Technical Note 20/4

LEG 172

NW Atlantic Sediment Drifts

Modified from Proposal 404 Submitted By

L.D. Keigwin, R. Flood, and E.A. Boyle

Staff Scientist: Gary Acton
Co-Chief Scientists: Lloyd D. Keigwin &;Domenico Rio


The Blake-Bahama Outer Ridge (BBOR) and Carolina Slope are located at the western boundary for deep water circulation in the North Atlantic and the surface waters of the Gulf Stream, which are important as a source of salt and heat to the northern North Atlantic. For this reason, the BBOR is the optimal location to monitor changes in North Atlantic Deep Water (NADW). It is also an excellent location to monitor Antarctic Bottom Water (AABW) because that water mass is recirculated in the subtropics, blending with the exposed NADW. At shallower depths the region is bathed by water from the Labrador Sea, which on geological time scales may be as important a water mass as is the upper NADW. Recently a shallow component to Labrador Sea water has been identified which interacts with the Gulf Stream, probably controlling the distribution of sediment on the Carolina Slope. 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 Carolina Slope. 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 Bermuda Rise site will be compared with data from sites located at deep, high-deposition-rate locations on the BBOR and Carolina Slope 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.

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 (Lonsdale, 1982; McCave and Tucholke, 1986). 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 North Atlantic Deep Water (NADW) on geological time scales using geochemical and sedimentological techniques (Keigwin and Jones, 1989). Leg 172 will core 12 sites: nine on the Blake-Bahama Outer Ridge (BBOR), two on the Carolina Slope, and one on the Bermuda Rise (Figs. 1 and 2) to distinguish between latitudinal changes in the mixing zone between southern- and northern-source waters and changes due to vertical migration of a benthic front. A North Atlantic depth transect at the BBOR is especially important because this feature forms a western boundary for deep currents (Stommel, 1958), which follow depth contours (Heezen et al., 1966). Above ~4000 m these waters are sourced mostly from the north, whereas at greater depths there is a greater proportion of recirculated southern-source water (Hogg, 1983). 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 orbital time scales.

Paleoceanography and Paleoclimatology
One of the most intensively studied sediment drifts in the North Atlantic lies on the northeast Bermuda Rise, where the overlying 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). The ultimate source of this terrigenous sediment is probably eastern Canada. During glaciation, deposition rates were as high as 200 cm/k.y. on the Bermuda Rise (Keigwin and Jones, 1989). Geochemical studies of cores from the Bermuda Rise have revealed the coupling of the ocean, the atmosphere, and ice sheets on the sub-millennial scale. For example, deep ocean circulation at the depth of the Bermuda Rise (~4500 m) responded to the Younger Dryas cooling episode (Boyle and Keigwin, 1987), as well as earlier oscillations in the climate system (Keigwin et al., 1991). Results from isotope stages 3-5 (23-80 k.y.) at the bottom of Giant Piston Core (GPC)-5 show similar climate oscillations. Unfortunately, the price for high resolution sedimentology is a short temporal record, even in a core 28 m long. Thus, it is important to core the much longer record on the Bermuda Rise (BR) using the Advanced Hydraulic Piston Corer (APC).

The most heavily studied core from the BBOR region, GPC-9, was taken in 1973 from the northwest flank of the Bahama Outer Ridge at a depth of 4758 m (Fig. 3). Initial stratigraphy of that core was discussed by Flood (1978), followed by unpublished benthic foraminiferal (Lohmann, unpublished) and stable isotope studies (Curry and Lohmann). Keigwin and Jones (1989) documented the planktonic, stable isotope stratigraphy and presented accelerator mass spectrometer (AMS) radiocarbon results. Using AMS and 18O stratigraphy (Fig. 4A), we have plotted percent carbonate results at 4 cm spacing from the upper 2200 cm of GPC-9 vs. age (Fig. 4B). A core at 4935 m (GPC-7; Figure 3) has too little carbonate to determine a useful stratigraphy (Keigwin and Jones, 1989), making the depth of GPC-9 (~4800 m) a lower limit for cores in the depth transect.

