Geochemical Cycles of 9-Be and 27-Al

点击翻译按钮获取中文摘要

This project began as an attempt to establish the systematics of the stable isotopes of Be and Al in the oceans with a view to using their cosmogenic radio-isotopes as geochronologic tools. There is an explicit analogy here to C-14. Carbon is involved in the biogeochemical cycle; hence the effect of decay on the distribution of the radio-isotope is overprinted by non- radiogenic processes. However the stable isotope C-12 is affected only by the biogenic processes and hence can be used to correct for their effects on the radio-isotope. The success of this approach is greatly facilitated by the fact that the carbon isotopes share a common, well-mixed reservior, the atmosphere. The latter is known not to be the case for Be and Al. The objective of this project is to establish whether the residence time and chemical behaviours of these elements in seawater is such that isotope equilibrium is achieved in the oceanic reserviour. If this were true, then direct radiometric dating of marine sediments would be feasible in the range 0 to -15 million years. Support is requested to complete the first phase of the investigations concerning the geochemical cycle Al with a view to developing Al and Be isotopes as geochronometers. The effort will extend coverage into several new basins in the South Atlantic and examine the evolution of the isotope distribution as the Atlantic water masses propagate from their northern source 8915831 Measures This project began as an attempt to establish the systematics of the stable isotopes of Be and Al in the oceans with a view to using their cosmogenic radio-isotopes as geochronologic tools. There is an explicit analogy here to C-14. Carbon is involved in the biogeochemical cycle; hence the effect of decay on the distribution of the radio-isotope is overprinted by non- radiogenic processes. However the stable isotope C-12 is affected only by the biogenic processes and hence can be used to correct for their effects on the radio-isotope. The success of this approach is greatly facilitated by the fact that the carbon isotopes share a common, well-mixed reservior, the atmosphere. The latter is known not to be the case for Be and Al. The objective of this project is to establish whether the residence time and chemical behaviours of these elements in seawater is such that isotope equilibrium is achieved in the oceanic reserviour. If this were true, then direct radiometric dating of marine sediments would be feasible in the range 0 to -15 million years. Support is requested to complete the first phase of the investigations concerning the geochemical cycle Al with a view to developing Al and Be isotopes as geochronometers. The effort will extend coverage into several new basins in the South Atlantic and examine the evolution of the isotope distribution as the Atlantic water masses propagate from their northern source areas.