# Maturation of Silicate Weathering Pathways Revealed by Germanium-Silicon Ratios

Andrew Guertin, Charlie Cunningham, Julien Bouchez, Marine Gelin, Jon Chorover, Louis Derry, Jennifer Druhan

► To cite this version:

Andrew Guertin, Charlie Cunningham, Julien Bouchez, Marine Gelin, Jon Chorover, et al. Maturation of Silicate Weathering Pathways Revealed by Germanium-Silicon Ratios. Geophysical Research Letters, 2025, 52 (20), pp.e2025GL118410. 10.1029/2025GL118410. hal-05382284

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL118410

**Key Points:**

* Dissolved Ge/Si ratios in mesoscale hillslopes were enriched relative to bedrock, in contrast to depleted values observed in natural streams

* Dissolved Ge/Si decreased through time, along with reduced weathering rates and the formation of more mature secondary phases

* Ge/Si offers a specialized utility in developing landscapes as a sensitive tracer for the formation of multiple new solid phases

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

A. Guertin,
andrew58@illinois.edu

**Citation:**

Gertuin, A., Cunningham, C., Bouchez, J., Gelin, M., Chorover, J., Derry, L. A., & Druhan, J. L. (2025). Maturation of silicate weathering pathways revealed by germanium-silicon ratios. *Geophysical Research Letters*, *52*, e2025GL118410.
https://doi.org/10.1029/2025GL118410

Received 29 JUL 2025
Accepted 6 OCT 2025

**Author Contributions:**

**Conceptualization:** Louis A. Derry,
Jennifer L. Druhan

**Data curation:** Andrew Guertin,
Charlie Cunningham, Julien Bouchez,
Marine Gelin, Louis A. Derry

**Formal analysis:** Andrew Guertin,
Julien Bouchez, Louis A. Derry, Jennifer
L. Druhan

**Funding acquisition:** Jon Chorover, Louis
A. Derry, Jennifer L. Druhan

**Investigation:** Andrew Guertin,
Julien Bouchez, Louis A. Derry, Jennifer
L. Druhan

**Methodology:** Andrew Guertin,
Charlie Cunningham, Marine Gelin,
Jennifer L. Druhan

© 2025. The Author(s).
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.

<figure><img src="image_2.png" /></figure>

## Maturation of Silicate Weathering Pathways Revealed by Germanium-Silicon Ratios

Andrew Guertin<fnref n="1" /> ID, Charlie Cunningham<fnref n="2" />, Julien Bouchez³ ID, Marine Gelin³, Jon Chorover<fnref n="2" /> ID, Louis A. Derry³,⁴ ID, and Jennifer L. Druhan<fnref n="1" />,³ ID

**Abstract** We document the evolution of fresh basalt subject to roughly a decade of chemical weathering through systematic temporal shifts in aqueous germanium-silicon ratios (Ge/Si) in a unique set of meso-scale artificial hillslopes. Elevated dissolved Ge/Si signatures relative to bedrock in this young system are associated with formation of high-Si amorphous phase(s) that preferentially exclude Ge. Over several years, dissolved Ge/Si ratios decrease, approaching those of natural watersheds, consistent with a shift toward formation of Ge-enriched secondary phases. Our study points to the critical control of secondary phase speciation on Ge/Si partitioning, while in contrast, stable silicon isotope ratios (δ³⁰Si) in the same samples show fractionation comparable to natural systems. Paired consideration of Ge/Si and δ³⁰Si tracers may thus offer a previously unrecognized means of interrogating the progression of secondary phase speciation in young landscapes, for example, following deglaciation, volcanism, and landslides, and in engineering applications such as enhanced weathering.

**Plain Language Summary** Where fresh rock is rapidly exposed to surface conditions through deglaciation, volcanism, landslides, human excavation, etc., chemical weathering is typically much faster than in more mature landscapes. Here we use a set of artificially constructed basalt hillslopes, which have been subject to routine irrigation over approximately a decade of exposure, to study the earliest stages of weathering. Dissolved germanium-silicon (Ge/Si) elemental ratios are elevated in our discharge samples relative to bedrock. This is in contrast to natural watersheds where they tend to be depleted. We argue that this unique behavior results from a transient balance between competing classes of new minerals forming from solution. Our results demonstrate the capacity for fresh rock weathering to form distinct geochemical signals compared to the weathered bedrock characteristic of more mature natural watersheds, thus offering Ge/Si ratios as a tool to decipher weathering pathways in young systems.

### 1. Introduction

Fresh bedrock can be rapidly exposed to surface environments through a variety of natural processes, for example, volcanism, deglaciation, and landslides (Börker et al., 2019; Emberson et al., 2018; Graly et al., 2014; Li et al., 2022), as well as by engineering methods, for example, mining, construction, or amendment of agricultural soils with mafic rock to promote enhanced carbon sequestration (Beerling et al., 2020; Hartmann et al., 2013; Kantzas et al., 2022). The incipient stages of chemical weathering and landscape development are relatively brief, and thus less well documented, analyzed, and understood relative to mature environments, yet they characteristically support much higher chemical weathering rates. For example, alkalinity fluxes from young Holocene basalts are roughly 10x greater than that from older inactive fields (Börker et al., 2019). The Landscape Evolution Observatory (LEO) in Arizona offers an extraordinarily rare glimpse into decadal-scale weathering of fresh basalt across a triplicate set of artificial hillslopes (Bauser et al., 2022; Cunningham et al., 2025; Kim & Harman, 2022; Kim & Troch, 2020; Kim et al., 2021). After 12 years of routine irrigation (~1,000 mm yr⁻<fnref n="1" />) interspersed by targeted experiments, LEO now presents a unique opportunity to document chemical weathering on intermediate time scales between those of fresh materials typically used in laboratory settings (e.g., Eick et al., 1996) and those of more mature natural landscapes (Cohen et al., 2013).

At such short timescales, direct evidence of the pathways and products of geochemical weathering are difficult to detect as alterations to the solid phase. This limitation has plagued direct documentation of the efficacy of crushed mafic rock applications to agricultural fields for enhanced weathering, leading to application of a suite of

<footnote n="1">¹Department of Earth Science & Environmental Change, University of Illinois at Urbana-Champaign, Urbana, IL, USA,</footnote>

<footnote n="2">²Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ, USA, ³Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France, ⁴Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA</footnote>
**Project administration:** Jon Chorover, Louis A. Derry, Jennifer L. Druhan

**Resources:** Julien Bouchez, Jon Chorover, Louis A. Derry, Jennifer L. Druhan

**Supervision:** Julien Bouchez, Jennifer L. Druhan

**Validation:** Andrew Guertin, Julien Bouchez, Louis A. Derry, Jennifer L. Druhan

**Visualization:** Andrew Guertin

**Writing – original draft:**

Andrew Guertin, Jennifer L. Druhan

**Writing – review & editing:**

Andrew Guertin, Julien Bouchez, Jon Chorover, Louis A. Derry, Jennifer L. Druhan

advanced geochemical tracer methods with varying degrees of success (e.g., Derry et al., 2025; Kantola et al., 2023). Water chemistry offers the capacity to resolve chemical weathering pathways in young landscapes. Stable isotopes and trace element ratios are particularly well suited to this purpose and are increasingly used to study young landscapes such as subglacial and deglaciated settings (Andrews & Jacobson, 2018; Baronas et al., 2021; Hatton, Hendry, Hawkings, Wadham, Kohler, et al., 2019; Hatton, Hendry, Hawkings, Wadham, Opfergelt, et al., 2019; Hindshaw et al., 2014; Rickli et al., 2017; Wimpenny et al., 2010, 2011).

