Eocene/Oligocene paleoceanography from ODP Hole 113-689B and Hole 113-690B

Middle Eocene to Late Oligocene sediments from near the crest (Site 689B, water depth 2080 m) and flank (water depth 2914 m) of the Maud Rise (62°S) have been investigated by coarse fraction analysis and have revealed the following: (1) The middle Eocene (50-40 Ma) was a period of pure carbonate sedimentation, with good preservation of carbonate microfossils. No opal > 40 µm is present. (2) In the late Eocene (40-36.5 Ma) opal fossils (mainly radiolaria, and some diatoms > 40 µm) appeared for the first time. Three maxima in opal sedimentation (Eocene/Oligocene boundary, middle early Oligocene and early/late Oligocene boundary) are separated by increases in carbonate sedimentation. The dissolution of carbonate fossils is strong in the opal-rich layers. Opal sedimentation is attributed to cooling and probably more vigorous atmospheric circulation and increased upwelling. (3) Carbonate dissolution increased with water depth in the Oligocene, whereas in the middle Eocene excellent carbonate preservation in the deeper Site 690B and stronger dissolution in the shallower Site 689B is attributed to different bottom-water characteristics. The middle Eocene bottom water probably was formed by strong evaporation at low latitudes, whereas by the earliest Oligocene formation of Antarctic Bottom Water (AABW) had set in. (4) Current influence, not on top but on the flank of the Maud Rise, could be recorded by means of larger grain sizes of benthonic and planktonic microfossils. (5) Ice-rafted debris was not found. Quartz and other minerals are very rare and not larger than 125 µm and may have been supplied by ice as well as by wind or by deep currents. Mica contents were up to 10 times higher in the middle Eocene on the flank compared to on the crest of the Maud Rise, indicating deep current supply.

Carbon and oxygen isotope data for carbonates and benthic foraminifers from ODP Hole 113-689B

At Ocean Drilling Program Site 689 (Maud Rise, Southern Ocean), d18O records of fine-fraction bulk carbonate and benthic foraminifers indicate that accelerated climate cooling took place following at least two closely spaced early late Eocene extraterrestrial impact events. A simultaneous surface-water productivity increase, as interpreted from d13C data, is explained by enhanced water-column mixing due to increased latitudinal temperature gradients. These isotope data appear to be in concert with organic-walled dinoflagellate-cyst records across the same microkrystite-bearing impact-ejecta layer in the mid-latitude Massignano section (central Italy). In particular, the strong abundance increase of Thalassiphora pelagica is interpreted to indicate cooling or increased productivity at Massignano. Because impact-induced cooling processes are active on time scales of a few years at most, the estimated 100 k.y. duration of the cooling event appears to be too long to be explained by impact scenarios alone. This implies that a feedback mechanism, such as a global albedo increase due to extended snow and ice cover, may have sustained impact-induced cooling for a longer time after the impacts.

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.

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.

Strontium isotope data from DSDP Sites 32-305 and 62-463 and ODP Sites 113-689 and 113-690

Fluctuations in oxygen (d18O) and carbon (d13C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71-70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal d18O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal d13C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal d18O values in the Indo-Pacific decreased, and benthic and planktic d13C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic d13C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.

Peak areas and heights for minerals of ODP Leg 113-696B (Table 1)

Seismic data acquired over the eastern shelf and margin of the South Orkney microcontinent, Antarctica, have shown a high-amplitude reflection lying at a sub-bottom two-way traveltime (TWT) of 0.5-0.8 s. There appear to be two causes for the reflection which apply in different parts of the shelf. The more widespread cause of the reflection is a break-up unconformity associated with the opening of Jane Basin to the east. This is clearly seen where reflections in the underlying sequence are discordant. In contrast, in Eotvos Basin and the southeastern part of Bouguer Basin, the high-amplitude reflection in places cuts across bedding and is interpreted to be caused by silica diagenesis. A post-cruise analysis of core samples from Site 696 in Eotvos Basin by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the presence of a silica diagenetic front at 520-530 mbsf. The position of the unconformity at this site is uncertain, but probably coincides with a change of detrital input near 548 mbsf. Fluctuations of physical properties related to the depth of the diagenetic front are difficult to separate from those related to the variation of detrital composition over the same depth interval. Correlation of the drilling record with the seismic record is difficult but with a synthetic seismogram it is demonstrated that diagenesis is the probable cause of the high-amplitude reflection. In Bouguer Basin at Site 695 the depth of the high-amplitude reflection was not reached by drilling; however, the reflection is probably also caused by silica diagenesis because of the biogenic silica-rich composition of the sediments cored. The estimated temperatures and ages of the sediments at the depths of the high-amplitude reflections at Sites 695 and 696 compare favorably with similar data from other diagenetic fronts of the world. The high-amplitude reflection in Bouguer Basin is commonly of inverse polarity, possibly caused either by interference between reflections from several closely-spaced reflecting layers, such as chert horizons, or by free gas trapped near the diagenetic front.

Strontium isotope geochemistry of ODP Leg 113 sites

The concentration of dissolved Sr and the distribution of 87Sr/86Sr isotope ratios in Leg 113 interstitial waters may be interpreted in terms of mixing of Sr from four different reservoirs: indigenous seawater, marine carbonate minerals, and basaltic and siliceous detrital material. The input to the pore water from these reservoirs is determined by the reactivity of the reservoir rather than its size. The presence of strontium derived from siliceous detrital material is unequivocally demonstrated in the pore waters of the hemipelagic deposits, and is also significant in the calcareous Maud Rise sediments due to the unusually low degree of carbonate recrystallization. Also, alteration of basic volcanic material is important at several sites.