JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. C2, PAGES 3211-3222, FEBRUARY 15, 1999

Eolian-transported freshwater diatoms and Phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns

Oscar E. Romero, 1 Carina B. Lange,2 Robert Swap, 3 and Gerold Weferl

Abstract. The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20øN-7øS) in the tropical and equatorial Atlantic, along a N-S transect in the eastern equatorial Atlantic around 10øW and in the western equatorial Atlantic around 25øW. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD - 9 x 104 valves m -2 d-•; P = 2 x 104 bodies m -2 d-•), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 x 104 valves m -2 d -• for FD, and 0.7 x 104 bodies m -2 d -1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira grantdata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.

1. Introduction

The Sahara and Sahel are the main sources of eolian material

into the Atlantic Ocean [Schtitz, 1980]. This dust is transported across the equatorial Atlantic to the Caribbean and northeast coast of South America, in a southwesterly direction toward the coast of Guinea, and across the Mediterranean to Europe and the Near East [e.g., Folger, 1970; Prospero and Carlson, 1972; Kalu, 1979; Sch•itz, 1980, and references therein; Swap et al., 1992, 1996a, b; Husar et al., 1997; Moulin et al., 1997]. This dust may be important ecologically, in that it may contain large amounts of nutrients [Lundholm, 1979; Sch•itz, 1980] that, when carried out to sea, may promote biological production [e.g., Honjo, 1982; Wefer and Fischer, 1993; Swap et al., 1996a].

•Fachbereich Geowissenschaften, Universitfit Bremen, Bremen, Germany.

2Geosciences Research Division and Marine Life Research Group Scripps Institution of Oceanography, La Jolla, California.

3Global Environmental Change Program, DePartment of Environmental Sciences, University of Virginia, Charlottesville.

Copyright 1999 by the American Geophysical Union.

Paper number 1998JC900070. 0148-0227/99/1998JC900070509.00

Saharan dust export is episodic, varying in total amounts in space and over timescales of weeks to tens of thousands of years [Swap et al., 1996a; Husar et al., 1997], and is affected by changes in precipitation and atmospheric circulation over the regions of dust mobilization and transport [Moulin et al., 1997]. In particular, increased precipitation may affect the African dust cycle through both a change in soil moisture in source regions that results in decreased surface deflation and increased washout

during transport [Tegen and Fung, 1994; Moulin et al., 1997]. Previous studies have associated low loadings of eolian material with wet conditions over sub-Saharan Africa and higher loadings with drier conditions [Prospero and Nees, 1986; Swap et al., 1996a]. In the Saharan/Sahelian region, rainfall is restricted to a relatively short period (June to September) and is highly variable from year to year [Nicholson, 1981; Tucker et al., 1991; Swap et al., 1996a; Moulin et al., 1997].

Over the past 4 decades, several attempts have been made to reconstruct long-range transport of Saharan and Sahelian dust using freshwater diatom and phytolith records in equatorial Atlantic cores [e.g., Kolbe, 1957; Folger et al., 1967; Pokras a•d Mix, 1985; Stabell, 1986; Gasse et al., 1989; Pokras, 1991;

deMenocal et al., 1993] and sediment traps [Lange et al., 1994, 1998; Treppke et al., 1996]. Freshwater diatoms and phytoliths are derived from land areas and can be transported by wind more

3211

3212 ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS

than 2000 km from their source [e.g., Folger et al., 1967; Folger, 1970].

Windblown diatoms originate mainly from deflation basins and Holocene lake deposits dispersed throughout the Sahara and the Sahel and from rivefine sediments in the Chad Basin. Gasse

et al. [ 1989] found that windblown diatoms may derive from the Tamanrasset area in central and northern Sahara or from the

alluvial plains of Bilma, Niger, and Faya Largeau in the Chad Basin at the southern edge of the Sahara (Figure 1). Phytoliths are discrete, solid bodies of opaline silica in epidermal cells of plants [Blackman, 1971]. They are injected into the atmosphere during dry-season brush fires [e.g., Folger et al., 1967; Pokras and Mix, 1985]. In central North Africa, grasses are dominant in the T6n6r6 and the Sahel (Figure 1, grass savanna) [Anhuf and Frankenberg, 1991] and in the transition zone between the northern edge of the Sahara and the southern boundary of the Mediterranean steppe (Figure 1, Mediterranean-Saharan domain) [Anhuf and Frankenberg, 1991 ].

