Chemical weathering produces alkalinity that, in conjunction with marine carbonate burial, mediates the return of carbon from planetary volcanism, metamorphism, and sedimentary recycling. However, clay formation during chemical denudation can result in reduced alkalinity fluxes. Because the 7Li/6Li ratio of dissolved lithium in river water (S7Liriver) traces the degree of clay mineral formation following silicate dissolution at the watershed scale, fluvial Li isotope ratios and their expression in sedimentary archives can potentially convey the strength of silicate weathering-driven CO2 drawdown. However, little attention has been given to the coupled dynamics that emerge when also considering the weathering of coexisting non-silicate minerals, such as carbonate and sulfide phases, that also modify alkalinity and carbon fluxes to the global ocean-atmosphere system. These additional phases potentially complicate attempts to relate S7Liriver values to changes in the partial pressure of atmospheric carbon dioxide (pCO2). Here we address this complexity by compiling a global dataset of S7Liriver values and major and trace ion concentrations (n = 413), attributing solutes among lithologic sources and clay sinks with the MEANDIR inversion model and comparing numerical results to watershed properties. The analyses demonstrate that S7Liriver values correlate with alkalinity consumption by clay formation but do not simply relate to the net impact of weathering on atmospheric pCO2 due to the weathering of carbonate and sulfide minerals. However, other weathering indices, like river Li/Na ratios, may more directly relate to pCO2 change. A simple regolith model demonstrates that mineral supply, driven by bedrock composition and uplift, impacts the balance of sulfuric and carbonic acid weathering, the degree of clay mineral formation, and the fraction of Li incorporated from solution into clays. As a result, where measured or calculated S7Liriver trends have previously been interpreted in terms of clay mineral formation, we interpret S7Liriver as reflecting mineral supply modulated by climate.

Methane is a potent greenhouse gas, an important energy source, and an important part of the global carbon cycle. The relative abundances of doubly substituted ("clumped") methane isotopologues (13CH3D and 12CH2D2) offer important information on the sources and sinks of methane. However, the clumped isotope signatures of microbially produced methane from different methanogenic pathways lack a systematic investigation. In this study, we provide a data set encompassing isotopic signatures of hydrogenotrophic, methylotrophic, acetoclastic, and methoxydotrophic methanogenesis. We find that a statistical "combinatorial effect" generates significant differences in 12CH2D2 compositions between hydrogenotrophic methanogenesis and the other pathways, while variations in the fractionation factors of clumped isotopologues result in differences in 13CH3D compositions between the methylotrophic, acetoclastic, and methoxydotrophic pathways. The energy yield of methanogenesis and the energy conservation approaches implemented by different microbial strains may also influence the isotope values of methane. Further analysis suggests that previously observed isotopic signatures of methane in freshwater environments are potentially due to mixing between hydrogenotrophic and other methanogenesis pathways. This study provides new experimental constraints on the isotope signatures of different microbial methanogenic pathways and evidence of the mechanisms responsible for the observed differences. This enables a better understanding of the sources and sinks of methane in the environment.

Some fraction of the total carbon (C) transported by rivers can enter the atmosphere as CO2 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 222Rn) reactive transport model to simulate CO2 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 222Rn, which yielded more accurate estimates of CO2 evasion by improving estimates of spatially-averaged groundwater pCO2 and generating a site-specific gas transfer scaling relationship. Our results demonstrate that riparian wetlands contribute to 19% of the computed CO2 evasion flux. Lastly, we find that CO2 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.

