送交者: 三谷 于 June 11, 2003 12:47:39:
回答: 我不懂呀 由 CANADAGE 于 June 10, 2003 22:29:37:
I took an isotope geology course some 13+ years ago in my graduate study, and hope I can still remember some thing.
Carbon-14 is made when cosmic rays knock neutrons out of atomic nuclei in the upper atmosphere. These displaced neutrons, now moving fast, hit ordinary nitrogen (14N) at lower altitudes, converting it into 14C. Geological evidences showed that sun's cosmic radiation hasn't changed more than 10% in the past 10,000 years.
10% deviation may mean several hundred years in accuracy. Therefore, carbon dating needs to be calibrated against other geological calendars. The most famous one is the tree-ring count, which is good up to 11,000 years. Beyond that other radioactive calibrations can be done, which I don't remember any more.
Carbon dating does not work well for short ages either because the C14 concentration varied abnormally each year due to nuclear tests and green house effect.