Geochemical and geophysical implications of the radiocarbon calibration
Geochemical and geophysical implications of the radiocarbon calibration
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DOI：
10.1016/s0016-7037(98)00130-6
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发表时间：
1998-06-01
影响因子：
5
通讯作者：
Bard, E
中科院分区：
文献类型：
--
作者：
Bard, E
一个精确的时间框架对于研究过去45，000年中发生的各种现象的动态至关重要。虽然C-14测年法自20世纪50年代以来已被广泛应用，但人们认识到，大气中的C-14/C-12比率在过去并不稳定。为了计算出准确的年龄，这些波动必须通过校准曲线进行校正，校准曲线是通过将原始C-14测量值与独立测年方法提供的真实日历年龄进行比较而获得的。迄今为止获得的校准曲线的特点是，在过去45，000年的大部分时间里，原始C-14年龄明显小于日历年龄。几个世纪到几千年来发生的C-14突变叠加在大气中C-14/C-12比率下降的长期趋势上。在一定程度上，通过考虑从不同纬度研究的其他宇宙成因核素获得的补充信息，有可能概述大气C-14变化的不同原因：（1）大气C-14/C-12比率的大多数高频变化与太阳起源的磁波动有关，这是通过研究最近三个世纪对太阳的直接观测所揭示的。通过将C-14/C-12事件与极地冰芯中Be-10和Cl-36浓度最大值进行比较，得出了类似的结论。(2)C-14年龄的长期趋势变化是由于地磁偶极场的屏蔽效应在10，000年至40，000年BP之间发生了长时间的减弱。这一解释得到了火山岩和沉积岩古地磁测量以及低纬度和高纬度记录中Be-10和Cl-36分析的支持。(3)在13，000和11，500 cal-yr-BP之间发生了一次显著和快速的大气C-14/C-12漂移，因此对应于新仙女木冷期。与上述与碳14产量变化有关的碳14变异性相比，这种所谓的碳14年龄高原可能是由于全球碳循环中交换率的突然变化造成的。这种解释得到了独立的地球化学代理和碳循环数值模拟的支持。版权所有（C）1998 Elsevier Science Ltd.
A precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45,000 years. Although the C-14 dating method has been widely applied since the 1950's, it is recognized that the atmospheric C-14/C-12 ratio has not been stable during the past. In order to calculate accurate ages, these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw C-14 measurements with true calendar ages provided by independent dating methods.The calibration curve obtained so far is characterized by a long-term trend with raw C-14 ages being significantly younger than calendar ages during most of the last 45,000 years. Abrupt C-14 shifts, which occurred over centuries to millennia, are superimposed on this long-term trend of decreasing atmospheric C-14/C-12 ratio. To a certain extent, it is possible to outline the different causes of atmospheric C-14 variations by considering complementary information obtained form other cosmogenic nuclides studied at different latitudes: (1) Most high-frequency changes in the atmospheric C-14/C-12 ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the Sun are available. A similar conclusion is derived by comparing C-14/C-12 events with Be-10 and Cl-36 concentration maxima in polar ice cores. (2) The long-term trend shift of C-14 ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10,000 and 40,000 years BP. This interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by Be-10 and Cl-36 analysed in low and high latitude records. (3) a prominent and rapid atmospheric C-14/C-12 excursion occurred between 13,000 and 11,500 cal-yr-BP, thus corresponding to the Younger Dryas cold period. By contrast with the C-14 variability mentioned above, which is linked to C-14 production changes, this so-called C-14 age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle. This interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle. Copyright (C) 1998 Elsevier Science Ltd.