How glaciation affects drawdown by chemical weathering influences the weathering-climate feedback strength, which controls the exogenic carbon cycle and planetary habitability. However, the role of glaciers remains elusive as glaciation alters multiple factors controlling weathering, the net effect of which is ambiguous even in directionality. To isolate and quantify the effect of glaciers, we developed a novel multi-proxy system for constraining catchment-scale weathering fluxes in the past. This approach utilizes the correlation between Ge/Si and Si isotope ratios in modern rivers and the preservation of these signals in lacustrine sediments. Reconstructed weathering fluxes in two Icelandic catchments with different glacial histories during the past 10,000 years show that chemical weathering fluxes are roughly 10 times higher when a catchment is glaciated versus ice-free. The synchronous variations in weathering fluxes with the expansion and contraction of glaciers indicate that glaciation may rapidly amplify climatic variations via a positive feedback.

Tropical watersheds are thought to exert a strong control on the global carbon cycle because elevated temperature and rainfall rates promote the chemical weathering of silicate rocks. However, the critical factors that control tropical weathering, such as the role of subsurface flowpaths in setting the sensitivity of weathering to climate change, remain obscure. Here, we relate solute dynamics to flowpath partitioning using new and existing data from the Luquillo Critical Zone Observatory (LCZO) in the tropical forests of eastern Puerto Rico. We used new measurements of deuterium excess in streamflow and rainfall to show that the fraction of young water (F yw , fraction of streamflow less than 1–3 months old) for each catchment increases with increasing discharge. We attribute F yw ‐Q behavior to the activation of shallow flowpaths that efficiently route incident rainfall to streamflow. Results from this 2‐year sampling period are comparable to results from end‐member mixing analysis of longer‐term solute records, suggesting that water routed via shallow flowpaths acquires little additional solutes from weathering reactions. Our findings of apparent mixing between flowpaths can be unified with time‐dependent weathering reactions and time‐variable transit time distributions. To estimate the response of the LCZO to climatic change, we compare F yw ‐Q behavior between sites that experience different mean annual precipitation amounts. Intriguingly, we find that climatically driven changes in flowpath partitioning reconcile watershed fluxes with regolith‐based studies. This suggests that shallow flowpath activation is the mechanism for maintaining constant weathering rates despite variable rainfall rates in the supply limited regions where weathering is already maximized.

During the burial of mudstones, the associated organic matter undergoes gradual thermal maturation, a key process that can influence the reactivity of organic matter during catagenesis, the formation of hydrocarbon deposits and the chemical weathering of mudstones. Conventional methods for assessing the thermal maturity of organic matter often fail to reflect the geochemical heterogeneity between individual organic phases in mudstone samples. Here, we report an alternative, non‐destructive, surficial and micro‐scale (analytical spot size of ~ 300 nm with about 4 μm diffusion depth for micrometre‐size organic grains) method to evaluate the thermal maturity of organic matter in mudstones using the carbon K α X‐ray spectrum measured by field emission‐electron probe microanalyser (FE‐EPMA). Using this method, we observed correlations between parameter values derived from FE‐EPMA spectra, including the peak position, the peak area and the intra‐sample heterogeneity of these measurements, and independently measured vitrinite/solid bitumen reflectance for a suite of mudstones, representing different age, geological context and burial depth. With the increased values in peak area and position, we identified an increase in the carbon mass fraction of organic matter and the mean nominal oxidation state of carbon approaching zero. These trends, which are consistent with aromatisation and graphitisation, provide the rationale for using FE‐EPMA to estimate the thermal maturity of organic matter. To explore some of these trends in more detail, we employed time‐of‐flight secondary ionisation mass spectrometry, X‐ray photoelectron spectroscopy and optical reflectance measurements on a subset of samples.

