Late Quaternary history of atmospheric and oceanic circulation in the eastern equatorial Pacific

Marine Micropaleontology, 7 (1982): 163--187

163

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

LATE QUATERNARY HISTORY OF ATMOSPHERIC THE EASTERN EQUATORIAL PACIFIC

AND

OCEANIC

CIRCULATION

IN

KAREN ROMINE

Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881 (U.S.A.) (Revised manuscript received August 26, 1981 ;approved August 27, 1981)

Abstract Romine, K., 1982. Late Quaternary history of atmospheric and oceanic circulation in the eastern equatorial Pacific. Mar. Micropaleontol., 7 : 163--187. F o u r radiolarian assemblages have been defined in recent seafloor sediments of the equatorial Pacific Ocean. The distribution of these assemblages corresponds to the modern pattern of oceanic circulation and water mass structure in this region: the eastern Pacific shallow permanent thermocline and the Equatorial Undercurrent; Peru Current upweUing and the oxygen minimum; the subtropical water mass; warm western tropical water and the North Equatorial Countercurrent. In twelve cores chosen to transect the region both longitudinally and latitudinally, the distribution of these four assemblages has been reconstructed for six time-intervals during the last 127,000 years: 18,000 B.P. (glacial Stage 2); 36,000 B.P. and 52,000 B.P. (interstadial Stage 3); 65,000 B.P. (glacial Stage 4); 82,000 B.P. and 120,000 B.P. (interglacial Stage 5). Atmospheric and oceanic circulation changes through time have been inferred from the reconstructed microfossil assemblage distributions. Changes in assemblage distributions indicate that variations in intensity, direction and mean position of the tradewinds caused marked changes in the oceanic circulation patterns through the last glacial cycle. Near the end of interglacial Stage 5, the disappearance of the North Equatorial Countercurrent from the eastern Pacific suggests that the mean position of the tradewinds was shifted to the south approximately 5° of latitude relative to the modern position, so that the Northeast trades prevented the flow of the North Equatorial Countercurrent into the eastern Pacific. Near the end of interstadial Stage 3, a change in wind direction occurred from predominantly zonal winds, which enhance equatorial divergence and surfacing of the Equatorial Undercurrent, to more meridional winds, which enhance coastal upwelling associated with the Peru Current. In the tropical Pacific Ocean, late Quaternary changes in atmospheric and oceanic circulation are linked with times of continental ice sheet growth in the Northern Hemisphere (i.e., the interglacial-to-glacial transitions across oxygen isotope stage boundaries 5/4 and 3/2). The major changes in circulation seem to occur a few thousand years in advance of the glacial episodes, at or near periods of ice sheet growth. This relationship indicates that changes in atmospheric circulation in the tropics led and influenced the development of conditions suitable for polar and continental ice sheet growth in the Northern Hemisphere.

Introduction Throughout the Pleistocene, fluctuations in t h e a m o u n t o f ice o n c o n t i n e n t s a n d in the polar regions occurred and were intimately a s s o c i a t e d w i t h c h a n g e s in t h e character of both oceanic and atmospheric

circulation (Luz, 1973; Moore, 1973, 1978; CLIMAP, 1976, 1981; Molina-Cruz, 1977a; Ruddiman and McIntyre, 1977; and others). The nature of the relationship between the i c e - a g e f l u c t u a t i o n s a n d c h a n g e s in c i r c u l a tion patterns has proven to be complex. Time-series studies of deep-sea sediment

164

cores have shown that in some regions there is not a simple relationship between the isotopically defined glacial and interglacial episodes and times of great change in atmospheric and oceanic circulation [e.g. in the tropical Pacific, Pisias et al. (1975); MolinaCruz (1977b); Romine and Moore ( 1 9 8 1 ) ] . The CLIMAP program has attempted to explore this relationship globally by utilizing both the time-series and time-slice techniques. By definition interpretation of a time series is restricted in geographic scope and detail, but provides instead a nearly continuous record of events in time as well as a record of the transitions between extreme conditions. In order to study oceanographic patterns associated with an individual event, a set of samples which constitute a "slice" of time is used. In such time-slice investigations, the geographic detail is limited only by the number of suitable samples (cores) available for study. Using microfossil assemblages in deep-sea sediments, CLIMAP members were able to reconstruct oceanographic conditions as they existed 18,000 years ago during the last glacial maximum (CLIMAP, 1976, 1981). This study focuses on the eastern equatorial Pacific from 30°N latitude to 20°S latitude. In this area cold eastern boundary currents meet and become incorporated into a major zonal circulation system. Coastal upwelling and upwelling associated with equatorial divergence make this a region of high productivity which is reflected in the microfossil-rich b o t t o m sediments. In addition, the eastern equatorial region plays an important role in the exchange of heat energy between the ocean and atmosphere. Perturbations in the system of heat input and heat loss in the late Quaternary are related to cycles of glacial and interglacial episodes, and thus the equatorial Pacific is an important and interesting area in which to study the effects of late Quaternary climatic change. The usefulness of microfossils in paleoceanographic research has been established

in many studies associated with the CLIMAP program (e.g. Cline and Hays, 1976). In this work radiolarian microfossils were utilized because of their greater diversity and better overall preservation relative to calcareous microfossils in sediments of this area. Radiolaria in modern sediments have been shown to reflect their life distribution patterns (Casey, 1971b; Petrushevskaya, 1971). Since the life distribution patterns of zooplankton assemblages are determined by the distribution of surface water masses (Bd and Tolderlund, 1971), changes in the distribution of radiolarian assemblages in the sedimentary record can be assumed to reflect changes in the distribution of water masses. Keeping this relationship in mind, radiolarian assemblages in six "time slices" from the last glacial--interglacial cycle (127,000 B.P. to present) were examined. The purpose of this study was to create geographically detailed reconstructions of oceanographic conditions and, in this way, provide a more complete picture of the oceanographic changes which accompany climatic change.

