Description: Pliocene and Miocene magnetostratigraphy from ODP Site 1218 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 50 discrete samples. U-channel samples were measured at 1 cm intervals and stepwise demagnetized in alternating fields up to a maximum peak field of 80 mT. The component magnetization directions were determined by principal component analysis for demagnetization steps in the 20-60 mT peak field range. A relatively small number of discrete samples were subject to both thermal and alternating field (AF) demagnetization and gave results compatible with u-channel measurements. Magnetostratigraphy from u-channel samples are compared with shipboard data that were based on blanket demagnetization at peak AF fields of 20 mT. U-channel measurements add more detail to the magnetostratigraphic record and allow identification of thin polarity zones especially in the upper part of the section were the sedimentation rates are very low (~2 m/Myr). The component magnetization directions determined from u-channel measurements also gave more reliable and precise estimates of inclination (paleolatitude). The magnetostratigraphy from Site 1218 can be unambiguously correlated with the reference geomagnetic polarity time scale and gives a means of dating the sedimentary sequence. Both Miocene-Pliocene and Oligocene-Miocene stage boundaries were easily identified from the magnetostratigraphic record. Although calculation of paleomagnetic poles is hindered by the low precision of the cores' azimuthal orientation, the data from both u-channel and discrete samples allow determination of the paleolatitude of the Site through time with good precision. Paleomagnetic data indicate that the paleolatitude of Site 1218 has increased form nearly equatorial latitude in the Oligocene to its present-day latitude close to 9°N. Within the precision of the paleomagnetic data, this is in agreement with current predictions of plate motion models based on fixed hotspots.

Description: Magnetic polarity stratigraphies from ODP Leg 177 'high resolution' sites indicate Brunhes sedimentation rates in the 12-25 cm/kyr range, with a trend of decreasing sedimentation rates with increasing age. Magnetite is the principal remanence-carrying mineral. Downcore alteration of magnetite and authigenic growth of iron sulfides introduces a high coercivity diagenetic remanence carrier (pyrrhotite). The change in pore water sulfate with depth in the sediment tends to be in step with the decrease in magnetization intensity, indicating the link between sulfate reduction and magnetite dissolution. Shipboard pass-through magnetometer data are generally very noisy due to a combination of weak magnetization intensities, drilling-related core deformation, and the influence of authigenic iron sulfides. Post-cruise progressive demagnetization of discrete samples aids the magnetostratigraphic interpretation, as these measurements are less influenced by low magnetization intensities and drilling-related deformation. The magnetostratigraphic interpretations provide much-needed calibration for biostratigraphic events in the high latitude southern oceans. Apart from the ODP Hole 745B (Kerguelen Plateau), published Plio-Pleistocene magnetostratigraphies from ODP sites in the Southern Ocean are poorly constrained. For this reason, we compare interpolated ages of 11 radiolarian events and one diatom event that occur at Hole 745B and Leg 177 sites.

Description: At Ocean Drilling Program (ODP) Site 1090 (lat 42°54.8'S, long 8°54.0'E) located in a water depth of 3702 m on the Agulhas Ridge in the sub-Antarctic South Atlantic, ~300 m of middle Eocene to middle Miocene sediments were recovered with the advanced piston corer (APC) and the extended core barrel (XCB). U-channel samples from the 70-230 meters composite depth (mcd) interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole-core and discrete sample data. The magnetostratigraphy can be interpreted by the fit of the polarity-zone pattern to the geomagnetic polarity time scale (GPTS) augmented by isotope data and bioevents with documented correlation to the GPTS. Three normal-polarity subchrons (C5Dr.1n, C7Ar.1n, and C13r.1n), not included in the standard GPTS, are recorded at Site 1090. The base of the sampled section is correlated to C19n (middle Eocene), although the interpretation is unclear beyond C17r. The top of the sampled section is correlated to C5Cn (late early Miocene), although, in the uppermost 10 m of the sampled section, a foraminifer (Globorotalia sphericomiozea) usually associated with the Messinian and early Pliocene has been identified. 87Sr/86Sr, d13C, and d18O values measured on foraminifera, including the d18O and d13C shifts close to the Eocene/Oligocene boundary, support the correlation to the GPTS. For the interval spanning the Oligocene/Miocene boundary, benthic d13C, d18O, and 87Sr/86Sr records from Site 1090 can be correlated to isotope records from ODP Site 929 (Ceara Rise), providing support for the recently-published Oligocene/Miocene boundary age (22.92 Ma) of Shackleton et al.

Description: We present mid-Pliocene (4.3-2.6 Ma) benthic stable oxygen and carbon isotope data from Ocean Drilling Program Site 1092 (ODP Leg 177) drilled in the sub-Antarctic sector of the Southern Ocean. The results are compared with the stable isotope results from nearby Site 704 (ODP Leg 114). Oxygen isotope data show that minimum values are about 0.5 per mil less than those of the Holocene, which is consistent with the results from Site 704, indicating only minor deglaciation of Antarctica during the studied interval. Oxygen isotope data from both Site 1092 and Site 704 are slightly higher relative to Pacific values during several intervals which could be related to the contribution of warm, saline North Atlantic Deep Water (NADW). Comparisons of benthic carbon isotope gradients between sites located in the North Atlantic, sub-Antarctic sector of the Southern Ocean, and Pacific indicate that at times, the gradient between the Southern Ocean and the Pacific evolved differently than the Atlantic-Pacific gradient. This suggests that variations in NADW strength alone might not be responsible for the observed carbon isotope values in the Southern Ocean.