Magnesium (Mg) stable isotope ratios reflect Mg turnover in ecosystems. At the Hailuogou glacial retreat chronosequence in SW China, about one-third of the initially present Mg was lost from the topsoil in 127 years. We determined bulk soil and exchangeable δ26Mg values at six sites exposed by the glacier from 0 to 127 years ago. Moreover, we conducted a weathering experiment (pHstat) at the youngest (0 years) and the oldest (127 years) sites and measured δ26Mg values in differently reactive pools. We found a close correlation between the δ26Mg values of the bulk topsoils (0–10 cm) and the Mg depletion rates (r = 0.98, p < 0.001, n = 5). The particularly fast Mg loss in the first 37 years was attributable to leaching of exchangeable Mg and the fast dissolution of chlorite as revealed by the lower δ26Mg values of the fast- (−1.28 ± 0.10‰) than the slow-reacting (−0.64 ± 0.11‰) pool at the 0 year-old site in our pHstat experiment. The low δ26Mg values of the fast-reacting pool matched those reported for chlorite. Our results indicate that the δ26Mg values might be used as proxy of Mg loss and to identify the mineral sources of this loss.

Understanding the uranium (U) cycle─reservoirs and processes─at the watershed scale is key to manage contaminated areas, to elucidate ore formation processes, as well as to implement paleoenvironmental research. Here, we investigated the different steps of the U cycle, from sources to sinks, and the relative roles of redox processes and organic matter in the control of U mobility in the naturally U-rich small mountainous watershed of Lake Nègre (France). We interpret the U repartition in U reservoirs through chemical, isotopic (δ238U and (234U/238U)), and speciation analyses, in light of anterior studies of the site. We show that U(VI) originates from the leaching of U-rich rock fractures and is transported in dissolved forms. Wetlands and meadow soils then act as intermediary sinks where U(VI) is complexed by organic matter (up to >5000 μg/g) and subsequently partly reduced to U(IV). Dissolved U is also supplied to the lake, in addition to particulate and colloidal U resulting from soil physical erosion. After entering the lake, most U(VI)-bearing organic particles settle in the sediments and U(VI) is reduced to U(IV), resulting in high sedimentary U concentrations (up to >1000 μg/g), while a fraction of U is potentially desorbed from particles. Remaining dissolved U is exported from the watershed through the lake outlet stream. In this high-mountain lake catchment, the U cycle is mainly controlled by organic matter complexation and particulate transport, though U reduction in the lake sediments may help its long-term immobilization.

The degradation of microcystin-LR (MC-LR) in cyanobacterial aerosol with atmospheric oxidants, such as ozone and OH radicals, was predicted by the Harmful Algal Aerosol Reaction (HAAR) model. The ozonolysis of MC-LR in cyanobacterial aerosol at nighttime and its photooxidation during the daytime was observed in an outdoor chamber. The HAAR model simulates the impact of humidity and aerosol compositions on MC-LR decay. In the model, gas-particle partitioning of atmospheric oxidants onto algal aerosol was kinetically treated using the absorption and desorption processes. In the model simulation, the half-life of MC-LR estimated with its ozonolysis rate constant (3 × 10–11cc/molecules/s) is 4.6 h ± 0.92 at 66 ppb ozone. With the reaction rate constant for MC-LR with OH radicals (6 × 10–7 cc/molecules/s), the estimated half-life of MC-LR during daytime under Florida’s typical summer sunlight is 6 minutes, suggesting that the reaction with OH radicals dominates daytime MC-LR decay. Under moderate sunlight with a typical wind speed (9.2 km/h), the dispersion and HAAR models predict that 25% of aerosolized MC-LR undergoes the atmospheric process within 0.92 km from a bloom source in Florida’s largest lake, suggesting the critical role of the atmospheric oxidation of MC-LR decay.

