| IODP Proceedings Volume contents Search | |||
| |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.303306.105.2006 Site U13051Expedition 303 Scientists2Background and objectivesThe Labrador Sea has been a conduit for meltwater from the Laurentide, Inuitian, and Greenland Ice Sheets into the North Atlantic Ocean. Labrador Sea sediments, therefore, provide valuable insights into ice sheet variability (Hesse and Chough, 1980; Hillaire-Marcel et al., 1994; Stoner et al., 1995b, 1996; Hiscott et al., 2001) and the response of thermohaline circulation to freshwater pulses (Hillaire-Marcel and Bilodeau, 2000). The Eirik Drift (Fig. F1) was built in the Pliocene and Quaternary by sediment deposition as deepwater currents from the Denmark Strait transporting Norwegian Sea Overflow Water were diverted around southern Greenland (see Arthur et al., 1989). The sediment pile hence provides a high-resolution archive for monitoring this component of North Atlantic Deep Water (NADW) as it enters the Labrador Sea. Integrated Ocean Drilling Program Site U1305 is located off southern Greenland at the southwestern extremity of the Eirik Drift (Fig. F1). The site is 82.2 km south of Ocean Drilling Program (ODP) Site 646 (Fig. F2) and in the same water depth (3459 m). Two holes were drilled at Site 646 during ODP Leg 105 (August–October, 1985) in poor weather conditions (Shipboard Scientific Party, 1987). The depths of the drilled intervals were 103.5 meters below seafloor (mbsf) in Hole 646A and 777 mbsf in Hole 646B. At this site, the sediments comprise 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). 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. Magnetostratigraphy of the upper Miocene–Holocene record was compromised by incomplete recovery and core disturbance, which was exacerbated by poor weather conditions during drilling. Planktonic oxygen isotope data (Aksu et al., 1989) are available from Site 646 sediments to marine isotope Stage (MIS) 23 (~900 ka). The Eirik Drift was built largely after the mid-Early Pliocene, and at Site 646, a prominent seismic Reflector R1 that correlates to the middle of the Upper Pliocene lies at 0.28 s two-way traveltime (~236 mbsf) (Arthur et al., 1989). The objectives at Site 646 were focused largely below R1; therefore, maximizing the sediment thickness above R1 was not a priority. For the purpose of generating a high-resolution record of the uppermost Pliocene and Quaternary, we targeted a location where R1 was deepest at the desired water depth within the multichannel seismic network obtained over the Eirik Drift during Knorr Cruise KN166-14 in summer 2002. Site U1305 was placed at common depth point 900 on seismic Line 25 (Figs. F3, F4) where seismic Reflector R1 is identified at ~540 mbsf (0.35 s), implying a mean sedimentation rate of ~18 cm/k.y. for the uppermost Pliocene and Quaternary section at this site, approximately twice that for the same interval at Site 646. The 3.5 kHz data yield penetration to ~50 mbsf (Fig. F5). The detrital layer stratigraphy within the Quaternary section at Site 646 is of particular interest because these detrital layers can be associated with instability of the Laurentide, Inuitian, and Greenland Ice Sheets. These detrital layers were described in the original studies of Site 646 sediments (Cremer, 1989; Hiscott et al., 1989); however, two piston cores collected subsequently within a few hundred meters of the Site 646 location have drawn attention to their importance. The first piston core (HU90-013-013) was collected by CCGS Hudson in 1991 in 3380 m water depth. A second core (MD99-2227) was collected at almost the same location in 3460 m water depth by the Marion-Dufresne during the 1999 Images cruise (Turon et al., 1999). In spite of evident core stretching in the upper part of Core MD99-2227, the magnetic susceptibility records from the two cores can be correlated one to another and to Site 646 (Fig. F6). A constant offset of 3.6 m was added to the mbsf depth of Site 646 to align the susceptibility high found at ~14 mbsf in Core HU90-13-013 (Fig. F6). This feature occurs within MIS 5e in both records (Aksu et al., 1989; Hall et al., 1989; Hillaire-Marcel