Five holes were drilled at Site 1091 to obtain a complete section deposited under high sedimentation rates. Hole 1091A was the deepest hole cored with the APC to a depth of 310.9 mbsf with a recovery rate of 90%. Basal sediments are early Pliocene (~3.4 Ma) in age. The remaining four holes provided overlap to fill coring gaps in Hole 1091A and resulted in a continuous spliced section to 234 mcd (~1.7 Ma). One lithologic unit was defined consisting of diatom-rich ooze, with minor and varying amounts of nannofossils, foraminifers, and mud. Calcium carbonate contents in Hole 1091A are relatively low, ranging from 0.2 to 58.9 wt% with an average value of 5.7 wt%. Despite the low carbonate content, planktonic and benthic foraminifers are sufficiently abundant in most samples for stable isotopic analysis. Total organic carbon contents vary between 0.17 and 0.91 wt% with an average value of 0.60 wt%.
Because of the close proximity of Site 1091 to the Polar Front, the underlying sediments should document past movements of this front. Extensive laminated Thalassiothrix diatom mat deposits, analogous to those of the eastern equatorial Pacific (Kemp and Baldauf, 1993), occur at several horizons. Sedimentation rates are high, averaging 140 m/m.y. in the Pleistocene section. Carbonate-rich, interglacial periods are easily recognized by their brightness in the signal of diffuse color reflectance. For example, peaks in red reflectance (650-700 nm) at 26.5, 38, and 49 mcd correlate to interglacial Stages 7, 9, and 11, respectively, which is supported by biostratigraphic information. Downhole variations of physical properties (diffuse spectral reflectance, GRA density, and magnetic susceptibility) show distinct evidence of cyclicity at Milankovitch and suborbital time scales.
A transition in sedimentation occurred at ~2.0 Ma that is marked by a change from rapidly accumulated diatomaceous sediments above to lower opal contents and lower sedimentation rates below. This event represents an important change in opal export production in the Southern Ocean during the latest Pliocene and was recognized previously in ODP Hole 704 (Froelich et al., 1991a; Hodell and Venz, 1992), which is located only ~60 km to the east of Site 1091 on the crest of Meteor Rise.
Natural remanent magnetization (NRM) at Site 1091 was affected by a drill-string overprint that was largely removed at peak demagnetization fields in excess of 10 mT; however, the resulting inclination values are highly scattered especially during the Matuyama Chron. The Brunhes/Matuyama boundary can be identified in the 95.5-102.4 mbsf interval in Hole 1091A. The Matuyama/Gauss boundary is tentatively identified between 285.8 and 288.7 mbsf in Hole 1091A.
The redox conditions in Site 1091 sediments can be classified as reducing on the basis of dissolved H2S (by smell), but sulfate concentrations decrease only modestly from near-bottom water concentrations of about 28 mM at 4 mbsf to about 22 mM around 300 mbsf.
In summary, Site 1091 represents the midpoint (at 47°S) of a Pliocene-Pleistocene latitudinal transect across the ACC. The high sedimentation rates during the Pleistocene at Site 1091 (145 m/m.y.) complement the records obtained at Sites 1089 (128 m/m.y.), 1093 (250 m/m.y.), and 1094 (140 m/m.y.) at 41°, 50°, and 53°S, respectively. This latitudinal transect of sites with high sedimentation rates will be used to reconstruct past movement of the ACC and Antarctic sea-ice field frontal boundaries during the Pliocene-Pleistocene, and to study the impact of climate variability on processes in the Southern Ocean on orbital and suborbital (millennial) time scales.
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