Leg 172 Scientific Report


The Blake-Bahama Outer Ridge (BBOR) and Carolina Slope (CS) form the western boundary for deep- and surface-water circulation in the subtropical 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. At the deepest levels of the western North Atlantic, Antarctic Bottom Water (AABW) flows northward and blends with several other water masses in the deep recirculating gyres to form North Atlantic Deep Water (NADW) (Fig. 1). The western intensification and flow of these waters erode the continental margin in some locations and preferentially deposit sediments, known as sediment drifts, at other locations.

The main focus of Ocean Drilling Program (ODP) Leg 172 was recovery of a sequence of high deposition rate sediment cores from sediment drifts in the western North Atlantic to document rapid changes in climate and ocean circulation during middle Pliocene to Pleistocene time (Figs. 2, 3). Because it is difficult to understand the ocean-climate system from the study of single core locations, Leg 172 was planned as a depth transect. For example, paleochemical results from Bermuda Rise (BR) cores illustrate changes in deep ocean circulation from only one water depth (4500 m), because the northeast BR is a plateau. Proxy data for deep-ocean nutrient content at that location show large changes with time, but we cannot distinguish between latitudinal changes in the mixing zone between southern- and northern-source waters and changes due to vertical migration of a benthic front. Groups of sites must be located across a range of depths to study the ocean in three dimensions.

Leg 172 cored 11 sites: two on the CS (Sites 1054 and 1055), six on the BBOR (Sites 1056 to 1062), one on the BR (Site 1063), and one on the Sohm Abyssal Plain (Site 1063; Figs. 2, 3; Site Summary Table). The main purpose of Leg 172 was to provide a latest Neogene depth transect to document changes in depth distribution of water masses (Fig. 4). 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. Sites 1054 and 1055 monitor the shallowest components of NADW, which originate in and near the Labrador Sea and which can be traced using chlorofluorocarbons (CFCs) (Pickart and Smethie, 1993). This water mass is expected to wax and wane on glacial/interglacial and millennial time scales, out of phase with production of Lower NADW (LNADW) (Boyle and Keigwin, 1987; Oppo and Lehman, 1993). Reconstructions of glacial North Atlantic hydrography show that the hinge point between better ventilated waters at intermediate depth and more poorly ventilated deep waters occurs at about 2000 m. This boundary is bracketed by Sites 1055 and 1056, which are positioned to detect changes in its depth. Likewise, Site 1057 is located at the interface between Upper (U) and LNADW. Sites 1058, 1059, and 1060, which lie within the core of LNADW, should be insensitive to all but the largest changes in benthic fronts between AABW, LNADW, and UNADW. Lastly, our deepest Sites 1061-1064 are situated to record more subtle changes in the AABW/LNADW front.

Complementing these paleoceanographic/paleoclimatic objectives, it was hoped that the Leg 172 sediments would reveal a high-resolution history of magnetic field behavior and a history of biotic change. Indeed, the paleomagnetic record reveal detailed geomagnetic field changes, including many excursions that were present in multiple holes and at multiple sites, as well as transitional field directions at the Brunhes/Matuyama polarity reversal that could be correlated at sites over 1000 km apart (Fig. 5).

In addition, sedimentary microstructures at Leg 172 sites reflect the combination of both downslope and along-slope depositional processes. Mud-wave dynamics have been a long standing interest of ODP, but have not been successfully studied by ODP until this leg. Mud-wave migration can be measured by determining the ratio of sediment thickness deposited on each wave flank during a time interval or between two correlated layers, from which a model-dependent flow speed can be estimated (Flood, 1988). Leg 172 successfully cored across a single mud wave, mapping the variations in sedimentation rates that occurred as it migrated eastward. Our pre drilling seismic survey also revealed with great clarity structures within the mud wave, which will be combined with the coring results to give a detailed dynamic history of this mud wave (Fig. 6).

The world's best-known marine gas hydrate occurrence is located within the operating area of Leg 172 on the Blake Ridge and Carolina Rise. Gas hydrate, present on continental margins world wide (e.g., Shipley et al., 1979; Kvenvolden, 1988), is important because it may (1) affect the Earth's climate through the storage and release of methane, a greenhouse gas (e.g., Nisbet, 1989; Paull et al., 1991); (2) cause sediment slumping on continental margins (e.g., Carpenter, 1981; Popenoe et al., 1993; Paull et al., 1996); and (3) influence the diagenesis of continental rise sediments (e.g., Lancelot and Ewing, 1972; Matsumoto, 1983; Borowski et al., 1996a, b). Gas hydrate occurrence is usually inferred from the appearance of a bottom-simulating reflector (BSR) on seismic reflection profiles (Tucholke et al., 1977). However, geochemical concentrations and isotopic profiles are potentially more sensitive indicators of underlying gas hydrate than established seismic detection methods (Borowski et al., 1996a). On Leg 172, seismic data collected showed a probable BSR under all the sites drilled along the Blake-Bahama Outer Ridge. Moreover, the pore water samples taken from cores give chloride concentrations that strongly suggest the presence of underlying gas hydrate. These data are critical to improve estimates of the size of the gas hydrate reservoir in the Blake Ridge area (and elsewhere), and to understand the geochemical processes involved in the development of extensive gas hydrate fields.

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