Geochemical investigation of ODP Leg 113 sites

Dissolved organic carbon (DOC) was determined in pore water extracted from pelagic and hemipelagic sediments recovered during Leg 113. DOC concentration varied between 1.82 and 13.6 mg C/L which is one to two orders of magnitude less than previously reported for hemipelagic sediments. It is argued that this difference is related to differences in the intensity of degradation of organic matter. As a first approximation it is found that in reducing sediments, the level of DOC is proportional to the intensity of sulfate reduction. It is suggested that DOC is formed by different mechanisms in oxic and reducing environments.

Investigations of Cretaceous and Tertiary kerogens in sediments of ODP Leg 113 holes

Seventy-one samples from nine sites were analyzed for total organic carbon (TOC). Fifty-six samples, containing 0.2% or more TOC, were evaluated by Rock-Eval to assess the nature of their kerogen and its petroleum source potential. Visual kerogen studies were carried out. Petroleum potential was encountered only in Valanginian calcareous claystones at Hole 692B close to the margin of Dronning Maud Land. A section of 44.7 m was penetrated. The unit possesses a revised mean TOC of 9.8% and petroleum potential of 43.2 kg/Mg, relatively high values in comparison to other Cretaceous anoxic oceanic sections and the totality of petroleum source rocks. At Sites 689 and 690, extremely low TOC levels, mean 0.07%, preclude kerogen analysis. Kerogens in Eocene to Pliocene sediments of the central and western Weddell Sea (Sites 694, 695, 696, and 697) are similar everywhere, largely comprising brown to black, granular, amorphous material of high rank, and generally possessing several reflectance populations of vitrinite particles. The latter are interpreted as indicative of the recycling of sediments of a variety of levels of thermal maturity.

Interstitial-water chemistry of ODP Leg 113 sites

Variations in the distribution of major elements and stable oxygen isotopes in ODP Leg 113 pore water are not related to lithology and thus appear to be controlled by minor constituents. Petrographic observations and geochemical considerations indicate that alteration of calc-alkalic volcanic material dispersed in the sediment is an important process. A diagenetic reaction is constructed that involves transformation of volcanic glass into smectite, zeolite (represented by phillipsite), chert, and iron sulfide. Mass balance calculations reveal that alteration of less than 10% (volume) of volcanogenic material may account for the observed depletion of magnesium, potassium, and 18O and enrichment of calcium. Alteration of this amount of volcanic glass produces less than 4% (volume) of smectite and zeolite. Hence, mass balance is obtained without having to invoke unreasonable large amounts of volcanic matter or interactions between seawater and basement.

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.

Isotopic composition and Strontium/Calcium ratios of foraminifera of ODP Holes 113-689B and 113-690C

Oxygen and carbon isotopic ratios were measured from Maestrichtian benthic and planktonic foraminifer species and bulk carbonate samples from ODP Sites 689 and 690, drilled on the Maud Rise during Leg 113. Careful scanning electron microscope observations reveal that test calcite in some intervals was diagenetically altered, although Sr/Ca and isotopic ratios of these tests do not appear to have been modified significantly. Foraminifer d18O values at both sites document a cooling trend during early Maestrichtian time, a rapid drop in water temperatures at the time of the first appearance of Abathomphalus mayaroensis in the high southern latitude regions (about 69.9 Ma), and lower water temperatures during late Maestrichtian time. d13C values record a depletion in 13C in the latest early Maestrichtian time beginning at about 72.2 Ma, just prior to the sharp late Maestrichtian increase in d18O values. These trends are similar to those previously reported for well-preserved benthic foraminifer species from Seymour Island, in the Antarctic Peninsula. Paleotemperature estimates are also comparable to those at Seymour Island and suggest temperate climatic conditions in Antarctica and that bottom waters in the southern South Atlantic region were of Antarctic origin. Benthic and planktonic foraminifer 613C values fluctuate sympathetically and are higher in upper Maestrichtian sediments than in the lower Maestrichtian sequence.

