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Site U13131 Expedition 306 Scientists2 Channell, J.E.T., Kanamatsu, T., Sato, T., Stein, R., Alvarez Zarikian, C.A., Malone, M.J., and the Expedition 303/306 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 303/306 Site U13131 Expedition 306 Scientists2 Chapter contents Background and objectives Integrated Ocean Drilling Program Site U1313 constitutes a reoc- Background and objectives. 1 cupation of Deep Sea Drilling Project (DSDP) Site 607 located at Operations. 2 the base of the upper western flank of the Mid-Atlantic Ridge in a Lithostratigraphy. 3 water depth of 3426 m, ~240 mi northwest of the Azores (Fig. F1). Biostratigraphy . 6 Seismic air gun profiles indicate a prevalent sediment cover of Paleomagnetism . 13 500–900 m above the acoustic basement in this region (Fig. F2). Stratigraphic correlation. 14 Two holes were drilled at this site during Leg 94 (June–August Geochemistry . 15 1983) using the variable-length hydraulic piston coring system Physical properties . 18 and the extended core barrel system (Ruddiman, Kidd, Thomas, et Downhole measurements . 19 al., 1987). Hole 607 penetrated to a total depth of 284.4 m and References . 20 Hole 607A to a total depth of 311.3 m. The sediments recovered Figures . 24 at Site 607 consist predominantly of calcareous biogenic oozes Tables. 74 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 interlay- ered 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, be- low ~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 sedimen- tation 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 Wa- ter (NADW) (Fig. F4), has been very important for generating δ18 δ13 benthic O, C, and CaCO3 records for the Pleistocene (Ruddi- 1 Expedition 306 Scientists, 2006. Site U1313. In man et al., 1989) and late Pliocene (Ruddiman et al., 1986; Raymo Channell, J.E.T., Kanamatsu, T., Sato, T., Stein, R., et al., 1989, 2004) (Fig. F3) and for interpreting these records in Alvarez Zarikian, C.A., Malone, M.J., and the terms of ice sheet variability and changes in NADW circulation, as Expedition 303/306 Scientists. Proc. IODP, 303/ well as for generating orbitally tuned timescales. Site 607, at a wa- 306: College Station TX (Integrated Ocean Drilling Program Management International, Inc.). ter depth of 3427 m, remains the only site in the high-latitude doi:10.2204/iodp.proc.303306.112.2006 North Atlantic that monitors NADW circulation throughout the 2Expedition 306 Scientists’ addresses. Pleistocene. Leg 94 drilling of this site preceded the advent of the Proc. IODP | Volume 303/306 doi:10.2204/iodp.proc.303306.112.2006 Expedition 306 Scientists Site U1313 shipboard capability for construction of composite construction of the phasing of the temperature sections and pass-through magnetometers for con- records and their relationship to ice sheet instability tinuous measurement of magnetic parameters. Pale- and changes in deepwater circulation. omagnetic data from this site indicate that the mag- netic properties are optimal for recording the geomagnetic field. The present condition of existing Operations DSDP cores collected in 1983 does not permit the We departed the Azores Islands for Site U1313 (pro- high-resolution studies proposed here. posed Site IRD3A) at 0645 h on Saturday, 26 March At the site of Core VM 30-97, located close to Site 2005, after spending 8 days in the lee of the islands 607, Heinrich events are marked by a distinctive de- during severe weather conditions. The 277 nmi tran- trital carbonate signature, providing a means of cor- sit to Site U1313 began with sea-state conditions still relation to other Expedition 303 and 306 sites. Based somewhat marginal but expected to continuously on census counts of planktonic foraminifers, sea-sur- improve over time. The transit was completed at an face temperature (SST) warmed markedly during the average speed of 8.9 kt, and we arrived at Site U1313 Heinrich events and during the Last Glacial Maxi- at 1425 h on Easter Sunday, 27 March. See Table T1 mum (LGM; 18 and 20 ka) in distinct contrast to the for a summary of coring at Site U1313. climate records from the subpolar North Atlantic (Bond et al., 1999b). This complex SST pattern, with Hole U1313A respect to the subpolar North Atlantic, has now been Hole U1313A was spudded at 0005 h on 28 March