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doi:10.2204/iodp.sp.303306.2004
Orphan Knoll is a seamount located between the Newfoundland continental slope and the Northwest Atlantic Mid-Ocean Canyon (NAMOC) (Fig. F4). Oceanic magnetic Anomaly 34 (Late Cretaceous) lies offshore to the east. Core HU90-045-094, collected by the Hudson (HU) in 1990, was located in a small channel east of Orphan Knoll within 1 km of a 30 m core (MD95-2024) collected during the 1995 Marion Dufresne (MD) (Images) campaign. An additional 30 m "calypso" piston core (MD99-2237) was collected nearby during the 1999 Images campaign.
The attraction of this region is that these MD and HU piston cores have provided the most detailed marine record of Laurentide Ice Sheet (LIS) instability over the last 110 k.y. in the form of rapidly deposited detrital layers intercalated in background hemipelagic sediments (Hillaire-Marcel et al., 1994; Stoner et al., 1995, 2000). The site is apparently well positioned to sample overflow deposits from the nearby NAMOC and is therefore a sensitive monitor of NAMOC activity and, hence, LIS instability. Drilling at this site will extend the record of LIS instability back beyond the last glacial cycle, allowing us to observe LIS instability in a variety of glacials and interstadial conditions, as well as in the "40-k.y. world" and the transition into the "100-k.y. world."
Drilled on the crest of Orphan Knoll in 1970, DSDP Site 111 lies ~36 km northwest of proposed Site ORPH2A. Site 111, located in 1797 m water depth, was drilled to 250 mbsf. The base of the Pleistocene was placed at 128 mbsf with 20 m of Pliocene overlying a series of unconformities between which lower Miocene, Eocene, and Mesozoic sediments were recovered.
Proposed Site ORPH2A is located at the site of cores MD95-2024 and HU91-045-094 (Fig. F5). A cruise of the Hudson in August 2001 obtained crossing seismic reflection lines for the proposed site using a Huntec deep-towed sparker system augmented by single-channel data (Toews and Piper, 2002). The ~200 ms TWT for the upper Quaternary sediment sequence at proposed Site ORPH2A is increased to ~300 ms TWT at proposed Site ORPH3A (Fig. F6). Correlation of reflectors to the HU and MD piston cores places the "pink" reflector at ~120 ka (Fig. F7). At proposed Site ORPH2A, ~5 km northeast from the scarp shown in Figure F7, the "pink" reflector is at ~30 ms TWT (Fig. F8). The Quaternary sedimentary section is expanded at proposed Site ORPH3A relative to ORPH2A.
Two holes were drilled at ODP Site 646 (Leg 105) on the northwest edge of Eirik Drift (Fig. F9) during October 1985. The penetration depths were 103.5 mbsf in Hole 646A and 777 mbsf in Hole 646B. Poor recovery (53% for Hole 646B) led to an incomplete record of the upper Miocene–Holocene sedimentary sequence. The sediments are silty clays in the upper Pliocene–Quaternary (0–188 mbsf), muddy sand and silty muds in part of the upper Pliocene (188–236 mbsf), and silty clay in the upper Miocene–upper Pliocene interval (236–766 mbsf). Thin-bedded detrital carbonate beds are present throughout the section and are of particular interest because they can be associated with LIS instability. Little significance was attached to these layers in the original studies of Site 646 sediments; however, we now know from nearby piston cores (e.g., HU90-013-013P) that these detrital layers correlate with Heinrich-type detrital layers in the North Atlantic IRD belt. One of the objectives of drilling at this site will be to place these detrital layers into a PAC and, hence, correlate them to similar layers at the other drill sites and to ice core records.
The Site 646 sediments carry a well-defined magnetization component resolved by alternating-field (AF) demagnetization. The Brunhes/Matuyama boundary was identified at 60 mbsf (Clement et al., 1989), indicating a mean Brunhes sedimentation rate of 7.7 cm/k.y. A complete magnetostratigraphy of the late Miocene–Holocene record was compromised by incomplete recovery and drilling disturbance, exacerbated by poor weather conditions. Planktonic oxygen isotope data (Aksu et al., 1989) are available from Site 646 sediments to MIS 23 (~900 ka).
The deeper-water proposed sites (LAB3A and LAB6A) are located south of ODP Site 646 (Fig. F9). Tracing of reflectors from Site 646 along seismic Line BGR-2 and KN166-14 Lines 21 and 25a indicates that proposed Sites LAB3A and LAB6A have expanded Pliocene–Quaternary sedimentation relative to Site 646. These deepwater sites are located below the Western Boundary Undercurrent (WBUC) and therefore are characterized by expanded interglacial sedimentation. The contrasting sedimentation between proposed Sites LAB3A, LAB6A, and LAB7A will allow us to document changes in the outflow of the WBUC (therefore in the production of NADW) during Pliocene–Quaternary time and also to reconstruct the deep-sea circulation patterns that prevailed during interglacial intervals. The composite record from the proposed sites will benefit from the contrasting sedimentation patterns, thereby maximizing the resolution of the composite record at this location.
