High Southern Latitude Magnetostratigraphy and Rock Magnetic Properties of Sediments from Sites 747, 749, and 751

Abstract

The magnetostratigraphy of Sites 747, 749, and 751 of Leg 120 that was established by shipboard measurements was extended and confirmed. Five hundred and eighty sediment cubes were stepwise demagnetized with alternating fields to determine their characteristic remanence. We obtained a rather complete magnetostratigraphy from the Oligocene to the Pliocene for the nannofossil oozes of Site 747. Alternating-field demagnetization experiments on laboratory-induced magnetization, hysteresis measurements, and low-temperature experiments showed that the magnetic carriers are single-domain and small pseudo-single-domain (titano)magnetite particles. A fraction of these Fe-Ti oxides seems to reside as microlites in the vitric volcanic ash particles that were enriched in the magnetic separates. The natural remanent magnetizations seem to be of detrital origin with small Fe-Ti oxide grains in the volcanic glass particles as carriers of the stable paleomagnetic signal.

Main text

“…The revised chronozones and diatom zones illustrated are based on reassessing the data sets of these workers (Table 37). Comparison of the chronozonal assignments of Heider et al (1992;Table 36) to those of this study (Table 37) indicates that the chronozonal placements are similar. Figure 53 shows an age-depth graph for Hole 120-747B using the stratigraphical markers determined in this study.…”

“…A partial palaeomagnetic polarity record was obtained from the sequence. The interval from 2-25 and 70-98 mbsf is difficult to interpret as a result of the lack of data and scatter Heider et al 1992). All three microfossil groups were recorded from this hole.…”

“…2) based on the comparison of Leg 120 results with previous stratigraphies of Weaver & Gombos (1981), Ciesielski (1983, 1986, Gersonde & Burckle (1990) and . Diatom datums were calibrated directly to the polarity sequence of Heider et al (1992) and for the last 31 million years (see also Harwood et al 1992). The Pleistocene to mid-Miocene portion of the diatom zonation represented an integration between the zonations of Gersonde & Burckle (1990) and .…”

“…Radiolarians were not observed in sediments older than Oligocene ). The integrated biostratigraphical and magnetostratigraphical data previously used (palaeomagnetics: Heider et al 1992;Harwood et al 1992;diatoms: Harwood & Maruyama 1992; calcareous nannofossils: Wei & Wise 1992;radiolarians: Abelmann 1992;Lazarus 1992) and calibrated to the GPTS of Berggren et al (1985a, b) are presented in Table 34. An age-depth graph for Hole 120-747A using these data is presented in Fig.…”

“…A palaeomagnetic polarity record was obtained for the Quaternary to upper Miocene sequence. Shipboard workers resolved their inability to recognize short polarity reversal by shorebase measurements of discrete samples Heider et al 1992). Diatoms, calcareous nannofossils and radiolarians are observed in the sequence recovered.…”