northerly winds derived from the eastem limb of the North Pacific high-pressure atmospheric cell, and varies seasonally along the northern part of the coast. South of 35-37”N the winds are northerly throughout the year, but at higher latitudes a seasonal shift occurs, with the Aleutian lowpressure cell dominating during the winter (producing southerly winds), and the North Pacific high dominating during summer (Huyer, 1983; Strub et al., 1987). While upwelling can occur any time that winds are favorable, it is more consistently present off Oregon during late spring and summer. The active upwelling zone lies within 25 km of the coast, but jets and eddies may extend as far as 200 km offshore (Mooers and Robinson, 1984; Ikeda and Emery, 1984). A change in strength or direction of seasonal winds, therefore, might be reflected in the fossil record of upwelling indicators found in deep-sea sediments. During the summer, particles in surface water advected offshore are dominated by cells of phytoplankton, while the intermediate nepheloid layer, which extends off the continental shelf at depths of 100-150 m, is enriched in mineral detritus (Small et al., 1989). During winter, particle distribution and chlorophyll concentrations are more uniform with depth, while silicate minerals remain concentrated in the nepheloid layer. At any season, resuspension events can introduce particles from the sediments into the nepheloid layer. Shelf sediments, there-

ET AL.

fore, can serve as a source for mineral detritus and phytoplankton cells (especially resting spores) transported offshore, in addition to the surface transport of actively growing phytoplankton during productive periods. MATERIALS

AND METHODS

Sediment traps were located at three moorings in a westerly transect (Fig. 1). Several cores were taken at the site of each mooring, including box cores (BC) and piston cores (PC). The cores reported here are W8709A- 13PC and W8709A-9BC, located at the Nearshore site in 2714 m of water (Table 1). Organic carbon, calcite, and opal were analyzed using standard methods of acidification/wet oxidation and NqCO, digestion (Lyle et al., 1988). Stable isotope analyses were carried out on benthic foraminifers Uvigerina senticosa and Cibicidoides wuellerstorj?. The analyses were run on a Finnigan MAT 251 mass spectrometer coupled online to an Autoprep Systems “common acid bath” carbonate preparation device. Sample preparation followed standard procedures (Zahn and Mix, 1991). Each sample contained from one to seven specimens. The al80 and 613C records of both species show reasonably stable interspecific offsets of 0.74 -+-0.06%0,(1 u) and 0.64 f 0.07%~ respectively, as calculated from 11 paired measurements. A A 6i80 of 0.74%0is relatively high compared to the widely accepted value of 0.64%0(Shackleton and Opdyke, 1973). However, higher than “normal” values of 0.6%0 have also been reported from the coastal upwelling area off northwest Africa (Zahn-Knoll, 1986), which may point to differential isotopic fractionation in response to varying productivity and foraminiferal growth rates. Diatoms were examined in smear slides, which best preserve a sense of the original proportions of different components. Each slide was examined at 1250~ ; planktonic diatoms were identified to the species level

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FIG. 1. Map showing locations of sediment-trap moorings with respect to regional bathymetry (in meters). Stars indicate locations of Nearshore (N), Midway (M), and Gyre (G) sites. Cores W8709A9BC and W8709A-13PC are located at the Nearshore site.

and others (benthic and continental diatoms) to the genus level. The percentages of marine diatoms reported here are based on total counts of marine forms only (i.e., excluding continental forms from the denominator); only when calculating occurrence of continental diatoms was that term included. An estimate of diatom accumulation was made by multiplying each taxon percentage in each sample by the accumulation rate of opal on the assumption that most of the biogenic opal was contributed by diatoms (radiolaria were not seen in the smear slides). Although this approach may underestimate the contribution of large taxa relative to small ones, it is adequate to TABLE

1. LOCATIONS OF SEDIMENT MOORINGS AND CORES

TRAP

Name

Latitude (“NJ

Longitude (“W)

Water depth (ml

Nearshore W8709A-9BC W8709A-13PC Midway Gyre

42”05’ 42%3 ‘ 42%4’ 42”lO’ 4150’

125’45’ 125’29’ 12527’ 127’35’ 132”OO’

2829 3107 2714 2830 3664

show the basic trends, and both more effrcient and less subject to preparation error than alternative procedures. Pollen samples were processed following our usual techniques: about 3 g of sediment, with a known quantity of exotic pollen added, was suspended successively in 0.001 M Na4P,0,, HCl, and HF, followed by actetolysis and safranin staining. Percentages of pollen are based on the sum of 300 terrestrial pollen grains. Concentration of pollen grains per gram dry weight sediment was determined from the ratio of exotic and fossil pollen grains. RESULTS

