172 Scientific Prospectus


Figure 1. Map of the western North Atlantic Ocean showing the location of Leg 172 sites on the northeast Bermuda Rise (Site BR-1), the Carolina Slope (CS), and Blake-Bahama Outer Ridge (BBOR). Note that alternate sites are not shown.

Figure 2. Map showing the location of the Carolina Slope and Blake-Bahama Outer Ridge sites. These sites form a depth transect in water depths from ~1200 to 4800 m. Sites BBOR-6 and BBOR-9 (~2 km apart) span an abrupt change in terrigenous sediment flux, and Site BBOR-1 and -1B will sample across a mud wave. Note: primary sites are shown in addition to the secondary Site BBOR-3.

Figure 3. Position of BBOR and CS Leg 172 sites with respect to water masses in the western subtropical North Atlantic. Water depths are indicated every 500 m along the temperature/salinity (T/S) plot. Sites were chosen so at least one lies within each modern water mass and one lies at the boundary between water masses. This depth distribution of sites is required to monitor the most likely changes in water masses and their boundaries through the late Neogene. U = upper. L = lower. KNORR 140/2 Hydro Sta. 1 refers to the site survey cruise and station number that were used to collect the data.

Figure 4. Schematic of circulation patterns in the deep western North Atlantic (updated from Schmitz and McCartney, 1993 by M.S. McCartney, pers. comm. to L. Keigwin, 1995). The thin lines represent streamlines of two recirculating gyres with approximate transport in Sverdrups (1 Sv = 106 m3s-1). Thick lines represent generalized flow direction of AABW and NADW, which contribute to the Deep Western Boundary Current (DWBC). The stippled pattern marks the region marked by high surface eddy kinetic energy (EKE), deep EKE, and deep suspended sediment. Note that in this scheme the southern recirculating gyre, over the Bermuda Rise, is the mixing zone for northern and southern origin waters, and that true "NADW" is formed in that mixing zone.

Figure 5. Age-depth relationship for Bermuda Rise Site BR-1 (core GPC-5, open circles) and for Bahama Outer Ridge site BBOR-1A (core GPC-9, solid circles). Solid line without data points is the calendar year age model for GPC-5 (Keigwin and Jones, 1994). AMS = accelerator mass spectrometer.

Figure 6. Percent-carbonate data for two Leg 172 sites (Core KNR31-GPC-5 for the Bermuda Rise and Core KNR31-GPC-9 for the Bahama Outer Ridge), plotted on the age model (solid line) of Fig. 5. The millennial-scale stadial/interstadial carbonate variability is thought to correlate with similar oscillations in ice core records of climate, and has been linked to variable production of NADW (Keigwin and Jones, 1994).

Figure 7. Map of mud-wave field on northwestern flank of Bahama Outer Ridge, based on Scripps deep tow with 4-kHz profiler, sidescan sonar, etc. (Flood, 1978). Note location of core GPC-9 in southwest corner. Sites BBOR-1 and -1B (asterisk) will be on the flanks of the mud wave just to the east of the GPC-9 location.

Figure 8. Map showing the outline of the Blake Outer Ridge gas hydrate field (stippled) for comparison with prospective Leg 172 core sites (Fig. 2). Gas hydrate is typically detected by the presence of a bottom-simulating reflector (BSR) on seismic reflection profiles. The gas hydrate outline is based on mapping the presence of BSRs (from Dillon and Paull, 1983; modified after Paull, Matsumoto, Wallace et al., in press).

Figure 9. Percent-core-recovery at ODP Hole 994C on the Blake Outer Ridge near Sites BBOR-6 and -9. This pattern is typical of other Leg 164 sites. Note the sudden drop in APC recovery at ~160 mbsf, and the generally poor recovery using the XCB. Difficult coring conditions are attributed to the presence of gas and to gas-related sediment diagenesis (carbonate nodules, drilling biscuits).

TO 172 Proposed Site Summaries
172 Table of Contents
Publications Home
ODP Home