Reconstruction of Aeolian Input to the Equatorial Atlantic: A One Million Year Record

R. Haese Univ. of Bremen, Dept. of Geosciences (FB 5), P.O. Box 330 440, 28334 Bremen, Germany

H. D. Schulz Univ. of Bremen, Dept. of Geosciences (FB 5), P.O. Box 330 440, 28334 Bremen, Germany

A 13.5 m long gravity core from the Sierra Leone Rise (5° N, 21° W, 2700 m water depth) revealed an undisturbed record of the past 1.2 million years. Stratigraphy for the past 750.000 years was established by correlation of carbonate content with core M 13519 (Sarnthein et al., 1984; Müller et al., 1983), older parts were dated by paleomagnetic reversals (Petermann, pers. com.).

In this study we focus on the following major aspects of chemical sedimentation: 1. Which elements are most characteristic for the aeolian fraction within the sediment? 2. How did these elements and thus the aeolian input vary during glacial and interglacial times within the Inner Tropical Convergence Zone (ITCZ)? 3. Which proxies can be found tracing paleoproductivity and events of organic rich rain within this organic poor environment?

In agreement with a previous study (Murray and Leinen, 1993) we found excellent correlation between the following elements: Ti, Al, Fe, K, Cr and V. In addition, the iron oxide content determined semiquantitatively by visible spectroscopy (Deaton and Balsam, 1991) correlated well with the named elements. These parameters turned out to be typical and most useful when reconstructing the variability of the terrigenous input to pelagic sediment. They are strongly negatively correlated with carbonate content which is generally lower during cold stages. Even on a carbonate free basis the elements of terrigenous origin increase by a factor up to 3 during cold periods. Since the correlation of these parameters persists throughout the whole core the chemical composition of the dust has been constant over time, indicating a constant mixture of dust material transported by NE- and SE-trade winds.

Biogenic Ba was calculated from total Ba content (Boström et al., 1973; Wedepohl, 1991) and compared to estimated paleoproductivity of core M 13519 (Müller et al., 1983) determined according to Müller and Seuss (1979). Babio is enriched by a factor of 2-3 during glacial stages relative to interglacial stages and agrees with the general pattern of paleoproductivity according to Müller et al. (1983). Bringing together the results of aeolian input and paleoproductivity estimations we find a direct correlation with each other. Whether this means an increase of wind intensity by a factor of 2 - 3 during glacial times and constant arid conditions of the source regions or even higher wind intensities and slightly vegetated source regions during glacial times remains open.

In addition, Cu and the Ce/La-ratio revealed striking phenomena: Cu is generally associated with organic matter and shows two distinctive enrichments at the base of Babio enriched parts (isotope stage 6 and 12). We assume a deliberation of cupper during dissimilation of organic rich matter at the sediment surface and diffusive transport into the sediment where it becomes associated with solid phase. At the end of isotope stage 11 Ce/La-ratio dropped significantly for a period of about 25 Ky. Ce anomaly has been shown to be a sensitve tracer for the redox condition in the water column as Ce(III) is soluble in contrast to Ce(IV) (German and Elderfield, 1990). Our observation implies at least suboxic conditions in the water column for this period although no further evidence is given.


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