In headwater streams the ratio of dissolved Ge to silicon (Ge/Si<sub>diss</sub>) consistently measures lower than local bedrock across a variety of catchments (Mortlock & Frohlich, 1987). This depletion is attributed to the preferential incorporation of Ge relative to Si in secondary aluminosilicates (primarily sheet-silicates and their precursors), driving down the residual Ge/Si<sub>diss</sub> (Kurtz et al., 2002; Murnane & Stallard, 1990). Ge/Si is often combined as a tracer with stable Si isotopes (δ<sup>30</sup>Si) to parse biological and chemical weathering fluxes (e.g., Baronas et al., 2018; Cornelis et al., 2010; Derry et al., 2005; Frings et al., 2021; López-Urzúa et al., 2025; Lugolobi et al., 2010; Opfergelt et al., 2010). Initial application of the Ge/Si<sub>diss</sub> tracer in freshly exposed, deglaciated landscapes, suggests unique chemical behavior compared to more evolved systems (Baronas et al., 2021). However, to date, decoupling the driving mechanisms for Ge/Si fractionation in these young systems remains a challenge due to the inherent complexity of natural environments.

Here we employ Ge/Si<sub>diss</sub> ratios of water draining from the LEO hillslopes as a means to interrogate chemical weathering pathway development in the earliest stages of landscape evolution. Immediately following construction, the initial geochemistry of the LEO system indicated high chemical weathering rates, incongruency, oversaturation in several secondary mineral phases, and depletion of most major cations (Pohlmann et al., 2016). Now after a decade of weathering, we document the utility of Ge/Si<sub>diss</sub> as a tracer of chemical weathering pathways and evolution across the earliest stages of hillslope development.

## 2. Methods

### 2.1. Experimental Facility

LEO was installed in 2012 and offers a uniquely controlled environment to explore the earliest stages of landscape development. The facility consists of three identical hillslopes (East, Center, and West), each 30-m long by 11-m wide, which reside within an insulated structure equipped with a variety of meteorological and climate sensors. Crushed basaltic tephra mined from a deposit associated with the Merriam Crater in northern Arizona (Table S1 in Supporting Information S1) was used to fill each hillslope in four compacted, 0.25-m increments (Pangle et al., 2015). The unweathered material had a loamy-sand texture with a particle size distribution of 84.6% sand, 12.2% silt, and 3.2% clay, a bulk density of 1.59 g cm<sup>-3</sup>, and an average porosity of 39% (Pangle et al., 2015). Each hillslope tilts to an average slope of 10° with a primary zone of convergence extending 18 m upslope to focus water drainage for collection (Figure S1 in Supporting Information S1). Irrigation water is first purified by reverse-osmosis filtration, and pre-equilibrated with atmospheric CO<sub>2</sub>, before application. Seven-hour irrigations have been conducted every 2 weeks since January 2014 at a rate of 12 mm hr<sup>-1</sup>. This routine irrigation ensures that the hillslopes are subjected to rainfall rates and frequencies characteristic of natural landscapes while simultaneously preventing dry out (Arevalo et al., 2020; Volkmann et al., 2018).

### 2.2. Irrigation Experiments and Sampling

Discharge at the bottom seepage face of each hillslope is funneled through a trench to produce a flux-weighted collection point. Two irrigation experiments are reported in the current study. The first was initiated on 4 December 2018, with the standard 7-hr irrigation. From the onset of irrigation, discharge was sampled manually every 4-hr for the subsequent 24-hr. As discharge slowed, sampling frequency progressively decreased to coarser timescales (max 168-hr) to allow 50 ml of water to accumulate. Sampling ceased on 14 January 2019. Regularly scheduled irrigations were suspended during the 41-day sampling campaign, resulting in a single continuous event hydrograph (Figure 1a). After collection, each sample was immediately filtered through a 0.45-micron polypropylene filter and subsampled for analysis. A select subset were further subjected to 0.025-micron cellulose acetate/nitrate filters.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Timeseries of (Ge/Si)~diss~ and specific discharge for 2018–2019 experiment. (b) 2022–2023. Lines represent specific discharge and points represent (Ge/Si)~diss~. Irrigation in mm/hr is plotted at the top of each figure. (Ge/Si)~diss~ remains relatively stable through the duration of both experiments. There is a mild increase in (Ge/Si)~diss~ during the recession of pulse 3 during the 2022–2023 experiment.</figcaption></figure>

The 2022–2023 experiment was comprised of three sequential 30-day irrigation periods of equivalent intensity, interspersed by increasingly longer dry periods of 30-, 60-, and 90-day (Figure 1b). Each 30-day irrigation varied between 3.1- and 10.5-mm $hr^{-1}$ as determined by a randomly generated schedule described in Guertin et al. (2024). Samples were collected for each slope every 2-hr using ISCO autosamplers during active drainage. Bottles were rotated and cleaned with nitric acid between deployments. Once discharge slowed to the point that 300 ml could not be collected within 8-hr, sampling ceased. Discharge samples were filtered at 0.22-microns and acidified to pH 3 using trace-metal grade nitric acid. A subset of water from each sampling campaign was separated for Ge/Si analysis: 233 total samples or ~1 per 12 to 24-hr across each hillslope for each irrigation period.

## 2.3. Analytical Methods

For the 2022–2023 samples, major cations and dissolved Si were analyzed at the PARI platform of the Institut de Physique du Globe de Paris (IPGP) by inductively coupled plasma mass spectrometry using an Agilent 7900 Q-ICP-MS. 2018–2019 samples were measured by equivalent instrumentation at the University of Arizona Lab for Emerging Contaminants. Instrumental accuracy was monitored using the certified river water reference standard SLRS-6 once every eight samples to check against intra-lab certified values and to track measurement uncertainty over time. Uncertainty in Si [4.21%] and Na [4.50%] are constrained by propagating two standard deviations of repeat SLRS-6 measurements throughout the course of analysis. Similarly, uncertainty for the molar ratio of Si/Na [4.14%] was separately constrained based on the ratios of the two elements obtained from the repeat SLRS-6 values.

Germanium concentrations were analyzed using hydride-generation ICP mass-spectrometry (HG-ICP-MS) (Aguirre et al., 2017) at Cornell University for the 2018–2019 samples and at IPGP for the 2022–2023 samples on a Thermo-Finnigan Element 2 ICP-MS. A $^{70}$Ge-enriched spike solution was used to reach a target $^{70}$Ge/$^{74}$Ge ratio
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ versus specific discharge (mm/hr) for the 2018–2019 and 2022–2023 experiments. Specific discharge has no correlation with $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ within or between experiments. (b) $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ versus $f_{\text{diss}}^{\text{Si}}$. There is a decrease from 2018–2019 to 2022–2023 in $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ as $f_{\text{diss}}^{\text{Si}}$ increases. Ultrafiltration of the 2018–2019 samples removes more Si than Ge, further reducing $f_{\text{diss}}^{\text{Si}}$ and increasing $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$.</figcaption></figure>

of 10 before introduction into the HG-ICP-MS, allowing Ge concentrations to be quantified using isotope dilution. A solid sample of fresh LEO basalt was prepared using alkali-fusion digestion (Georg et al., 2006b). Analytical uncertainty is approximately 3% RSD (Derry et al., 2005; Kurtz et al., 2002, 2011; Lugolobi et al., 2010).

