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1. Chemical Weathering and Physical Erosion Fluxes From Serpentinite in Puerto Rico

   https://doi.org/10.1029/2024JF007776  

   Moore, Angus K; Méndez_Méndez, Kimberly; Hughes, K Stephen; Granger, Darryl E (January 2025, Journal of Geophysical Research: Earth Surface)

   Abstract Weathering of ultramafic rocks emplaced at low latitude during arc‐arc and arc‐continent collisions may provide an important sink for atmospheric CO₂over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ³⁶Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km⁻² yr⁻¹), chemical weathering fluxes are large (55–143 tons km⁻² yr⁻¹), and weathering intensities are among the highest yet reported for silicate‐rock weathering (up to 0.88). We use these data to parameterize power‐law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO₂consumption from the best‐fit power‐law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO₂consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time. 

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2. When Snowpacks Shrink: Declining Weathering and Export Rates but Rising Stream Solute Concentrations in Warming Mountains

   https://doi.org/10.1002/hyp.70539  

   Knapp, Abigail S; Stewart, Bryn; Kerins, Devon; Sullivan, Pamela L; Barnard, Holly R; Li, Li (May 2026, Hydrological Processes)

   ABSTRACT Silicate weathering produces solutes, consumes CO₂, and regulates Earth's long‐term climate. Rates of silicate weathering are changing, particularly in rapidly warming mountains with dwindling snowpacks. Although hydrologic shifts in warming mountains have been extensively studied, their impacts on water chemistry and weathering have been sparsely examined. Here we ask the question:How does changing mountain hydrology, in particular shrinking snowpacks and early snowmelt, alter silicate weathering, solute export, and stream chemistry?We answer this question using a catchment‐scale reactive transport model (BioRT‐HBV) calibrated with streamflow and chemistry data (2010–2020) from the Sagehen Creek Experimental Forest Catchment, a snow‐dominated headwater catchment in Sierra Nevada Mountains, CA, USA. We show that snow‐dependent water storage controls silicate weathering and solute export, especially in the shallow subsurface, contrasting the general perception that silicate weathering is slow and kinetically controlled with limited dependence on water storage. In the driest year with the least snow water equivalent (SWE, 10% of the wettest year), rates of silicate weathering and export rates of weathering products (such as calcium) were about half and a third of those in the wettest year with the highestSWE. In fact, annual rates of weathering and solute export scale linearly with annual total precipitation and maximumSWE. The timing of weathering and solute export closely followed that of snowmelt, with rates peaking about 3 months earlier in the driest years compared to those in the wettest years. Mean stream calcium concentrations however were about 1.6 times higher in the driest year, with highest concentrations in the dry summer. These results suggest that as mountains shift to a low‐to‐no snow future, catchments transition from exporters to reactors with a lower weathering capacity and even lower export capacity, leading to higher solute concentration in streams. 

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3. Concentration-runoff relationships of contrasting small mountainous rivers in the Pacific Northwest, USA: Insights into the weathering of rhenium relative to other weathering products

   https://doi.org/10.1016/j.gca.2022.09.036  

   Ghazi, Layla; Goñi, Miguel; Haley, Brian A.; Muratli, Jesse M.; Pett-Ridge, Julie C. (November 2022, Geochimica et Cosmochimica Acta)

   This study examines dissolved rhenium (Re) concentrations as a function of water runoff using river samples from two contrasting mountainous watersheds, the Eel and Umpqua Rivers in the Pacific Northwest, USA. These watersheds share many key characteristics in terms of size, discharge, climate, and vegetation, but they have a 15-fold difference in sediment yield due to differences in their tectonic setting and uplift and erosion rates. We evaluate concentration-runoff (C-R) relationships and ratios of coefficients of variation (CVC/CVR) for major cations, anions, dissolved inorganic carbon, selected trace elements including Re, and 87Sr/86Sr ratios. Recent research outlines the potential of Re to serve as a tracer for the oxidation of ancient/fossil organic matter because of its close association with petrogenic carbon (OCpetro) in rocks. In both the Eel and Umpqua Rivers, our measurements show that Re behaves similarly to major weathering derived-solutes corrected for atmospheric input, such as Ca2+*, Mg2+*, and Na+* with modest dilution across all tributaries with increasing runoff. Rhenium behaves dissimilarly from other trace elements, such as Mo and U, and is also dissimilar to biologically-cycled nutrients, such as NO3 – , PO4 3 , and K+*, suggesting differences in sources, solute generation mechanisms, and flowpaths. Rhenium behavior is also distinct from that of colloids, which have increasing concentrations with increasing runoff. We find that Re and sulfate corrected for atmospheric input (SO4 2 *) have distinct CR relationships, in which SO4 2 * undergoes greater dilution with increasing runoff. This implies that Re is not dominantly sourced from sulfide weathering, which leaves primary bedrock minerals and OCpetro hosted in bedrock of these watersheds as the likely dominant sources of dissolved Re release. At mean discharge, Re concentration in the Eel river (3.5 pmol L-1) is more than two times greater than Re concentrations in the Umpqua River (1.5 pmol L-1). Furthermore, comparison of two tributary watersheds with similar bedrock but marked differences in erosion rates show higher Re concentrations in Bull Creek (erosion rate of 0.5 mm yr 1) relative to Elder Creek (erosion rate of 0.2 mm yr 1). The results of this study suggest that dissolved Re in the Eel and Umpqua River basins is likely derived from primary mineral dissolution or OCpetro oxidation, and Re fluxes are higher in areas with higher erosion rates, suggesting that tectonic setting is one factor that controls Re release and therefore OCpetro oxidation. 