Sporomorphs and dinoflagellate cysts of ODP Leg 113 holes

Palynological studies were carried out on Paleogene sections from Sites 693 and 696 of Ocean Drilling Project Leg 113 in the Weddell Sea region. Dinoflagellate cysts and sporomorphs were recovered at Site 696 (61°S, 42°W) indicating a middle Eocene to late Eocene/earliest Oligocene age for a glauconitic silt/sandstone. At Site 693 (70°S, 14°W) early Oligocene siliciclastic mud contains a low diversity palynoflora. In an upper Oligocene section (Site 693) only rare, reworked Mesozoic palynomorphs were encountered. Palynological data from Kerogen analyses, dinocysts, and sporomorphs are used to reconstruct the climatic change on the South Orkney microcontinent from the middle Eocene to the late Eocene/earliest Oligocene at Site 696 and the late early Oligocene/early late Oligocene time interval at Site 693 near the continental margin. The middle Eocene was a warm period in the Orkney region with good growing conditions for a warm temperate Nothofagus/conifer forest with an admixture of Proteaceae. Temperate surface water masses, which allowed the growth of a reasonably diverse dinocyst assemblage (ca. 15-20 species), persisted until the end of the Eocene at Site 696. Late early Oligocene sediments of Site 693 (Antarctic continental margin) contain only a low diversity dinocyst flora (two species). The major Cenozoic cooling event in the Weddell Sea region probably occurred at the Eocene/Oligocene boundary. A second dramatic climatic deterioration seems to have taken place during the late early/early late Oligocene, when dinocysts disappeared at the Dronning Maud Land margin area.

Geochemistry of basalts from ODP Hole 113-690C

Basalts from Maud Rise, Weddell Sea, are vesicular and olivine-phyric. Major, trace, and rare earth element concentrations are similar to those of alkali basalts from ocean islands and seamounts. The rocks are low in MgO, Cr, Ni, and Sc, and high in TiO2, K2O, P2O5, Zr, and LREE contents. The abundance of "primary" biotite and apatite in the matrix indicates the melting of a hydrous mantle. Prevalence of olivine and absence of plagioclase in the rocks suggests that the volatile in the melt was an H2O-CO2 mixture, where H2O was <0.5. Mantle derived xenocrysts in the basalt include corroded orthopyroxene, chromite, apatite, and olivine. Olivine (Fo90) is too magnesian to be in equilibrium with the basalts, as they contain only 5-6 wt% MgO. Based on the presence of mantle xenocrysts, the high concentration of incompatible elements, the spatial and chemical affinity with other ocean island basalts from the area, and the relative age of the basalt (overlain by late Campanian sediments), it is suggested that Maud Rise was probably generated by hot-spot activity, possible during a ridge crest jump prior to 84 Ma (anomaly 34 time). Iddingsite, a complex intergrowth of montmorillonite and goethite, is the major alteration product of second generation olivine. It is suggested that iddingsite crystallized at low temperatures (<200°C) from an oxidized fluid during deuteric alteration. Vesicles are commonly filled by zeolites which have been replaced by K-feldspars.

Middle Eocene to Pleistocene calcareous nannofossils of ODP Leg 113 holes

Long sequences of Upper Cretaceous through Quaternary sediments rich in calcareous and siliceous microfossils were recovered at Ocean Drilling Program Sites 689 and 690 on Maud Rise off East Antarctica. These sites have become the southernmost anchor in the Atlantic Basin for bio-, magneto-, chemostratigraphic, and paleobiogeographic studies. ODP Sites 692 and 693 on the Weddell Sea margin of East Antarctica and Site 696 on the South Orkney microcontinent of West Antarctica yielded calcareous nannofossils within some stratigraphic intervals. Sites 691, 692, 694, 695, and 697 did not recover Cenozoic calcareous nannofossils. Calcareous nannofossil biostratigraphy suggests a major hiatus across the Paleogene/Neogene boundary at Sites 689 and 690, and two additional hiatuses in the middle Eocene-lower Oligocene section at Site 690. Correlation with magnetostratigraphy reveals: the last occurrence (LO) of Reticulofenestra umbilica at Maud Rise is over 1 m.y. younger than that at the middle-latitude sites; the LO of Isthmolithus recurvus is synchronous in the middle-latitude and high-latitude areas (about 34.8 Ma); Reticulofenestra oamaruensis ranges from 38.0 to 36.0 Ma at Maud Rise; Reticulofenestra reticulata has a shorter range at Maud Rise (42.1 to 38.9 Ma) than at the middle-latitude DSDP Site 516; the range of Chiasmolithus oamaruensis is diachronous over different latitudes; and the LO of Chiasmolithus solitus is a good datum at 41.3 Ma from 30°S to 65°S in the South Atlantic Ocean. Comparison of calcareous nannofossil abundances in a latitudinal transect shows: Reticulofenestra bisecta is a temperate-water species and its LO, which crosses below that of Chiasmolithus altus at Maud Rise, is not applicable for the Paleogene/Neogene boundary in high southern latitude areas; Clausicoccus fenestratus is rare or absent at Maud Rise and can not be used as a marker; Coccolithus formosus is a warm-water species which disappeared earlier toward higher latitudes. Calcareous nannofossil assemblages indicate that by at least the middle Eocene, surface water temperatures became considerably lower in the high southern latitudes than in the middle-latitude areas and that there have been more extreme cold events in the high latitudes during the Neogene. Bicolumnus ovatus n. gen., n. sp. is proposed in this paper.