Plio-Pleistocene diatom biostratigraphy from ODP Leg 177, Atlantic sector of the Southern Ocean

Seven sites were drilled during Ocean Drilling Program Leg 177 in the Atlantic sector of the Southern Ocean (SO) on a transect over the Antarctic Circumpolar Current from the Subantarctic to the Antarctic Zone. At four sites sediments were recovered with a Pliocene/Pleistocene sediment package of up to 580 m allowing the refinement of previous diatom zonation concepts. Samples were analyzed on stratigraphic distribution and abundance of diatom species. A refined diatom biozonation tied to the geomagnetic polarity record is proposed. For the middle and late Pleistocene two zonations applicable to the northern and southern area of the SO were constructed, considering different latitudinal distributions of biostratigraphic diatom marker species. The southern zonation for the Pleistocene relies on the occurrence of species of the genus Rouxia, R. leventerae and R. constricta n. sp. as well as on a revised last occurrence datum (LOD) of Actinocyclus ingens (0.38 Ma, late marine isotope stage (MIS) 11). The use of these new stratigraphic marker species refines the temporal resolution for biostratigraphic age assignment to up to 0.1 Myr. In particular the LOD of R. leventerae as an indicator for the MIS 6/5 boundary (Termination II) will improve future dating of carbonate-free Antarctic sediments. These new data were obtained from sediments of Sites 1093 and 1094 (Antarctic Zone). The northern zonation for the middle and late Pleistocene time interval is based on the Pleistocene abundance pattern of Hemidiscus karstenii which was already proposed by previous investigations (e.g. Gersonde and Barcena, 1998). One new species (R. constricta) and two new combinations (Fragilariopsis clementia, Fragilariopsis reinholdii) are proposed in this study.

Stable isotope record of benthic and planktonic foraminifera in sediment cores of ODP Leg 177, Southern Ocean

While onboard ship during Leg 177, we used variations in sediment physical properties (mainly percent color reflectance) in conjunction with biomagnetostratigraphy to correlate among sites and predict the position of marine isotope stages (MISs) (e.g., see fig. F11 in Shipboard Scientific Party, 1999, p. 45). Our working assumption was that physical properties of Leg 177 sediments are controlled mainly by variations in carbonate content. Previous studies of Southern Ocean sediment cores have shown that carbonate concentrations are relatively high during interglacial stages and low during glacial stages at sites located within the Polar Frontal Zone (PFZ). Today, the PFZ marks a lithologic boundary in underlying sediment separating calcareous oozes to the north and silica-rich facies to the south (Hays et al., 1976). Although there is debate whether the position of the "physical" PFZ actually moved during glacial-interglacial cycles (Charles and Fairbanks, 1990; Matsumoto et al., 2001), the "biochemical" PFZ, as expressed by the CaCO3/opal boundary in sediments, certainly migrated north during glacials and south during interglacials. This gave rise to lithologic variations that are useful for stratigraphic correlation. At Leg 177 sites located north of the PFZ and at sublysoclinal depths, we expected the same pattern of carbonate variation because cores in the Atlantic basin are marked by increased carbonate dissolution during glacial periods and increased preservation during interglacials (Crowley, 1985).

Plio-Pleistocene magnetic polarity stratigraphies of ODP Leg 177 Sites from the South Atlantic

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.

Pleistocene calcareous nannofossil stratigraphy for ODP Leg 177, Atlantic sector of the Southern Ocean

Seven Ocean Drilling Program (ODP) sites recovered during ODP Leg 177 in the Atlantic sector of the Southern Ocean were analyzed to study the Pleistocene calcareous nannofossil record. Calcareous nannofossil events previously described from intermediate and low latitudes were identified and calibrated with available geomagnetic and stable isotope stratigraphic data. In general, Pleistocene southern high latitude calcareous nannofossil events show synchronicity with those observed from warm and temperate latitudes. The first occurrence (FO) of Emiliania huxleyi and the last occurrence (LO) of Pseudoemiliania lacunosa are observed in marine isotope stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed at MIS 5. MIS 6 is characterized by an increase in G. muellerae and MIS 7 features a dramatic decrease in the proportion of Gephyrocapsa caribbeanica. This latter species began to increase its proportions from MIS 14 to 13. The LO of Reticulofenestra asanoi is observed within subchron C1r.1r and the FO of R. asanoi occurs at the top of C1r.2r. A reentry of medium-sized Gephyrocapsa can be identified in some cores during subchron C1r.1n. The LO of large morphotypes of Gephyrocapsa is well correlated through the studied area, and occurs during the middle-low part of subchron C1r.2r,synchronous with other oceanic regions. The FO of Calcidiscus macintyrei and FO of medium-sized Gephyrocapsa occur in the studied area close to 1.6 Ma.

Concentrations of Ti, Fe, and Al and radiogenic isotope data for sediments of ODP Leg 177

Identifying terrigenous sources in deep-sea sediments may reveal temporal trends in paleocirculation and the relative role of eolian, upwelled, and hemipelagic Fe sources to surface waters. Bulk elemental and isotopic geochemistry of deep-sea sediments recovered during Ocean Drilling Program Leg 177 in the southeastern Atlantic sector of the Southern Ocean reveal several important aspects of paleocirculation and terrigenous provenance. The sites studied span 43°-53°S and represent different oceanographic settings relative to regional hydrography and sediment type. Bulk sediment geochemistry indicates that terrigenous provenance varied over the past 600 k.y. Site 1089, the northernmost site, exhibits clear glacial-interglacial variability in provenance, while provenance appears to vary regardless of climate state at the more southerly sites (Site 1093 and 1094). Nd and Sr isotopes and Sm/Nd ratios of the terrigenous fraction indicate that study sites have geochemically distinguishable provenance. Nd and Sr isotopes further suggest that Sites 1089 and 1094 both contain detrital components that originated in South America over the past 30 k.y.; however, Site 1089 is also influenced by southern African sources and the strength of the Agulhas Current. The e-Nd data support a more hemipelagic source for the terrigenous material rather than an eolian source based on comparisons with Antarctic ice core data and known sea-ice extent.

(Table 1) Diatom mats from ODP Leg 177 Sites in the Southern Ocean, Atlantic Sector

The interaction between biogenic silica export and burial, paleoceanography, diatom species succession and mats formation was examined based on relative abundances data of Plio/Pleistocene diatoms from six cores recovered during ODP Leg 177 on a transect across the Antarctic Circumpolar Current (ACC) in the Atlantic sector of the Southern Ocean. Fragilariopsis kerguelensis, Actinocyclus ingens and species of the genus Thalassiothrix were the main contributors to the diatom assemblages. Three main steps marked the development of the silica system in the Southern Ocean: Step 1 (at ca. 2.77 Ma), establishment of increased biogenic silica burial in the Antarctic Circumpolar Current area, following the large-scale oceanic reorganization connected to the increased northern hemisphere glaciation; Step 2 (at ca. 1.93 Ma), the Antarctic Polar Front becomes the main biogenic silica sink, diatom mats are widespread, and are also found slightly to the north and south of the APF; Step 3 (at ca. 0.63 Ma), with the strong drop in abundance (and later extinction at 0.38 Ma) of A. ingens and the rise to dominance of F. kerguelensis, the system enters a glacial-interglacial mode, with diatom mats occurring during interglacials at the APF and in the Antarctic Zone, but disappearing north of it.