Physical oceanography Major surface currents The North and South Equatorial Currents are the major westward-flowing currents in the area (Fig. 1). Some of the water transported westward by these currents comes from the eastern boundary currents which flow towards the Equator, the California Current in the Northern Hemisphere and the Peru--Chile Current system in the Southern Hemisphere. Upwelling along the coasts of North and South America occurs in association with these eastern boundary flows. Separating the North and South Equatorial Currents, the North Equatorial Countercurrent flows out of the western Pacific bringing warm water into the eastern Pacific. A South Equatorial Countercurrent which also flows eastward has been d o c u m e n t e d by Reid (1959). Very little information on the

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speed and transport of this current is available but the evidence to date suggests that it is a weaker flow than its counterpart, the North Equatorial Countercurrent. The Northeast and Southeast tradewinds drive the major westward-flowing currents. These winds vary seasonally in both latitudinal position and strength (Wyrtki, 1974; Fig. 2). In the Northern Hemisphere winter and spring, the NE trades are closer to the E q u a t o r and centered a b o u t 9°N. They are strong at this time and in a position to oppose the eastward flow of the North Equatorial Countercurrent. Consequently, the Countercurrent is weakened and no longer brings warm water into the eastern Pacific east of approximately 120°W longitude (Wyrtki, 1965, 1974). The North Equatorial Current is also weaker at this time, as the NE trades are shifted southward and do not act on it

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166

(Fig. 1). This current flows along the Equator at 150--250 m depths in the western Pacific, shallowing in the eastern Pacific to less than 50 m depth. Here it is involved in windinduced divergence and upwelling. The Undercurrent also has been observed surfacing both west and east of the Galapagos Islands in circumstances which appear unrelated to wind-induced divergence and upwelling (Pak and Zaneveld, 1973; Stevenson and Taft, 1971). A characteristic feature of eastern Pacific subsurface thermal structure is the permanent shallow thermocline (Fig. 3). It is maintained by upwelling, divergence and the surfacing of the Equatorial Undercurrent in the eastern Pacific. The close relationship between the flow of the Equatorial Undercurrent and the thermocline is shown in Fig. 4. A third feature of importance in this region is the oxygen minimum layer. The subsurface minimum in oxygen is outlined by the less than 1 ml 1-1 contours of Fig. 5. In both Northern and Southern Hemisphere lobes,

fully. Later in the year the NE tradewindbelt is in a more northerly position (approximately 15°N) and the North Equatorial Current is intensified even though the wind stress is weaker. The North Equatorial Countercurrent is no longer opposed by the tradewind stress and is stronger. At this time, the Southern Hemisphere winter and spring (June to December), the SE trades extend north of the Equator and are strong (Wyrtki, 1974; Wyrtki and Meyers, 1976). Under the influence of these winds, the South Equatorial Current and upwelling are strengthened in the eastern Pacific. In the Southern Hemisphere summer the SE trades are weaker and in their southernmost position (approximately 13°S). Circulation in the eastern Pacific at this time is much less intense. Subsurface structure The major subsurface current in the equatorial Pacific is the Equatorial Undercurrent 160 ° 30°4

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long residence time of the water at these depths, the high oxygen consumption within the water column as a consequence of the large production of organic matter in the surface waters of the region (Reid, 1962; Wyrtki, 1967), and the large organic content of sediments at these depths on the nearby margins of Central and South America (Heath et al., 1977; Berger, 1974).

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the " t o p " of the layer reaches to within 50 m of the surface while the actual minimum lies between 300 and 500 m (Wyrtki, 1967). Along the Equator the influence of t~c oxygen-rich water from the Equatorial Undercurrent displaces the oxygen minimum layer so that its upper boundary is as deep as 250 m. The oxygen minimum layer is maintained through the relatively

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Percent abundance data on 45 radiolarian species in 99 surface sediment samples were mathematically analyzed using Q-mode factor analysis (Imbrie and Kipp, 1971). This technique resolves each sample into orthogonal vectors (factors) which can be described by the contribution of each species to each factor. Each factor then is essentially a faunal assemblage. The geographic distribution of each of these faunal assemblages can be determined by contouring the factor values at each sample location. The mapped distributions are then related to the near-surface circulation of the overlying waters.

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Time-slice data set Six time intervals were sampled in twelve cores from the study area (Fig. 6). The radiolarian percent abundance data from these 72 samples were classified in terms of the modern assemblages (Klovan and Imbrie, 1971). For each of these six prerecent levels, the time-slice samples were used to map assemblages which were described in terms of their modern analogs. These distributions can then be compared to the modern ones and, because of the relationship between assemblage distributions and modern water-mass distributions, the comparison allows resolution of past changes in surface oceanic circulation patterns.

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correlative in the tropical region (Arrhenius, 1952; Hays et al., 1969; Berger, 1970; Broecker, 1971). Similarly, the 5lsO record of the Quaternary reflects glacial-to-interglacial fluctuations in global ice volume (Shackleton and O p d y k e , 1973) and leads changes in carbonate concentration b y approximately 6,000 yr (Ninkovich and Shackleton, 1975; Moore et al., 1977). Thus, the approximate synchroneity of calcium carbonate maxima and minima with maxima and minima in global ice volume (~lsO record) makes these records useful and complementary tools in stratigraphic correlations (Thompson and Saito, 1974; Shackleton and Opdyke, 1976). The correlation of the twelve cores was done using an adapted version of the graphic correlation technique of Shaw (1964). This technique utilizes the total range of fossils to develop stratigraphic control and was originally used for correlation of sedimentary rock sequences. In this study, correlative intervals in 51sO or CaCO3 records are used instead of fossil ranges. The records of CaCO~ or 5~sO in all the cores are compared to a single reference core with a well-defined CaCO3 and ~ ~sO record. The records are correlated visually and depth ranges around

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c o r e s w i t h ~ 180 r e c o r d s correlated with V19-29 5 2 , 0 0 0 B.P.; D = 6 5 , 0 0 0 a n d o n l y t w o time-slice

