Late Quaternary Upwelling Variations in the Eastern Equatorial Atlantic Ocean as Inferred from Dinoflagellate Cysts, Planktonic Foraminifera, and Organic Carbon Content

Quaternary Research 54, 58 – 67 (2000) doi:10.1006/qres.2000.2139, available online at http://www.idealibrary.com on

Late Quaternary Upwelling Variations in the Eastern Equatorial Atlantic Ocean as Inferred from Dinoflagellate Cysts, Planktonic Foraminifera, and Organic Carbon Content Christine Ho¨ll Historische Geologie/Pala¨ontologie, Universita¨t Bremen, Fachbereich-5 Geowissenschaften, Postfach 330 440, 28334 Bremen, Germany E-mail: [email protected]

and Sylvia Kemle-von Mu¨cke Meeresgeologie, Universita¨t Bremen, Fachbereich-5 Geowissenschaften, Postfach 330 440, 28334 Bremen, Germany Received February 18, 1999

In this paper we employ different paleoenvironmental indicators to make a detailed reconstruction of the equatorial divergence. Some of the indicators are relatively new, while others are well accepted. Our combined data are based on planktonic foraminifera, dinoflagellate cysts, and total organic carbon content (Ho¨ll et al., 1998, 1999; Kemle-von Mu¨cke, 1994; Wefer et al., 1996). Statistical transfer-function sea surface temperatures of the cold season are considered to reflect the sea surface temperatures of the upwelling season in the present study. A large difference in stable isotope composition between Globorotalia crassaformis and Globigerinoides ruber (pink) and high relative abundance of Neogloboquadrina dutertrei provide indicators of enhanced upwelling intensity. As indicators for higher paleoproductivity, we use high weight percentages of total organic carbon (TOC), high values of the ratio between cysts of peridinoid and oceanic organic-walled dinoflagellate cyst species (p/o ratio), and low accumulation rates of calcareous dinoflagellates.

Analysis of multiple proxies shows that eastern equatorial Atlantic upwelling was subdued during isotope stage 5.5, more intense during stages 4, 5.2, 5.4, and 6, and most intense early in stage 2. These findings are based on proxy measures from a core site about 600 km southwest of Liberia. The proxies include total organic carbon content, the ratio of peridinoid and oceanic organic-walled dinoflagellate cyst species, accumulation rates of calcareous dinoflagellates, estimates of sea surface paleotemperatures, the difference in stable oxygen isotope composition between two species of planktonic foraminifera that live at different water depths, and the abundance of the planktonic foraminifera Neogloboquadrina dutertrei. Most of these parameters consistently vary directly or inversely with one another. Slight discrepancies between the individual parameters show the usefulness of a multiple proxy approach to reconstruct paleoenvironments. Our data confirm that northern summer insolation strongly influences upwelling in the eastern equatorial Atlantic Ocean. © 2000 University of Washington. Key Words: dinoflagellates; planktonic foraminifera; stable oxygen isotopes; eastern equatorial Atlantic upwelling; late Quaternary.

MODERN WINDS AND CURRENTS

The modern spatial and temporal extent of the equatorial upwelling in the eastern equatorial Atlantic Ocean is controlled by seasonal variations of the trade wind system. During the boreal summer, the SE trade winds are strongest and the Intertropical Convergence Zone (ITCZ) reaches its northernmost position (Philander and Pacanowsi, 1986). During this time the wind-driven South Equatorial Current (SEC; Fig. 1) reaches an annual maximum in velocity. Furthermore, the greater westward wind stress in the tropical Atlantic Ocean leads to a tilting of the thermocline, which rises in the eastern equatorial Atlantic Ocean and sinks in the west (Hastenrath and Merle, 1987). Because the thermocline depth and the nitracline

INTRODUCTION

The tropical regions of the oceans are important parts of the global climatic system. Because of intensive insolation, these regions form important heat reservoirs. The tropical Atlantic Ocean is the connection in heat transport from the southern to the northern hemisphere (Peterson and Stramma, 1991). This heat flow is strongly influenced by climatic variations. The complex oceanographic structure of the equatorial divergence, where cooler, nutrient-enriched water masses ascend, is directly influenced by the seasonally varying intensity of SE and NE trade winds (McIntyre et al., 1989). 0033-5894/00 $35.00 Copyright © 2000 by the University of Washington. All rights of reproduction in any form reserved.

