Peat wetlands at high latitudes have been a key focus for geochemical and hydrologic research due to their roles in carbon storage and greenhouse gas emissions. At the local scale, fens facilitate nutrient cycling and methane production via microbial processes. At the watershed scale, their distribution and hydrologic connectivity with surface waters can substantially influence water quality. However, the role of mid-latitude montane fens in moderating watershed functions remains understudied, despite their susceptibility to pronounced redox shifts driven by high-elevation seasonal dynamics. We conducted a comprehensive analysis of (1) base cations, (2) biologically active, redox-sensitive nutrients and metals, and (3) carbon compounds in Rocky Mountain fens and surface waters along an elevational gradient in the East River watershed (Upper Gunnison Basin, Colorado, USA). Our aim was to evaluate whether fens act as passive transporters or active transformers of these solute groups. Fen base cation chemistry was largely indistinguishable from surface water and groundwater, suggesting transport-dominated behavior controlled by bedrock lithology. In contrast, anoxic conditions in fens produced distinct redox-sensitive nutrient signatures. A quasi-linear relationship between Cl-normalized NO3 2- and PO4 3- in high-elevation fens and surface waters suggests a possible mixing signal originating from nutrient-enriched fen waters and more dilute surface or groundwater sources. Stable isotope data for dissolved methane, unlike conservative cations, indicate active carbon transformation via methanogenesis, with limited microbial methane oxidation. Collectively, our study offers an integrated assessment of the aqueous geochemistry of montane fens and their potential to modulate surface water chemistry across mid-latitude alpine watersheds.

Mountainous watersheds are characterized by variability in functional traits, including vegetation, topography, geology, and geomorphology, which together determine nitrogen (N) retention, and release. Coal Creek and East River are two contrasting catchments within the Upper Colorado River Basin that differ markedly in total nitrate (NO3-) export. The East River has a diverse vegetation cover, sinuous floodplains, and is underlain by N-rich marine shale, resulting in a three to twelve times greater total NO3- export relative to the conifer-dominated Coal Creek. While this can partly be explained by the larger size of the East River, the distinct watershed traits of these two catchments imply different mechanisms controlling the aggregate N-export signal. A causality analysis shows biogenic and geogenic processes were critical in determining NO3- export from the East River catchment. Stable isotope ratios of NO3- (δ15NNO3 and δ18ONO3) show the East River catchment is a strong hotspot for biogeochemical processing of NO3- at the soil-saprolite interface and within the floodplain prior to export. By contrast, the conifer-dominated Coal Creek retained nearly all (~97 %) atmospherically-deposited NO3-, and its export was controlled by catchment hydrological traits (i.e., snowmelt periods and water table depth). The conservative N-cycle within Coal Creek is likely due to the abundance of conifer trees, and a smaller riparian region, retaining more NO3- overall and reduced processing prior to export. This study highlights the value of integrating isotope systematics to link watershed functional traits to mechanisms of watershed element retention and release.

Methane clumped isotope compositions signify the relative natural abundances of rare, doubly substituted isotopic species of methane (13CH3D and 12CH2D2) and have emerged as a new isotopic tool to trace the sources, sinks, and lifecycles of methane in the environment. Such measurements can identify equilibration (or reequilibration) temperatures if found to be in isotopic equilibrium or non-equilibrium processes (e.g., kinetically controlled reactions or mixing) if not in isotopic equilibrium. Naturally occurring thermogenic methane-formed by the thermally activated breakdown of larger organic molecules-has been found to have clumped isotope compositions consistent with equilibrium at reasonable gas formation temperatures in some settings and non-equilibrium processes occurring during either formation, migration, storage, or extraction in others. To explore the potential controls on the isotopic composition of thermogenic methane, we conducted isothermal time-series ethane pyrolysis experiments at 550 and 600 & DEG;C to measure methane and ethane 13C/12C and D/H fractionations and methane clumped isotope compositions (resolved 13CH3D and 12CH2D2). We explore the effects of modifying the initial clumped isotope composition of ethane and the addition of water vapor to pyrolysis experiments. We observe that ethane and methane 13C/12C are controlled by kinetic isotope effects and Rayleigh distillation processes. In contrast, ethane and methane D/H and methane clumped isotope compositions appear to be controlled by a combination of these processes and hydrogen isotope exchange. The hydrogen isotope exchange processes lead to isotopic equilibrium as reaction completion is approached for both D/H (ethane/methane) and methane clumped isotope compositions. We develop a chemical model based on a mass balance approach that accounts for inheritance vs. hydrogen-abstraction formation pathways for singly and doubly substituted isotopologues of ethane and methane that is compared to the experimental data. The model allows the determination of carbon and hydrogen kinetic isotope effects associated with ethane cracking and hydrogen abstraction reactions that, where applicable, we compare to prior theoretical constraints. From the comparison of the model to the experimental data, we infer that the kinetically controlled ethane and methane bulk isotope compositions and methane clumped isotope compositions are controlled by kinetic isotope effects (both primary and secondary) associated with both C-C bond and C-H bond cleavage reactions. Specifically, the methane clumped isotope compositions likely result from a combination of clumped isotope effects associated with ethane breakdown and/or assembly of methane isotopologues (expressed in terms of & gamma;-factor parameters =/ 1) and combinatorial effects that arise probabilistically. We discuss our experimental results in the context of recent pyrolysis experiments and observations of naturally occurring thermogenic methane. We consider a proposal consistent with observations from nature that the hydrogen isotope exchange reactions that promote equilibration of methane isotopic molecules at or near formation temperature may be facilitated by free radicals generated by pyrolysis reactions. In this framework, isotope exchange effectively ceases when pyrolysis effectively ceases locking in compositions that can be consistent with peak formation temperatures.

