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Abstract
Studies from the equatorial Pacific Ocean have shown that the sedimentary A1/Ti ratio in open Oceans can be extremely different from those in natural terrestrial source material (e.g. Murray and Leinen 1993). The observed enrichment of A1 has been accounted to the carbonate accumulation by Murray and Leinen (1996) and recently to the sedimentation of biogenic opal by Dymond et al. (1997). In contrast to these investigations results presented here give evidence for a quite different situation in the equatorial Atlantic Ocean. Three gravity cores from oligotrophic areas along an E W transect (3~176 were geochemically investigated. The most westerly core (GeoB 1523) was recovered from the eastern flank of the Cearfi Rise, the central site (GeoB 1505) is located on the western slope of the Mid Atlantic Ridge, whereas easterly samples (GeoB 2910) come from the Sierra Leone Rise. Generally the AI/Ti ratios (13-24) are similar to values of the terrestrial source material. Two regions may serve as source area: 1) the African deserts Sahara and Sahel, and 2) the catchment of the Amazon River. Whereas especially the westward dust transport and its range of deposition is well defined by numerous works, the quantity as well as the regional influence of river suspended particles on the composition of adjacent deep-sea deposits are not yet known. However, sediments in the western Atlantic Ocean (Cearfi Rise GeoB 1523) contain high amounts of Amazon material (Riihlemann et al., 1996), whereas the terrestrial portion of the eastern sediments (Sierra Leone Rise GeoB 2910) is controlled by the eolian particle transport from the African continent. Figure I shows the three records of the A1/Ti ratio together with the carbonate accumulation rates. Additionally results of spectra analysis of the AI/Ti variations are given. Whereas marine proxies such as TOC/CaCO3 indicate forcing by the orbital 100 kyr and 41 kyr cycles (not shown), changes within the AI/Ti ratio occur in precessional University of Bremen, FB 5 Geowissenschaften, Dept of Geochemistry and Hydrogeology, PBox 330 440, 28 334 Bremen, Germany
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