170

TABLE 1 Scaled v a r i m a x f a c t o r scores Species

Tropical

Peru C u r r e n t Upwelling

Subtropical

Spongurus sp. Spongurus elliptica Actinomma leptodermum Hexacontium enthacanthum. Hexacontium laevigatum } Stylatractus sp. Axoprunum stauraxonium Ommatartus tetrathalamus Polysolenia flammabunda Polysolenia lappacea Polysolenia murrayana Siphonosphaera polysiphonia Heliodiscus asteriscus Larcopyle butschlii Larcospira quadrangula Lithelius minor Pylospira octopyle Stylochlamidium asteriseus Porodiscus sp. A Porodiscus sp. B Stylodictya validispina Spongaster tetras Dictyocoryne profunda Dictyocoryne truncatum Euchitonia triangulum Euchitonia elegans Euchitonia furcata • Hymenastrum euclidis Tetrapyle octacantha Octopyle stenozoa } Giraffospyris angulata Phormospyris stabilis scaphiphes Carpocanium sp. Anthocyrtidium ophirense Lamprocyrtis nigriniae Lamprocyclas maritalis maritalis Lamprocyclas m. maritalis ventricosa Lamprocyelas m. maritalis polypora J Pterocorys minythorax Pterocorys zancleus Theocorythium trachelium Botryostrobus auritus/australis g r o u p Botryostrobus aquilonaris Dictyophimus crisiae Dictyophimus hirundo g r o u p } Pterocanium sp. Pteroeanium praetextum eueolpum Pterocanium praetextum praetextumJ Theocalyptra bicornis } Theoealyptra bicornis vat. Theocalyptra davisiana Eucyrtidium hexagonatum

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---0.06 0.49 --0.77 -0.02 -0.08 4.94 0.05 -0.65 0.12 O. 01 1.41 0.24 0.26 -0.08 0.13 0.29 -0.56 --0.20 -0.06 --0.03

5.18 0.44 2.75 --0.04 0.16 1.16 O. 11 1.19 0.05 O. 12 1.23 0.19 0.17 0.55 -0.26 0.06 0.86 0.31 -0.02 0.16

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each correlative point are plotted on a graph of depth-in-core of the reference section versus depth-in-core of the compared record (e.g. V19-29 vs. RC10-65 in Fig. 7, from Romine and Moore, 1981). Line segments are drawn to best fit the point correlations. The resulting graph is useful for checking the "goodness" of the correlation as well as providing graphic resolution of differences in sedimentation rates. When correlating the time,slice cores, V19-29 was used as the reference core. The results of the correlation are shown in Fig. 8. Modern radiolarian assemblage distributions The first four factors (assemblages) account for 93.5% of the variance in the data. They form coherent, mappable patterns which can be related to modern oceanographic conditions in the equatorial Pacific (Romine and Moore, 1981). The contribution of individual species to each factor is shown in Table I by the "score" of the species for each factor. Tropical assemblage (Factor 1 -- 54.1% o f the variance)

Tropical mixed-layer water of western Pacific origin (including North and South Equatorial Countercurrents) is indicated by the distribution of the first factor (Fig. 9a). The distribution of this assemblage along the Equator is associated with the change in the position of the thermocline in an east--west direction. The thermocline shallows to the east as divergence increases and the Equatorial Undercurrent comes closer to the surface (Fig. 4; Wyrtki, 1965). East of approximately 120°W, divergence and upwelling brings cooler (Undercurrent) waters to the surface. Hence, the presence of tropical radiolarian species diminishes and is replaced by another assemblage, the Equatorial assemblage.

Equatorial assemblage (Factor 4 -- 10.2% o f the variance)

The distribution of the Equatorial assemblage (Fig. 9d) seems to describe rather closely the limits of the permanent shallow thermocline typical of the eastern Pacific (Wyrtki, 1966, 1967, 1975; Fig. 3). This oceanographic feature is associated with the surfacing of the Equatorial Undercurrent, as well as divergence and upwelling along the Equator. Cyclonic circulation and upwelling also associated with the Undercurrent are responsible for the shallow thermocline in the region north of the Equator (Wyrtki, 1964, 1965, 1966). Peru Current Upwelling/Oxygen Minimum assemblages (Factor 2 -- 24.0% o f the variance)

This assemblage closely follows the subsurface pattern formed by the oxygen minimum layer. The highest factor values, however, are associated with the productive and economically important upwelling region off Peru in the flow of the Peru Current (Fig. 9b). Upwelling associated with this factor involves cold, nutrient-rich and oxygenpoor water; equatorial upwelling as described above involves cool, relatively oxygen-rich water from the Equatorial Undercurrent. Subtropical assemblage (Factor 3 -- 5.2% o f the variance)

The Subtropical assemblage accounts for a much smaller proportion of the variance than the previously described assemblages. The distribution of this assemblage, however, does show a pattern which describes approximately the low-latitude limits of the Subtropical water masses of both the Northern and Southern Hemispheres (Fig. 9c). Subtropical water is relatively high in salinity but variable in temperature.

173

Time-slice views of the last 127,000 years Previous work

During the last 127,000 years, climate passed through a complete cycle from interglacial through glacial to interglacial once again. This cycle was accompanied by changes in atmospheric circulation which were responsible for concurrent changes in the intensity and geographic extent of the oceanic circulation system (CLIMAP, 1976, 1981). In order to adequately examine the history of change from fully interglacial conditions through glacial conditions leading back to present interglacial conditions, both timeseries and time-slice data from radiolarian assemblages of the past 127,000 years are useful. Molina-Cruz (1977a) studied the paleoceanography of the subtropical southeastern Pacific using downcore time-series data of radiolarian assemblages spanning the last 127,000 years. He related changes in the assemblage compositions (downcore) to variations in climate and associated atmospheric and oceanic circulation changes. Romine and Moore (1981) presented a history of oceanographic and atmospheric circulation in the eastern equatorial Pacific Ocean based on two time-series of changes in the distribution of radiolarian assemblages in the past 127,000 years. (See Fig. 6 for location of two cores used in detailed time-series study.) The history of oceanic and atmospheric circulation developed from the time-series shows large changes in the factors which are interpreted as changes in flow patterns and water-mass boundaries. These oceanographic changes, in turn, are thought to be associated with changes in the tradewind system. The timing of a presumed change in meridionality of atmospheric circulation and of intensified winds has been previously described by Molina-Cruz {1977a, b) in a study of paleoceanography in the southeastern subtropical Pacific Ocean. The timeseries study reviewed here corroborates the timing of the atmospheric circulation changes

suggested by Molina-Cruz. Further, movements in the boundaries of specific equatorial water masses have been interpreted in the time-series study as shifts in the mean position of the NE and SE tradewind maxima. The timing of these shifts in position, direcATMOSPHERIC AND OCEANIC CIRCULATION HISTORY MODERN POSITION

