DAVID K. REA AND STEVEN A. HOVAN
University of Michigan
THOMAS R. JANECEK
Texas A&M University
The eolian flux of terrigenous minerals to the pelagic ocean has been measured in about 30 locations in the Pacific, Atlantic, and Indian Oceans. Dust fluxes vary by three orders of magnitude from more than 1000 mg/cm2/kyr directly downwind from the major dust sources in China and North Africa to 1 mg/cm2/kyr over much of the South Pacific and southern Indian Oceans. In both the Atlantic and Pacific oceans there is an order of magnitude reduction in the deposition rates of eolian dust as one proceeds south across the Intertropical Convergence Zone (ITCZ). Dividing the total dust flux value of Prospero (1981) by the area of the global ocean results in an average deposition rate of 200 mg/cm2/kyr, but this value is too low for the nearshore regions and a factor of 4 or 5 high for the pelagic ocean.
Detailed downcore records of eolian flux to the northwest Pacific document much higher fluxes during glacial times and accumulation minima during interglacials; this accumulation pattern matches the loess/soil stratigraphy of central China. Cores raised from just south of the ITCZ in the Pacific exhibit greater dust fluxes during interglacial times, matching the known aridity pattern of northwest South America. We observe a shift from the present southern hemisphere dust flux pattern to that of the northern hemisphere about 300,000 years ago at core RC11-210 at 1.8° N, implying a 5° or greater latitudinal shift in the position of the ITCZ from south to north at the time of the Mid-Brunhes Climate Event.
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8 Late Quaternary Flux of Eolian Dust to the Pelagic Ocean DAVID K. REA AND STEVEN A. HOVAN University of Michigan THOMAS R. JANECEK Texas A&M University ABSTRACT The eolian flux of terrigenous minerals to the pelagic ocean has been measured in about 30 locations in the Pacific, Atlantic, and Indian Oceans. Dust fluxes vary by three orders of magnitude from more than 1000 mg/cm2/kyr directly downwind from the major dust sources in China and North Africa to 1 mg/cm2/kyr over much of the South Pacific and southern Indian Oceans. In both the Atlantic and Pacific oceans there is an order of magnitude reduction in the deposition rates of eolian dust as one proceeds south across the Intertropical Convergence Zone (ITCZ). Dividing the total dust flux value of Prospero (1981) by the area of the global ocean results in an average deposition rate of 200 mg/cm2/kyr, but this value is too low for the nearshore regions and a factor of 4 or 5 high for the pelagic ocean. Detailed downcore records of eolian flux to the northwest Pacific document much higher fluxes during glacial times and accumulation minima during interglacials; this accumulation pattern matches the loess/soil stratigraphy of central China. Cores raised from just south of the ITCZ in the Pacific exhibit greater dust fluxes during interglacial times, matching the known aridity pattern of northwest South America. We observe a shift from the present southern hemisphere dust flux pattern to that of the northern hemisphere about 300,000 years ago at core RC11-210 at 1.8° N, implying a 5° or greater latitudinal shift in the position of the ITCZ from south to north at the time of the Mid-Brunhes Climate Event.
