SITE 1093

Site 1093 (proposed site TSO-6A) is located north of Shona Ridge, near the present Polar Front and north of the average winter sea-ice edge. The siteis characterized by moderately laminated pelagic sediments deposited at high sedimentation rates (25 cm/k.y.) throughout the Pleistocene within the circumantarctic biogenic silica belt (Fig. 9). The high burial rates of biosiliceous sediment offer an excellent opportunity for the study of millennial- and submillennial-scale climate variability. Site 1093 represents the first time that cores have been recovered from the Antarctic silica belt in multiple APC holes, permitting construction of a complete composite section. At 3624 m, the site is located within lower CPDW.

Six holes were drilled at Site 1093 with a maximum penetration of 597.7 mbsf. A continuous spliced section was constructed to a depth of 252 mcd, representing the early Pleistocene (~1.0 Ma) to Holocene. For Holes 1093A and 1093B, the Brunhes/Matuyama boundary (0.78 Ma) was found in the interval between 205 and 210 mcd, yielding an average sedimentation rate of 250 m/m.y. Shipboard multisensor track (MST) results (natural gamma radiation [NGR], GRA bulk density, and magnetic susceptibility) and diffuse color reflectance document lithologic variations on orbital and suborbital time scales, which can be used to interpret climatic changes during the past 1.0 m.y. Preliminary age models based on shipboard MST results indicate that in some interglacial intervals sedimentation rates reached as high as 300 to 700 m/m.y., permitting a temporal sampling resolution of 100 yr or less. The remarkably expanded and relatively complete section is well suited for paleoceanographic studies on millennial-to-centennial time scales over the last 1 m.y.

The lithology at Site 1093 consists almost exclusively of diatom ooze, including distinctive intervals of laminated diatom mats up to several meters thick. Calcium carbonate contents at Site 1093 were generally low (<15 wt%) with occasional peaks of up to 56.9 wt%. Despite the low carbonate content, it appears from core-catcher samples that a nearly continuous planktonic foraminiferal isotope stratigraphy (Neoglobiquadrina pachyderma sinistral) and a more-or-less continuous benthic isotope record should be possible over the last 1 m.y.

Relatively carbonate-rich, interglacial intervals are recognized by their brightness in the signal of diffuse color reflectance. On this basis, we were able to predict MISs 1 through 11 (~400 k.y.). MIS 11, at ~124 mcd, stands out as the "brightest," most carbonate-rich interglacial of the Pleistocene (Howard and Prell, 1994). The transition from glacial MIS 12 to interglacial MIS 11 (Termination V) occurs over an 8-m interval (from ~133 to 125 mcd) and is marked by a thick laminated interval of Thalassiothrix diatom mats. This section provides an unprecedented opportunity to study changes in sedimentation controlled by abrupt climate changes associated with Termination V.

The recovery of the deeper section below ~255 mcd, averaging only 26%, was disappointing in Hole 1093D. This is thought to be the result of thick intervals of laminated diatom mats that were difficult to recover using the APC and particularly the XCB coring systems. Apparently, a brief hiatus (spanning 0.2 m.y.) marks the Pliocene/Pleistocene boundary, and sedimentation rates decrease to 57 m/m.y. below this level. A hiatus spanning ~2.5 m.y. also marks the latest early Pliocene to latest late Miocene. The oldest sediment recovered at 595 mcd was latest Miocene (6.3-6.9 Ma) in age and contained a Neobrunia mirabilis diatom ooze with similar composition to that recovered at ODP Site 701.

Closely spaced (one per section) interstitial water samples were taken from cores from Hole 1093A between 0 and 63 mbsf for major ion and stable isotopic analysis. One to three samples per core were taken at Holes 1093A and 1093D to a maximum depth of 498 mbsf. Shipboard analyses show that, as normally expected, chlorinity increases downhole, with a well defined maximum in the interval from 50 to 60 mbsf, probably resulting from diffusion of higher salinity water associated with the last glaciation. The chloride profile is identical (within analytical uncertainty) to its lower resolution counterpart in Site 1091. Both sites are characterized by high sedimentation rates, and the presence of diatom mats may be responsible for creating such a distinct chloride maximum. Additional shore-based isotopic analyses and modeling of pore-water profiles may permit the estimation of the oxygen isotopic composition and salinity of bottom waters at Site 1093 during the last Ice Age.

Hole 1093D was wireline-logged between 70 and 560 mbsf using the Triple Combination tool and the Geological High-sensitivity Magnetic Tool (GHMT). Good quality resistivity, NGR, and magnetic susceptibility data were obtained that should permit core-log integration using the MST core-logging data. The magnetic susceptibility record obtained from the Pliocene section of Site 1093 shows close similarities to the Pliocene record obtained at lower sedimentation rates at Site 1092.

In summary, the purpose of Site 1093 was to obtain an expanded record of biosiliceous sediments south of the present-day position of the Polar Front to study interactions between rapid climate change on suborbital time scales and the Antarctic surface waters and sea-ice field. We succeeded in obtaining Pleistocene sediments that were deposited at the highest sedimentation rates yet recovered in any pelagic deep-sea section, affording the opportunity to study paleoceanographic processes in response to climate variability on millennial and even centennial time scales. In particular, the high accumulation rates and associated temporal resolution of the sedimentary record will permit detailed correlation of the paleoceanographic history at Site 1093 with results from the Greenland and Antarctic ice cores, especially the Vostok ice core that has now been extended to a depth representing the last 420 k.y. (Petit et al., 1997).

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