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Fig. 10. Summary of atmospheric and oceanic circulation history of the eastern equatorial Pacific. The solid line indicates position of the tradewinds relative to the modern position, i.e. 5 ° to the north or south of the modern position (not latitude). Shaded region indicates times of increased wind intensity (times of increased quartz accumulation in V19-29). Arrows indicate relative dominance of zonal (more horizontal arrows) or meridional (more vertical arrows) co m p o n en t of the wind stress. Significant changes in surface oceanic circulation are indicated to the right of the diagram. Oxygen isotope stages are indicated on the right side of the diagram.

174

tion and strength of the trades is presumed to be related to the global climatic changes which occurred in the last 127,000 years; but studies to date (Molina-Cruz, 1977a, b; Romine and Moore, 1981) have shown no direct relationship between the time-series results and the times of maxima or minima in global ice volume. Instead, the inferred changes in atmospheric circulation tended to precede glacial episodes and may be more directly linked to insolation changes. A review of the oceanic and atmospheric circulation history of the eastern equatorial Pacific follows. Time-series review

From the beginning of the interglacial oxygen isotope Stage 5 (127,000 B.P.)until approximately 85,000 B.P., more Peru Current upwelling, strong surfacing of the Equatorial Undercurrent east of the Galapagos Islands and the presence of subtropical waters near the Equator suggest that the mean position of the trades was north of the present mean position (Romine and Moore, 1981; fig. 10). At 85,000 B.P. factor values of the radiolarian assemblages resemble the modern values in both of the time-series cores (V19-29 and RC10-65), indicating that the mean position of the tradewinds was moving southward. By 80,000 B.P. the southward shift of the tradewinds ceased at a position to the south of the present mean position. High values of quartz in the downcore record of quartz abundance (Molina-Cruz, 1977a; Fig. 1 1 ) i n d i c a t e an increase in wind intensity in glacial Stage 4 (73,000--61,000 B.P.), but no detectable oceanographic response appeared until the end of this glacial period. Gradual strengthening of Peru Current upwelling and the North Equatorial Countercurrent at the Equator suggests that the mean position of the tradewinds was shifting northward. For 65,000 years, this trend continued slowly northward, then briefly accelerated to a maximum northern position similar to that of Stage 5 at the Stage 2/1 boundary (11,000 B.P.)

and thereafter moved southward to the modern position. Wind intensity was low from 61,000 B.P. to about 45,000 B.P., when it increased again (Fig. 11). This intensified atmospheric circulation coincided with increases in both tropical mixed-layer flow of the North Equatorial Countercurrent and Peru Current upwelling at the Equator. The intensification of both atmospheric and oceanic circulation is concomitant with the more gradual build-up of glacial conditions leading into Stage 2. At about 35,000 B.P., surfacing of the Equatorial Undercurrent diminished permanently. This change is attributed to a change in the meridionality of the winds from more parallel to the Equa-

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175 tor (i.e. more zonal) before 35,000 B.P. to more perpendicular to the Equator (i.e. more meridional) after that time. More meridional winds after 35,000 B.P. are compatible with the increased coastal upwellhlg (e.g. Peru Current) and Countercurrent flow observed. In fact, a maximum in Peru Current upweUing occurs at 18,000 B_P. in the westernmost core of the two used in the time,series study, indicating that the westward extent of coastal upweUing was greatest at this time of maximum glacial conditions (as defined by the oxygen isotope record). After 18,000 B.P. the intensity of atmospheric circulation lessened and Peru Current upweUing no longer reached as far west. However, Peru Current upwelling remained a dominant influence in the eastern Pacific until approximately 3,000 B_P. when tropical influence of the North Equatorial Countercurrent became dominant. At the end of glacial Stage 2, subtropical water neared the Equator along the South American coast as in Stage 5, indicating a northern extreme in the mean position of the tradewind maxima; this was followed in Stage 1 (the present interglacial) by a slight southward shift and the establishment of modern atmospheric and oceanic circulation patterns.

Time-slice results The use of two cores in the study of Romine and Moore (1981) provides a detailed history at two critical locations, but does not provide a complete picture of changes in circulation patterns. Once key times in the past are defined, however, a history of change of geographic patterns can be developed by looking at a suite of samples representing these key times. The twelve cores selected for this purpose include the two used in the time,series study and are arranged in longitudinal and latitudinal transects of the equatorial current system (Fig. 6). Six time slices were picked from the last 127,000 years, so that expanded views of equatorial Pacific oceanography as it existed at each time level might be obtained. Two of the times were chosen because they represent

