949 resultados para Avicenna, 980-1037.
Resumo:
Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate-to-deep delta13C gradients (commonly reaching >1.2?) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric delta13C gradients covary with benthic foraminiferal delta18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on delta13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.
Resumo:
We have analysed the concentrations of Li, K, Rb, Cs, and B, and the isotopic ratios of Li and B of a suite of pore fluids recovered from ODP Sites 1037 (Leg 169; Escanaba Trough) and 1034 (Leg 169S; Saanich Inlet). In addition, we have analysed dissolved K, Rb, and Cs concentrations for estuarine mixing of the Ganges-Brahmaputra river system. Together, these data sets have been used to assess the role of sediments in the marine geochemical cycles of the alkali elements and boron. Uptake onto clay minerals during estuarine mixing removes 20-30% of the riverine input of dissolved Cs and Rb to the oceans. Prior to this study, the only other recognised sink of Rb and Cs was uptake during low-temperature alteration of the oceanic crust. Even with this additional sink there is an excess of inputs over outputs in their modern oceanic mass balance. Pore fluid data show that Li and Rb are transferred into marine sediments during early diagenesis. However, modeling of the Li isotope systematics of the pore fluids from Site 1037 shows that seawater Li taken up during marine sedimentation can be readily returned to solution in the presence of less hydrated cations, such as NH4+. This process also appears to result in high concentrations of pore fluid Cs (relative to local seawater) due to expulsion of adsorbed Cs from cation exchange sites. Flux calculations based on pore fluid data for a series of ODP sites indicate that early diagenesis of clay sediments removes around 8% of the modern riverine input of dissolved Li. Although NH4+-rich fluids do result in a flux of Cs to the oceans, on the global scale this input only augments the modern riverine Cs flux by ~3%. Nevertheless, this may have implications for the fate of radioactive Cs in the natural environment and waste repositories.
Resumo:
"January 1980."
Resumo:
"September 1968."
Resumo:
"June 1985."
