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At least 559 records · Page 31Linked to original sources

USGS1980

Stable carbon isotopes of HCO3- in oil-field waters-implications for the origin of CO2

The δ 13 C values of dissolved HCO 3 − in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO 2 of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ 13 C values of total HCO 3 − indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin. The range of δ 13 C values for the HCO 3 − of these waters is −20–28 per mil relative to PDB. This wide range of δ 13 C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ 13 C values of HCO 3 − in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ 13 C values (< −10 per mil) are obtained in waters where concentrations of SO 4 2− are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria ( SO 4 2− + CH 3 COO − → 2 HCO 3 − + HS − ). The high δ 13 C values probably result from the degradation of these anions by methanogenic bacteria ( CH 3 COO − + H 2 O ai HCO 3 − + CH 4 ). Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO 2 and CH 4 is probably the major source of CO 2 for production zones with subsurface temperatures greater than 80°C. The δ 13 C values of HCO 3 − for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO 2 and CH 4 . At these high temperatures, δ 13 C values of HCO 3 − decrease with increasing temperatures and decreasing concentrations of these acid anions.

Geochimica et Cosmochimica Acta↗

USGS1979

Relationship between physical condition of the carbonate fraction and sediment environments: Northern Puerto Rico shelf

Each of three sediment types recognizable on the northern Puerto Rico shelf and beaches is characterized by calcareous material in different physical condition. Dark terrigenous sand is accumulating at a relatively rapid rate and has a carbonate fraction containing fresh-appearing angular particles. The pure-carbonate skeletal-sand sediment type has a much lower rate of deposition, and its old-appearing calcareous particles have a dull surface luster. The third sediment type, a mixture of the first two, is largely from beaches and is characterized by highly polished, very well-rounded grains. Differences in length of sea-floor exposure and intensity of abrasion appear to be responsible for the variable physical condition of the carbonate fractions. We suggest that on other shelves having less distinctive sedimentary regimes, the physical condition of the carbonate grains could be used to interpret present shelf processes.

Puerto Rico↗

USGS1989

Characterization of alkyl carbon in forest soils by CPMAS 13C NMR spectroscopy and dipolar dephasing

Samples obtained from forest soils at different stages of decomposition were treated sequentially with chloroform/methanol (extraction of lipids), sulfuric acid (hydrolysis), and sodium chlorite (delignification) to enrich them in refractory alkyl carbon. As revealed by NMR spectroscopy, this treatment yielded residues with high contents of alkyl carbon. In the NMR spectra of residues obtained from litter samples, resonances for carbohydrates are also present, indicating that these carbohydrates are tightly bound to the alkyl carbon structures. During decomposition in the soils this resistant carbohydrate fraction is lost almost completely. In the litter samples the alkyl carbon shows a dipolar dephasing behavior indicative of two structural components, a rigid and a more mobile component. As depth and decomposition increase, only the rigid component is observed. This fact could be due to selective degradation of the mobile component or to changes in molecular mobility during decomposition, e.g., because of an increase in cross linking or contact with the mineral matter of the soil.

Science of the Total Environment↗

USGS1996

Occurrence and morphology of carbonate concretions in the Beulah-Zap coal bed, Williston basin, North Dakota

Carbonate concretionary bodies were encountered during mining of the Beulah-Zap lignite seam in the Coteau Properties' Freedom mine, Mercer County, North Dakota. Preliminary studies show that areal and vertical distribution of the concretions are variable. All concretions examined are composed almost entirely of calcite. They occur as thin tabular bodies, as more or less elliptical forms, or as tear shaped bodies, and may occur individually or as clusters of buff-colored, poorly consolidated to solidly crystalline material. The carbonate masses vary in size from a few millimeters to tens of centimeters. Bedding in the lignite may display some compactional folding over dense spheroidal to elliptical concretions, indicating formation of the concretions prior to compaction. Internal morphology of the concretions is complex, and includes cone-in-cone structure, cross-cutting calcite veinlets, and multiple generations of calcite. Carbon isotope values suggest the concretions are composed of biogenic carbonate, probably related to early diagenesis and decomposition of organic matter (peat); oxygen isotope values are light, and consistent with a freshwater origin.

