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Before Present (BP) years is a time scale used mainly in geology and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1 January 1950 as commencement date of the age scale, reflecting the fact that radiocarbon dating became practicable in the 1950s. The abbreviation "BP", with the same meaning, has also been interpreted as "Before Physics"; that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable. ==Usage of "BP"== The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy.〔. 〕〔 〕 This usage differs with the recommendation by van der Plicht & Hogg,〔 〕 followed by the ''Quaternary Science Reviews'',〔 〕 both of which requested that publications should use the unit "a" (year ) and reserve the term "BP" for radiocarbon estimations. Some archaeologists use the lowercase letters ''bp'', ''bc'' and ''ad'' as terminology for uncalibrated dates for these eras. 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「Before Present」の詳細全文を読む スポンサード リンク
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