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are assumed to be still comparable. The stratigraphy is based on the correlation of the ␦ 18O record of G. ruber (pink) (Meinecke, 1992) with the SPECMAP oxygen isotope curve (Imbrie et al., 1984). All the samples come from material believed to date from the last 145,000 yr B.P. Data used in this study are available in digital form at the data bank PANGAEA at the Alfred Wegner Institute, Bremerhaven (http://www.pangaea.de; E-mail: [email protected]/ info.pangaea.de). METHODS FIG. 1. Eastern equatorial Atlantic Ocean, showing positions of cores and schematic summary of oceanic currents (after Peterson and Stramma, 1991): NECC, North Equatorial Countercurrent; SEC, South Equatorial Current; SECC, South Equatorial Countercurrent; EUC, Equatorial Undercurrent; GC, Guinea Current; AC, Angola Current; BCC, Benguela Coastal Current; BOC, Benguela Oceanic Current; (Black arrows, surface currents; white arrows, undercurrent). Scale is valid only at the equator.

depth are almost identical in the eastern equatorial Atlantic Ocean (Herbland and Voituriez, 1978), the upwelling of cooler subthermocline waters leads to nutrient transport into the surface water, where productivity increases and sea surface temperatures (SST) fall. During the boreal winter, when the SE trades are less intensive, the westward wind stress is weaker, and as a result the thermocline deepens in the eastern equatorial Atlantic Ocean, leading to diminished upwelling and thus to less productivity and higher SST. The equatorial Atlantic Ocean is also influenced by the SW–NE monsoon system over tropical Africa (Prell and Kutzbach, 1987). Seasonal variation of northern hemisphere insolation forces the monsoon intensity. Strong summer heating produces a low-pressure cell over northern Africa, which strengthens the flow of air from ocean to land. This onshore flow diminishes the zonal component of the SE trades and thus weakens equatorial upwelling. CORE SAMPLES

Most of our samples come from core GeoB 1105-4 (01°39.95⬘S, 12°25.7⬘W; water depth, 3225 m; Fig. 1). Because the top of gravity core GeoB 1105-4 was missing, we also used box core GeoB 1105-3 from the same site. The sediments mainly consist of fine-grained foraminiferal ooze with varying amounts of nannofossils and little bioturbation (Wefer et al., 1989). Samples studied for calcareous and organic-walled dinoflagellate cysts were taken only from gravity core GeoB 1105-4. These samples were taken every 10 cm, whereas the sediment material used for investigations on planktonic foraminifera and TOC was sampled every 5 cm. Although samples were collected at different intervals, the trends in the distribution patterns of the different parameters

Calcareous Dinoflagellates Preparation methods used for calcareious dinoflagellates are described in detail in Ho¨ll et al. (1998). Among the calcareous dinoflagellates, we distinguished between the coccoid, vegetative stage of Thoracosphaera heimii and the calcareous resting cysts (Sphaerodinella albatrosiana, Sphaerodinella tuberosa, Calciodinellum operosum, Orthopithonella granifera, and Rhabdothorax spp.). For convenience, we use the term “calcareous dinoflagellates” for both the vegetative stage of T. heimii and the calcareous resting cysts. The individual species of the calcareous resting cysts can be treated as one group because the individual species have fairly similar distribution patterns throughout the investigated time interval (Ho¨ll et al., 1998). Accumulation rates of calcareous dinoflagellates have been calculated as calcDino accumulation rate (individuals per cm 2/ 1000 yr B.P.) ⫽ calcDinos/g * DBD * SR (1), where calcDinos/g is the number of calcareous dinoflagellates per gram of dried sediment, DBD is the dry bulk density in g/cm 3, and SR is the average sedimentation rate of core GeoB 1105-4 for the last 145,000 yr B.P. Accumulation rates that are based on linear interpolation between oxygen isotope control points may be affected by errors inherent in the SPECMAP standard isotope ␦ 18O stack itself (⫹/⫺2500 yr B.P.; Martinson et al., 1987) as well as by artifacts in age modeling (Lyle, 1988). Therefore, mean sedimentation rates of core GeoB 1105-4 for the last 145,000 yr B.P. have been used to estimate accumulation rates independent of sediment compaction during burial. Data for DBD and SR are given in Meinecke (1992). Based on statistical analyses, Ho¨ll et al. (1998, 1999) inferred that preservation and redeposition have little effect on the accumulation rates of calcareous dinoflagellates in core GeoB 1105-4. Surface sediment investigations have shown higher concentrations of calcareous dinoflagellates in more oligotrophic regions of the South Atlantic Ocean (Vink et al., 2000; Zonneveld et al., in press). These findings, together with the correlation of calcareous dinoflagellates with paleoproductivity in core GeoB 1105-4 (Ho¨ll et al., 1998, 1999), suggest that accumulation rates of calcareous dinoflagellates vary inversely with paleoproductivity.

