Surface ages and weathering rates from ¹⁰Be (meteoric) and ¹⁰Be/⁹Be: Insights from differential mass balance and reactive transport modeling

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• 作者：Kate Maher^a ; ^{kmaher@stanford.edu" class="auth_mail" title="E-mail the corresponding author} ; Friedhelm von Blanckenburg^b ; ¹
• 关键词：Cosmogenic isotopes ; Beryllium ; Chemical weathering ; Quaternary geochronology ; Reactive transport modeling
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：23 December 2016
• 年：2016
• 卷：446
• 期：Complete
• 页码：70-86
• 全文大小：1753 K

文摘

To reconstruct the dynamic evolution of weathered material at the Earth's surface requires techniques that record the fluxes of material into and out of the weathering zone. Cosmogenic isotopes, formed in situ within the Earth's surface or in the atmosphere (“meteoric”) are important chronometers for both time and fluxes. Meteoric ¹⁰Be has been used to evaluate surface ages, soil residence times and rates of sediment transport; however, the meteoric ¹⁰Be dating tool relies on complete retention of ¹⁰Be on mineral surfaces within the weathered regolith, an assumption that will not be valid where soil pH is low or mineral surface area available for sorption is lacking. Here we evaluate whether the stable isotope of ⁹Be that is released upon weathering (“reactive” Be) can be used to correct for ¹⁰Be mobility. This approach relies on the inventory of reactive ⁹Be in the weathering zone as a measure of the mass depletion, whereas the ¹⁰Be inventory is a function of age. To explore the spatial and temporal relationship between the Be isotopes, we use a multi-component reactive transport model framework to evaluate a set of differential mass balance equations for the ¹⁰Be/⁹Be composition of the bulk solid and sorbed reservoirs on non-eroding surfaces. We first find that the (¹⁰Be/⁹Be)_reac is inversely proportional to the long-term chemical weathering rate, even in cases of incomplete Be retention. Second, we find that coupling ¹⁰Be and ⁹Be to provide a tool for surface age dating (¹⁰Be) is complex because incomplete retention affects each isotope to a different extent. Beryllium-10 is supplied at the surface of soil, where sorption onto secondary minerals is more extensive, while ⁹Be is released from primary minerals at the weathering front where secondary minerals are less abundant. If ⁹Be loss is less than about 15%, we find ⁹Be inventories can be used to reliably correct ¹⁰Be inventories to within 10%. The error associated with the correction also increases with increasing age, and thus the correction is more favorable for younger surfaces. Over longer timescales, the error on the age introduced by the loss correction using ⁹Be is magnified, whereas the error associated with uncorrected ages decreases because the ¹⁰Be is more strongly retained as chemical weathering progresses. We also find that the reactive transport model confirms a set of simple mass balance descriptions, providing guidance for future data collection. Collectively, the modeling results suggest that measurement of Be isotopes in weathered regolith provides a unique approach to determine both surface ages and chemical weathering rates.