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Seasonal impact of mineral dust on deep-ocean particle flux in the eastern subtropical Atlantic Ocean
Marine Geology 159 Ž1999. 241–252 www.elsevier.nlrlocatermargeo
Seasonal impact of mineral dust on deep-ocean particle flux in the eastern subtropical Atlantic Ocean V. Ratmeyer
a,)
, W. Balzer b, G. Bergametti c , I. Chiapello c , G. Fischer a , U. Wyputta a
a
c
Fachbereich Geowissenschaften, UniÕersitat ¨ Bremen, Klagenfurter Strasse, 28359 Bremen, Germany b Fachbereich Biologier Chemie, UniÕersitat ¨ Bremen, Bibliothekstrasse, 28359 Bremen, Germany Laboratoire InteruniÕersitaire des Systemes Atmospheriques, URA CNRS 1404, UniÕersites 61 ` ´ ´ Paris, Centre Multidisciplinaire de Creteil, ´ aÕ du General ´ ´ de Gaulle, 94010 Creteil ´ Cedex, France Received 9 January 1998; accepted 24 November 1998
Abstract Seasonal lithogenic particle and Al fluxes were obtained from a deep-ocean sediment trap deployment during 1992 and 1993 off NW Africa, and were compared concurrently with atmospheric Al concentrations and two-dimensional backward trajectories of windfields from two barometric levels in the lower and mid troposphere. Marine Al fluxes, lithogenic particle fluxes and grain size distributions in the area were found to be directly linked to airmass pathways and surface mineral aerosol concentrations. At 1000 m water depth, highest Al fluxes Ž10.77 mg my2 dayy1 ., lithogenic particle fluxes Ž99.25 mg my2 dayy1 . and smallest mean grain sizes Ž11.9 mm. occurred during the winter and spring season, concurrent with highest atmospheric dust load and Al-concentrations Ž15 300 ng my3 . in the lower troposphere. A strong seasonal change of the main atmospheric dust transport from low altitude winds during winterrspring to higher altitudes during summer is clearly reflected at depth by a significant coarsening of mean grain sizes Ž18.6 mm. and lowest Al Ž0.81 mg my2 dayy1 . and lithogenic particle fluxes Ž11.3 mg my2 dayy1 . found in the sediment traps. The comparison of marine, atmospheric and model derived data used within this study highlights the close temporal coupling between atmospheric dust transport and the deep-ocean particle stock. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Atlantic Ocean; Cape Verde; atmospheric dust; particle flux; grain size
1. Introduction Atmospheric transport of mineral aerosol from continental sources is currently discussed to have an essential impact on the global climate variability both via albedo effects on the atmospheric heat budget ŽTegen et al., 1996. and through a variety of )
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effects on the marine organic carbon cycle. Long term changes of the dust supply to the deep-sea over timescales of 10 3 to 10 6 years are recorded in marine sediments and provide valuable informations for paleoclimatic reconstructions such as glacialrinterglacial climatic changes ŽSarnthein et al., 1982; Stein and Sarnthein, 1984; Ruddiman, 1997., paleoCO 2 variations ŽBerger and Keir, 1984. and watermass distributions ŽPetschick et al., 1996.. On shorter time scales, the marine particle inventory is partly
0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 8 . 0 0 1 9 7 - 2
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determined by the aeolian supply of limiting nutrients and their effect on marine biomass production ŽMartin and Fitzwater, 1988; Donaghay et al., 1991., effects on nitrogen fixation ŽMichaels et al., 1996. and the scavenging of heavy dust grains by organic particles. The latter is discussed to increase the settling speed of aggregates built in the upper ocean ŽRamaswamy et al., 1991. and thus may enhance the downward transport of particulate organic carbon to the deep-sea, namely the efficiency of the biological pump ŽFowler and Knauer, 1986.. A strong coupling of particle fluxes in the deep-sea with seasonal changes of surface winds and primary productivity in the upper ocean was found during sediment trap experiments in many regions of the world ocean Že.g., Deuser, 1986; Wefer and Fischer, 1993.. But, so far only few data show to what extent vertical fluxes of particulate matter reflect short-term or seasonal changes of the atmospheric dust input. In the Mediterranean Sea ŽBuat-Menard et al., 1989., atmospheric Al fluxes could be traced down to 200 m water depth with a time delay in the order of 1 week and imply a fast biogenic vertical transport in the upper ocean layer. Results from long-term experiments in the Sargasso Sea ŽJickels et al., 1998. indicate that atmospheric Al and deep-sea lithogenic particle fluxes are linked on an annual basis, with variations for both compartments in the subannual to seasonal timescale. In a general view, prominent sites for studying how atmospherically controlled dust deposition affects the deep-sea particle inventory are areas where high productivity coincides with significant atmospheric dust input, such as the eastern tropical Atlantic. Here, along the northwest African coast Ž11– 308N., extensive atmospheric dust transport from the dry areas of the coastal Morocco, the Sahel and Sahara regions is well known Žsummary in Morales, 1979; Prospero, 1990. and contributes up to 660 Mt of mineral aerosol to the subtropical and tropical Atlantic ŽMarticorena and Bergametti, 1996.. On a global scale, this amount supplies one third to half of the roughly estimated 0.9 to about 2 billion tons of global aeolian dust input to the world ocean Žd’Almeida and Jaenicke, 1984; Duce et al., 1991; IPCC, 1994.. Concurrently, nutrient upwelling leads to almost year-round high biological productivity ŽMittelstaedt, 1991.. This enhanced primary production is
discussed to provide an effective vehicle to transport dust to the deep-ocean via biogenic aggregation and fast vertical flux, with aggregate settling velocities in the order of 100 m dayy1 ŽFischer et al., 1996; Neuer et al., 1997; Ratmeyer et al., in press.. Further, the imprint of mineral dust on the marine sediments in this area reflects past global and regional climatic changes and was subject to a number of investigations Že.g., Koopmann, 1981; Sarnthein et al., 1982; Stein, 1985.. In this study we show that deep-ocean Al fluxes, lithogenic particle fluxes and their grain sizes can reflect short-term, seasonal changes in the aeolian dust supply. This also implies that the vertical transport through the watercolumn may be fast and effective enough to reflect mass concentrations and grain size spectra of dust entering the surface ocean. The study area is situated within the first 1500 km off the African coast, a distance where most of the dust leaving the Saharan sources is assumed to settle out.
2. Material and methods 2.1. Sediment trap samples Particle fluxes were recorded with sediment traps between October 1992 and October 1993 at 1000 m water depth near the Cape Verde Žsite CV: 11829.0X N; 21801.0X W; 4900 m waterdepth.. Traps were of the type KIEL SMT 234 and cone-shaped with 0.5 m2 opening. Prior to and after deployment, sample cups were poisoned with mercuric chloride. After trap recovery, samples were wet-sieved Ž1 mm., splitted, treated and analyzed as described elsewhere ŽFischer and Wefer, 1991.. The lithogenic fraction was calculated as: Lith flux s Total flux y Ž opal flux q carbonate flux qorganic matter flux . where the analytical errors associated with opal and carbonate determination were negligible. Organic matter was converted from C org by factor 2.0. Due to changes in the plankton composition, this conversion includes a maximum error of "10%. Al and Fe in the trap samples were determined by flame AAS after subjecting the trapped particles to a
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total dissolution pressure digestion in a microwave oven using HFrHNO 3 . Based on the analysis of certified reference materials the precision was estimated to be better than 6% for Al and Fe. For grain size measurements on the lithogenic fraction, 1r16 to 1r5 split of each sample was treated with acetic acid and H 2 O 2 to remove carbonate and organic matter. However, dust from Morocco and Sahel may contain neglible amounts of dolomite ŽJohnson, 1979; Pye, 1987. which is not preserved after acid treatment of the samples. The remaining fraction was neutralized with NH 3 and wet sieved. Biogenic opal contributed between 5 and 14% Žmean 6.9%. to the total flux and was not removed due to preservation of clay minerals for later analysis. Grain size analysis was performed on a CIS-GALAI laser particle analyzer ŽAharonson et al., 1986.. McCave et al. Ž1986. and McCave et al. Ž1995. found a significant underestimation of the clay-fraction Ž- 2 mm. concurrent with a constant ‘ghost’-peak in the fraction 2–10 mm as a general problem of the laser diffraction method Žsee also Bayvel and Jones, 1981; Dodge, 1984.. Although the instruments based on laser diffraction are not directly comparable to the CIS-GALAI particle analyzer used in this study, we decided to exclude the smallest silt-sized fraction Ž2–6 mm. of particles from the dataset. Due to the very low absolute particle concentration in most samples, the clay fraction Žgrains - 2 mm. could not be quantitatively separated by decantation or filtration. Thus, a direct measurement of the total size fraction was not possible. Percentages were normalized to the fraction 6–63 mm. Due to these limitations a direct comparison with grain sizes of atmospheric dust Žmean median size of 1.8 mm at Sal Island; Chiapello, 1996., however, remained impossible. None of the samples contained a measurable amount of grains ) 63 mm. A more detailed description of the method is published elsewhere ŽRatmeyer et al., in press.. 2.2. Atmospheric dust samples Dust samples at Sal Island, Cape Verde Archipelago Ž16845X N; 22857X W., were obtained between 1992 and 1994 by bulk filtration on Nuclepore@ 0.4 mm pore size using a flow rate of about 1 m3 hy1 Ž24 h sampley1 .. Aluminum particulate
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concentrations in atmospheric dust were determined directly on the filter by X-ray spectrometry fluorescence ŽSiemens SRS 303. ŽLosno et al., 1987; Chiapello et al., 1995.. 2.3. Trajectory computations Daily atmospheric backward trajectories were computed for site CV from October 1992 through October 1993 at 850 and 500 hPa barometric levels with a global two-dimensional isobaric model. The precursor of this model was developed at the Norwegian Meteorological Institute ŽNMI. ŽEliassen, 1976.. The model computes 4-day isobaric backward trajectories from zonal and meridional wind components ŽPetterssen, 1956.. Wind data were obtained from the European Centre for Medium Range Weather Forecasts ŽECMWF..