Conference Paper↗

USGS1995

Comparison of hydrothermal alteration of carboniferous carbonate and siliclastic rocks in the Valles caldera with outcrops from the Socorro caldera, New Mexico

Continental Scientific Drilling Program (CSDP) drill hole VC-2B [total depth 1761.7 m (5780 ft); maximum temperature 295 °C] was continuously cored through the Sulphur Springs hydrothermal system in the western ring-fracture zone of the 1.14 Ma Valles caldera. Among other units, the hole penetrated 760.2 m (2494.1 ft) of Paleozoic carbonate and siliciclastic strata underlying caldera fill and precaldera volcanic and epiclastic rocks. Comparison of the VC-2B Paleozoic rocks with corresponding lithologies within and around the 32.1 Ma Socorro caldera, 192 km ( 119 miles) to the south-southwest, provides insight into the variability of alteration responses to similar caldera-related hydrothermal regimes. The Pennsylvanian Madera Limestone and Sandia Formation from VC-2B preserve many of the sedimentological and diagenetic features observed in these units on a regional basis and where unaffected by high temperatures or hydrothermal activity. Micrites in these formations in VC-2B are generally altered and mineralized only where fractured or brecciated, that is, where hydrothermal solutions could invade carbonate rocks which were otherwise essentially impermeable. Alteration intensity (and correspondingly inferred paleopermeability) is only slightly higher in carbonate packstones and grainstones, low to intermediate in siltstones and claystones, and high in poorly cemented sandstones. Hydrothermal fracture-filling phases in these rocks comprise sericite (and phengite), chlorite, allanite, apatite, an unidentified zeolite and sphene in various combinations, locally with sphalerite, galena, pyrite and chalcopyrite. Terrigenous feldspars and clays are commonly altered to chlorite and seriate, and euhedral anhydrite “porphyroblasts” with minor chlorite occur in Sandia Formation siltstone. Fossils are typically unaltered, but the walls of some colonial bryozoans in the Madera Limestone are altered to the assemblage chlorite-sericite-epidote-allanite. La, Ce and Nd are present in an unidentified hydrothermal mineral occurring throughout much of the VC-2B Pennsylvanian sequence. Carboniferous carbonate and siliciclastic formations within and around the Socorro caldera show a similar style of alteration and mineralization to their Valles caldera counterparts, but by contrast locally host commercial, caldera-related, base-metal sulfide deposits. As in the Valles rocks, mineralization and alteration in those of the Socorro caldera were strongly controlled by porosity. Unless disrupted by fractures, breccias, or karst cavities ( not identified in Valles caldera drill holes), the rocks remained relatively unaltered. Where these features allowed ingress of mineralizing hydrothermal solutions, base-metal sulfides and rare-earth-element-bearing minerals were precipitated.

New Mexico↗

USGS1994

Simulation of gas phase transport of carbon-14 at Yucca Mountain, Nevada, USA

We have simulated gas phase transport of Carbon-14 at Yucca Mountain, Nevada. Three models were established to calculate travel time of Carbon-14 from the potential repository to the mountain surface: a geochemical model for retardation factors, a coupled gas-flow and heat transfer model for temperature and gas flow fields, and a particle tracker for travel time calculation. The simulations used three parallel, east-west cross-sections that were taken from the Sandia National Laboratories Interactive Graphics Information System (IGIS). Assuming that the repository is filled with 30- year-old waste at an initial areal power density of 57 kw/acre, we found that repository temperatures remain above 60??C for more than 10,000 years. For a tuff permeability of 10-7 cm2, Carbon-14 travel times to the surface are mostly less than 1,000 years, for particles starting at any time within the first 10,000 years. If the tuff permeability is 10-8 cm2, however, Carbon- 14 travel times to the surface range from 3,000 to 12,000 years, for particle starting within the 10,000 years.