times of extremes in climatic conditions: 18,000 B2., the last maximum in glacial conditions; and 120,000 B.P., the last maximum in interglacial conditions which was most similar to today. A time slice at 82,000 B2. was chosen because it is at the end of interglacial Stage 5 and represents the beginning of an interval of rapid build-up of the continental glaciers in the Northern Hemisphere (Ruddiman et al., 1980), and just follows the major changes in the equatorial circulation patterns noted by Romine and Moore (1981). Stage 4 is a time of increased ice volume on continents following the build-up late in Stage 5. Although Stage 4 is glacial in character, it is not as intense a glacial episode as Stage 2, and appears to be oceanographicaUy quite different from Stage 2 (Romine and Moore, 1981). Therefore, a sampling level at 65,000 B2. was chosen in glacial Stage 4 so that the two glacial episodes may be compared. The two remaining time slices were chosen from early and late Stage 3 at 52,000 B_P. and 36,000 B.P. Stage 3 is not considered to be either a full interglacial or glacial interval because of the intermediate nature of isotopic values. The time,series studies (Molina-Cruz, 1977a, b; Romine and Moore, 1981) found that Stage 3 was a time of oceanographic and atmospheric transition separating two quite different glacial intervals (Stages 4 and 2). Because oceanic and atmospheric circulation were very different in each glacial, two time slices in Stage 3 were chosen in order to study more closely the oceanographic patterns of the equatorial Pacific at intervals marking the recovery from glacial Stage 4 (52,000 B.P.) and during the return to glacial conditions which began near the end of Stage 3 (36,000 B.P.). Altogether, the six time slices cover the complete range of extremes in climatic and oceanographic conditions during the last 127,000 years.

120,000 B.P. time slice (early interglacial Stage 5) The area delineated by the 0.8 contours on the map of the Tropical assemblage distribution (Fig. 12a) is essentially the same

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as that of the modern distribution. East of 110°W and in particular in the southeast, the 0.6 contour of factor values is displaced approximately 15 ° to the west and 3--4 ° to the north relative to modern configuration. This displacement appears to be related to a change in the dominance of one of the remaining three factors which are most important east of 120°W. The distribution of the Equatorial Undercurrent assemblage (Fig. 12d) was similar to that in modern times; however, much higher values (greater than 0.6) occur along the Equator west of 100°W longitude. This indicates a greater dominance of the Equatorial Undercurrent assemblage at this time, which may be responsible for the displacement of the Tropical assemblage. The assemblage which seems most different in its geographic position relative to the present is the Subtropical factor (Fig. 12c), whose 0.2 contour is shown approximately 5 ° north of today's position. The Peru Current Upwelling/Oxygen Minimum assemblage (Fig. 12b) had the same general configuration as that of the present with a slightly more northern influence along the South American coast. Based on the record of quartz abundance, wind intensity was low during the Stage 5 interglacial episode (Fig. 11). At 120,000 B.P., the distribution of the Equatorial Undercurrent assemblage indicates that the Undercurrent surfaced more strongly than it does today, displacing the bounds of western tropical mixed layer water to the west along the Equator and inhibiting intrusion of the North Equatorial Countercurrent east of the Galapagos Islands. The occurrence of the Subtropical assemblage and Peru Current Upwelling assemblage north of their modern locations implies that the Subtropical water mass (specifically in the Southern Hemisphere} and the locus of Peru Current upwelling were several degrees of latitude north of their modern positions. From this evidence it is inferred that the SE tradewind maximum was approximately 5° north of its modern position.

82,000 B.P. time slice (late interglacial Stage 5) Along the Equator west of approximately 120°W longitude, the Tropical assemblage (Fig. 13a) remained dominant as it was during early Stage 5 and is in the present. In the east, however, displacement of the Tropical assemblage is matched by the increased westward expansion of the Equatorial Undercurrent assemblage as indicated by its larger area of dominance (greater than 0.6 factor values, Fig. 13d). North of the Equator, the 0.4 contour of the Equatorial Undercurrent assemblage extended approximately 10 ° of longitude further west than it does today. The Subtropical assemblage is not significantly changed in its distribution (Fig. 13c). Compared to the 120,000 B.P. distribution there was a southward shift of 5 ° in latitude in the distribution of the Peru Current Upwelling assemblage (Fig. 13b). The southward shift of this assemblage at this time corroborates the time-series evidence of a southward shift in the position of the tradewind maxima between approximately 90,000 and 80,000 years ago. The hypothesized southward movement of the SE tradewind maximum, if matched by the NE trades, would place the NE trades in opposition to the North Equatorial Countercurrent causing the decrease in Countercurrent flow into the eastern Pacific which may have allowed increased surfacing of the Equatorial Undercurrent. The observed displacement of the Tropical assemblage by the Equatorial Undercurrent assemblage suggests that this was the case.

65,000 B.P. time slice (glacial Stage 4) In glacial Stage 4 wind intensity was greatly increased (indicated by high quartz abundance, Fig. 11}; and in response, the Equatorial assemblage expanded even more to the west (Fig. 14d}, further displacing the Tropical assemblage west along and north of the Equator. The northern branch of the Tropical assemblage {associated with the North Equatorial Countercurrent) is definitely cut off from the region east of 100°W longitude (Fig. 14a). The northern

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52,000 B.P. time slice (early interstadial Stage 3) Along the Equator the 0.6 and 0.8 contour lines of the Tropical assemblage (Fig. 15a) shifted even further westward (approximately 10 ° of longitude). The displacement of this assemblage in this way still appears related to the expansion of the Equatorial Undercurrent assemblage (Fig. 15d), which reached maximum influence with highest concentration extending as far west as 110°W longitude. This relationship suggests a westward shift in the thermal boundary between the warm waters of the tropical mixed-layer and the cooler waters associated with surfacing of the Equatorial Undercurrent. The Northern Hemisphere area occupied by the Equatorial Undercurrent assemblage seems to be unchanged. The Peru Current Upwelling assemblage (Fig. 15b) shifted northward along the coast. Simultaneously, the southern extent of the region bounded by the 0.4 contour line of this assemblage moved northward. Based on the actual factor values of the southernmost core, a northward shift of the 0.2 contour of the Subtropical assemblage (Fig. 15c} also occurred at this time (remaining still south of the present position). The apparent magnitude of the shift is only 2 ° to 3 ° in latitude. The Subtropical and Peru Current Upwelling assemblage distributions indicate the early stages of a probable northward progression of the tradewind maxima from the extreme southern position noted in Stage 4. The NE trades still opposed the flow of the North Equatorial Countercurrent and allowed maximal surfacing of the Equatorial Undercurrent. Tradewind intensity at this time was low as in Stage 5 (based on low quartz abundance, Fig. 11).