3. Results and discussion 3.1. Seasonal Õariability Atmospheric Al concentrations of the mineral aerosol ŽFig. 1a. in the surface layer of Sal Island were found to exhibit a strong seasonal pattern. Monthly averaged concentrations were highest from December Ž4600 ng my3 . through January Ž15 300 ng my3 ., and decreased significantly during the summer months Žminimum in June: 290 ng my3 .. Although situated in a more marginal position of the general dust outbreak path, a similar seasonal change was recorded in sediment trap samples at the CV site off Cape Verde. Here, changes of the particulate Al flux at a water depth of 1000 m ŽFig. 1b. follow the overall pattern of atmospheric Al concentrations. High mean fluxes of between 8.1 ŽFebruary 1993. and 10.8 mg my2 dayy1 ŽMarch 1993. prevailed during winter and spring, while much lower mean fluxes of Al down to 0.8 mg my2 dayy1 ŽAugust 1993. occurred during summer. However, a detailed peak-to-peak correlation between atmospheric Al concentrations and marine Al fluxes remains impossible, both due to the different sampling intervals as well as different positions of atmospheric and marine sampling sites relative to the atmospheric path. Yet
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X X Fig. 1. Ža. Al concentrations measured at surface level on Sal Island, Cape Verde Archipelago Ž16845 N; 22857 W.: monthly mean values are calculated from discontinuous daily data obtained during 1992 and 1993. Žb. Al fluxes measured in sediment traps at 1000 m water X X depth near the Cap Verde Žsite CV: 11829.0 N; 21801.0 W.. Žc. Lithogenic particle fluxes recorded in the sediment trap samples. Žd. Lithogenic particle fluxes of the size class 20–63 mm. Solid lines represent a 3rd order polynom.
we believe, that the overall pattern of seasonal changes in the dust supply to the area is reflected in both data sets. Marine Al fluxes show a significant correlation Ž r 2 s 0.77, n s 28. with the lithogenic particle fluxes during the entire sampling period ŽFig. 1c and Fig. 2a.. FerAl ratios in trapped material varied from
0.43 ŽAugust 1993. to 0.58 ŽOctober 1992., similar to the range of atmospheric ratios between 0.41–0.63 from January and September 1993. These ratios might be changed by differential dissolution in the ocean’s mixed layer after the deposition of dust and further during settling through the water column. Although it is not possible to assign definite values to the
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Fig. 2. Linear regression between Ža. Al and Lith flux and Žb. C org and Lith flux in the sediment trap samples.
solubility of the individual elements, a review of existing data demonstrates that Al and Fe which are predominantly associated with mineral aerosol have comparatively low solubilities ŽDuce et al., 1991..
These authors assume that 5% of the Al and 10% of the Fe that enters the ocean from the atmosphere is soluble. These percentages might be higher when the dust particles were subjected to the low pH regime
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of rain before being deposited Že.g., Jickels, 1995; Helmers and Schrems, 1995.. Since only very few
rain events of short duration occurred on Sal Island during the trap deployment, it is likely that for these
Fig. 3. Backward trajectories to the CV site of airmass pathways in the trade wind layer Ž850 hPa, solid line. and the Saharan Air Layer ŽSAL. Ž500 hPa, dashed line. in January Ža. and July Žb. 1993. Transport of dust from continental sources during winter occurs primarily at low altitudes Ža., while during summer dust is carried by high altitude winds within the SAL Žb..
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two metals most of the atmospheric signature has survived the contact with sea water until settling to the traps. In contrast to the Al fluxes, the flux of coarse lithogenic particles in the size class 20–63 mm shows a reverse pattern ŽFig. 1d.. Coarse grains contributed generally less than 12 mg my2 dayy1 Ž20–30% of the measured fraction 6–63 mm. to the lithogenic flux during winter, but increased significantly from April 1993 through July 1993 to fluxes between 15 and 32 mg my2 dayy1 Ž30–55%.. Possible changes in the vertical transport from the ocean’s surface to the trap depth, however, do not provide a satisfying explanation for this contradictional behaviour. Organic carbon and lithogenic particle fluxes correlated well during both seasons Ž r 2 s 0.63, n s 28; Fig. 2b., and indicate a rapid and
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efficient biogenic particle transport without lateral advection of terrigenous fractions at depth during the entire deployment time. Instead, we suggest that the seasonal pattern of coarse grain fluxes is mainly caused by the input of mineral dust from different atmospheric pathways, which reflects a seasonal change of atmospheric circulation patterns in this region. 3.2. Atmospheric transport Atmospheric dust transport from the African continent between 10 and 208N is known to occur either in the Saharan Air Layer ŽSAL. above the trade wind inversion Žpressure levels around 500 hPa., or in the trade winds below 2 km altitude Žaround 850 hPa. ŽSemmelhack, 1934; Carlson and Prospero, 1972;
Fig. 4. Comparison of atmospheric trajectories Ža,b. and sediment trap Žc. data from site CV between October 1992 and 1993. Frequency index of continental backward trajectories Ža. at the 500 hPa and Žb. at the 850 hPa pressure level: 1.0 stands for continental provenance and possible dust transport during all days of a sampling period. Numbers were counted from directional plots, classifying sources as continental vs. oceanic. Žc. Equivalent spherical mean diameters of the fraction 6–63 mm of lithogenic particles trapped at 1000 m water depth.
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Fig. 5. Ža. Linear regression between the abundance of lithogenic grains in the size class 20–63 mm from 1000 m water depth Žplotted as percent of the fraction 6–63 mm. and the monthly abundance of continental winds within the 500 hPa pressure level Žplotted as weeks per month in 1993.. Žb. Linear regression between the abundance of lithogenic grains in the size class 6–11 mm from 1000 m water depth Žplotted as percent of the fraction 6–63 mm. and the monthly abundance of continental winds within the 850 hPa pressure level Žplotted as weeks per month in 1993..