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INTRODUCTION Terrigenous materials are being supplied to the world's oceans by rivers at a flux rate of about 1.5 to 2 x 1016 g/yr (Holland, 1981). Most of this material is trapped in estuaries or on continental shelves and only a minor portion reaches the deep sea. Eolian dust, perhaps only 5 percent of the total lithogenous flux (approximately 0.53 to 0.85 x 1015 g/yr, Prospero, 1981), dominates the mineral component of pelagic sediments in regions away from the influence of turbidites, equatorward of the effects of ice rafting and seaward of the influence of hemiplegic deposition (Windom, 1969, 1975; Rea et al., 1985). The eolian input estimate of Prospero (1981) when divided over the 360 x 1016 cm2 of the oceans and marginal seas results in an average dust flux value for the entire ocean of about 200 mg/cm2/kyr. Dust is raised by storms from the arid and semi-arid surfaces of continents, elevated to the upper portion of the troposphere, transported global distances by the zonal winds and washed out of the atmosphere by rain. The long-term pattern of this sedimentary process is shown clearly in maps of the quartz content of surficial sediments of the sea floor (Leinen et al., 1986). Regions relatively enriched in quartz, which makes up roughly 10 to 20 percent of the total eolian load, extend downwind from the major deserts of the world—Gobi, Sahara, Arabia, Australia. The short-term record of dust transport is seasonal. Spring storms are responsible for most of the annual transport. The best record of present-day eolian transport is that developed by Prospero for materials crossing the Atlantic from the Sahara. That record shows an order of magnitude variation in the amount of dust transported in any given year with maxima during the late spring or early summer. On a longer term basis the flux of dust crossing the North Atlantic shows a three- to fivefold increase during the height of the Sahelian droughts in 1973-1974 and 1983-1984, demonstrated times of significantly reduced rainfall (Nicholson, 1985; Middleton, 1985) in comparison to more normal years (Prospero and Nees, 1977, 1986). This sort of seasonality in dust transport with maxima in the late spring occurs wherever dust fluxes have been measured (Parrington et al., 1983; Uematsu et al., 1983; Merrill et al., 1989). Useful summaries of eolian processes and dust deposition have been given by Windom (1975), Prospero (1981), Rea et al., (1985), and Pye (1987). THE GEOLOGIC RECORD OF DUST DEPOSITION A Note on Methodology The mass input of any sedimentary component to the sea floor can be quantified. The parameters necessary to determine the mass accumulation rate (MAR) in g/cm2/kyr, or flux, of dust are: the linear sedimentation rate (LSR) in cm/kyr, the dry bulk density (DBD) of the sediment in g/cm3, and the weight percent of the eolian component of the bulk sediment. Eolian flux values are the product of these three. The eolian component itself is isolated from the total sediment by a series of extractions that remove all other sedimentary components, leaving the minerals (Rea and Janecek, 1981). The amount of any volcanic ash remaining after extraction is estimated visually from smear slides and deducted from the total to give values for continentally derived material. Sediment samples usually span one or two centimeters of any given core and so represent many hundreds to thousands of years of deposition. Furthermore, any initially discrete event or signal is smoothed by the bioturbation process that acts to homogenize the uppermost several centimeters of deep-sea sediments. Short-term climatic variability on time scales of decades to centuries is therefore smoothed and the records we present are representative of truly long-term climatic trends and events. Our working assumption, based largely on the data of Prospero on transport from Africa (Prospero, 1981; Prospero and Nees, 1986), is that the amount of dust transported to the oceans is a function of source area aridity; drier, less vegetated regions provide more dust to the atmosphere than do moist, well vegetated regions. This assumption is also consistent with information available for the pedology of the great loess-soil sequences in China (Kukla, 1987; Kukla and An, 1989) and the distal record of those sequences (Hovan et al., 1989). The supply of dust to the oceans is independent of the intensity of transporting winds (Chuey et al., 1987), that aspect of eolian deposition is recorded by the grain size variation of the dust (Rea et al., 1985; Pisias and Rea, 1988). Geographic Variation in Eolian Fluxes The input rate of dust to the pelagic oceans today (Holocene) ranges through three orders of magnitude, from low values of 1 or 2 mg/cm 2/kyr in the center of the South Pacific to more than 1000 mg/cm2/kyr just downwind from the North African and Asian source regions. There are perhaps 35 locations in all the oceans where flux values for clearly pelagic eolian minerals have been determined (Figure 8.1; Table 8.1), locations more than 500 of 1000 km offshore and thus beyond the effective range of hemipelagic deposition. All values calculated for near continent regions may include a significant hemipelagic component. When compared with information from nearby sediment traps or the data from island-based collectors, the dust flux values from sediment cores agree to within the combined errors of the two kinds of measurements (Rea et al., 1985).