### 3. Results

The irrigation schedule for the 2018–2019 experiment was shorter than in the 2022–2023 experiment but reached a slightly higher maximum intensity. As a result, the earlier study produced a lower maximum drainage rate $(0.4 \text{ mm hr}^{-1})$ with a shorter recession period compared to the later $(1.2 \text{ mm hr}^{-1})$ (Figure 2a). The majority of samples across the two studies were still associated with comparable discharge ranges, allowing comparison between the behavior of the system in 2018–2019 and 2022–2023.

The shorter 2018–2019 experiment produced Si concentrations of 218–314 µmol L⁻¹ with a range of $(\text{Ge/Si})_{\text{diss}}$ between 4.3 and 6.7 µmol mol⁻¹ (Figure 1a and Figure S2a in Supporting Information S1). $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ values ranged from 2.1 to 3.2 when normalized to the unweathered basalt ratio of 2.1 (Figure 2a). The measured $(\text{Ge/Si})_{\text{rock}}$ falls within the typical range for igneous rocks (~1–3 µmol/mol) (Bernstein, 1985; De Argollo & Schilling, 1978). The water samples further subjected to ultrafiltration yielded a further decrease in Si concentrations reaching 154–254 µmol L⁻¹, while $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ increased to a range of 2.9–3.9. Discharge flux showed no correlation with Si or $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ in either the standard or ultra-filtered samples (Figure 2a).

During the 2022–2023 experiment, dissolved Si concentrations were slightly higher than in 2018–2019 (230–375 µmol L⁻¹) with a corresponding range of $(\text{Ge/Si})_{\text{diss}}$ ratios between 2.0 and 4.9. Approximately 90% of these $(\text{Ge/Si})_{\text{diss}}$ measurements fell between 3.19 and 4.55 (Figure 1b and Figure S2b in Supporting Information S1). $(\text{Ge/Si})_{\text{diss}}/(\text{Ge/Si})_{\text{rock}}$ ranged from 0.99 to 2.33 (Figure 2a). The $(\text{Ge/Si})_{\text{diss}}$ ratios remained largely stable across the range of observed variation in Si concentration throughout the course of the experiment,
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Landscape Evolution Observatory (LEO) Ge/Si measurements plotted together with results from Fernandez et al. (2022) from a diverse set of upland catchments. Individual measurements are shown as points, flux-weighted averages are shown as diamonds. Dotted line intersection represents congruent dissolution. LEO (Ge/Si)_{diss}/(Ge/Si)_{rock} decreased from 2018–2019 to 2022–2023 while $f_{diss}^{Si}$ increased. (b) Unlike (Ge/Si)_{diss}/(Ge/Si)_{rock}, LEO $\Delta^{30}$Si fractionates in the same direction as natural watersheds. This contrasting behavior is due to consistent preferential uptake of the lighter $^{28}$Si while Ge uptake varies between secondary phases at LEO.</figcaption></figure>

indicating covariation between the two independently measured elements (Figure S3a in Supporting Information S1). Concentrations and (Ge/Si)_{diss} ratios were similar across replicate hillslopes and irrigation pulses, though we note some minor increase in (Ge/Si)_{diss} during the recession of pulse 3 on the East hillslope (Figure 1b). As in the 2018–2019 data set, discharge flux showed no systematic influence on (Ge/Si)_{diss}/(Ge/Si)_{rock} (Figure 2a).

# 4. Discussion

## 4.1. Ge Release and Retention in the LEO Hillslopes

LEO discharge exhibits unusually high (Ge/Si)_{diss}/(Ge/Si)_{rock} compared to natural streams draining low-order watersheds (Fernandez et al., 2022). This increase in Ge_{diss} relative to Si_{diss} occurs despite significant retention of both Ge and Si in the hillslopes (Figure 3). Retention is estimated relative to sodium (Na), given that Na is released stoichiometrically with Si and Ge during dissolution, but is largely excluded from secondary solid precipitation and plant uptake (Dellinger et al., 2015; Georg et al., 2007; Gislason et al., 1996; Millot et al., 2010). Normalizing the (Si/Na)_{diss} and (Ge/Na)_{diss} to that of the basalt thus offers an estimate of the fraction remaining in solution after congruent dissolution and subsequent secondary solid (and/or biotic) retention (Brichau et al., 2008; Charbonnier et al., 2024; Fernandez et al., 2022; Georg et al., 2006a; Gislason et al., 1996, Equation 1).

$$f_{\text{diss}}^{X} = \frac{\left(\frac{X}{\text{Na}}\right)_{\text{discharge}}}{\left(\frac{X}{\text{Na}}\right)_{\text{bedrock}}} \qquad (1)$$

In natural systems, glass is often rapidly weathered during basalt dissolution (Brichau et al., 2008; Georg et al., 2006a; Gislason et al., 1996), and thus, for LEO, we use an $(Si/Na)_{bedrock}$ ratio based on the glass phase of the
basalt (6.92 mol mol⁻¹; Table S1 in Supporting Information S1, Guertin et al., 2024). This (Si/Na)bedrock is smaller than that of the bulk solid (8.41), and is employed to account for the relatively high reactivity of basaltic glass, which appears to form a lattice around the rest of the primary minerals (Pohlmann et al., 2016). Additionally, elevated δ³⁰Si isotope ratios in our discharge samples (Guertin et al., 2024; discussed further in Section 4.5) suggest formation of new ²⁸Si enriched secondary phases that are stable, or at least subject to only partial redissolution. Thus we attribute the principal source Si influx into solution as primary phase dissolution. For (Ge/Si)bedrock we employ a bulk rock measurement of 2.1 µmol mol⁻¹, given that, across a variety of volcanic rock compositions, the Ge/Si ratio of the bulk rock tends to be comparable to that in the glass component (Harris, 1954).

$$ \left(\frac{\text{Ge}}{\text{Si}}\right)_{\text{glass}} * \left(\frac{\text{Si}}{\text{Na}}\right)_{\text{glass}} = \left(\frac{\text{Ge}}{\text{Na}}\right)_{\text{glass}} \quad (2) $$

Inserting these values into Equation 2, we find a (Ge/Na)glass ratio of 14.5 µmol mol⁻¹.

The resulting fSi/diss ranges from 0.02 to 0.04 for 2018–2019 discharge samples and 0.07 to 0.14 for 2022–2023 samples. The fGe/diss ranges are slightly higher, between 0.08–0.14 in 2018–2019 and 0.11–0.31 in 2022–2023. These values indicate that in the 2018–2019 experiment, ~96%–98% of the Si(diss) and ~86%–92% of the Ge(diss) were retained due to secondary phase formation. In the 2022–2023 experiment, retention of both elements was lower, reaching 85%–93% Si(diss) retained in solid phases, and 69%–89% of Ge(diss). These estimates depend on the relative dissolution rates between the two experiments, but serve as a reasonable indicator for significant retention of Ge and Si at LEO. The formation of secondary phases at LEO during the 2022–2023 experiment was independently confirmed by characteristic fractionation of stable silicon isotope ratios (Guertin et al., 2024). The absence of plants at LEO eliminates the possibility of a competing biological effect overprinting the uptake of Si or Ge, and the strong retention of Ge is consistent with the formation of Ge-incorporating secondary phases as seen across natural systems (Kurtz et al., 2002). Our data indicate that the extent of Si retention outpaces substantial retention of Ge, driving up (Ge/Si)diss. This creates the clear distinction between the LEO (Ge/Si)diss ratios and those observed in natural systems (Figure 3a).

