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doi:10.2204/iodp.proc.303306.111.2006 Site U13121Expedition 306 Scientists2Background and objectivesSite U1312 (proposed Site IRD4A) constitutes a reoccupation of Deep Sea Drilling Project (DSDP) Site 608 located northeast of the Azores on the southern flank of the King’s Trough tectonic complex in a water depth of 3554 m (Fig. F1). During DSDP Leg 94, two principal holes (Hole 608 and Hole 608A) were drilled (Ruddiman, Kidd, Thomas, et al., 1987). Hole 608 was continuously cored with the variable-length hydraulic piston coring (VLHPC) system and the extended core barrel system down to the basement at 515.4 meters below seafloor (mbsf) (42 Ma) (Fig. F2), and Hole 608A was continuously cored with the VLHPC to refusal at 146.4 mbsf (3.4 Ma). At this site, a nearly continuous bio- and magnetostratigraphic section of Quaternary to middle upper Oligocene sediments was recovered to 455 mbsf (Baldauf et al., 1987). Below this depth, some coring gaps and the presence of a major hiatus representing at least 7.5 m.y. (late Eocene–early Oligocene) cause the record to be less complete through the Oligocene and into the Eocene. Upper middle Eocene (Zone NP16) sediments lie upon the basaltic basement at 515.4 mbsf. Mean sedimentation rates at Site 608 are 2–3 cm/k.y., with the higher values generally occurring in the late Neogene–Quaternary time intervals. Together with Sites 607 and 609, Site 608 was part of a north–south paleoceanographic transect from 53° to 37°N in the carbonate-rich sediments along the middle and upper flanks of the Mid-Atlantic Ridge. The objectives of this transect were to study the response of the North Atlantic to (1) increases in volume of Antarctic ice sheet at 14 and 6.5 Ma, (2) the closure and reopening of Atlantic/Mediterranean connections at the end of the Miocene (6–5 Ma), and (3) the closing of the Isthmus of Panama (4.5–3 Ma), as well as to document the magnitude and spectral character of the surface response to high-latitude climate change in the northern hemisphere during times of major northern hemisphere glaciation (i.e., the last at ~2.5 m.y. [Ruddiman et al., 1987]). Sites 607 and 609 especially constitute benchmark sites for generating deep-ocean climate records from the North Atlantic for the Pleistocene (Ruddiman et al., 1989) and late Pliocene (Ruddiman et al., 1986; Raymo et al., 1989, 2004) and interpreting these records in terms of ice sheet variability and oceanic circulation changes and for generating orbitally tuned timescales. The surface North Atlantic between 40° and 50°N was the most thermally reactive ocean area in the world during the Quaternary, undergoing glacial–interglacial oscillations of sea-surface temperatures (SST) of >10°C (Ruddiman, Kidd, Thomas, et al., 1987; Pflaumann et al., 2003). Sites 607, 608, and 609 are located in the area of maximum glacial–interglacial differences in SST as shown for the Last Glacial Maximum (LGM)/modern North Atlantic Ocean (Fig. F3) (Pflaumann et al., 2003). Partly incomplete recovery and the present condition of the existing DSDP cores collected in 1983 do not permit the high-resolution studies proposed here. The main objective at Site U1312 was to obtain detailed records of surface and deepwater characteristics and their interactions with ice sheet instabilities during Neogene–Quaternary times using gamma ray attenuation (GRA) density, natural gamma radiation (NGR), and other parameters measured using the archive multisensor track and multisensor track (MST) for complete composite section construction as well as high-resolution magnetic, sedimentological, and geochemical techniques both for shipboard and postcruise studies which were not available during Leg 94. Furthermore, an excellent upper Miocene section composed of nannofossil ooze and foraminifer nannofosssil ooze has been recovered at Site 608 at 140–260 mbsf. Thus, an important target at this site is the recovery of a complete undisturbed upper Miocene section using the advanced piston coring (APC) system that will allow the study of climate variability and ocean-atmosphere interactions under very different boundary conditions (see above). |
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