The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low-and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376-420 degrees C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the dD of CH4 over the time course of an experiment. Regression of derived second order rate constants (k(r)) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(k(r)) =-17.32 (+/- 4.08, 1 s.e.) x 1000/T + 3.19 (+/- 6.01, 1 s.e.) (units of k(r) of sec(-1) [mol/L](-1)), equivalent to an activation energy of 144.0 +/- 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100-125 degrees C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 degrees C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur. (C) 2022 The Authors. Published by Elsevier Ltd.

Sulfur (S) is an essential macronutrient and important component of the earth’s crust, and its cycling has critical impacts on trace metal mobility, water quality, and human health. Pyrite weathering is the primary pathway by which sulfur enters surface waters. However, biogeochemical cycling of sulfur in soils and the river corridor mediates sulfate exports. In this study, we identified the major forms of sulfur across multiple compartments and scales in a pristine mountainous watershed, including shale bedrock weathering profiles, hillslope soils, and alluvial floodplain sediments, in order to provide insight into biogeochemical sulfur cycling in a hydrologically variable alpine system. X‐ray absorption near‐edge spectroscopy (XANES) analysis of shale weathering profiles showed clear evidence of pyrite oxidation to sulfate, with large accumulations of intermediate S(0) (20%–53%). Micro‐scale XANES showed evidence of reprecipitation of pyrite at fracture surfaces within the permanently saturated zone. Organic sulfur dominated S speciation in shallow hillslope soil and floodplain sediment, with little evidence of reduced inorganic S. However, mackinawite formation, representing active sulfate reduction, was observed in saturated oxbow sediments and saturated weathered shale underlying floodplain sediments. Further evidence of sulfate reduction from aqueous sulfur isotopic analysis was observed in shallow groundwater transects across an Fe‐reducing meander, whereas increases in pore water sulfate concentrations implied sulfur oxidation at other locations. The data present an integrated picture of sulfur cycling in a shale‐dominated watershed, where riverine sulfate exports are mediated by biological cycling, particularly in redox‐stratified and temporally dynamic hyporheic zone sediments.

The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ``floes" forming at the atmosphere-water interface, "snow" of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5-12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in delta C-13 (-9 to -28 parts per thousand VPDB) and delta(18) O (0 to -20 parts per thousand VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the delta C-13-delta O-18 endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (similar to 0.5 km(2)) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction-diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe Delta Ca-44 data (similar to 0.75 +/- 0.07%), direct mineral growth rate measurements (4.8 to 8 x 10(-7) mol/m(2)/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (%) = (alpha - 1) x 1000) accompanying CO2 hydroxylation, estimated here to be -17.1 +/- 0.8 parts per thousand (vs. CO2(aq)) for carbon and -7.1 +/- 1.1% (vs. 'CO2(aq)+ H2O') for oxygen at 17.4 +/- 1.0 degrees C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of -17.0 +/- 0.3 parts per thousand (vs. CO2(aq)) for carbon and -6.8 +/- 0.8% for oxygen (vs. 'CO2(aq)+ H2O') over the 17-28 degrees C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO3-, pH = 7.8-8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate delta C-13 and delta O-18 values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water delta O-18 and atmospheric delta C-13 values. There is evidence that older travertine at The Cedars recorded atmospheric delta C-13 that predates large-scale combustion of fossil fuels. (C) 2021 Elsevier Ltd. All rights reserved.