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4. Isotope Ratio – Discharge Relationships of Solutes Derived From Weathering Reactions

   https://doi.org/10.2475/001c.84469  

   Druhan, Jennifer L.; Benettin, Paolo (January 2023, American Journal of Science)

   To date, the vast majority of studies seeking to link discharge to solute concentrations have been based on representations of fluid age distributions in watersheds that are time-invariant. As increasingly detailed spatial and temporal datasets become available for weathering-derived riverine solute concentrations, the capacity to link this mass flux to transient routing of reactive fluids through Critical Zone environments is vital to quantitative interpretation. Relationships between fluid age distributions and the stable isotope ratios of these geogenic solutes are even less developed, yet these signatures are vital to parsing the suite of water-rock-life interactions that create concentration-discharge relationships. Here we offer the first merging of a hydrological model featuring time-variant fluid age distributions with a geochemical model for isotopically fractionating weathering reactions. Using SiO_2(aq)and the corresponding silicon isotope ratio $\delta$ ³⁰Si as an example, we show that the stable isotope signatures of riverine solutes produced by weathering reactions reflect a component of the fluid age distribution that is unique to the corresponding solute concentrations. This distinct sensitivity is the result of a stronger link between isotope ratios and the age distribution parameters describing a given watershed. This novel modeling framework is used to provide a quantitative basis for the interpretation of SiO_2(aq)and $\delta$ ³⁰Si in six low-order streams spread across a diversity of climates, geologies, and ecosystems. To our knowledge, this is the first forward and process-based model to describe the isotopic signatures of solutes derived from weathering reactions in watersheds subject to time-varying discharge. 

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5. Expanded constraints on chemical weathering reactions and carbon cycling beneath the Antarctic Ice Sheet

   https://doi.org/10.31223/X51H65  

   Piccione, Gavin; Blackburn, Terrence; Hain, Mathis; Tulaczyk, Slawek; Rafter, Patrick; Gagliardi, Jessica; Carney, Colin (August 2024, Earth ArXiv)

   Ice sheets may influence the global carbon cycle by releasing chemical weathering products and carbon from basal environments. However, limited data describing subglacial biogeochemical cycles beneath Antarctic and Greenland ice leaves open fundamental questions regarding the feedbacks between climate, ice sheets, and the carbon cycle. Most notably: does subglacial chemical weathering beneath ice sheets act as a source or sink of atmospheric CO2? Here, we present constraints on biogeochemical reactions beneath the Antarctic ice sheet using geochemical and geochronologic measurements of carbonate precipitates, which formed from basal waters in ten locations along the ice sheet margins and Transantarctic Mountains (TAM), since the late Miocene. Precipitate carbon and oxygen isotopic compositions reveal a consistent geographic pattern in subglacial carbon cycling and chemical weathering, where parent waters in catchments draining through the TAM source CO2 through microbial metabolism of organic matter that drives silicate weathering, while waters beneath the ice sheet periphery acquire carbon mainly through the dissolution of carbonate minerals. Proxies for parent water pH (P/Ca) and solute source (87Sr/86Sr) show that bedrock composition and reactivity are the main factors determining intensity of silicate weathering in TAM waters. While the dominance of subglacial silicate versus carbonate weathering is determined by the presence of carbonate minerals and/or basal water residence time, which can fluctuate across global climate cycles. Because carbonate and silicate dissolution have quantitively different effects on the carbon cycle, the balance between them dictates whether Antarctica acts as a positive or negative climate-carbon cycle feedback. 

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