traced from the early Holocene to Heinrich event 5 2005, establishing a rig floor–corrected seafloor at ~46 ka. Recent coupled ocean-atmosphere models depth of 3423.3 meters below rig floor (mbrf). Core suggest that during full shutdowns of NADW pro- 1H recovered 5.2 m of sediment, indicating a sea- duction, likely during Heinrich events, complex pat- floor depth of 3412.3 meters below sea level. The ad- terns of SST may appear globally (Schiller et al., vanced piston coring (APC) system, utilizing non- 1997) with antiphased warm anomalies appearing magnetic core barrels, continued through Core 31H. south of Newfoundland (Manabe and Stouffer, Alloy steel barrels were utilized for Cores 32H and 1999). Reconstruction of SSTs in the North Atlantic 33H, and drillover of stuck APC barrels was required indicates that the Polar Front was situated between for these last two cores. Tensor core orientation was ~42° and 46°N during glacial times, extending in an used for all cores beginning with Core 3H from a east–west direction and resulting in a steep south– depth of 14.7 mbsf. All core barrels fully stroked ex- north SST gradient (CLIMAP, 1976; Pflaumann et al., cept for the last one, Core 33H. The cored interval 2003) (Fig. F5A). Alkenone SST estimates determined for this hole was 308.6 m, and 319.64 m of sediment in sediment cores from areas south of and within the was recovered (recovery = 103.6%). The drill string Polar Front resulted in very different values for dif- was pulled clear of the seafloor at 1300 h on 29 ferent glacials (Fig. F5B) (Calvo et al., 2001), indicat- March, concluding operations in Hole U1313A. ing different climatic conditions (e.g., the location of the Polar Front) in these glacial periods. Hole U1313B Site U1313 (especially in combination with similar Hole U1313B was offset 25 m due north (000°) from records from other Expedition 303 and 306 sites) will Hole U1313A and was spudded at 1430 h on 29 document the evolution of the complex surface tem- March 2005. The bit was positioned at a depth of perature phasing over time, addressing questions 3421.0 mbrf, or 2.0 m deeper than Hole U1313A, such as whether the patterns are a peculiarity of the and Core 1H recovered 5.90 m of sediment, estab- last glaciation, whether they were present in the 41 lishing a rig floor–corrected seafloor depth of 3424.6 k.y. world, and whether they appeared at the onset mbrf. APC coring continued through Core 20H to a of northern hemisphere glaciation. Antiphased pat- depth of 186.4 mbsf. At that time, operations per- terns of ocean surface temperatures are documented sonnel were advised that the core breaks between in Core VM 30-97 for the 10–40 ka interval (Bond et Holes U1313A and U1313B were possibly becoming al., 1999a). Redrilling Site 607 will provide a long- aligned. As a precaution to ensure adequate overlap term record of this apparent antiphase pattern, between holes, the driller was advised to advance the which is now beginning to emerge in the western bit 2.0 m further before shooting Core 21H. Coring North Atlantic. By placing the surface temperature then continued until 1930 h on 30 March, through signals into a chronological framework based on a Core 32H, to a total depth of 302.4 mbsf. Tensor core combination of oxygen isotopic stratigraphy, detrital orientation was used for all cores beginning with carbonate-bearing Heinrich events, and geomagnetic Core 3H from a depth of 15.4 mbsf. All core barrels paleointensity, we expect to obtain an optimal re- fully stroked, and no drillover was required in this Proc. IODP | Volume 303/306 2 Expedition 306 Scientists Site U1313 hole. Nonmagnetic core barrels were used in the re- Hole U1313D covery of all 32 cores. The cored interval for this hole The drillship was offset 25 m due south (180°) of was 300.4 m, and 306.54 m of sediment was recov- Hole U1313C. The bit was positioned at a depth of ered (recovery = 102.0%). 3423.0 mbrf, or 4.0 m deeper than for Hole U1313A. After completing coring operations, Hole U1313B Hole U1313D was spudded at 2255 h on 1 April was prepared for logging. A wiper trip was conducted 2005. Core 1H was fully recovered (9.80 m); there- from total depth to 74.9 mbrf and back to total fore, the seafloor depth of 3423.0 mbrf was consid- depth without difficulty.
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