摘要
宇宙成因核素10Be(半衰期1.5M a)不但是古环境变化的示踪剂,而且具有确定地质年龄的能力。80个全岩样品的10Be和化学分析数据表明,黄土高原约6M a以来的红粘土和黄土序列中年龄校正的10Be浓度与沉积物粒度和风化程度的化学指标具有显著的关系。根据这种关系建立的10Be浓度与化学指标的经验回归线性模型,可以估计红粘土和黄土形成时的10Be浓度,确定红粘土和黄土形成的10Be放射性年龄。
As we reported early, there are very close relationships between ^10Be concentration, chemical composition, and grain size of the wind blown dust sediments of Chinese loess, paleosol, and red clay, implying that these relationships could be employed to estimate the ^10Be concentration at dust deposition or soil formation. In this paper, using the chemical data we attempt to construct a linear model to estimate the initial ^10Be concentrations at burial of the dust sediments for ^10Be decay dating.
^10Be concentrations and major element oxides have been measured on total 80 samples from the well studied the sequences of loess-paleosol and red clay at Weinan, Baoji, and Leijiahe in the Loess Plateau. The results show that the measured ^10Be concentrations in the samples generally decrease with decay, but much scatter in about five folds. However, variations in the ^10Be concentrations corrected for decay are very much correlated with grain size and weathering intensity of the dust sediments. For example, the ^10Be/Al ratio increases respectively with Ti/Si ratio as an index of grain size, and with decreases of ratios of Na/Al, Mg/Al, K/Al etc. as indices of weathering intensity. These relations are reasonable since ^10Be is rich in fine particles of the dust sediments, and for abundance of the ^10Be carried by rain to a soil increase with exposure time of the soil, which is one of major factors as dominant control on the soil maturation.
Using multiple regression techniques, we have constructed a linear model to link the decay corrected （^10Be/Al） dc ratios to the major elemental ratios：
（^10Be/Al）est = 127.4 （Ti/Si） ＋ 0. 560 （Na/Al） - 0. 237 （Na/K） - 0. 802 （1）
where the （^10Be/Al）e.t is an estimate of （^10Be/Al）dc in unit of 10-12 atom/atom; and （Ti/Si） , （Na/Al） , and （Na/K） are in molecular ratio unit. Coefficient of determination of this model is as high as 0.910 （r^2, n = 80） ,indicating that this model could be used to date on the red clay and loess-paleosol sequences in the Loess Plateau.
T=-1/λ In（^10Be/Al）m/（^10Be/Al）est （2）
where T is age （million years）, λ ^10Be decay constant （0. 4621Ma^-1）, and （^10Be/Al）m the measured ^10Be/Al ratio. Although this dating model （equation 2） remains an uncertainty of 11% （1 σ, equatively to ca. 0.24Ma）, it is much better than the simple dating model that assumed a constant ^10Be concentration.
出处
《第四纪研究》
CAS
CSCD
北大核心
2006年第2期244-249,共6页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40073031和40373046)资助
