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Ge and Si Isotope behavior during intense tropical weathering and ecosystem cycling

Ge and Si Isotope behavior during intense tropical weathering and ecosystem cycling
Ge and Si Isotope behavior during intense tropical weathering and ecosystem cycling
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.
0886-6236
Baronas, J. Jotautas
15be653c-d7d7-4fca-842f-414a2a629a4d
West, A. Joshua
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Burton, Kevin W.
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Hammond, Douglas E.
3c237cdb-35a2-4328-8d2c-950b77bdb068
Opfergelt, Sophie
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Pogge Von Strandmann, Philip A. E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
James, Rachael H.
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Rouxel, Olivier J.
c68deadf-01db-450a-9cc8-97f24ddb1d77
Baronas, J. Jotautas
15be653c-d7d7-4fca-842f-414a2a629a4d
West, A. Joshua
0283c977-efd7-47fe-9bc7-240724a6e7a1
Burton, Kevin W.
b17a2651-0697-4369-bfa7-ece9a9f0a3f1
Hammond, Douglas E.
3c237cdb-35a2-4328-8d2c-950b77bdb068
Opfergelt, Sophie
48c611cc-f57f-489a-9c18-75177ff7e9de
Pogge Von Strandmann, Philip A. E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Rouxel, Olivier J.
c68deadf-01db-450a-9cc8-97f24ddb1d77

Baronas, J. Jotautas, West, A. Joshua, Burton, Kevin W., Hammond, Douglas E., Opfergelt, Sophie, Pogge Von Strandmann, Philip A. E., 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]. (doi:10.1029/2019GB006522).

Record type: Article

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

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2019GB006522 - Version of Record
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Accepted/In Press date: 23 July 2020
Published date: 1 August 2020

Identifiers

Local EPrints ID: 444814
URI: http://eprints.soton.ac.uk/id/eprint/444814
ISSN: 0886-6236
PURE UUID: f147a99c-8a4e-469c-a17e-bf4bb48193b0
ORCID for Rachael H. James: ORCID iD orcid.org/0000-0001-7402-2315

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Date deposited: 05 Nov 2020 17:32
Last modified: 26 Nov 2021 02:53

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Contributors

Author: J. Jotautas Baronas
Author: A. Joshua West
Author: Kevin W. Burton
Author: Douglas E. Hammond
Author: Sophie Opfergelt
Author: Philip A. E. Pogge Von Strandmann
Author: Olivier J. Rouxel

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