Grain size distribution and bulk chemistry of eolian dust

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their physical (grain-size distribution), mineralogical, and chemical (major elements) composition. On the basis of these data the samples were grouped into sets of samples that most likely originated from the same source area. In addition, shipboard-collected atmospheric meteorological data, modeled 4-day back trajectories for each sampling day and location, and Total Ozone Mapping Spectrometer aerosol index data for the time period of dust collection (February-March 1998) were combined and used to reconstruct the sources of the groups of dust samples. On the basis of these data we were able to determine the provenance of the various dust samples. It appears that the bulk of the wind-blown sediments that are deposited in the proximal equatorial Atlantic Ocean are transported in the lower level (>~900 hPa) NE trade wind layer, which is a very dominant feature north of the Intertropical Convergence Zone (ITCZ). However, south of the surface expression of the ITCZ, down to 5°S, where surface winds are southwesterly, we still collected sediments that originated from the north and east, carried there by the NE trade wind layer, as well as by easterly winds from higher altitudes. The fact that the size of the wind-blown dust depends not only on the wind strength of the transporting agent but also on the distance to the source hampers a direct comparison of the dust's size distributions and measured wind strengths. However, a comparison between eolian dust and terrigenous sediments collected in three submarine sediment traps off the west coast of NW Africa shows that knowledge of the composition of eolian dust is a prerequisite for the interpretation of paleorecords obtained from sediment cores in the equatorial Atlantic.

Os isotope ratios of Late Eocene sediments from the tropical Pacific

High resolution records (ca. 100 kyr) of Os isotope composition (187Os/188Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene 187Os/188Os excursion and confirm that the Late Eocene 187Os/ 188Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink (2003, doi:10.1016/S0012-821X(03)00137-7), is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene 187Os/188Os minimum can be placed at 34.5 +/- 0.1 Ma in the marine records. In addition, two other distinct features of the 187Os/188Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. +/-0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO2. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the 187Os/188Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; 187Os/188Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of 187Os/188Os records with high resolution benthic foraminiferal delta18O records across the Eocene-Oligocene transition suggests that 187Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater 187Os/188Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187Os/188Os records are obscured by recovery from the Late Eocene 187Os/188Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.