The sediment in W8709A-13PC is dominated throughout by lithogenic silty clay, with varying percentages of diatoms and calcite (Fig. 2). Calcite percentages vary between 1 and 9%, with a broad maximum between 200 and 300 cm. Oxygen isotope analysis of benthic foraminifera indicates that the upper 5.5 m of core span the Holocene and last glacial maximum. The top of the piston core overpenetrated and the sediment section is estimated to begin at

362

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ET AL. CaCO3(%)

b '80

5 mSilty

diatom

5.20

4.20

3.20

clay

q

Calcareous

diatom

/ZEJ

Celcareous

clay

HSilty

IO

(%e)

clay

clay

FIG. 2. Stratigraphy and age control for W870!9A-13PC, plotted against depth in core. Calcite concentration below 400 cm is estimated, based upon examination of smear slides. Numbers in lower part of oxygen-isotope plot represent AMS r4C yr B.P.

4600 yr. The age model for this core was initially constrained by benthic foraminiferal oxygen isotope stratigraphy. Three AMS 14Cdates for bulk samples of planktonic foraminifera were used to constrain the glacial time scale, while three tie points to a regional calcite stratigraphy (R. Karlin, M. Lyle, and R. Zahn, unpublished data) constrain the Holocene section. Sedimentation rate variations were kept to the minimum necessary to match the ages of the tie points. This age model indicates that the lowest sample (440 cm) corresponds to an age of 23,500 yr B.P., with sedimentation rates being slightly higher during the late Pleistocene interval (24 vs 21 cm/kyr). The age model is supported by the implied timing of pollen events which have been independently dated elsewhere (maximum of alder (Afnus)pollen at 10,008 yr; lack of latest Holocene patterns of coastal redwood (Sequoia) and oak (Quercus); Gardner et al., 1988). The maximum accumulation of bulk organic carbon occurred during late-glacial time (16,00&12,000 yr B.P.), which might be interpreted as indicating higher production during this period (Fig. 3A). However, analysis of lipid geochemistry indicates that approximately half of the carbon in this in-

terval is of terrigenous origin versus about 20% in the Holocene (M. Lyle et al., unpublished data). In the same interval, accumulation of Al, a good tracer of rock detritus, is twice as high as during the Holocene. The lithology in this section is overwhelmingly dominated by silty clay, with very few microfossils, and pollen concentration is at A

B

WAR Corg(mg/cm*/103yr) 110

130

150

170

190

-0.2

d'3C

(%.)

-0.6

-1.0

-1.4

20 t

FIG. 3. Calculated accumulation of organic carbon and carbon isotopes for W8709A-13PC, plotted against age. (A) Mass accumulation of total organic carbon. (B) Carbon isotope values for Uvigerina (black) and Cibicidoides (white).

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(Pinus),

Oak(%)

Redwood(%) 8

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as well as subalpine taxa such as mountain hemlock (Tsuga mertensiana) and herbs (Gramineae, Compositae, Chenopodiaceae, and Cyperaceae) (Fig. 4). In this region, the ratio of western hemlock (7’. heterophyllu) to spruce (Piceu sitchensis), which has been used as a general indicator of coastal temperatures (Heusser and Shackleton, 1979), suggests that the interval 22,000-16,000 yr B.P., corresponding to the early part of the Fraser Glaciation, was the coldest achieved. The hemlock/spruce ratio indicates that prior to 22,000 yr B.P., during the Olympia nonglacial interval, the climate was slightly warmer. Grasses and herbs reached a maximum about 14,000 yr B.P., corresponding to the time of the maximum extent of the Cordilleran Ice Sheet during the Vashon Stade. Samples deposited during deglaciation (14,000-10,000 yr

a maximum. The increase in bulk accumulation rate evidently is a function of greater influx of terrigenous material. Rates of marine organic carbon accumulation in the late Pleistocene interval are about half those of the Holocene, indicating that average annual production may have been similarly reduced. The lack of Cibicidoides during most of the Holocene interval makes it difficult to estimate possible changes in carbon flux by comparing 613C values of Uvigerina and Cibicidoides. However, the single such value in the earliest Holocene (9000 yr B.P.) shows a marked change in offset relative to Uvigerina (Fig. 3B). This implies higher organic carbon flux at this level relative to that of the underlying glacial interval. Glacial pollen assemblages are characterized by maximum representation of pine 4