Here we report on fluxes in the water column and accumulation rates in surface sediments of freshwater diatoms

(FD) and phytoliths (P) over a large area encompassing the eastern tropical Atlantic and the eastern and western equatorial Atlantic. As the sampling sites in this study are located in the open ocean, distant from any river influence, eolian processes are believed to be the main deflation and transport mechanism for FD and P. Our data define the temporal flux of these land-

derived components, which are, in turn, related to seasonal changes in Saharan dust export. Given the geographical extension of our sampling sites, our data do not, however, expand spatially our knowledge about the geographical extension of the Saharan dust plume across the Atlantic. Our results may help in the interpretation of the sedimentary record by providing an understanding of the modern eolian transport mechanism, where it is assumed that the combined record of continent-derived,

windblown FD and P in deep-sea sediments may reflect (1) periods of enhanced deflation and wind transport and therefore of aridity [e.g., Bukry, 1980; Pokras and Mix, 1985; Gasse et al., 1989; deMenocal et al., 1993] and (2) changes in grass belt cover associated with changes in rainfall [Pokras and Mix, 1985; Tucker et al., 1991 ].

2. Methods

Cone-shaped traps with 20-cup collectors and a collection area of 0.5 m2 (SMT 230 and SMT 234, Salzgitter Electronics, Kiel) and 13-cup collectors (Mark VI) attached to moored arrays were used (Figure 1 and Table 1). Collection cups were poisoned with HgCI2 before deployment, and NaC1 was added to reach a final salinity of 38-40%o. Samples were poisoned again after recovery with HgC12 and stored at 4øC. The splitting procedure and chemical analyses were carried out at Bremen University (described by Fischer and Wefer [ 1991 ] and Wefer and Fischer [1993]).

w

50 ø 40 ø 30 ø 20 ø I I I

Atlantic

Ocean

Summer Plume

E 10 ø 0 ø 10 ø 20 ø

CB1•1121

EA2 •1101 ....... '" ........ 'EA'3•e3 •'f0•-• • 03"-"- ......

1105[] •'EA4 1117a .... ' .... '" 2904 al•WA7 111 _2• l•WA3 ........... ',"'

2903[] ..........

.... Winter plume

40øN

30 ø

20 ø

10 ø

0 ø

i I i i 20øS

Figure 1. Locations of mooring sites (triangles) and surface sediments (squares). Solid lines are Intertropical Convergence Zone (ITCZ) northern (August) and southernmost (February) positions [after HOfiich, 1984]. Dotted lines represent the areal extension of the Saharan dust plume and dust fallout over the Atlantic during summer and winter (modified from Schiitz [ 1980]). Arrows denote main dust trajectories from two source areas in the African continent [Kalu, 1979].

ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS 3213

For this study, 1/25, 1/64, and 1/125 splits of the original samples were used, rinsed with distilled water, acid cleaned, and prepared as described by Lange et al. [1994]. Qualitative and quantitative FD and P analyses were carried out on permanent slides (Mountex mounting medium) of acid-cleaned material, at x400 using a Zeiss-Photomicroscope III and a Zeiss Axiophot, both with phase-contrast illumination. Several traverses across the coverslip were examined, depending on the abundance of FD and P. For diatoms, each individual was identified to the lowest

possible taxonomic level. Since few grasses contain silica bodies that are unique in shape [Blackman, 1971], phytoliths were counted as a group, and a distinction of different morphologies was attempted based on the morphological classes outlined by Bukry [1980]. The resulting counts yielded estimates of daily fluxes of FD valves and P bodies per m-2 d-1 (Table 2) calculated according to Sancetta and Calvert [1988], as well as relative abundances of individual taxa (Table 3). Counting of replicate slides indicated that the error for the flux estimates is < 15%.

Surface sediment samples were collected with a box corer of 50 x 50 cm2 surface area and were taken from the uppermost 0.5- 1 cm of the sediment surface (Table 1). Box core samples were prepared after the method of Schrader and Gerso•;de [1978]. Counts of FD and P in the sediments were transformed to

concentrations in 1 g of dry sediment and to accumulation rates (see Table 4, where FDAR is freshwater diatom accumulation rate and PAR is phytolith accumulation rate), based on concentrations, sedimentation rates, and dry bulk densities after Van Andel et al. [1975].