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FIGURE 8.1 Locations of Pacific and Atlantic Ocean cores discussed in text. FIGURE 8.2 Pleistocene-Holocene eolian flux to the Pacific Ocean. The most flux data exist for the North Pacific where essentially all of the eolian dust deposited comes from central and western China and Mongolia. That material is transported to the east by the westerlies and the westerly jet stream, then drifts south towards the equator and dominates eolian deposition all the way south to the Intertropical Convergence Zone (Merrill et al., 1989). Flux rates decline all along this transport path (Figure 8.2). Rea and Leinen (1988) reported on the late Glacial to Holocene eolian records of a suite of six cores along a latitudinal profile extending from 28.4°N to 46.6°N between 155° and 160°E. Data from those cores (Figure 8.3) show distinct latitudinal variations in dust flux with a present-day maximum of about 1000 mg/cm2/kyr at 38° to 40°N and flux values falling off to the north and south. The latitudinal position of the flux maxima has remained unchanged through the last 30,000 years, although the amount has varied (Rea and Leinen, 1988). Farther east, at the same latitude of 37 to 40°N and between 174 and 179°E, the flux of dust in uppermost samples (i.e., Pleistocene, not Holocene) of DSDP and piston cores is approximately 250 mg/cm2/kyr, a significant decline in about 1500 km of transport distance. To the south of the main region of the westerlies, the eolian fluxes in the central North Pacific decline to less than 100 mg/cm2/kyr at about 30 to 35°N, depending on longitude. Flux values from three equatorial cores whose late Glacial and Holocene records were studied in detail, all from about 1°N and spaced between 109 and 179°W, remain quite constant at 10 to 20 mg/cm2/kyr over the past 30 kyr (Figure 8.4). The Holocene flux value from DSDP Core 503B at about 4°N, 96°W, somewhat closer to the presumed South American source area, is slightly greater (Rea et al., 1986). We have determined the flux of eolian dust to the South Pacific for five of the DSDP Leg 92 drill sites which were spaced along 19°S from 130 to 117°W (Figure 8.1; Bloomstine and Rea, 1986). The flux value of the uppermost sample from each of these cores averaged 1 mg/cm2/kyr and was never greater than 1.8 mg/cm2/kyr. Eolian fluxes to the South Pacific are very low and have been so at least since the Oligocene (Rea and Bloomstine, 1986). Results from the southern Indian Ocean indicate similarly low fluxes for most of the Cenozoic (Hovan and Rea, 1991). The Pacific data are adequate to construct a map of the flux of eolian dust to the ocean (Figure 8.2). The data mapped (Table 8.1) include both Holocene and whole-Pleistocene values so the map does not represent a true (i.e., restricted) time slice. Nevertheless the primary flux patterns are clear. High eolian accumulation rates near Asia decrease to the east along a latitude band approximately 35 and 45°N from over 1000 to less than 250 mg/cm2/kyr in the central North Pacific. Isopleths of eolian flux trend east-west, matching the pattern of present-day transport (Merrill et al., 1989) and the pattern of mineralogy of sea-floor surface sediments (Leinen et al., 1986). There is an order of magnitude decline in dust input along about 30° N from the high values to the north to values of a few tens of mg/cm2/kyr in the northern subtropics. Another order of magnitude decline occurs at the Intertropical Convergence Zone, where the rainfall associated with the equatorial low serves as an effective barrier to the interhemispherical transport of dust.