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Jaenicke and Schutz, ¨ 1978.. The summerly transport within the SAL is widely accepted as the main agent of dust export from the Saharan sources towards the subtropical North Atlantic Ocean Že.g., Prospero, 1990.. However, recent studies ŽChiapello et al., 1995; Chiapello et al., 1997. show the significance of low-layer dust transport in the trade winds in particular for the proximal sites within the first 1500 km off the NW African coast. Among the different regions all along the coast, the study area exhibits the strongest seasonal patterns for both atmospheric transport layers ŽU. Wyputta, Bremen, unpublished data.. Strongly differing situations were found during January and July 1993, when airmasses in both atmospheric layers followed opposite directions towards the CV site at both times ŽFig. 3.. During intermediate times, a dominant increase of continental airmass trajectories at the 500 hPa level occurred between winterrspring 1993 and summer 1993 ŽFig. 4a.. Since airmasses with continental provenance imply possible dust transport, highest amounts of dust within the SAL should be expected during summer from March through October 1993 ŽFig. 4a., coinciding with relatively low dust transport in the trade winds ŽFig. 4b.. Frequent trade winds of continental origins indicate that dust transport during winter is mainly routed through the lower atmosphere ŽFig. 4b.. These major changes in atmospheric circulation appear to have relevant implications for the marine lithogenic particle inventory. We assume that the initial grain size distribution of the dust Ždominated by particles between 1 and 10 mm ŽSchutz ¨ and Sebert, 1987., but with significant admixture of grains - 40 mm. is the same for both atmospheric transport layers. In fact, it has been shown by comparing measurements obtained in various desert regions that the initial size distribution of the desert dust is quite typical ŽSchutz, ¨ 1979.. Consequently, low altitude atmospheric transport leads to a significant fallout of coarse grains during winter before reaching the CV site. High altitude transport during summer causes a different situation: at the same distance from land, only grains large and heavy enough to have sufficient deposition velocities reach the ocean’s surface. The resulting mean grain size of the particle fraction entering the ocean is
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shifted towards the coarse end, and fluxes are lower due to the fact that only a small part of the mineral aerosol reach the ocean at the CV site due to mainly dry deposition. In this area, it could be considered that most of the deposition flux of atmospheric dust operates via dry deposition since there is no rainfall between January and July. During the rainy period ŽAugust to December. the rainfall is very limited both in intensity Žless than 80 mm. and in number of rainy days Žbetween 5 and 11. Žfrom meteorological data recorded by the Servicio Nacional de Meteorologia and Geofisica for 1993, 1994, 1995.. A further reduction of fine grains may occur when low layer winds blowing from the opposite directions winnow out the fine material from the falling fraction. Dust fallout from high altitudes then has to pass several layers of moving air during its way down, so it seems likely that relatively large and heavy particles arrive faster at surface level without less horizontal drift compared to smaller fractions. Concurrent with the increase of continental winds at high altitudes Žaround 500 hPa., mean grain sizes in the trapped lithogenic particles increase between spring and summer 1993 from 11 to 19 mm ŽFig. 4c.. The link between atmospheric transport and grain size is corroborated by the good correlation of the percentage of a size fraction in the trapped lithogenic particles vs. the frequency of continental winds at both pressure levels: highly abundant fine grains Žsize fraction 6–11 mm. correspond to frequent continental trade winds ŽFig. 5a., while a high concentration of coarse grains Žsize fraction 20–63 mm. correlates with frequent continental winds in the SAL ŽFig. 5b.. 4. Conclusions From the comparison of the different datasets we can conclude that two general atmospheric transport situations are reflected in the lithogenic particle signal recorded in the deep-ocean at the CV site. Ža. During winter and spring, dust is mainly transported in continental trade winds and causes high atmospheric Al and dust concentrations at the surface level. Dust fallout leads to a high input of lithogenic particles to the upper ocean and high Al and lithogenic particle fluxes at 1000 m water depth. The flux of coarse grains and the resulting mean
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diameters of the lithogenic particle fraction are low at this time of the year. Žb. During summer, continental winds at high altitudes in the SAL are the main agent of dust transport. Only a small fraction of dust particles settles out from the transport layer bringing about low Al and dust concentrations at surface level. Consequently, lower Al and lithogenic particle fluxes are observed in the trap concurrent with higher fluxes of coarse grains and larger mean diameters as compared to the winter situation. Although the data obtained in this study represent only one year sampling time, the seasonal situations found here confirm results from recent studies ŽSwap et al., 1996; Chiapello et al., 1997. and clearly show the significance of winterly dust emission and lowlayer transport in change with high altitude dust outbreaks during summer as a general pattern in this area. From a more general point of view, our results imply that different fractions of dust enter the pelagic ocean probably depending on the altitude level in which atmospheric transport occurs. From continental sources, proximal and distant ocean regions within the atmospheric transport path receive different fractions of dust, not only depending on the distance from the source but also on the seasonal change of the atmospheric circulation. This may have implications for the interpretation of deep-sea particle fluxes and the sedimentary record, in particular regarding the question wether increased glacial dust fluxes result from orbital-scale changes in continental humidity. However, results reported here confirm studies which attribute changes in glacial dust fluxes to changes in wind path and strength ŽSarnthein et al., 1982; Ruddiman, 1997. rather than to changes in source area aridity ŽTiedemann et al., 1989; Rea, 1990.. Because Saharan dust has been traced over the entire tropical Atlantic Ocean finally reaching the Caribbean ŽProspero, 1990. and the Amazon Basin ŽSwap et al., 1992., it is likely that open ocean particle fluxes do as well react to short-term dust fallout from atmospheric long range transport, as found during recent studies in the Sargasso Sea ŽJickels et al., 1998.. But regarding mass fluxes and sediment accumulation rates, proximal deep-ocean areas receive a higher impact of low altitude, short
range-transported dust, probably concurrent with a higher temporal variability and shorter response time compared to the impact of the fraction further transported at higher altitudes. A further investigation of the trapped material regarding its mineralogical composition may give valuable informations on both the atmospheric and oceanic transport pathways as well as the source regions of the dust. In particular, clay ratios appear to be a stable and relevant proxy in assessing the origin of dust from arid and semi-arid regions in marine sediments Že.g., Chamley, 1989. and atmospheric dust samples ŽCaquineau et al., 1998.. This approach will be followed in the future and compared to results from grain size analysis as published in this paper. Acknowledgements We thank G. Wefer for initial support and helpful discussion, and G. Kuhn and B. Diekmann for support during grain-size analysis. R. Stein and two anonymous reviewers gave valuable suggestions which improved the manuscript. Parts of this work were financed by the Deutsche Forschungsgemeinschaft in the frame of the Sonderforschungsbereich 261 at the University of Bremen ŽContribution No. 210.. References Aharonson, N., Karasikov, N., Roitberg, M., Shamir, J., 1986. GALAI-CIS-1—A novel approach to aerosol particle size analysis. Journal of Aerosol Science 17, 530–536. Bayvel, L.P., Jones, A.R., 1981. Electromagnetic Scattering and Its Applications. Applied Science, London, 289 pp. Berger, W.H., Keir, R.S., 1984. Glacial–Holocene changes in atmospheric CO 2 and the deep-sea record. In: Hansen, J.E., Takahashi, T. ŽEds.., Climate Processes and Climate Sensitivity. AGU Geophysical Monograph 29 Ž5. 337–351. Buat-Menard, P., Davies, J., Remoudaki, E., Miquel, J.C., Bergametti, G., Lambert, C.E., Etzat, U., Quetel, C., La Rosa, J., Fowler, S.W., 1989. Non-steady-state biological removal of atmospheric particles from Mediterranean surface waters. Nature 340, 131–134. Caquineau, S., Gaudichet, A., Gomes, L., Magontheir, M.C., Chatenet, B., 1998. Saharan dust: clay ratio as a relevant tracer to assess the origin of soil-derived dust. Geophysical Research Letters 25, 983–986. Carlson, T.N., Prospero, J.M., 1972. The large-scale movement of
