LEG 154

Ceara Rise

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 earthÕs 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.