Plant stress alters emissions of volatile organic compounds. However, little is known about how this could influence climate-relevant properties of secondary organic aerosol (SOA), particularly from complex mixtures such as real plant emissions. In this study, the chemical composition and viscosity were examined for SOA generated from real healthy and aphid-stressed Canary Island pine (Pinus canariensis) trees, which are commonly used for landscaping in Southern California. Healthy Canary Island pine (HCIP) and stressed Canary Island pine (SCIP) aerosols were generated in a 5 m3 environmental chamber at 35–84% relative humidity and room temperature via OH-initiated oxidation. Viscosities of the collected particles were measured using an offline poke-flow method, after conditioning the particles in a humidified air flow. SCIP particles were consistently more viscous than HCIP particles. The largest differences in particle viscosity were observed in particles conditioned at 50% relative humidity where the viscosity of SCIP particles was an order of magnitude larger than that of HCIP particles. The increased viscosity for the aphid-stressed pine tree SOA was attributed to the increased fraction of sesquiterpenes in the emission profile. The real pine SOA particles, both healthy and aphid-stressed, were more viscous than α-pinene SOA particles, demonstrating the limitation of using a single monoterpene as a model compound to predict the physicochemical properties of real biogenic SOA. However, synthetic mixtures composed of only a few major compounds present in emissions (<10 compounds) can reproduce the viscosities of SOA observed from the more complex real plant emissions.

Ozone (O3) oxidation chemistry on proxy compounds of the sea surface microlayer (SML) generates volatile organic compounds (VOCs) in the atmosphere. To shed light on the proposed significance of this chemistry, we investigated the formation of VOCs through heterogeneous chemistry of O3 (100 ppb) with authentic SML collected from 10 sites in the South China Sea using a reactor coupled to proton transfer reaction–time of flight–mass spectrometry (PTR–TOF–MS) and subsequently identified by off-line techniques. On the basis of the semi-quantitative data of the identified compounds, we estimated the production rates of acetone, acetaldehyde, propanal, hexanal, heptanal, octanal, and nonanal, which correspond to the experimental conditions applied in this study. These results provide a significant update to our understanding of abiotic formation of VOCs in the marine atmosphere, which should be considered in future model studies to properly evaluate the VOC contribution of ozone heterogeneous chemistry with the SML.

Terrestrial enhanced weathering of alkaline silicate minerals is a promising climate change mitigation strategy with the potential to limit the global temperature rise. The formation and accumulation of pedogenic carbonate and bicarbonate in soils/subsoils and groundwater offers a large sink for C storage; the amount of soil inorganic carbon (SIC) presently held within soils has been estimated to be 720–950 Gt of C. These values can be augmented by the addition of a variety of calcium and magnesium silicates via enhanced weathering. While the concept of the application of finely milled silicate rocks for faster weathering rates is well established, there has been limited discussion on the role of local climate, natural SIC content (i.e., the SIC innately present in the soil), and soil pH (among other important agronomic factors) on silicate weathering when applied to croplands, especially in view that the aim is to establish terrestrial enhanced weathering as a carbon dioxide removal (CDR) strategy on a global scale. In this work, we emphasized the importance of soil pH and soil temperature on silicate weathering and looked to estimate an upper limit of (i.e., constrain) the global capacity until the year 2100 for enhanced rock weathering (ERW) to draw down CO2 in the form of accumulated pedogenic carbonate or soluble bicarbonate. We assessed the global spatial distribution of cropland soil pH, which serves as a proxy for local innate SIC; annual rate of pluvial (rainfall) precipitation; and soil temperature, and found that the potential CO2 drawdown difference between faster and slower weathering silicates is narrower in Asia, Africa, and South America, while the gap is larger for Europe, North America, and Oceania.

A detailed quantum chemical investigation of a new reaction mechanism possibly leading to the formation of cyanoketene (NC–CH═C═O) in the interstellar medium (ISM) was carried out. Different reaction channels have been found by the AutoMeKin program, and the structures and harmonic force fields of the key stationary points have been characterized at the density functional theory level employing last-generation double-hybrid functionals. Finally, single-point computations at those geometries by state-of-the-art composite wave function methods provided accurate energies for the evaluation of thermochemical and kinetic parameters in the framework of an Ab Initio Transition State Theory based Master Equation (AITSTME) strategy. Our results indicate that the barrier-less association reaction of the formyl radical (HCO•) to the cyanocarbene radical (HCCN) can lead to the formation of cyanoketene under the harsh conditions of the ISM. Canonical rate constants computed for temperatures up to 600 K show that the most abundant product is indeed cyanoketene. The formation of other, even more stable, species involves higher activation energies and/or less favorable multi-step processes. Furthermore, to aid the search of cyanoketene, still undetected in the ISM, its rotational spectrum was recorded up to 530 GHz. The refined set of spectroscopic constants obtained in this way allows for spectral predictions from the microwave to the terahertz region, particularly for the bright b-type transitions, which can be targeted for the identification of cyanoketene in spectral line surveys. Despite cyanoketene was already sought without success in a variety of astronomical sources, we suggest to look for it in those sources where HCO or HCCN have already been detected, namely, W3, NGC2024, W51, K3-50, IRC + 2016, and TMC-1.