CURRENT

Alder(%)

12

o"

Pine(%) 0

2040

6080100

Mountain 0

Hemlock(%) 1

2

3

W. Hemlock/Spruce 4

0

1

2

Herbs

(%I

3

~~~~~~

FIG. 4. Concentrations and ratios of pollen types in W8709A-13PC, plotted against age. HerrdocW spruce ratio is based upon western hemlock only. Note different scales on axes.

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ET AL.

B.P.) contain evidence of the diagnostic expansion of alder, reflecting first-stage succession upon newly deglaciated soils, followed by the initial expansion of temperate coastal forest components. This sequence agrees closely with regional vegetational and climatic reconstructions previously described from terrestrial and marine records (Florer-Heusser, 1975; Heusser, 1977, 1983). Coastal redwood and oak pollen are essentially absent from the glacial section, and show a two-step increase during deglaciation, the first occurring about 13,000 yr B.P. and the second, full expansion after 9000 yr B.P. Today, coastal redwood is restricted to northern California; its distribution is associated with availability of moisture produced by coastal fogs (Azevedo and Morgan, 1974). Oak, which is best developed in northern California today, extends north in warm, relatively dry environments on the western side of the Oregon central valley (within the rain shadow of the coastal range). The absence of these taxa during the glacial interval therefore implies a climate cooler than present and lacking the summer coastal fogs produced by upwelling. Diatoms are common to abundant in the upper 100cm and very rare below about 130 cm (10,ooOyr B.P.). This trend reflects that of diatom accumulation rate for the dominant species (Fig. 5), which is a factor of three higher in the Holocene. The dominant taxa throughout the record are ThalasChdetocefos

o” 5 z k

200

400

sionema nitzschioides and spores of the genus Chaetoceros (including C. debile, C. radicans, and C. vanheurckii). These taxa

are also common in the sediment trap material where they show distinct seasonal variations (C. Sancetta, unpublished data). Chaetoceros spores are found only in traps of the nearshore and midway sites (very minor at the latter), with highest fluxes and greatest relative abundance during the summer and fall seasons(July-November). The taxon is a characteristic component of the flora during late-stage upwelling in many coastal regions, including the California Current system (Guillard and K&am, 1977; Garrison, 1979; Anderson et al., 1987). We conclude that its accumulation in the sediments is a reflection of the relative intensity, frequency, or duration of summer upwelling conditions. The significance of T. nitzschioides is less clear. In the sediment trap material, it consistently shows highest fluxes and maximum relative abundance during the early spring (March-April) at all three sites, even at the Gyre site which lies outside the region affected by coastal upwelling (C. Sancetta, unpublished data). It thus appears to be a reliable indicator of the spring increase in production, but the combination of conditions supporting the increase is uncertain. Classically, the spring bloom is related to increasing stabilization of the water column and solar irradiance (Riley, 1942), but in this area the switch from net southerly

7: nit’zschioides 600

Benthics

P doliola

Pliocene

0

20

40

60

80

t

I

f

;

'

0

5

10 15 20

25

.i

10

kn m. 15 al 20 7 25 =I

30

FIG. 5. Accumulation of diatom taxa in W8709A-13PC, plotted against age. Units represent the proportion of biogenic silica, in mg/cm2/103 yr, accounted for by the diatom taxa.

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(winter) to northerly (summer) winds may play an important part (Thomas and Strub, 1989). We conclude that the accumulation of this taxon reflects spring-season production, but cannot infer the physical mechanisms responsible. The much lower accumulation rates of both taxa prior to 10,000 yr B.P. indicate a marked reduction in annual production, relative to that of the Holocene. Accumulation of Chaetoceros spp. during the glaciation was about one-third that of the Holocene, implying a 60% reduction in frequency or intensity of summer upwelling. Since the upwelling is driven today by northerly winds, this in turn implies a weakening of such winds during the glacial interval. The accumulation of T. nitzschioides was similarly low during most of the late Pleistocene. However, between 23,000 and 18,000 yr B.P. the accumulation of T. nitzschioides increased markedly, implying a period of relatively greater spring production. This may reflect the occurrence of northerly winds during early spring, but, as noted above, the mechanisms supporting spring production in the region are imperfectly understood today, so that this conclusion remains tentative. Throughout the latest Pleistocene (30,00&l 1,000 yr B.P.) there is a significant component of displaced benthic diatoms, varying from 5 to 20% of the total marine diatom assemblage. This group includes taxa diagnostic of coarse sediments from waters less than 50 m deep (Delphineis, Rhaphoneis, Cocconeis); the increased accumulation of this group implies an increase in lateral advection of sediment from the shelf, in agreement with the evidence for greater fluxes of terrigenous rock detritus and organic matter during this time period (M. Lyle et al., unpublished data). We suggest that the high flux of benthic diatoms, and perhaps partly also that of terrigenous sediment is a result of increased erosion of the continental shelf exposed during the low stand of sea level. Also notable is the occurrence of lacus-

