Abstract:
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.
