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doi:10.2204/iodp.proc.303306.103.2006 Sites U1302 and U13031Expedition 303 Scientists2Background and objectivesOrphan Knoll, located 650 km northeast of St. John’s (Newfoundland, Canada), is a topographic high consisting of a fragment of continental crust that detached from North America during continental rifting (Keen and Beaumont, 1990; Chian et al., 2001). It rises 2 km above the Labrador Sea abyssal plain and coincides with the junction of oceanic and continental crust. Integrated Ocean Drilling Program (IODP) Sites U1302 and U1303 are separated by 5.68 km in 3250–3500 m water depth ~35–45 km southeast of the crest of Orphan Knoll (1800 m water depth), the location of Deep Sea Drilling Project Site 111 drilled in 1970. The axis of the Northwest Atlantic Mid-Ocean Channel (NAMOC) (see Chough et al., 1987), at ~4000 m water depth, lies 200 km to the northeast (Fig. F1). Sites U1302 and U1303 are on the crest of a small ridge to the east of a fault scarp marking the eastern side of Orphan Knoll (Fig. F2). Surrounding canyons partially protect this location from debris flow wedges associated with advances of ice tongues to the edge of the shelf (Aksu and Hiscott, 1992). The impetus for IODP sampling at Sites U1302 and U1303 stems from sedimentologic observations made in piston Core HU91-045-094P recovered close to Site U1303 (Fig. F3) by the CCGS Hudson (HU) in 1991. This 11 m piston core revealed the presence of numerous detrital layers deposited during the last glacial cycle (Hillaire-Marcel et al., 1994; Stoner et al., 1995, 1996), providing a proximal record of Laurentide Ice Sheet (LIS) instability, equivalent to the more distal Heinrich layers of the central Atlantic ice-rafted debris (IRD) belt (Heinrich, 1988; Broecker et al., 1992). For Core HU91-045-094P, Stoner et al. (1995) introduced a labeling scheme for detrital carbonate (DC1–DC6) and low detrital carbonate (LDC 5–6) layers. The detrital layers can be detected by a range of proxies such as gamma ray attenuation (GRA) density, magnetic susceptibility (Fig. F4), color reflectance, and grain-size-sensitive magnetic parameters (see Stoner et al., 1996). Low planktonic δ18O and δ13C values coincide with detrital layers, indicating the presence of meltwater and low productivity in surface water (Hillaire-Marcel et al., 1994; Hillaire-Marcel and Bilodeau, 2000). Following the important results from Core HU91-045-094P, the Site U1303 location was sampled during the 1995 and 1999 Images cruises of the Marion-Dufresne (MD; Cores MD95-2024 and MD99-2237). The stratigraphy in these MD cores can be correlated to the HU core (Fig. F4) and extends the environmental record into marine isotope Stage (MIS) 6, although the record is incomplete beyond MIS 5d (Turon et al., 1999; Stoner et al., 2000). The detrital layers have been shown to correlate to cold stadials in the Greenland Summit GRIP/GISP2 ice core on the basis of correlation of both marine/ice core δ18O records and relative geomagnetic paleointensity data from Core MD95-2024 to cosmogenic isotope flux at GRIP/GISP2 (Stoner et al., 2000). An enigma in the correlation between Core HU91-045-094P and the two MD cores is the apparent contrast in accumulation rates among cores, whereby the MD cores have approximately twice the apparent accumulation rates of the HU core (Fig. F4). This may be explained partly by the stretching in the MD “calypso” cores. Two piston cores were also collected in the Orphan Basin, 85.6 km (46.17 nmi) southwest of Sites U1302 and U1303. One of these piston cores (Core HU92-045-11P) is 11.7 m long and was described by Hiscott and Aksu (1996). The other (Core MD95-2025) was collected during the 1995 Images cruise of the Marion-Dufresne (Hiscott et al., 2001). These cores have mean sedimentation rates of 10 cm/k.y. and record both detrital carbonate and low detrital carbonate “Heinrich-like” events back to 340 ka. The identification of detrital events is based on the “Heinrich variable,” the ratio of undifferentiated rock fragments and silicate minerals to foraminifers in the 180–3000 µm grain-size fraction. Although it is generally believed that detrital layers at Sites U1302 and U1303 denote LIS instability and are a proximal analog to the central Atlantic Heinrich layers, the depositional origin of the layers remains open to speculation. The layers appear to have multiple components including IRD and sediments deposited from suspension possibly related to turbiditic flow along the NAMOC or spillover turbidites derived from the NAMOC (Hillaire-Marcel et al., 1994; Stoner et al., 1995, 1996). Fine-grained detrital carbonate (glacial flour) does not flocculate as readily as detrital clay and may be transported over 100 km in suspension (see Hesse et al., 1997), possibly supporting the NAMOC origin of the carbonate-rich suspended sediment component of these detrital layers. The objectives at Sites U1302 and U1303 were to document the detrital layer stratigraphy at the sites, understand the structure and origin of individual detrital layers, and extend the sediment record beyond MIS 5d. We aim to place this important environmental record into a paleointensity-assisted chronology, building on the Labrador Sea chronostratigraphy developed by Stoner et al. (1998, 2000) for the last glacial cycle. Are detrital carbonate events present in glacial stages beyond the last glacial period? How did the occurrence and pacing of millennial-scale IRD events evolve during the late Pleistocene under changing orbital and glacial boundary conditions? In addition to these objectives, isotopic data from epipelagic, deeper dwelling, and benthic foraminifers, as well as dinocyst and microfossil abundance, will be used to document changes in hydrography and structure of water masses in the Labrador Sea (Hillaire-Marcel and Bilodeau, 2000; Hillaire-Marcel et al., 2001a, 2001b). The rationale for drilling at Site U1302, located 5.68 km southeast of Site U1303 (Fig. F3), was that Site U1302 appeared to have a thicker sediment drape above the underlying mudwaves (Fig. F5). The Pollution Prevention and Safety Panel authorized penetration to 300 meters below seafloor (mbsf) at Site U1302 and 200 mbsf at Site U1303. These maximum penetration depths were not achieved because of a debris flow encountered at ~90 mbsf at both sites. Although the drilling did not achieve the proposed penetration, the record extends the existing piston core records at this site from MIS 5d to MIS 17. Site positioning successfully avoided debris flows associated with MIS 5 and MIS 7 (Fig. F6). |
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