The evolution of lithium isotope signatures in fluids draining actively weathering hillslopes
The evolution of lithium isotope signatures in fluids draining actively weathering hillslopes
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DOI：
10.1016/j.epsl.2021.116988
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发表时间：
2021-05-18
影响因子：
5.3
通讯作者：
Druhan, Jennifer L.
中科院分区：
文献类型：
--
作者：
Golla, Jon K.;Kuessner, Marie L.;Druhan, Jennifer L.
锂(Li)的稳定同位素是硅酸盐风化的有力代表。这些同位素在主要河流溶解负荷中的去向和迁移已被表征，以推断当代风化制度和古条件的变化。在这一贡献中，我们通过直接测量通过快速侵蚀的一级山坡的水域(Li-7三角洲)的Li同位素比率，将这一综合信号反卷积到硅酸盐风化开始时分离Li的个别过程中。我们利用这些数据建立了一个多组分反应输运框架，该框架表明，风化物质的净溶解在包气带的浅部产生了轻微的三角洲Li-7信号(低至-9.2‰)。Li-7三角洲向包气带深处的增加(高达+18ppm)反映了次生成矿作用的增加。在地下水位以下，发生了一致的风化作用，导致阳离子浓度升高和基岩三角洲Li-7。随着地下水向下(Li-7增量=+13到+24‰)流向溪流，饱和带内的硅酸盐风化作用仍在继续。河流信号(Delta Li-7=+28-+29 ppm)反映了硅酸盐风化反应网络的终点和每个作用过程的相对大小(例如，次生矿物形成的转变，风化物质的溶解)。我们发现，流体穿过这一一级山坡的风化剖面是通过一系列特征Li同位素特征来区分的，这些特征可以在一个向前的、基于过程的模型框架中再现。这一模型的发展为金属(LOID)稳定同位素在解缠流域尺度的化学风化通量中的使用提供了改进的量化基础。(C)2021年提交人(S)。爱思唯尔出版公司(Elsevier B.V.)
The stable isotopes of lithium (Li) serve as a robust proxy of silicate weathering. The fate and transport of these isotopes in the dissolved load of major rivers have been characterized to infer changes in both contemporary weathering regimes and paleo-conditions. In this contribution, we deconvolve this integrated signal into the individual processes that fractionate Li at the inception of silicate weathering by directly measuring Li isotope ratios of waters (delta Li-7) transiting through a rapidly eroding first-order hillslope. We use these data to develop a multicomponent reactive transport framework, which shows that net dissolution of weathered material generates light delta Li-7 signatures (as low as -9.2 parts per thousand) in the shallow portion of the vadose zone. An increase in delta Li-7 deeper into the vadose zone (as much as +18 parts per thousand) reflects an increasing contribution of secondary mineral formation. Below the water table, congruent weathering occurs and imparts elevated cation concentrations and bedrock delta Li-7. Silicate weathering continues within the saturated zone as groundwater travels downslope (delta Li-7 =+13 to + 24 parts per thousand) to the stream. The stream signatures (delta Li-7 =+28 to +29 parts per thousand) reflect the terminus of this network of silicate weathering reactions and the relative magnitude of each contributing process (e.g., transitions in secondary mineral formation, dissolution of weathered material). We show that fluid progressing through the weathering profile of this first-order hillslope is distinguished by a sequence of characteristic Li isotope signatures, which can be reproduced in a forward, process-based model framework. This model development offers an improved quantitative basis for the use of metal(loid) stable isotopes in disentangling catchment-scale chemical weathering fluxes. (C) 2021 The Author(s). Published by Elsevier B.V.