## 4.2. Ge Retention Pathways

There are multiple possible secondary phases that could host the Ge retained in the LEO hillslopes. These include a variety of clay minerals, high-Si amorphous phases, and Fe-oxyhydroxides. Multiple phases are oversaturated and poised to form from the LEO porewater (e.g., montmorillonite, allopheane, goethite, kaolinite, and nontronite; Table S3 in Supporting Information S1). Where mineral phases are saturated, amorphous precursors like hydroxy aluminosilicates and hydrous ferric silicates likely form first (Baldermann et al., 2024). Precipitation of amorphous smectite-like or proto-kaolinite species is consistent with natural basaltic systems in Iceland and Hawaii (Crovisier et al., 1992; Gislason et al., 1996; Kurtz et al., 2002). Clay-like silicates tend to be enriched in Ge/Si, up to 4–7 µmol mol⁻¹ (Aguirre et al., 2017; Bernstein, 1985; Evans & Derry, 2002; Kurtz et al., 2002) and are commonly accredited with Ge retention in catchments with mafic bedrock (Baronas et al., 2020; Kurtz et al., 2002; Scribner et al., 2006). Precipitation of smectite-like phases would also account for the observed retention of Ca and Mg in the hillslopes (Table S2 in Supporting Information S1). Laboratory experiments forming allopheane (Kurtz et al., 2002) and Fe-oxyhydroxide minerals show a similar preferential incorporation of Ge through isomorphic substitution and adsorption (Anders et al., 2003; Bernstein, 1985; Bernstein & Waychunas, 1987; Scribner et al., 2006). Despite retention in both aluminosilicates and Fe-oxyhydroxide phases, several studies have demonstrated that aluminosilicate formation overpowers the role of Fe-oxides in the overall Ge mass balance in basaltic systems (Kurtz et al., 2002; Scribner et al., 2006). In our 2022 study, Ge(diss) covaries with Si(diss) (Figure S3a in Supporting Information S1) and, to a lesser extent, Al(diss) (Figure S3b in Supporting Information S1). Fe(diss) remains low throughout the experiment and shows no significant correlation with Ge(diss) (Figure S3c in Supporting Information S1). While Fe-bearing phases are supersaturated and thus likely to accumulate at LEO, the lack of correlation with Ge(diss) suggests a tighter coupling to the behavior of Si regardless of attendant Fe-phase formation. Formation of either secondary aluminosilicates or Fe-oxyhydroxides offers a sink for retained Ge but cannot explain the elevated (Ge/Si)diss/(Ge/Si)rock. We are unaware of any field data or
laboratory experiments suggesting the preferential exclusion of Ge in clay-like silicates, and thus an alternative Si sink is necessary to reconcile this (Ge/Si)_{diss}/(Ge/Si)_{rock} signature.

The simplest plausible mechanism driving elevated (Ge/Si)_{diss}/(Ge/Si)_{rock} is the formation of a high-Si phase preferentially excluding Ge. Benchtop experiments have shown that the precipitation of amorphous SiO₂ in the absence of Al and Fe excludes Ge (Fernandez et al., 2021). These observations are consistent with a variety of field studies showing stronger incorporation of Ge by alumino-silicates than by quartz or opal (Ameijeiras-Mariño et al., 2018; Kurtz et al., 2002; Lugolobi et al., 2010). Formation of a high-Si phase excluding Ge offers a reasonable explanation for elevated (Ge/Si)_{diss}/(Ge/Si)_{rock} values at LEO but alone cannot explain this observation in tandem with relatively low f<sub>diss</sub><sup>Ge</sup> ratios.

In total, these observations lead us to suggest that the combination of low f<sub>diss</sub><sup>Ge</sup> and elevated (Ge/Si)_{diss}/(Ge/Si)_{rock} at LEO reflects the formation of two contemporaneous classes of secondary phases, one preferring and one disfavoring Ge. Based on saturation indices (Table S3 in Supporting Information S1) and observations taken in natural mafic systems, Ge is likely being retained by some combination of a smectite-like amorphous phase (Crovisier et al., 1992) and allophane/imogolite (Kurtz et al., 2002), with some contribution from Fe-oxyhydroxides. Additional Si is then retained in another amorphous, high-Si phase (e.g., Baronas et al., 2021; Fernandez et al., 2021) which preferentially excludes Ge, thus driving the system toward elevated (Ge/Si)_{diss}/(Ge/Si)_{rock}. Further evidence for the precipitation of a high-Si phase preferentially excluding Ge is offered by subjecting the 2019 samples to 0.025-micron filtration, which increased the mean (Ge/Si)_{diss}/(Ge/Si)_{rock} ratio from 2.7 to 3.5, without contemporaneous loss of Al from solution. This increase results from a higher (Ge/Si)_{diss} ratio in all 24 ultrafiltered water samples, and hence a lower Ge/Si (sample mean of 2.5 ± 1.55 µmol mol⁻¹) in the colloids removed from solution. While both Ge and Si are removed by ultrafiltration, there is no correlation between them in the resulting filtrate (Figure S4 in Supporting Information S1), implying an Si sink during ultrafiltration which is dissociated from that of Ge or Al, consistent with Ge exclusion from a high-Si phase.

## 4.3. LEO Through Time and Maturation

Systematic differences in discharge chemistry between the two experiments support the concept of a competition between alumino-silicate/Fe-oxyhydroxide phase(s) which incorporate Ge and high-Si phase(s) which preferentially exclude Ge. Between the 2018–2019 and 2022–2023 experiments, the (Ge/Si)_{diss}/(Ge/Si)_{rock} of water draining the hillslopes decreased while f<sub>diss</sub><sup>Si</sup> increased (Figure 2b) despite comparable drainage rates. Na<sub>diss</sub>, a metric for the extent of basalt dissolution, decreased from 829 µmol L⁻¹ in 2018–2019 to 383 µmol L⁻¹ in 2022–2023 (Table S2 in Supporting Information S1). This apparent decrease in primary solid phase dissolution rates corresponds with relatively more Si<sub>diss</sub> remaining compared to Ge<sub>diss</sub>. These observations are consistent with a decline in the formation (or partial redissolution) of high-Si amorphous solids and a relative increase in Ge-bearing secondary phase(s). As the fresh basalt tephra covering the hillslopes has evolved over roughly a decade of chemical weathering, our data trend toward depleted (Ge/Si)_{diss}/(Ge/Si)_{rock} more characteristic of natural systems. It is likely that the balance of secondary silicate phases will further shift toward Ge-enriched alumino-silicates as reaction rates continue to slow, leading to the eventual production of (Ge/Si)_{diss}/(Ge/Si)_{rock} signatures consistent with natural systems.