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The methods used for preparation of organic-walled dinoflagellate cysts are given in Ho¨ll et al. (1998). For convenience, we use “organic cysts” instead of “organic-walled dinoflagellate cysts.” High relative amounts of the cysts of peridinoid dinoflagellates are commonly associated with high nutrient contents in upwelling areas, coastal areas, arctic ice margins, and fronts between water masses, while Impagidinium species (such as Impagidinium species which cannot be identified on the species level, I. aculeatum, I. paradoxum, I. patulum, I. sphaericum, I. strialatum) and Nematosphaeropsis labyrinthus are characteristic of oligotrophic oceanic areas (Wall et al., 1977; Lewis et al., 1990; Aksu et al., 1992; Edwards and Anderle, 1992). The ratio between peridinoid cysts and the oceanic cysts (p/o ratio) can therefore be used as an indicator for more eutrophic or oligotrophic conditions. Higher values of p/o ratio refer to enhanced nutrient concentration in the surface waters. The ratio is calculated as p/o ⫽ nP/共nP ⫹ nO兲,

(1)

where n is the number of specimens counted, P is the number of cysts of peridinoid dinoflagellates (Brigantedinium spp. also including cysts with a reticulate to granulate wall, Multispinula quanta, Selenopemphix nephroides, Protoperidinium cf. thorianum), and O is the number of oceanic cysts (Nematosphaeropsis labyrinthus and Impagidinium spp. such as Impagidinium species which cannot be identified on the species level, I. aculeatum, I. paradoxum, I. patulum, I. sphaericum, I. strialatum). The assemblage of the organic cysts of the investigated core was probably not much affected by selective preservation or by redeposition (Ho¨ll et al., 1998). We therefore assume that variations in the p/o ratio reflect changes in the composition of the organic cyst assemblage, and that higher p/o ratios reflect enhanced upwelling intensity. Foraminifera For determination of the stable oxygen isotope composition of G. ruber (pink), the size class of 250 –350 ␮m was picked every 5 cm down the core. For G. crassaformis, the size class of 400 –550 ␮m was selected. The isotope ratios were measured with a FINNIGAN MAT 251 mass spectrometer equipped with a Kiel Automated Carbonate Device. The standard error was ⬍0.07‰ for ␦ 18O. These two planktonic foraminifera species calcify at different water depths. Globigerinoides ruber lives in the mixed surface water layer between 25 and 35 m water depth (Hemleben et al., 1989). The maximum of plasma-containing shells was found in the upper 50 m of eastern equatorial Atlantic Ocean plankton net hauls (Kemle-von Mu¨cke, 1994). The oxygen isotope composition of G. ruber indicates a calcifica-

FIG. 2. The ␦ 18O values of G. ruber (pink) and G. crassaformis and the difference between them. Arrows mark large differences. Glacial stages are shaded. The isotope stages and age scale are after Wefer et al. (1996).

