171B Scientific Prospectus


The origin of deep waters is a fundamental control on biogeochemical cycles and global climate. Analysis of time periods in which deep-water formation and global temperature gradients may have been much different from well-known Pleistocene variability offers a test of the models developed to explain climate change. Oceanic circulation in the Paleogene and Cretaceous was quite different from that of the modern ocean in part because of the greenhouse conditions that existed during part of this time and the apparent absence of major centers for deep-water formation in the northern basins. Many authors have suggested that deep-water circulation was enhanced in the Cretaceous and Paleogene by the production of warm, saline waters by evaporation in marginal seas such as the basins of the Tethys, but there are few observational data to provide unequivocal support for this "Warm Saline Deep Water" hypothesis.

The sediments on the Blake Plateau and Blake Nose in the Western North Atlantic offer an ideal record for reconstructing water-mass chemistry and circulation in the Cretaceous and early Cenozoic. The plateau's location in the Northern Hemisphere, proximal to the western end of the Tethys seaway, makes the deposited sediments ideal for determining northern sources of deep and intermediate waters. Ocean Drilling Program Leg 171B will drill five sites in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment. Paleogene and Barremian-Maastrichtian strata crop out or are present at shallow burial depths in present water depths of 1200 m to more than 2700 m across the plateau. Today this depth range spans deep thermocline water to upper North Atlantic Deep Water. The plateau spanned a similar range of depths in the early Cenozoic because margin subsidence was largely complete by the Early Cretaceous, and minor subsidence since then is partly offset by reduced sea level after the Eocene.

The proposed transect of cores will be used to (1) interpret the sea-level history and associated vertical structure of the Paleogene and Cretaceous oceans and test the Warm Saline Deep Water hypothesis near the proposed source areas; (2) provide critically needed low-latitude sediments for interpreting tropical sea-surface temperature (SST) and climate cyclicity in the Cretaceous and Paleogene; (3) study the effects of sea-level change on sedimentation patterns down the continental slope during the Cretaceous and Paleogene; (4) provide well-preserved microfossils for refinement of low-latitude Paleogene and Cretaceous chronologies and evolutionary dynamics; (5) recover complete Cretaceous/Paleogene and Paleocene/Eocene boundaries along a depth transect to describe the events surrounding the boundary and water depth-related changes in sedimentation of the boundary beds; (6) recover sections suitable for magneto-stratigraphy so that low-latitude biochronologies may be tied directly to the magnetic reversal record; and (7) interpret the thermocline and intermediate water structure of low-latitude, Early Cretaceous oceans and refine the biochronology of this period.

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