Graduate Thesis Or Dissertation
 

Biogenic silica sedimentation in the central equatorial Pacific during the Cenozoic

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j098zf25z

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  • A new technique for determining the amount of opal in deep-sea sediments of any age is described. Using a normative calculation, a portion of the analytical silica concentration of sediments is subtracted as non-biogenic in proportion to the concentration of aluminum in the sample. The ratio of SiO₂:Al₂O₃ used to characterize the non-biogenic sediment fraction was determined by X-ray diffraction analysis of opal-free sediments. The procedure was tested against the X-ray diffraction method for determining opal in deep-sea sediments The biogenic silica content of Cenozoic sediments from 20 Deep Sea Drilling Project sites in the central equatorial Pacific was determined using the normative calculation technique for opal determination. The equatorial Pacific lies beneath the equatorial current system where upwelling of nutrient-rich waters results in high plankton productivity. Accumulation rates of biogenically produced silica were calculated from the opal contents. Maps of these accumulation rates for time intervals during the Cenozoic show that opal accumulation was highest near the equator or paleoequator during the last 50 m.y. Superimposed on this pattern are fluctuations in the rate of opal accumulation in the entire area with time. Regional maxima in opal accumulation in the entire area with time. Regional maxima in opal accumulation occurred during the middle Eocene (42-45 m.y. ago) and the late Miocene (7-10 m.y. ago). The accumulation rates during these maxima are an order of magnitude higher than those during times of minimum accumulation: the late Oligocene (25 m.y. ago) and the present. The percent of biogenic silica in the sediments varies synchronously with the accumulation rates, but is low to the east due to dilution by non-biogenic sediment from terrigenous and volcanic sources. Surface productivity controls the accumulation of opal in the equatorial Pacific and opaline sediments are not subject to differential solution with depth. The opal productivity indicated by opal accumulation rates is not related to changes in sea surface or bottom water temperatures and is therefore not directly governed by climate. The association of equatorial productivity and upwelling suggests that changes in circulation which cause upwelling were the principal factors controlling productivity and accumulation of biogenic silica in the past.
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