tion in surface waters as well (Kemle-von Mu¨cke and Oberha¨nsli, 1999). Globorotalia crassaformis instead prefers water depths between 100 and 300 m, which coincides with the depth of the oxygen minimum zone (Jones, 1967; Kemle-von Mu¨cke, 1994). The ␦ 18O values from plankton samples of the study area show that G. crassaformis precipitates most of the calcite below the seasonal thermocline between 100 and 300 m water depth (Kemle-von Mu¨cke, 1994). Other isotopic data (Fairbanks et al., 1982; Ravelo and Fairbanks, 1992) support these interpretations. According to the linear relationship between the ␦ 18O of the calcite shell and sea water temperature (Epstein et al., 1953), the ␦ 18O-difference between these two species (abbreviated as ⌬␦ 18O crass.-rp.) is a measurement of the temperature gradient between surface waters and waters underneath the thermocline. Comparison of the two ␦ 18O records reveals converging and diverging isotope values, primary modulated by isotope values of G. crassaformis (Fig. 2). Large differences result mainly from high ␦ 18O values of G. crassaformis, and small differences result from significant low ␦ 18O peaks. The glacial to interglacial isotope change in the ␦ 18O record of G. crassaformis is even larger than that of G. ruber, although the temperature change should be larger at the sea surface than at the lower thermocline. We suppose that G. crassaformis records cool temperatures of water masses below the thermocline base during intensive upwelling events and comes into warmer thermocline waters when the thermocline deepens. This scenario would explain the temperature leap of about 3°C shown by G. crassaformis not only between glacial and interglacial periods but also between cold and warm substages of stage 5. It is unlikely that water below the thermocline should have

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varied as much as 3°C and the surface water only about 2°C (shown by the ␦ 18O values of G. ruber). Thus, we infer that a larger ␦ 18O difference reflects a rise of the thermocline. Another explanation for high G. crassaformis ␦ 18O values could be dissolution of shells in glacial intervals and cold stages of interglacial periods. However, we doubt this explanation because G. ruber is more sensitive to dissolution than G. crassaformis. Furthermore, the ␦ 13C values of G. crassaformis do not increase contemporaneously with the ␦ 18O values (Wefer et al., 1996), which would be expected in the case of dissolution processes (Berger, 1967; Bonneau et al., 1980). The frequency of the planktonic foraminifera Neogloboquadrina dutertrei (N. dutertrei %) probably provides additional information about upwelling intensity. Several authors describe its commonness in tropical upwelling areas (Van Leeuwen, 1989; Ravelo et al., 1990). This species mainly secretes its shell in the thermocline, where the deep chlorophyll maximum occurs (Fairbanks et al., 1982). It appears that it prefers areas where the seasonal thermocline is shallow and the chlorophyll maximum is not deeper than 25–50 m (Kemle-von Mu¨cke and Hemleben, 1999). This hydrographic feature is typical of tropical upwelling areas. The relative abundance of this species is therefore assumed to vary directly with upwelling intensity.

(Figs. 3 and 4). High accumulation rates of calcareous dinoflagellates correlate with low values of the p/o ratio. High values of the p/o ratio are recorded at the beginning of isotope stage 2 and during 4 and 6 with further peaks during isotope stages 3 and 5 (between 5.2 and 5.3 and around 5.4). The lowest values are visible in stages 5.5, 5.3, 5.1, and 1. The ⌬␦ 18O crass.-rp. is greatest during glacial stages and during cold substages of interglacial periods, for example, in isotope stages 6, 5.4, 5.2, 4, 3, and 2, though it is less pronounced in stage 2 (Fig. 3). Neogloboquadrina dutertrei shows higher frequencies in isotope stage 6, in stage 5.3, at the end of stages 5 and 4, and in stage 2. Especially low percentages are reached at the transitions from isotope stage 6 to 5, at the beginning of stage 5.2, in stage 5.1, in stage 3 (around 50,000 yr B.P.), and in stage 1 (Fig. 3). SST shows cooler temperatures during glacial and cold substages of interglacial periods (isotope stages 2, 4, 5.2, 5.4, and 6, with a further peak during stage 3 around 42,000 yr B.P.). TOC covaries with SST, showing higher values in glacial and cold substages of interglacial periods. TOC values are particularly high in isotope stage 2 and are lowest during isotope stages 1 and 5.5 (Fig. 3).

