172 Scientific Prospectus


Paleoceanography and Paleoclimatology
One of the most intensively studied sediment drifts in the North Atlantic lies on the northeast BR, where the overlying deep water is the most turbid in the basin (Biscaye and Eittreim, 1977) due to advection of clays and silts by the deep Gulf Stream return flow (Laine and Hollister, 1981; see also Hogg, 1983; Schmitz and McCartney, 1993). The ultimate source of this terrigenous sediment is probably eastern Canada, although Laine et al. (1994) document local erosion on the eastern scarp of the BR and redeposition on the BR plateau. During glaciation, deposition rates were as high as 200 cm/k.y. on the BR (Fig. 5). Geochemical studies of cores from the BR have revealed the coupling of the ocean, the atmosphere, and ice sheets on the submillennial scale (Fig. 5). For example, deep ocean circulation at the depth of the BR (~4500 m) responded to the Younger Dryas cooling episode (Boyle and Keigwin, 1987), as well as to earlier oscillations in the climate system (Keigwin et al., 1991; Keigwin and Jones, 1994). Unfortunately, the price for high resolution studies is a short temporal record, even in a core 28 m long such as KNR 31 GPC-5 (Fig. 6, Table 2). Recent coring by the International Marine Global Change Study (IMAGES) program on the Marion Dufresne has produced a 53-m core extending into isotope Stage 6 near Site BR-1. Thus, it is important to core a much longer interval on the BR using the Advanced Hydraulic Piston Corer (APC) in order to document more than just one glacial/interglacial cycle.

In the BBOR/CS region at the western boundary of the Sargasso Sea, available evidence indicates patterns of climate change similar to those on the BR. The most heavily studied core from the BBOR region, KNR 31 GPC-9, was taken in 1973 from the northwest flank of the Bahama Outer Ridge at a depth of 4758 m (GPC-9 is located near Site BBOR-1 in Fig. 2, Table 2). Initial stratigraphy of that core was discussed by Flood (1978), followed by unpublished benthic foraminiferal (Lohmann, unpublished) and stable isotope studies (Curry and Lohmann, 1983). Keigwin and Jones (1989) documented the planktonic, stable isotope stratigraphy and presented accelerator mass spectrometer (AMS) radiocarbon results. Using AMS and delta18O stratigraphy (Fig. 5), they plotted percent-carbonate results at 4-cm spacing from the upper 2200 cm of GPC-9 vs. age (Fig. 6). It is apparent that many of the same millennial-scale climate oscillations are present on both the BBOR and the BR. These oscillations are thought to be a useful proxy for deep-ocean circulation changes (Keigwin et al., 1994; Keigwin and Jones,1994). Grain size results on the Blake Outer Ridge are consistent with nutrient proxy results as monitors of deglaciation changes in deep-ocean circulation, indicating there were important glacial/interglacial changes in the intensity and position of the Deep Western Boundary Current (DWBC) on the Blake Outer Ridge (Haskell, 1991; Haskell et al., 1991). Two pairs of Leg 172 sites from identical water depths, but very different sedimentary regimes, will document directly the effects of current controlled sedimentation.

Large vertical gradients can be expected in the Tertiary and Quaternary oceans. Using benthic foraminiferal chemistry, the BBOR region will monitor southern source waters entering the North Atlantic basin as well as northern source deep and intermediate waters exported in the depth range of 2000 to 4800 m. We also can expect to monitor important basin-wide changes in the position of the lysocline, which may be additionally influenced at this location by the position of the Deep Western Boundary current (Balsam, 1982).

Mud Wave Dynamics
Recent studies of mud-wave dynamics suggest that mud waves migrate because there are cross wave changes in bed shear stress (Flood, 1988; Blumsack and Weatherly, 1989). In the case of fine-grained cohesive sediment, accumulation rate decreases as shear stress increases (McCave and Swift, 1976), thus, less sediment accumulates on the wave flank with the higher flow speed. In the case of a lee-wave flow pattern, flows on the upcurrent, upslope wave flank are weaker than those on the downcurrent wave flank, leading to upslope and upcurrent wave migration. Enhanced wave migration is expected at higher flow speeds because currents on the downcurrent flank approach the critical shear stress for deposition before those on the upcurrent flank (Flood, 1988).