Waste Management↗

USGS2012

Advances in spectroscopic methods for quantifying soil carbon

The current gold standard for soil carbon (C) determination is elemental C analysis using dry combustion. However, this method requires expensive consumables, is limited by the number of samples that can be processed (~100/d), and is restricted to the determination of total carbon. With increased interest in soil C sequestration, faster methods of analysis are needed, and there is growing interest in methods based on diffuse reflectance spectroscopy in the visible, near-infrared or mid-infrared spectral ranges. These spectral methods can decrease analytical requirements and speed sample processing, be applied to large landscape areas using remote sensing imagery, and be used to predict multiple analytes simultaneously. However, the methods require localized calibrations to establish the relationship between spectral data and reference analytical data, and also have additional, specific problems. For example, remote sensing is capable of scanning entire watersheds for soil carbon content but is limited to the surface layer of tilled soils and may require difficult and extensive field sampling to obtain proper localized calibration reference values. The objective of this chapter is to discuss the present state of spectroscopic methods for determination of soil carbon.

Book chapter↗

USGS2022

Greenhouse gas balances in coastal ecosystems: Current challenges in “blue carbon” estimation and significance to national greenhouse gas inventories

Coastal wetlands are defined herein as inundated, vegetated ecosystems with hydrology, and biogeochemistry influenced by sea levels, at timescales of tides to millennia. Coastal wetlands are necessary components of global greenhouse gas estimation and scenario modeling, both for continental and oceanic mass balances. The carbon pools and fluxes on coastal lands, especially those influenced by tidal drivers and sea level rise, are distinct in their magnitude, rates, and uncertainties. We describe herein the pathways taken for a US scale estimation of blue carbon based on annual timesteps and bottom-up modeling, as appropriate for the first effort to include coastal wetlands in the Intergovernmental Panel on Climate Change (IPCC) guidelines for a National Greenhouse Gas Inventory (NGGI). As such, we summarize multiple efforts to reconcile mapping, modeling, and measurement issues and we report the assumptions we made based on data availability. Provided as requested feedback to the IPCC. Subsidiary Body for Scientific and Technological Advice (SBSTA) evaluation of guidance criteria, these analyses synergistically point scientists, practitioners, and policy makers toward the greatest uncertainties to address in future assessments: coastal wetland methane emissions and carbon dioxide emissions associated with the fate of eroded soil. This is a story of what was learned in the 2014–2018 NASA Carbon Monitoring System project (https://carbon.nasa.gov/cgi-bin/cms_projects.pl), how it informs “good practice” (IPCC 2006) in reporting coastal wetland emissions and removals, and where it points scientifically toward data needs at different temporal and spatial scales.

Book chapter↗

USGS2018

Estimating the societal benefits of carbon dioxide sequestration through peatland restoration

The Great Dismal Swamp National Wildlife Refuge (GDS) is a forested peatland that provides a number of ecosystem services including carbon (C) sequestration. We modeled and analyzed the potential capacity of the GDS to sequester C under four management scenarios: no management, no management with catastrophic fire, current management, and increased management. The analysis uses the Land Use and Carbon Scenario Simulator developed for the GDS to estimate net ecosystem C balance. The model simulates net C gains and losses on an annual time-step from 2013 through 2062 which is converted to carbon dioxide equivalent (CO2-eq) and monetized using the Interagency Working Group's Social Cost of Carbon. Our analysis incorporates compounded uncertainty including variation in ecological processes, temporal and spatial heterogeneity, and uncertainty in the discount rate. The no management scenario results in 2.4 million tons of CO2 emissions with a Net Present Value (NPV) under a 3% discount rate of −\$67 million. No management with catastrophic fires emits 6.5 million tons of CO2 with an NPV of −\$232 million. Current management avoids 9.9 million tons of emissions (via sequestration) with an NPV of \$326 million. Increased management avoids 16.5 million tons of emissions with an NPV of \$524 million.

