Terrigenous sedimentation at Ceara Rise

Abstract

A series of 47 samples was taken at Ceara Rise from Sites 925 and 929 during Ocean Drilling Program Leg 154. The samples spanned the entire cored section from the late Paleocene to the present. A series of chemical extractions was performed on these samples to isolate the terrigenous component. The magnitude of the terrigenous component was calculated both as weight percent and as a mass accumulation rate, which is not affected by fluctuations in nonterrigenous deposition. The mass accumulation rate of terrigenous materials at Ceara Rise has varied widely over the period studied, culminating in a tenfold increase since 8 Ma. This was probably caused by Andean uplift and increased Amazon river flow. Thirty of the extracted samples were then analyzed for elemental compositions by instrumental neutron activation analysis. The results divide the terrigenous materials into three chemically distinct groups. These chemical distinctions also split the samples by age, implying that the source of terrigenous materials for Ceara Rise has changed through time. Interestingly, the three chemical groupings are not on the same mixing line and must then represent at least three independent terrigenous signals. The timing of the shifts between groups is consistent with South American tectonic history. Finally, the terrigenous fraction (by weight percent) of the extracted samples was regressed against high-resolution shipboard measurements. This allows estimation of weight percent terrigenous materials at any depth or age from pre-existing natural gamma, magnetic susceptibility, and color reflectance. Only samples from the last 12 Ma produced useful estimates.