SITE 1094

Site 1094 is located in a small sedimentary basin north of Bouvet Island. It was the highest latitude site drilled during Leg 177 and represents the southernmost anchor of sites drilled along a latitudinal transect across the ACC. The site is located in the southern part of the ice-free Antarctic Zone, but it was covered by sea ice during the last Ice Age and preceding glacial intervals. The water depth of 2808 m places the site within the core of CPDW.

Four APC holes were drilled to depths of 159.6 mbsf (Hole 1094A), 38.0 mbsf (Hole 1094B), 73.1 mbsf (Hole 1094C), and 171.1 mbsf (Hole 1094D). The oldest sediments recovered are Pleistocene in age (~1.4 to 1.5 Ma) and the section consists predominantly of olive-gray to gray diatom oozes, with minor and varying amounts of foraminifers, nannofossils, and siliciclastic mud. A continuous spliced section was constructed to ~121 mcd, representing the last 1 m.y., with one gap at the bottom of Core 177-1094A-7H.

Lithologic variations (as expressed in signals of magnetic susceptibility, GRA bulk density, color reflectance, and NGR) mirror in great detail the glacial and interglacial cycles of the late Pleistocene (Fig. 10). MISs 1-12 are readily identifiable in the upper 80 mcd. Glacial stages are marked by relatively high susceptibility, high NGR, low GRA bulk density, and low color reflectance (650-770 nm). Glacial terminations are marked by abrupt decreases of susceptibility and NGR and by increases in GRA bulk density and color reflectance. Sedimentation rates average ~140 m/m.y. in the diatom dominated middle to upper Pleistocene sequence above ~80 mcd. Below MIS 12, sedimentation rates decreased to about 91 m/m.y. during the early middle and early Pleistocene. The transition between the Brunhes and Matuyama Chrons is identified between 98.20 and 101.58 mcd in Hole 1094A.

Fragments of porcellanite (opal-CT) were recovered at 68 mbsf (Holes 1094A and 1094D), 104 mbsf (Hole 1094A), and 164 mbsf (Hole 1094D), and a porcellanite concretion was found at 141 mbsf (Hole 1094A). These layers were also detected in Parasound sediment echosounding lines and are characterized by distinct high-amplitude reflectors. The upper porcellanite occurs in the lower portion of MIS 11 and is the same as that previously described in piston cores from the area (Bohrmann et al., 1994). An anomalously low temperature gradient (~7°/km) was measured at Site 1094, indicating that these porcellanites formed under low (near-bottom-water) temperatures. The upper porcellanite (at 68 mbsf) coincides with a sharp discontinuity in interstitial chloride concentrations, suggesting that the porcellanite layer may have acted as a diffusion barrier. Shore-based geochemical analyses of pore-water and solid phase samples taken from near these porcellanite beds will be important for studying early silica diagenesis in Site 1094.

The purpose of Site 1094 was to obtain a high-resolution record of biosiliceous sediments south of the present-day position of the Polar Front. Together with Sites 1089 (41°S), 1091 (47°S), and 1093 (50°S), Site 1094 (53°S) represents the southernmost anchor of high-resolution sites across the ACC needed to reconstruct past changes in frontal boundaries and sea-ice distribution during glacial-interglacial cycles of the Pleistocene. The expanded upper and mid Pleistocene sediments at Site 1094 will permit study of rapid climate change on suborbital time scales, including comparison with paleoclimatic signals from Antarctic and Greenland ice cores.

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