Results from about six conventional diameter piston cores have been reported from the crest of the Blake Outer Ridge between water depths of 2600 m and 4400 m on cruises of the Cape Hatteras (Fig. 3). Most of these cores have lower rates of deposition than GPC-9, based on the available low-resolution (20-cm spacing) percent carbonate curves and 18O curves on planktonic foraminifera (Johnson et al., 1988, and Haskell et al., 1991). The major emphasis on these cores was to study grain-size variations as a measure of Western Boundary Undercurrent velocity and position. Grain size results were consistent with nutrient proxy results (Keigwin et al., 1991) as monitors of deglaciation changes in deep-ocean circulation, indicating there have been important changes in the intensity and position of the Deep Western Boundary Current on the Blake Outer Ridge (Haskell et al., 1991). A summary of percent carbonate in the Blake Outer Ridge piston cores (Fig. 5; Haskell, 1991) shows that had they been sampled closely enough, they might reveal variability similar to that of GPC-9 (Fig. 4B). Although the Blake Outer Ridge has the potential for benthic paleochemical reconstructions with resolution similar to that on the Bermuda Rise and Bahama Outer Ridge, it has not yet been demonstrated.

Large vertical gradients can be expected in the Tertiary and Quaternary oceans. Using benthic foram 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 2000 to 4800 m. We also can expect to monitor important basin-wide changes in the position of the lysocline, which may have an additional influence at this location by the position of the Deep Western Boundary current (Balsam, 1982). Sedimentological studies also indicate considerable depth-related variability, which may reflect changes in sediment provenance and current speed (Haskell et al., 1991).

On the Bermuda Rise we found evidence for oscillations in surface and deep water properties which appear to be related to millennial-scale variability in carbonate content characteristic of the past 80 k.y. By comparison with the core from the Bahama Outer Ridge, we speculate that similar oceanographic change is typical of the deep western basin of the North Atlantic in general. Specific questions to be answered include:

For a bathymetric reconstruction to be most useful it should range from the deepest waters (the location of GPC-9, ~4800 m) to intermediate depths. For this part of the western North Atlantic we have good intermediate coverage to 1500 m from the thesis transect of Slowey (1990) at Little Bahama Bank (Fig. 6). Slowey's glacial-interglacial 13C comparisons clearly show that the glacial thermocline was better ventilated (more nutrient-depleted) than today. Other results on widely distributed Atlantic Ocean cores show that the nutrient depletion extended as deep as ~2400 m (Boyle and Keigwin, 1987; Oppo and Fairbanks, 1987), but there has been no concentrated transect of cores to make detailed reconstruction for the western North Atlantic. Because the Blake Outer Ridge first detaches from the continental margin at ~2000 m (Fig. 7), the proposed coring transect for this leg will begin there, making a patch with Slowey's nearby results. Coring will extend deeper than the deepest Cape Hatteras core, onto the Bahama Outer Ridge where we will fill in the depth range between the Bermuda Rise (~4500 m) and core GPC-9 (~4800 m). Data obtained from Site BR-1 on the Bermuda Rise will be compared with data from sites located at deep, high deposition rate locations on the BBOR and Carolina Slope to document late Neogene oceanographic change in the western North Atlantic for millennial as well as Milankovitch time scales over the entire deep- and intermediate-water column.

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 has been 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, each flank) in order to choose the best locations for 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 drilling program, but it has not yet been successfully studied by ODP. 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 DSDP Sites 610 and 611 were found to be surprisingly stable, migrating on the million year time scale (Kidd and Hill, 1986). 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 models suggests that the largest difference in sedimentation rates is to be expected on the flanks. The Bahama Outer Ridge wavefield (Fig. 8) 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).

The major objectives of this program 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, O), carbonate, 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, this proposal seeks to investigate:
€ sediment wave migration and drift sedimentation processes
€ detailed variations of the Earth's magnetic field (secular variations, reversals)
€ geotechnical/acoustic properties of the deep-sea sediments
€ structure of stable isotope Milankovitch cycles during the Pleistocene/Pliocene

The goals of Leg 172 can best be achieved by APC/XCB coring of one site on the Bermuda Rise (BR), coring a depth transect on the Blake-Bahama Outer Ridge (BBOR, nine sites) and the Carolina Slope (CS, two sites), the first such ODP transect in the North Atlantic. The original drilling plan called for APC/XCB coring at all the sites given in the table below. At the annual Planning Committee (PCOM) meeting in December 1995, PCOM considered this proposal in the light of results from North Atlantic-Arctic Gateways II Leg 162 and decided that ODP's long-term planning for climate and ocean circulation studies would be best served by extending the objectives of some of the sites in the transect (see Site Table).