Recent studies in snowmelt-dominated catchments have documented changes in nitrogen (N) retention over time, such as declines in watershed exports of N, though there is a limited understanding of the controlling processes driving these trends. Working in the mountainous headwater East River Colorado watershed, we explored the effects of riparian hollows as N-cycling hotspots and as important small-scale controls on observed watershed trends. Using a modeling-based approach informed by remote sensing and in situ observations, we simulated the N-retention capacity of riparian hollows with seasonal and yearly hydrobiogeochemical perturbations imposed as drivers. We then implemented a scaling approach to quantify the relative contribution of riparian hollows to the total river corridor N budget. We found that riparian hollows primarily serve as N sinks, with N-transformation rates significantly limited by periods of enhanced groundwater upwelling and promoted at the onset of rainfall events. Given these observed hydrologic controls, we expect that the nitrate ( NO3- ) sink capacity of riparian hollows will increase in magnitude with future climatic perturbations, specifically the shift to more frequent rainfall events and fewer snowmelt events, as projected for many mountainous headwater catchments. Our current estimates suggest that while riparian hollows provision ~5–20% of NO3- to the river network, they functionally act as inhibitors to upland NO3- reaching the stream. Our work linking transient hydrological conditions to numerical biogeochemical simulations is an important step in assessing N-retaining features relative to the watershed N budget and better understanding the role of small-scale features within watersheds.

The stable isotopic composition of methane (CH4) is commonly used to fingerprint natural gas origins. Over the past 50 years, there have been numerous proposals that both microbial and thermogenic CH4 can form in or later attain hydrogen isotopic equilibrium with water (H2O) and carbon isotopic equilibrium with carbon dioxide (CO2). Evaluation of such proposals requires knowledge of the equilibrium fractionation factors between CH4 and H2O or CO2 at the temperatures where microbial and thermogenic CH4 form in or are found in the environment, which is generally less than 200 degrees C. Experimental determinations of these fractionation factors are only available above 200 degrees C, requiring extrapolation of these results beyond the calibrated range or the use of theoretical calculations at lower temperatures. Here, we provide a calibration of the equilibrium hydrogen isotopic fractionation factor for CH4 and hydrogen gas (H-2) ((D)alpha(CH4( g)-H2(g))) based on experiments using gamma-Al2O3 and Ni catalysts from 3 to 200 degrees C. Results were regressed as a 2nd order polynomial of 1000 x ln(D) alpha(CH4(g)-H2(g)) vs. 1/T (K-1) yielding: 1000 x ln D alpha(CH4(g)-H2(g)) = 3.5317x10(7)/T-2 + 2.7749x10(5)/T - 179.48 We combine this calibration with previous experimental determinations of hydrogen isotope equilibrium between H-2, H2O(g), and H2O(l) and we provide an interpolatable experimental calibration of 1000 x ln(D) alpha(CH4( g)-H2O(l)) from 3 to 200 degrees C. Our resulting 4th order polynomial is the following equation: 1000 x ln(D) alpha(CH4(g)-H2O(l)) = -7.9443x10(12)/T-4 + 8.7772x10(10)/T-3 + 3.4973x08/T-2 + 5.4398x10(5)/T - 382.05 At 3 degrees C, the value from our calibration differs by 93% relative to what would be calculated based on the extrapolation of the only experimental calibration currently available to temperatures below its calibrated range (lowest temperature of 200 degrees C; Horibe and Craig, 1995). We additionally provide new theoretical estimates of hydrogen isotopic equilibrium between CH4(g), H-2(g), and H2O(g) and carbon isotopic equilibrium between CH4(g) and CO2(g) using Path Integral Monte Carlo (PIMC) calculations. Our PIMC calculations for hydrogen isotopic equilibrium between CH4 and H-2 agree 1:1 with our experiments. Finally, we compile carbon and hydrogen isotopic measurements of CH4, CO2, and H2O from various environmental systems and compare observed differences between carbon and hydrogen isotopes to those expected based on isotopic equilibrium. We find that isotopic compositions of some microbial gases from marine sedimentary, coalbed, and shale environments are consistent with those expected for CH(4)AH(2)O(l) hydrogen and CH(4)ACO(2) carbon isotopic equilibrium. In contrast, microbial terrestrial and pure culture gases are not consistent with both CH(4)AH(2)O(l) hydrogen and CH(4)ACO(2) carbon isotopic equilibrium. These results are explained qualitatively using previously developed conceptual models that link free energy gradients available to microorganisms to the degree that their enzymes can promote isotope-exchange reactions between CH4, CO2, and H2O. (C) 2021 Published by Elsevier Ltd.

Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that, at this site, bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification probably occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadose zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that, under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle. Weathering of deep bedrock releases reactive nitrogen into the subsurface, which contributes to the flux of nitrous oxide to the atmosphere, according to a field study that combines soil, rock and groundwater data within a river catchment.

In hyperalkaline (pH>10) fluids that have participated in low-temperature (degrees C) serpentinization reactions, the dominant form of C is often methane (CH4), but the origin of this CH4 is uncertain. To assess CH4 origin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid-hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (CH313D and CH212D2) relative abundances less than equilibrium, consistent with substantial microbial CH4 production. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations and delta 13CCH4 across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source of CH4 is evidenced by the presence of CH4-bearing fluid inclusions in the Samail Ophiolite and our measurement of high delta 13C values of ethane and propane, which are similar to those reported in studies of CH4-rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption of CH4 in the ophiolite may further enrich CH4 in C-13. We conclude that substantial microbial CH4 is produced under varying degrees of C limitation and mixes with abiotic CH4 released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.

Mineral weathering plays a primary role in the geologic carbon cycle. Silicate weathering by carbonic acid consumes CO2 and stabilizes Earth's climate system. However, when sulfuric acid drives weathering, CO2 can be released to the atmosphere. Recent work has established that sulfuric acid weathering resulting from sulfide mineral oxidation is globally significant and particularly important in rapidly eroding environments. In contrast, if SO42- produced by sulfide oxidation is reduced during continental transit, then CO2 release may be negated. Yet, little is known about how much SO42- reduction takes place in terrestrial environments. We report oxygen and sulfur stable isotope ratios of SO42- in river waters and mass budget calculations, which together suggest that SO42- released from pyrite oxidation in the Peruvian Andes mountains is conservatively exported across similar to 300 km of the Amazon floodplain. In this system, floodplain SO42- reduction does not counteract the large SO42- flux from Andean pyrite weathering or measurably affect the stable isotope composition of riverine SO42-. These findings support the hypothesis that uplift and erosion of sedimentary rocks drive release of CO2 from the rock reservoir to the atmosphere.

Atmospheric nitrous oxide (N2O) contributes directly to global warming, yet current models1-5 overlook bedrock-contained nitrogen (rock-N), the largest terrestrial N pool6, as a N2O source. Although rock-N release rates are large6-9, incomplete understanding on the fate of released rock-N has obscured connections between rock-N and atmospheric N2O. This connection emerged through our field studies of a hillslope underlain by marine shale. Bedrock weathering within the zone of the seasonally fluctuating water table controls the weathering depth, hence the release of rock-N. At this site, rock-N weathering contributes 78% of the subsurface reactive-N, with ~22% derived from atmospheric deposition and biological nitrogen fixation, commonly regarded as the sole sources of reactive-N in pristine environments10,11. About 56% of reactive-N denitrifies, including 14% emitted as N2O into the atmosphere. The remaining reactive-N discharges in porewaters to a floodplain where additional denitrification likely occurs. Using global rock-N releases of 11–18 Tg y-1 8, our measurements extrapolate to a weathering driven efflux of 1.3–2.1 Tg N-N2O y-1, consistent with a flux of 1.0–1.7 Tg N- N2O y-1 solely derived from the literature. Thus, bedrock weathering contributes approximately 10-17 % of nitrous oxide to the current global estimate of ~10 Tg y-1.

Mountainous terrains exhibit rapid weathering rates due to physical erosion that supplies fresh minerals. When this weathering involves carbonic acid, it leads to CO 2 consumption. Sulfide minerals such as pyrite are common in rapidly eroding environments, and when oxidized produce sulfuric acid. As a weathering agent sulfuric acid can lead to CO 2 release if carbonate rocks are weathered. When sulfate released from pyrite oxidation is reduced to H 2 S or a secondary sulfide mineral, some of the associated CO 2 release is negated. Previous work has shown that the majority of weathering in the Andes is from sulfuric acid (and therefore releases CO 2 ), while in the Amazon floodplain the majority of weathering is from carbonic acid (and therefore consumes CO 2 ). The open question remains, however, of what happens to the sulfate that is released from pyrite oxidation as it moves from its mountainous origin across a tropical floodplain. Here, we employ dual isotopic measurements of aqueous sulfate ( d 34 S and d 18 O) to understand the fate of sulfate released from pyrite oxidation and the associated carbon cycle implications. We employ three different interpretive frameworks (dual isotopes, oxygen isotope gradient and sulfate mass balance) to show that sulfate released from pyrite oxidation in the Andes does not undergo any quantitatively significant redox recycling of sulfate – and therefore no negation of CO 2 release. Our results support the hypothesis that uplift and erosion of sedimentary rocks rich in sulfide minerals drives the release of CO 2 on geologic time scales.

