The median grain size of eolian material extracted from deep-sea sediment can be used to constrain past zonal wind intensity (e.g., Janecek and Rea, 1983; Rea et al., 1985; Miller et al., 1987; Hovan and Rea, 1992; Rea, 1994). Sediment sequences from several locations in the Northern Hemisphere have been examined for changes in eolian grain size over the Cenozoic and specifically the early Paleogene. These records show a major fining in eolian material from the Paleocene to the Eocene, an observation interpreted as a relative decrease in the intensity of zonal winds and, more generally, atmospheric circulation (Rea and Janecek, 1983; Rea et al., 1985; Miller et al., 1987; Rea, 1994). To date, one eolian grain-size record has been generated using lower Paleogene sediment deposited in the Southern Hemisphere. This record from Deep Sea Drilling Project (DSDP) Site 215 adjacent to Ninetyeast Ridge in the central Indian Ocean also shows a significant drop in grain size across the Paleocene/Eocene (P/E) transition (Hovan and Rea, 1992). More records from the Southern Hemisphere, however, are clearly needed to confirm these results and further substantiate broad zonal interpretations.
One major hurdle facing studies of eolian material is site selection. Sediment sequences must contain biogenic carbonate, which can be dated, and cannot contain chert, which limits the chemical isolation of all siliciclastic material, including eolian grains. Appropriate sites also must be located far away from continents so that the terrigenous fraction of sediment is not completely dominated by noneolian (riverine/hemipelagic) material. Often, the principal concern is hemipelagic material (Rea, 1994). At locations beyond the continental shelf, reworked riverine material settles to water depths of supportable density (deeper than ~2000 meters below sea level [mbsl]) and travels as a hemipelagic cloud hundreds of kilometers from the margin (Honjo, 1982; Rea, 1994).
Walvis Ridge is located in the South Atlantic Ocean and extends west from Africa to the mid-ocean ridge. The sites of interest to this study were all drilled on Walvis Ridge >1200 km from Namibia. DSDP Site 527, cored in 1980 at ~4430 mbsl, recovered a thick Cenozoic sediment sequence on a deep flank of this ridge (Moore, Rabinowitz, et al., 1984). Preliminary work at this site indicated that hemipelagic material heavily dominates the terrigenous component of the lower Paleogene section (Rea and Hovan, 1995), making it problematic for studies of eolian material.
Ocean Drilling Program (ODP) Leg 208 returned to Walvis Ridge, recovering good lower Paleogene sediment sequences at five proximal sites that span a depth transect of ~2 km (Zachos, Kroon, Blum, et al., 2004). Site 1267 was drilled close to Site 527 and presumably holds a Paleogene terrigenous record dominated by hemipelagic material. Site 1263, however, was drilled near the crest of the ridge at ~2700 mbsl and could contain a terrigenous record with significant amounts of eolian material. The object of this work is to evaluate whether the newly acquired lower Paleogene sequence at Site 1263 is appropriate for eolian grain-size studies. Our results suggest that a high–age resolution eolian grain-size record could be generated at Site 1263, providing long-desired information regarding early Paleogene atmospheric circulation in the Southern Hemisphere.