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1. Rapid Forward and Reverse Weathering Reactions Drive Cryptic Silica and Cation Cycling in Arctic Fjord Sediments

   https://doi.org/10.1029/2025GB008650  

   Wehrmann, Laura M; Aller, Robert C; Kasten, Sabine; Dotzler, Jack; Steinhoefel, Grit (December 2025, Global Biogeochemical Cycles)

   Early diagenetic forward and reverse weathering reactions play a significant role in controlling alkalinity fluxes and silica, alkali metal and alkaline earth metal cycling in coastal systems. In Kongsfjorden, Svalbard, the inputs of autochthonous biogenic debris (diatomaceous silica) and allochthonous lithogenic material of varying reactivity (dominated by clays, especially illite and chlorite, and primary aluminosilicates, mostly plagioclase) drive complex balances of diagenetic silicate reactions in sediments. The rapid dissolution of reactive silica results in the release of dissolved silica (Si_d) into pore‐waters and sustains elevated benthic Si_dfluxes (−0.2 to −0.8 mmol m⁻² d⁻¹), which are on the upper end of values previously determined for Arctic environments. Increases with depth in pore‐water lithium (Li⁺), potassium, magnesium, and barium concentrations within the top centimeters provided evidence for forward weathering of clays quickly upon burial. Due to the prevalent occurrence of forward weathering, the benthic net Li⁺flux was associated with a light isotope signal. Decreases in pore‐water rubidium concentrations with depth at the near‐glacier station, elevated ratios of the authigenically altered silica to the biogenic silica pool at all sites, and small increases of pore‐water δ⁷Li values with depth showed that reverse weathering also takes place. Anoxic incubation of diatom frustule probes provided further evidence for the neoformation of cation‐rich clays. The superposition of reverse and forward weathering results in cryptic silica and cation cycling that muted net benthic fluxes. In deeper sediments, changes in pore‐water solute patterns indicated an interconnected occurrence of reverse and forward weathering, potentially driven by reactive silica‐limitation. 

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2. Lithium isotope evidence for a plumeworld ocean in the aftermath of the Marinoan snowball Earth

   https://doi.org/10.1073/pnas.2407419121  

   Gan, Tian; Tian, Meng; Wang, Xi-Kai; Wang, Shijie; Liu, Xiao-Ming; Jiang, Ganqing; C_Gill, Benjamin; Nolan, Morrison; Kaufman, Alan J; Luo, Taiyi; et al (November 2024, Proceedings of the National Academy of Sciences)

   The snowball Earth hypothesis predicts that continental chemical weathering diminished substantially during, but rebounded strongly after, the Marinoan ice age some 635 Mya. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition from underlying hypersaline seawater, generating both vertical and lateral salinity gradients. Here, we test the plumeworld hypothesis using lithium isotope compositions in the Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along a proximal–distal (nearshore–offshore) transect in South China. Our data show an overall decreasing δ⁷Li trend with distance from the shoreline, consistent with the variable mixing of a meltwater plume with high δ⁷Li (due to incongruent silicate weathering on the continent) and hypersaline seawater with low δ⁷Li (due to synglacial distillation). The evolution of low δ⁷Li of synglacial seawater, as opposed to the modern oceans with high δ⁷Li, was likely driven by weak continental chemical weathering coupled with strong reverse weathering on the seafloor underneath silica-rich oceans. The spatial pattern of δ⁷Li is also consistent with the development and then collapse of the meltwater plume that occurred at the time scale of cap dolostone accumulation. Therefore, the δ⁷Li data are consistent with the plumeworld hypothesis, considerably reduced chemical weathering on the continent during the Marinoan snowball Earth, and enhanced reverse weathering on the seafloor of Precambrian oceans. 

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3. Maturation of Silicate Weathering Pathways Revealed by Germanium‐Silicon Ratios

   https://doi.org/10.1029/2025GL118410  

   Guertin, Andrew; Cunningham, Charlie; Bouchez, Julien; Gelin, Marine; Chorover, Jon; Derry, Louis A; Druhan, Jennifer L (October 2025, Geophysical Research Letters)

   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. 

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4. Biogeochemical reactivity in the unsaturated zone revealed by in-situ measurements

   https://doi.org/10.3897/aca.8.e155241  

   Osorio-Leon, Ivan; Rempe, Daniella; Bouchez, Julien; Druhan, Jennifer (May 2025, ARPHA Conference Abstracts)

