土壤中次生与碎屑组分的差异性剥蚀
收稿日期: 2020-05-28
修回日期: 2020-07-10
网络出版日期: 2020-09-15
基金资助
国家自然科学基金国际合作与交流项目“基于中国和南非玄武岩流域的风化比较学研究”(41761144058);国家自然科学基金重点项目“颗粒破碎铀同位素年代技术的发展及其在风尘系统中的应用”(41730101)
Decoupled Erosion of Authigenic and Detrital Components in Soil
Received date: 2020-05-28
Revised date: 2020-07-10
Online published: 2020-09-15
Supported by
the National Natural Science Foundation of China "A comparative study of weathering processes based on the basaltic fields in China and South Africa"(41761144058);"Developing the uranium comminution age in the eolian system and its associated applications"(41730101)
在地表过程研究中，一般以表层土壤可溶元素的亏损程度代表风化强度，以表层土壤粗颗粒矿物中宇生核素的积累计算土壤存留时间，进而推算土壤剥蚀速率。这两种方法都基于一个重要的假设，即土壤中不同组分的一致剥蚀。相对于新鲜基岩，土壤中可溶元素的亏损主要体现在细颗粒次生黏土组分，而碎屑矿物的亏损程度较弱。细颗粒次生黏土的优先剥蚀会导致风化强度的低估，还会导致粗颗粒矿物存留时间大于土壤的总存留时间，从而低估土壤剥蚀速率。最近的研究表明，土壤不同组分可能存在差异性剥蚀。通过莱索托高地玄武岩风化土壤和流域地表水的铀同位素组成验证土壤不同组分是否存在差异性剥蚀。结果表明，碎屑组分在土壤中的存留时间为(543 ± 32) ka，而次生黏土组分的存留时间为（22±11） ka，二者相差一个数量级。次生组分的存留时间与由区域风化通量计算得到的剥蚀速率24~33 t/(km2?a)相匹配，表明次生黏土是土壤剥蚀的主要组成部分。该发现表明土壤中不同组分的差异性剥蚀可能是普遍存在的。次生黏土的优先剥蚀会导致风化强度和剥蚀速率被低估，引起风化通量的估算偏低，进而低估化学风化在全球碳循环中的作用。
李旭明 , 李来峰 , 王浩贤 , 王野 , 陈旸 . 土壤中次生与碎屑组分的差异性剥蚀[J]. 地球科学进展, 2020 , 35(8) : 826 -838 . DOI: 10.11867/j.issn.1001-8166.2020.065
In the study of surface processes, it is generally assumed that erosion occurs equally throughout the soil profile so that chemical depletion of the topsoil can represent the intensity of chemical weathering and the duration of surface exposure to cosmogenic radiation can reflects the soil residence time, and then the rate of erosion can be calculated. In comparison with fresh bedrock, the depletion of soluble elements in soil mainly comes from fine-grained secondary clay components, while the depletion degree of detrital minerals is weak. The preferential erosion of fine-grained secondary clay will lead to the underestimation of weathering intensity, and the retention time of detrital mineral will be longer than the total retention time of soil， and thus the soil erosion rate will be underestimated. Based on the uranium isotope comminution ages of soil in the Lesotho Highlands, we found that erosion operates differentially between the detrital and authigenic components of the soil. Uranium isotope comminution ages show a soil residence time of (543±32) ka for the detrital particles. In contrast, soil residence time of the authigenic phases is constrained to be (22±11) ka according to the accumulation of recoiled 234U from the absorbed 238U to river water. The residence time of secondary clay matches with the regional erosion rate 24-33 t/(km2·a) calculated from weathering flux, indicating that secondary clay is the main component of soil erosion. The results indicate that the decoupled erosion of different components in soil may be common. This finding implies that the intensity of weathering based on bulk soil erosion and the rate of soil erosion determined by exposure dating of coarse soil grains may be invalidated due to the preferential erosion of authigenic particles. As a result, a lower estimate of weathering flux may be made, and therefore the role of chemical weathering in the global carbon cycle could be underestimated.
Key words： Regolith; Erosion; Carbon cycle; Landscape evolution; Uranium isotope
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