高分辨10Be记录与黄土地层定年

利用Ｂ／Ｍ地磁极性倒转层位０．７３ＭａＢ．Ｐ．时间参考点，通过黄土剖面_１０Ｂｅ变化曲线与深海ＳＰＥＣＭＡＰδ_１８Ｏ变化曲线匹配拟合，求得洛川黄土剖面０．７５Ｍａ以来的相对连续时标。时标精度约５０００—１００００ａ。由于０．１５Ｍａ以来洛川剖面加密取样以及_１０Ｂｅ、δ_１８Ｏ两曲线之间更好的相关性，_１０Ｂｅ相对时标精度与ＳＰＥＣＭＡＰδ_１０Ｏ时标精度大体相同，即３０００—５０００ａ。各时期_１０Ｂｅ通量与黄土堆积速率之间存在十分好的线性相关关系，展示了一条崭新的地层定年途径。黄土_１０Ｂｅ记录３５０００ａＢ．Ｐ．处的Ｒａｉｓｂｅｃｋ峰可被用作地层时间参考点。黄土_１０Ｂｅ／(２６)Ａｌ值、黄土石英中“就地”成因_１０Ｂｅ／(２６)Ａｌ值以及“就地”成因_１０Ｂｅ浓度等均可望应用于黄土地层精确定年。

10Be RECORDS IN LOESS WITH HIGH RESOLUTION AND THE DATING OF LOESS STRATA

Abstract:There are several interesting relations between （10）Be records in loess with highresolution and the dating of loess strata.The agreement relationship between （10）Be curve of loess and δ（18）O curve of deepsea sediments is astonishingly good for both Luochuan profile and Xifen profile.The position of Brunhes-Matuyama boundary has been determined in Luochuan profile at a depth of 50.5m. Keeping this boundry fixed and assuming that the age ofthe boundary is 0.73 Ma B.P., the main fluctuations of the （10）Be records in loess arematched with the variations of the δ（18）O in deep-sea records by stretching and compressing the loess depth scale. In this way, a （10）Be continuous time scale of Luochuan profile for last 0.75 Ma has been established with precision of 5 000-10 000 a. Afterthe sampling with 5 cm interval, a （10）Be time scale of Luochuan profile for last 0.15Ma has also been established with precision of 3 000-5 000 a, which is about thesame as SPECMAP δ（18）O time scale precision. The agreement among the differenttime scales for the dating of loess strata is very good.The linearity of relationship between accumulation rate of loess and （10）Be fluxis also astonishingly good. It reveals a new way for the dating of loess strata.where Tm,To represent ages of m layer and modern layer respectively, △Xi=Xi-X（i-1）, sampling intervals; Ai, average accumulation rate of i-th layer. As the relation between （10）Be concentration and accumulation rate has been determined quantitatively, the ages of corresponding loess strata can be calculated according to themeasured （10）Be concentration directly. And then the continuous time scale for wholeloess section would be established. Obviously, this new time scale is independent ofthe uniformity of accumulation rate.The event of Raisbeck peak happened at 35 000 a B. P. is a global event. So, theRaisbeck peak can be used as a precision time mark for the dating of loess strataif this peak can be recognized in （10）Be records of loess.Since both （10）Be and 26Al are cosmogenic nuclides and have about the same geochemistry behaviour, （10）Be/（26）Al ratio dating has a important advantage, that is theeffects of intensity variations of both cosmic ray and geomagnetic field on production rates of cosmogenic nuclides can offset each other. The measurement of （26）Al/（27）Alin samples of earth surface by AMS is very difficult since （27）Al concentration inearth's crust is rather high （⁷% ）. But （27）Al in loess exist mainly as aluminium silicate and cosmogenic （26）Al is absorbed on grain surface. A special chemical procedurecan make （26）Al measurement successfully, in which crily （26）Al can be dissociatedbut aluminium silicate remain stationary.The determination of“in situ cosmogenic （10）Be, （26）Al” in quartz from loess canbe also used for dating of loess strata. Since （27）Al concentration in quartz is generally M 300 ppm and （26）Al/（27）Al ratio in quartz is much higher than that in loess, themeasurement of “in situ cosmogenic （26）Al” in quartz is as easy as in extraterrestrialsamples. On the other hand, “atmospheric （10）Be, （26）Al” absorbed on the surface of quartz can been completely eliminated through the cleaning by HCl and weak HFrepeatedly, so that “in situ cosmogenic （10）Be, （26）Al” in quartz can be measuredaccurately. The “in situ cosmogenic （10）Be” in loess quartz is built up of only twoparts: preexisting （10）Be before rock weathering and （10）Be produced during accumulationand burial. And during the process of accumulation and butal, the former contributes decreasingly to the “in situ cosmogenic （10）Be” in loess quartz and the latter tendsto increase with increasing burial depth and age. Therefore, the “in situ cosmlogenic（10）Be” concentrations in quartz are expected to be applied to the dating of loess strata.