Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling

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.

Modulation of Riverine Concentration‐Discharge Relationships by Changes in the Shape of the Water Transit Time Distribution

“…Other conceptual models for C‐Q behavior exist including some that attribute it to end‐member mixing (e.g., Calmels et al, 2011; Chanat et al, 2002; Christophersen et al, 1990; Johnson et al, 1969; Neal et al, 1990). The generation of multiple water masses with distinct chemical compositions could arise from spatial differences in the composition of solid phases (e.g., different compositions for surface soils versus deeper bedrock) and would help explain patterns in hydrochemcial data sets that appear to be consistent with conservative mixing (Baronas et al, 2020; Calmels et al, 2011; Christophersen et al, 1990; Lee et al, 2017). Similarly, differences in C‐Q behavior for different elements in the same system (i.e., variations in elemental ratios with discharge) are sometimes attributed to variations in the solid‐phase composition between and along flow paths (Calmels et al, 2011; Kurtz et al, 2011; Torres et al, 2015; Winnick et al, 2017; Zhi et al, 2019).…”

Ge/Si and Ge Isotope Fractionation During Glacial and Non-glacial Weathering: Field and Experimental Data From West Greenland

“…It is theoretically possible that subglacially formed aSi is Ge-poor and isotopically heavy, as there is little data on Ge behavior during aSi formation (and no data on δ 74 Ge), especially during glacial weathering. However, we deem this unlikely, given 1) the lack of Ge/Si and δ 74 Ge fractionation in the field samples, as outlined above in Section 4.1; and 2) the fact that an overwhelming majority of secondary phases documented to date exhibit the opposite fractionation, i.e., high Ge/Si and low δ 74 Ge (Murnane and Stallard, 1990;Froelich et al, 1992;Pokrovsky et al, 2014;Rouxel and Luais, 2017;Baronas et al, 2018;Qi et al, 2019;Baronas et al, 2020). Therefore, our preferred hypothesis is that the change in dissolved Ge/Si and δ 74 Ge during the experiments reflects both the dissolution of aSi or silicates (or both) and Ge precipitation (or adsorption) coupled to isotopic fractionation.…”

“…The uniqueness of subglacial weathering is also reflected in the signatures of silicate weathering intensity proxies such as germanium to silicon (Ge/Si) or Si isotope (δ 30 Si) ratios, which in non-glacial settings are strongly fractionated by secondary weathering phases, as well as biological uptake by plants (e.g., Froelich et al, 1992;Derry et al, 2005;Cornelis et al, 2011;Frings et al, 2016;Baronas et al, 2018;Baronas et al, 2020). Germanium is a trace element primarily present in part-permillion concentrations in silicate rocks.…”

“…Globally, dissolved riverine Ge/Si ratios have been observed to vary from 0.1 to 3 μmol/mol, often significantly lower than silicate rock ratios of 1-3 μmol/mol, which are the primary source of Ge and Si to solution (Froelich et al, 1985;Mortlock and Froelich, 1987;Murnane and Stallard, 1990;Froelich et al, 1992;Rouxel and Luais, 2017;Baronas et al, 2018). In contrast, secondary weathering phases (such as Fe oxides and aluminosilicates like kaolinite) exhibit Ge/Si ratios commonly above 5 μmol/mol, reflecting the preferential incorporation of Ge (Mortlock and Froelich, 1987;Murnane and Stallard, 1990;Kurtz et al, 2002;Lugolobi et al, 2010;Baronas et al, 2020).…”

Resiliency of Silica Export Signatures When Low Order Streams Are Subject to Storm Events

“…) is in this case inverted, leading to lower fluid Ge/Si ratios and enriched secondary mineral Ge/Si ratios as the reaction proceeds (Aguirre et al, 2017;Ameijeiras-Mariño et al, 2017;Baronas et al, 2020Baronas et al, , 2018Gaspard et al, 2021;Kurtz et al, 2002;Lugolobi et al, 2010;Perez-Fodich & Derry, 2020;Qi et al, 2019;Scribner et al, 2006). In contrast, evidence based primarily on phytoliths suggests Si uptake by vegetation strongly discriminates against Ge, causing the remaining fluid Ge/Si ratio to increase (Blecker et al, 2007;Derry et al, 2005;Meek et al, 2016).…”