Proposed Site LAB3A is located close to the LA-5 site (surveyed for ODP Leg 105) on Eocene oceanic crust. Proposed Site LAB3A lies at SP 4000 on seismic Line BGR-2 at 3350 m water depth (Fig. F10). Hudson (84-030) Line 14 passes through ODP Site 646 (Fig. F11) and crosses Line BGR-2 at SP 3900 (Fig. F10). KN166-14 Line 21 crosses Line BGR-2 at SP 3697 on Line BGR-2 (Fig. F12). The multichannel seismic (MCS) profile for KN166-14 Line 21 at the BGR-2 crossing is shown in Figure F13.
Maximum penetration at ODP Site 646 was 767 mbsf (reaching into the upper Miocene) in Hole 646B. At this site, reflector R1 (late Pliocene) lies at 236 mbsf and the base of the Pliocene (R2) at 480 mbsf (Fig. F11) (Arthur et al., 1989; Shipboard Scientific Party, 1987). We expect the sedimentary section at proposed Site LAB3A to be thicker than that at Site 646. APC coring was discontinued at Site 646 after recovery of Core 105-646B-14H (~130 mbsf) as pull-up was becoming "difficult" in poor weather conditions. We expect to achieve a maximum of ~ 300 mbsf using the APC drillover technique.
In the 767 m section recovered at Site 646, there was no evidence of gas pockets or expansion of sediment within core liners. Methane was not detected above 180 mbsf (within the zone of pore water sulfate depletion). Below 180 mbsf, methane occurrences were very sporadic, with three maximum values in the 25,000–30,000 ppm range for gas collected by vacutainer sampling the 300–400 mbsf depth range. Low organic carbon contents in the sediment (average ~0.35 wt%, mainly of terrigenous origin) (Stein et al., 1989) indicate little biogenic activity.
Proposed Site LAB6A is located at CDP 900 on KN166-14 Line 25a (Figs. F14, F15, F16) on Eocene oceanic crust at 3485 m water depth. Sedimentation rates are enhanced relative to proposed Site LAB3A, providing the opportunity to recover an expanded Pliocene–Quaternary section at sedimentation rates estimated to be ~50 cm/k.y., based on a provisional age model for a gravity core collected at the site during the KN166-14 cruise (Henderson and Wright, 2004).
Proposed Site LAB7A is located at the crossing of KN166-14 Lines 19 and 24c (Fig. F9) on Eocene oceanic crust at a water depth of 2273 m. MCS profiles for Line 19 (Fig. F17) and Line 24c (Fig. F18) indicate that this relatively shallow water site provides the opportunity to recover an expanded Pliocene–Quaternary section at sedimentation rates in excess of 15 cm/k.y. The 3.5 kHz profile is shown in Figure F19.
Alternate proposed Sites LAB8A, LAB8B, and LAB8C are located on KN166-14 Line 25a at CDPs 13975, 15000, and 14375, respectively (Figs. F20, F21). The objective here is to sample an expanded Pliocene–Quaternary sediment package at relatively shallow water depths (water depth = 2556 m at proposed Site LAB8B) and the mudwaves at proposed Sites LAB8A and LAB8C. Drilling in the mudwaves (Fig. F20) permits access to the older part of the sedimentary section by APC drilling. In addition, the results will provide important information on the sedimentary architecture of the mudwaves and, hence, on sedimentary evolution of the Eirik Drift.
Gardar Drift (GAR)
ODP Leg 162 (Sites 983 and 984) drilling in the Iceland Basin indicated that northern Gardar/Bjorn Drift sediments are devoid of major hiatuses, at least during the Quaternary. Sites 983 and 984 are located outside the main IRD belt (sensu Ruddiman, 1977) and do not contain a robust detrital carbonate (Heinrich layer) signal. We propose two sites in the southern part of the Gardar Drift (Fig. F1) where sedimentation rates are high (9–11 cm/k.y.) and yet close enough to the IRD belt to record the Heinrich-type detrital layers, which monitor ice sheet instability. The water depths at GAR1B and GAR2A (2820 and 3024 m) and the ability to derive a benthic stable isotope record from this region (see Chapman and Shackleton, 1999) will allow ice sheet–ocean interaction to placed on a benthic isotopic record. Carbon isotope data will allow high-resolution monitoring of NADW. ODP Leg 162 drift sites (Sites 980–984) are all at shallower water depths (<2000 m) and therefore monitor the intermediate water. At ODP Site 982, Venz et al. (1999) proposed that GNAIW production ceased during terminations and NADW production increased. There was apparently a time lag between the shutdown of GNAIW and the renewed production of upper NADW. ODP Site 607 presently provides the best monitoring of NADW (Raymo et al., 1992). One of the objectives of proposed Sites GAR1B and GAR2A is to provide NADW monitoring at higher resolution in the southern part of the Gardar Drift.
