| Co-Chiefs: Lloyd Keigwin and Dominico Rio | Cruise Dates: 19 February- 16 April, 1997 |
| Staff Scientist: Gary Acton | Operations Superintendent: Ron Grout |
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 western-most North Atlantic. 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 CS (~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 CS. 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.
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. 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 NADW on geological time scales using geochemical and sedimentological techniques.
Leg 172 will core 11 sites: seven primary and one alternate on the BBOR, two on the CS, and one on the BR. The main purpose of Leg 172 is to provide a latest Neogene depth transect for documenting changes in depth distribution of water masses. 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 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, which follow depth contours. 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.
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.
The major objectives of Leg 172 are to obtain a detailed history of late Neogene paleoceanography and paleoclimate in the North Atlantic by investigating 1) millennial scale oscillations of stable isotopes (C and O), faunal and floral abundance, percent carbonate and other lithologic components, and trace metals in drift deposits; 2) the nature of cyclicity of these oscillations; and 3) how these cycles are related to the history of Northern Hemisphere glaciations during the late Neogene.
In addition, scientists on Leg 172 will investigate:
[ Contents of the Semiannual Report, No. 2, June-November 1996 |
| Program Updates | New Initiatives | Project Summaries | Laboratory Working Groups |
| Panel Recommendations | Appendixes |
| Semiannual Report, No. 1, December-May 1996 ]