North Carolina, Virginia↗

USGS2018

Sediment accretion and carbon storage in constructed wetlands receiving water treated with metal-based coagulants

In many regions of the world, subsidence of organic rich soils threatens levee stability and freshwater supply, and continued oxidative loss of organic matter contributes to greenhouse gas production. To counter subsidence in the Sacramento-San Joaquin Delta of northern California, we examined the feasibility of using constructed wetlands receiving drainage water treated with metal-based coagulants to accrete mineral material along with wetland biomass, while also sequestering carbon in wetland sediment. Nine field-scale wetlands were constructed which received local drainage water that was either untreated (control), or treated with polyaluminum chloride (PAC) or iron sulfate (FeSO4) coagulants. After 23 months of flooding and coagulant treatment, sediment samples were collected near the inlet, middle, and outlet of each wetland to determine vertical accretion rates, bulk density, sediment composition, and carbon sequestration rates. Wetlands treated with PAC had the highest and most spatially consistent vertical accretion rates (~6 cm year-1), while the FeSO4 wetlands had similarly high accretion rates near the inlet but rates similar to the untreated wetland (~1.5 cm year-1) at the middle and outlet sites. The composition of the newly accreted sediment in the PAC and FeSO4 treatments was high in the added metal (aluminum and iron, respectively), but the percent metal by weight was similar to native soils of California. As has been observed in other constructed wetlands, the newly accreted sediment material had lower bulk densities than the native soil material (0.04-0.10 g cm-3 versus 0.2-0.3 g cm-3), suggesting these materials will consolidate over time. Finally, this technology accelerated carbon burial, with rates in PAC treated wetland (0.63 kg C m-2 yr-1) over 2-fold greater than the untreated control (0.28 kg C m-2 yr-1). This study demonstrates the feasibility of using constructed wetlands treated with coagulants to reverse subsidence by accreting the resulting organo-metal flocculent and storing carbon at rates exceeding untreated wetlands. Management and design questions remain for how to best integrate this technology into heavily subsided land to lower the risks and consequences associated with levee failure, improve water quality, and ultimately restore these lands to tidal wetlands.

California↗

USGS2024

Birdwatching preferences reveal synergies and tradeoffs among recreation, carbon, and fisheries ecosystem services in Pacific Northwest estuaries, USA

Coastal ecosystems provide multiple ecosystem services that are valued in diverse ways. The Nisqually River Delta (the Delta), an estuary in Puget Sound, Washington, U.S.A., is co-managed by the Nisqually Indian Tribe and the Billy Frank Jr. Nisqually National Wildlife Refuge. In an ecosystem services assessment, we used different service-appropriate methods including citizen science, statistical and geospatial models, and scenario analysis to evaluate three ecosystem services – recreational birdwatching, soil carbon accumulation and fishery production – indicated as priorities for the Refuge, Nisqually Indian Tribe, and surrounding communities. We developed a generalized additive mixed model set based on eBird mobile application birdwatching observations to understand the biological and landscape features that influence birdwatching and to project birdwatching visitation based on scenarios of Delta habitat change. We evaluated ecosystem service synergies and tradeoffs associated with habitat change for three coastal habitat types using scenario outputs from the birdwatching model and published results on Delta soil carbon accumulation and fishery production. The highest-ranked birdwatching models explained 88 % of the deviance and showed that visitation was greatest in winter months when distance to major cities was approximately 20 km. Recreational birdwatching increased with increasing area of forested wetland, emergent wetland, aquatic vegetation bed, open access, and total estuary. With increasing forested and emergent wetland area, recreational birdwatching, out-migrating juvenile Chinook salmon weight and soil carbon accumulation all increased. With increasing aquatic vegetation bed (resulting from sea level rise), recreational birdwatching increased, but salmon weight and soil carbon accumulation decreased. We identified practical ways in which ecosystem services may be incorporated into adaptive management frameworks that support climate adaptation decision making. This study illustrated how use of ecosystem services can help managers make decisions that have greater benefit for wildlife and people, communicate the societal value of decisions and increase local support and participation.