The contribution of lithogenic material was taken from Wefer and Fischer [1993] and Fischer a•d Wefer [1996]. Grain size measurements on the lithogenic part of the samples were taken from Ratmeyer et al. [1999]. For standardization purposes, we refer to the Northern Hemisphere seasons, regardless of sampling site location.

3. Results

3.1. Sediment Traps

The lithogenic fraction contributed 32-52% of the total mass flux off Cape Blanc (CB1, 1988-1989), 21-58% at Cape Verde (CV1-2, 1992-1993), 9-25% at the eastern equatorial sites (EA1- 5, 1991), and 6-21% at the western equatorial sites (WA3, 1993- 1994; WA7, 1994-1995).

Mean daily fluxes of lithogenic particles, FD, and P decrease as a function of distance from the African source areas (Table 2). A sharp contrast is seen between the northern (north of the equator) and southern (south of the equator) sites, with lowest values south of the equator and in the western Atlantic. Settling velocities of the particulate material are available for the Cape Blanc and eastern equatorial sites and are believed to be 150-280 m d-I [Ratmeyer et al., 1999] and 31-158 m d-1 [Treppke et al., 1996], respectively.

Temporal changes within a sampling year and for each area are depicted in Figure 2. "Seasonality" is marked in the tropical and eastern equatorial Atlantic, mainly at and north of the equator.

At the Cape Blanc mooring (CB1, Figures 1, 2, and 4), lithogenic fluxes were highest in spring and summer (88.8 and

3214 ROMERO ET AL.: WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS

Table 2. Comparison of Mean Daily Fluxes of Lithogenic Particles, Freshwater Diatoms, and Phytoliths

Mooring Lithogenics, Sites mg m -2 d -•

Mean Daily fluxes

Freshwater Diatoms, Phytoliths, valves x 10 4 m -2 d -t bodies x 102 m -2 d -I

Eastern Tropical Atlantic Cape Blanc 79.93 + 32.34 3.06 + 1.25 72.89 + 34.52 Cape Verde 60.24 + 27.21 9.17 + 6.93 201.40 + 129.87

Eastern Equatorial Atlantic EA1 28.51 + 16.65 3.70 + 2.25 86.38 + 62.75 EA2 24.12 + 18.42 4.55 + 4.12 67.73 + 51.29 EA3 6.30 + 5.49 1.83 + 2.29 5.85 + 10.79 EA4 4.10 + 5.54 0.44 + 0.65 0.60 4- 0.85 EA5 6.42 + 9.34 0.42 4- 0.51 2.86 + 6.12

Western Equatorial Atlantic WA7 4.10 + 2.89 0.70 + 0.94 5.24 4- 2.67 WA3 2.39 + 3.01 0.15 + 0.18 0.08 + 0.23

Values are averages + standard deviations. See Table 1 for mooring locations.

Lithogenics (mg m -2 d -• )

0 30 60 90

I I I I

Freshwater diatoms

(valves * 104 m -2 d -• ) 0 5 10 15 I I I I

Phytoliths (bodies * 102 m -2 d-• ) 0 lOO 2oo 3oo I I I I

CB1 20ø55'N

CV1-2 11 ø29'N

EA1

03ø10'N

EA2 01 ø47'N

EA3

00ø05'S

EA4 02 ø 11 'S

EA5

04ø20'W

WA7 04øS

WA3 - 07ø31 'S

•-•s

Su*•---•W S•F ----'•S

EQ

.sSu Figure 2. Mean daily fluxes for each season of lithogenic particles, freshwater diatoms, and phytoliths along a north-south transect in the tropical and equatorial Atlantic. Abbreviations are S, spring; Su, Summer; F, Fall; W, Winter. For standardization purposes, we refer to the Northern Hemisphere seasons, regardless of sampling site location.

ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS 3215

78.5 mg m-2 d-l, respectively), FD peaked in winter (4.2 x 104 valves m-2 d-I ), and P peaked in summer (104.3 x 102 bodies m-2 d-i). Seasonality is well defined at the Cape Verde mooring (CVI-2, Figures 1-3). Flux values were highest in winter (74.3 mg m-2 d-l for lithogenics, 14.3 x 104 valves m-2 d-I for FD, and 283.1 x 102 bodies m-2 d-I for P) and second highest in spring (lithogenics and phytoliths) and fall (lithogenics and freshwater diatoms). Of all the sites studied, Cape Verde is the one with the largest contribution of FD and P (Table 2). At the eastern equatorial sites in the Guinea Basin (Figures 1 and 2) north of the equator, flux maxima were observed in spring (EA1) and spring and summer (EA2). Although our data are limited to the period April to November 1991 and we lack data for the winter season, Wefer and Fischer's [1993] and Lange et al.'s [1994] results for a nearby site in the Guinea Basin (01 ø 47.5'N, 11 ø 07.6'W), covering the period March 1989 to March 1990, also show that lithogenic components and FD had a pronounced maximum in March. Remotely sensed aerosol products from the National Oceanic and Atmospheric Administration advanced very high

resolution radiometer (AVHRR) also support the finding of pronounced seasonal aerosol maximum in March [Swap et al., 1996a; Husar et al., 1997]. Flux values for all three components were about an order of magnitude lower at and south of the equator (Table 2, sites EA3- EAS), yet they still show the same pattern of increased spring sedimentation (Figure 2).

In the western equatorial Atlantic (Figures 1 and 2), fluxes were low and similar in magnitude to the eastern equatorial sites south of the equator and revealed only weak seasonality during the sampling period. Higher lithogenic fluxes were observed in fall (site WA3) and winter (site WA7), FD peaked in winter and spring at 4øS (WA7), and P did not show a seasonal pattern. The low loadings found in the western equatorial Atlantic are to be expected, as indicated by the results of air parcel, aerosol, and trace gas transport research by Swap et al. [1996b] and Husar et al. [1997]. They find a consistently diminishing atmospheric transport of aerosols from Africa over this same region.

A total of 39 FD species were observed (Figure 3), and their presence and relative contribution of > 0.5% are summarized in

Figure 3. Freshwater diatoms, (a) Aulacoseira granulata, (b) A. islandica, (c) Stephanodiscus astraea, (d) Pinnularia borealis, and (e) Hantzschia amphyoxis, and phytoliths (f) trilobate panicold, (g) paired regular panicoid, (h) regular chloridoid, (i) slant panicoid, and (j) chain of regular panicolds, collected in sediment traps deployed in the tropical and equatorial Atlantic Ocean (see Table I for locations). All scale bars are 10 pm.

3216 ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS

Table 3. Aulacoseira granulata and A. islandica dominated the freshwater assemblage in all areas; highest relative abundance (12% of total diatom assemblage) was observed at Cape Verde in January. Most of the species fall within "group II" of Gasse et al. [1989], a group that is characteristic of slightly alkaline to hyperalkaline water bodies of the Chad Basin (Figure 1). Only a few taxa fall within their "group I", with assemblages derived from the central and northern Sahara (Tamanrasset; see Figure 1), where saline chloride water bodies are widespread [Gasse et al., 1989]. Almost twice as many species are present in the areas of Cape Blanc and Cape Verde as in the equatorial sites (23-25 versus 9-14; see Table 3). As with the flux data, the relative contribution of freshwater diatoms to the total diatom assemblage decreased with increasing distance from the African continent.

Diverse assemblages of opal phytoliths were observed over the entire study area (Figure 3). In an attempt to discern seasonal and geographical differences, P were divided into four morphological classes [Bukry, 1980]: elongate (large, simple rod shape, 40-60 gin), panicoid (dumbell shape, 15-36 gin), festucoid (cubold and oblong shape, 7-21 lam), and chloridoid (small, oblong-to-square shape, 7-15 lain). Off Cape Blanc, the summer flux maximum (Figure 2) was mainly composed of elongate and panicold phytoliths (> 30% each). At Cape Verde, spring and winter flux peaks were composed of > 40% of panicold phytoliths (Figure 2). Panicoid forms also dominated the spring and early summer flux maxima in the Guinea Basin.

