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doi:10.2204/iodp.proc.303306.214.2010

Site U1314

The objective of drilling Site U1314 was to obtain a deepwater record of past oceanographic conditions to compare to deepwater Site U1304 from the southern Gardar Drift and the intermediate depth site on the northern part of the Gardar Drift sampled during Leg 162 at Site 983. Site U1314 is located north of Site U1304 in the southern Gardar Drift (Fig. F1), at a water depth of 2820 meters below sea level (mbsl), and just north of the Ruddiman (1977) IRD belt. As at Site U1304, the stratigraphic record at Site U1314 will allow high-resolution monitoring of NADW variability and ice sheet instability over the last ~3 m.y.

Three holes were drilled at Site U1314 using the APC system to a maximum depth of 280 mbsf. Hole-to-hole stratigraphic correlation was straightforward to 281 mcd (mid-Gauss Chron; ~3 Ma) using magnetic susceptibility and NGR, apart from one tenuous tie between Holes U1314B and U1314C at 25.90 mcd. The sedimentary sequence is composed of mainly greenish gray nannofossil oozes with clays, with abundant siliceous and calcareous microfossils. Calcium carbonate content ranges from <10 to >70 wt%, with an average of 34 wt%, and the variations are in step with nannofossil abundance. A variability of 20–30 wt% in CaCO₃ around the mean persists throughout the entire section. Variations in sediment color/reflectance follow CaCO₃ variations but do not mimic an oxygen isotope record. Biogenic silica abundances, dominated by diatoms, reach 25% of visible grains in shipboard smear slides. Diatom-rich laminae, dominated by Thalassiothrix, recur periodically throughout the stratigraphic sequence. J. Grützner and S.M. Higgins (unpubl. data) have derived an orbital age model for Site U1314 by correlating sediment physical properties from this site with equivalent data from Site 983. Mean sedimentation rates are estimated at ~7–8 cm/k.y. in the Pleistocene and 11–12 cm/k.y. in the late Pliocene. The magnetic polarity stratigraphy is unambiguous to the Gauss Chron at ~3 Ma, and the fidelity of the magnetic record is exemplified by detailed study of the Gauss–Matuyama polarity transition (Ohno et al., 2008). The longer polarity subchrons (Jaramillo and Olduvai) and brief subchrons such as the Cobb Mountain and Réunion are recorded.

Rich nannofossil (Hagino and Kulhanek), radiolarian, and planktonic foraminifer and diatom (see the “Site U1314” chapter) assemblages occur throughout the sedimentary sequence and are suitable for reconstruction of SSTs for the late Pliocene–Pleistocene interval. Benthic foraminifers (see the “Site U1314” chapter) and ostracodes (Alvarez Zarikian) are diverse and generally well preserved. The good preservation of benthic microfossils makes this site suitable for developing a benthic isotopic record and reconstructing deep-sea environmental conditions. Alvarez Zarikian et al. (2009) studied the ostracode assemblage at this site over the past 170 k.y. and showed that changes in ostracode abundance, diversity, and assemblage composition are apparently linked to changes in deep sea water masses and glacial–interglacial cyclicity. The ostracod assemblage found during glacial stages comprises shelf and intermediate water Arctic/subarctic species, and abyssal North Atlantic species, which possibly reflect temporal shifts in the influence of high-nutrient southern source waters (e.g., Antarctic Bottom Water) and low-nutrient waters (e.g., Glacial North Atlantic Intermediate Water) at these times. In contrast, interglacial ostracode assemblages are characterized by deep cosmopolitan species that are known to be associated with NADW and increased food supply to the sediments (Dingle and Lord, 1990; Cronin et al., 1999; Didié and Bauch, 2000). Glacial and interglacial ostracode assemblages found at Site U1314 show a strong affinity to those found on the Rockall Plateau (Didié and Bauch, 2000) and the mid-ocean ridge in the North Atlantic (Cronin et al., 1999), but species diversity is higher during the last two glacial intervals at Site U1314. Initial results by Sorrell and Judge (2007) and Alvarez Zarikian et al. (2009) have shown that sediments at Site U1314 contain a record of ice-rafted debris that can be used for monitoring ice sheet instability and reconstructing the glacial climate history of the North Atlantic Ocean.

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