## 4.4. LEO as an Intermediate Between Environments and Lab Experiments

Germanium has been used extensively as a weathering tracer because it tends to preferentially partition into neoformed clays (e.g., Anders et al., 2003; Baronas et al., 2018; Mortlock & Frohlich, 1987; Murnane & Stallard, 1990), based on the observation that the (Ge/Si)_{diss}/(Ge/Si)_{rock} ratio is <1.0 in most natural non-hydrothermal systems and from analysis of clays and clay size fractions (Aguirre et al., 2017; Cornelis et al., 2010; Frings et al., 2021; Kurtz et al., 2002; Lugolobi et al., 2010). Individual watersheds each appear to have their own distinct (Ge/Si)_{diss} signature (Fernandez et al., 2022), which has been interpreted through a variety of perspectives including subsurface flow paths, the extent of secondary precipitation, and incorporation into plant phytoliths (Ameijeiras-Mariño et al., 2018; Kurtz et al., 2011; Murnane & Stallard, 1990). Our understanding of the Ge/Si tracer depends on a combination of these field observations and laboratory experiments.

LEO functions as an intermediate between natural watersheds and benchtop-scale lab experiments, both in terms of size (~30 m flow path), water transit time (~10 days) and duration of weathering (~12 years of exposure). LEO
offers a rare view into the earliest stages of landscape development, exhibiting both significant incongruent weathering (low $f_{diss}^{Si}$) and a shifting balance in weathering products (decreasing $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$). Such evolution is indecipherable in major rock-derived solute concentrations or standard solid phase analyses (e.g., X-ray diffraction detection limits are ca. 5% by mass, Bunaciu et al., 2015), requiring the use of advanced tracers like Ge/Si.

While species dependence in Ge/Si partitioning has been well documented for a variety of mineral phases in both the field and lab (De Argollo & Schilling, 1978; Fernandez et al., 2021; Frings et al., 2021; Harris, 1954; Kurtz et al., 2002), to our knowledge this is the first evidence of systematic shifts in $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$ signatures over a decadal period of weathering attributable to secondary speciation. These data offer compelling evidence for the development of $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$ as a means to trace secondary phase formation in the earliest stages of weathering. Quantitative applications of the Ge/Si tracer in young landscapes, as proposed in Baronas et al. (2018), may consider a range of fractionation factors as secondary phases precipitate including those preferentially excluding Ge. In this sense, our work subdivides secondary precipitation pathways for young catchments and expands the interpretation of secondary phase precipitation.

## 4.5. Paired Application of Ge/Si and Si Isotopes

In mature natural watersheds, the combined utility of Ge/Si and $\delta^{30}$Si is based on the distinct fractionating effects associated with secondary phase formation and plant uptake (De La Rocha et al., 2000; Ziegler, Chadwick, Brzezinski, & Kelly, 2005; Ziegler, Chadwick, White, & Brzezinski, 2005) or as a comparable weathering tracer (Frings et al., 2021). During secondary phase formation, Ge and $^{28}$Si are preferentially incorporated, such that Ge/Si$_{diss}$ is lowered while $\delta^{30}$Si$_{diss}$ is enriched. During certain plant uptake pathways, Ge and $^{30}$Si are excluded, leading to enrichment in Ge/Si$_{diss}$ and $\delta^{30}$Si$_{diss}$. At LEO, where biotic uptake is negligible, $\delta^{30}$Si$_{diss}$ and Ge/Si$_{diss}$ would be expected to exhibit simultaneous enrichment and depletion, respectively. Clearly our Ge/Si$_{diss}$ data diverge from this expectation, yet $\delta^{30}$Si$_{diss}$ data from the 2022 to 2023 LEO experiment displayed enrichment characteristic of natural watersheds (Figure 3b).

The paired $\delta^{30}$Si and $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$ data sets from our mesoscale experiment suggests, in agreement with the Fernandez et al. (2021) study, that Ge/Si$_{diss}$ ratios respond to different drivers than $\delta^{30}$Si$_{diss}$ in the early stages of weathering. This distinction in timescale is likely reflective of greater species dependence in Ge/Si fractionation. We suggest that such contrasting behavior between $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$ and $\delta^{30}$Si tracers owes to the transition in prevalent secondary phase formation from amorphous high-Si solids to higher alumino-silicates. Unlike Ge/Si, the kinetic fractionation of stable Si isotopes during precipitation is largely phase independent, with both clays (Ziegler, Chadwick, White, & Brzezinski, 2005) and amorphous silica (Fernandez et al., 2019) exhibiting similar fractionating effects across experiments. The result is that more precipitation of high-Si phases during the early stages of weathering increases both $(Ge/Si)_{diss}$/(Ge/Si)$_{rock}$ and $\delta^{30}$Si$_{diss}$, creating signatures unique from any observed in natural catchments. Such species sensitive fractionation in Ge/Si opens the possibility to juxtapose these two tracers, traditionally used in parallel, for application in developing systems (e.g., deglaciated catchments, steep thin soils, volcanic islands) to observe transitions from initial weathering products to the alumino-silicates observed in mature systems.

## 5. Conclusions

($Ge/Si)_{diss}$/$($Ge/Si)_{rock}$ signatures in LEO discharge reveal systematic evolution in silicate weathering pathways over the first decade of fresh basalt exposure to surface conditions. These results offer insight into a unique ($Ge/Si)_{diss}$/$($Ge/Si)_{rock}$ behavior potentially characteristic of young landscapes at the start of surface exposure during rapid silicate weathering product maturation. In just a few years, overall rock-derived elemental mass flux from the LEO basalt significantly slowed and more Ge was incorporated into secondary phases. The divergence in ($Ge/Si)_{diss}$/$($Ge/Si)_{rock}$ versus $\delta^{30}$Si signatures functions as a potential tool to trace geochemical maturation in young natural landscapes. We propose that ($Ge/Si)_{diss}$/$($Ge/Si)_{rock}$ ratios progressively become depleted, approaching natural stream ratios, as the predominant Si-weathering products transition from high-Si amorphous phases toward more chemically diverse alumino-silicate phases.

This rapid evolution in LEO's geochemistry is highly relevant to enhanced rock weathering strategies based on application of mafic rock to agricultural systems to accelerate weathering and draw down atmospheric $CO_2$ (e.g.,
Beerling et al., 2020; Seifritz, 1990). Like LEO, this material is fresh, supporting high chemical weathering rates relative to more evolved natural systems. Our work suggests that models focused on this behavior will require the capacity to describe chemical evolution on the timescale of months to years from initial conditions with consideration of multiple competing secondary phases (Cunningham et al., 2025). LEO's deviation from traditional watershed behavior motivates a more nuanced consideration of species-dependent fractionation in these engineering strategies as well as in the early evolution of young landscapes. This study motivates further quantification of species-specific Ge/Si fractionation to unlock a potentially powerful diagnostic of solid phase evolution based on fluid measurements when and where system maturation is insufficient to allow for solid phase characterization methods.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

Accompanying data are available on Hydroshare at: http://www.hydroshare.org/resource/cc024a9854914983bd0e7a467cc9409a (Guertin, 2025).

$f_{\text{diss}}^{\text{Si}}$ values for natural watersheds use $\text{Si}/\text{Na}_{\text{bedrock}}$ values from an updated repository correction published with Fernandez et al. (2025): https://www.hydroshare.org/resource/a6e5952cd87749b78a5eb74eb3caaa86/.