36,000 B.P. time slice (late interstadial Stage 3) By approximately 40,000 B.P., increased quartz abundance (Fig. 11) indicates that the intensity of the tradewind regime had again increased. Major changes in the assemblage distributions can be observed in this

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time slice which emphasize the subtle balances and interactions of the assemblages. The changes in Tropical and Equatorial Undercurrent assemblages (Fig. 16a, d) are a good example. In the east (east of ll0°W), the Tropical assemblage shows increasing influence (cf. position of 0.6 and 0.4 contour lines relative to that of early Stage 3, Figs. 15a, 16a). Along the Equator the area of high factor values in the Equatorial Undercurrent assemblage distribution drastically decreased (Fig. 16d). In the west, however, the 0.4 contour of the Equatorial assemblage extended as far west as 140°W, matching the recession of the 0.8 contour of the Tropical assemblage along the Equator to 150°W (Figs. 16a, d). A shift to the south of the Equator in the 0.4 and 0.6 contours of the Tropical assemblage (Fig. 16a) is also evident in the late Stage 3 reconstruction. During Stage 4, these contours were symmetrical about the Equator. The reduction of the influence of Peru Current upweUing along the Equator, which occurred in Stage 4, was the condition for this symmetry. These distribution patterns indicate increased flow and influence of North Equatorial Countercurrent in the extreme eastern equatorial Pacific with concomitantly decreased surfacing of the Equatorial Undercurrent west of 100°W. The increase in Peru Current upwelling (Fig. 16b) to the north of the Equator (along the South American coast) concurrent with the changes in surface flow adds support to the interpretation that tradewind maxima were shifting northward. The NE trades exerted less and less influence on Equatorial Countercurrent flow and allowed reassertion of its influence in the east. Increased wind intensity seems not to strongly affect the assemblages except by enlarging the area of Peru Current upweUing to the south. The Subtropical water mass (cf. Subtropical assemblage, Fig. 16c) showed no equatorward shift, probably because it was suppressed by this expanded area of upwelling. The decrease in the surfacing of the Equatorial Undercurrent at 36,000 B.P. is enigmatic. A theory which may contribute to

an explanation of this change involves changing the meridionality of atmospheric circulation. Tradewinds blow at an angle to the Equator and the nearer this angle is to perpendicular, the larger the meridional component of air flow. A larger meridional component of the trades results in more coastal upweUing (in the case of the SE trades, Peru Current UpweUing); and a weaker meridional flow (more parallel to the Equator) should result in greater Equatorial divergence and/or surfacing of the Equatorial Undercurrent (Molina-Cruz, 1977b). The time-slice data prior to 36,000 B.P. shows much stronger influence of the Equatorial Undercurrent assemblage associated with surfacing of the Equatorial Undercurrent and at 36,000 B.P. shows drastically reduced influence. This observed change in influence of Equatorial Undercurrent flow was confirmed through the interval of the 18,000 B.P. time slice into m o d e m times (Romine and Moore, 1981). Therefore, it is proposed that prior to approximately 36,000 B.P. the flow of the tradewinds was generally more parallel to Equator (i.e. less meridional) resulting in stronger surfacing of the Equatorial Undercurrent.

18,000 B.P. time slice (glacial Stage 2) This time slice represents the time of the maximum in global ice volume in the last 127,000 year period. At this time, the Equatorial Undercurrent assemblage diminished almost to the modern configuration with factor values of 0.6 or better in only one core (Figs. 9d, 17d). Concurrently, the Tropical assemblage (Fig. 17a) reappeared in greater strength in the east and north of the Equator. These assemblage distributions imply reduced influence of the Equatorial Undercurrent at the same time the North Equatorial Countercurrent reappeared in the eastern Pacific. Judging from the reappearance in the east of the Equatorial Countercurrent, the tradewind regime must have moved to a position similar to the present in which the NE trades only seasonally oppose the flow of the Countercurrent. The Subtropical assemblage (Fig. 17c)

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185 showed no movement; b u t again, the Peru Current Upwelling assemblage (Fig. 17b) had expanded somewhat southward (perhaps due to the increased wind intensity and the presumed change in meridionality of the SE trades). The Peru Current Upwelling assemblage was also more strongly expressed along the coast with factor values greater than 0.6 appearing further north than at any time previously. Higher factor values as far west as l l 0 ° W indicate that the Upwelling assemblage extended further west along the E q u a t o r than at any other time. The decreased influence of the Equatorial Undercurrent and the signs of increased upwelling along the South American coast argue for stronger meridionality of the trades (and in particular the SE trades). Simulation of the tropical climate of the 18,000 B.P. CLIMAP reconstruction (CLIMAP, 1976) has been done by Manabe and Hahn (1977). Their study predicts a strong meridional c o m p o n e n t in tropical atmospheric circulation during 18,000 B.P., supporting the interpretations in this study. However, in their simulation meridionality was greater 18,000 years ago relative to the present. Based on the reconstructions presented here, surfacing of the Equatorial Undercurrent t o d a y is diminished and Peru Current upwelling is stronger relative to times prior to 36,000 B 2 . ; thus, the meridional c o m p o n e n t of the tradewinds must be more important now than it was before 36,000 B.P. and only slightly less important than at 18,000 B.P. Summary The scenarios described for each time slice are consistent with the summarized timeseries history of atmospheric circulation from Romine and Moore (1981). Changes in intensity, direction and mean position of the tradewinds are inferred from changes in surface circulation indicated by temporal variations in the distribution of radiolarian assemblages. Wind intensity (as deduced from quartz abundance) was generally strongest during times of increased ice on continents,