Most of the proposed holes in the depth transect will not exceed 350 m penetration and will not be logged, although deeper holes may be logged. The primary objectives of Leg 172 will depend on good core recovery of material for stable isotope measurements, as well as other paleoclimate proxies. Downhole logging may help achieve the primary goals of this proposal by ensuring that full stratigraphic coverage is achieved through the integration of core and log measurements. Where core recovery is incomplete or core disturbance is high, usually in XCB cored sections, downhole logs will provide continuously sampled physical properties that can serve as proxies for paleoclimatic indicators.

Geophysical logs (sonic, porosity density, and resistivity) may also provide the acoustic characterization of the sediments and further quantification of gas and gas hydrate present in the sediments. In addition, Formation MicroScanner (FMS) images may provide detailed characterization of sedimentary structures (particularly difficult to obtain in XCB cores) for interpretation of sedimentary processes in the drift deposits, help orient XCB cores for magnetic studies, as well as provide a very high-resolution record of resistivity (although not in absolute values) for cyclicity studies. In summary, the total number of logged holes will depend on penetration, core recovery and quality, available time, and the specific objectives at the site.

This site is in the well-known deep mud wave field just northwest of the Bahama Outer Ridge (~4700 m). The waters there are ~20% AABW that has mixed with the southward-flowing NADW and follows the bathymetric contours. The deposition rates are very high, which we know from the site survey and other cores. For example, for the latest Quaternary a typical wave accumulates at an average of 262 m/m.y. At this site we plan to core six holes to 200 m, three on the high-depositional rate east flank, and three on the lower rate western flank. 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 will not be affected by gas hydrate, this hole may be cored as deep as 350 mbsf if problems are experienced at shallower sites or if we do not core as deep as 200 m at the lower-sedimentation rate side of the wave.

No information at this time.

No information at this time.

No information at this time.

This site will be triple APC cored to 200 m at the tip of the Blake Outer Ridge at ~4250 m. Sediments there should be free of gas hydrate, so we are requesting clearance to extend holes 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 Bermuda Rise site (BR1) far to the northeast in the Sargasso Sea.

No information at this time.

This site will be triple APC cored to refusal and double XCB cored to 350 m. It is located at the position of DSDP Site 102 (3430 m), a high sedimentation rate location in the heart of the lower NADW. From Site 102 we know that the sedimentation rates are nearly constant, with the ~3 Ma level at 350 m (based on the LA of Sphaeroidinellopsis and G. altispira). Significant gas expansion was noted at Site 102 beginning at ~100 m, but carbonate diagenesis like that which interfered with APC operations on Leg 164 was not noted.

BBOR-6 and -9
Like the deeper mud wave field, this pair of sites reveals striking physical evidence of current controlled sedimentation. At present, the plan calls for three APC cores to 150 m at a high deposition-rate location where the boundary current at ~3000 m has expanded the section, and three comparison holes to the same depth at a relatively lower sedimentation-rate location less than a mile away. These two pairs of triple-core locations 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 in 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 foraminifera are not subject to traction transport, then the foram fluxes should be identical regardless of sedimentation rate. We are considering extending the penetration of the relatively low sedimentation-rate holes at the expense of the higher sedimentation-rate holes to get farther into the Pliocene.

This Blake Ridge site was chosen for its high sedimentation rate and its modern day location between the upper and lower limbs of the NADW. BBOR-7 will be triple APC cored to 100 m.

As a result of a Detailed Planning Group (DPG) study, this site has been moved slightly to a location with better seismic coverage. In addition, this site will be extended to 200 m by XCB which may be necessary judging from the experiences of Leg 164. That should give a bottom age well within the Pliocene, which is important considering that at 2164 m this site lies close to a bathymetric front during late Quaternary time.

CS-2 and CS-3
These sites will complete the depth transect in the BBOR region by providing high-resolution sections from water depths of 1790 m and 1203 m, respectively. CS-3 has been moved slightly by the DPG (now CS-3B) to lie on an existing seismic line. Because of the prevalence of slumps and slides on the Carolina Slope, it is recommended that these sites receive a quick seismic survey by the drillship before coring.

This is the last site to be cored by Leg 172, on the way to port in Lisbon, Spain. The location is very well surveyed, and the facies should be the same as those cored by DSDP Site 9. Goals at 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 acoustically stratified facies began with Northern Hemisphere glaciation.

No information at this time.

* Tentative logging plan
** Sites were not finalized at time of publication

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