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TABLE 8.1 Core Locations and ''Surfical" (Holocene Average or Youngest Pleistocene) Eolian Flux Values (in mg/cm2/kyr). Core Reference Latitude Longitude Flux Comment V20-122 1 6.6°N 161.7°E 642 Holocene V20-126 1 42.2°N 155.9°E 737 Holocene RC14-105 1 39.7°N 157.5°E 988 Holocene V-129 1 37.7°N 156.6°E 1299 Holocene RC10-167 1 33.4°N 150.4°E 431 Holocene V28-294 1 28.4°N 140.0°E 55 Holocene V21-146 2 37.7°N 163.0°E 206 Holocene KK75-02 3 38.6°N 179.3°E 241 Holocene DSDP 310 4 36.9°N 176.9°E 253 Pleistocene DSDP 466 5 34.2°N 179.3°E 81 Pleistocene DSDP 578 6 33.9°N 151.6°E 1790 Probably hemipelagic DSDP 465 5 33.8°N 178.9°E 9 Unreliable sediment rate DSDP 576 6 32.4°N 164.3°E 603 Pleistocene DSDP 305 4 32.0°N 157.8°E 86 Pleistocene DSDP 463 7 21.4°N 174.7°E 41 Pleistocene LL44-GPC3 8 30.3°N 157.8°W 183 Pleistocene RC11-210 9 1.8°N 140.0°W nd Near K7905-16GC DSDP 503B 10 4.1°N 95.6°W 28 Holocene V28-203 11 1.0°N 179.4°W 13 Holocene K7905-16GC 11 1.1°N 138.9°W 19 Holocene RC10-65 11 0.7°N 108.6°W 21 Holocene DSDP 597 12 8.8°S 129.8°W 1.0 Pleistocene DSDP 598 12 19.0°S 124.7°W 0.6 Pleistocene DSDP 599 12 19.5°S 119.9°W 0.4 Pleistocene DSDP 600C 12 18.9°S 116.8°W 0.8 Pleistocene DSDP 601 12 18.9°S 116.9°W 1.8 Pleistocene V30-36 13 5.4°S 27.3°W nd — V30-40 13 0.2°S 23.1°W 450 Holocene RC24-09 13 1.9°S 11.4°W 474 Holocene RC24-16 13 5.0°S 10.2°W 89 Holocene V22-174 13 10.8°S 12.0°W 38 Holocene ODP 664 14 0.1°N 23.2°W 450 Pleistocene ODP 663A 14 1.2°S 11.9°W 380 Pleistocene RC27-61 15 16.6°S 59.9°E 500 Holocene ODP 756B 11 27.4°S 87.6°E 0.3 Holocene Note: References to original data are: 1. Rea and Leinen (1988); 2. Hovan et al. (1989); 3. Janecek (1983); 4. Rea and Janecek (1982); 5. Rea and Harrsch (1981); 6. Janecek (1985); 7. Rea and Janecek (1981); 8. Janecek and Rea (1983); 9. Chuey et al. (1987); 10. Rea et al. (1986); 11. Rea, unpublished data; 12. Bloomstine and Rea (1986); 13. Janecek, unpublished data; 14. Ruddiman et al. (1989); 15. Clemens and Prell (1990). The flux of dust to the Atlantic has been measured in several cores, most of which are downwind from the Sahara (Table 8.1). Holocene flux values in cores well away from the problems attendant to continental margin sedimentation are 400 to 500 mg/cm2/kyr in the region between the equator and about 10°N. There is a marked drop-off south of the equator to values of about 90 mg/cm2/kyr at 5°S and about 40 mg/cm 2/kyr at 11° S, an order of magnitude reduction in values, a similar amount to the trans-equatorial flux reduction in the Pacific. The westward moving dust plume from the deserts of North Africa is centered at about 20° N (Sarnthein et al., 1981; Leinen et al., 1986), so flux values greater than 500 mg/cm2/kyr might be expected at that latitude.
