黄土和红粘土中宇宙成因核素定年的潜力: 10 Be浓度与化学成分的关系*

宇宙成因核素¹⁰ Be(半衰期1.5Ma)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¹⁰ Be和化学分析数据表明,黄土高原约6Ma以来的红粘土和黄土序列中年龄校正的¹⁰ Be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¹⁰ Be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¹⁰ Be浓度,确定红粘土和黄土形成的¹⁰ Be放射性年龄。

10 Be CONCENTRATION RELATION TO CHEMICAL COMPOSITIONS OF CHINESE LOESS AND RED CLAY AS A POTENTIAL DATING METHOD

As we reported early , there are very close relationships between ¹⁰ Be 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 ¹⁰ Be 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 ¹⁰ Be concentrations at burial of the dust sediments for ¹⁰ Be decay dating.¹⁰ Be 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 ¹⁰ Be concentrations in the samples generally decrease with decay, but much scatter in about five folds. However, variations in the ¹⁰ Be concentrations corrected for decay are very much correlated with grain size and weathering intensity of the dust sediments. For example, the ¹⁰ Be/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 ¹⁰ Be is rich in fine particles of the dust sediments, and for abundance of the ¹⁰ Be 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 (¹⁰ Be/Al)_dc ratios to the major elemental ratios: (¹⁰ Be/Al)_est=127.4(Ti/Si)+0.560(Na/Al)-0.237(Na/K)-0.802 (1) where the (¹⁰ Be/Al)_est is an estimate of (¹⁰ Be/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 (r², 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 ln (¹⁰ Be／Al)_m (¹⁰ Be／Al)_est (2)where T is age (million years), λ ¹⁰ Be decay constant (0.4621Ma^-1), and (¹⁰ Be/Al)_m the measured ¹⁰ Be/Al ratio. Although this dating model (equation 2) remains an uncertainty of 11 ％ (1σ,equatively to ca.0.24Ma), it is much better than the simple dating model that assumed a constant ¹⁰ Be concentration^7].