Site 1133

Site 1133 is located on the middle-upper slope in 1037.2 m of water. It was one of two paleoceanographic sites located to intersect pelagic sections that collectively span the entire Cenozoic succession and form the deeper water component of the shelf-to-basin transect. The principal objectives at this site were to (1) recover pelagic ooze from the middle-upper slope to construct a Cenozoic paleoceanographic record of the opening of the Southern Ocean and the development of the Circum-Antarctic Current, (2) determine the history of Cenozoic CCD fluctuations and intermediate water mass variations, and (3) determine depositional and diagenetic facies on the middle-upper slope.

Sediments recovered at Site 1133 were divided into two major lithostratigraphic units. Unit I (0–28.55 mbsf) consists of gray and white, moderately to strongly bioturbated calcareous ooze with varying amounts of calcareous nannofossils and planktonic foraminifers. This unit was subdivided into three subunits based on color changes, textural differences, firmgrounds, and a scoured surface. The boundary between Unit I and II is marked by a firmground that separates overlying white nannofossil ooze from underlying unlithified bioclastic wackestone. Unit II (28.55–142.59 mbsf) consists mainly of (1) gray to light gray, poorly sorted, unlithified bioclastic wackestone, with very fine-grained silt- to sand-sized particles; (2) gray, light gray to light olive gray, uniform, unlithified to partially lithified bioclastic packstone, with poorly to well-sorted silt- to sand-sized grains; and (3) gray to dark gray chert/porcellanite (silicified wackestone). Most of the recovered sediments are pebble- to cobble-sized fragments. Some porcellanite/chert fragments are draped with a thin layer of unlithified to partially lithified bioclastic packstone. It is likely that the entire unit consists of bioclastic packstone or wackestone containing beds or lenses of preferentially silicified limestone (formerly nannofossil planktonic foraminiferal ooze/chalk).

Drilling at Site 1133 revealed the presence of two major biostratigraphic units, dated by nannofossils and planktonic foraminifers as Pleistocene and middle–early Miocene. The Pleistocene sequence extends down to 21.49 mbsf and overlies a thin, highly condensed interval with upper Miocene assemblages. This disconformity represents a hiatus of at least 3 m.y., and a second disconformity at 28.55 mbsf separates upper Miocene from lower–middle Miocene (Zone NN5/NN4). Three main benthic foraminifer assemblages are recognized—a diversified Pleistocene calcareous assemblage, a diversified upper Miocene assemblage, and an impoverished middle–lower Miocene assemblage. All three assemblages indicate middle to lower bathyal paleodepths.

Magnetic measurements of archive-half cores establish a tentative Pleistocene magnetostratigraphy to 40 mbsf in Hole 1133B, which includes the Brunhes Chron (0–12 mbsf), Matuyama Chron (12–24 mbsf), and Gauss Chron (24–? mbsf). The interpretation is problematic because the same result was not reproduced in Hole 1133A, which yielded uniform normal polarity magnetizations to a depth of 20 mbsf and anomalously shallow magnetizations of both polarities below this depth. Intensities are low; after demagnetization to 20 mT, median values are ~7910 5 A/m. Intensity fluctuations are closely correlated with lithology in both holes but appear to be inversely correlated with magnetic susceptibility. Anomalously high intensities and susceptibilities are observed in both holes at ~30 mbsf. Rock magnetic properties are consistent with single-domain biogenic magnetite or greigite as the remanence carrier. Magnetic susceptibility is dominated by contributions from the diamagnetic carbonate matrix.

Construction of the composite and spliced section from Holes 1133B and 1133C indicates that a complete record of the cored interval between 0 and 39 mcd (34 mbsf) was recovered. This depth interval includes Holocene and Pleistocene–early Miocene age sediments. Distinctive high amplitude events occur in the reflectance and GRA density records, enabling easy correlation between holes. Difficulties in correlation result from a slumped interval at ~20–32 mcd; however, the base of the slumped interval is clearly defined and provides a good correlation horizon.

Except for two samples with slightly higher methane contents, generally low concentrations of methane (<12 ppm) were detected at Site 1133. Calcium carbonate content ranges from 78.5 to 94.4 wt%, with most samples between 85 and 92 wt%. Organic carbon values are <0.4 wt%.

Seven interstitial water samples were collected at Site 1133. These data indicate that Site 1133 may be, as was the case with most other sites, underlain by high-salinity fluids, although the maximum salinity value measured was 41 at 123.4 mbsf. Compared to shallower sites, the sulfate reduction rate is substantially reduced and the resulting maximum alkalinity values are <5.24 mM. This, together with the small variations in excess calcium, suggests that the rate of carbonate diagenesis is slower than at other Leg 182 sites.

Physical properties measurements at Site 1133 were limited because of poor core recovery. Data were divided into two units, with the boundary correlating with the base of the Pleistocene. Unit 1 (0–21.8 mbsf) is characterized by increasing NGR (5–22 cps), bulk density (1.65–1.95 g/cm3), velocity (1.57–1.62 km/s), and decreasing porosity (62%–45%). At the base of Unit 1, NGR values decrease to 5 cps, bulk density decreases to 1.8 g/cm3, and porosity shows a general increase to 58%. Unit 2 (21.8–152.1 mbsf) is mainly composed of neritic carbonate with low NGR values (~5 cps), variable density, and variable velocities, correlating with an alternation between high-velocity silicified limestones and packstones and lower velocity partially lithified/unlithified packstones.


To 182 Principal Results Site 1134

To 182 Table of Contents