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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Further carbon isotope measurements of LEW 88516

Douglas et al. have previously analyzed the carbon content and isotopic composition of a crushed sample (sub-sample 13) of the shergottite, LEW 88516. The powder, which was from a relatively large portion of the meteorite in order to obtain a representative sample, was distributed amongst the scientific community. However, it is probable that the preparation procedure was not optimized for the purposes of carbon measurements. Indeed, it was found that LEW 88516,13 contained over 1200 ppm carbon, a concentration which is greater than that present in any other SNC meteorite. That a close relative, ALH A77005, contains only 141 ppm carbon seems to implicate the preparation procedure as being responsible for the apparently high carbon content of LEW 88516. However, it may also be the nature of the fine powder which has resulted in contamination. The observation of high carbon content in LEW 88516,13 highlights the extreme difficulty of trying to obtain representative samples of whole meteorites for this kind of investigation. Presented herein are some further measurements of LEW 88516 which should serve to clarify some of the issues raised by the previous investigation.

Wright, I. P.↗
DOE OSTI2020

A genetic link between leaf carbon isotope composition and whole-plant water use efficiency in the C 4 grass Setaria

Genetic selection for whole-plant water use efficiency (yield per transpiration; WUE plant ) in any crop-breeding programme requires high-throughput phenotyping of component traits of WUE plant such as intrinsic water use efficiency (WUE i ; CO 2 assimilation rate per stomatal conductance). Measuring WUE i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ 13 C leaf ) has been suggested as a potential time-ntegrated proxy for WUE i that may provide a tool to screen for WUE plant . However, a genetic link between δ 13 C leaf and WUE plant in a C 4 species has not been well established. Thus, to confirm if there is a genetic relationship in a C 4 plant between δ 13 C leaf and WUE plant under well watered and water-limited growth conditions, a high-throughput phenotyping facility was used to measure WUE plant in a recombinant inbred line (RIL) population created between the C 4 grasses Setaria viridis and S. italica . Three quantitative trait loci (QTL) for δ 13 C leaf were found and co-localized with transpiration, biomass accumulation, and WUE plant . Additionally, WUE plant for each of the δ 13 C leaf QTL allele classes was negatively correlated with δ 13 C leaf , as would be predicted when WUE i influences WUE plant . These findings demonstrate that δ 13 C leaf is genetically linked to WUE plant , likely to be through their relationship with WUE i , and can be used as a high-throughput proxy to screen for WUE plant in these C 4 species.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1994

Carbon isotopic fractionation in lipids from methanotrophic bacteria: relevance for interpretation of the geochemical record of biomarkers

Experiments with cultured aerobic methane oxidising bacteria confirm that their biomarker lipids will be significantly depleted in 13C compared to the substrate. The methanotrophic bacteria Methylococcus capsulatus and Methylomonas methanica, grown on methane and using the RuMP cycle for carbon assimilation, show maximum 13C fractionation of approximately 30% in the resultant biomass. In M. capsulatus, the maximum fractionation is observed in the earliest part of the exponential growth stage and decreases to approximately 16% as cells approach stationary phase. This change may be associated with a shift from the particulate form to the soluble form of the methane monooxygenase enzyme. Less than maximum fractionation is observed when cells are grown with reduced methane availability. Biomass of M. capsulatus grown on methanol was depleted by 9% compared to the substrate. Additional strong 13C fractionation takes place during polyisoprenoid biosynthesis in methanotrophs. The delta 13C values of individual hopanoid and steroid biomarkers produced by these organisms were as much as l0% more negative than total biomass. In individual cultures, squalene was 13C-enriched by as much as 14% compared to the triterpane skeleton of bacteriohopaneaminopentol. Much of the isotopic dispersion in lipid metabolites could be attributed to shifts in their relative abundances, combined with an overall reduction in fractionation during the growth cycle. In cells grown on methanol, where there was no apparent effect of growth stage on overall fractionation there were still significant isotopic differences between closely related lipids including a 5.3% difference between the hopane and 3 beta-methylhopane skeletons. Hopane and sterane polyisoprenoids were also 13C-depleted compared to fatty acids. These observations have significant implications for the interpretation of specific compound isotopic signatures now being measured for hydrocarbons and other lipids present in sediments and petroleum. In particular, biomarker lipids produced by a single organism do not necessarily have the same carbon isotopic composition.