Proposed Sites GAR1B and GAR2A were surveyed from Knorr during summer 2002 using the Lamont-Doherty HiRes MCS system: dual 45/45 in3 generator injector guns towed at 2 m, recording 48 channels, and then stacking the data 24-fold. Continuous hull-mounted 3.5 kHz sonograms and Hydrosweep bathymetry were also recorded during the survey and during transit between proposed Sites GAR1B and GAR2A.
On the crest of the Gardar Drift, sedimentation rates are ~11 cm/k.y. for a 33 m core (MD99-2253) collected by the Marion Dufresne in 1999 (56°21.78'N, 27°48.95'W; water depth = 2840 m). This is close to the location of proposed Site GAR1B (Fig. F22). It lies on oceanic crust close to Anomaly 10 (~30 Ma), and the total sediment thickness is ~600 m. The MD99-2253 piston core has a moderately high sedimentation rate (~9 cm/k.y. for the last glacial cycle) and well-defined planktonic 
18O and geomagnetic paleointensity records. We present the Seabeam survey at proposed Site GAR1B (Fig. F23), the MCS track map (Fig. F24), the MCS profiles (Figs. F25, F26, F27), and the 3.5 kHz data (Fig. F28).
Proposed Site GAR2A is located at the site of Core HU91-045-080 (53°3.40'N, 33°31.78'W; water depth = 3024 m). It lies north of the Charlie Gibbs Fracture Zone and east of Reykjanes Ridge. It is located on oceanic crust close to Anomaly 5 (~10 Ma), and the total sediment thickness is ~600 m. The Seabeam survey (Fig. F29), the MCS track map (Fig. F30), the MCS profiles (Figs. F31, F32), and the 3.5 kHz data (Fig. F33) are presented.
Proposed Site IRD1A is at DSDP Site 609. The site is an obvious candidate for drilling using modern techniques to recover a demonstrably complete record of the sediment sequence. DSDP Hole 609 penetrated to 400 mbsf and recovered an upper Miocene to Holocene sedimentary sequence comprising nannofossil ooze and chalk. Unfortunately, the present state of these cores, collected in 1983, is poor. This classic site must be redrilled for the high-resolution studies proposed here. The mean sedimentation rate at Site 609 was ~5.7 cm/k.y. Vema cruise 23 passed over the site. Vema single-channel seismic (SCS) data were augmented by crossing lines (Ch94) at the time of DSDP drilling. The sediment thickness was estimated to be ~800 m. Our proposed penetration at proposed Site IRD1A (300 mbsf to within the upper Miocene) is less than the penetration in DSDP Hole 609 (400 mbsf); therefore, we believe that no further site survey data are required. The base of the Pliocene lies at ~295 mbsf and represents an attainable target depth for APC drilling.
Proposed Site IRD3A is placed at DSDP Site 607. Mean sedimentation rates at this location are 5–10 cm/k.y. for the last few million years. Two holes were drilled at this site during DSDP Leg 94 (June–August 1983) using the hydraulic piston corer (HPC). A 311 m section of foraminiferal-nannofossil ooze and nannofossil ooze was recovered. The rationale for reoccupying this site is essentially the same as that for DSDP 609. Together Sites 609 and 607 constitute benchmark sites for the long-term (millions of years) surface and deep ocean climate records from the subpolar North Atlantic. DSDP Leg 94 drilling of this site preceded the shipboard capability for construction of composite sections (and pass-through magnetometers for continuous measurement of magnetic parameters). Paleomagnetic data from this site indicate that the magnetic properties are optimal for recording the geomagnetic field. The present condition of existing DSDP cores, collected in 1983, does not permit the high-resolution studies proposed here.
At the site of core VM 30-97, located close to DSDP Site 607, Heinrich events are marked by the distinctive detrital carbonate signature, thereby providing a means of correlation to the other proposed sites. At this locality, antiphased patterns of ocean surface temperatures were found in core VM 30-97 for the 10–40 ka interval (Bond et al., 1999). The proposed redrilling of Site 607 will provide a long-term record of this apparent antiphase pattern that is beginning to emerge in the western North Atlantic. By placing the surface temperature signals into a chronological framework based on a combination of oxygen isotopic stratigraphy, detrital carbonate-bearing Heinrich events, and geomagnetic paleointensity, we expect to obtain an optimal reconstruction of the phasing of the temperature records. Penetration at Site 607 reached 311 mbsf (Hole 607A). We propose the same penetration depth, reaching upper Miocene sediments (base of Pliocene at ~240 mbsf).
At DSDP Site 608 (proposed Site IRD4A), the total penetration was 530 mbsf, reaching basaltic basement (capped by middle Eocene sediments) at 515 mbsf. The target at proposed Site IRD4A is the excellent upper Miocene section in the 140–260 mbsf interval. The base of Pliocene lies at ~140 mbsf and the base of the Miocene at ~410 mbsf.