Washington↗

USGS2011

Development of a carbonate absorption-based process for post-combustion CO2 capture: The role of biocatalyst to promote CO2 absorption rate

An Integrated Vacuum Carbonate Absorption Process (IVCAP) for post-combustion carbon dioxide (CO2) capture is described. IVCAP employs potassium carbonate (PC) as a solvent, uses waste or low quality steam from the power plant for CO2 stripping, and employs a biocatalyst, carbonic anhydrase (CA) enzyme, for promoting the CO2 absorption into PC solution. A series of experiments were performed to evaluate the activity of CA enzyme mixed in PC solutions in a stirred tank reactor system under various temperatures, CA dosages, CO2 loadings, CO2 partial pressures, and the presence of major flue gas contaminants. It was demonstrated that CA enzyme is an effective biocatalyst for CO2 absorption under IVCAP conditions. ?? 2011 Published by Elsevier Ltd.

Conference Paper↗

USGS2011

U.S. Department of Energy's regional carbon sequestration partnership initiative: Update on validation and development phases

The U.S. Department of Energy (DOE) is the lead federal agency for the development and deployment of carbon sequestration technologies. The Regional Carbon Sequestration Partnerships (RCSPs) are the mechanism DOE utilizes to prove the technology and to develop human capital, stakeholder networks, information for regulatory policy, best practices documents and training to work toward the commercialization of carbon capture and storage (CCS). The RCSPs are tasked with determining the most suitable technologies, regulations, and infrastructure for carbon capture, transport, and storage in their respective geographic areas of responsibility. The seven partnerships include more than 400 state agencies, universities, national laboratories, private companies, and environmental organizations, spanning 43 states and four Canadian provinces. The Regional Partnerships Initiative is being implemented in three phases: Characterization, Validation, and Development. The initial Characterization Phase began in 2003 and was completed in 2005 and focused on characterization of CO2 storage potential within each region. It was followed by the Validation Phase, which began in 2005 and is nearing completion in 2011. The focus of the Validation Phase has been on small-scale field tests throughout the seven partnerships in various formation types such as saline, oil-bearing, and coal seams. The Validation Phase has characterized suitable CO2 storage reservoirs and identified the need for comprehensive legal and regulatory frameworks to enable commercial-scale CCS deployment. Finally, the Development Phase will consist of a series of large-scale, one-million-ton, injection tests throughout the United States and Canada. The objective of these large-scale tests is to identify the regulatory path or challenges in permitting CCS projects, to demonstrate the technology can inject CO2 safely, and to verify its permanence in geologic formations in preparation for the commercialization of geologic sequestration. ?? 2010 Elsevier Ltd. All rights reserved. ?? 2011 Published by Elsevier Ltd.

Conference Paper↗

USGS2014

Dynamics of submarine groundwater discharge and associated fluxes of dissolved nutrients, carbon, and trace gases to the coastal zone (Okatee River estuary, South Carolina)