NASA Center ARC↗
NASA NTRS1990

Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons

The organic matter found in sedimentary rocks must derive from many sources; not only from ancient primary producers but also from consumers and secondary producers. In all of these organisms, isotope effects can affect the abundance and distribution of 13C in metabolites. Here, by using an improved form of a previously described technique in which the effluent of a gas chromatograph is continuously analysed isotopically, we report evidence of the diverse origins of sedimentary organic matter. The record of 13C abundances in sedimentary carbonate and total organic carbon can be interpreted in terms of variations in the global carbon cycle. Our results demonstrate, however, that isotope variations within sedimentary organic mixtures substantially exceed those observed between samples of total organic carbon. Resolution of isotope variations at the molecular level offers a new and convenient means of refining views both of localized palaeoenvironments and of control mechanisms within the global carbon cycle.

Non-NASA Center↗
NASA NTRS2011

Carbon Isotope Systematics in Mineral-Catalyzed Hydrothermal Organic Synthesis Processes at High Temperature and Pressures

Observation of methane in the Martian atmosphere has been reported by different detection techniques. Reduction of CO2 and/or CO during serpentization by mineral surface catalyzed Fischer-Tropsch Type (FTT) synthesis may be one possible process responsible for methane generation on Mars. With the evidence a recent study has discovered for serpentinization in deeply buried carbon rich sediments, and more showing extensive water-rock interaction in Martian history, it seems likely that abiotic methane generation via serpentinization reactions may have been common on Mars. Experiments involving mineral-catalyzed hydrothermal organic synthesis processes were conducted at 750 C and 5.5 Kbars. Alkanes, alcohols and carboxylic acids were identified as organic compounds. No "isotopic reversal" of delta C-13 values was observed for alkanes or carboxylic acids, suggesting a different reaction pathway than polymerization. Alcohols were proposed as intermediaries formed on mineral surfaces at experimental conditions. Carbon isotope data were used in this study to unravel the reaction pathways of abiotic formation of organic compounds in hydrothermal systems at high temperatures and pressures. They are instrumental in constraining the origin and evolution history of organic compounds on Mars and other planets.

Fu, Qi↗
NASA NTRS1972

Carbon isotopic studies of organic matter in Precambrian rocks.

A survey has been undertaken of the carbon composition of the total organic fraction of a suite of Precambrian sediments to detect isotopic trends possibly correlative with early evolutionary events. Early Precambrian cherts of the Fig Tree and upper and middle Onverwacht groups of South Africa were examined for this purpose. Reduced carbon in these cherts was found to be isotopically similar to photosynthetically produced organic matter of younger geological age. Reduced carbon in lower Onverwacht cherts was found to be anomalously heavy; it is suggested that this discontinuity may reflect a major event in biological evolution.

Oehler, D. Z.↗
NASA NTRS1988

Carbon isotope geochemistry of hydrocarbons in the Cerro Prieto geothermal field, Baja California Norte, Mexico

Hydrocarbon abundances and stable-isotopic compositions were measured in wells M5, M26, M35 and M102, which represent a range of depths (1270-2000 m) and temperatures (275-330 degrees C) in the field. In order to simulate the production of the geothermal hydrocarbons, gases were collected from the pyrolysis of lignite in the laboratory. This lignite was obtained from a well which sampled rock strata which are identical to those occurring in the field, but which have experienced much lower subsurface temperatures. In both the well and the laboratory observations, high-temperature environments favored higher relative concentrations of methane, ethane and benzene and generally higher delta 13C-values in the individual hydrocarbons. The best correlation between the laboratory and well data is obtained when laboratory-produced gases from experiments conducted at lower (400 degrees C) and higher (600 degrees C) temperatures are mixed. This improved correlation suggests that the wells are sampling hydrocarbons produced from a spectrum of depths and temperatures in the sediments.