   Bedrock mineral breakdown in mountainous landscapes sustains long-term atmospheric CO₂ drawdown and releases solutes that sustain ecosystems and set the chemistry of upland stream water. In the unsaturated Bedrock Vadose Zone (BVZ) of hillslopes, minerals, water, reactive gases, and deep roots interact to produce a hotspot of these weathering reactions. New evidence suggests that, in the BVZ, rock moisture sustains evapotranspiration, while deep roots drive deep CO₂ production. However, the contribution of the BVZ to catchment-scale chemical weathering fluxes, and particularly the role of deep roots as drivers of this reactivity, remains poorly constrained due to the difficulty of direct access and measurement of fluids in this compartment. Quantifying this BVZ chemical reactivity is crucial to advance our understanding of upland landscapes' influence on the global carbon cycle and river solute fluxes. Here, we leverage an innovative Vadose-zone Monitoring System (VMS) to investigate these processes and their biogeochemical implications. We present four years of in-situ monitoring of water and pore gas composition in a ~16-meter weathered bedrock profile at the Eel River Critical Zone Observatory (ERCZO) in California. Using this novel instrumentation, we show that the average geogenic chemical solute efflux from the BVZ is 80 ± 34 t km⁻² yr⁻¹, representing approximately 68% of the solute flux measured in the creek draining the hillslope. This highlights the substantial role of the BVZ in catchment-scale weathering. To further interpret these data, we develop a multi-component reactive transport model (RTM) calibrated with batch-reactor experiments using drilling cuttings from the hillslope. Results indicate that solute concentrations arise from a coupled set of primary mineral dissolution and secondary mineral precipitation reactions, which evolve with depth and align with the mineralogical composition gradient from soil to fresh bedrock. Our RTM demonstrates that water storage and drainage timescales in the vadose zone are sufficient to allow significant evolution of pore water due to chemical weathering. However, the model cannot reproduce observed solute concentrations unless elevated CO₂ from deep rhizosphere respiration is incorporated. We find that organic carbon respiration in the deep rhizosphere enhances chemical weathering fluxes at the ERCZO by a factor of ~1.8 ± 0.5, providing direct evidence of accelerated chemical weathering in the Critical Zone due to the presence and function of deeply rooted vegetation. A key characteristic of the linkage between deep rhizosphere respiration and weathering reactions is that, despite strong seasonal variations in water content and O₂ profiles, CO₂ concentrations in the BVZ remain stable, leading to steady weathering rates across seasons and years. Sensitivity analyses with our calibrated RTM reveal that this stability results from the interplay between labile organic carbon oxidation and seasonal hydrological conditions. During dry periods, O₂ is abundant throughout the BVZ due to opening of pore space, but DOC availability is constrained by low water content. Conversely, wet periods enhance availability of labile organic carbon, but O₂ becomes limiting. This balance maintains stable CO₂ profiles and, ultimately, steady weathering rates. We propose that this buffering mechanism allows watersheds to modulate seasonal variability in ecological, hydrological, and climatic inputs, contributing to the pervasive observation of stable concentrations of geogenic solutes in streams draining these landscapes despite their strong variations in discharge. These findings underscore the critical role of the BVZ in regulating solute fluxes from hillslopes, and suggest that deep vadose zone processes must be integrated into biogeochemical and hydrological models to accurately predict catchment responses to environmental change. 

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5. Signatures of the deep rhizosphere: Novel instrumentation and predictive models

   https://doi.org/10.3897/aca.8.e156623  

   Druhan, Jennifer; Osorio-Leon, Ivan; Benettin, Paolo; Rempe, Daniella; Bouchez, Julien (May 2025, ARPHA Conference Abstracts)

   Vegetation simultaneously drives transpiration, a significant component of the hydrological balance, and stimulates the breakdown of bedrock and formation of soil. Together these actions impact both the magnitude of streamflow and the chemical or solute load of the stream. Yet, it is still unknown under what conditions deeply rooted plants enhance or impede chemical weathering of rocks. An unestablished link at the heart of this gap in knowledge are the ways in which coupling between plant water demand, plant nutrient demand and recycling of these elements through the ecosystem manifest in the geochemical composition of watersheds and the rivers that drain them. Addressing this unknown is of utmost practical importance to water resource management, environmental stewardship, ecosystem resilience to disturbance (storms, fire, drought), and ultimately nutrient effluxes from watersheds. This presentation introduces two recent advancements in our collective capacity to deconvolve these vital linkages between ecology, hydrology and chemical weathering: novel observational tools and ecologically informed chemical weathering models. First, we present hydrologic and geochemical observations from within the deep root-zone gained from the successful deployment of a Vadose zone Monitoring System (VMS). The VMS allows for real-time moisture content monitoring as well as discrete sampling of water and reactive gases across partially saturated bedrock.This novel capability has now revealed that the mature, deeply rooted forest relies on water stored in bedrock above the water table during the extended dry season. The VMS has also shown CO₂concentrations and production rates in the deep root zone comparable to what is typically observed in shallow soils. This deep CO₂is radiocarbon modern and thus associated with recent photosynthetically fixed carbon. Water chemistry observations from these depths indicate that this CO₂production in the deep root-zone enhances chemical weathering by increasing carbonic acid formation. Thus, the extension of water and carbon fluxes to depths meters below soils leads to a hotspot of chemical weathering in the deep root zone where meteoric water, carbonic acid weathering potential, and primary minerals all intersect. Second, we present the derivation and testing of a new ecologically informed reactive transport model (RTM) which directly simulates the uptake of both water and nutrients across the deep rhizosphere. A key aspect of our modeling framework is that, unlike transpired water, rock-derived nutrients taken up by plants are not lost to the atmosphere but rather recycled into litter and soils, creating a biological pool in the Critical Zone cycling of rock-derived nutrients. Our model leads to the hypothesis that the water and nutrient demands of ecosystems, coupled with the capacity to partially or fully recycle elements to the soil surface, regulate the observed rates and depth of chemical weathering reactions. These results reveal the capacity for plant water and nutrient demands to both enhance and impede mineral weathering reactions, to drive formation of secondary minerals, and to redistribute elements across the vertical weathering profile. Ultimately, our model allows us to demonstrate how plant water and nutrient requirements manifest in the export of water and solutes by streams. The development of this forward model in tandem with critical advancements in direct observation and sampling of the deep rhizosphere is now poised to provide a foundation upon which to improve our understanding of reactive transport processes in watersheds and Critical Zone systems, which in turn supports advancements in ecohydrology, global elemental budgets, watershed stewardship, and water quality resources. 

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