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trine taxa, including Melosira (Aulacoseira) solida, and several species of Stephanodiscus and Cyclotella (C. antiqua, C. elgeri, S. carconensis, and S. oregonicus). These forms are present in low numbers throughout the Pleistocene section, with small maxima around 11,000 and 18,000 yr B.P. Two of these species (C. elgeri and S. CUYconensis) are extinct forms, occurring only in Pliocene lacustrine deposits of the western interior (Fig. 6). The others are found across a wider geographic and stratigraphic range. One of us (J.P.B.) has examined more than 100 such deposits and has found all of these taxa in the Yonna Formation of south-central Oregon (Newcomb, 1958; Pickthorn and Sherrod, 1990; Bradbury, in press) and the Glenns Ferry Formation of southwest Idaho (Bradbury, 1982; Krebs et al., 1987). In particular, C. elgeri is abundant in both of these formations but has not been found elsewhere. We therefore conclude that although several localities are of the same general age and taxonomic composition, the presence of C. elgeri in our

FIG. 6. Map showing localities of Pliocene lacustrine diatoms discussed in text. Stars indicate late Quaternary sites in which displaced lacustrine taxa have been observed: NS, Nearshore core W8709A13PC; BG, Battle Ground Lake; H, Humptulips bog. Letters without stars indicate lacustrine deposits that contain some or ah of the taxa discussed (J. P. Bradbury, unpublished data): D, Deschutes Formation; FRV, deposits in Fort Rock Valley; Y, Yonna Formation; GF, Glenns Ferry Formation; A, younger part of Alturas Formation; LB, deposits at Lake Britton.

366

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samples strongly favors the Yonna and Glenns Ferry formations as the most probable sources. The presence of Pliocene lacustrine fossils in marine sediments 120 km offshore might be explained by either fluvial or atmospheric transport. If fluvial, the only feasible mechanism is the combined Snake River/Columbia River system, which drains regions in which these Pliocene deposits occur (Fig. 6). The low-salinity plume of the Columbia outflow can be detected as far south as the Nearshore site today (Anderson, 1964), such that if outflow were at least as great as that of the present, the plume could serve as the source. In addition, there is strong evidence for repeated massive flooding of the Columbia River system when ice dams blocking Glacial Lake Missoula failed catastrophically during glacial times (Bretz, 1923; Waitt, 1985). Such floods could have eroded the Pliocene deposits and supplied a large enough volume of water to transport particles as far as the Nearshore site. A problem with this hypothesis is that one would expect to find common Quaternary freshwater taxa as well as eroded Pliocene forms in the resultant sediments, but this is not the case. A majority of the specimens we found are uniquely Pliocene and although two of the taxa (C. antiqua and S. oregonicus) do occur in Quaternary lakes, they have also been found in Pliocene deposits and therefore are not diagnostic of age. Eolian transport is also a feasible mechanism, in view of the dry nature of the interior during the late-glacial interval (Barnosky et al., 1987). That eolian dust containing diatoms can travel for hundreds of miles at an altitude sufficient to cross mountain ranges has been demonstrated by the occurrence of African-derived dust as far north as southern France (Styve and Fourtanier, 1985). Furthermore, the same redeposited Pliocene diatom taxa have been identified from two other late-glacial localities for which eolian transport is the only possible mechanism (J. P. Bradbury,

ET

AL.

unpublished data). One is the Humptulips peat bog on the Olympic Peninsula (Fig. 6), which contains a silt layer dated by radiocarbon and by pollen correlation to a nearby site to between 24,000 and 10,000 yr B.P. (Heusser and Heusser, 1990). The Pliocene diatoms are restricted to the silt layer (J. P. Bradbury, unpublished data), and are associated with a minimum in westem hemlock and a maximum in herbs and grasses representing full-glacial conditions. The silt has not been studied in detail, but its general appearance is consistent with that of loess (C. J. Heusser, personal communication, 1991), lending powerful support to the inference of eolian transport. Even if the silt is not loess, eolian transport is the most reasonable mechanism, for the Columbia River has no direct access to the area, and there are no known local Pliocene lacustrine deposits. Even more persuasive evidence is the occurrence of the same diatom taxa in Battle Ground Lake (Fig. 6), which occupies a small maar basin. Only one sample was examined for diatoms, so that the