NASA Center ARC↗
NASA NTRS1998

Carbon-Isotopic Dynamics of Streams, Taylor Valley, Antarctica: Biological Effects

We have investigated the role of biological processes in the C-isotopic dynamics of the aquatic ecosystems in Taylor Valley, Antarctica. This cold desert ecosystem is characterized by the complete lack of vascular plants, and the presence of algal mats in ephemeral streams and perennially ice covered lakes. Streams having abundant algal mats and mosses have very low sigma CO2 concentrations, as well as the most depleted delta C-13 values (-4%). Previous work has shown that algal mats in these streams have delta C-13 values averaging -7.01%. These values are similar to those observed in the algal mats in shallow areas of the lakes in Taylor Valley, where CO2 is thought to be colimiting to growth. These low Sigma CO2 concentrations, and delta C(13) signatures heavier than the algal mats, suggest that CO2 may be colimiting in the streams, as well. Streams with little algal growth, especially the longer ones in Fryxell Basin, have higher Sigma CO2 concentrations and much more enriched isotopic signatures (as high as +8%). In these streams, the dissolution of isotopically enriched, cryogenic CaCO3 is probably the major source of dissolved carbonate. The delta C(13) geochemistry of Antarctic streams is radically different from the geochemistry of more temperate streams, as it is not affected by terrestrially produced, isotopically depleted Sigma CO2. These results have important implications for the understanding of "biogenic" carbonate that might have been produced from aquatic ecosystems in the past on Mars.

Neumann, K.↗
DOE OSTI2021

Proton decays in 16 Ne and 18 Mg and isospin-symmetry breaking in carbon isotopes and isotones

We report that proton-rich nuclei possess unique properties in the nuclear chart. Due to the presence of both continuum coupling and Coulomb interaction, phenomena such as halos, Thomas-Ehrman shift, and proton emissions can occur. Relevant experimental data are difficult to obtain, so that theoretical calculations are needed to understand nuclei at drip lines and to guide experimentalists. In particular, the 16 Ne and 18 Mg isotopes are supposed to be one-proton and/or two-proton emitting nuclei, but associated experimental data are either incomplete or even unavailable. Consequently, we performed Gamow shell model calculations of carbon isotones bearing A = 15 - 18 . Isospin-symmetry breaking occurring in carbon isotones and isotopes is also discussed. It is hereby shown that the mixed effects of continuum coupling and Coulomb interaction at drip lines generate complex patterns in isospin multiplets. Added to that, it is possible to determine the one-proton and two-proton widths of 16 Ne and 18 Mg . Obtained decay patterns are in agreement with those obtained in previous experimental and theoretical works. Moreover, to our knowledge, this is the first theoretical calculation of binding energy and partial decay widths of 18 Mg in a configuration interaction picture.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2000

The Precambrian Biogeochemical Carbon Isotopic Record: Contributions of Thermal Versus Biological Processes

Superplumes offer a new approach for understanding global C cycles. Isotopes help to discern the impacts of geological, environmental and biological processes ujpun the evolution of these cycles. For example, C-13/C-12 values of coeval sedimentary organics and carbonates give global estimates of the fraction of C buried as organics (Forg), which today lies near 0.2. Before Oxygenic photosynthesis arose, our biosphere obtained reducing power for biosynthesis solely from thermal volatiles and rock alteration. Thus Forg was dominated by the mantle redox state, which has remained remarkably constant for greater than Gy. Recent data confirm that the long-term change in Forg had been small, indicating that the mantle redox buffer remains important even today. Oxygenic photosynthesis enabled life to obtain additional reducing power by splitting the water molecule. Accordingly, biological organic production rose above the level constrained by the mantle-derived flux of reduced species. For example, today, chemoautotrophs harvesting energy from hydrothermal emanations can synthesize at most between 0.2 x 10(exp 12) and 2x 10(exp 12) mol C yr-1 of organic C globally. In contrast, global photosynthetic productivity is estimated at 9000 x 10(exp 12) mol C yr-1. Occasionally photosynthetic productivity did contribute to dramatically -elevated Forg values (to 0.4 or more) as evidenced by very high carbonate C-13/C-12. The interplay between biological, tectonic and other environmental factors is illustrated by the mid-Archean to mid-Proterozoic isotopic record. The relatively constant C-13/C-12 values of Archean carbonates support the view that photosynthetically-driven Forg increases were not yet possible. In contrast, major excursions in C-13/C-12, and thus also in Forg, during the early Proterozoic confirmed the global importance of oxygenic photosynthesis by that time. Remarkably, the superplume event at 1.9 Ga did not trigger another major Forg increase, despite the favorable conditions for organic burial that were evidenced by elevated sea levels and higher black shale abundances. Perhaps those superplume-rated processes that favored enhanced organic burial were offset by the ability of mantle-derived redox buffering, which was enhanced during the superplume event, to limit excursions in Forg.

