172 Scientific Prospectus


Sites are discussed in the order they will be drilled.

CS-2 and CS-3B
These proposed sites will begin the depth transect in the BBOR/CS region by providing high resolution sections from water depths of ~1800 m and ~1300 m, respectively. Site CS-3B is presently in the shallowest waters of the Deep Western Boundary Current, which are thought to originate in the southern Labrador Sea. During glacial times the CS sites probably monitor glacial North Atlantic Intermediate Water. CS sites have much higher deposition rates than nearby sites on the Blake Outer Ridge; a pair of accelerator 14C dates from a site survey core close to Site CS-3B indicates zero years at the core top and 4500 yrs at 300 cm for a rate of 670 m/m.y. Together with BBOR-8C, these sites will test models of continental margin sedimentation during glacial lowering of sea level (see Paull et al., 1996). From the pore-water geochemical perspective, Site CS-2 is the first priority site for having a dedicated fourth hole as deep as 50 mbsf. Data from site survey cores show low foraminiferal abundances, giving added importance to quadruple coring at this site.

This site is located at a water depth (~2164 m) where the Blake Outer Ridge first becomes noticeable as a bulge on the continental slope. Presently that location lies within the core of upper NADW, close to the hinge between nutrient-enriched deeper waters and nutrient-depleted intermediate waters in hydrographic reconstructions of the last glaciation. By correlation to nearby ODP Site 996, Site BBOR-8C might extend back to 2.6 Ma if it is cored to 150 m.

This Blake Outer Ridge site was chosen for its high sedimentation rate and its modern day location between the upper and lower limbs of the NADW (Fig. 3). Site BBOR-7A will be triple APC cored to at least 100 m.

BBOR-6 and BBOR-9
Like the deeper mud wave field (Site BBOR-1/-1B), this pair of sites reveals striking physical evidence of current-controlled sedimentation. At present, the plan calls for three APC cores to at least 100 m at Site BBOR-6 where the boundary current at ~3000 m water depth has expanded the section, and three comparison APC holes cored to at least 150 m at Site BBOR-9, a location with a relatively lower sedimentation-rate, that is about a mile away. For reference, the site survey piston core at "lower-resolution" Site BBOR-9 has a higher rate of sedimentation than well-known "high resolution" DSDP Site 609 in the northern North Atlantic. The Site BBOR-6 and -9 pair will allow us to directly observe the history of boundary current movement by comparing sedimentation rates for short-time intervals. Such a comparison might reveal, for example, that the boundary current is only active at this location during interglacial time. Sediment flux studies at these sites will test some fundamental assumptions in sediment drift paleoceanography. If the only difference between the two groups of holes is the lateral flux of sediment brought by the boundary current, and if the foraminifers are not subject to traction transport, then the foraminiferal fluxes should be identical regardless of sedimentation rate. More cores might be taken to greater depth at Site BBOR-9 in order to recover a longer temporal sequence.

This site will be triple APC cored to at least 150 m, projected to an age of ~1.5 Ma. It is located very close to the position of DSDP Site 102 (3430 m), a high sedimentation rate location in the heart of the lower NADW. Unless the benthic front migrates by more than 500 m, this site will always be bathed by lower NADW (Fig. 3). From Site 102 we know that the sedimentation rates are nearly constant, with the ~3 Ma level at 350 m (based on the last appearance [LA] of Sphaeroidinellopsis and G. altispira). Significant gas expansion was noted at Site 102 beginning at ~100 m, but carbonate diagenesis similar to that which interfered in part with APC operations on Leg 164 was not noted. A fourth shallow (~50 m) hole at this site would be valuable in documenting the shallowest depth-of-no-sulfate on the Blake Outer Ridge. This peculiar occurrence implies a very high upward flux of methane at this site.

This second priority site on the tip of the Blake Outer Ridge at ~4250 m water depth is not currently part of the drilling plan. Sediments there might be free of gas hydrate, and clearance has been granted to core as deep as 350 m, if the situation permits. This location has the highest late Quaternary sedimentation rate of the entire BBOR region (~339 m/m.y.). It should make a useful complement to the BR site far to the northeast in the Sargasso Sea.

At ~4 km water depth, proposed Site BBOR-4B lies close to the boundary between AABW and lower NADW in the western North Atlantic (Fig. 3). This position makes it sensitive to changes in the balance between these two water masses, and its record will complement that of Site BR-1 at ~4.5 km. No BSR is evident in the seismic data, so it is hoped that triple APC penetration will be greater than 150 m and that XCB cores will have complete recovery and little disturbance. If time permits, double or triple XCB coring to 350 m will be attempted. If XCB recovery is high, the cores should contain Pliocene sediments that record a high sedimentation rate. This record could then be compared with the cores from Legs 154 and 162, which are located to the south and north, respectively, of this study.

BBOR-1 and BBOR-1B
This site is in the well-known deep mud-wave field just northwest of the Bahama Outer Ridge (~4700 m). The waters there are composed of ~20% AABW (Fig. 3) that has mixed with the southward-flowing NADW and follows the bathymetric contours. The deposition rates are very high and foraminiferal abundance is very low, which we know from the site survey and other cores. For example, during the latest Quaternary a typical wave accumulated at an average of 262 m/m.y. and benthic foraminifers are frequently absent during glacial and stadial climates. At this site we plan to core six holes to at least 100 m. The minimum plan is for three holes on the high depositional-rate east flank (Site BBOR-1), and two on the lower-rate western flank (Site BBOR 1B). These sites will be the deepest high-resolution paleoceanography sites in the North Atlantic (and perhaps the deepest anywhere), and will be useful for monitoring the evolution of the blend of NADW and AABW in the North Atlantic, for studying magnetic reversals in high resolution and for directly measuring paleocurrent speed through ratios of sedimentation rates on either flank of the wave. As these holes are the last to be cored in the CS/BBOR region and may be less affected by gas hydrate than sites in shallower water, this site may be cored as deep as 350 mbsf if problems are experienced at shallower sites. Other possible sites in this sedimentary environment include a wave crest site (Site BBOR-1C) and another east-flank site on a nearby mud wave (Site BBOR-1A). One of the two holes on the low-deposition-rate flank will be available for pore-water sampling once per section in the upper few tens of meters.

This is the last site to be cored by Leg 172, on the way to port in Lisbon, Portugal. The location is very well surveyed, and the facies should be the same as those cored by DSDP Site 9 (Hays, et al., 1972). Goals for that site were to date the bottom of the prominent acoustic reflectors at ~300 mbsf and to date sediment overlying basalt as a test of seafloor spreading. Neither objective was achieved because of spot coring, core catcher failure, and generally unfossiliferous sediments at great depth. It is expected that the onset of Northern Hemisphere glaciation led to increased terrigenous flux which may account for the acoustically stratified facies. Site BR-1 will be triple cored to not less than 350 m. If time is available, a fourth hole will be cored at this site for pore water studies and to ensure enough sample for high-resolution studies.

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