Morphological diversity was lowest in the western equatorial Atlantic, where small festucoid forms represented > 50% of the

Table 3. Diatoms at Tropical, Eastern Equatorial and Western Equatorial Mooring Sites

Freshwater Taxa Tropical Eastern Equatorial CB 1 CV 1-2 EA 1 EA 2 EA 3

Achnanthes delicatula A. hauckiana

Anomoeoneis sphaeorophora Aulacoseira spp. (A. granulata, A. islandica) Campylodiscus clypeus Cocconeis speciosa Cyclotella meneghiniana C. ocellata

Ellerbeckia arenaria

Epithemia sp. Eunotia monodon v. mortorlon

E. papilio Fallacia nyella Hantzschia amphioxys Luticola mutica f. mutica

Lyrella lyra Pinnularia borealis

Stauroneis anceps Staurosirella lapponica S. leptostauron S. pinnata Stephanodiscus astraea Anomoeoneis sp. Cyclotella kuetzingiana Cymbella sp. Eunotia mortorlon v. bidens

Mastogloia smithii Nitzschia sp. 3 Nitzschia sp. 4 Pinnularia major Pinnularia sp. 1 Pinnularia sp. 2 Pseudostaurosira brevistriata

Stauroneis sp. S. borrichii

Nitzschia sp. 1 Nitzschia sp. 2 Pinnularia microstauron

Number of species Season of highest % abundance Season of highest flux

P

P

P

(4%)

EA 4

P

P

P

P

P

P

P

P

P

P

P

P

P

P

P

P

P

(0.5%)

EA 5

Western Equatorial

WA 7 WA 3

(12%) (3.5%) (4%) (5%) (1.4%) (1.4%) (1.4%) (2.7%)

P P P P P P P P P P (0.6%) P p p p p

P P

(1%) P p p (0.6%) P p p

P P P P

P

(0.7%) (1%)

P

P

P

P

P

P

P

P

P

P

P

P

P P

P P P

P

P P

P P

P P

P P P P P P

(0.5%) (1.3%) (0.7%)

P P

P P

P

P P P P

23 25 14 13 10 11 10 10 9 Su W S S NS NS NS S S + Su W F+W S S S NS NS W+S NS

Data indicate freshwater taxa recorded at mooring sites, reflect/ng presence (P) and relative abundance of dominant taxa (percentage of total diatom assemblage). Values in parentheses represent_>0.5% in at least one sample. Other abbreviations are NS, no seasonality; S, spring; Su, summer; F, fall; and W, winter. For standardization purposes, we refer to the Northern Hemsiphere seasons, regardless of sampling site location.

ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS 3217

phytolith flux throughout the sampling year and large elongate forms were rarely present.

Grain size data on the lithogenic fraction of sinking particulate matter for the size spectrum 6-63 •m are available for Cape Blanc and Cape Verde mooring sites only [Ratmeyer et al., 1999]. We compared our record of FD and P fluxes with the data of Ratmeyer et al. on the normalized particle flux of size classes 6-63 •m (Figures 4 and 5), and observed that the load of FD and P correlates well with the < 20 pm fraction. This is especially true for the Cape Verde site (Figure 4), where grain size frequency distribution shows a well-sorted spectrum with high abundances in small size classes in the fall, winter, and early spring [Ratmeyer et al., 1999]. Correlations are somewhat higher for FD (r = 0.89 with the 6-11 •m class and r = 0.79 with the 11- 20 •m class) than for P (r = 0.64 and 0.69, respectively). Mean diameters or valve lengths of the dominant diatom species (Table 3) fall between 4 and 20 •m. In contrast, grain size distributions off Cape Blanc appear to be less well sorted (Figure 5), and a clear seasonal pattern cannot be resolved [Ratmeyer et al., 1999].

3.2. Surface Sediments

To determine if FD and P in surface sediments would provide information on regional variability, the accumulation rates of freshwater diatoms (FDAR) and phytoliths (PAR) were calculated for surface sediment samples collected at or near mooring sites (Table 1). Sedimentation rates vary between 1.5

and 22 cm kyr-1 (Table 4). Our FD and P data are comparable to those of Treppke et al. [ 1996] and are about 1 order of magnitude lower than those reported by Pokras and Mix [1985].

