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.
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° .
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.
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