172 Scientific Prospectus


Sediment drifts are widespread in the North Atlantic basin and reflect both the abundant sources of sediment and the focusing of the sediments by deep currents (Lonsdale, 1982; McCave and Tucholke, 1986). There is at least one sediment drift associated with every water mass in the North Atlantic, suggesting a potential for tracing the individual components of North Atlantic Deep Water (NADW) on geological time scales using geochemical and sedimentological techniques (Keigwin and Jones, 1989; Mienert et al., 1994).

Leg 172 will core 11 sites: seven primary and one alternate on the Blake-Bahama Outer Ridge (BBOR), two on the Carolina Slope (CS), and one on the Bermuda Rise (BR) (Figs. 1, 2, Table 1). The main purpose of Leg 172 is to provide a latest Neogene depth transect for documenting changes in depth distribution of water masses (Fig. 3). The geographic range of sites may also help distinguish between latitudinal changes in the mixing zone between southern and northern source waters and changes due to vertical migration of a benthic front, especially when considered in the context of other recent Ocean Drilling Program (ODP) legs such as 154 and 162. A North Atlantic depth transect at the BBOR is especially important, because this feature forms a western boundary for deep currents (Stommel, 1958), which follow depth contours (Heezen et al., 1966). Above ~4000 m depth, these waters mostly have a northern source, whereas, at greater depths there is a greater proportion of recirculated southern-source water (Hogg, 1983). The latest scheme depicting the role of deep recirculating gyres in mixing northern and southern source waters (after Schmitz and McCartney, 1993) is shown in Figure 4. BBOR coring is essential to document and understand first-order changes in the ocean-climate system such as glacial-interglacial variability in the production and flow of North Atlantic water masses and changes in terrigenous, authigenic, and biogenic fluxes. In addition, coring on sediment drifts with high deposition rates is especially important in order to understand North Atlantic climate on millennial and even centennial time scales.

To 172 Background
172 Table of Contents
Publications Home
ODP Home