V. Ratmeyer et al.r Marine Geology 159 (1999) 241–252 Saharan air outbreaks over the northern equatorial Atlantic. Journal of Applied Meteorology 11, 283–297. Chamley, H., 1989. Clay Sedimentology. Springer, Berlin, 623 pp. Chiapello, I., 1996. Les aerosols atmospheriques au-dessus de ´ ´ l’Atlantique nord tropical: approche physico-chimique et meteorogique. Evaluation de la contribution des differentes ´´ ´ especes a` l’epaisseur optique en aerosol. Thesis, Universite´ ` ´ ´ Paris VII-Denis Diderot. Chiapello, I., Bergametti, G., Gomes, L., Chatenet, B., Dulac, F., Pimenta, J., Santos Suares, E., 1995. An additional low layer transport of Sahelian and Saharan dust transport over the north-eastern Tropical Atlantic. Geophysical Research Letters 22, 3191–3194. Chiapello, I., Bergametti, G., Chatenet, B., Bousquet, P., Dulac, F., Soares, E.S., 1997. Origins of African dust transported over the northeastern tropical Atlantic. Journal of Geophysical Research 102 D12, 13701–13709. d’Almeida, G.A., Jaenicke, R., 1984. Sahara desert dust transport to the south. Proc. 11th Int. Conf. on Atm. Aerosols, Condension and Ice Nuclei, 3–8 September, Budapest, Preprint Vol. 1, pp. 185–190. Deuser, W.G., 1986. Seasonal and interannual variations in the deep-water particle fluxes in the Sargasso Sea and their relation to surface hydrography. Deep-Sea Research 33, 225–246. Dodge, L.G., 1984. Calibration of the Malvern particle sizer. Applied Optics 23, 2415–2419. Donaghay, P.L., Liss, P.S., Duce, R.A., Kester, D.R., Hanson, A.K., Villareal, T., Tindale, N.W., Gifford, D.J., 1991. The role of episodic atmospheric nutrient inputs in the chemical and biological dynamics of oceanic ecosystems. Oceanography 4, 62–70. Duce, R.A., Liss, P.S., Merill, J.T., Atlas, E.L., Buat-Menard, P., Hicks, B.B., Miller, J.M., Prospero, J.M., Arimoto, R., Church, T.M., Ellis, W., Galloway, J.N., Hansen, L., Jickels, T.D., Knap, A.H., Reinhardt, K.H., Schneider, B., Soudine, A., Tokos, J.J., Tsunogai, S., Wollast, R., Zhou, M., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochemical Cycles 5, 193–259. Eliassen, A., 1976. The Trajectory Model: A Technical Description. Norwegian Institute of Air Research, Kjeller, Norway. Fischer, G., Wefer, G., 1991. Sampling, preparation and analysis of marine particulate matter. In: Hurd, D.C., Spencer, W. ŽEds.., The Analysis and Characterization of Marine Particles. AGU, WA, pp. 391–397. Fischer, G., Donner, B., Ratmeyer, V., Davenport, R., Wefer, G., 1996. Distinct year-to-year particle flux variations off Cape Blanc during 1988-1991: relation to d18 O-deduced sea-surface temperatures and trade winds. Journal of Marine Research 54, 73–98. Fowler, S.W., Knauer, G.A., 1986. Role of large particles in the transport of elements and organic compounds through the oceanic water column. Prog. Oceanogr. 16, 147–194. Helmers, E., Schrems, O., 1995. Wet deposition of metals to the tropical North and the South Atlantic ocean. Atmospheric Environment 29, 2475–2484. IPCC ŽIntergovernmental Panel on Climate Change., 1994. Radia-
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tive forcing of climate change 1994. Report from the Scientific Assessment Working Group ŽWGI., UNEPrWMO. Jaenicke, R., Schutz, ¨ L., 1978. Comprehensive study of physical and chemical properties of the surface aerosols in the Cape Verde Island region. Journal of Geophysical Research 83, 3585–3599. Jickels, T.D., 1995. Atmospheric inputs of metals and nutrients to the oceans: their magnitude and effects. Marine Chemistry 48, 199–214. Jickels, T.D., Dorling, S., Deuser, W.G., Church, T.M., Arimoto, R., Prospero, J.M., 1998. Air-borne dust fluxes to a deep water sediment trap in the Sargasso Sea. Global Biogeochemical Cycles 12, 311–320. Johnson, L.R., 1979. Mineralogical dispersal patterns of North Atlantic deep-sea sediments with particular reference to eolian dusts. Marine Geology 29, 335–345. Koopmann, B., 1981. Die Sedimentation von Saharastaub im subtropischen Nordatlantik wahrend der letzten 25.000 Jahre. ¨ Meteor Forschungs-Ergebnisse C 35, 23–59. Losno, R., Bergametti, G., Movier, G., 1987. Determination of optima conditions for atmospheric aerosol analyses by X-ray fluorescence. Environ. Technol. Lett. 8, 77–87. Marticorena, B., Bergametti, G., 1996. Two-year simulations of seasonal and interannual changes of the Saharan dust emission. Geophysical Research Letters 23, 1921–1924. Martin, J.H., Fitzwater, S.E., 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subantarctic. Nature 331, 341–343. McCave, I.N., Bryant, R.J., Cook, H.F., Coughanowr, C.A., 1986. Evaluation of a laser-diffraction-size analyzer for use with natural sediments. Journal of Sedimentary Petrology 56, 561– 564. McCave, I.N., Manighetti, B., Robinson, S.G., 1995. Sortable silt and fine sediment sizercomposition slicing: parameters for palaeocurrent speed and paleoceanography. Palaeoceanography 10, 593–610. Michaels, A.F., Olson, D., Sarmiento, J.L., Ammerman, J.W., Fanning, K., Jahnke, R., Knap, A.H., Lipschultz, F., Prospero, J.M., 1996. Inputs, losses and transformation of nitrogen and phosphorous in the pelagic north Atlantic Ocean. Biogeochemistry 35, 27–73. Mittelstaedt, E., 1991. The ocean boundary along the northwest African coast. Prog. Oceanogr. 26, 307–355. Morales, C., 1979. Saharan Dust: Mobilisation, Transport, Deposition—SCOPE Report 14. Wiley, New York, 298 pp. Neuer, S., Ratmeyer, V., Davenport, R., Fischer, G., Wefer, G., 1997. Deep water particle flux in the Canary Island region: seasonal variation in relation to long-term satellite derived pigment data and lateral sources. Deep-Sea Research I 44, 1451–1466. Petschick, R., Kuhn, G., Gingele, F., 1996. Clay mineral distribution in surface sediments of the South Atlantic: sources, transport and relation to oceanography. Marine Geology 130, 203–339. Petterssen, S., 1956. Weather Analysis and Forecasting. McGraw-Hill, New York. Prospero, J.M., 1990. Mineral–aerosol transport to the North
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Atlantic and North Pacific: the impact of African and Asian sources. In: Knap, A.H. ŽEd.., The Long Range Atmospheric Transport of Natural and Contaminant Substance. Kluwer, the Netherlands, pp. 59–86. Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London, 334 pp. Ramaswamy, V., Nair, R.R., Manganini, S., Haake, B., Ittekott, V., 1991. Lithogenic fluxes to the deep Arabian Sea measured by sediment traps. Deep-Sea Research 38, 169–184. Ratmeyer, V., Fischer, G., Wefer, G., in press. Lithogenic particle fluxes and grain size distributions in the deep ocean off NW Africa: implications for seasonal changes of aeolian dust input and downward transport. Deep-Sea Research Part I. Rea, D.K., 1990. Aspects of atmospheric circulation: the Late Pleistocene Ž0–950,000 yr. records of eolian deposition in the Pacific Ocean. Palaeogeogr. Plaeoclimatol. Palaeoecol. 78, 217–227. Ruddiman, W.F., 1997. Tropical Atlantic terrigenous fluxes since 25,000 yrs B.P. Marine Geology 136, 189–207. Sarnthein, M., Thiede, J., Pflaumann, U., Erlenkeuser, H., Futterer, ¨ D., Koopmann, B., Lange, H., Seibold, E., 1982. Atmospheric and oceanic circulation patterns off Northwest Africa during the past 25 million years. In: Rad, U., Hinz, K., Sarnthein, M., Seibold, E. ŽEds.., Geology of the Northwest African Continental Margin. Springer, Berlin, pp. 545–604. Schutz, ¨ L., 1979. Saharan dust transport over the north Atlantic Ocean. In: Morales, C. ŽEd.., Saharan Dust. Wiley, New York, pp. 267–277.
Schutz, ¨ L., Sebert, M., 1987. Mineral aerosol and source identification. Journal of Aerosol Science 18, 1–10. Semmelhack, W., 1934. Die Staubfalle ¨ im nordwest-afrikanischen Gebiet des Atlantischen Ozeans. Annalen der Hydrographie 62, 273–277. Stein, R., 1985. The Post-Eocene sediment record of DSDP-site 316: implications for African climate and plate tectonic drift. Geol. Soc. Am. Memoir 163, 305–315. Stein, R., Sarnthein, M., 1984. Late Neogene events of atmospheric and oceanic circulation offshore Northwest Africa: high-resolution record from deep-sea sediments. Paleoecology of Africa 16, 9–36. Swap, R., Garstang, M., Greco, S., Talbot, R., Kallberg, P., 1992. Saharan dust in the Amazon Basin. Tellus 44B, 133–149. Swap, R., Ulanski, S., Cobbett, M., Garstang, M., 1996. Temporal and spatial characteristics of Saharan dust outbreaks. Journal of Geophysical Research 101 D2, 4205–4220. Tegen, I., Lewis, A.A., Fung, I., 1996. The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419–422. Tiedemann, R., Sarnthein, M., Stein, R., 1989. Climatic changes in the western Sahara: aeolo-marine sediment record of the last 8 million years ŽSites 657–661.. Proc. ODP Sci. Results 108, 241–261. Wefer, G., Fischer, G., 1993. Seasonal patterns of vertical particle flux in equatorial and coastal upwelling areas of the eastern Atlantic. Deep-Sea Research 40, 1613–1645.
