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doi:10.2204/iodp.proc.303306.108.2006 Site U13081Expedition 303 Scientists2Background and objectivesIntegrated Ocean Drilling Program Site U1308 constitutes a reoccupation of Deep Sea Drilling Project (DSDP) Site 609 (Figs. F1, F2). Two principal holes (Holes 609 and 609B) were drilled with the variable-length hydraulic piston corer and extended core barrel (XCB) during DSDP Leg 94 (June–August 1983). Two cores were collected from Hole 609A to recover the mudline, and seven XCB cores were collected from Hole 609C to recover the 123–190 meters below seafloor (mbsf) interval. The magnetic stratigraphy at Site 609 was resolved down to polarity Chron 3An (~6 Ma) at 350 mbsf using discrete samples collected shipboard (Clement and Robinson, 1987). Mean sedimentation rates at Site 609 are therefore 5–6 cm/k.y. for the last 6 m.y. The marly foraminiferal nannofossil ooze at this site exhibits distinctive varicolored glacial–interglacial cycles above the base of the Matuyama Chronozone, excellent preservation of calcareous and siliceous microfossils, and the necessary attributes for the generation of high-resolution isotopic and paleointensity-based chronostratigraphies. DSDP Sites 607 and 609 have 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) and interpreting these records in terms of ice-sheet variability and oceanic circulation changes and for generating orbitally tuned timescales. Almost all the δ18O and δ13C data for these studies were derived from Site 607, partly because of enhanced continuity of the recovered section at this site. The CaCO3 data at both sites were used to correlate the δ18O and δ13C data to Site 609. Site 607 (3427 m water depth) remains the only site in the high-latitude North Atlantic that monitors North Atlantic Deep Water (NADW) throughout the Pleistocene. In a recent compilation of North Atlantic δ18O and δ13C vertical gradients during the Pleistocene, Raymo et al. (2004) utilized Site 607 (3427 m water depth), with the next deepest site being Site 552 (2300 m water depth), presently within North Atlantic Intermediate Water (Fig. F1). These authors concluded that the water mass structure at intermediate depths in the North Atlantic during the Quaternary did not change significantly on glacial–interglacial timescales and that the δ13C signal at Site 607 and other deep Atlantic sites may be more influenced by variations in production of deep water around Antarctica than changes in NADW production and the strength of the “conveyer belt.” The large volumetric flux of Norwegian Sea Overflow Water observed today may be caused by current open sea-ice conditions in the Norwegian-Greenland Sea and atypical of Pleistocene interglacials. The water depths at Sites U1308 (3900 m) and U1304 (3024 m) and the ability to derive a benthic stable isotope record at both sites will allow ice sheet/ocean interaction to be placed on a benthic isotopic record that mostly reflects changes in global ice volume. Carbon isotope data will allow high-resolution monitoring of NADW. Ocean Drilling Program (ODP) Leg 162 drift sites (Sites 980–984), from south of Iceland, were all at water depths <2000 m and therefore monitored the intermediate water (Fig. F1). At Site 982 (Rockall Plateau, 1145 m), Venz et al. (1999) proposed that Glacial North Atlantic Intermediate Water (GNAIW) production ceased during terminations and lower NADW production increased. There was apparently a time lag between the shutdown of GNAIW and the renewed production of upper NADW. One of the major objectives at Sites U1308 and U1304 is to provide NADW monitoring at high resolution (high sedimentation rates) in the central Atlantic. Together with Leg 162 sites (980–984) (Fig. F1), Sites U1308 and U1304 will provide a detrital record of millennial-scale changes in the vertical δ13C structure of the water column in the North Atlantic. The importance of Site 609 was accentuated by the early recognition of detrital (Heinrich-type) layers by lithic counts (Broecker et al., 1992; Bond et al., 1992) and by sea-surface temperature (SST) proxies such as grayscale (Broecker et al., 1990) and percent Neogloboquadrina pachyderma (sinistral) (Bond et al., 1993, 1999). The proposed match of SST minima at Site 609 to cold stadials in the δ18O Greenland Summit ice core record led to the proposal that Dansgaard–Oeschger (ice core) cycles are grouped into so-called Bond cycles. Each cycle is terminated by the massive iceberg discharges from Hudson Strait that constitute Heinrich events (Bond et al., 1993; see Hemming, 2004, for comprehensive review). Partly from petrologic characteristics observed at Site 609, Bond and Lotti (1995) showed that Heinrich events are superimposed on another, higher-frequency rhythm of ice-rafting events with detrital sources not only in Hudson Strait but also in Greenland, Iceland, and Europe (see Snoeckx et al., 1999; Grousset et al., 2000). Within marine isotope Stages (MIS) 4–5d at Site 609, the ice-rafted debris (IRD) record and the percent N. pachyderma (sinistral) SST proxy can be correlated with the record of climate instability in the Greenland Summit δ18O ice core record (McManus et al., 1994). Oxygen isotope data from the North Greenland Ice Core Project (2004) implies that the climate on Greenland during at least the younger part of MIS 5e was stable, consistent with the marine record (McManus et al., 1994), with temperatures ~5°C warmer than today. More recently, the 1500 y cycle has been identified at Site 609 and in a number of piston cores from this part of the central Atlantic, using a number of lithologic characteristics including the percent of hematite-stained grains and percent of Icelandic glass (Bond et al., 1997, 1999). Site 609 is an obvious candidate for redrilling using modern techniques to recover a demonstrably complete record of the sediment sequence. In the 22 y since the site was originally drilled, much has changed in high-resolution magnetic, sedimentologic, and geochemical techniques both for shipboard and postcruise studies. Shipboard composite section construction did not become routine until after ODP Leg 138 in 1991 (Mayer, Pisias, Janecek, et al., 1992). Apart from the gamma ray attenuation (GRA) densitometer, archive multisensor track, and multisensor track (MST), tools for modern composite section construction were not available during Leg 94. For Sites 607–611, Ruddiman et al. (1987) constructed composite sections postcruise using visual color and magnetic polarity reversals. The record from the two holes (Holes 609 and 609B) could be spliced together down to about Core 94-609-8H with a tenuous correlation to 94-609-9H, and then again down to 94-609-14H, below which the record is demonstrably incomplete (Ruddiman et al., 1987). The archive-half cores of Site 609 (stored until recently at Lamont-Doherty Earth Observatory of Columbia University) are now in poor condition caused by desiccation/contraction, mold growth, and prior sampling and are no longer suitable for most types of further study. Samples from Site 609 have played a major role in driving some of the most exciting developments in paleoceanographic research during the last 10–15 y, such as the recognition and understanding of Heinrich layers, the recognition of the 1500 y pacing in hematite-stained grains and Icelandic glass, and the correlation of ice-core δ18O to SST proxies. The majority of the analyses from Site 609 have dealt with the record younger than MIS 6, partly because of the lack of a continuous pristine composite record. A primary objective at Site U1308 was to recover a demonstrably complete composite record and, hence, considerably enhance the potential for Pliocene–Quaternary climatic records from this site. |
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