Heavy metals or metalloids (HMs) in agricultural soils at high geological background areas could inherit the characteristics of parent materials, which would affect HM accumulation in crops. However, the specific impact of different parent materials on the behavior of HMs was unclear. This study focused on the bioavailability and risk variations of HMs in paddy soils derived from eluvial and alluvial parent materials in the Xitiaoxi catchment based on the total concentration, fraction, and risk assessment of HMs. The results showed that the paddy soil in the alluvial area had a higher bioavailability and risk level of HMs than those in the eluvial area, due to the increased proportion of activated HMs in the alluvial paddy soil. The increase in activated HMs was associated with a decrease in soil acidity and fertility as well as intensified anthropogenic activities within the alluvial study area. The spatial distribution of HM risk in soil samples demonstrated that the risk level of HMs was related to the exposure of black shale in eluvial soil, while the risk level of HMs in alluvial soil was negatively correlated with distance from the riverside. The source apportionment results further indicated that the alluvial paddy soil had lower geological but higher anthropogenic sources, especially for Cd, Pb, and Cu. In summary, the alteration in the properties of parent materials during river transportation and more anthropogenic sources raised the bioavailability and risk of HMs in the alluvial paddy ecosystem. This study provided new insights into HM risk assessment and control in agricultural soils developed from different parent materials within a catchment with a high geological background.

Dissolved organic matter (DOM) is a complex mixture of organic compounds with chemical heterogeneity in natural soils, sediments, and waters. Predicting the dissolved organic carbon (DOC)–water partition coefficients (KDOC) of organic pollutants to DOMs from various sources is important for assessing their fate and bioavailability. However, the accuracy of the recently developed polyparameter linear free energy relationship (pp-LFER) KDOC models is generally restricted by the small data set, inclusion of experimental data from unreliable measurement methods, or undefined selection standards. The aim of this study was to establish pp-LFER models for predicting the KDOC of nonionic organic chemicals to a variety of sources of DOMs and to get an understanding of DOM chemodiversity. A reliable and expanded experimental KDOC data set was compiled. Improved pp-LFER models were developed and assessed for All DOM (DOM from all sources), Aldrich humic acid (HA), Roth HA, soil porewater DOM, sediment porewater DOM, natural aquatic DOM, natural terrestrial DOM, natural DOM, and commercial DOM. The models developed in this study were reasonably robust and accurate with a root mean square error (RMSE) of 0.538 for the All DOM model and RMSE from 0.196 to 0.860 for the specific DOMs. Also, the models generally performed better than previously published ones. Moreover, the system parameters of the models well described the chemical variabilities between soil porewater DOM and sediment porewater DOM, between natural aquatic DOM and natural terrestrial DOM, and between natural DOM and commercial DOM regarding polarizability, dipolarity, H-bond-accepting and -donating ability, and cavity formation energy. This study provides effective tools to assess the tendencies of organic chemicals to DOMs and improves our understanding of the chemical heterogeneity of DOMs from different sources.

This study establishes a theoretical foundation for the oxidation pathway of monovalent thallium (Tl(I)) to trivalent thallium (Tl(III)) on birnessite, which is responsible for the over million-times enrichment of Tl in marine ferromanganese deposits over seawater concentration. Tl(I) oxidation occurs on vacant Mn(IV) sites located on the basal planes of the birnessite layers and on the edge sites, in agreement with experiment. Two Mn(IV) atoms are reduced to Mn(III) when Tl(I) gives up two electrons in two one-electron steps with formation of an intermediate Tl(II) inner-sphere complex. Tl(I) oxidation is facilitated at pH > 4–5 by the partial hydrolysis of the Tl(III) inner-sphere product on the reactive basal and edge sites. Oxidation by O2 is thermodynamically unfavorable. Although density functional theory has predictive power for an intermediate Tl(II) complex, it would be difficult to characterize it as Tl(II) is highly reactive and therefore probably short-lived. These findings provide the first atomic-scale description of the oxidation of Tl(I) by a manganese oxide and fill gaps in our understanding of global thallium sequestration in natural systems.