Total Organic Carbon (TOC) and Sea Surface Temperature Estimates (SST)

Productivity Proxies

TOC data were taken from Meinecke (1992). TOC in the eastern equatorial Atlantic Ocean can be related to (paleo)productivity in surface waters (Meinecke, 1992; Wefer et al., 1996) and can therefore be used as an indicator of upwelling intensity. Higher TOC values are assumed to reflect higher (paleo)productivity and thus higher upwelling intensity. Meinecke (1992) estimated SST with a transfer-function technique (TF; Imbrie and Kipp, 1971) based on counts of planktonic foraminifera. The calculated SST values for the cold season (TFcold) are used here, because they are assumed to reflect the sea surface temperatures of the upwelling season. These transfer functions are referred to raw data, in percentages, from 777 core top samples from the Atlantic Ocean (Pflaumann et al., 1996). The sea surface temperature at the core site was taken from Levitus (1982). The statistical error of the TFcold is ⫹/⫺1.4°C (Meinecke, 1992). RESULTS

Analyses of the proxies show that the accumulation rates of T. heimii and the calcareous resting cysts vary similarly throughout the past 145,000 yr, with increasing accumulation rates during deglaciation (transition from isotope stage 6 to 5 and from stage 2 to 1), reaching maximum values around isotope stage 5.5 (Fig. 3). Furthermore, calcareous cysts have slightly enhanced values around isotope stages 5.3 and 5.1

DISCUSSION

Because accumulation rates of the calcareous dinoflagellates probably vary inversely with paleoproductivity, one should expect a negative correlation of these accumulation rates with p/o ratio and TOC. Indeed, the maxima of calcareous dinoflagellate accumulation rates occur during absolute minima in TOC, as in isotope stage 5.5 and in isotope stage 1. Lower values in TOC around isotope stage 5.1 also correlate with enhanced values of calcareous dinoflagellates, whereas a correlation in isotope stage 5.3 is less obvious. The same holds for the comparison of the calcareous dinoflagellates with the p/o ratio, for the p/o ratio and the TOC tend to correlate with one another despite somewhat larger fluctuations of the p/o ratio. However, the pronounced maxima of the calcareous dinoflagellates are not reflected to the same degree in the p/o ratio and TOC. Sea Surface Temperature Proxy In the modern eastern equatorial Atlantic Ocean, productivity is enhanced when cooler, deeper, nutrient-enriched waters ascend, leading to a decrease of SST. If this analogue applies to the late Quaternary equatorial Atlantic Ocean, higher SST reflect a decrease in upwelling intensity. Maximum accumulation rates of calcareous dinoflagellates correlate with highest SST (referring to particularly reduced upwelling intensity; Fig. 3). However, based on statistical analyses carried out on data from the same core, sea surface temperature lacks a significant influence on the calcareous dinoflagellates (Ho¨ll et al., 1998).

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FIG. 3. Paleoenvironmental indicators for the last 145,000 yr B.P. in core GeoB 1105-4: p/o ratio (ratio between cysts of peridinoid dinoflagellates and oceanic organic cyst species), accumulation rates of calcareous dinoflagellate cysts and T. heimii, relative abundance of N. dutertrei, difference in stable isotope composition between G. crassaformis and G. ruber (pink) (⌬␦ 18O G. crassaformis–G. ruber p.), sea surface temperatures of the cold season estimated with the transfer function technique, and total organic carbon (TOC) weight percentage. Glacial stages are shaded. The isotope stages and age are after Wefer et al. (1996). For ease of comparison, some of the axes are inverted.

Thermocline Proxy Late Quaternary thermocline depth has also been reconstructed in two recent studies from the tropical Atlantic Ocean. One study has been carried out on core GeoB 1105-4, using planktonic foraminifera abundance to reconstruct paleothermocline depth (Wolff et al., 1999). The result shows a deeper thermocline at the beginning of stage 5, at about 100,000 yr B.P., 80,000 yr B.P., 52,000 yr B.P., 25,000 yr B.P., and 4,000 yr B.P. An adjacent core (GeoB 1117-2; Fig. 1) was examined

for planktonic foraminifera assemblages and coccolithophores (Dittert, 1998). Dittert (1998) inferred a deep thermocline at about 125,000 yr B.P., 100,000 yr B.P., 80,000 yr B.P., 58,000 yr B.P., and stage 1. These results support our hypothesis that a small ⌬␦ 18O crass.-rp. indicates a deep thermocline, and vice versa. In our reconstructions, the thermocline shallows around 118,000 and 90,000 yr B.P. and in stage 4, stage 3, and early stage 2, and a short upwelling period about 2,000 yr B.P. (Fig. 2).

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FIG. 4. Percentages of N. dutertrei and accumulation rates of T. heimii and the calcareous cysts. Gray shading shows correlative minima and maxima.