## References

Aguirre, A. A., Derry, L. A., Mills, T. J., & Anderson, S. P. (2017). Colloidal transport in the Gordon Gulch catchment of the Boulder Creek CZO and its effect on C-Q relationships for silicon. *Water Resources Research*, 53(3), 2368–2383. https://doi.org/10.1002/2016WR019730

Ameijeiras-Mariño, Y., Opfergelt, S., Derry, L. A., Robinet, J., Govers, G., Minella, J. P. G., & Delmelle, P. (2018). Ge/Si ratios point to increased contribution from deeper mineral weathering to streams after forest conversion to cropland. *Applied Geochemistry*, 96, 24–34. https://doi.org/10.1016/j.apgeochem.2018.06.002

Anders, A. M., Sletten, R. S., Derry, L. A., & Hallet, B. (2003). Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments. *Journal of Geophysical Research*, 108(F1), 6005. https://doi.org/10.1029/2003JF000026

Andrews, M. G., & Jacobson, A. D. (2018). Controls on the solute geochemistry of subglacial discharge from the Russell glacier, Greenland ice sheet determined by radiogenic and stable Sr isotope ratios. *Geochimica et Cosmochimica Acta*, 239, 312–329. https://doi.org/10.1016/j.gca.2018.08.004

Arevalo, J., Zeng, X., Durcik, M., Sibayan, M., Pangle, L., Abramson, N., et al. (2020). Highly sampled measurements in a controlled atmosphere at the Biosphere 2 Landscape Evolution Observatory. *Scientific Data*, 7(1), 306. https://doi.org/10.1038/s41597-020-00645-5

Baldermann, A., Stamm, F. M., Farkaš, J., Löhr, S., Ratz, B., Letofsky-Papst, I., & Dietzel, M. (2024). Precipitation of short-range order hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) at ambient temperature: Insights into mineral formation pathways, crystal chemistry and solubility-stability relationships. *Chemical Geology*, 646, 121911. https://doi.org/10.1016/j.chemgeo.2023.121911

Baronas, J. J., Hammond, D. E., Bennett, M. M., Rouxel, O., Pitcher, L. H., & Smith, L. C. (2021). Ge/Si and Ge isotope fractionation during glacial and non-glacial weathering: Field and experimental data from west Greenland. *Frontiers in Earth Science*, 9, 551900. https://doi.org/10.3389/feart.2021.551900

Baronas, J. J., Torres, M. A., West, A. J., Rouxel, O., Georg, B., Bouchez, J., et al. (2018). Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach. *Earth and Planetary Science Letters*, 503, 194–215. https://doi.org/10.1016/j.epsl.2018.09.022

Baronas, J. J., West, A. J., Burton, K. W., Hammond, D. E., Opfergelt, S., Pogge von Strandmann, P. A. E., et al. (2020). Ge and Si isotope behavior during intense tropical weathering and ecosystem cycling. *Global Biogeochemical Cycles*, 34(8), e2019GB006522. https://doi.org/10.1029/2019GB006522

Bauser, H. H., Kim, M., Ng, W.-R., Bugaj, A., & Troch, P. A. (2022). Richards equation at the hillslope scale: Can we resolve the heterogeneity of soil hydraulic material properties? https://doi.org/10.1002/essoar.10510743.1

Beerling, D. J., Kantzas, E. P., Lomas, M. R., Wade, P., Eufrasio, R. M., Renforth, P., et al. (2020). Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. *Nature*, 583(7815), 242–248. https://doi.org/10.1038/s41586-020-2448-9

Bernstein, L. R. (1985). Germanium geochemistry and mineralogy. *Geochimica et Cosmochimica Acta*, 49(11), 2409–2422. https://doi.org/10.1016/0016-7037(85)90241-8

Bernstein, L. R., & Waychunas, G. A. (1987). Germanium crystal chemistry in hematite and goethite from the Apex Mine, Utah, and some new data on germanium in aqueous solution and in Stottite. *Geochimica et Cosmochimica Acta*, 51(3), 623–630. https://doi.org/10.1016/0016-7037(87)90074-3

Börker, J., Hartmann, J., Romero-Mujalli, G., & Li, G. (2019). Aging of basalt volcanic systems and decreasing CO₂ consumption by weathering. *Earth Surface Dynamics*, 7(1), 191–197. https://doi.org/10.5194/esurf-7-191-2019

Brichau, S., Respaut, J.-P., & Monié, P. (2008). New age constraints on emplacement of the Cévenol granitoids, South French Massif Central. *International Journal of Earth Sciences*, 97(4), 725–738. https://doi.org/10.1007/s00531-007-0187-x

Bunaciu, A. A., Udriştiou, E. G., & Aboul-Enein, H. Y. (2015). X-Ray diffraction: Instrumentation and applications. *Critical Reviews in Analytical Chemistry*, 45(4), 289–299. https://doi.org/10.1080/10408347.2014.949616

Charbonnier, Q., Rickli, J., Archer, C., & Vance, D. (2024). The influence of secondary weathering processes on dissolved nickel isotope compositions under cold climatic conditions – Observations from the Mackenzie Basin. *Geochimica et Cosmochimica Acta*, 364, 10–21. https://doi.org/10.1016/j.gca.2023.10.026

## Acknowledgments

This work is supported by a multi-institutional collaborative research grant awarded by the US National Science Foundation: EAR-2135405 to J.L.D., EAR-2134453 to J.C., and EAR-2141989 and Agence Nationale de la Recherche project ANR-17-MPGA-0009 (France) to L.D. and M.G. was supported by Centre National de la Recherche Scientifique Grants TOF-COL and NanoCZ. The authors thank the LEO team at Biosphere 2 for their contributions to sample collection. We thank Sofia López Urzúa and the IPGP group for assistance in analytical measurements. This work benefited from the Région Île-de-France SESAME Grant 12015908 awarded to the PARI platform of IPGP.
Cohen, K. M., Finney, S. C., Gibbard, P. L., & Fan, J.-X. (2013). The ICS international chronostratigraphic chart. *Episodes*, **36**(3), 199–204.
https://doi.org/10.18814/epiungs/2013/v36i3/002

Cornelis, J. T., Delvaux, B., Cardinal, D., André, L., Ranger, J., & Opfergelt, S. (2010). Tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using Si isotopes and Ge/Si ratios. *Geochimica et Cosmochimica Acta*, **74**(14), 3913–3924. https://doi.org/10.1016/j.gca.2010.04.056

Crovisier, J.-L., Honnorez, J., Fritz, B., & Petit, J.-C. (1992). Dissolution of subglacial volcanic glasses from Iceland: Laboratory study and modelling. *Applied Geochemistry*, **7**, 55–81. https://doi.org/10.1016/S0883-2927(99)80064-4

Cunningham, C. J., Guertin, A., Gelin, M., Derry, L. A., Bauser, H. H., Kim, M., et al. (2025). Carbon dioxide removal during dissolution of granular basalt: A mass balance test of enhanced rock weathering at the hillslope scale. *Earth and Planetary Science Letters*, **671**, 119662.
https://doi.org/10.1016/j.epsl.2025.119662

De Argollo, R., & Schilling, J.-G. (1978). Ge-Si and Ga-Al fractionation in Hawaiian volcanic rocks. *Geochimica et Cosmochimica Acta*, **42**(6), 623–630. https://doi.org/10.1016/0016-7037(78)90007-8

De La Rocha, C. L., Brzezinski, M. A., & DeNiro, M. J. (2000). A first look at the distribution of the stable isotopes of silicon in natural waters. *Geochimica et Cosmochimica Acta*, **64**(14), 2467–2477. https://doi.org/10.1016/S0016-7037(00)00373-2

Dellinger, M., Gaillardet, J., Bouchez, J., Calmels, D., Louvat, P., Dosseto, A., et al. (2015). Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes. *Geochimica et Cosmochimica Acta*, **164**, 71–93. https://doi.org/10.1016/j.gca.2015.04.042