full range of occurrence is unknown (J. P. Bradbury, unpublished data). This sample occurs in a disturbed interval of silty clay having a radiocarbon age of between 15,700 and 14,800 yr B.P. (Bamosky, 1985). Considering the location of the site-a crater with a rim 72 m high located in Pleistocene volcanic rocks 155 m above sea level-the diatoms could only have been transported to the site by wind. Although fluvial transport to the marine Nearshore site cannot be conclusively eliminated, we strongly favor an eolian mechanism. This requires frequent occurrence of strong easterly winds between 25,000 and 11,000 yr B.P. The Holocene section is characterized by a rapid increase in accumulation of Chaetocet-oSspores about 10,000 yr B.P., with a more gradual increase upward. Influx of displaced benthic diatoms decreases markedly, and the Pliocene taxa disappear. Pseudoeunotia doliola is restricted to the

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Holocene section, with maximum values between 10,000 and 7000 yr B.P. This is a pelagic species rarely found in the coastal zone and most commonly reported from tropical and subtropical waters of the central ocean. In the Multitracer series (C. Sancetta, unpublished data), it is characteristic of the summer (June-August) samples, with greatest relative abundance at the Gyre site (17%) and decreasing shoreward (7% at Midway and 3% at Nearshore). We infer that it represents warm, highly stratified waters with low nutrient availability and production largely based upon regenerated nutrients. Accumulation of this taxon is quite low, compared with that of Chaetoceros spores, but the trend is nonetheless striking and suggests that during some years winds may have been too weak to cause upwelling, or at least to carry upwelled waters as far as the site. DISCUSSION

The last glacial maximum, the period of deglaciation, the earliest Holocene, and later Holocene climates have all left distinctive records in the sediments at this site. Our data indicate that on land the glacial period from 22,000 to 14,000 yr B.P. was colder and drier than today. Terrigenous inorganic and organic sediment fluxes were much higher than present, which may have been partly a result of fluvial transport of glacially eroded debris, but also from the greater erosion and transport of coastal sediments. Off the coast, marine productivity was considerably lower during the late Pleistocene than it is today, with weaker coastal upwelling in summer. During the time roughly corresponding to the maximum advance of the Laurentide Ice Sheet (ca. 23,OOO-18,000yr B.P.) conditions favored high rates of production during early spring. In addition, there is evidence for strong easterly winds from the continental interior throughout the late Pleistocene. Our observations support predictions from an atmospheric model that suggests that the presence of the Laurentide and

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Cordilleran ice sheets produced a strong high-pressure atmospheric cell over northern North America during the glacial maximum (Kutzbach and Guetter, 1986). The model predicts strong easterly surface winds during the winter and very weak westerlies during the summer in the region of our site. In contrast, the modern climate is characterized by winter westerlies and summer winds from the northwest (parallel to the coast). The prediction of weak summer winds is supported by our evidence from pollen and diatom taxa indicating weak summer coastal upwelling, as well as the evidence of marine organic carbon which implies relatively low rates of annual production, The prediction of easterly winds is verified by the occurrence of the displaced Pliocene lacustrine diatoms at our site and in two other localities which could only have been affected by wind transport from the east. Our evidence does not permit us to determine the seasonof the easterlies, nor does it require constant strong winds. Occasional outbursts of cold air from the ice sheet are adequate to explain the data, and the model indicates that conditions in winter would favor such outbreaks. We emphasize that the easterly wind referred to is surface wind; the model of Kutzbach and Guetter (1986) predicts that winds aloft (12 km altitude) would be westerly throughout the year. Although this model was run only to predict conditions at 3000-yr intervals since 18,000 yr, our evidence indicates that easterly winds must have occurred frequently throughout the late Pleistocene. Sediments representing the deglacial and early Holocene intervals contain evidence for major climatic shifts in the region. Evidence of atmospheric warming combined with summer fogs first appears about 13,000 yr B.P. Studies from continental sites indicate that summer atmospheric temperatures increased throughout the Pacific Northwest at this time (Heusser et al., 1985; Barnosky et al., 1987). We suggest that the appearance of redwood and oak at

368

