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doi:10.2204/iodp.proc.303306.214.2010

Site U1313

Site U1313 is a reoccupation of Site 607, drilled in 1983 during Leg 94. The rationale for reoccupying this site is essentially the same as that for Site 609 (Site U1308). Together, Sites 609 and 607 constitute benchmark sites for the long-term (millions of years) surface and deep ocean climate records from the subpolar North Atlantic (Ruddiman et al., 1986, 1989; Raymo et al., 1989). Leg 94 drilling of this site preceded the shipboard capability for construction of composite sections and pass-through magnetometers for continuous measurement of magnetic parameters, and the objective was to recover a complete composite section and generate a high-resolution stratigraphic record.

Four holes were drilled at Site U1313, reaching a maximum depth of 308 mbsf and providing the means of constructing an apparently complete composite section back into the late Miocene (~6 Ma) at mean sedimentation rates of ~5 cm/k.y. for the Pliocene–Quaternary and perhaps as high as ~13–14 cm/k.y. for the upper Miocene. The Holocene to upper Pliocene sediments are composed of nannofossil ooze and silty clay nannofossil ooze. Below these sediments, the Pliocene to upper Miocene sediments are homogeneous nannofossil oozes with high (>90 wt%) carbonate values.

Shipboard correlation among holes was unambiguous in the upper 168 m, based largely on sediment color/reflectance data that mimic the marine oxygen isotope curve (Stein et al., 2006). The homogeneity of the nannofossil oozes below this level resulted in less secure hole-to-hole correlations. The shipboard magnetic stratigraphy is interpretable through most of the Gauss Chron to ~150 mbsf. Below this depth, the magnetic stratigraphy is hampered by low magnetization intensities, although postcruise U-channel studies have helped to resolve the magnetic stratigraphy through the Gilbert Chron and into the late Miocene (Evans et al., 2006).

Diatom flora are present within the upper ~70 mbsf (Pliocene–Pleistocene interval). Calcareous nannofossils are abundant and moderately well preserved to well preserved, and a complete succession of nannofossil datums has been determined for the upper 158 mbsf, to ~3 Ma (Hagino and Kulhanek). Sierro et al. studied Pliocene–Pleistocene planktonic foraminifer events at this site and assigned ages using the correlation of sediment color/reflectance data to an oxygen isotope reference curve. These ages have been compared with the same datums in the Mediterranean where calibration was previously achieved through astrochronology. An oxygen isotope stratigraphy has been generated from planktonic and benthic foraminifers for part of the record, specifically the MIS 10–16 interval (Voelker et al., 2009, 2010), providing both chronological control and information about surface and deep water conditions. Planktonic and benthic oxygen isotope data, lithic fragment, and physical grain size data at Site U1313 indicate that major Heinrich-type ice-rafting events were associated with Terminations V and VII and with MIS 10 and 12 (Voelker et al., 2010). Lower benthic δ¹³C values indicate weakened meridonial overturning circulation (MOC) and the presence of Antarctic Bottom Water during all glacial periods in this interval (MIS 10–16) with strong MOC during all interglacials.

First results from biomarker analyses, performed shipboard, indicate that alkenone-derived SSTs show variability from ~13° to 19°C during the Pleistocene (Stein et al., 2006). A few data points from the late Pliocene interval display SST values of ~17° to 22°C. With new shore-based data representing the 320–640 ka (MIS 9–16) interval, Stein et al. (2009) are able to compare SST variability, surface water productivity, and IRD deposition during the more severe glacials (MIS 16, 12, 10) with the weaker glacial MIS 14, and, on the other hand, the extreme interglacials MIS 11 and the less extreme interglacial MIS 15 and 13. Alkenone-based SST reached an absolute maximum of ~19°C during MIS 11.3 and absolute minima of <10°C during MIS 12 and 10. Within MIS 11, prominent cooling events (MIS 11.22 and 11.24) occurred. The absolute SST minima, recorded directly before and after the glacial maxima MIS 12.2 and 10.2, are related to Heinrich-like meltwater pulses, as supported by the coincidence of SST minima with maxima in C_37:4 alkenones and dolomite (Fig. F6). Based on these data, ice sheet disintegration and subsequent surges and outbursts of icebergs and meltwater discharge appear to have been triggered by increased insolation forcing at high northern latitudes (Stein et al., 2009).

Ferretti et al. (2010) documented the MIS 20–23 interval at Site U1313 using δ¹⁸O from both benthic and planktonic foraminifers. MIS 21 can be broken into four interstadial periods that can be correlated to other North Atlantic isotopic records from Site U1308 (Hodell et al., 2008), Site 983 (Channell and Kleiven, 2000), and ODP Site 1063 (Ferretti et al., 2005). These variations in MIS 21 are evident in both the planktonic and benthic isotope records, although they are more muted in the benthic records. Ferretti et al. (2010) matched these variations to insolation variations at the Equator and suggested a forcing mechanism related to low-latitude precession variations and advection to high latitudes by tropical convective processes.

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