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, and a model-dependent flow speed can be estimated (Flood, 1988). This approach was used with success in the Argentine Basin where a mud wave appears to have become inactive during the last 20-30 k.y. (Manley and Flood, 1992). Although our present understanding suggests that only two core sites are required to make this comparison (one on each wave flank), this needs to be explicitly tested by sampling at least four places across the wave profile (crest, trough, and each flank) in order to choose the best locations for future ODP cores. Independent evidence of changes in flow speed will supplement interpretations of circulation change made on the basis of ocean paleochemistry.

Wave migration on sediment drifts has been a long-standing interest of the ODP, but it has not yet been successfully studied. As the Sedimentary and Geochemical Processes Panel (SGPP) White Paper (JOIDES Journal, 1990) states:

"The history of thermohaline bottom current processes is preserved in sediment drifts and sediment waves molded under relatively steady currents. Drilling transects will test sedimentation models for sediment structure and bottom current depositional processes and use these models to determine past variations in the bottom flow regime of the ocean."

Waveforms observed at Deep Sea Drilling Project (DSDP) Sites 610 and 611 were found to be surprisingly stable, migrating on the million-year time scale (Kidd and Hill, 1987). However, that study sampled wave crests and troughs, not wave flanks. Evidence from the Bahama Outer Ridge (Flood, 1978) and the Argentine Basin (Manley and Flood, 1992), as well as wave models suggest that the largest difference in sedimentation rates is to be expected on the flanks. The Bahama Outer Ridge wave field (Fig. 7) is mapped with much greater precision than those on Gardar and Feni Drifts and carbonate content in small free-fall cores indicates that sedimentation rates did indeed change between upstream and downstream wave flanks during the latest Quaternary (Flood, 1978).

Gas Hydrate and Pore-Water Geochemistry
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. Three DSDP-ODP legs in the Blake Ridge area (Fig. 8) have recovered gas hydrate and/or found pore-water signatures indicating its presence: DSDP Leg 11 (Sites 102, 103, and 104; Hollister, Ewing, et al., 1972), DSDP Leg 76 (Site 533; Sheridan, Gradstein, et al., 1983), and ODP Leg 164 (Paull, Matsumoto, Wallace, et al., in press). 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 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).

The presence of gas hydrate in the BBOR/CS region is of particular concern to Leg 172 for two reasons. First, both DSDP Leg 76 and ODP Leg 164 found that gas charging and sediment diagenesis associated with gas hydrate prevented successful operation of the APC at sub-bottom depths greater than ~150 m. Second, although the acoustic signature of gas hydrate is not present in the BBOR region at water depths >4 km, Leg 164 scientists found that the absence of the acoustic signature (the bottom-simulating reflector [BSR], below) does not mean the absence of hydrate. Thus, hydrate may compromise the Leg 172 objective of coring a high-resolution Pliocene section deep on the Blake Outer Ridge.

Gas hydrate occurrence is usually inferred from the appearance of a bottom-simulating reflector on seismic reflection profiles (Tucholke et al., 1977). However, geochemical concentration and isotopic profiles are potentially more sensitive indicators of underlying gas hydrate than established seismic detection methods (Borowski et al., 1996a). For example, Site 994 (Leg 164) displays no BSR but possesses the following pore-water anomalies that strongly suggest the presence of underlying gas hydrate (Paull, Matsumoto, Wallace, et al., in press): (1) chloride concentration decreases with depth, probably reflecting dissociation of gas hydrate at the base of the stability zone and upward migration of fresher fluids (Hesse and Harrison, 1981; Ussler and Paull, 1995); (2) the sulfate concentration profile decreases linearly, suggesting an upward methane flux from gas hydrates below (Borowski et al., 1996a); and (3) extreme depletion of 13C occurs within the interstitial methane and CO2 pools at shallow depths (Borowski et al., 1996b).

ODP Leg 172 will drill holes both inside and outside of the mapped distribution of BSRs of the Blake Ridge hydrate field (Fig. 8). The leg, thus, provides an opportunity to: (1) assess the lateral distribution of gas hydrate and its related geochemical signatures within the continental rise; and (2) assess the linkage of various geochemical patterns to diagenetic processes which may be directly or indirectly caused by 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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