ANALYSIS OF EFFECTS OF GEOMAGNETIC INTENSITY VARIATIONS ON EXPOSURE DATING
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摘要: 在103~105a的尺度上,地磁场强度变化是影响陆地宇生核素生成速率的主要因素,其影响程度取决于样品的地理位置和暴露时间。根据已有的磁场古强度数据,模拟200 ka以来海拔2 km、25°N和40°N的地表10Be生成速率的变化,进而分析地表宇生核素生成速率变化对岩石暴露年代测定的影响及其模式年龄的校正。校正磁场强度变化后,海拔2 km、25°N上,50~200 ka的模式年龄可被压缩14%~19%,大于1σ的误差,相同海拔40°N上的模式年龄可减小约8%。对中低纬两组模式年龄的校正充分证明,磁场强度引起的陆地宇生核素生成速率变化是暴露年代测定中主要误差源之一,尤其在低纬高海拔地区这一影响更不容忽视。Abstract: The production rates of in-situ terrestrial cosmogenic nuclides are functions of the cosmic-ray flux that is largely modulated by geomagnetic intensity on time scales of 103 to 105 year range. However, the effects of temporal variations in the geomagnetic field on production rates of cosmogenic nuclides are often neglected in exposure dating researches. According to the available paleogeomagnetic field intensity data, this study simulated instantaneous and time-integrated 10Be production rates over the last 200 ka, and calibrated the assumed model ages with an iterative method. The quantitative analysis suggests that changing production rates due to geomagnetic field intensity variations is one of the major uncertainty sources involved in exposure dating, especially at low latitudes and high altitudes.For example, the exposure ages between 50~200 ka, at 25 N and 2 km altitude, are reduced by 14%~19%, and at 40 N with the same altitude, they are reduced by 8% roughly. So, we should take account of the uncertainty of production rates of in-situ cosmogenic nuclides, and use time-integrated rates in exposure dating research.
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