Sebastian Muñoz

Rock weathering influences Earth's atmosphere and climate. The dissolution of silicate minerals draws down atmospheric CO ₂ while sulfide oxidation coupled with carbonate dissolution releases CO ₂ . Alpine glacier rock weathering causes high sulfide oxidation and carbonate dissolution. This influence on weathering suggests a potential negative climate feedback where ice growth leads to CO ₂ release, warming, and limitations on ice extent. However, the role continental ice sheets play in this feedback remains unclear. Here, we investigate global measurements of proglacial meltwater and find that ice sheets favor silicate weathering over sulfide oxidation and carbonate dissolution. This contrast in weathering mode from alpine glaciers and ice sheets means that the proportion of discharge coming from each determines their net effect on the carbon cycle, which has likely varied across Pleistocene glacial interglacial timescales. Additionally, we find that the influence of glacial weathering reactions is greatest during periods of increased ice loss.

The top meter of the Earth’s crust holds nearly equal amounts of rock-derived organic carbon (OC _petro ) and soil organic carbon. Due to this large reservoir size, the oxidation of OC _petro may be an important flux of CO ₂ to the atmosphere over geologic time. However, determining the magnitude and drivers of OC _petro oxidation remains difficult at the watershed scale. To understand the balance of OC sources eroded from a landscape and prone to breakdown, we measured the OC concentrations, stable isotope ratios, and the radiocarbon content of suspended sediment samples from the East River: a shale-dominated alpine watershed near Crested Butte, Colorado. Suspended sediment samples were studied because they integrate multiple sources of sediment across a watershed. With these OC measurements, we employ a Monte Carlo mass balance inversion to constrain inputs of carbon from plants, soil, and bedrock. We find that along the main stem of the river, δ13C values range from -27 to -26 ‰ VPDB and fraction modern OC (FmC) ranges from 0.73 to 0.95, suggesting millennial-scale floodplain residence times. In contrast, lighter δ13C values (-29 to -27 ‰) and older FmC values are observed within tributaries. Notably, samples collected at different river stages at the downstream-most locality exhibit the widest variability in carbon isotope data. A decrease in sediment OC abundances and FmC values with increasing discharge is likely the result of an increased proportion in bedrock carbon in transport. These data illuminate hydrologic and geomorphic controls on the balance of OC export, which has consequences for watershed-scale OC fluxes. Whereas the river main stem primarily transports young, labile, plant- and soil-derived organic carbon, small steeper tributaries appear to transport older, rock-derived organic carbon, likely due to erosion of bedrock. However, the relative contributions of bedrock and soil-derived carbon in floodplains can fluctuate in response to changing hydrologic states, which are influenced by climate. Ongoing measurements of rhenium and carbon isotopes on suspended sediment, bedrock, and soil will help refine endmembers and shed light on the role of bedrock OC in riverine carbon fluxes.

Some fraction of the total carbon (C) transported by rivers can enter the atmosphere as CO ₂ via gas evasion as water transits from source to sink. Quantifying this evaded portion can be challenging due to the need to constrain chemical and physical parameters along an entire stream network using a limited number of point measurements. To address this challenge, we employed an tracer‐enabled (C and ²²² Rn) reactive transport model to simulate CO ₂ evasion along an entire stream network in the Little Deschutes River in the Eastern Cascades, Oregon, USA. We sampled the river network in distinct lanscape regimes and measured potential C sources including soil gas, groundwater springs, and wetland waters. Using these data, we first evaluated the reactive transport model using empirical gas transfer scaling relationships and measured groundwater chemistry. We then employed a Monte‐Carlo optimization using riverine observations of , C and ²²² Rn, which yielded more accurate estimates of CO ₂ evasion by improving estimates of spatially‐averaged groundwater pCO ₂ and generating a site‐specific gas transfer scaling relationship. Our results demonstrate that riparian wetlands contribute to 19% of the computed CO ₂ evasion flux. Lastly, we find that CO ₂ evasion only accounts for 12% of the total riverine C flux, with the remaining fraction contributed by advective flux of DIC (50%) and DOC (38%) through the watershed outlet.

High latitude regions across the globe are undergoing severe modifications due to changing climate. A high latitude region of concern is the Gulf of Alaska (GoA), where these changes in hydroclimate undoubtedly affect the hydrogeochemistry of freshwater discharging to the nearshore ecosystems of the region. To fill the knowledge gap of our understanding of freshwater stream geochemistry with the GoA, we compile stream water chemistry data from 162 stream sites across the region. With an inverse model, we estimate fractional contributions to solute fluxes from weathering of silicate, carbonate, and sulfide minerals, and precipitation. We assess weathering rates across the region and compare them against global river yields. The median fractional contribution of carbonate weathering to total weathering products is 78% across all stream sites; however, there are several streams where silicate weathering is a dominant source of solutes. Weathering by sulfuric acid is elevated in glacierized watersheds. Finally, cation weathering rates are lower in GoA streams compared to the world's largest rivers; however, weathering rates are similar when compared to a global dataset of glacier fed streams. We suggest that hydrologic changes driven by glacier ice loss and increased precipitation will alter river water quality and chemical weathering regimes such that silicate weathering may become a more important source of solutes and sulfide oxidation may decrease. This contribution provides a platform to build from for future investigations into changes to stream water chemistry in the region and other high latitude watersheds.

Rock weathering impacts atmospheric CO₂ levels with silicate rock dissolution removing CO_2, and carbonate dissolution, pyrite oxidation, and organic rock carbon oxidation producing CO₂. Glacierization impacts the hydrology and geomorphology of catchments and glacier retreat due to warming can increase runoff and initiate landscape succession. To investigate the impact of these changes on catchment scale weathering CO₂ balances, we report monthly samples of solute chemistry and continuous discharge records for a sequence of glacierized watersheds draining into Kachemak Bay, Alaska. We partition solute and acid sources and estimate inorganic weathering CO₂ balances using an inverse geochemical mixing model. Furthermore, we investigated how solutes vary with discharge conditions utilizing a concentration‐runoff framework. We develop an analogous fraction‐runoff framework which allows us to investigate changes in weathering contributions at different flows. Fraction‐runoff relationships suggest kinetic limitations on all reactions in glacierized catchments, and only silicate weathering in less glacierized catchments. Using forest cover as a proxy for landscape age and stability, multiple linear regression shows that faster reactions (pyrite oxidation) contribute less to the solute load with increasing forest cover, whereas silicate weathering (slow reaction kinetics) contributes more. Overall, in glacierized catchments, we find elevated weathering fluxes at high runoff despite significant dilution effects. This makes flux estimates that account for dilution more important in glacierized catchments. Our findings quantify how glaciers modify the inorganic weathering CO₂ balance of catchments through hydrologic and geomorphic forcings, and support the previous hypothesis that deglaciation will be accompanied by a shift in inorganic weathering CO₂ balances.