Multiple techniques, including thermal infrared aerial remote sensing, geophysical and geological data, geochemical characterization and radium isotopes, were used to evaluate the role of groundwater as a source of dissolved nutrients, carbon, and trace gases to the Okatee River estuary, South Carolina. Thermal infrared aerial remote sensing surveys illustrated the presence of multiple submarine groundwater discharge sites in Okatee headwaters. Significant relationships were observed between groundwater geochemical constituents and 226 Ra activity in groundwater with higher 226 Ra activity correlated to higher concentrations of organics, dissolved inorganic carbon, nutrients, and trace gases to the Okatee system. A system-level radium mass balance confirmed a substantial submarine groundwater discharge contribution of these constituents to the Okatee River. Diffusive benthic flux measurements and potential denitrification rate assays tracked the fate of constituents in creek bank sediments. Diffusive benthic fluxes were substantially lower than calculated radium-based submarine groundwater discharge inputs, showing that advection of groundwater-derived nutrients dominated fluxes in the system. While a considerable potential for denitrification in tidal creek bank sediments was noted, in situ denitrification rates were nitrate-limited, making intertidal sediments an inefficient nitrogen sink in this system. Groundwater geochemical data indicated significant differences in groundwater chemical composition and radium activity ratios between the eastern and western sides of the river; these likely arose from the distinct hydrological regimes observed in each area. Groundwater from the western side of the Okatee headwaters was characterized by higher concentrations of dissolved organic and inorganic carbon, dissolved organic nitrogen, inorganic nutrients and reduced metabolites and trace gases, i.e. methane and nitrous oxide, than groundwater from the eastern side. Differences in microbial sulfate reduction, organic matter supply, and/or groundwater residence time likely contributed to this pattern. The contrasting features of the east and west sub-marsh zones highlight the need for multiple techniques for characterization of submarine groundwater discharge sources and the impact of biogeochemical processes on the delivery of nutrients and carbon to coastal areas via submarine groundwater discharge.

South Carolina↗

USGS2014

Compositional and stable carbon isotopic fractionation during non-autocatalytic thermochemical sulfate reduction by gaseous hydrocarbons

The possibility of autocatalysis during thermochemical sulfate reduction (TSR) by gaseous hydrocarbons was investigated by examination of previously reported laboratory and field data. This reaction was found to be a kinetically controlled non-autocatalytic process, and the apparent lack of autocatalysis is thought to be due to the absence of the required intermediate species. Kinetic parameters for chemical and carbon isotopic fractionations of gaseous hydrocarbons affected by TSR were calculated and found to be consistent with experimentally derived values for TSR involving long-chain hydrocarbons. Model predictions based on these kinetic values indicate that TSR by gaseous hydrocarbon requires high-temperature conditions. The oxidation of C 2–5 hydrocarbons by sulfate reduction is accompanied by carbon isotopic fractionation with the residual C 2–5 hydrocarbons becoming more enriched in 13 C. Kinetic parameters were calculated for the stable carbon isotopic fractionation of gaseous hydrocarbons that have experienced TSR. Model predictions based on these kinetics indicate that it may be difficult to distinguish the effects of TSR from those of thermal maturation at lower levels of hydrocarbon oxidation; however, unusually heavy δ 13 C 2+ values (>−10‰) can be diagnostic of high levels of conversion (>50%). Stoichiometric and stable carbon isotopic data show that methane is stable under the investigated reaction conditions and is likely a product of TSR by other gaseous hydrocarbons rather than a significant reactant. These results indicate that the overall TSR reaction mechanism for oxidation of organic substrates containing long-chain hydrocarbons involves three distinct phases as follows: (1) an initial slow and non-autocatalytic stage characterized by the reduction of reactive sulfate by long-chain saturated hydrocarbons; (2) a second autocatalytic reaction phase dominated by reactions involving reduced sulfur species and partially oxidized hydrocarbons; (3) and a final, or late-stage, TSR reaction in which hydrocarbon oxidation continues at a slower rate via the non-autocatalytic reduction of sulfate by gaseous hydrocarbons.