N. dutertrei Percentage as Upwelling Proxy High frequency of N. dutertrei, which may record enhanced upwelling intensity, correlates with cooler SST, higher values of TOC, and high p/o ratios during isotope stages 2, 3, 4, and 6 (Fig. 3). Lower percentages at the end of isotope stage 6, in stage 5.1, at the end of stage 3, and in stage 1 also agree with a smaller ⌬␦ 18O crass.-rp. The low N. dutertrei % in stage 3 seems to agree with the other proxies that show that upwelling intensity was rather low around 50,000 yr B.P. Discrepancies occur in stage 4, where low percentages of N. dutertrei do not coincide with high p/o ratio, low accumulation rates of calcareous dinoflagellates, large ⌬␦ 18O crass.-rp., and relatively high TOC values. These proxies, in contrast, indicate stronger upwelling intensity. On the other hand, low ␦ 18Ovalues of G. ruber (Fig. 2) and mean sea surface temperatures

(Fig. 3) indicate relatively warm sea surface temperatures during this time period. Thus, the N. dutertrei % imply reduced upwelling intensity between approximately 65,000 and 67,000 yr B.P. The planktonic foraminifera assemblage of the adjoining core GeoB 1117-2 (Dittert, 1998) hints also toward a short decline in the upwelling intensity during this time period. However, these discrepancies show the usefulness of multiple proxies in paleoceanographic reconstructions. If accumulation rates of calcareous dinoflagellates are high during intervals of reduced upwelling intensity, when the thermocline is deep, these rates should correlate with low N. dutertrei % and with a smaller ⌬␦ 18O crass.-rp. That this is mainly the case can be seen in Figure 3. High accumulation rates of calcareous dinoflagellates do correlate with particularly low N. dutertrei % in stages 5.5, 5.3, 5.1, and 1 (Fig. 4). These

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FIG. 5. (a) Comparison of the ⌬␦ 18O difference between G. crassaformis and G. ruber (⌬␦ 18O G. crassaformis–G. ruber p.), SST, and total organic carbon (TOC) with the July insolation at 15°N. (b) Comparison of p/o ratio, accumulation rates of T. heimii, and calcareous dinoflagellate cyst accumulation rates with July insolation at 15°N. Glacial stages are shaded.

peaks also correlate with lowest TOC values and highest SST during the warmest periods of interglacials, as described above. These results point to a thriving of calcareous dinoflagellates whenever upwelling intensity is particularly low. During these time intervals the equatorial upwelling area is less eutrophic, SST is high, the thermocline is deep, and, therefore, stratification of the upper water masses is fairly stable. Equatorial Upwelling and Monsoon Intensity Striking changes in many parameters, especially accumulation rates of calcareous dinoflagellates, occur at the stage 6/5 and 2/1 transitions and, less prominently, between stages 5.2 and 5.1. Major changes in the global circulation system probably occurred during these transitions from glacial to interglacial times (Boyle and Keigwin, 1982; Shackleton et al., 1983). Planktonic foraminifera assemblages imply that equatorial up-

welling was least intense during these deglaciations (Mix and Morey, 1996). The decrease of TOC, SST, ⌬␦ 18O crass.-rp., N. dutertrei %, and p/o at our core site also implies less upwelling during the deglaciations. The increase in calcareous dinoflagellate accumulation rates likewise points to a weakening of upwelling intensity, but the minimum is reached around isotope stage 5.5, as also implied by the p/o ratio, TOC, and SST. According to the model of Mix and Morey (1996), the greatest upwelling intensity occurs during waxing glaciation. In the sediments of core GeoB 1105-4, our parameters indicate an increase in upwelling intensity at the transition from isotope stage 3 to 2, reaching highest intensity at the beginning of isotope stage 2. TOC, N. dutertrei %, and p/o ratios also reach maxima during stage 2. In spectral analyses of core GeoB 1105-4, spectra for ␦ 18O of G. ruber and TOC agree less with the obliquity period than

