977 resultados para Atomic bomb.


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CaCO3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial-interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic delta13C values of ?23? indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0-140 ka) and 17 through 19 (660-790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic delta13C values to ca. ?29? in portions of isotope stage 2 suggest that the local concentration of dissolved CO2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments.

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Carbon isotopic composition of predominantly marine kerogen in latest Oligocene mudstones of the Peru Margin ODP 682A Hole shows an about 3.5? increase with decreasing age. Py-GC and elemental (C=N ratio) analysis of the kerogen plus sulphur isotopic study together with earlier knowledge on geological setting and organic geochemistry results in a better understanding of depositionary environment and allows to separation of the influence of concentration of water dissolved carbon dioxide (ce) on kerogen delta13C from that of other factors (bacterial degradation, sea surface temperature, DIC delta13C, productivity, and admixture of land plant OM). Based on this analysis, the major part of the kerogen shift is considered as a result of the latest Oligocene decrease of marine photosynthetic carbon isotopic fractionation in the Peru Margin photic zone, which in turn possibly reflects a simultaneous drop in atmospheric CO2 level. Uncertainties in the evaluation of the factors affecting the marine photosynthetic carbon isotopic fractionation and the extent of ocean-atmosphere disequilibrium do not permit calculation of the decrease of the atmospheric CO2.

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CHN analyses of sediments and rocks sampled during DSDP Leg 75 in the South Atlantic have provided concentrations of organic carbon and atomic C/N ratios of organic matter from two sites. High values of organic carbon were measured in sediments deposited during Neogene and Cretaceous times at Site 530 in the Angola Basin; sediments deposited at other times contain less than 0.5% organic carbon. Development of the Benguela Current and associated upwelling-supported biological productivity is recorded in late Miocene to Holocene sediments which contain 1 to 7% organic carbon. These sediments include debris flows and turbidites composed of predominantly biogenic materials originally deposited on the Walvis Ridge and on the African continental margin. Organic-carbon-rich black shales containing up to 17% organic carbon occur in late Albian to Coniacian turbidite sequences. These Cretaceous black shale layers are commonly several centimeters thick and are separated by bioturbated fine-grained organic-carbon-poor turbidites which are usually much thicker. At Site 532 on the Walvis Ridge, biogenic sediments deposited between late Miocene and Holocene times contain 1 to 9% organic carbon. Fluctuations in the intensity of high biological productivity associated with the Benguela Current are preserved in alternating light and dark layers of sediments. C/N ratios of organic matter in sediments from both sites are typical of marine sources

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The organic matter contents of sediments and rocks sampled during DSDP Leg 93 have been characterized by CHN and Rock-Eval analyses. Most samples from Sites 604 and 605 on the New Jersey continental slope and from Site 603 on the Hatteras outer continental rise contained less than 0.5% organic carbon. Some Neogene samples from the slope contained 1 to 2% organic carbon, and Cretaceous samples from the outer rise were as rich as 13.6% organic carbon by weight. Thin layers of black claystones of Santonian, Cenomanian, and Albian age were found interbedded in organiccarbon- lean, bioturbated, turbiditic claystones. Similar layers of turbiditic black marlstones were interspersed among Neocomian limestones and sandstones. Although the organic matter in many of the samples appeared to be detrital continental material, according to Rock-Eval and C/N values, Cenomanian black shales, in particular, contained substantial proportions of marine-derived organic matter.

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The four sites drilled on the Irish continental margin (Goban Spur) yielded sediments ranging in age from Holocene to Barremian. Most of the sediments were deposited in well oxygenated waters, and the small amounts of organic matter they contain are highly oxidized. During a few time intervals from the Cenomanian to earliest Turonian, however, the oxygen content of the bottom waters reached very low levels, resulting in the deposition of homogeneous or laminated black sediments containing from 0.5 to 11% total organic carbon (TOC). The original organic matter was of mixed marine and terrestrial origin. The oxidizing-reducing cycles represented by interbedded black and light sediments are probably a result of changes in both circulation and productivity. The black sediments at Sites 550, 551, and 549 were probably deposited near the lower end, middle, and upper end, respectively, of an expanded oxygen-minimum layer. The oil and gas source potential of the laminated black sediments is very good to excellent. The organic-carbon-lean sediments deposited under oxidizing conditions have no oil or gas source potential. The thermal maturity of all sediments is low.

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Mode of access: Internet.

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"TID-3507 Chemistry."