between 61,000 and 73,000 B.P. and between approximately 45,000 and 16,000 B.P. A permanent and major change in wind direction seems to have occurred at approximately 36,000 B.P. when winds became more meridional than zonal. This change was reflected b y increased coastal upwelling and a decreased influence of the Equatorial Undercurrent. The oceanic circulation patterns based on the assemblage patterns presented here indicate that the mean latitudinal position of the tradewinds varied throughout the last 127,000 years. During most of interglacial Stage 5, the mean position of the trades was shifted north b y a b o u t 5 ° of latitude. This location was reflected in the position of the Subtropical water mass which was approximately 5 ° north of today's position. By 65,000 B.P., however, the Subtropical water mass had shifted approximately 5 ° south of today's position. This shift, in addition to the diminished influence of the North Equatorial Countercurrent in the eastern Pacific, suggests that the mean position of the trades had also shifted 5 ° to the south. The timing of the inferred changes in latitudinal position and direction of the tradewinds coincides with t w o times of growth in continental ice sheets in the Northern Hemisphere which resulted in glacial episodes, i.e. oxygen isotope Stages 4 and 2 (Shackleton and Opdyke, 1973; Ruddiman et al., 1980). The southward shift in mean latitudinal position of the trades which occurred in latest Stage 5 was probably a result of much decreased summer insolation at all latitudes at this time, analogous to the seasonal change observed as winter begins in the Northern Hemisphere. Ruddiman and McIntyre (1981) propose that times of decreased summer insolation which coincide with times of increased winter insolation at mid to high latitudes are optimal for growth of ice in higher latitudes. This situation maximizes ice retention on continents in summer while minimizing the growth of sea ice in winter so as to maintain a large surface area of open ocean from which moisture may be extracted to feed

186

the ice sheets. When the NE tradewinds were in their southernmost position, the warm water normally flowing eastward as the North Equatorial Countercurrent may have been diverted northward along the western side of the North Pacific Ocean, acting as a moisture source for high latitude ice sheets. Once the ice sheets were initiated, continued growth was insured by increased albedo due to the snow and ice accumulations. As the albedo increased, the insolation gradient from Equator to pole steepened. This steepening gradient was ultimately responsible for the increased meridionality of the tradewind system (at 36,000 B.P.) and probably for increased wind strength as well. Studies of modern global ocean---atmosphere interaction indicate that the air--sea system of the tropical regions exerts a strong influence on atmospheric and oceanic conditions in higher latitudes (e.g., Bjerknes, 1966; Rowntree, 1972). The results of this study indicate that, at least during the last 127,000 years, changes in atmospheric and oceanic circulation, which are indicated by changes in the distribution of radiolarian assemblages of the low latitude Pacific, were influential in modifying the ocean--atmosphere system of heat transport so that ice accumulation occurred in high latitudes.

Acknowledgements Thanks go to T.C. Moore, Jr. and W.F. Ruddiman for critical review of the manuscript and to Delores Smith for typing it. The research was supported by a grant from the National Science Foundation, funded jointly by the International Decade of Ocean Exploration and The Climate Dynamics Research Section to the CLIMAP project at the University of Rhode Island (OCE 78-26881).

References Arrhenius, G., 1952. Sediment cores from the east Pacific. Rep. Swed. Deep-Sea Exped., 1947-48, 5 : 1--228. B~, A.W.H. and Tolderlund, D.S., 1971. Distribution and ecology of living planktonic Foraminifera in surface waters of the Atlantic and Indian

Oceans. In: B.M. Funnel and W.R. Riedel (Editors), The Micropaleontology of Oceans. Cambridge University Press, London, pp. 105--149. Berger, W.H., 1970. Selective solution and the lysocline. Mar. Geol., 8: 111--138. Berger, W.H., 1974. Deep-sea sedimentation. In: C.A. Burk and C.L. Drake (Editors), The Geology of Continental Margins. Springer-Verlag, New York, N.Y., pp. 213--241. Bjerknes, J., 1966. A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus, 18: 820--829. Broecker, W.S., 1971. Calcite accumulation rates and glacial to interglacial changes in oceanic mixing. In: K.K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale University Press, New Haven, Conn., pp. 239--265. Casey, R.E., 1971b. Radiolaria as indicators of past and present water masses. In: B.M. Funnel and W.R. Riedel (Editors), The Micropaleontology of Oceans. Cambridge University Press, London, pp. 331--342. CLIMAP,' 1976. The surface of the Ice-Age earth. Science, 191: 1131--1136. CLIMAP, 1981. Seasonal reconstructions of the earth's surface at the last glacial maximum. Palaeogeogr., Palaeoclimatol., Palaeoecol., vol. 35. Cline, R.M. and Hays, J.D. (Editors), 1976. Investigations of Late Quaternary Paleoceanography and Paleoclimatology. Geol. Soc. Am. Mere., 145. EASTROPAC Atlas, 1972. C.M. Love (Editor), Volume 1, Washington, D.C. Hays, J.D., Saito, T., Opdyke, N.D. and Burckle, L.H., 1969. Pliocene--Pleistocene sediments of the equatorial Pacific: Their paleomagnetic, biostratigraphic and climatic record. Geol. Soc. Am. Bull., 80 : 1481--1514. Heath, G.R., Moore, T.C., Jr., and Dauphin, J.P., 1977. Organic carbon in deep-sea sediments. In: N.R. Anderson and A. Malahoff (Editors), The Fate of Fossil Fuel CO 2 in the Oceans. Plenum Press, New York, N.Y., p. 605. Imbrie, J. and Kipp, N.G., 1971. A new micropaleontological method of quantitative paleoclimatology: application to a late Pleistocene Caribbean core. In: K.K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale University Press, New Haven, Conn., pp. 71--181. Klovan, J.E. and Imbrie, J., 1971. An algorithm and FORTRAN-IV program for large-scale Q-mode factor analysis and calculation of factor scores. Math. Geol., 3(1): 61--77. Knauss, J.A., 1966. Further measurements and observations on the Cromwell Current. J. Mar. Res., 24 : 205--240. Kominz, M.A., Heath, G.R., Ku, T.-L. and Pisias, N.G., 1979. Brunhes time scales and the interpretation of climatic change. Earth Planet. Sci. Lett., 45: 394--410.