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FIG. 6. Schematic view of the equatorial Atlantic Ocean surface-layer structure during glacial and interglacial periods (after Meinecke, 1992; Baumann et al., 1999). During glacial periods, SE trade winds and South Equatorial Current (SEC) speeds reach their maximum and the thermocline ascends beneath the equator. The upwelling of nutrient-enriched water into the photic zone leads to increased primary productivity and a decrease in sea surface temperature (SST). During interglacial periods, SE trade winds are less intensive and SEC speed is at its minimum. The thermocline deepens in the eastern equatorial Atlantic Ocean, leading to diminished upwelling and thus to reduced productivity and higher SST. Proxies: rp, G. ruber (pink); cra, G. crassaformis; du and du, high and low frequency of N. dutertrei; p/o and p/o, high and low p/o ratio (ratio between cysts of peridinoid dinoflagellates and oceanic organic cyst species); cd and cd, high and low accumulation rates of calcareous dinoflagellates; TOC and TOC, high and low total organic carbon content; SST and SST, warm and cold sea surface temperatures; shaded area, equatorial divergence. For abbreviations of oceanic currents see Figure 1.

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with precession and eccentricity. If the ␦ 18O of G. ruber is subtracted from the ␦ 18O of G. crassaformis (⌬␦ 18O crass.-rp), both the 100,000 yr B.P. cycle and the 41,000 yr B.P. cycles disappear, and what remains most is power at 23,000 yr B.P. (Wefer et al., 1996). Accordingly, Wefer et al. (1996) suggested that temperature gradients between surface and subsurface waters are entirely dominated by local tropical dynamics, and have little or nothing to do with ice-age dynamics. Precession and consequently tropical insolation (e.g., July insolation variability at 15°N) strongly influence the equatorial upwelling that is driven by trade winds. Previous studies suggested that African monsoon intensity and trade wind strength are forced by tropical boreal summer insolation (Prell and Kutzbach, 1987; McIntyre et al., 1989). When boreal summer insolation is low, equatorial divergence is maximal due to a stronger zonal component of the SE trade wind tied to weak North African monsoon. Our data support this idea by showing a good correlation of ⌬␦ 18O crass.-rp., SST, TOC, and p/o ratio with July insolation at 15°N (Figs. 5a and 5b). Whenever the insolation is low, ⌬␦ 18O crass.-rp., TOC, and p/o ratio are high and SST is low, indicating a shallow thermocline, cool sea surface temperatures and higher productivity in the eastern equatorial Atlantic Ocean (Fig. 6). The calcareous dinoflagellates show no orbital components, but they do correlate with maxima of boreal summer insolation at 125,000 yr B.P., 80,000 yr B.P., and ⬃10,000 yr B.P. (Fig. 5b). These time periods coincide with strong monsoon over North Africa (Prell and Kutzbach, 1987) and weak equatorial Atlantic upwelling (Fig. 6). However, the calcareous dinoflagellates apparently are not sensitive to the wind-driven equatorial upwelling processes, else they would vary with tropical insolation. It rather appears that they flourish when threshold values are reached, like particularly low productivity, high SST, and stable stratification. We therefore infer that they indicate times of anomalous weak equatorial upwelling with reduced productivity. CONCLUSIONS

Late Quaternary variations in equatorial upwelling intensity can be inferred from many proxy parameters that show good overall correlation with one another. Equatorial upwelling was enhanced during glacial periods or cold substages of interglacials (isotope stages 2, 4, 6, 5.2, and 5.4), on the basis of calcareous and organic-walled dinoflagellate cysts, TOC weight percentages, and planktonic foraminifera. Our proxies show that the weakest upwelling intensity was reached in isotope stage 5.5 and the strongest occurred at the beginning of isotope stage 2. Comparison of the time series with the tropical boreal summer insolation shows that all parameters, except calcareous dinoflagellates, are closely linked to North African monsoon and SE trade wind strength. The calcareous dinoflagellates may reflect especially weak intensity of the

equatorial divergence, but they are not directly influenced by the wind-driven upwelling processes. ACKNOWLEDGMENTS We thank Helmut Willems, Gerold Wefer, Karin Zonneveld, and Christopher Moos for helpful discussions and critical reading of the manuscript, Erna Friedel and Emma Eades for correcting the English, and two anonymous reviewers for constructive comments. This research was funded by the Deutsche Forschungsgemeinschaft in the Graduierten Kolleg project “Stoff-Flu¨sse in marinen Geosystemen” with a fellowship for C. Ho¨ll and within the Sonderforschungsbereich 261 at Bremen University (contribution 215).

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