FDAR and PAR patterns coincided with those in the traps, with highest values (of the order of 106 valves m-2 yr-1 for FD, and 105 bodies m-2 yr-1 for P) in the Cape Verde area, off Cape Blanc, and in the Guinea Basin north of the equator (Figure 6 and Table 4). At all sites, the relative contribution of FD (Table 5) as well as the ratio of FD to marine diatoms were higher in surface sediments than in sediment traps (Table 5).

The number of FD species in sediments was considerably reduced. A total of 12 freshwater taxa were identified in surface

sediments versus 39 in the traps (Table 3). This reduction was especially noticeable off Cape Blanc (23 species in the traps versus 9 in sediments) and at the Cape Verde site (25 species versus 8). As was the case for the trap assemblages, Aulacoseira granulata was the most abundant freshwater species in surface sediments, similar to data previously published by Pokras and Mix [1985], Stabell [1986], Pokras [1991], and deMenocal et al. [1993].

4. Discussion

4.1. Sediment Traps

Although interannual variability in the load of FD and P is to be expected, our results suggest that a general regional pattern

CAPE VERDE (CV1-2)

70-

• •'", 35

ß g

_

Sample

26

I Phytoliths 13;-

,," ',,,, •!, / / \ Freshwater 0

I I I I I I I I I I I I I I I I I I II I I I I I I

4 8 12 16 20 24 28

- 52O

"•"'"1

..c:o

o

- 2e0. o

Season W S Su

1992 1993

Figure 4. Normalized particle flux of size classes 6-11 pin, 11-20 pm, and 20-63 pm [from Ratmeyer et al., 1998] compared to fluxes of freshwater diatoms and phytoliths at Cape Verde mooring, from October 1992 to October 1993. Note that the seasonality of size classes at Cape Verde is clearly defined in fall, winter, and early spring.

3218 ROMERO ET AL.: WINDBLOWN FRESHWATER DIATOMS AND pHyTOLITHS

CAPE BLANC (CB1)

60-

-o • 30-

0•4_, -- • o

6 Freshwater

35• o

\\;,, Phytoliths • >e 1-20 IJm ./..•,\ 0

.... / -- 6-11 IJm "'% '-:'=• • • •

'20-63 IJm

Sample 2 4 6 8 10 12

Season Su w

- 180

• X 04m •E

,• 0

-90 •.,• o

I0

1988 1989

Figure 5. Normalized particle flux of size classes 6-11 prn, 11-20 pm, and 20-63 pm [from Ratmeyer et al., 1998] compared to fluxes of freshwater diatoms and phytoliths at the Cape Blanc mooring, from March 1988 to March 1989. At this site, a clear seasonal pattern of size classes cannot be resolved and grain size distributions appear to be less well sorted.

can be drawn that is tightly coupled with the seasonal migration of the Intertropical Convergence Zone (ITCZ) and the latitudinal and meridional extension of the Saharan dust layer (Figure 1) [Kalu, 1979; Schfitz, 1980; Swap et al., 1996a; Husar et al., 1997; Moulin e! al., 1997]. Highest FD and P fluxes were recorded at the Cape Verde site, and moderately high off Cape Blanc and in the North Guinea Basin. A sharp drop in fluxes is seen south of the equator (Figures 2 and 6 and Table 2). During the northern summer, when the ITCZ moves northward, the dust layer is usually confined to a latitudinal band between 15 ø and 25øN (summer plume) [Schlitz, 1980; Moulin et al., 1997]. This northward shift is reflected in data from our Cape Blanc site [Lange et al., 1998], where we recorded the highest P fluxes during this season along with a greater contribution of P of the elongated class from the North African Mediterranean-Saharian Domain (Figure 1). As the ITCZ moves south in winter, the dust plume moves in a southwesterly direction and is located approximately between the equator and 15øN (winter plume), with dust distributed from the Guinea Basin across the Atlantic to

South America [Schlitz, 1980; Prospero et al., 1981; Swap e! al., 1992, 1996a, b] (Figure 1). The geographical extent of the plume affected our site WA7 in the western equatorial Atlantic, where FD flux peaks were recorded from January to May, coincident with the strongest dust events in the area [Prospero et al., 1981; Prospero and Nees, 1986; Swap e! al., 1992, 1996a, b]. No

South American location as a source region has been found for those months of the year [Swap e! al., 1992, 1996a, b].

