983 resultados para Ac eff rock fragm


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The book summarizes data on distribution and composition of sedimentary material suspended in waters of the Atlantic Ocean and its seas. Results of observations of Soviet and foreign expeditions are given. Distribution of suspended matter in sections across the ocean, as well as in the most studied seas are shown. New data on grain size, mineral and chemical composition of suspended matter are published. Summary of history of investigation of bottom sediments from the Atlantic Ocean from the first scientific cruises to the present is done. A brief description of sediment types in the ocean and a detailed description of Mediterranean Sea sediments are given.

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The basalts in Holes 519A, 522B, and 524 were studied for intensity of natural remanent magnetization, magnetic hysteresis, magnetic susceptibility, stability of isothermal remanence, and thermomagnetic behavior. Some of these properties are sensitive to both the composition and the microstructure of the magnetic minerals, others to composition only. Thus it is possible to separate the two effects and to trace the variation of effective magnetic grain size and degree of alteration within a lithologic unit or over a yet larger distance or time interval. The flow in Hole 519A is highly maghemitized at the top, the degree of maghemitization decreasing with depth in the flow. Effective grain size increases with increasing depth. Electron microprobe analysis of the titanomaghemite grains in these samples provides no support for the leaching out of iron during alteration. The pillows and flows in Hole 522B are distributed among a number of cooling units, and no systematic downhole variations are apparent. The inferred magneto-petrology is consistent with the cooling and alteration history that might be expected within the units. The upper and lower sills in Hole 524 are more uniform and have a larger concentration of well-developed magnetic mineral grains than the pillows and flows in Holes 519A and 522B. Maghemitization appears to have developed from the boundaries of the sills that are in contact with the sediments between the sills.

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Petrologic descriptions will follow a division related to both grain size and lithology like that adopted in Cape Roberts investigation (Cape Roberts Science Team 2000, hdl:10013/epic.28287.d001, and references therein). First, we describe the petrology of the sand fraction of the core. Second, we report the distribution of clasts with diameter larger than 2 millimeter (mm) and the petrology of basement clasts. Third, we focus on the occurrence and petrology of McMurdo Volcanic Group sediments, which represent the largest component in the core. The final section continues with the compositional characterization of bulk sediments by continuous X-ray fluorescence measurement (XRF scan) integrated by carbonate geochemistry and concludes with the porewater geochemistry. All of these data will contribute to infer palaeoclimatic information and to understand provenance history of detritus, changes in depositional environment, and spatial and temporal evolution of magmatism in McMurdo Ice Shelf area.

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Results of study of bottom sediments near Iceland and on the Jan Mayen Island are reported. It was found that in recent sediments chemical elements are mainly associated with pyro- and volcanoclastics. In some areas adjusted to deep-seated faults ancient iron-manganese crusts and sediments occur. They are rich in Ni, Co, V, Cu, Mo, Cd and other elements associated with endogenic matter.

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The book is devoted to regularities of spatial distribution, mineralogy and geochemistry of hydrothermal and hydrothermal-sedimentary manifestations of the Mid-Atlantic Ridge rift zone.

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Geomorphology of the Guinea Basin is described along with sediments from cores collected on the abyssal plain, within the abyssal hill zone, and in the eastern part of the Chain Fracture Zone. Stratigraphic differentiation of deep-sea sediments was based on diatom analysis, geochemical and lithological data. Holocene and Pleistocene were identified by these criteria. The lower boundary of Holocene is was found from a marked decrease in CaCO3 concentration and total diatom count. Mineral and chemical compositions are given for coarse silt fraction of various Late Pleistocene sediments. It is shown that this facial complex is determined by tectonic position of the Guinea Basin.

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At Site 582, DSDP Leg 87, turbidites about 560 m thick were recovered from the floor of the Nankai Trough. A turbidite bed is typically composed of three subdivisions: a lower graded sand unit, an upper massive silt unit, and an uppermost Chondrites burrowed silt unit. The turbidites intercalate with bluish gray hemipelagic mud which apparently accumulated below the calcite compensation depth. In order to investigate the nature and provenance of the turbidites, we studied the grain orientation, based on magnetic fabric measurements and thin-section grain counting, and grain size, using a photo-extinction settling tube and detrital modal analysis. The following results were obtained: (1) grain orientation analysis indicates that the turbidity current transport parallels the trench axis, predominantly from the northeast; (2) Nankai Trough turbidites generally decrease in grain size to the southwest; (3) turbidite sands include skeletal remains indicative of fresh-water and shallow-marine environments; and (4) turbidites contain abundant volcanic components, and their composition is analogous to the sediments of the Fuji River-Suruga Bay area. Considering other evidence, such as physiography and geometry of trench fill, we conclude that the turbidites of Site 582 as well as Site 583 were derived predominantly from the mouth of Fuji River and were transported through the Suruga Trough to the Nankai Trough, a distance of some 700 km. This turbidite transport system has tectonic implications: (1) the filling of the Nankai Trough is the direct consequence of the Izu collision in Pliocene- Pleistocene times; (2) the accretion of trench fill at the trench inner slope observed in the Nankai Trough is controlled by collision tectonics; and (3) each event of turbidite deposition may be related to a Tokai mega-earthquake.

