Lithogenic particle fluxes and grain size distributions in the deep ocean off northwest Africa: Implications for seasonal changes of aeolian dust input and downward transport

Between 1988 and 1994, twenty time-series sediment traps were deployed at d ifferent water depths in the Canary Island region, off Cape Blanc (Mauritan ia), and off Cape Verde (Senegal). Lithogenic particle fluxes and grain siz e distributions of the carbonate-fret fraction of the trapped material show a high impact of dust transported either in the northeast trade winds or t he Saharan Air Layer (SAL). Highest annual mean lithogenic fluxes (31.2-56. 1 mg m(-2) d(-1)) were observed at the Cape Blanc site, and largest annual mean diameters (> 6 mu m) were found off Cape Verde (14.5-16.9 mu m) and of f Cape Blanc (15.2-16.7 mu m). Lowest annual lithogenic fluxes (11.4-21.2 m g m(-2) d(-1)) and smallest mean diameters (13.5-13.7 mu m) occurred in the Canary Island region. A significant correlation of organic carbon and lith ogenic Auras was observed at all sites. Off Cape Blanc, fluxes and mean dia meters correlated well between upper (around 1000 m depth) and lower traps (around 3500 m depth), indicating a fast and mostly undisturbed downward tr ansport of particulate matter. In contrast, a major correlation of fluxes w ithout correlating mean diameters occurred in the Canary Island region, whi ch translates into a Fast vertical transport plus scavenging of laterally a dvected material with depth at this site, The seasonality of lithogenic flu xes was highest in the Canary Island region and off Cape Verde, reflecting strong seasonal patterns of atmospheric circulation, with highest occurrenc e of continental winds in the trade wind layer during winter. In addition, grain size statistics reflect a dominant change of dust transport in the tr ade winds during winter/spring and transport in the SAL during summer 1993 at the Cape Verde site. Highest lithogenic fluxes during winter were correl ated with mean diameters around 10-13 mu m, whereas lower fluxes during sum mer consisted of coarse grains around 20 mu m. Annual mean dust input was c alculated from lithogenic fluxes in the range 0.7 x 10(6)-1.4 x 10(6) t yr( -1), roughly confirming both sediment accumulation rates and atmospheric mo del calculations reported previously from this area. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.