Shales contain high levels of organic carbon (OC) and represent a large fraction of the Earth's reduced carbon stocks. While recent evidence suggests that shale-derived OC may be actively cycled in riverine systems, this process is poorly understood and not currently considered in global C models. Through the use of sediment density fractionations, extractions, radiocarbon measurements, and chemical characterization, we provide information on the abundance, chemistry, and mobility of shale-derived OC in floodplain sediments of a shale-rich mountainous watershed. The heavy fraction of the sediment, representing mineral-associated OC, is the largest (84 6% of TOC) and oldest (Delta C-14 values -224 to -853) OC pool. Evidence of shale-derived OC is observed in all sediment C pools (i.e., occluded light fraction, water-soluble, and pyrophosphate-extractable) except the free light fraction, which is entirely modern. Relatively consistent chemistry was observed across samples for extracted and density-separated OC, despite wide ranges of Delta C-14 values. Carbon spectroscopy revealed that floodplain sediments had a higher degree of functionalized aromatic groups and lower carbonate content compared to shale collected nearby, consistent with chemical alteration and mixing with other C sources in the floodplain. We estimate that approximately 23-34% of sediment OC is derived from shale, with implications for other shale-derived elements (e.g., N). This study demonstrates the important contribution of shale-OC, particularly in environments with low litter inputs. The large impact of radiocarbon-dead shale-OC, which has a thermally altered chemical structure distinct from plant litter, on Delta C-14 values and reactivity of sediment-OC must be considered. Plain Language Summary Shales contain high levels of organic carbon (OC) and represent a large fraction of the Earth's total carbon stocks. While recent evidence suggests that shale-derived OC, which is millions of years old, may be actively cycled in riverine systems, this process is poorly understood and not currently considered in global C models. In this study, we analyze sediments collected from the floodplain of the East River, CO, located in a high-elevation mountainous watershed underlain by shale bedrock, to determine the importance and mobility of shale-derived OC in this environment. OC closely associated with sediment minerals is the largest (84 6%) and oldest OC pool, containing a large, but variable, amount of shale-derived OC. Evidence of shale-derived OC is also observed in other sediment OC pools which are considered to be more mobile and more easily degraded to carbon dioxide by bacteria (e.g., water-soluble). This study concludes that there are two primary OC sources in floodplain sediments, plant-litter and shale-derived OC, each with distinct chemical characteristics and reactivity. We estimate that 23-34% of the sediment OC is derived from shale, demonstrating the important contribution of shale-OC to the carbon cycle at this site, particularly in environments with low plant-litter inputs. Key Points Shale-derived organic carbon (OC) accounts for an estimated 23-34% of total OC in floodplain sediments of a shale-dominated landscape The mineral fraction contains the largest and oldest pool of OC, and shale-derived OC is also observed in pools considered to be actively cycled Implications for shale as an unrecognized source of global C and other rock-derived elements and for interpretation of bulk C-14 data

Beneficial microbial associations enhance the fitness of most living organisms, and wood-feeding insects offer some of the most striking examples of this. Odontotaenius disjunctus is a wood-feeding beetle that possesses a digestive tract with four main compartments, each of which contains well-differentiated microbial populations, suggesting that anatomical properties and separation of these compartments may enhance energy extraction from woody biomass. Here, using integrated chemical analyses, we demonstrate that lignocellulose deconstruction and fermentation occur sequentially across compartments, and that selection for microbial groups and their metabolic pathways is facilitated by gut anatomical features. Metaproteogenomics showed that higher oxygen concentration in the midgut drives lignocellulose depolymerization, while a thicker gut wall in the anterior hindgut reduces oxygen diffusion and favours hydrogen accumulation, facilitating fermentation, homoacetogenesis and nitrogen fixation. We demonstrate that depolymerization continues in the posterior hindgut, and that the beetle excretes an energy- and nutrient-rich product on which its offspring subsist and develop. Our results show that the establishment of beneficial microbial partners within a host requires both the acquisition of the microorganisms and the formation of specific habitats within the host to promote key microbial metabolic functions. Together, gut anatomical properties and microbial functional assembly enable lignocellulose deconstruction and colony subsistence on an extremely nutrient-poor diet.