Seasonal impact of mineral dust on deep-ocean particle flux in the eastern subtropical Atlantic Ocean V. Ratmeyer
a,)
, W. Balzer b, G. Bergametti c , I. Chiapello c , G. Fischer a , U. Wyputta a
a
c
Fachbereich Geowissenschaften, UniÕersitat ¨ Bremen, Klagenfurter Strasse, 28359 Bremen, Germany b Fachbereich Biologier Chemie, UniÕersitat ¨ Bremen, Bibliothekstrasse, 28359 Bremen, Germany Laboratoire InteruniÕersitaire des Systemes Atmospheriques, URA CNRS 1404, UniÕersites 61 ` ´ ´ Paris, Centre Multidisciplinaire de Creteil, ´ aÕ du General ´ ´ de Gaulle, 94010 Creteil ´ Cedex, France Received 9 January 1998; accepted 24 November 1998
Abstract Seasonal lithogenic particle and Al fluxes were obtained from a deep-ocean sediment trap deployment during 1992 and 1993 off NW Africa, and were compared concurrently with atmospheric Al concentrations and two-dimensional backward trajectories of windfields from two barometric levels in the lower and mid troposphere. Marine Al fluxes, lithogenic particle fluxes and grain size distributions in the area were found to be directly linked to airmass pathways and surface mineral aerosol concentrations. At 1000 m water depth, highest Al fluxes Ž10.77 mg my2 dayy1 ., lithogenic particle fluxes Ž99.25 mg my2 dayy1 . and smallest mean grain sizes Ž11.9 mm. occurred during the winter and spring season, concurrent with highest atmospheric dust load and Al-concentrations Ž15 300 ng my3 . in the lower troposphere. A strong seasonal change of the main atmospheric dust transport from low altitude winds during winterrspring to higher altitudes during summer is clearly reflected at depth by a significant coarsening of mean grain sizes Ž18.6 mm. and lowest Al Ž0.81 mg my2 dayy1 . and lithogenic particle fluxes Ž11.3 mg my2 dayy1 . found in the sediment traps. The comparison of marine, atmospheric and model derived data used within this study highlights the close temporal coupling between atmospheric dust transport and the deep-ocean particle stock. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Atlantic Ocean; Cape Verde; atmospheric dust; particle flux; grain size
1. Introduction Atmospheric transport of mineral aerosol from continental sources is currently discussed to have an essential impact on the global climate variability both via albedo effects on the atmospheric heat budget ŽTegen et al., 1996. and through a variety of )
Corresponding author. Tel.: q49-421-218-7762; Fax: q49421-218-3116; E-mail: [email protected]
effects on the marine organic carbon cycle. Long term changes of the dust supply to the deep-sea over timescales of 10 3 to 10 6 years are recorded in marine sediments and provide valuable informations for paleoclimatic reconstructions such as glacialrinterglacial climatic changes ŽSarnthein et al., 1982; Stein and Sarnthein, 1984; Ruddiman, 1997., paleoCO 2 variations ŽBerger and Keir, 1984. and watermass distributions ŽPetschick et al., 1996.. On shorter time scales, the marine particle inventory is partly
0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 8 . 0 0 1 9 7 - 2
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determined by the aeolian supply of limiting nutrients and their effect on marine biomass production ŽMartin and Fitzwater, 1988; Donaghay et al., 1991., effects on nitrogen fixation ŽMichaels et al., 1996. and the scavenging of heavy dust grains by organic particles. The latter is discussed to increase the settling speed of aggregates built in the upper ocean ŽRamaswamy et al., 1991. and thus may enhance the downward transport of particulate organic carbon to the deep-sea, namely the efficiency of the biological pump ŽFowler and Knauer, 1986.. A strong coupling of particle fluxes in the deep-sea with seasonal changes of surface winds and primary productivity in the upper ocean was found during sediment trap experiments in many regions of the world ocean Že.g., Deuser, 1986; Wefer and Fischer, 1993.. But, so far only few data show to what extent vertical fluxes of particulate matter reflect short-term or seasonal changes of the atmospheric dust input. In the Mediterranean Sea ŽBuat-Menard et al., 1989., atmospheric Al fluxes could be traced down to 200 m water depth with a time delay in the order of 1 week and imply a fast biogenic vertical transport in the upper ocean layer. Results from long-term experiments in the Sargasso Sea ŽJickels et al., 1998. indicate that atmospheric Al and deep-sea lithogenic particle fluxes are linked on an annual basis, with variations for both compartments in the subannual to seasonal timescale. In a general view, prominent sites for studying how atmospherically controlled dust deposition affects the deep-sea particle inventory are areas where high productivity coincides with significant atmospheric dust input, such as the eastern tropical Atlantic. Here, along the northwest African coast Ž11– 308N., extensive atmospheric dust transport from the dry areas of the coastal Morocco, the Sahel and Sahara regions is well known Žsummary in Morales, 1979; Prospero, 1990. and contributes up to 660 Mt of mineral aerosol to the subtropical and tropical Atlantic ŽMarticorena and Bergametti, 1996.. On a global scale, this amount supplies one third to half of the roughly estimated 0.9 to about 2 billion tons of global aeolian dust input to the world ocean Žd’Almeida and Jaenicke, 1984; Duce et al., 1991; IPCC, 1994.. Concurrently, nutrient upwelling leads to almost year-round high biological productivity ŽMittelstaedt, 1991.. This enhanced primary production is
discussed to provide an effective vehicle to transport dust to the deep-ocean via biogenic aggregation and fast vertical flux, with aggregate settling velocities in the order of 100 m dayy1 ŽFischer et al., 1996; Neuer et al., 1997; Ratmeyer et al., in press.. Further, the imprint of mineral dust on the marine sediments in this area reflects past global and regional climatic changes and was subject to a number of investigations Že.g., Koopmann, 1981; Sarnthein et al., 1982; Stein, 1985.. In this study we show that deep-ocean Al fluxes, lithogenic particle fluxes and their grain sizes can reflect short-term, seasonal changes in the aeolian dust supply. This also implies that the vertical transport through the watercolumn may be fast and effective enough to reflect mass concentrations and grain size spectra of dust entering the surface ocean. The study area is situated within the first 1500 km off the African coast, a distance where most of the dust leaving the Saharan sources is assumed to settle out.
2. Material and methods 2.1. Sediment trap samples Particle fluxes were recorded with sediment traps between October 1992 and October 1993 at 1000 m water depth near the Cape Verde Žsite CV: 11829.0X N; 21801.0X W; 4900 m waterdepth.. Traps were of the type KIEL SMT 234 and cone-shaped with 0.5 m2 opening. Prior to and after deployment, sample cups were poisoned with mercuric chloride. After trap recovery, samples were wet-sieved Ž1 mm., splitted, treated and analyzed as described elsewhere ŽFischer and Wefer, 1991.. The lithogenic fraction was calculated as: Lith flux s Total flux y Ž opal flux q carbonate flux qorganic matter flux . where the analytical errors associated with opal and carbonate determination were negligible. Organic matter was converted from C org by factor 2.0. Due to changes in the plankton composition, this conversion includes a maximum error of "10%. Al and Fe in the trap samples were determined by flame AAS after subjecting the trapped particles to a
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total dissolution pressure digestion in a microwave oven using HFrHNO 3 . Based on the analysis of certified reference materials the precision was estimated to be better than 6% for Al and Fe. For grain size measurements on the lithogenic fraction, 1r16 to 1r5 split of each sample was treated with acetic acid and H 2 O 2 to remove carbonate and organic matter. However, dust from Morocco and Sahel may contain neglible amounts of dolomite ŽJohnson, 1979; Pye, 1987. which is not preserved after acid treatment of the samples. The remaining fraction was neutralized with NH 3 and wet sieved. Biogenic opal contributed between 5 and 14% Žmean 6.9%. to the total flux and was not removed due to preservation of clay minerals for later analysis. Grain size analysis was performed on a CIS-GALAI laser particle analyzer ŽAharonson et al., 1986.. McCave et al. Ž1986. and McCave et al. Ž1995. found a significant underestimation of the clay-fraction Ž- 2 mm. concurrent with a constant ‘ghost’-peak in the fraction 2–10 mm as a general problem of the laser diffraction method Žsee also Bayvel and Jones, 1981; Dodge, 1984.. Although the instruments based on laser diffraction are not directly comparable to the CIS-GALAI particle analyzer used in this study, we decided to exclude the smallest silt-sized fraction Ž2–6 mm. of particles from the dataset. Due to the very low absolute particle concentration in most samples, the clay fraction Žgrains - 2 mm. could not be quantitatively separated by decantation or filtration. Thus, a direct measurement of the total size fraction was not possible. Percentages were normalized to the fraction 6–63 mm. Due to these limitations a direct comparison with grain sizes of atmospheric dust Žmean median size of 1.8 mm at Sal Island; Chiapello, 1996., however, remained impossible. None of the samples contained a measurable amount of grains ) 63 mm. A more detailed description of the method is published elsewhere ŽRatmeyer et al., in press.. 2.2. Atmospheric dust samples Dust samples at Sal Island, Cape Verde Archipelago Ž16845X N; 22857X W., were obtained between 1992 and 1994 by bulk filtration on Nuclepore@ 0.4 mm pore size using a flow rate of about 1 m3 hy1 Ž24 h sampley1 .. Aluminum particulate
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concentrations in atmospheric dust were determined directly on the filter by X-ray spectrometry fluorescence ŽSiemens SRS 303. ŽLosno et al., 1987; Chiapello et al., 1995.. 2.3. Trajectory computations Daily atmospheric backward trajectories were computed for site CV from October 1992 through October 1993 at 850 and 500 hPa barometric levels with a global two-dimensional isobaric model. The precursor of this model was developed at the Norwegian Meteorological Institute ŽNMI. ŽEliassen, 1976.. The model computes 4-day isobaric backward trajectories from zonal and meridional wind components ŽPetterssen, 1956.. Wind data were obtained from the European Centre for Medium Range Weather Forecasts ŽECMWF..