Derry, L. A., Chadwick, O. A., & Porder, S. (2025). Estimation of carbon dioxide removal via enhanced weathering. *Global Change Biology*, **31**(2), e70067. https://doi.org/10.1111/gcb.70067

Derry, L. A., Kurtz, A. C., Ziegler, K., & Chadwick, O. A. (2005). Biological control of terrestrial silica cycling and export fluxes to watersheds. *Nature*, **433**(7027), 728–731. https://doi.org/10.1038/nature03299

Eick, M. J., Grossl, P. R., Golden, D. C., Sparks, D. L., & Ming, D. W. (1996). Dissolution of a lunar basalt simulant as affected by pH and organic anions. *Geoderma*, **74**(1–2), 139–160. https://doi.org/10.1016/S0016-7061(96)00055-9

Emerson, R., Galy, A., & Hovius, N. (2018). Weathering of reactive mineral phases in landslides acts as a source of carbon dioxide in Mountain belts. *Journal of Geophysical Research: Earth Surface*, **123**(10), 2695–2713. https://doi.org/10.1029/2018JF004672

Evans, M. J., & Derry, L. A. (2002). Quartz control of high germanium/silicon ratios in geothermal waters. *Geology*, **30**(11), 1019. https://doi.org/10.1130/0091-7613(2002)030<1019:QCOHGS>2.0.CO;2

Fernandez, N. M., Bouchez, J., Derry, L., Chorover, J., Gaillardet, J., Giesbrecht, I., et al. (2025). Cross-CZO storm-event stream water silicon stable isotope - Discharge datasets from manuscript (USA, France, Canada) [Dataset]. *HydroShare*. https://doi.org/10.4211/hs.a6e5952cd87749b78a5eb74eb3caaa86

Fernandez, N. M., Bouchez, J., Derry, L. A., Chorover, J., Gaillardet, J., Giesbrecht, I., et al. (2022). Resiliency of Silica export signatures when low order streams are subject to storm events. *Journal of Geophysical Research: Biogeosciences*, **127**(5), e2021JG006660. https://doi.org/10.1029/2021JG006660

Fernandez, N. M., Perez-Fodich, A., Derry, L. A., & Druhan, J. L. (2021). A first look at Ge/Si partitioning during amorphous silica precipitation: Implications for Ge/Si as a tracer of fluid-silicate interactions. *Geochimica et Cosmochimica Acta*, **297**, 158–178. https://doi.org/10.1016/j.gca.2021.01.007

Fernandez, N. M., Zhang, X., & Druhan, J. L. (2019). Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies. *Geochimica et Cosmochimica Acta*, **262**, 104–127. https://doi.org/10.1016/j.gca.2019.07.029

Frings, P. J., Schubring, F., Oelze, M., & von Blanckenburg, F. (2021). Quantifying biotic and abiotic Si fluxes in the critical zone with Ge/Si ratios along A gradient of erosion rates. *American Journal of Science*, **321**(8), 1204–1245. https://doi.org/10.2475/08.2021.03

Georg, R. B., Reynolds, B. C., Frank, M., & Halliday, A. N. (2006a). Mechanisms controlling the silicon isotopic compositions of river waters. *Earth and Planetary Science Letters*, **249**(3–4), 290–306. https://doi.org/10.1016/j.epsl.2006.07.006

Georg, R. B., Reynolds, B. C., Frank, M., & Halliday, A. N. (2006b). New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS. *Chemical Geology*, **235**(1–2), 95–104. https://doi.org/10.1016/j.chemgeo.2006.06.006

Georg, R. B., Reynolds, B. C., West, A. J., Burton, K. W., & Halliday, A. N. (2007). Silicon isotope variations accompanying basalt weathering in Iceland. *Earth and Planetary Science Letters*, **261**(3–4), 476–490. https://doi.org/10.1016/j.epsl.2007.07.004

Gislason, S. R., Arnorsson, S., & Armannsson, H. (1996). Chemical weathering of basalt in Southwest Iceland; effects of runoff, age of rocks and vegetative/glacial cover. *American Journal of Science*, **296**(8), 837–907. https://doi.org/10.2475/ajs.296.8.837

Graly, J. A., Humphrey, N. F., Landowski, C. M., & Harper, J. T. (2014). Chemical weathering under the Greenland ice sheet. *Geology*, **42**(6), 551–554. https://doi.org/10.1130/G35370.1

Guertin, A. (2025). Maturation of silicate weathering pathways revealed by germanium-silicon ratios [Dataset]. *HydroShare*. Retrieved from
http://www.hydroshare.org/resource/cc024a9854914983bd0e7a467cc9409a

Guertin, A., Cunningham, C., Bouchez, J., Gelin, M., Chorover, J., Bauser, H., et al. (2024). Stable silicon isotope fractionation reflects the routing of water through a mesoscale hillslope. *Earth and Planetary Science Letters*, **648**, 119098. https://doi.org/10.1016/j.epsl.2024.119098

Harris, P. G. (1954). The distribution of germanium among coexisting phases of partly glassy rocks. *Geochimica et Cosmochimica Acta*, **5**(4), 185–195. https://doi.org/10.1016/0016-7037(54)90032-0

Hartmann, J., West, A. J., Renforth, P., Köhler, P., De La Rocha, C. L., Wolf-Gladrow, D. A., et al. (2013). Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. *Reviews of Geophysics*, **51**(2), 113–149. https://doi.org/10.1002/rog.20004

Hatton, J. E., Hendry, K. R., Hawkings, J. R., Wadham, J. L., Kohler, T. J., Stibal, M., et al. (2019). Investigation of subglacial weathering under the Greenland ice sheet using silicon isotopes. *Geochimica et Cosmochimica Acta*, **247**, 191–206. https://doi.org/10.1016/j.gca.2018.12.033

Hatton, J. E., Hendry, K. R., Hawkings, J. R., Wadham, J. L., Opfergelt, S., Kohler, T. J., et al. (2019). Silicon isotopes in Arctic and sub-Arctic glacial meltwaters: The role of subglacial weathering in the silicon cycle. *Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences*, **475**(2228), 20190098. https://doi.org/10.1098/rspa.2019.0098

Hindshaw, R. S., Rickli, J., Leuthold, J., Wadham, J., & Bourdon, B. (2014). Identifying weathering sources and processes in an outlet glacier of the Greenland ice sheet using Ca and Sr isotope ratios. *Geochimica et Cosmochimica Acta*, **145**, 50–71. https://doi.org/10.1016/j.gca.2014.09.016

Kantola, I. B., Blanc-Betes, E., Masters, M. D., Chang, E., Marklein, A., Moore, C. E., et al. (2023). Improved net carbon budgets in the US midwest through direct measured impacts of enhanced weathering. *Global Change Biology*, **29**(24), 7012–7028. https://doi.org/10.1111/gcb.16903

Kantzas, E. P., Val Martin, M., Lomas, M. R., Eufrasio, R. M., Renforth, P., Lewis, A. L., et al. (2022). Substantial carbon drawdown potential from enhanced rock weathering in the United Kingdom. *Nature Geoscience*, **15**(5), 382–389. https://doi.org/10.1038/s41561-022-00925-2
Kim, M., & Harman, C. J. (2022). Transit times and StorAge selection functions in idealized hillslopes with steady infiltration. *Water Resources Research*, 58(5), e2019WR025917. https://doi.org/10.1029/2019WR025917