We assessed the spatial distribution of 35 elements in aquifer sediments and groundwater of a crude-oil-contaminated aquifer and show evidence of the dissolution of barium (Ba), strontium (Sr), cobalt (Co), and nickel (Ni) during hydrocarbon oxidation coupled to historic microbial Fe(III)-reduction near the oil. Trace element plumes occur in the crude-oil-contaminated aquifer, where 50% Co, 47% Ni, 24% Ba, and 15% Sr have been mobilized from the sediment near the oil into groundwater, resulting in dissolved masses >33, 18, three, and two times greater than estimated dissolved masses prior to contamination, respectively. Ba2+ and Ni2+ concentrations exceeded the World Health Organization’s drinking-water guidelines of 700 and 20 μg/L, respectively. Sediments attenuate trace element plumes in two geochemically distinct zones, resulting in <0.01% total trace element masses dissolved in groundwater, despite the substantial mobilization near the oil body. Geochemical modeling of the modern Fe(III)-reducing zone suggests trace elements are likely attenuated via coprecipitation with/without sorption on iron carbonate precipitates. In the suboxic transition zone at the leading edge of the plume, Fe(III)-hydroxides sorb Ba2+, Sr2+, Co2+, and Ni2+. This study emphasizes that slow but persistent biogeochemical activity can substantially alter aquifer chemistry over decadal timeframes, a phenomenon we term biogeochemical gradualism.

Stable oxychlorides, mainly composed of ClO4–, ClO3–, or their mixture, are significantly enriched on the Martian surface (up to ∼0.5%) compared to Earth. ClO4– and ClO3– have different physicochemical properties and biological activities. They are also important candidates for in situ resource utilization (ISRU) and even the food sources of potential Martian lives. Therefore, it is of great scientific significance to determine the species of oxychlorides, especially the ratio of ClO4–/ClO3– on the Martian surface, not only for Martian environmental evolution and life search but also for Martian resource development. In this study, first, a simulation of oxychloride generation by electrostatic discharge (ESD) of the Martian dust storm was performed under simulated Martian conditions. Second, the spectral features of the Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra and liquid transmission (FTIR-TR) spectra of ClO4– and ClO3– were characterized, and a quantitative model between their IR spectra and their concentrations was built. Finally, the concentration of ClO4– and ClO3– in the ESD reaction product under various reaction times and the ratio of ClO4–/ClO3– was detected and analyzed. Our results provide valuable information for the study of the origin of the mechanism, the scientific estimation of the abundance and the species of oxychlorides on the Martian surface, and even the feasibility of ISRU on the Martian surface.

The non-ideal solid solutions between calcite and smithsonite, [(Ca1–xZnx)CO3], were synthesized, and their interaction with different aqueous solutions at 25 °C was experimentally investigated. The X-ray diffraction spectra indicated that all synthesized minerals exhibited the calcite structure exclusively. After 180–240 days of dissolution in N2-degassed water (NDW) and air-saturated water (ASW), the aqueous Zn concentrations reached a constant value ranging from 0.002565 to 0.006133 and 0.002710 to 0.006374 mmol/L for the solid solutions with low Zn/(Zn + Ca) mole ratios (XZn 0.864), respectively. After 180–240 days of dissolution in CO2-saturated water (CSW), the aqueous Zn concentrations reached a constant value ranging from 0.005938 to 0.081753 mmol/L for all solid solutions. The aqueous Zn/(Ca + Zn) mole ratios were considerably lower than the solid XZn. The aqueous Zn and Ca concentrations generally increased with increasing XZn for solid solutions with XZn 0.864. The average solubility products (Ksp) (≈ ion activity products at the constant state) were determined to be 10–8.36±0.10, 10–8.33±0.03, and 10–8.28±0.06 for calcite [CaCO3] in NDW, ASW, and CSW, respectively. Similarly, the average solubility products were determined to be 10–10.65±0.12, 10–10.60±0.08, and 10–10.47±0.06 for smithsonite [ZnCO3] in NDW, ASW, and CSW, respectively. The logarithm of Ksp showed a slight increase with increasing XZn for solid solutions with XZn 0.864. In the Lippmann diagram constructed with the Guggenheim coefficients a0 = 2.72 and a1 = −0.266 for the [(Ca1–xZnx)CO3] solid solutions, it was observed that the solid solutions dissolved non-stoichiometrically and moved progressively up to the minimum stoichiometric saturation curve for pure smithsonite and the solutus curve and then along them from right to left, finally reaching the saturation curve for calcite. The coexistence of Zn-poor aqueous solutions with ZnCO3-rich solids highlights the findings of the [(Ca1–xZnx)CO3] mineral–water reaction and its significance in the zinc geochemical cycle in Earth’s surface environments, contributing to a comprehensive understanding of these processes.

