SITE 1090

Site 1090 is located in the central part of the Subantarctic Zone on the southern flank of the Agulhas Ridge. The water depth (3699 m) places it near the boundary between NADW and underlying lower CPDW, and above the calcium-carbonate compensation depth (CCD). Together with Sites 1088 (2083 m) and 1089 (4624 m) it forms a depth transect that intersects most of the major water masses of the South Atlantic.

Five holes were drilled to 397.5 mbsf, spanning the Holocene to middle Eocene (~46 Ma), and including a ~14-m.y. hiatus at ~70 mcd that spans much of the lower Pliocene to lower Miocene record. From Hole 1090A we obtained one core that overpenetrated the mudline and recovered 7 m of sediment. The deepest hole (1090B) penetrated to a depth of 397.5 mbsf with APC coring to Core 177-1090B-20H (184.7 mbsf) and XCB coring thereafter. Hole 1090C was drilled to a depth of 69.3 mbsf (Core 177-1090C-8H) to recover a continuous Pleistocene-Pliocene section and the interval containing the lower Pliocene-lower Miocene hiatus. Holes 1090D and 1090E were drilled to APC refusal depths of 225.9 and 236.7 mbsf, respectively. The strategy for Holes 1090B, 1090D, and 1090E was to recover the lower Miocene-Oligocene record in its entirety by APC coring. We constructed a continuous spliced record to 212 mcd (and perhaps 245 mcd), corresponding to the early Oligocene.

Quaternary sediments, consisting of alternating foraminiferal nannofossil ooze, diatom bearing nannofossil ooze, and mud-bearing nannofossil ooze, extend to 44 mcd at sedimentation rates averaging 33 m/m.y. In Hole 1090C, the Brunhes/Matuyama boundary (0.78 Ma) lies between 18.0 and 19.2 mbsf. The top (0.99 Ma) and base (1.07 Ma) of the Jaramillo Subchron lie in the 24.6-25.4 and 27.7-28.4 mbsf intervals, respectively, in Hole 1090C. Two hiatuses may occur between 0.42 and 0.64 Ma and from 1.3 to 1.8 Ma; shore-based analysis is needed for confirmation. Variations in color reflectance permit the identification of glacial and interglacial Stages 1 to 12 in the upper 18 mcd, supported by identification of biostratigraphic events. Isotope Stage 11 is particularly prominent because of its exceptionally white color and high nannofossil carbonate content. Cyclic variations in the color reflectance and gamma-ray attenuation (GRA) bulk density signals may reflect the shift from the 41-k.y. world to the 100 k.y. world at about 30 mcd.

The upper Pliocene sequence was deposited at sedimentation rates of 11 to 13 m/m.y. In Hole 1090C, the top (1.77 Ma) and base (1.99 Ma) of the Olduvai Subchron were recognized in the 35.3-36.0 and 37.6-38.2 mbsf intervals, respectively. Diatom biostratigraphy indicates a hiatus at ~55 mcd that spans the Gauss/Matuyama boundary from 2.5 to 2.6 Ma.

A hiatus was encountered at ~70 mcd, marked by a lithologic change from white nannofossil ooze to reddish muddy nannofossil ooze and a tephra layer composed of vitric ash with greenish brown volcanic glass shards. Sediments above the hiatus are early Pliocene in age and contain manganese nodules. Below the hiatus, approximately 330 m of sediment was recovered consisting of mud-bearing diatom ooze and mud- and diatom-bearing nannofossil ooze and chalk ranging in age from early Miocene to middle Eocene. Sedimentation rates average 10 m/m.y. in the early Miocene to middle Eocene, and increase to 30 m/m.y. in the upper Eocene opal-rich sediments that include intervals of well-laminated diatom ooze. Middle Eocene carbonate-rich sediments have lower sedimentation rates (~10m/m.y.).

The potential for paleomagnetic reversal stratigraphy below the hiatus is superb, even in the cores that were recovered by XCB. Site 1090 holds much promise for detailed correlations of biostratigraphic datums to the geomagnetic polarity time scale during the early Miocene to middle Eocene.

Pore waters of Site 1090 can be characterized as suboxic, with sulfate reduction occurring at very low rates. Pore-water profiles indicate a sharp break at ~290 mbsf, corresponding to an impermeable layer (presumably a chert recovered as fragments in the top of Core 177-1090B 32X) that posed a barrier to diffusion in interstitial waters. Pore-water regimes above and below the diffusion barrier evolved independently because of the isolation imposed by the impermeable chert.

In summary, the importance of Site 1090 is twofold: (1) the Pleistocene to upper Pliocene section above the hiatus will be useful for reconstruction of high-latitude Southern Hemisphere paleoclimate at moderate resolution (30 m/m.y.) and (2) the middle Eocene-lower Miocene section below the hiatus will potentially be an important section for biomagnetostratigraphic correlations, astronomical tuning, and paleoceanographic studies of the late Paleogene and early Miocene. Cyclic variations in lithology may permit the development of an astronomically tuned time scale for the late Paleogene-early Neogene, similar to that developed during Leg 154 (Weedon et al., 1997). The shallow burial of the section at Site 1090 offers an opportunity to produce a stable isotope stratigraphy that has not been compromised by diagenetic alteration. The Paleogene interval is especially significant because it spans the time period associated with the onset of Antarctic glaciations, early production of cold surface and bottom waters, and paleogeographic changes (e.g., the separation of Australia and Antarctica and opening of the Drake Passage) which led to the establishment of the ACC.

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