Kim, M., & Troch, P. A. (2020). Transit time distributions estimation exploiting flow-weighted time: Theory and proof-of-concept. *Water Resources Research*, 56(12), e2020WR027186. https://doi.org/10.1029/2020WR027186

Kim, M., Volkmann, T. H. M., Bugaj, A., Wang, Y., Meira Neto, A. A., Matos, K., et al. (2021). Uncovering the hillslope scale flow and transport dynamics in an experimental hydrologic system. *Hydrological Processes*, 35(8), e14337. https://doi.org/10.1002/hyp.14337

Kurtz, A. C., Derry, L. A., & Chadwick, O. A. (2002). Germanium-silicon fractionation in the weathering environment. *Geochimica et Cosmochimica Acta*, 66(9), 1525–1537. https://doi.org/10.1016/S0016-7037(01)00869-9

Kurtz, A. C., Lugolobi, F., & Salvucci, G. (2011). Germanium-silicon as a flow path tracer: Application to the Rio Icacos watershed. *Water Resources Research*, 47(6), W06516. https://doi.org/10.1029/2010WR009853

Li, L., Li, G. K., Li, L., & Li, G. (2022). Landslide-induced weathering in tectonically active Mountains: Evidence from dissolved radiogenic uranium isotopes. *Frontiers in Earth Science*, 10, 825818. https://doi.org/10.3389/feart.2022.825818

López-Urzúa, S., Derry, L., & Bouchez, J. (2025). Quantifying the agricultural footprint on the silicon cycle: Insights from silicon isotopes and Ge/Si ratios. https://doi.org/10.5194/egusphere-2025-78

Lugolobi, F., Kurtz, A. C., & Derry, L. A. (2010). Germanium-silicon fractionation in a tropical, granitic weathering environment. *Geochimica et Cosmochimica Acta*, 74(4), 1294–1308. https://doi.org/10.1016/j.gca.2009.11.027

Millot, R., Vigier, N., & Gaillardet, J. (2010). Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada. *Geochimica et Cosmochimica Acta*, 74(14), 3897–3912. https://doi.org/10.1016/j.gca.2010.04.025

Mortlock, R. A., & Frohlich, P. N. (1987). Continental weathering of germanium: In the global river discharge. *Geochimica et Cosmochimica Acta*, 51(8), 2075–2082. https://doi.org/10.1016/0016-7037(87)90257-2

Murnane, R. J., & Stallard, R. F. (1990). Germanium and silicon in rivers of the Orinoco drainage basin. *Nature*, 344(6268), 749–752. https://doi.org/10.1038/344749a0

Opfergelt, S., Cardinal, D., André, L., Delvigne, C., Bremond, L., & Delvaux, B. (2010). Variations of δ³⁰Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture. *Geochimica et Cosmochimica Acta*, 74(1), 225–240. https://doi.org/10.1016/j.gca.2009.09.025

Pangle, L. A., DeLong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., et al. (2015). The landscape evolution observatory: A large-scale controllable infrastructure to study coupled Earth-surface processes. *Geomorphology*, 244, 190–203. https://doi.org/10.1016/j geomorph.2015.01.020

Pohlmann, M., Dontsova, K., Root, R., Ruiz, J., Troch, P., & Chorover, J. (2016). Pore water chemistry reveals gradients in mineral transformation across a model basaltic hillslope. *Geochemistry, Geophysics, Geosystems*, 17(6), 2054–2069. https://doi.org/10.1002/2016GC006270

Rickli, J., Hindshaw, R. S., Leuthold, J., Wadham, J. L., Burton, K. W., & Vance, D. (2017). Impact of glacial activity on the weathering of Hf isotopes – Observations from southwest Greenland. *Geochimica et Cosmochimica Acta*, 215, 295–316. https://doi.org/10.1016/j.gca.2017.08.005

Scribner, A. M., Kurtz, A. C., & Chadwick, O. A. (2006). Germanium sequestration by soil: Targeting the roles of secondary clays and Fe-oxyhydroxides. *Earth and Planetary Science Letters*, 243(3–4), 760–770. https://doi.org/10.1016/j.epsl.2006.01.051

Seifritz, W. (1990). CO₂ disposal by means of silicates. *Nature*, 345(6275), 486. https://doi.org/10.1038/345486b0

Volkmann, T. H. M., Sengupta, A., Pangle, L. A., Dontsova, K., Barron-Gafford, G. A., Harman, C. J., et al. (2018). Controlled experiments of hillslope coevolution at the biosphere 2 landscape evolution observatory: Toward prediction of coupled hydrological, biogeochemical, and ecological change. In *Hydrology of artificial and controlled experiments*. InTech. https://doi.org/10.5772/intechopen.72325

Wimpenny, J., Burton, K. W., James, R. H., Gannoun, A., Mokadem, F., & Gíslason, S. R. (2011). The behaviour of magnesium and its isotopes during glacial weathering in an ancient shield terrain in west Greenland. *Earth and Planetary Science Letters*, 304(1–2), 260–269. https://doi.org/10.1016/j.epsl.2011.02.008

Wimpenny, J., James, R. H., Burton, K. W., Gannoun, A., Mokadem, F., & Gíslason, S. R. (2010). Glacial effects on weathering processes: New insights from the elemental and lithium isotopic composition of west Greenland rivers. *Earth and Planetary Science Letters*, 290(3–4), 427–437. https://doi.org/10.1016/j.epsl.2009.12.042

Ziegler, K., Chadwick, O. A., Brzezinski, M. A., & Kelly, E. F. (2005). Natural variations of δ³⁰Si ratios during progressive basalt weathering, Hawaiian Islands. *Geochimica et Cosmochimica Acta*, 69(19), 4597–4610. https://doi.org/10.1016/j.gca.2005.05.008

Ziegler, K., Chadwick, O. A., White, A. F., & Brzezinski, M. A. (2005). δ³⁰Si systematics in a granitic saprolite, Puerto Rico. *Geology*, 33(10), 817. https://doi.org/10.1130/G21707.1

## References From the Supporting Information

Dontsova, K., Steefel, C. I., Desilets, S., Thompson, A., & Chorover, J. (2009). Solid phase evolution in the Biosphere 2 hillslope experiment as predicted by modeling of hydrologic and geochemical fluxes. *Hydrology and Earth System Sciences*, 13(12), 2273–2286. https://doi.org/10.5194/hess-13-2273-2009

Ibarra, D. E., Caves, J. K., Moon, S., Thomas, D. L., Hartmann, J., Chamberlain, C. P., & Maher, K. (2016). Differential weathering of basaltic and granitic catchments from concentration–discharge relationships. *Geochimica et Cosmochimica Acta*, 190, 265–293. https://doi.org/10.1016/j.gca.2016.07.006

Kim, M., Volkmann, T. H. M., Wang, Y., Meira Neto, A. A., Matos, K., Harman, C. J., & Troch, P. A. (2022). Direct observation of hillslope scale StorAge selection functions in experimental hydrologic systems: Geomorphologic structure and preferential discharge of old water. *Water Resources Research*, 58(3), e2020WR028959. https://doi.org/10.1029/2020WR028959

Wolery, T. J., Jackson, K. J., Bourcier, W. L., Bruton, C. J., Viani, B. E., Knauss, K. G., & Delany, J. M. (1990). Current status of the EQ3/6 software package for geochemical modeling (pp. 104–116). https://doi.org/10.1021/bk-1990-0416.ch008