AN ISOCHRON METHOD FOR COSMOGENIC-NUCLIDE DATING OF BURIED SOILS AND SEDIMENTS
AN ISOCHRON METHOD FOR COSMOGENIC-NUCLIDE DATING OF BURIED SOILS AND SEDIMENTS
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DOI：
10.2475/10.2008.02
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发表时间：
2008-12-01
影响因子：
2.9
通讯作者：
Rovey, Charles W., II
中科院分区：
文献类型：
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作者：
Balco, Greg;Rovey, Charles W., II
我们描述了一种改进的方法，通过测量石英颗粒中宇宙射线产生的放射性核素 (10)Be 和 (26)Al 来测定埋藏古土壤的年代，并将其应用于美国密苏里州中部的一系列夹层土和古土壤，记录了劳伦泰德冰盖的上里奥-更新世进展。埋藏的古土壤意味着一段时期的地表暴露和核素积累，随后是埋藏和核素生产的停止。如果古土壤是在沉积单元（例如耕地）中形成的，则该单元也可能含有从过去的表面暴露继承的未知的 (26)Al 和 (10)Be 浓度。如果继承的核素浓度在土壤中的所有深度都相同（对于充分混合的沉积物如直到），那么古土壤中不同深度的 (26)Al 和 (10)Be 浓度将呈现线性关系。这条线的斜率取决于古土壤埋藏的持续时间，而不取决于继承的核素浓度或样本深度。因此，通过测量古土壤中多个深度的 (26)Al 和 (10)Be 并根据所得等时线计算古土壤的埋藏年龄，可以确定埋藏古土壤上覆地层的年代。我们专注于将这种方法应用于直到古土壤序列，但是 2 使用具有相同埋藏年龄但在暴露历史的其他方面不同的一组样本形成 (26)Al-(10)Be 埋藏等时线的基本思想也适用于其他地层设置。该方法得出了密苏里州四个冰层的年龄，这些年龄在地层学上是一致的，符合古地磁年龄限制，并表明冰在接近 1.25 Ma、接近 0.8 Ma 和大约 1.25 Ma 之间推进到密苏里州。 0.4 和 0.2 Ma。
We describe an improved method for dating buried paleosols using measurements of the cosmic-ray-produced radionuclides (10)Be and (26)Al in quartz grains, and apply it to a sequence of intercalated tills and paleosols in central Missouri, USA, that record Plio-Pleistocene advances of the Laurentide Ice Sheet. A buried paleosol implies a period of surface exposure and nuclide accumulation, followed by burial and a halt to nuclide production. If the paleosol is formed in a sedimentary unit such as till, this unit may also have been emplaced with unknown (26)Al and (10)Be concentrations inherited from past surface exposure. If the inherited nuclide concentrations are the same at all depths in the soil-as is true for well-mixed sediments such as till-then the (26)Al and (10)Be concentrations at different depths in the paleosol will show a linear relationship. The slope of this line depends on the duration of burial of the paleosol, but not on the inherited nuclide concentrations or on the sample depths. Thus, one can date strata overlying buried paleosols by measuring (26)Al and (10)Be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron. We focus on applying this approach to till-paleosol sequences, but 2 the basic idea of forming an (26)Al-(10)Be burial isochron with a set of samples that share the same burial age, but differ in other aspects of their exposure history, applies to other stratigraphic settings as well. The method yields ages for four tills in Missouri that are stratigraphically consistent, agree with paleomagnetic age constraints, and show that ice advanced into Missouri near 1.25 Ma, near 0.8 Ma, and twice between ca. 0.4 and 0.2 Ma.