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 +/- 1 sigma:delta Si-30 = -2.1 +/- 0.3 parts per thousand,delta Ge-74 = -0.13 +/- 0.12 parts per thousand) compared to the parent rock (delta Si-30 = -0.11 +/- 0.05 parts per thousand,delta Ge-74 = 0.59 +/- 0.07 parts per thousand). Neoforming clays have even lower values (delta Si-30 = -2.5 +/- 0.2 parts per thousand,delta Ge-74 = -0.16 +/- 0.09 parts per thousand), demonstrating a whole-system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (delta Si-30 = 0.2 - 0.6 parts per thousand,delta Ge-74 = 2.2 - 2.6 parts per thousand) with solute-rich interbasin groundwater (delta Si-30 = 1.0 +/- 0.2 parts per thousand,delta Ge-74 = 4.0 parts per thousand). Using a Ge-Si isotope mass balance model, we calculate that91 +/- 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays,9 +/- 9% by vegetation, and only0.2 +/- 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering39 +/- 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soil delta Ge-74 closely tracks the depletion of Ge by chemical weathering (tau(Ge)), whereas delta Si-30 and Ge/Si both reflect the loss of Si (tau(Si)). Because of the different chemical mobilities of Ge and Si, a delta Ge-74-delta Si-30 multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.