Tropical coastal systems play a vital role in sustaining biodiversity, performing ecological functions, and providing ecosystem services. They are also home to 75% of people in the tropics. Given that coasts face intense anthropogenic pressures including climate change, human population growth, and land-use change, it is critical to develop an understanding of the linkages between physical processes, biological interactions, and social dynamics in the complex environment where land and sea meet. Here, we review and synthesize 40 years of research from the Bahia Almirante region on the Caribbean coast of Panama, summarizing the large knowledge base of marine ecology, paleontology, ecosystem science and social science and adding newer information on physical processes. We describe how the system experiences both global and local drivers that are common to many tropical coastal systems and examine the crosscutting linkages that shape the system's response to change. To accomplish this, we utilized the Press-Pulse Dynamics framework as a lens to organize the many strands of research and to allow the interdisciplinary research team to generate explicit illustrative hypotheses about important socioecological linkages related to stressors such as the variability in precipitation and increased migration and tourism. The goal for this review and synthesis is to encourage researchers in Bahia Almirante and other estuarine systems to consider the landscape and seascape more broadly, to reach beyond their immediate field of expertise, and to consider both social and environmental aspects as they seek to increase system understanding in ways that can enable more productive public discourse surrounding policy, infrastructural change, and conservation.

Per- and polyfluoroalkyl substances (PFAS) are persistent and bioaccumulative pollutants that can easily accumulate in soil, posing threat to environment and human health. Current PFAS degradation processes often suffer from low efficiency, high energy and water consumption, or lack of generality. Here, we develop a rapid electrothermal mineralization (REM) process to remediate PFAS-contaminated soil. With environmentally compatible biochar as conductive additive, the soil temperature increases to >1000 °C within seconds by direct current pulse input, converting PFAS to calcium fluoride with inherent calcium compounds in soil. The general electrical mineralization process is applicable for remediating various PFAS contaminants in soil, with high removal efficiencies (>99.9%) and mineralization ratios (>90%). While retaining soil particle size, composition and water infiltration rate, REM facilitates an increase of exchangeable nutrient supply and arthropod survival in soil. REM has a significant reduction of energy consumption and greenhouse gas emission over existing soil remediation practices.

Molybdenum (Mo) abundance in marine shales/mudstones has been widely used to reconstruct depositional redox conditions. Prior laboratory experiments have observed Mo association with iron-sulfide and organic matter in the presence of aqueous H2S (euxinic conditions). The results of these experiments imply that sulfide minerals and organic matter are the primary hosts of Mo in the rock record, and this notion has been corroborated in some individual measurements on shales. At the same time, a lack of correlation between bulk-rock Mo concentration and either pyrite or organic matter abundances in some mudstones has been observed, which challenges the experiment-based understanding of Mo enrichment mechanisms and therefore related paleoredox interpretations. Here, we use electron microprobe to analyze individual phases in a suite of mudstones to examine the application of bulk-rock Mo abundance as a redox proxy. The microprobe data reveal that pyrite, organic matter, and matrix (clay minerals) can all host Mo in our mudstone samples. We interpret the Mo distributions in these three hosts to reflect differences in early diagenetic conditions (e.g., H2S concentrations) and the aqueous Mo availability. We infer that pyrite and clay minerals host a large fraction of bulk-rock Mo concentrations when aqueous H2S and Mo are abundant. In contrast, with less available aqueous H2S or Mo, organic matter becomes more important as the Mo host. More generally, it appears that constraints on the partitioning of bulk Mo between sedimentary phases can improve paleoredox interpretations especially in conjunction with independent proxies (e.g., S to C ratios of organic matter). Lastly, comparisons of Mo abundance between organic matter from different mudstones indicate potential Mo preservation in the organic matter during thermal maturation, and variations in Mo concentrations between different pyrites qualitatively suggest variations in the porewater Mo inventory during diagenesis.

Biogeochemical reactions modulate the chemical composition of the oceans and atmosphere, providing feedbacks that sustain planetary habitability over geological time. Here, we mathematically evaluate a suite of biogeochemical processes to identify combinations of reactions that stabilize atmospheric carbon dioxide by balancing fluxes of chemical species among the ocean, atmosphere, and geosphere. Unlike prior modeling efforts, this approach does not prescribe functional relationships between the rates of biogeochemical processes and environmental conditions. Our agnostic framework generates three types of stable reaction combinations: closed sets, where sources and sinks mutually cancel for all chemical reservoirs; exchange sets, where constant ocean-atmosphere conditions are maintained through the growth or destruction of crustal reservoirs; and open sets, where balance in alkalinity and carbon fluxes is accommodated by changes in other chemical components of seawater or the atmosphere. These three modes of operation have different characteristic timescales and may leave distinct evidence in the rock record. To provide a practical example of this theoretical framework, we applied the model to recast existing hypotheses for Cenozoic climate change based on feedbacks or shared forcing mechanisms. Overall, this work provides a systematic and simplified conceptual framework for understanding the function and evolution of global biogeochemical cycles.

Silicate weathering and organic carbon (OC) burial in soil regulate atmospheric CO2, but their influence on each other remains unclear. Generally, OC oxidation can generate acids that drive silicate weathering, yet clay minerals that form during weathering can protect OC and limit oxidation. This poses a conundrum where clay formation and OC preservation either compete or cooperate. Debate remains about their relative contributions because quantitative tools to simultaneously probe these processes are lacking while those that exist are often not measured in concert. Here we demonstrate that Li isotope ratios of sediment, commonly used to trace clay formation, can help constrain OC cycling. Measurements of river suspended sediment from two watersheds of varying physiography and analysis of published data from Hawaii soil profiles show negative correlations between solid-phase delta 7Li values and OC content, indicating the association of clay mineral formation with OC accumulation. Yet, the localities differ in their ranges of delta 7Li values and OC contents, which we interpret with a model of soil formation. We find that temporal trends of Li isotopes and OC are most sensitive to mineral dissolution/clay formation rates, where higher rates yield greater OC stocks and lower delta 7Lisolivalues. Whereas OC-enhanced dissolution primarily dictates turnover times of OC and silicate minerals, clay protection distinctly modifies soil formation pathways and is likely required to explain the range of observations. These findings underscore clay mineral formation, driven primarily by bedrock chemistry and secondarily by climate, as a principal modulator of weathering fluxes and OC accumulation in soil.

The composition and preservation state of biogenic carbonate archives, such as foraminiferal tests, record ocean chemistry during the lifetime of the organism and post-depositional changes in ambient conditions via carbonate compensation. Depending upon the specific paleoclimate proxy, post-depositional processes, including dissolution, may alter original paleoenvironmental signals captured by the foraminifer's test composition. Accordingly, quantifying dissolution independent of geochemical measurements can improve proxy interpretation. Developing independent tools may also be useful for investigating whether changes in paleoclimatic conditions are associated with changes in seawater carbonate chemistry. Such approaches can be improved further if they are applied to individual foraminiferal tests, as specimen-to-specimen differences can record higher-frequency environmental changes compared to conventional bulk-scale analyses. Here, we combine individual foraminiferal carbon and oxygen isotopic analyses (IFA) with X-ray MicroCT Scanning to generate paired analyses of test density (a proxy for the extent of post-depositional dissolution) and isotopic composition. As a proof-of-concept application of this approach, we analyze Globigerina bulloides tests from both coretop and latest Miocene/earliest Pliocene-aged sediment from Ocean Drilling Project (ODP) Site 1088 (Agulhas Ridge). Our measurements and mixing model calculations show that within-population differences in carbon and oxygen isotopic ratios are largely independent of dissolution extent. By comparing population averages from coretop and downcore sediments, we find that lower oxygen isotopic ratios (likely driven by higher calcification temperatures) are associated with greater extents of dissolution at ODP Site 1088. We interpret this finding to reflect coupled changes in carbonate chemistry and climatic conditions over million-year timescales. Foraminifera are microscopic marine protists that produce shells made of calcium carbonate. The chemical composition of these shells reflects the temperature and chemistry of the seawater in which they live. However, after foraminifera die, their shells may start to dissolve depending upon the depth of the seafloor they sink to and the local chemistry of the seawater. During dissolution, the chemical and physical composition of the calcium carbonate shell changes, impacting how we quantify past environmental conditions. Ideally, we would have some way of measuring how much each shell was dissolved to see which chemical analyses are affected and which are not. In this study, we used Micro CT scanning to measure how much dissolution individual shells of foraminifera experienced at the bottom of the ocean offshore of South Africa. By measuring the chemical composition of the exact same shells, we found that our specific measurements were unaffected by dissolution. However, we also found that ancient shells were more dissolved than modern shells, which likely reflects changes in ocean chemistry. We present the first pairing of X-ray Micro CT scans and individual foraminiferal analyses of stable C and O isotopes CT Numbers generated from X-ray Micro CT scans of foraminifera are used as a dissolution proxy Drivers of dissolution and isotopic variation are decoupled over similar to 103 years timescales but are related over similar to 105 years timescales

The oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OCpetro) emits nearly as much CO2 as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OCpetro oxidation. However, these settings also export un-weathered material that may continue to react in downstream floodplains. The relative importance of OCpetro oxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OCpetro oxidation from the Andes to the Amazon floodplains using a common approach. Dissolved rhenium concentrations (n=131) range from 0.01 to 63 pmol.L-1 and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OCpetro oxidation and that this proportion increases downstream. We estimate that in the Andes, OCpetro oxidation releases 11.2+4.5/-2.8 tC km-2 yr-1 of CO2, which corresponds to ~41% of the total OCpetro denudation (sum of oxidized and solid OCpetro). A Re mass balance across the Rio Madre de Dios shows that 46% of OCpetro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OCpetro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OCpetro oxidation and CO2 release.

Soil contamination is an environmental issue due to increasing anthropogenic activities. Existing processes for soil remediation suffer from long treatment time and lack generality because of different sources, occurrences, and properties of pollutants. Here, we report a high-temperature electrothermal process for rapid, water-free remediation of multiple pollutants in soil. The temperature of contaminated soil with carbon additives ramps up to 1000 to 3000 °C as needed within seconds via pulsed direct current input, enabling the vaporization of heavy metals like Cd, Hg, Pb, Co, Ni, and Cu, and graphitization of persistent organic pollutants like polycyclic aromatic hydrocarbons. The rapid treatment retains soil mineral constituents while increases infiltration rate and exchangeable nutrient supply, leading to soil fertilization and improved germination rates. We propose strategies for upscaling and field applications. Techno-economic analysis indicates the process holds the potential for being more energy-efficient and cost-effective compared to soil washing or thermal desorption.

Chemical weathering of silicate minerals and burial of organic carbon (

Organic carbon buried in marine sediment serves as a net sink for atmospheric carbon dioxide and a source of oxygen1,2. The rate of organic carbon burial through geologic history is conventionally established by using the mass balance between inorganic and organic carbon, each with distinct carbon isotopic values (δ13C)3,4. This method is complicated by large uncertainties, however, and has not been tested with organic carbon accumulation data5,6. Here we report a ‘bottom-up’ approach for calculating the rate of organic carbon burial that is independent from mass balance calculations. We use data from 81 globally distributed sites to establish the history of organic carbon burial during the Neogene (roughly 23–3 Ma). Our results show larger spatiotemporal variability of organic carbon burial than previously estimated7–9. Globally, the burial rate is high towards the early Miocene and Pliocene and lowest during the mid-Miocene, with the latter period characterized by the lowest ratio of organic-to-carbonate burial rates. This is in contrast to earlier work that interpreted enriched carbonate 13C values of the mid-Miocene as massive organic carbon burial (that is, the Monterey Hypothesis)10,11. Suppressed organic carbon burial during the warm mid-Miocene is probably related to temperature-dependent bacterial degradation of organic matter12,13, suggesting that the organic carbon cycle acted as positive feedback of past global warming. A ‘bottom-up’ approach for calculating the rate of organic carbon burial in the global ocean shows larger variability than has been previously estimated, suggesting that the organic carbon cycle acted as positive feedback of past global warming.