For the Cape Verde site, highest fluxes of FD and P were observed in fall and winter-early spring (Figure 4), coincident with the highest frequency of winds of continental provenance [Ratmeyer et al., 1999]. This is consistent with the annual migration of the North African aerosol plume from north to south to north, which impacts the region of the Cape Verde Islands and our sampling site CV 1-2 most fully between November and April [Chiapelio et al., 1995; Swap et al., 1996a].

In contrast to the Cape Verde area, eolian input appears to be present throughout the year off Cape Blanc [Ratmeyer e! al., 1999], with largest impact in summer [Moulin e! al., 1997]. The fact that FD and P were constantly present throughout the year and seasonality in their fluxes was less marked (Figure 5) may suggest that the Cape Blanc site experiences the influence of both the summer and winter plume [Lange et al., 1998]. Off Cape Blanc, higher FD fluxes in winter and early spring coincided with the continental provenance of the trade winds and the dry season in the Chad Basin (Figure 1), where freshwater diatoms thrive in alluvial plains [Gasse et al., 1989]. For 1989, heavy aerosol loadings impacted the Cape Blanc region in January and February (sample 12, Figure 5) [Swap e! al., 1996a]. The summer phytolith flux maximum (sample 5, Figure 5) may be a consequence of the peak period of biomass burning (March to

ROMERO ET AL.: WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS 3219

3220 ROMERO ET AL.' WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS

Cape _ Blanc

Cape _ Verde

N-Guinea Basin

S-Guinea Basin

Western

equatorial Atlantic

-

Freshwater diatoms Phytoliths

Trap Yearly flux

(valves * 10 6 m -2 y J) 0 20 40 t i i i i

Sediment Trap Sediment Accumulation rate Yearly flux Accumulation rate (valves * 106 m -2 y-'•) (bodies * 10 s m -2 y_l) (bodies * 105 m -2 y-l) 0 20 40 0 40 80 0 15 30 i i i i i i i i i I i i i I i

Figure 6. Freshwater diatom and phytolith annual fluxes in sediment traps compared to accumulation rates in surface sediments off Cape Blanc, at Cape Verde, in the Guinea Basin, and in the western equatorial Atlantic.

June, north of the equator) [Hao and Liu, 1994], including both dry-season brush fires and anthropogenic effects in the Sahel region [Herman et al., 1997], which release phytoliths for later transport. It may also be associated with the high-level dust transport in the Saharan Air Layer (SAL) [Ratmeyer et al., 1999].

For both tropical sites (Cape Blanc and Cape Verde), significant correlation of freshwater diatoms and phytoliths with the lowest normalized grain size classes (6-11 •m and 11-20 •m) (Figures 4 and 5) corresponds well with the intensity of wind blowing from the continent. Although mean yearly fluxes in the eastern equatorial Atlantic resemble those off Cape Blanc, FD diversity is lower (Table 3) and the P assemblage is dominated by panicoids forms, possibly indicating diminished wind intensity. In the western equatorial Atlantic, the low diversity seen in phytolith morphology, the predominance of small festucoid forms (size range, 6-16 •m), and the low contribution of the larger elongate forms also provide evidence of the decreasing ability of the atmosphere to transport FD and P with increasing distances from the source (Figure 1).

4.2. Surface Sediments

The geographical extent of the Saharan dust plume is readily apparent in surface sediments. The overall pattern of FD and P accumulation rates in surface sediments mirrors that of the yearly fluxes in the traps, with highest loadings in the Cape Verde area that decrease westward and southward with increasing distance from Africa (Figure 6). Preserved assemblages in sediments also reflect those collected in traps with A. granulata as the main contributor (up to 22%) to the freshwater assemblage between

20øN and 7øS (Table 3). Although caution has been advised with the use of Aulacoseira-rich sediments for reconstructing phases of aridity/humidity on the African continent [Gasse et al., 1989], we agree with Pokras [ 1991 ] that for open ocean areas far from land where significant rivefine input can be ruled out, Aulacoseira values are useful for reconstructing the overall climatic history of North Africa.