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this time is primarily an effect of atmospheric warming, rather than of surfacewater cooling, for there is no corresponding increase in summer-upwelling diatoms. The high sedimentation rate (>20 cm/ 1000 yr) provides an opportunity to examine in detail the record of the deglacial period. Vegetation inferred from marine pollen shows a unidirectional decline of pinedominated conifer forest with subalpine hemlock, and an exponential rise of alder and redwood beginning about 14,000 yr B.P. No significant reversal is seen in mountain hemlock during the interval between 11,000 and 10,000 yr B.P., an interval equivalent to the Younger Dryas chronozone of Europe. Although some lateglacial records of vegetation on the northwest coast of Washington State show small maxima in the relative abundance of mountain hemlock, implying colder conditions near sea level (Heusser and Heusser, 1990), the age of these events is not well constrained (C. J. Heusser, personal communication, 1991), nor is a similar event in coastal British Columbia (Hebda,. 1983). The diatom data also do not show a temporary return to glacial-age conditions. By 9000 yr B.P. summer upwelling activity was well developed. However, there may have been some years when coastal winds were weaker than they are today, so that nutrient-rich coastal waters were not transported as far as the site (or possibly were shut off completely). As a result, there was a greater extent onshore of warm and stratified waters, at a time when summers were warmer and drier in northwestem North America (Heusser et al., 1985; Bamosky et al., 1987). Summer insolation was at a maximum during this interval (Berger, 1978). Simulation of conditions at this time by the atmospheric model does not predict wind patterns different from those of the present (Kutzbach and Guetter, 1986). However, the simulation was based upon modem sea-surface temperature values, which would tend to reproduce modem atmospheric circulation. We are

ET AL.

uncertain about the interpretation of these data, but believe that the trend is distinctive enough that it should be noted. The above conclusions may not be applicable for the entire California Current system. Although our evidence, and the predictions of the atmospheric model (Kutzbath and Guetter, 1986), imply weakened summer upwelling along the entire west coast of North America, conflicting evidence exists in sediment located along the outer continental shelf of California (Anderson et al., 1987). Sapropelic layers at these sites contain laminae which appear to reflect seasonal alternations between high upwelling (inferred to be summer) and high runoff (inferred as winter) conditions. The sapropels, which are discontinuous in various cores, appear to occur in both early Holocene and late Pleistocene intervals, but precise ages have not been determined. This evidence appears to be incompatible with the findings reported in this paper. Note that our site is located 120 km offshore, whereas the sapropelic sediments are much closer to land. We infer a weakening of upwelling, but not a complete cessation. Upwelling may still have been sufficiently strong to leave a distinctive signal in the coastal zone. Alternatively, the atmospheric model may be incorrect in this regard, and northerly winds may have occurred south of 40°N (i.e., off California) as they do today (Huyer, 1983). The greatest uncertainty of this study remains the significance of the increase in accumulation of T. nitzschioides during the glacial maximum. J.fwe accept the 3-yr record of sediment-trap data as a reliable indicator of long-term trends, we may conclude that spring conditions at this time were similar to those of today, with relatively high rates of primary production, while prior and subsequent to the interval spring production was much lower. If the spring bloom in the region is light dependent (Riley, 1942; Small et al., 1972), this would imply either stabilization of surface waters (reflecting a weakening of winds) or high

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it-radiance at the sea surface (possibly resulting from decreased cloud cover). If the spring bloom is dependent on increased nutrient supply (Thomas and Strub, 1989), this would indicate a spring period of winds favorable for mixing, followed by stabilization. In the latter case, coastal upwelling would be driven by northerly winds, while vertical mixing could result from any strong winds, regardless of direction. It seems probable, therefore, that the distinctive pattern of this diatom is related to a seasonal shift in the winds, but further work on the conditions controlling the spring bloom today will be necessary to determine the interplay of mechanisms responsible. ACKNOWLEDGMENTS We thank Larry Small for information concerning phytoplankton production and ecology, and Elaine Stock for preparation of the sediment for pollen analysis. Donn Gorsline, Leigh Welling, and an anonymous reviewer made several helpful comments which have greatly improved the paper. This research was supported by NSF Grants OCE89-19956 and OCE9000945 to M. Lyle, and OCE88-10962 to C. Sancetta. Core material was provided by the Core Repository of Oregon State University, supported by NSF Grant OCE88-00458. AMS i4C dates were kindly provided by Nicklas Pisias, supported by NSF Grant OCE8600936.

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