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FIGURE 8.3 Late Glacial and Holocene eolian fluxes in four Northwest Pacific cores. FIGURE 8.4 Late Glacial and Holocene eolian fluxes in three Equatorial Pacific cores. FIGURE 8.5 Eolian flux to North Pacific core V21-146 during the past 530,000 years. Left: benthic δ18O record, glacial aged (positive δ18O) peaks to the right; right: flux record. The Downcore Record of Quaternary Eolian Deposition The best record of eolian flux from the North Pacific ocean has been developed for core V21-146 raised from 3968 meters depth at 37.7°N, 163.0°E, about 3500 km downwind from the dust-generating regions of China (Figure 8.5; Hovan et al., 1989, 1991). That core has a detailed δ18O record derived from benthic foraminifera, which can be linked to the SPECMAP δ18O time scale (Imbrie et al., 1984), providing the temporal basis for the eolian flux analyses. During the last 530,000 yr, the flux of dust to core V21-146 has averaged about 250 mg/cm2/kyr, ranging from a low of about 55 to a peak value of about 670 mg/cm2/kyr. There is a distinct correlation between flux maxima and times of glaciation as indicated by the δ18O record; on the average, flux values increase by a factor of 3.9 from interglacial minima to glacial-aged maxima (Figure 8.5). Furthermore, the dust flux variations in V21-146 also correlate with the loess-soil stratigraphy of the China Loess Plateau (Kukla, 1987; Kukla and An, 1989). The correlation of periods of low flux to times of soil development and of times of high flux to periods of loess activity permits us to make a direct tie between this classic continental record of Quaternary climates and the δ18O record of climate change using the information from core V21-146 (Hovan et al., 1989, 1991). A longer-term trend of increasing dust flux to the North Pacific in younger sediments during the past 500,000 yr is apparent (Figure 8.5). This observation, which suggests increasing late Pleistocene aridity in the eolian source region, is consistent with the observations of Pye and Li (1989) of increased rates of dust accumulation in the Xifeng loess sequence. Equatorial core RC11-210 (1.8°N, 140.0°W, 4420 m), from the east-central Pacific south of the Intertropical Convergence Zone, provides a detailed, 946,000-yr-long record of paleoclimatic proxy indicators (Figure 8.6; Chuey et al.,
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FIGURE 8.6 Late Pleistocene eolian flux to Equatorial Pacific cores RC 11-210 (left) and DSDP 503B (right). Planktic δ18O records; glacial aged (positive δ18O) peaks to the right. FIGURE 8.7 Late Pleistocene eolian flux to Equatorial Atlantic cores V30-36, V30-40, RC24-09, RC24-16, and V22-174. Note that flux scales vary. 1987; Pisias and Rea, 1988; Rea et al., 1991). To the east, the uppermost hydraulic piston cores from DSDP Site 503B (4.0° N, 95.6° W, 3672 m) provide a somewhat less detailed record of the past 420,000 yr (Figure 8.6; Rea et al., 1986). At RC11-210 eolian dust accumulated during the late Pleistocene at rates of 4 to 90 mg/cm2/kyr (the high values at the top of the core may be spurious; the sharp peak at 75 kyr is an ash layer, see discussion in Chuey et al., 1987). Flux maxima occur during both interglacial, Stages 7, 15 and perhaps 19, and glacial, Stages 10 and 12, times. At 503B the flux pattern is similar, maxima occur in interglacial Stages 5 and 7, and glacial Stage 10. The shift in the flux pattern from interglacial maxima to glacial maxima occurs about 300,000 yr ago, the time of the Mid-Brunhes Climate Event (Chuey et al., 1987; Pisias and Rea, 1988). Five piston cores raised from the Equatorial Atlantic between about 5° N and 11° S provide a record of eolian dust flux to that part of the ocean over the past 300,000 yr (Figures 8.1 and 8.7). Flux values at ODP drillsites north of the ITCZ have averaged 400 to 500 mg/cm2/kyr for the past 0.5 myr and may have been 50 percent or more higher in earlier portions of the Pleistocene (Ruddiman et al., 1989). The piston cores appear to show an irregular pattern of heightened flux of dust during both the younger portions of