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Early Pliocene to middle late Miocene hemipelagic and distal turbidite sediments from Hole 1095B, near the Antarctic Peninsula, yield moderately abundant, moderately well preserved radiolarian faunas and other biosiliceous material (diatoms, silicoflagellates, and sponge spicules). Preservation characteristics, however, vary strongly even between closely related samples, and there are many intervals of poor preservation. In the 140- to 460-meters below seafloor interval studied, it was possible to identify the following standard Southern Ocean radiolarian zones: Upsilon, Tau, Amphymenium challengerae, Acrosphaera? labrata, Siphonosphaera vesuvius, and upper Acrosphaera australis (total age range ~4-10 Ma). Some normally common radiolarian groups, such as actinommids, are unusually rare in the studied material, and the relative ranges of several individual species, such as Acrosphaera labrata vs. A. australis, appear to be somewhat anomalous. These observations imply that the ranges of taxa in this section may be somewhat diachronous, due to either local ecologic factors and/or the highly variable preservation of the faunas. Thus, the ages of events reported are probably only approximate, although they are still useful for constraining the age of sediments in this section.

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During Leg 188 of the Ocean Drilling Program (ODP), employing JOIDES Resolution, we drilled holes at three sites in the southern Indian Ocean in and near Prydz Bay, East Antarctica, between 28 January and 29 February 2000. The objectives of the voyage were to: - Core through sediments deposited when Antarctica underwent the transition from "greenhouse" to the modern "icehouse" state late in the Eocene or early in the Oligocene, at sites obtaining their sediment from the currently subglacial Gamburtsev Mountains that probably were the site of nucleation of the ice sheet (principally Site 1166); - Obtain a sediment record from times at which major changes in the ice sheet volume and characteristics took place as judged from oxygen isotope records, especially at ~23.7 Ma (Oligocene/Miocene boundary), 12-16 Ma (middle Miocene), and 2.7 Ma (late Pliocene) (mainly Site 1165); and - Sample through the upper Pliocene and Quaternary in an attempt to document fluctuations in the extent of the ice sheet over the continental shelf during the Quaternary (especially Site 1167). Paleogene foraminifer-bearing marine sections were not intersected, and thus discussion of marine sections is restricted to the Neogene. Foraminifers are not major contributors to Leg 188 chronostratigraphy but contribute to paleoenvironmental interpretation, to issues such as carbonate compensation depth (CCD) effects and source and history of sediment, and provide a basis for Sr and d18O studies. Chronostratigraphy for the various sections was compiled from diatoms, radiolarians, and paleomagnetism (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.101.2001). Foraminifers were sporadic rather than continuous except in short intervals; however, the Neogene foraminifers from the region are very poorly known and the new records proved to be of significant value in paleoenvironmental interpretation. Only at Site 1167 did drilling intersect a section that yielded foraminifers virtually throughout. Other than for the very young section at each site, there is virtually no continuity of assemblages between sites and thus each section is treated here as separate and unrelated.

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A core of foraminiferal-coccolithic oozes filling a valley of the transform fault located at 29°40'S on the South Atlantic Ridge contains layers composed of angular fragments of igneous and metamorphic rocks. They include many serpentinites deriving from serpentinized ultrabasic rocks, probably exposed on the lower section of the southern slope of the fault valley. A mineral and chemical description of these serpentinites is given.

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The long-term record of glacial/interglacial cycles indicates three major paleoceanographic regimes in the Norwegian Sea. The period since the first major glaciation over Scandinavia at 2.56 Ma is characterized by high-frequency, low-amplitude oscillations of ice-rafted debris inputs, a lowered salinity, and decreased carbonate shell production in surface waters as well as overall strong carbonate dissolution at the sea floor. These conditions indicate a more zonal circulation pattern in the Northern Hemisphere and a relative isolation of surface and bottom waters in the Norwegian Sea. The generally temperate glacial climate was only interrupted by episodic weak intrusions of warm Atlantic waters. These intrusions have been detected in considerable magnitude only at Site 644, and thus are restricted to areas much closer to the Norwegian shelf than during earlier periods. The interval from 1.2 to 0.6 Ma is characterized by an increase in carbonate shell production and a better preservation, as well as a change in frequency patterns of ice-rafted debris inputs. This pattern reflects increasing meridionality in circulation-strengthening contrasts in the Norwegian Sea between strong glaciations and warm interglacials. The past 0.6 Ma reveal high-amplitude oscillations in carbonate records that are dominated by the 100-k.y. frequency pattern. Glacial/interglacial sedimentary cycles in the ODP Leg 104 drill sites reveal a variety of specific dark lithofacies. These dark diamictons reflect intense iceberg rafting in surface waters fed by surges along the front of marine-based parts of the continental ice sheets in the southeastern sector of the Norwegian Sea and are associated with resuspension of reworked fossil organic carbon and strong dissolution at the sea floor. Piling up of huge iceberg barriers along the Iceland-Faeroe-Scotland Ridge might have partially blocked off surface water connections with the North Atlantic during these periods

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New data on bottom sediments and igneous rocks of the Philippine Trench are under consideration. They show differences in geological structures of the island slope and the ocean slope of the trench. The island slope is comparable to the accretionary prism formations on the Philippines; there processes of gravitational re-deposition of sediments occur. The ocean slope is an edge of the Philippine Plate sinking into the trough, where basalts of the oceanic crust are exposed.

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Geomorphology, geology, stratigraphy, lithology and geochemistry of bottom sediments in the South Ocean are under consideration. Regularities of distribution of iron-manganese nodules, features of occurrence of ore components in the nodules, nodule abundance in bottom sediments have been studied.