This paper summarizes the magnetostratigraphic and biostratigraphic results obtained with siliceous microfossils (diatoms, radiolarians, silicoflagellates) on Neogene sections recovered in the Weddell Sea (Antarctic Ocean) during Ocean Drilling Program Leg 113 (Sites 689, 690, 693, 694, 695, 696, and 697).The biostratigraphic studies resulted in the establishment of an improved and revised Neogene biosiliceous zonation for the Antarctic Ocean.The zones are calibrated directly to the geomagnetic time scale.This is the first attempt at direct calibration of Miocene Antarctic biostratigraphic zones with the geomagnetic time scale.
Leg 150 drilling on the New Jersey slope recovered thick (110-350 m) middle Pleistocene sections at Sites 902 (811 m water depth), 903 (444 m), and 904 (1123 m). The physical properties records (gamma-ray attenuated porosity evaluator [GRAPE] and magnetic susceptibility) show distinct glacial-interglacial changes. Ages for these sections are constrained by calcareous nannofossil datums. A stratigraphy for each site was established by using GRAPE and magnetic susceptibility data that were calibrated to the SPECMAP oxygen isotope time scale (Imbrie et al., 1984). We improved the shipboard chronologies by tuning the physical properties records to the SPECMAP δ 18 θ stack in order to generate astronomically tuned stratigraphies for Sites 902, 903, and 904 at the oxygen isotope substage level. There is a high degree of similarity between the records at Sites 902, 903, and 904, despite the fine-scale variability unique to each hole. The most complete physical properties records are at Site 902, which provides the foundation for the Leg 150 studies. We extend the relationships established at Site 902 to Sites 903 and 904. Spectral and cross-spectral analysis of the tuned records were performed on the physical properties and the SPECMAP records. These analyses provide statistical evidence for the correlation of GRAPE and the SPECMAP records and link sedimentation on the New Jersey slope with orbitally controlled climate change. Although the relationship is best developed at Site 902, spectral analysis of the magnetic susceptibility records for Holes 903A and 904A support the interpretation of climatic forcing of sediment deposition at these sites. High coherency at the 100, 41, and 23-k.y. periodicities at Sites 902 and 903 indicate that our correlations are sound. The Site 904 correlations are less certain, though comparison to the SPECMAP stack suggests that our correlations are reasonable to at least the stage level. We suggest that the GRAPE signal for the Leg 150 sites reflects changes in grain size and variations in opal and carbonate content. Correlations of opal and carbonate content and the weight percentage of sand to the GRAPE records at Hole 902D indicate a consistent relationship between physical properties records and sedimentological components. Our studies suggest that Pleistocene sedimentation on the New Jersey slope was dominated by regular, climatically driven changes in grain size. This affects sediment density and porosity and permits the construction of substage-level stratigraphies (e.g., Sites 902 and 903). Occasional mass-wasting events have blurred these changes in portions of the physical properties records (e.g., the upper 13 m of Site 904), but chronologies to the stage level can still be constructed. The consistent relationship between the physical properties records and glacial-interglacial cycles at all three sites illustrates the utility of GRAPE as a valuable stratigraphic tool, even in an environment as complex as the New Jersey slope.
Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
Branches, stems and roots of Nothofagus (southern beech) were reported from the Beardmore Glacier area (Sirius Formation) some 500 km from the South Pole. The Sirius Formation is a glacially derived unit which is considered Pliocene in age. We considered two scenarios by which a Nothofagus forest could flourish so close to the South Pole during the Pliocene. One scenario calls for the disappearance of this genus from Antarctica during the early Tertiary (Oligocene) and its re-introduction during a warm Pliocene interval. Seeds from modern Nothofagus , however, are not viable after submersion in sea water, are not carried by migrating birds and are not designed for long-distance wind transport. The second scenario involves survival of Nothofagus in Antarctic refugia through middle Tertiary glacial advances and its flourishing during a Pliocene warming. Excluding its occurrence in the Beardmore Glacier area, the known range of this genus in Antarctica is Cretaceous to Oligocene with some questionable occurrences in the early Miocene. The existing data do not support the refugia scenario. We also considered published speculations in which the closest modern analogue to the Beardmore Glacier Region, during the time that the Nothofagus represented by these specimens lived, was the coastal region of southern Chile where average annual temperatures are approximately 5°C and summer temperatures are of the order of 8–10°C. We incorporated these observations and speculations into a palaeoenvironmental model, assuming them to be valid; the results require Pliocene temperatures as warm as, or warmer than, the Cretaceous, a scenario which is not supported by the literature. We conclude, therefore, that the occurrence of Nothofagus in the Beardmore Glacier area is older than Pliocene. Instead, we suggest that it represents a relict assemblage which is probably no younger than Oligocene but which may have persisted into the early Miocene.
Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