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glacial stages and occasionally during interglacial periods. Pokras and Mix (1985, 1987) have studied the flux of the windblown freshwater diatom Melosira to core V30-40. Their data do not match the dust flux patterns shown on Figure 8.7; the diatom fluxes may be recording the initial drying and deflation of African lakes at the onset of arid conditions (Pokras and Mix, 1987). The decrease in dust flux by a factor or eight to ten to the south of the Intertropical Convergence Zone persists throughout the downcore records. Estimating the long-term flux of volcanogenic material to the ocean is more difficult. Studies of ash layers are common but provide information on discrete events and not the ongoing, on geologic time scales, influx of ash and other volcanic debris to the deep sea. A geochemical approach to this problem has been taken recently by Olivarez (Olivarez, 1989; Olivarez et al., 1991), who used methods of multivariate analysis to apportion the assemblage of rare earth elements (REE) in the eolian material between continental and oceanic crust (presumably volcanic) end members. She found that near the west Pacific island arcs, the ocean-crust component was perhaps 20 to 25 percent of the REE assemblage, declining to 5 to 10 percent in the central North Pacific. The REE abundance patterns of the low dust-flux cores along the equator commonly indicate 30 to 50 percent of the oceanic crust end member. Although not definitive, these results are indicative of an important background of disseminated volcanic material entering the ocean via the wind. Quantification is problematic; Olivarez' data suggest that volcanogenic flux values might be as much as 10 to 30 percent of those shown on Figure 8.2, with the greater values nearer the volcanic arcs of the Pacific rim. Partitioning and provenance of the eolian material is an ongoing research topic. DISCUSSION In the introduction we noted that the average flux of eolian dust to the ocean was 200 mg/cm2/kyr. The data presented here, however, shows that this value is exceeded only in regions immediately downwind from important dust sources, China, North Africa, and Arabia. All the remaining pelagic ocean has fluxes of only a few tens of mg/cm2/kyr, and the entire South Pacific and probably the southern Indian Ocean receives almost no dust. For the pelagic ocean, therefore, the average value of eolian flux is far too high, probably by a factor of four or five. This does not negate the value of the total flux number, 0.7 x 1015 g/yr, because just as for rivers, most of the dust carried to the oceans will be deposited within a few hundred kilometers of the shoreline and become indistinguishable from the hemipelagic and turbidite materials accumulating at those locations at much larger rates. The reader should be aware, however, that over most of the open sea the input of dust to the ocean is no more than a few tens of mg/cm2/kyr, and that this value is much less in the southern hemisphere (Figure 8.2, Table 8.1). The provenance of dust in Pacific equatorial cores is a significant problem. If we infer a southern hemisphere source for these cores presently south of the Intertropical Convergence Zone, then the flux records of Site 503B and RC11-210 may be related to the paleoclimatology of the northern Andes. Pollen data from that region show that throughout the Quaternary the Andean lakes were full during glacial times and dry salars or playas during interglacial times (Hooghiemstra, 1984; van der Hammen, 1985). This is consistent with the eolian flux data at 503B, which show interglacial flux maxima during Stages 5 and 7. The shift in accumulation maxima from the assumed southern hemisphere pattern of interglacial maxima to the known northern hemisphere pattern of glacial-aged maxima implies a quasi-permanent shift in the latitude of the Intertropical Convergence Zone at the time of the Mid-Brunhes Climate Event, from an earlier more southerly location to a younger position further north. The location RC11-210 constrains this latitudinal shift in the position of the ITCZ to be at least 5° . ACKNOWLEDGMENTS This chapter was reviewed by J.M. Prospero and an anonymous reviewer, and we are grateful to them for their thoughtful remarks. Most of the samples we report on here were provided by either the New Core Laboratory of the Lamont-Doherty Geological Observatory or the Deep Sea Drilling Project and those organizations are thanked for their cooperation. Our work on the flux of dust to the Quaternary oceans has been supported by the Climate Dynamics Program of the National Science Foundation for a number of years, and we appreciate their continuing support. REFERENCES Bloomstine, M.K., and D.K. Rea (1986). Post-middle Oligocene eolian deposition from the trade winds of the southeast Pacific, in Initial Reports of the Deep Sea Drilling Project 92, M. Leinen, D.K. Rea et al., U.S. Government Printing Office, Washington, D.C., pp. 331-340. Chuey, J.M., D.K. Rea, and N.G. Pisias (1987). Late Pleistocene paleoclimatology of the central Equatorial Pacific: A quantitative record of eolian and carbonate deposition, Quaternary Research 28, 323-339. Clemens, S.C., and W.L. Prell (1990). 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