et al., 1994; Stoner et al., 1996). Differences in the depths of correlative features (Fig. F6) can be largely attributed to stretching in the upper part of Core MD99-2227. Core HU90-013-013 has been studied in more detail than MD99-2227, for which published information is limited to the Images cruise report (Turon et al., 1999). Core HU90-013-013 is 1760 cm long and extends into MIS 7 (Hillaire-Marcel et al., 1994). High-resolution planktonic δ18O and acceleration mass spectrometry (AMS) 14C data (Hillaire-Marcel et al., 1994) and magnetic stratigraphy based on paleointensity (Stoner et al., 1998) indicate that over the last glacial cycle sedimentation rates were ~7–10 cm/k.y. during glacial intervals and >30 cm/k.y. during the Holocene. Core HU90-013-013 preserves a composite record of NADW entering the Labrador Sea and Laurentide- and Greenland-derived detrital events (Hillaire-Marcel et al., 1994; Stoner et al., 1998). In Core HU90-013-013, susceptibility peaks and magnetic grain size proxies indicating coarsening document detrital layers associated with instability of the Greenland Ice Sheet (Stoner et al., 1995b). In the Quaternary at Site 646, carbonate-rich detrital layers are often delicately laminated, have sharp bases and bioturbated tops, and are associated with spillover turbidites from the Northwest Atlantic Mid-Ocean Channel (NAMOC) (Hesse and Chough, 1980; Chough et al., 1987) and/or detritus shedding off the Greenland margin (Hiscott et al., 1989). Other siliciclastic laminated silts and very fine sands may indicate velocity fluctuations in the contour currents that account for the silty clays that constitute the bulk of the Pliocene–Quaternary sediment pile on the Eirik Drift (Chough, 1985; Hiscott et al., 1989). The sedimentation pattern in Core HU90-013-013, recovered in 3380 m water depth, contrasts with that in Core HU90-013-012, recovered in 2830 m water depth 108 km northeast of Core HU90-013-013 (Hillaire-Marcel et al., 1994; Stoner et al., 1998). At this shallower water depth site, deposition is expanded during glacials (>10 cm/k.y.) and condensed during interglacials. This contrasting sedimentation pattern in Core HU90-013-012 and Core HU90-013-013 is due to the position of these cores relative to the present high-speed axis of the Western Boundary Undercurrent (WBUC) (Fig. F1) (Hillaire-Marcel et al., 1994; Hillaire-Marcel and Bilodeau, 2000). Similarly, the deeper site (Site U1305) sampled during Expedition 303 is located below the WBUC (Fig. F1) and is therefore characterized by expanded interglacial sedimentation (Hillaire-Marcel et al., 1994). The shallower water site (Site U1306), located in a water depth of 2273 m, exhibited high sedimentation rates during glacial intervals and more condensed deposition during interglacial periods. The contrasting sedimentation patterns between Sites U1305 and U1306 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 two sites will benefit from the contrasting sedimentation patterns, thereby maximizing the resolution of the composite record at this location. The objective of drilling at Site U1305 is to recover a complete uppermost Pliocene–Quaternary sedimentary section with sedimentation rates that allow high-resolution studies of the environmental history of surrounding ice sheet instability, the WBUC, and, hence, the formation of NADW and the West Greenland (surface) Current. The chosen water depth for Site U1305 (3459 m) is designed such that deposition occurred predominantly below the WBUC (Fig. F1), accounting for expanded interglacial intervals and relatively condensed glacial intervals. Using a paleointensity-assisted chronology (PAC), detrital layers (carbonate rich and carbonate poor) in Core HU90-013-013 have been correlated to North Atlantic Heinrich-type layers in the H1–H6 interval and to episodes of Greenland deglaciation (Stoner et al., 1998). The overall objective of drilling at Site U1305 is to place the detrital layer stratigraphy into a PAC that can be exported outside the region with millennial-scale precision. In so doing, we aim to reconstruct ice sheet instability as well as the isotopic characteristics of surface and deep waters. |
|||
