植被在确定临界区锶稳定同位素比率中的作用

IF 1.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY American Journal of Science Pub Date : 2021-10-01 DOI:10.2475/08.2021.04
J. Bouchez, F. von Blanckenburg
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引用次数: 7

摘要

在地球表面,锶的稳定同位素比率(88Sr/86Sr)主要由锶的生物吸收及其在植物垃圾中的储存决定。这一结论是根据稳定的同位素质量平衡得出的,该平衡通过直接测定瑞士阿尔卑斯山、美国内华达山脉和斯里兰卡热带高地三个现场的临界区隔间(岩石、土壤、植被和溪水)之间的元素通量而得到独立验证。这些地点的侵蚀率呈梯度,与固体在临界区的停留时间呈反比,从而构成“侵蚀序列”。对于侵蚀景观,以前的稳定同位素模型预测,同位素比率是由岩石转化为风化层期间次生固体形成的速率决定的。与这一预期相反,我们发现,在从原生矿物中释放后,Sr被分配为一部分,被植物吸收,其余部分被分解为溶解的Sr通量。次生风化产物(如粘土和氧化物)的形成在决定Sr预算方面起着次要作用。根据散装植物中Sr稳定同位素的平均组成及其在溪流中的溶解同位素,确定了三个生态系统中每一个生态系统的生物吸收Sr同位素分馏因子。这种分馏因子的范围从阿尔卑斯山和内华达山脉的约-0.3‰到热带斯里兰卡的约0‰。这些由生物吸收引起的同位素指纹被保存下来,这意味着植物垃圾中物理去除的Sr比回收的Sr更多。植物垃圾中的这种Sr去除似乎在缓慢侵蚀的地点最强,而溪流中溶解的Sr出口在侵蚀速度最快的地点最高。在那里,植物吸收的所有Sr都从垃圾中返回到溶液中。固体停留时间短的地点是母体物质和溶解出口Sr同位素组成不同的唯一地点。我们的研究表明,Sr在临界区的行为与同位素分馏不受生物吸收影响(例如锂,主要由次生固体的形成决定)或不受次生固体形成和生物吸收影响的金属(例如硅)形成鲜明对比。锶稳定同位素特征为量化作为环境和生态参数函数的临界区营养循环提供了新的机会。
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The role of vegetation in setting strontium stable isotope ratios in the Critical Zone
At Earth's surface the stable isotope ratio of strontium (88Sr/86Sr) is predominantly set by biological uptake of Sr and its storage in plant litter. This conclusion was reached from a stable isotope mass balance that was independently validated by direct determination of elemental fluxes between the Critical Zone compartments (rock, soil, vegetation, and stream water) of three field sites located in the Swiss Alps, the US Sierra Nevada, and the tropical highlands of Sri Lanka. These sites cover a gradient in erosion rates, which is inversely related to the residence time of solids in the Critical Zone thereby constituting an “erodosequence”. For eroding landscapes, previous stable isotope models predicted that isotope ratios are set by the rate at which secondary solids form during the conversion of rock to regolith. Counter to this expectation we found that, after release from primary minerals, Sr is partitioned into one fraction taken up by plants and the remainder into dissolved Sr flux. The formation of secondary weathering products such as clays and oxides plays a subordinate role in determining the Sr budget. A Sr isotope fractionation factor for biological uptake was determined for each of the three ecosystems from the average Sr stable isotope composition in bulk plants and its dissolved counterpart in stream water. This fractionation factors range from ca. −0.3 ‰ for the Alps and Sierra Nevada to ∼0 ‰ for the tropical Sri Lanka site. That these isotope fingerprints caused by biologic uptake are preserved means that more Sr was physically removed in plant litter than recycled. Such Sr removal in plant litter appears to be strongest at the slowly-eroding site, whereas the dissolved Sr export by streams is highest at the site with the fastest erosion rate. There, all Sr taken up by plants is returned from litter back into solution. The site with short residence time of solids is the only one at which parent material and dissolved export differ in their Sr isotope composition. Our study shows that the behavior of Sr in the Critical Zone is in stark contrast to that of metals of which the isotope fractionation is not affected by biological uptake (for example lithium, mostly set by formation of secondary solids) or affected by both secondary solid formation and biological uptake (for example silicon). Strontium stable isotope signatures offer the new opportunity to quantify nutrient cycling in the Critical Zone as a function of environmental and ecological parameters.
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来源期刊
American Journal of Science
American Journal of Science 地学-地球科学综合
CiteScore
5.80
自引率
3.40%
发文量
17
审稿时长
>12 weeks
期刊介绍: The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.
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