Paleocene to middle Eocene calcareous nannofossils of Maude Rise, Weddell Sea

Cores from Sites 689 and 690 of Ocean Drilling Program Leg 113 provide the most continuous Paleocene and Eocene sequence yet recovered by deep sea drilling in the high latitudes of the Southern Ocean. The nannofossil-foraminifer oozes and chalks recovered from Maud Rise at 65°S in the Weddell Sea provide a unique opportunity for biostratigraphic study of extremely high southern latitude carbonate sediments. The presence of warm water index fossils such as the discoasters and species of the Tribrachiatus plexus facilitate the application of commonly used low latitude calcareous nannofossil biostratigraphic zonation schemes for the upper Paleocene and lower Eocene intervals. In the more complete section at Site 690, Okada and Bukry Zones CP1 through CP10 can be identified for the most part with the possible exception of Zone CP3. Several hiatuses are present in the sequence at Site 689 with the most notable being at the Cretaceous/Tertiary and Paleocene/Eocene boundaries. Though not extremely diverse, the assemblage of discoasters in the upper Paleocene and lower Eocene calcareous oozes is indicative of warm, relatively equable climates during that interval. A peak in discoaster diversity in uppermost Paleocene sediments (Zone CP8) corresponds to a negative shift in 5180 values. Associated coccolith assemblages are quite characteristic of high latitudes with abundant Chiasmolithus, Prinsius, and Toweius. Climatic cooling is indicated for middle Eocene sediments by assemblages that contain very abundant Reticulofenestra, lack common discoasters and sphenoliths and are much less diverse overall. Two new taxa are described, Biscutum? neocoronum n. sp. and Amithalithina sigmundii n. gen., n. sp.

Lower Cretaceous nannofossils of ODP Holes 113-682B and 113-693A

Organic-rich, moderately to sparsely nannofossiliferous Lower Cretaceous claystones ("black shales") were cored at two Ocean Drilling Program Leg 113 sites on the continental slope of East Antarctica off Dronning Maud Land. A 39 m section at Site 692 yielded a Neocomian assemblage of limited diversity with rare Cyclagelosphaera deflandrei, Diadorhombus rectus, and Cruciellipsis cuvillieri, and is probably Valanginian in age. A 70-m section at Site 693 is assigned to the Rhagodiscus angustus Zone (late Aptian-early Albian in age). The latter zone is represented at DSDP sites on the Falkland Plateau, but equivalents to the Neocomian section are absent there, probably due to a disconformity. Watznaueria barnesae is the dominant species at both ODP sites, but it shares dominance with Repagulum parvidentatum at Site 693, where they total 70%-90% of the assemblage; their dominance is attributed to a paleogeographic setting within a restricted basin rather than to postdepositional dissolution of other species. The evolutionary development of this restricted basin and its eventual ventilation in early Albian times is discussed in terms of the regional stratigraphy and the breakup and dispersal of southwestern Gondwanaland. One new species, Corollithion covingtonii, is described.

Datum events and their estimated magnetochronology at DSDP Site 72-516

Calcareous plankton biostratigraphy (foraminifers and nannoplankton) and magnetostratigraphy of the upper Oligocene to Pleistocene have been studied in hydraulic piston Cores 516-1 to 516-44, 516A-5 to 516A-11, and 516F-1 to 516F-11, Rio Grande Rise (water depth 1313 m). Some 80 biostratigraphic datum events have been correlated to the magnetic polarity stratigraphy over an interval representing the Matuyama to Chron 5, and Chrons 16 to 23. Coring disturbance and biostratigraphic evidence of a condensed section preclude unambiguous identification of polarity or biostratigraphic events over an approximately 30-m interval in the middle and upper Miocene. Sedimentation rates varied considerably during the Neogene, but an abnormally thick upper Oligocene and lower Miocene section allows a high degree of magnetobiochronologic resolution. A new planktonic foraminiferal zonation for the Miocene completes the midlatitude Neogene zonation of the South Atlantic. Important magnetobiostratigraphic correlations at Site 516 and their estimated magnetochronology include: (1) Oligocene/ Miocene boundary = first appearance datum (FAD) Globorotalia kugleri = last appearance datum (LAD) Reticulofenestra bisecta = mid-Anomaly 6C (Chron 23) = 23.7 Ma; (2) Aquitanian/Burdigalian boundary = LAD G. kugleri = between base Anomaly 6A and top of unnumbered anomaly between 6A and 6B (Chron 21) = 21.8 Ma; (3) Zone N6/N7 boundary = LAD Catapsydrax dissimilis (= FAD G. pseudomiozea and G. zealandica) = Chron 16/17 boundary = 17.6 Ma; (4) early/middle Miocene (= Burdigalian/Langhian) boundary = FAD Praeorbulina sicana = midpart of Anomaly 5C (Chron 16) = 16.6 Ma or FAD P. glomerosa = just above Anomaly 5C (inferred) = 16.3 Ma; (5) Zone N8/N9 boundary = FAD Orbulina suturalis above Anomaly 5C (later part Chron 16, inferred); (6) Miocene/ Pliocene boundary = LAD Globoquadrina dehiscens LAD Globorotalia lenguaensis = basal Gilbert Chron = 5.3 Ma.

Displaced diatom, discoasters and benthic foraminifers at DSDP Hole 72-515A

Late Pliocene to Recent sediments from the southern Brazil Basin (DSDP Hole 515A, hydraulic piston core) were analyzed for evidence of episodic flow of Antarctic Bottom Water (AABW) through the Vema Channel. Carbonate-enriched layers punctuate the post-Pliocene section, otherwise composed predominantly of terrigenous silt and clay. Carbonate enrichment is thought to result from rapid deposition of fine-grained calcareous turbidites, originating in canyons incised on the northern margin of the Rio Grande Rise. The composition of benthic foraminiferal assemblages and the presence of stratigraphically displaced discoasters is consistent with a turbidite origin. Based on the presence of displaced Antarctic diatoms, AABW flow through the Vema Channel apparently has had a major influence on this site for only four periods during the last 2.7 Ma (about 45 to 250; 375 to 430; 700 to 780; 1320 to 1345 thousand yr. ago).