172 Scientific Prospectus
ABSTRACT
The Blake-Bahama Outer Ridge (BBOR) and Carolina Slope (CS) form the western boundary for deep- and surface-water circulation in the North Atlantic. Between the northward-flowing surface waters of the Gulf Stream and the net southerly flow of intermediate and deep waters, most of the climatically important exchanges of heat, salt, and water with other ocean basins occur in the westernmost North Atlantic. Ocean Drilling Program Leg 172 is designed to monitor changes in these water masses and their fluxes through the late Pliocene and Quaternary. Virtually all water transported by the Gulf Stream to sites of convection in the northern North Atlantic comes from the western subtropical North Atlantic. The Carolina Slope (~1-2 km depth) underlies the axis of the Gulf Stream, as well as the shallowest component of Labrador Sea water. At depths greater than ~2 to ~4 km, the Blake Outer Ridge monitors changes in North Atlantic Deep Water (NADW), and the Bahama Outer Ridge (~4 to 5 km depth) extends coverage from the deepest components of NADW to Antarctic Bottom Water (AABW). On geological time scales it is known that during cold epochs the North Atlantic switches from today's circulation mode of deep nutrient-depleted water mass production (i.e., NADW) to production of a less dense nutrient-depleted water mass at intermediate depths. A shallow component of Labrador Sea water was identified recently, which interacts with the Gulf Stream and probably controls the distribution of sediment on the Carolina Slope. This water mass may be the modern equivalent of glacial NADW. According to the "Great Ocean Conveyor" paradigm, knowledge of the history of these surface, intermediate, and deep-water masses is essential to understanding the world ocean's role in climate change. Drilling on Leg 172 will provide paleoenvironmental records for late Neogene hemipelagic sediments that are deposited at accelerated rates on western North Atlantic sediment drifts on the BBOR and CS. These two areas may represent the only sediment drift locations in the world's oceans where it is possible to conduct high-resolution paleoclimate studies through a 3500-m range of water depths. Data obtained from the gyre-center, which will be sampled at the Bermuda Rise (BR) site at a depth of ~4.5 km, will be compared with data from sites located at deep, high-deposition-rate locations on the BBOR and CS to document late Neogene oceanographic changes in the western North Atlantic for millennial, as well as Milankovitch times scales over the entire deep and intermediate water column. In addition to geochemical and micropaleontological studies of climate change, the hemipelagic composition and high deposition rates of BBOR, CS, and BR sites will enable high resolution studies of magnetic reversals and excursions and studies of current-controlled sedimentation.
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