Figure F24. A. Quartz distribution in surface sediments of the subtropical southeast Pacific, expressed as weight percentage (carbonate- and opal-free basis, reproduced from Molina-Cruz, 1977). Gray arrows indicate trade winds. Sites 1236, 1237, 1238, and 1239 are shown with their plate tectonic backtrack. B. Siliciclastic accumulation rates and hematite peak height at Sites 1236 (red line) and 1237 (blue line). The accumulation rates of siliciclastics were estimated as (100% wt% CaCO3 wt% organic matter) x sedimentation rate x dry bulk density. The shipboard magnetostratigraphy and biostratigraphy were used to calculate sedimentation rates that were smoothed to avoid overinterpretations of the preliminary age model. Note that the siliciclastic accumulation rates at Site 1237 include biogenic silica in the interval of the last ~8 m.y. Based on smear slide data (that overestimate weight percent biogenic opal), biogenic opal values increased from ~1 to 10 vol% over the last 8 m.y. This, however, will not change the general trends observed in the siliciclastic accumulation rate record that is used as a proxy for dust flux. The hematite peak height is derived from color reflectance spectra and serves as a proxy for changing concentrations of hematite.
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