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.

Molecular ionization potentials (IP) and photoionization cross sections (σ) can affect the sensitivity of photoionization detectors (PIDs) and other sensors for gaseous species. This study employs several methods of machine learning (ML) to predict IP and σ values at 10.6 eV (117 nm) for a dataset of 1251 gaseous organic species. The explicitness of the treatment of the species electronic structure progressively increases among the methods. The study compares the ML predictions of the IP and σ values to those obtained by quantum chemical calculations. The ML predictions are comparable in performance to those of the quantum calculations when evaluated against measurements. Pretraining further reduces the mean absolute errors (ε) compared to the measurements. The graph-based attentive fingerprint model was most accurate, for which εIP = 0.23 ± 0.01 eV and εσ = 2.8 ± 0.2 Mb compared to measurements and computed cross sections, respectively. The ML predictions for IP correlate well with both the measured IPs (R2 = 0.88) and with IPs computed at the level of M06-2X/aug-cc-pVTZ (R2 = 0.82). The ML predictions for σ correlated reasonably well with computed cross sections (R2 = 0.66). The developed ML methods for IP and σ values, representing the properties of a generalizable set of volatile organic compounds (VOCs) relevant to industrial applications and atmospheric chemistry, can be used to quantitatively describe the species-dependent sensitivity of chemical sensors that use ionizing radiation as part of the sensing mechanism, such as photoionization detectors.

Ecological water replenishment projects have been implemented in the Yongding River (YDR) Basin since 2019 to adjust the contradiction between regional ecology and social development. To clarify the water chemistry, sources of riverine solutes, and the water quality assessment of the YDR under the ecological water replenishment, concentrations of major ions (Na+, K+, Ca2+ Mg2+, SO42–, HCO3–, Cl–, and NO3–) of the YDR collected in August 2021 are determined. The dominant water chemical type of the YDR water was SO42–·Cl––Na+. Anthropogenic activities were the dominant factors affecting concentrations of riverine solutes, making up 71% of the total cations, and the main type was urban domestic sewage inputs. According to the evaluation of irrigation water quality, the YDR was facing a low-level sodium hazard and moderate salinity hazard, and the harm to soil aggregates was limited according to irrigation. The health threats to adult residents were limited in YDR, while potential health risks for children remained. Ecological water replenishment projects have been implemented to improve the ecological function of the YDR with significant effects observed near the replenishment outlets, leading to a significant improvement in the irrigation water quality. This study highlights the disturbance of ecological replenishment to water chemical characteristics and ecological restoration function to the dry-up river and provides a valuable example for further studies of water chemistry under ecological water replenishment.

The search for life on other bodies in our solar system is currently focused on Ocean Worlds, those bodies known to contain liquid water, as water is one of the requirements for life as we understand it. In the search for organic biosignatures that would indicate the presence of past or current life, liquid samples from these bodies would utilize an initial sample preparation step in which interfering substances such as salts can be removed before analysis. Previous work on potential sample preparation techniques for these samples evaluated solid phase extraction (SPE) on cation exchange media, but only explored amino acid analytes at low salt to analyte ratios and used high concentration eluents. This work utilized mixed-mode ion exchange solid phases, developing methods for elutions in low concentration solutions and evaluating polar analytes with a range of functionalities. Method development revealed the need for a pre-elution step in the process to decrease the elution volume required. Both anion and cation exchange media were evaluated for the capture of analytes from solutions that simulated Earth’s oceans. The Oasis MCX and Strata X-C cation exchange media provided the best results, with >90% retention of all analytes including amino acids, organic amines, nucleotides, peptides, and an oligonucleotide. These cation exchange media retained even anionic components, including glutamic acid and organic acids, with >90% efficiency. These analytes were released in the wash step, but salt ion removal was completed before release, allowing this technique to be used for desalting of these analytes. Extraction of a 14 component mixed analyte solution also showed retention of all analytes, with testing of analyte concentrations down to 100 nM in 35 g/L simulated ocean solution. Oasis MAX and Strata X-A anion exchange media did not retain glutamic acid, fumaric acid, or dipicolinic acid when the salt concentration was high; these anionic analytes were easily extracted from low salt solutions. The anion exchange media showed a range of functionality for extracting other analytes from the simulated ocean water, capturing >90% of tryptophan and phenylalanine, but retaining 20% of the glycine. Irradiation exposure, as a model for the solar irradiation expected during deployment to extraterrestrial locations, did not affect the performance of any ion exchange media tested. A reversed-phase SPE column was directly coupled to a cation exchange column, to investigate removal of nonpolar compounds that might bind to and block the mixed-mode ion exchange media.

