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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Δ18O ： ウィキペディア英語版 Δ18O In geochemistry, paleoclimatology and paleoceanography ''δ''18O or delta-O-18 is a measure of the ratio of stable isotopes 18O:16O (oxygen-18:oxygen-16). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature. The definition is, in "per mil" (‰, parts per thousand): :$\backslash delta\; ^O\; =\; \backslash Biggl(\; \backslash frac\; \backslash bigr)\_\}\; \backslash bigr)\_\}\; -1\; \backslash Biggr)$ * 1000\ ^\!/\!_ where the standard has a known isotopic composition, such as Vienna Standard Mean Ocean Water (VSMOW).〔 (【引用サイトリンク】title= USGS -- Isotope Tracers -- Resources -- Isotope Geochemistry )〕 The fractionation can arise from kinetic, equilibrium, or mass-independent fractionation. == Mechanism == Foraminifera shells are composed of calcium carbonate (CaCO3) and are found in many common geological environments. The ratio of 18O to 16O in the shell is used to indirectly determine the temperature of the surrounding water at the time the shell was formed. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets. also reflects local evaporation and freshwater input, as rainwater is 16O enriched—a result of the preferential evaporation of the lighter 16O from seawater. Consequently, the surface ocean contains greater amounts of 18O around the subtropics and tropics where there is more evaporation, and lesser amounts of 18O in the mid-latitudes where it rains more. Similarly, when water vapor condenses, heavier water molecules holding 18O atoms tend to condense and precipitate first. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of 18O. Snow falling in Canada has much less H218O than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter signature than that at its margins, since heavier 18O precipitates first. Changes in climate that alter global patterns of evaporation and precipitation therefore change the background δ18O ratio. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Δ18O」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.