Geochimica et Cosmochimica Acta↗

USGS2019

Bright carbonate surfaces on Ceres as remnants of salt-rich water fountains

Vinalia and Cerealia Faculae are bright and salt-rich localized areas in Occator crater on Ceres. The predominance of the near-infrared signature of sodium carbonate on these surfaces suggests their original material was a brine. Here we analyze Dawn Framing Camera's images and characterize the surfaces as composed of a central structure, either a possible depression (Vinalia) or a central dome (Cerealia), and a discontinuous mantling. We consider three materials enabling the ascent and formation of the faculae: ice ascent with sublimation and carbonate particle lofting, pure gas emission entraining carbonate particles, and brine extrusion. We find that a mechanism explaining the entire range of morphologies, topographies, as well as the common composition of the deposits is brine fountaining. This process consists of briny liquid extrusion, followed by flash freezing of carbonate and ice particles, particle fallback, and sublimation. Subsequent increase in briny liquid viscosity leads to doming. Dawn observations did not detect currently active water plumes, indicating the frequency of such extrusions is longer than years.

Icarus↗

USGS2004

Land use change and terrestrial carbon stocks in Senegal

Environmental degradation resulting from long-term drought and land use change has affected terrestrial carbon (C) stocks within Africa's Sahel. We estimated Senegal's terrestrial carbon stocks in 1965, 1985, and 2000 using an inventory procedure involving satellite images revealing historical land use change, and recent field measurements of standing carbon stocks occurring in soil and plants. Senegal was divided into eight ecological zones containing 11 land uses. In 2000, savannas, cultivated lands, forests, and steppes were the four largest land uses in Senegal, occupying 70, 22, 2.7, and 2.3 percent of Senegal's 199,823 km 2 . System C stocks ranged from 9 t C ha −1 in degraded savannas in the north, to 113 t C ha −1 in the remnant forests of the Senegal River Valley. This approach resulted in estimated total C stocks of 1019 and 727 MT C between 1965 and 2000, respectively, indicating a loss of 292 MT C over 35 years. The proportion of C residing in biomass decreased with time, from 55 percent in 1965 to 38 percent in 2000. Calculated terrestrial C flux for 1993 was −7.5 MT C year −1 and had declined by 17 percent over the previous 18 years. Most of the terrestrial C flux in 1993 was attributed to biomass C reduction. Human disturbance accounted for only 22 percent of biomass C loss in 1993, suggesting that the effects of long-term Sahelian drought continue to play an overriding role in ecosystem change. Some carbon mitigation strategies for Senegal were investigated, including potential C sequestration levels. Opportunities for C mitigation exist but are constrained by available knowledge and access to resources.

Journal of Arid Environments↗

USGS2021

Uranium(VI) attenuation in a carbonate-bearing oxic alluvial aquifer

Uranium minerals are commonly found in soils and sediment across the United States at an average concentration of 2–4 mg/kg. Uranium occurs in the environment primarily in two forms, the oxidized, mostly soluble uranium(VI) form, or the reduced, sparingly soluble reduced uranium(IV) form. Here we describe subsurface geochemical conditions that result in low uranium concentrations in an alluvial aquifer with naturally occurring uranium in soils and sediments in the presence of complexing ligands under oxidizing conditions. Groundwater was saturated with respect to calcite and contained calcium (78–90 mg/L) with elevated levels of carbonate alkalinity (291–416 mg/L as HCO 3 − ). X-ray adsorption near edge structure (XANES) spectroscopy identified that sediment-associated uranium was oxidized as a uranium(VI) form (85%). Calcite was the predominant mineral by mass in the ultrafine fraction in uranium-bearing sediments (>16 mg/kg). Groundwater geochemical modeling indicated calcite and/or a calcium-uranyl-carbonate mineral such as liebigite in equilibrium with groundwater. The δ 13 C (0.57‰ ± 0.15‰) was indicative of abiotic carbonate deposition. Thus, solid-phase uranium(VI) associated with carbonate is likely maintaining uranium(VI) groundwater levels below the maximum contaminant level (MCL; 30 µg/L), presenting a deposition mechanism for uranium attenuation rather than solely a means of mobilization.

Nebraska↗