Organic aerosol in the atmosphere has an impact on climate, visibility, and human health. Oxidation of biogenic volatile organic compounds forms secondary organic aerosols by lowering the volatility of the product molecules and thus enhancing partitioning to the particle phase. The NO3-initiated oxidation of Δ-3-carene was studied because it connects the interaction between biogenic and anthropogenic emissions to form aerosols. This work characterized the first-generation gas-phase products of the NO3-initiated oxidation of Δ-3-carene in a 7.4 m3 Teflon FEP chamber using a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus) and a high-resolution time-of-flight chemical-ionization mass spectrometer (CIMS) using iodide adducts. The mass spectra not only show the presence of most of the expected products, including dicarbonyl, hydroxy nitrate, carbonyl nitrate, hydroxy dicarbonyl, and dicarbonyl nitrate but also show significant fragmentation of the parent ions in the Vocus through the loss of water and/or nitric acid and other neutral fragments. Parent and fragment ions were grouped together, taking advantage of gas-wall interactions in Teflon tubing, which separate the molecules by their inlet delay time. After grouping product ions, the Vocus signals were used to determine the gas-phase product yields for the NO3 + Δ-3-carene reaction. A comparison between the Vocus and iodide CIMS data allowed the sensitivity of the iodide CIMS to be investigated. Understanding the mechanism of the oxidation of Δ-3-carene by NO3 radicals allows for a better understanding of the sources of organic nitrate in the atmosphere and can improve the interpretation of field data and the representation of this chemistry in models.

Manganese (Mn) oxides are strong oxidants and sorbents of nutrients and contaminants, thus their formation mechanisms impact multiple biogeochemical cycles. Manganese in oxic surface waters is often found as dissolved Mn species, such as Mn(II) and Mn(III)-ligand complexes, rather than Mn oxides. This is believed to be a result of Mn oxide reduction by hydrogen peroxide associated with photolysis of organic matter and direct organic matter-mediated reduction within sunlit waters. Nevertheless, Mn oxides can persist in some surface environments, which indicates an incomplete understanding of controls on Mn oxide distributions. Here, we couple field- and lab-based analyses to explore Mn oxide distributions, Mn oxidation rates, and underlying controls on Mn oxide formation within Siders Pond, a brackish and meromictic pond on Cape Cod (Massachusetts, USA) during the summer and fall of 2020. Manganese oxides were observed consistently in sunlit surface waters with concentrations declining to undetectable at the base of the chemocline. Surface Mn oxide concentrations were highest in late summer, reaching concentrations of ∼1 μM, and lowest in late fall, reaching only ∼50 nM. Minerals identified using synchrotron-based absorbance measurements were structurally similar to δ-MnO2 and feitknechtite. Substantial light-mediated Mn oxidation only took place in live incubations of Siders Pond water while net Mn reduction proceeded in killed incubations. Thus, total particle-mediated oxidation by microbes and minerals combined outpaced photoreduction, leading to net accumulation of Mn oxides within Siders Pond. Our results identify important roles for microbial- and mineral-mediated oxidation in determining Mn oxide distributions within surface waters of a natural setting, a finding that may help explain comparable distributions in other locations.

Bare rock surfaces in dry to semiarid places of the world often host a black-brown accretion rich in Mn and Fe known as rock varnish. The varnish surface presents an ideal environment for microbial development. A burgeoning interdisciplinary arena of scholarship focuses on the biogeochemical fingerprints of life in severe settings. Given that a large number of researchers hypothesize that varnish formation is a key process by microorganisms, the high altitude Ladakh remains a largely unexplored research setting. Thus, as one of the world’s harshest dry deserts, we selected Ladakh as the focus for this investigation into the nature of organic biomarkers found in subaerial rock varnish in this severe climate. Microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy reveals the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids that we interpret as resulting from mineral–microbial interactions. We hypothesize that a newly discovered change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) might be important in facilitating the development of microbial processes that could be related to varnish formation.