991 resultados para 113-690A


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A remarkable oxygen and carbon isotope excursion occurred in Antarctic waters near the end of the Palaeocene (~57.33 Myr ago), indicating rapid global warming and oceanographic changes that caused one of the largest deep-sea benthic extinctions of the past 90 million years. In contrast, the oceanic plankton were largely unaffected, implying a decoupling of the deep and shallow ecosystems. The data suggest that for a few thousand years, ocean circulation underwent fundamental changes producing a transient state that, although brief, had long-term effects on environmental and biotic evolution.

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At Site 697 a 320 m thick Pleistocene and Pliocene section was recovered, consisting of hemipelagic terrigenous mud with varying amounts of diatoms, thin altered ash layers, and ice-rafted debris (IRD). Sedimentation rates range from 41 m/m.y. (upper Pleistocene) to 150 m/m.y. (lower Pliocene). Diatom percentage and sediment grain-size have been measured for the whole section with approximately one sample per 5,000 yr. IRD is most abundant in the lower Pliocene (sediments older than 4.5 Ma) following the first major West Antarctic glaciation. A decrease in IRD to near-zero above 3.2 Ma may record a transition from valley glaciers to a grounded ice-sheet on West Antarctica. Bottom current flow, recorded in sediments as the proportion of silt, was at a maximum around 3.0-3.3 Ma then gradually decreased until 0.5 Ma. In the upper Pleistocene, maxima in diatom percentage are assumed to occur during interglacials, implying reduced sea-ice cover; maxima in silt percentage correspond to diatom maxima, implying stronger bottom water flow during interglacials.

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From late middle Eocene through earliest Oligocene, high-latitude regions cooled, and by the end of the period, continental ice sheets existed in Antarctica. Diversity of planktonic microorganisms declined, and modern groups of terrestrial vertebrates originated. Coeval faunal changes in deep-sea benthic foraminifers have been related to cooling of deep waters and increased oxygenation. Cooling, however, occurred globally, whereas species richness declined at high latitudes and not in the tropics. The late Eocene and younger lower-diversity, high-latitude faunas typically contain common Epistominella exigua and Alabaminella weddellensis, opportunistic phytodetritus-exploiting species that indicate a seasonally fluctuating input of organic matter to the sea floor. We speculate that the species-richness gradient and increase in abundance of phytodetritus-exploiting species resulted largely from the onset of a more unpredictable and seasonally fluctuating food supply, especially at high latitudes.

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Publisher's no.: Bote & Bock Kleine Partitur-Ausgabe No. 4.

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"Physics thesis under the direction of Professor Sherwood K. Haynes."

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Continues the numbering and the pagination of Interstate migration, hearing before the Select Committee to Investigate the Interstate Migraion of Destitute Citizens, which Committee was the predecessor of the Select Committee Investigating National Defense Migration.

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Translation of Le dossier no. 113.

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Translation of Le dossier no. 113.