3. Results and discussion 3.1. Seasonal Õariability Atmospheric Al concentrations of the mineral aerosol ŽFig. 1a. in the surface layer of Sal Island were found to exhibit a strong seasonal pattern. Monthly averaged concentrations were highest from December Ž4600 ng my3 . through January Ž15 300 ng my3 ., and decreased significantly during the summer months Žminimum in June: 290 ng my3 .. Although situated in a more marginal position of the general dust outbreak path, a similar seasonal change was recorded in sediment trap samples at the CV site off Cape Verde. Here, changes of the particulate Al flux at a water depth of 1000 m ŽFig. 1b. follow the overall pattern of atmospheric Al concentrations. High mean fluxes of between 8.1 ŽFebruary 1993. and 10.8 mg my2 dayy1 ŽMarch 1993. prevailed during winter and spring, while much lower mean fluxes of Al down to 0.8 mg my2 dayy1 ŽAugust 1993. occurred during summer. However, a detailed peak-to-peak correlation between atmospheric Al concentrations and marine Al fluxes remains impossible, both due to the different sampling intervals as well as different positions of atmospheric and marine sampling sites relative to the atmospheric path. Yet
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X X Fig. 1. Ža. Al concentrations measured at surface level on Sal Island, Cape Verde Archipelago Ž16845 N; 22857 W.: monthly mean values are calculated from discontinuous daily data obtained during 1992 and 1993. Žb. Al fluxes measured in sediment traps at 1000 m water X X depth near the Cap Verde Žsite CV: 11829.0 N; 21801.0 W.. Žc. Lithogenic particle fluxes recorded in the sediment trap samples. Žd. Lithogenic particle fluxes of the size class 20–63 mm. Solid lines represent a 3rd order polynom.
we believe, that the overall pattern of seasonal changes in the dust supply to the area is reflected in both data sets. Marine Al fluxes show a significant correlation Ž r 2 s 0.77, n s 28. with the lithogenic particle fluxes during the entire sampling period ŽFig. 1c and Fig. 2a.. FerAl ratios in trapped material varied from
0.43 ŽAugust 1993. to 0.58 ŽOctober 1992., similar to the range of atmospheric ratios between 0.41–0.63 from January and September 1993. These ratios might be changed by differential dissolution in the ocean’s mixed layer after the deposition of dust and further during settling through the water column. Although it is not possible to assign definite values to the
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Fig. 2. Linear regression between Ža. Al and Lith flux and Žb. C org and Lith flux in the sediment trap samples.
solubility of the individual elements, a review of existing data demonstrates that Al and Fe which are predominantly associated with mineral aerosol have comparatively low solubilities ŽDuce et al., 1991..
These authors assume that 5% of the Al and 10% of the Fe that enters the ocean from the atmosphere is soluble. These percentages might be higher when the dust particles were subjected to the low pH regime
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of rain before being deposited Že.g., Jickels, 1995; Helmers and Schrems, 1995.. Since only very few
rain events of short duration occurred on Sal Island during the trap deployment, it is likely that for these
Fig. 3. Backward trajectories to the CV site of airmass pathways in the trade wind layer Ž850 hPa, solid line. and the Saharan Air Layer ŽSAL. Ž500 hPa, dashed line. in January Ža. and July Žb. 1993. Transport of dust from continental sources during winter occurs primarily at low altitudes Ža., while during summer dust is carried by high altitude winds within the SAL Žb..
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two metals most of the atmospheric signature has survived the contact with sea water until settling to the traps. In contrast to the Al fluxes, the flux of coarse lithogenic particles in the size class 20–63 mm shows a reverse pattern ŽFig. 1d.. Coarse grains contributed generally less than 12 mg my2 dayy1 Ž20–30% of the measured fraction 6–63 mm. to the lithogenic flux during winter, but increased significantly from April 1993 through July 1993 to fluxes between 15 and 32 mg my2 dayy1 Ž30–55%.. Possible changes in the vertical transport from the ocean’s surface to the trap depth, however, do not provide a satisfying explanation for this contradictional behaviour. Organic carbon and lithogenic particle fluxes correlated well during both seasons Ž r 2 s 0.63, n s 28; Fig. 2b., and indicate a rapid and
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efficient biogenic particle transport without lateral advection of terrigenous fractions at depth during the entire deployment time. Instead, we suggest that the seasonal pattern of coarse grain fluxes is mainly caused by the input of mineral dust from different atmospheric pathways, which reflects a seasonal change of atmospheric circulation patterns in this region. 3.2. Atmospheric transport Atmospheric dust transport from the African continent between 10 and 208N is known to occur either in the Saharan Air Layer ŽSAL. above the trade wind inversion Žpressure levels around 500 hPa., or in the trade winds below 2 km altitude Žaround 850 hPa. ŽSemmelhack, 1934; Carlson and Prospero, 1972;
Fig. 4. Comparison of atmospheric trajectories Ža,b. and sediment trap Žc. data from site CV between October 1992 and 1993. Frequency index of continental backward trajectories Ža. at the 500 hPa and Žb. at the 850 hPa pressure level: 1.0 stands for continental provenance and possible dust transport during all days of a sampling period. Numbers were counted from directional plots, classifying sources as continental vs. oceanic. Žc. Equivalent spherical mean diameters of the fraction 6–63 mm of lithogenic particles trapped at 1000 m water depth.
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Fig. 5. Ža. Linear regression between the abundance of lithogenic grains in the size class 20–63 mm from 1000 m water depth Žplotted as percent of the fraction 6–63 mm. and the monthly abundance of continental winds within the 500 hPa pressure level Žplotted as weeks per month in 1993.. Žb. Linear regression between the abundance of lithogenic grains in the size class 6–11 mm from 1000 m water depth Žplotted as percent of the fraction 6–63 mm. and the monthly abundance of continental winds within the 850 hPa pressure level Žplotted as weeks per month in 1993..
