Magnetobiostratigraphy and stable isotope record of Paleocene sediments from ODP Hole 122-761B in the subtropical Indian Ocean

Surface and deep water circulation patterns in the eastern Indian Ocean during the Paleocene Epoch are inferred based on an integrated magnetobiostratigraphic and stable isotope investigation of Ocean Drilling Program Hole 761B, drilled on the Wombat Plateau. A combination of magnetostratigraphy, biostratigraphy and isotope stratigraphy demonstrates that numerous deep sea sites that have been considered to show continuous, or nearly continuous sedimentation through the Paleocene are punctuated by a series of hiatuses, some of which exceeding a duration of 1 Myr. Therefore, our study is based on a detailed temporal interpretation of the stratigraphic successions we used for paleoceanographic reconstructions. We compare detailed planktonic and benthic foraminiferal carbon and oxygen isotope records from Hole 761B with several temporally correlative records published from different oceanic provinces in order to distinguish between local and global patterns within the eastern Indian Ocean. Although Site 761 was situated at low latitudes during the Paleocene, its surface waters were predominantly influenced by circulation originating from the Southern Ocean as indicated by inferred cool sea surface temperatures and reduced surface to deep water temperature gradients. We suggest that deep waters in the eastern Indian Ocean were not directly fed by the Southern or Tethys Oceans. Rather, the more negative delta13C composition of the bottom waters recorded by benthic foraminifera implies the presence and/or active contribution of aged deep waters from the Pacific during this time, at least prior to ~60.2 Ma and subsequent to ~59.0 Ma. The Indian continent, Ninetyeast Ridge, Kerguelen Plateau and Broken Ridge may have played a significant role as submarine barriers to deep water circulation during the Paleocene.

Benthic foraminifera in Northwest Indian Ocean surface sediments

The relationship between the distribution of benthic foraminifera and sediment type and depositional environment in the Arabian Sea is discussed. The benthic foraminiferal fauna were sampled in nineteen Recent surface sediment samples, and geochemical variables of the sediment of the same samples were measured. The water depths for the box core samples varies from 440 to 4040 m. A total of 103 species and six species-complexes were identified. The geochemical properties were found to correspond well to the sediment type and depositional environment and six different sediment/depositional environment types could be distinguished. Analysis of the benthic foraminiferal fauna reveals specific faunal assemblages that are closely related to these sediment/depositional environment types.

Biomass of cysts and active infusoria in waters of the Central Indian Ocean and Peru coast area

The number of cysts of marine planktic infusoria was determined in oligotrophic waters of the central Indian Ocean and productive waters of the Southeast Pacific. Cyst biomass at stations studied varied from 1.2 to 23.4 ?g/l, which was 9.9-115.8% of free infusoria biomass in the 0-15 m layer in the Indian Ocean and 0.3-19.3% in the Southeast Pacific.

Radiocarbon dataing, stable oxygen isotopes, and Mg/Ca ratios in surface sediment samples of the tropical eastern Indian Ocean

Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone-derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed-layer between 20 m and 50 m, whereas Globigerina bulloides records mixed-layer conditions at ~50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ~75 m, 75-100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca-temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca-temperatures of the mixed-layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño-Southern Oscillation, and the Indian Ocean Dipole Mode.

Hydroclimate records and radiocarbon dating of sediment cores SO189/2 _39KL, SO189/2 _119KL, and SO189/2 _144KL from the eastern tropical Indian Ocean

The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells1, 2, 3, 4, 5, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.

Chemical composition of ultrabasites and gabbro from rift zones of the Indian Ocean

Results of petrographic studies of ultrabasites and gabbro from rift zones of the Indian Ocean are discussed using materials of Cruise 36 of R/V Vityaz. Rocks sampled from two sites 2700 km apart are close to each other in their composition. Petrographically ultrabasic rocks are divided into four subgroups: I - dunite; II - harzburgite, serpentinite; III - plagioclase lherzolite; and IV - metamorphically altered rocks. Petrographic description and chemical composition of basic rock varieties are presented as well as description of rock-forming minerals and their optical properties. Formation of pyroxene and plagioclase is shown to be related to autometasomatosis. Formation of ultrabasite in rift zones is related to complicated processes.