A Test of Secular Variations in Cosmic Radiation Using 36C1 in Pack Rat Middens

Bard等人最近的U/TH和14C测量。(1990)对巴巴多斯珊瑚的研究表明，20,000年前，大气中14C的活度大约比现代活度高50%。他们将这种活跃性的增加归因于14C产生的增加，这是由于较弱的地球偶极磁场导致更强烈的宇宙辐射。我们建议通过重建另一种宇宙源放射性核素36C1的沉积来测试14C增强的这一来源。对36C1的类似改进将表明这两种核素都对生产波动做出了反应。我们建议对内华达州西部一个干旱地区的化石袋鼠(Neotoma)尿液中保存的大气36C1进行采样。该遗址包含了一只14C年代的侏儒，其年代涵盖了距今29,000到3,000年的时间。如果发现36c1和巴巴多斯14C记录之间有很强的相关性，则36c1数据可能有助于重建30,000至20,000年B.P.期间的大气14C活动，而其他数据很少。我们还将比较来自内华达州西部和其他广泛分布地区的中期样本中的36c1/c1比值与相同地区的陨石36c1比值，以建立鼠尿与土壤-水36c1之间的关系。除了解决宇宙成因核素产生的长期变化问题外，陨石36c1的变化记录可能为地下水和土壤水提供一个有价值的测年试剂。

Recent U/TH and 14C measurements by Bard et al. (1990) on corals from Barbados have indicated that 20,000 years ago the activity of 14C in the atmosphere was approximately 50% higher than the modern activity. They ascribed this increase in activity to enhanced 14C productions due to more intense cosmic radiation as a result of a weaker terrestrial dipole magnetic field. We propose to test this origin for the 14C enhancement by reconstructing the deposition of another cosmogenic radionuclide, 36C1. A similar enhancement of the 36C1 would demonstrate that both nuclides were responding to production fluctuations. We propose to sample atmospheric 36C1 preserved in fossil packrat (Neotoma) urine in an arid site in western Nevada. This site contains a midden that has been 14C dated to cover the time interval 29,000 to 3,000 years before present. If strong correlation of the 36C1 and the Barbados 14C records is found, the 36C1 data might help to reconstruct atmospheric 14C activity during the period 30,000 to 20,000 years B.P., for which other data are scarce. We will also compare 36C1/C1 ratios in midden samples from western Nevada and other widely spread localities with meteoric 36C1 ratios in the same localities in order to establish the relationship between the rat urine and soil-water 36C1. In addition to addressing the problem of secular variations in cosmogenic nuclide production, the record of variation in meteoric 36C1 may provide a valuable dating agent for goundwater and soil water.