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doi:10.2204/iodp.proc.303306.112.2006 Site U13131Expedition 306 Scientists2Background and objectivesIntegrated Ocean Drilling Program Site U1313 constitutes a reoccupation of Deep Sea Drilling Project (DSDP) Site 607 located at the base of the upper western flank of the Mid-Atlantic Ridge in a water depth of 3426 m, ~240 mi northwest of the Azores (Fig. F1). Seismic air gun profiles indicate a prevalent sediment cover of 500–900 m above the acoustic basement in this region (Fig. F2). Two holes were drilled at this site during Leg 94 (June–August 1983) using the variable-length hydraulic piston coring system and the extended core barrel system (Ruddiman, Kidd, Thomas, et al., 1987). Hole 607 penetrated to a total depth of 284.4 m and Hole 607A to a total depth of 311.3 m. The sediments recovered at Site 607 consist predominantly of calcareous biogenic oozes with variable amounts of fine-grained terrigenous material. The sedimentary sequence can be divided into two major lithologic units (Fig. F3). Unit I, from 0 to ~116 meters below seafloor (mbsf) (late Pliocene–Pleistocene), is characterized by cyclically interlayered intervals of dark sediment rich in fine-grained terrigenous material (silty clay foraminiferal-nannofossil ooze) and lighter colored sediment with relatively small amounts of terrigenous compounds (mainly foraminiferal-nannofossil ooze). Unit II, below ~116 mbsf (late Miocene–early late Pliocene), is composed of pale gray to white foraminiferal-nannofossil ooze and nannofossil ooze. Based on magneto- and biostratigraphy, the mean sedimentation rate at Site 607 is ~5 cm/k.y. for the Pliocene–Pleistocene time interval (Baldauf et al., 1987). The rationale for reoccupying this site is essentially the same as that for Site U1308 (recoring of Site 609; see the “Site U1308” chapter). Together, Sites 607 and 609 constitute benchmark sites for the long-term (millions of years) as well as short-term surface and deep ocean climate records from the subpolar North Atlantic. Site 607, situated under the influence of North Atlantic Deep Water (NADW) (Fig. F4), has been very important for generating benthic δ18O, δ13C, and CaCO3 records for the Pleistocene (Ruddiman et al., 1989) and late Pliocene (Ruddiman et al., 1986; Raymo et al., 1989, 2004) (Fig. F3) and for interpreting these records in terms of ice sheet variability and changes in NADW circulation, as well as for generating orbitally tuned timescales. Site 607, at a water depth of 3427 m, remains the only site in the high-latitude North Atlantic that monitors NADW circulation throughout the Pleistocene. Leg 94 drilling of this site preceded the advent of 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 Site 607, Heinrich events are marked by a distinctive detrital carbonate signature, providing a means of correlation to other Expedition 303 and 306 sites. Based on census counts of planktonic foraminifers, sea-surface temperature (SST) warmed markedly during the Heinrich events and during the Last Glacial Maximum (LGM; 18 and 20 ka) in distinct contrast to the climate records from the subpolar North Atlantic (Bond et al., 1999b). This complex SST pattern, with respect to the subpolar North Atlantic, has now been traced from the early Holocene to Heinrich event 5 at ~46 ka. Recent coupled ocean-atmosphere models suggest that during full shutdowns of NADW production, likely during Heinrich events, complex patterns of SST may appear globally (Schiller et al., 1997) with antiphased warm anomalies appearing south of Newfoundland (Manabe and Stouffer, 1999). Reconstruction of SSTs in the North Atlantic indicates that the Polar Front was situated between ~42° and 46°N during glacial times, extending in an east–west direction and resulting in a steep south–north SST gradient (CLIMAP, 1976; Pflaumann et al., 2003) (Fig. F5A). Alkenone SST estimates determined in sediment cores from areas south of and within the Polar Front resulted in very different values for different glacials (Fig. F5B) (Calvo et al., 2001), indicating different climatic conditions (e.g., the location of the Polar Front) in these glacial periods. Site U1313 (especially in combination with similar records from other Expedition 303 and 306 sites) will document the evolution of the complex surface temperature phasing over time, addressing questions such as whether the patterns are a peculiarity of the last glaciation, whether they were present in the 41 k.y. world, and whether they appeared at the onset of northern hemisphere glaciation. Antiphased patterns of ocean surface temperatures are documented in Core VM 30-97 for the 10–40 ka interval (Bond et al., 1999a). Redrilling Site 607 will provide a long-term record of this apparent antiphase pattern, which is now 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 and their relationship to ice sheet instability and changes in deepwater circulation. |
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