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Jaenicke and Schutz, ¨ 1978.. The summerly transport within the SAL is widely accepted as the main agent of dust export from the Saharan sources towards the subtropical North Atlantic Ocean Že.g., Prospero, 1990.. However, recent studies ŽChiapello et al., 1995; Chiapello et al., 1997. show the significance of low-layer dust transport in the trade winds in particular for the proximal sites within the first 1500 km off the NW African coast. Among the different regions all along the coast, the study area exhibits the strongest seasonal patterns for both atmospheric transport layers ŽU. Wyputta, Bremen, unpublished data.. Strongly differing situations were found during January and July 1993, when airmasses in both atmospheric layers followed opposite directions towards the CV site at both times ŽFig. 3.. During intermediate times, a dominant increase of continental airmass trajectories at the 500 hPa level occurred between winterrspring 1993 and summer 1993 ŽFig. 4a.. Since airmasses with continental provenance imply possible dust transport, highest amounts of dust within the SAL should be expected during summer from March through October 1993 ŽFig. 4a., coinciding with relatively low dust transport in the trade winds ŽFig. 4b.. Frequent trade winds of continental origins indicate that dust transport during winter is mainly routed through the lower atmosphere ŽFig. 4b.. These major changes in atmospheric circulation appear to have relevant implications for the marine lithogenic particle inventory. We assume that the initial grain size distribution of the dust Ždominated by particles between 1 and 10 mm ŽSchutz ¨ and Sebert, 1987., but with significant admixture of grains - 40 mm. is the same for both atmospheric transport layers. In fact, it has been shown by comparing measurements obtained in various desert regions that the initial size distribution of the desert dust is quite typical ŽSchutz, ¨ 1979.. Consequently, low altitude atmospheric transport leads to a significant fallout of coarse grains during winter before reaching the CV site. High altitude transport during summer causes a different situation: at the same distance from land, only grains large and heavy enough to have sufficient deposition velocities reach the ocean’s surface. The resulting mean grain size of the particle fraction entering the ocean is
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shifted towards the coarse end, and fluxes are lower due to the fact that only a small part of the mineral aerosol reach the ocean at the CV site due to mainly dry deposition. In this area, it could be considered that most of the deposition flux of atmospheric dust operates via dry deposition since there is no rainfall between January and July. During the rainy period ŽAugust to December. the rainfall is very limited both in intensity Žless than 80 mm. and in number of rainy days Žbetween 5 and 11. Žfrom meteorological data recorded by the Servicio Nacional de Meteorologia and Geofisica for 1993, 1994, 1995.. A further reduction of fine grains may occur when low layer winds blowing from the opposite directions winnow out the fine material from the falling fraction. Dust fallout from high altitudes then has to pass several layers of moving air during its way down, so it seems likely that relatively large and heavy particles arrive faster at surface level without less horizontal drift compared to smaller fractions. Concurrent with the increase of continental winds at high altitudes Žaround 500 hPa., mean grain sizes in the trapped lithogenic particles increase between spring and summer 1993 from 11 to 19 mm ŽFig. 4c.. The link between atmospheric transport and grain size is corroborated by the good correlation of the percentage of a size fraction in the trapped lithogenic particles vs. the frequency of continental winds at both pressure levels: highly abundant fine grains Žsize fraction 6–11 mm. correspond to frequent continental trade winds ŽFig. 5a., while a high concentration of coarse grains Žsize fraction 20–63 mm. correlates with frequent continental winds in the SAL ŽFig. 5b.. 4. Conclusions From the comparison of the different datasets we can conclude that two general atmospheric transport situations are reflected in the lithogenic particle signal recorded in the deep-ocean at the CV site. Ža. During winter and spring, dust is mainly transported in continental trade winds and causes high atmospheric Al and dust concentrations at the surface level. Dust fallout leads to a high input of lithogenic particles to the upper ocean and high Al and lithogenic particle fluxes at 1000 m water depth. The flux of coarse grains and the resulting mean
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diameters of the lithogenic particle fraction are low at this time of the year. Žb. During summer, continental winds at high altitudes in the SAL are the main agent of dust transport. Only a small fraction of dust particles settles out from the transport layer bringing about low Al and dust concentrations at surface level. Consequently, lower Al and lithogenic particle fluxes are observed in the trap concurrent with higher fluxes of coarse grains and larger mean diameters as compared to the winter situation. Although the data obtained in this study represent only one year sampling time, the seasonal situations found here confirm results from recent studies ŽSwap et al., 1996; Chiapello et al., 1997. and clearly show the significance of winterly dust emission and lowlayer transport in change with high altitude dust outbreaks during summer as a general pattern in this area. From a more general point of view, our results imply that different fractions of dust enter the pelagic ocean probably depending on the altitude level in which atmospheric transport occurs. From continental sources, proximal and distant ocean regions within the atmospheric transport path receive different fractions of dust, not only depending on the distance from the source but also on the seasonal change of the atmospheric circulation. This may have implications for the interpretation of deep-sea particle fluxes and the sedimentary record, in particular regarding the question wether increased glacial dust fluxes result from orbital-scale changes in continental humidity. However, results reported here confirm studies which attribute changes in glacial dust fluxes to changes in wind path and strength ŽSarnthein et al., 1982; Ruddiman, 1997. rather than to changes in source area aridity ŽTiedemann et al., 1989; Rea, 1990.. Because Saharan dust has been traced over the entire tropical Atlantic Ocean finally reaching the Caribbean ŽProspero, 1990. and the Amazon Basin ŽSwap et al., 1992., it is likely that open ocean particle fluxes do as well react to short-term dust fallout from atmospheric long range transport, as found during recent studies in the Sargasso Sea ŽJickels et al., 1998.. But regarding mass fluxes and sediment accumulation rates, proximal deep-ocean areas receive a higher impact of low altitude, short
range-transported dust, probably concurrent with a higher temporal variability and shorter response time compared to the impact of the fraction further transported at higher altitudes. A further investigation of the trapped material regarding its mineralogical composition may give valuable informations on both the atmospheric and oceanic transport pathways as well as the source regions of the dust. In particular, clay ratios appear to be a stable and relevant proxy in assessing the origin of dust from arid and semi-arid regions in marine sediments Že.g., Chamley, 1989. and atmospheric dust samples ŽCaquineau et al., 1998.. This approach will be followed in the future and compared to results from grain size analysis as published in this paper. Acknowledgements We thank G. Wefer for initial support and helpful discussion, and G. Kuhn and B. Diekmann for support during grain-size analysis. R. Stein and two anonymous reviewers gave valuable suggestions which improved the manuscript. Parts of this work were financed by the Deutsche Forschungsgemeinschaft in the frame of the Sonderforschungsbereich 261 at the University of Bremen ŽContribution No. 210.. References Aharonson, N., Karasikov, N., Roitberg, M., Shamir, J., 1986. GALAI-CIS-1—A novel approach to aerosol particle size analysis. Journal of Aerosol Science 17, 530–536. Bayvel, L.P., Jones, A.R., 1981. Electromagnetic Scattering and Its Applications. Applied Science, London, 289 pp. Berger, W.H., Keir, R.S., 1984. Glacial–Holocene changes in atmospheric CO 2 and the deep-sea record. In: Hansen, J.E., Takahashi, T. ŽEds.., Climate Processes and Climate Sensitivity. AGU Geophysical Monograph 29 Ž5. 337–351. Buat-Menard, P., Davies, J., Remoudaki, E., Miquel, J.C., Bergametti, G., Lambert, C.E., Etzat, U., Quetel, C., La Rosa, J., Fowler, S.W., 1989. Non-steady-state biological removal of atmospheric particles from Mediterranean surface waters. Nature 340, 131–134. Caquineau, S., Gaudichet, A., Gomes, L., Magontheir, M.C., Chatenet, B., 1998. Saharan dust: clay ratio as a relevant tracer to assess the origin of soil-derived dust. Geophysical Research Letters 25, 983–986. Carlson, T.N., Prospero, J.M., 1972. The large-scale movement of
