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: δ 30Si = –2.1 ± 0.3‰, δ 74Ge = –0.13 ± 0.12‰) compared to the parent rock (δ 30Si = –0.11 ± 0.05‰, δ 74Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ 30Si = –2.5 ± 0.2‰, δ 74Ge = –0.16 ± 0.09‰), demonstrating a whole-system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ 30Si = 0.2 – 0.6‰, δ 74Ge = 2.2 – 2.6‰) with solute-rich interbasin groundwater (δ 30Si = 1.0±0.2‰, δ 74Ge = 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 δ 74Ge closely tracks the depletion of Ge by chemical weathering (τGe), whereas δ 30Si and Ge/Si both reflect the loss of Si (τSi). Because of the different chemical mobilities of Ge and Si, a δ 74Ge-δ 30Si multi-proxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation