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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 carbonKα 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. 

Abstract 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 analyzeGlobigerina bulloidestests 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. 

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 d7Li values and OC content, indicating the association of clay mineral formation with OC accumulation. Yet, the localities differ in their ranges of d7Li 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 d7Li values. 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 oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OC_petro) emits nearly as much CO₂as 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 OC_petrooxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OC_petrooxidation 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 OC_petrooxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L⁻¹and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OC_petrooxidation and that this proportion increases downstream. We estimate that in the Andes, OC_petrooxidation releases 11.2^+4.5/_−2.8tC km⁻²y⁻¹of CO₂, which corresponds to ~41% of the total OC_petrodenudation (sum of oxidized and solid OC_petro). A Re mass balance across the Rio Madre de Dios shows that 46% of OC_petrooxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OC_petrooxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OC_petrooxidation and CO₂release. 

Land use and land cover (LULC) can significantly alter river water, which can in turn have important impacts on downstream coastal ecosystems by delivering nutrients that promote marine eutrophication and hypoxia. Well-documented in temperate systems, less is known about the way land cover relates to water quality in low-lying coastal zones in the tropics. Here we evaluate the catchment LULC and the physical and chemical characteristics of six rivers that contribute flow into a seasonally hypoxic tropical bay in Bocas del Toro, Panama. From July 2019 to March 2020, we routinely surveyed eight physical and chemical characteristics (temperature, specific conductivity, salinity, pH, dissolved oxygen (DO), nitrate and nitrite, ammonium, and phosphate). Our goals were to determine how these physical and chemical characteristics of the rivers reflect the LULC, to compare the water quality of the focal rivers to rivers across Panama, and to discuss the potential impacts of river discharge in the Bay. Overall, we found that the six focal rivers have significantly different river water characteristics that can be linked to catchment LULC and that water quality of rivers 10 s of kilometers apart could differ drastically. Two focal catchments dominated by pristine peat swamp vegetation in San San Pond Sak, showed characteristics typical of blackwater rivers, with low pH, dissolved oxygen, and nutrients. The remaining four catchments were largely mountainous with >50% forest cover. In these rivers, variation in nutrient concentrations were associated with percent urbanization. Comparisons across Panamanian rivers covered in a national survey to our focal rivers shows that saltwater intrusions and low DO of coastal swamp rivers may result in their classification by a standardized water quality index as having slightly contaminated water quality, despite this being their natural state. Examination of deforestation over the last 20 years, show that changes were <10% in the focal catchments, were larger in the small mountainous catchments and suggest that in the past 20 years the physical and chemical characteristics of river water that contributes to Almirante Bay may have shifted slightly in response to these moderate land use changes. (See supplementary information for Spanish-language abstract). 

Abstract Basaltic watersheds such as those found in Iceland are thought to be important sites of CO₂sequestration via silicate weathering. However, determining the magnitude of CO₂uptake depends on accurately interpreting river chemistry. Here, we compile geochemical data from Iceland and use them to constrain weathering processes. Specifically, we use a newly developed inverse model to quantify solute supply from rain and hydrothermal fluids as well as allow for variable silicate end‐member compositions, solutes to be removed via secondary phase formation, and some Ca to be sourced from carbonate dissolution. While some of these processes have been considered previously, they have not been considered together allowing us to newly determine their relative contributions. We find that weathering in Iceland is incongruent in two ways. First, solute release from primary silicates is characterized by a higher proportion of Na than would be expected from bulk basalts, which may reflect preferential weathering or some contribution from rhyolites. This Na enrichment is further enhanced by preferential Mg and K uptake by secondary phases. No samples in our data set (n = 537) require carbonate dissolution even if isotopic data (δ²⁶Mg,δ³⁰Si,δ⁴⁴Ca, and/or⁸⁷Sr/⁸⁶Sr) are included. While some carbonate weathering is allowable, silicate weathering likely dominates. The complexity we observe in Iceland underscores the need for inverse models to account for a wide range of processes and end‐members. Given that riverine fluxes from Iceland are more Na‐rich than expected for congruent basalt weathering, the characteristic timescale of CO₂drawdown is likely affected.