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Ge and Si isotope behavior during intense tropical weathering and ecosystem cycling.

Baronas, J. Jotautas and West, A. Joshua and Burton, Kevin W. and Hammond, Douglas E. and Opfergelt, Sophie and Pogge von Strandmann, Philip A. E. and James, Rachael H. and Rouxel, Olivier J. (2020) 'Ge and Si isotope behavior during intense tropical weathering and ecosystem cycling.', Global biogeochemical cycles., 34 (8). e2019GB006522.

Abstract

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 multi‐proxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1029/2019GB006522
Publisher statement:© 2020. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Date accepted:23 July 2020
Date deposited:04 August 2020
Date of first online publication:21 August 2020
Date first made open access:27 August 2020

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