V. Ratmeyer et al.r Marine Geology 159 (1999) 241–252 Saharan air outbreaks over the northern equatorial Atlantic. Journal of Applied Meteorology 11, 283–297. Chamley, H., 1989. Clay Sedimentology. Springer, Berlin, 623 pp. Chiapello, I., 1996. Les aerosols atmospheriques au-dessus de ´ ´ l’Atlantique nord tropical: approche physico-chimique et meteorogique. Evaluation de la contribution des differentes ´´ ´ especes a` l’epaisseur optique en aerosol. Thesis, Universite´ ` ´ ´ Paris VII-Denis Diderot. Chiapello, I., Bergametti, G., Gomes, L., Chatenet, B., Dulac, F., Pimenta, J., Santos Suares, E., 1995. An additional low layer transport of Sahelian and Saharan dust transport over the north-eastern Tropical Atlantic. Geophysical Research Letters 22, 3191–3194. Chiapello, I., Bergametti, G., Chatenet, B., Bousquet, P., Dulac, F., Soares, E.S., 1997. Origins of African dust transported over the northeastern tropical Atlantic. Journal of Geophysical Research 102 D12, 13701–13709. d’Almeida, G.A., Jaenicke, R., 1984. Sahara desert dust transport to the south. Proc. 11th Int. Conf. on Atm. Aerosols, Condension and Ice Nuclei, 3–8 September, Budapest, Preprint Vol. 1, pp. 185–190. Deuser, W.G., 1986. Seasonal and interannual variations in the deep-water particle fluxes in the Sargasso Sea and their relation to surface hydrography. Deep-Sea Research 33, 225–246. Dodge, L.G., 1984. Calibration of the Malvern particle sizer. Applied Optics 23, 2415–2419. Donaghay, P.L., Liss, P.S., Duce, R.A., Kester, D.R., Hanson, A.K., Villareal, T., Tindale, N.W., Gifford, D.J., 1991. The role of episodic atmospheric nutrient inputs in the chemical and biological dynamics of oceanic ecosystems. Oceanography 4, 62–70. Duce, R.A., Liss, P.S., Merill, J.T., Atlas, E.L., Buat-Menard, P., Hicks, B.B., Miller, J.M., Prospero, J.M., Arimoto, R., Church, T.M., Ellis, W., Galloway, J.N., Hansen, L., Jickels, T.D., Knap, A.H., Reinhardt, K.H., Schneider, B., Soudine, A., Tokos, J.J., Tsunogai, S., Wollast, R., Zhou, M., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochemical Cycles 5, 193–259. Eliassen, A., 1976. The Trajectory Model: A Technical Description. Norwegian Institute of Air Research, Kjeller, Norway. Fischer, G., Wefer, G., 1991. Sampling, preparation and analysis of marine particulate matter. In: Hurd, D.C., Spencer, W. ŽEds.., The Analysis and Characterization of Marine Particles. AGU, WA, pp. 391–397. Fischer, G., Donner, B., Ratmeyer, V., Davenport, R., Wefer, G., 1996. Distinct year-to-year particle flux variations off Cape Blanc during 1988-1991: relation to d18 O-deduced sea-surface temperatures and trade winds. Journal of Marine Research 54, 73–98. Fowler, S.W., Knauer, G.A., 1986. Role of large particles in the transport of elements and organic compounds through the oceanic water column. Prog. Oceanogr. 16, 147–194. Helmers, E., Schrems, O., 1995. Wet deposition of metals to the tropical North and the South Atlantic ocean. Atmospheric Environment 29, 2475–2484. IPCC ŽIntergovernmental Panel on Climate Change., 1994. Radia-
251
tive forcing of climate change 1994. Report from the Scientific Assessment Working Group ŽWGI., UNEPrWMO. Jaenicke, R., Schutz, ¨ L., 1978. Comprehensive study of physical and chemical properties of the surface aerosols in the Cape Verde Island region. Journal of Geophysical Research 83, 3585–3599. Jickels, T.D., 1995. Atmospheric inputs of metals and nutrients to the oceans: their magnitude and effects. Marine Chemistry 48, 199–214. Jickels, T.D., Dorling, S., Deuser, W.G., Church, T.M., Arimoto, R., Prospero, J.M., 1998. Air-borne dust fluxes to a deep water sediment trap in the Sargasso Sea. Global Biogeochemical Cycles 12, 311–320. Johnson, L.R., 1979. Mineralogical dispersal patterns of North Atlantic deep-sea sediments with particular reference to eolian dusts. Marine Geology 29, 335–345. Koopmann, B., 1981. Die Sedimentation von Saharastaub im subtropischen Nordatlantik wahrend der letzten 25.000 Jahre. ¨ Meteor Forschungs-Ergebnisse C 35, 23–59. Losno, R., Bergametti, G., Movier, G., 1987. Determination of optima conditions for atmospheric aerosol analyses by X-ray fluorescence. Environ. Technol. Lett. 8, 77–87. Marticorena, B., Bergametti, G., 1996. Two-year simulations of seasonal and interannual changes of the Saharan dust emission. Geophysical Research Letters 23, 1921–1924. Martin, J.H., Fitzwater, S.E., 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subantarctic. Nature 331, 341–343. McCave, I.N., Bryant, R.J., Cook, H.F., Coughanowr, C.A., 1986. Evaluation of a laser-diffraction-size analyzer for use with natural sediments. Journal of Sedimentary Petrology 56, 561– 564. McCave, I.N., Manighetti, B., Robinson, S.G., 1995. Sortable silt and fine sediment sizercomposition slicing: parameters for palaeocurrent speed and paleoceanography. Palaeoceanography 10, 593–610. Michaels, A.F., Olson, D., Sarmiento, J.L., Ammerman, J.W., Fanning, K., Jahnke, R., Knap, A.H., Lipschultz, F., Prospero, J.M., 1996. Inputs, losses and transformation of nitrogen and phosphorous in the pelagic north Atlantic Ocean. Biogeochemistry 35, 27–73. Mittelstaedt, E., 1991. The ocean boundary along the northwest African coast. Prog. Oceanogr. 26, 307–355. Morales, C., 1979. Saharan Dust: Mobilisation, Transport, Deposition—SCOPE Report 14. Wiley, New York, 298 pp. Neuer, S., Ratmeyer, V., Davenport, R., Fischer, G., Wefer, G., 1997. Deep water particle flux in the Canary Island region: seasonal variation in relation to long-term satellite derived pigment data and lateral sources. Deep-Sea Research I 44, 1451–1466. Petschick, R., Kuhn, G., Gingele, F., 1996. Clay mineral distribution in surface sediments of the South Atlantic: sources, transport and relation to oceanography. Marine Geology 130, 203–339. Petterssen, S., 1956. Weather Analysis and Forecasting. McGraw-Hill, New York. Prospero, J.M., 1990. Mineral–aerosol transport to the North
252
V. Ratmeyer et al.r Marine Geology 159 (1999) 241–252
Atlantic and North Pacific: the impact of African and Asian sources. In: Knap, A.H. ŽEd.., The Long Range Atmospheric Transport of Natural and Contaminant Substance. Kluwer, the Netherlands, pp. 59–86. Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London, 334 pp. Ramaswamy, V., Nair, R.R., Manganini, S., Haake, B., Ittekott, V., 1991. Lithogenic fluxes to the deep Arabian Sea measured by sediment traps. Deep-Sea Research 38, 169–184. Ratmeyer, V., Fischer, G., Wefer, G., in press. Lithogenic particle fluxes and grain size distributions in the deep ocean off NW Africa: implications for seasonal changes of aeolian dust input and downward transport. Deep-Sea Research Part I. Rea, D.K., 1990. Aspects of atmospheric circulation: the Late Pleistocene Ž0–950,000 yr. records of eolian deposition in the Pacific Ocean. Palaeogeogr. Plaeoclimatol. Palaeoecol. 78, 217–227. Ruddiman, W.F., 1997. Tropical Atlantic terrigenous fluxes since 25,000 yrs B.P. Marine Geology 136, 189–207. Sarnthein, M., Thiede, J., Pflaumann, U., Erlenkeuser, H., Futterer, ¨ D., Koopmann, B., Lange, H., Seibold, E., 1982. Atmospheric and oceanic circulation patterns off Northwest Africa during the past 25 million years. In: Rad, U., Hinz, K., Sarnthein, M., Seibold, E. ŽEds.., Geology of the Northwest African Continental Margin. Springer, Berlin, pp. 545–604. Schutz, ¨ L., 1979. Saharan dust transport over the north Atlantic Ocean. In: Morales, C. ŽEd.., Saharan Dust. Wiley, New York, pp. 267–277.
Schutz, ¨ L., Sebert, M., 1987. Mineral aerosol and source identification. Journal of Aerosol Science 18, 1–10. Semmelhack, W., 1934. Die Staubfalle ¨ im nordwest-afrikanischen Gebiet des Atlantischen Ozeans. Annalen der Hydrographie 62, 273–277. Stein, R., 1985. The Post-Eocene sediment record of DSDP-site 316: implications for African climate and plate tectonic drift. Geol. Soc. Am. Memoir 163, 305–315. Stein, R., Sarnthein, M., 1984. Late Neogene events of atmospheric and oceanic circulation offshore Northwest Africa: high-resolution record from deep-sea sediments. Paleoecology of Africa 16, 9–36. Swap, R., Garstang, M., Greco, S., Talbot, R., Kallberg, P., 1992. Saharan dust in the Amazon Basin. Tellus 44B, 133–149. Swap, R., Ulanski, S., Cobbett, M., Garstang, M., 1996. Temporal and spatial characteristics of Saharan dust outbreaks. Journal of Geophysical Research 101 D2, 4205–4220. Tegen, I., Lewis, A.A., Fung, I., 1996. The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419–422. Tiedemann, R., Sarnthein, M., Stein, R., 1989. Climatic changes in the western Sahara: aeolo-marine sediment record of the last 8 million years ŽSites 657–661.. Proc. ODP Sci. Results 108, 241–261. Wefer, G., Fischer, G., 1993. Seasonal patterns of vertical particle flux in equatorial and coastal upwelling areas of the eastern Atlantic. Deep-Sea Research 40, 1613–1645.