Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling
Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling
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DOI：
10.1029/2019gb006522
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发表时间：
2020-01
影响因子：
5.2
通讯作者：
J Jotautas Baronas;A. West;K. Burton;D. Hammond;S. Opfergelt;P. V. Strandmann;R. James;O. Rouxel
中科院分区：
文献类型：
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作者：
J Jotautas Baronas;A. West;K. Burton;D. Hammond;S. Opfergelt;P. V. Strandmann;R. James;O. Rouxel
温暖潮湿气候下火山岩的化学风化对全球溶质通量的贡献不成比例。在此过程中形成的溶质和固体的地球化学特征可以帮助量化和重建过去的风化强度。在这里，我们测量了哥斯达黎加热带低地雨林的土壤、粘土和液体的硅 (Si) 和锗 (Ge) 同位素比率。与母岩（δ30Si = -0.11 ± 0.05‰，δ74Ge = 0.59 ± 0.07‰）相比，大块表土风化严重，同位素较轻（平均值±1σ：δ30Si = -2.1 ± 0.3‰，δ74Ge = -0.13 ± 0.12‰）。新形成粘土的值甚至更低（δ30Si = -2.5 ± 0.2‰，δ74Ge = -0.16 ± 0.09‰），表明极端风化系统中存在全系统同位素变化。低地溪流代表稀释的当地流体（δ30Si = 0.2−0.6‰，δ74Ge = 2.2−2.6‰）与富含溶质的流域间地下水（δ30Si = 1.0±0.2‰，δ74Ge = 4.0‰）的混合。使用Ge-Si同位素质量平衡模型，我们计算出通过低地土壤风化释放的Ge有91±9%被新生粘土封存，9±9%被植被封存，只有0.2±0.2%仍然溶解。植被在硅循环中发挥着重要作用，直接封存 39 ± 14% 释放的硅，并通过添加无定形植硅体二氧化硅增强表层土壤中的粘土新生。在全球范围内，火山土壤δ74Ge密切跟踪化学风化（τGe）导致的Ge消耗，而δ30Si和Ge/Si都反映了Si（τSi）的损失。由于Ge和Si的化学迁移率不同，δ74Ge-δ30Si多代理系统比每个孤立的同位素系统对更广泛的风化强度敏感。
Chemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ: δ30Si = −2.1 ± 0.3‰, δ74Ge = −0.13 ± 0.12‰) compared to the parent rock (δ30Si = −0.11 ± 0.05‰, δ74Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ30Si = −2.5 ± 0.2‰, δ74Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ30Si = 0.2 − 0.6‰, δ74Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ30Si = 1.0 ± 0.2‰, δ74Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that 91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays, 9 ± 9% by vegetation, and only 0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering 39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soil δ74Ge closely tracks the depletion of Ge by chemical weathering (τGe), whereas δ30Si and Ge/Si both reflect the loss of Si (τSi). Because of the different chemical mobilities of Ge and Si, a δ74Ge‐δ30Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.