Differences in relative abundance of the freshwater

assemblage between the traps (FD% = 0.6-4.5) and the surface sediments (FD% = 1.6 - 26.1) (Table 5) may be attributed, in part, to selective dissolution of delicate diatom species typical of pelagic areas in the equatorial Atlantic and their low fluxes [Treppke et al., 1996; Romero et al., 1999]. Lightly silicified

Table 5. Comparison of the Relative Contribution of Freshwater Diatoms and the Ratio of Freshwater Diatoms to Marine Diatoms

in Sediment Traps and Underlying Sediments.

Study Area

Cape Blanc Cape Verde

North Guinea Basin

South Guinea Basin

Abundance, % FD Ratio, FD/MD x 100

Trap Sediment Trap Sediment ,

2.8 8.5 2.3 9.3

4.5 26.1 4.3 35.3

1.7 11.8 2.9 13.4

2.6 20.5 4.1 25.8

3.9 5.9-8.4 6.3 ---

0.8 5.4 1.0 5.7

0.6 1.6-2.8 0.9 2.9

Western equatorial Atlantic 0.9 0.7

4.4 0.5 4.6

7.8 0.2 ---

ROMERO ET AL.: WINDBLOWN FRESHWATER DIATOMS AND PHYTOLITHS 3221

diatoms of the Nitzschia bicapitata group made up 60-65% of the total diatom assemblage in sediment traps south of Cape Verde, the Guinea Basin, and the western Atlantic but never exceed 10%

of the diatom associations in the sediments of these areas [e.g., Romero e! al., 1999]. As a consequence, the abundance of robust species (most of the freshwater taxa fall within this category), quantitatively less important in the traps, is enhanced in the sediments. Thus FD and P accumulation rates in sediments are

far better indicators of the input of eolian-transported material than their relative abundances and/or concentrations [Pokras, 1991; deMenocal e! al., 1993]. For example, off Cape Blanc and in the western equatorial Atlantic, similar relative abundances of FD do not mirror the fluxes in the water column and would

wrongly suggest that the intensity of the eolian input is similar in these two geographically separate locations (Table 5).

Information on the role of the different wind systems operating seasonally in the Atlantic can be of help when interpreting the land-derived freshwater diatoms and phytoliths assemblages preserved in sedimentary records. FD and P accumulation rates are considered as paleoindicators of the trade wind layer and the winter dust plume since these appear to be the main mechanism of transport across the Atlantic (Figure 1). In addition, they furnish information on the role played by the SAL and the summer dust plume carrying land-derived organisms to the Cape Blanc area.

5. Outlook

This work has contributed to the beginning of the quantification of fluxes of terrestrial material to the oceans' sediment over seasonal and annual timescales, as well as to the

development of the understanding of modern mechanisms of eolian transport and deposition of North African material. Future research directions will involve continued exploration toward an understanding of the modern-day mechanism of eolian deflation, transportation, and deposition so that it can be used as an analog for paleoclimatic research. Perhaps an analogy to the strong FD and P records associated with eolian transport from the Sahara and Sahel exists in the South Atlantic and Indian Ocean, off sub-

Sahelian Africa. There are many areas south of the Sahel, particularly in central and southern Africa, that contain seasonally flooded grassland savannas. Not only may these areas produce large quantities of FD and P associated with the seasonal wetlands, but they are also subjected to frequent, large-scale biomass burning [Justice e! al., 1996]. Further observations of FD and P distributions and sedimentation rates in the oceans

around central and southern Africa may add to our present understanding of the land-atmosphere-ocean system.

Acknowledgments. The authors wish to thank Lydie Dupont, Fred Jansen, Volker Ratmeyer, and Freda Reid for useful comments on a first draft of the manuscript. The final version greatly benefited from comments of Lloyd Burckle and John Milliman. We are grateful to the crew of the R/V Meteor for professional expertise in deployment and recovery of the mooring arrays and collection of sediment samples. This work was t•nded by the Deutsche Forschungsgemeinschaft (Sonderibrschungsbereich 261 at the University of Bremen, contribution 212), and by the Marine Life Research Group and the Geosciences Research Division of the Scripps Institution of Oceanography.

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(Received May 21, 1998; revised September 18, 1998; accepted November 12, 1998.)