187

Luz, B., 1973. Stratigraphic and paleoclimate analysis of late Pleistocene tropical southeast Pacific cores. Quaternary Res., 3(1 ): 56--72. Manabe, S. and Hahn, D.G., 1977. Simulation of the tropical climate of an Ice Age. J. Geophys. Res., 82: 3889. Molina-Cruz, A., 1977a. Late Quaternary Oceanic Circulation Along the Pacific Coast of South America. Dissertation, Oregon State University, Corvallis, Ore., 246 pp. Molina-Cruz, A., 1977b. The relation of the southern tradewinds to upwelling processes during the last 75,000 years. Quaternary Res., 8: 324--338. Moore, T.C., Jr., 1973. Late Pleistocene--Holocene oceanographic changes in the northeastern Pacific. Quaternary Res., 3 : 99--109. Moore, T.C., Jr., 1978. The distribution of radiolarian assemblages in the modern and ice-age Pacific. Mar. Micropaleontol., 3: 229--266. Moore, T.C., Jr., Pisias, N.G. and Heath, G.R., 1977. Climate changes and lags in Pacific carbonate preservation, sea surface temperature and global ice volume. In: N.R. Andersen and A. Malahoff (Editors), The Fate of Fossil Fuel CO2 in the Oceans. Plenum Press, New York, N.Y., pp. 145-165. Ninkovich, D. and Shackleton, N.J., 1975. Distribution, stratigraphic position and age of ash layer " L " , in the Panama Basin region. Earth Planet. Sci. Lett., 27: 20--34. Pak, H. and Zaneveld, J.R., 1973. The Cromwell Current on the east side of the Galapagos Islands. J. Geophys. Res., 78(33): 7845--7859. Petrushevskaya, M.G., 1971. Spumellarian and Nassellarian Radiolaria in the plankton and bottom sediments of the Central Pacific. In: B.M. Funnel and W.R. Riedel (Editors), The Micropaleontology of Oceans. Cambridge University Press, London, pp. 309--318. Pisias, N., Heath, G.R. and Moore, T.C., Jr., 1975. Lag times for oceanic responses to climatic change. Nature, 256: 716--717. Reid, J.L., Jr., 1959. Evidence of a South Equatorial Countercurrent in the Pacific Ocean. Nature, 184: 209--210. Reid, J.L., Jr., 1962. On circulation, phosphatephosphorus content and zoo-plankton volumes in the upper part of the Pacific Ocean. Limnol. Oceanogr., 7(3): 287--306. Robinson, M.K. and Bauer, R.A., 1976. Atlas of North Pacific Ocean monthly mean temperatures and mean salinities of the surface layer. Naval Oceanographic Office Res. Publ. 2, Washington, D.C. Romine, K. and Moore, T.C., Jr., 1981. Radiolarian assemblage distribution and paleoceanography of the eastern equatorial Pacific Ocean during the last 127,000 years. Palaeogeogr., Palaeoclimatol., Palaeoecol., 35 : 281--314.

Rowntree, P.R., 1972. The influence of tropical east Pacific Ocean temperature on the atmosphere. Q. J. R. Meteorol. Soc., 98: 290--321. Ruddiman, W.F. and McIntyre, A., 1977. Late Quaternary surface ocean kinematics and climatic change in the high-latitude North Atlantic. J. Geophys. Res., 82: 3877--3887. Ruddiman, W.F. and McIntyre, A., 1981. Oceanic mechanisms for amplification of the 23,000year ice-volume signal. Science, 212: 617--627. Ruddiman, W.F., McIntyre, A., Niebler-Hunt, V. and Durazzi, J.T., 1980. Oceanic evidence for the mechanism of rapid Northern Hemisphere glaciation. Quaternary Res., 13: 33--64. Shackleton, N.J. and Opdyke, N.D., 1973. Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: Oxygen isotope temperatures and ice volumes on a l 0 s and 106 year scale. Quaternary Res., 3: 39--55. Shackleton, N.J. and Opdyke, N.D., 1976. Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239, late Pliocene to latest Pleistocene. In: R.M. Cline and J.D. Hays (Editors), Investigation of Late Quaternary Paleoceanography and Paleoclimatology. Geol. Soc. Am. Mere., 145: 449--464. Shaw, A.B., 1964. Time in Stratigraphy. McGrawHill, New York, N.Y., 365 pp. Stevenson, M.R. and Taft, B.A., 1971. New evidence of the Equatorial Undercurrent east of the Galapagos Islands. J. Mar. Res., 29: 103--115. Thompson, P.R. and Saito, T., 1974. Pacific Pleistocene sediments: Planktonic Foraminifera dissolution cycles and geochronology. Geology, 2(7): 333--335. Tsuchiya, M., 1968. Upper Waters of the Intertropical Pacific Ocean. John Hopkins Oceanographic Studies No. 4. The Johns Hopkins Press, Baltimore, Md., 50 pp. Wyrtki, K., 1964. Upwelling in the Costa Rica Dome. Fish. Bull., 63(2): 355--372. Wyrtki, K., 1965. Surface currents of the eastern tropical Pacific Ocean. Inter-Amer. Trop. Tuna Comm. Bull., 9(5): 207--304. Wyrtki, K., 1966. Oceanography of the eastern equatorial Pacific Ocean. Oceanogr. Mar. Biol. Annu. Rev., 4: 33--68. Wyrtki, K., 1967. Circulation and water masses in the eastern equatorial Pacific Ocean. Int. J. Oceanol. Limnol., 1: 117--147. Wyrtki, K., 1974. Equatorial currents in the Pacific 1950--1970 and their relations to the trade winds. J. Phys. Oceanogr., 4: 372. Wyrtki, K., 1975. Fluctuations of the dynamic topography in the Pacific Ocean. J. Phys. Oceanogr., 5(3): 450--459. Wyrtki, K. and Meyers, G., 1976. The trade wind field over the Pacific Ocean. J. Appl. Meteorol., 15(7): 698.