The geological record of climatic change consistently documents variability both on a time scale of
104 to 105 years and 106 years and upward. The higher frequency variability is associated with
changes in Earth-sun orbital geometry at least over the last few million years. Variability on longer
time scales probably has a wider range of causes, although each may ultimately be attributed to
tectonic processes. During Leg 154, five sites were drilled on the Ceara Rise (Site 925 to Site 929)
and 5.8 km of sediment was recovered, documenting sedimentation patterns and changes in
accumulation which reflect both orbital and tectonic scales of climate variability.
High frequency changes in lithology at Ceara Rise are primarily forced by the orbital precession
cycle and represent variations in the contribution of terrigenous material from an Amazon source.
The evolution of this response to orbital forcing can be traced back through at least the last 7
million years in a continuously deposited pelagic drape of sediments. The increased influence of the
Amazon on Ceara Rise sediments, marked by an increase in the mass accumulation rate of
terrigenous material as well as a change in its clay mineral composition, began at about 8 Ma, as a
result of changes in climate in the region or changes in uplift of the Andes.
On board measurements of magnetic susceptibility, reflectance, and natural gamma provided an
estimate of the varying ratio of biogenic carbonate to siliciclastic components. Middle and late
Miocene deep waters in this region were extremely corrosive to carbonate. After about 4.5 to 5 Ma,
the modern bathymetric gradients of the carbonate facies began and, since then, there has been a
decrease in the carbonate concentration of the sediments at all depths in the water column,
reflecting a gradually increasing rate of weathering in the Amazon drainage basin as earth's climate
deteriorated and late Pleistocene glaciations became more severe. Superimposed on this long-term
decrease in carbonate percent is a narrowing of the difference in carbonate between the shallow and
deep water sites, implying improved carbonate preservation (a deepening lysocline) since that time.
The measurements of magnetic susceptibility, reflectance, and natural gamma also recorded
variation on shorter time scales, forced by changes in the earthÍs orbital geometry.
The high precision with which the sites on Ceara Rise can be correlated with one another permits
reconstruction of detailed bathymetric transects of both carbonate burial and benthic microfossil
chemistry for the Pliocene and Pleistocene and the potential for correlation with equally high
precision records recovered from the eastern equatorial Pacific during ODP Leg 138.
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