DesMarais, David J.↗
NASA NTRS2012

Observations of Carbon Isotopic Fractionation in Interstellar Formaldehyde

Primitive Solar System materials (e.g. chondrites. IDPs, the Stardust sample) show large variations in isotopic composition of the major volatiles (H, C, N, and O ) even within samples, witnessing to various degrees of processing in the protosolar nebula. For ex~ ample. the very pronounced D enhancements observed in IDPs [I] . are only generated in the cold. dense component of the interstellar medium (ISM), or protoplanetary disks, through ion-molecule reactions in the presence of interstellar dust. If this isotopic anomaly has an interstellar origin, this leaves open the possibility for preservation of other isotopic signatures throughout the form ation of the Solar System. The most common form of carbon in the ISM is CO molecules, and there are two potential sources of C-13 fractionation in this reservoir: low temperature chemistry and selective photodissociation. While gas-phase chemistry in cold interstellar clouds preferentially incorporates C-13 into CO [2], the effect of self-shielding in the presence of UV radiation instead leads to a relative enhancement of the more abundant isotopologue, 12CO. Solar System organic material exhibit rather small fluctuations in delta C-13 as compared to delta N-15 and delta D [3][1], the reason for which is still unclear. However, the fact that both C-13 depleted and enhanced material exists could indicate an interstellar origin where the two fractionation processes have both played a part. Formaldehyde (H2CO) is observed in the gas-phase in a wide range of interstellar environments, as well as in cometary comae. It is proposed as an important reactant in the formation of more complex organic molecules in the heated environments around young stars, and formaldehyde polymers have been suggested as the common origin of chondritic insoluable organic matter (IOM) and cometary refractory organic solids [4]. The relatively high gas-phase abundance of H2CO observed in molecular clouds (10(exp- 9) - 10(exp- 8) relative to H2) makes it feasible to observe its less common isotopologues. As a step in our investigation of C-13 fractionation patterns in the ISM, we here present comparisons between observations of the C-13 fraction in formaldehyde, and chemical fractionation models.

Wirstrom, E. S.↗
DOE OSTI2020

Stable and radioactive carbon isotope partitioning in soils and saturated systems: a reactive transport modeling benchmark study

This benchmark provides the first rigorous test of a three-isotope system [ 12 C, 13 C, and 14 C] subject to the combined effects of radioactive decay and both stable equilibrium and kinetic fractionation. We present a series of problems building in complexity based on the cycling of carbon in both organic and inorganic forms. The key components implement (1) equilibrium fractionation between multiple coexisting carbon species as a function of pH, (2) radioactive decay of radiocarbon with associated mass-dependent speciation demonstrating appropriate correction of the Δ 14 C value in agreement with reporting convention, and (3) kinetic stable isotope fractionation due to the oxidation of organic carbon to inorganic forms as a function of time and space in an open, through-flowing system. Participating RTM codes are CrunchTope, ToughReact, Hytec, and The Geochemist’s Workbench. Across all problem levels, simulation results from all RTMs demonstrate good agreement.

58 GEOSCIENCES↗
NASA NTRS1975

Evolution of carbon isotopes, agglutinates, and the lunar regolith

Apollo 17 light-mantle soils and Apollo 15 Apennine Front soils are compared with respect to isotopic enrichment of C-13 and the maturity of the site. Analyses of soil-size fractions indicate that while the carbon concentration on particle surfaces remains relatively constant with increasing soil maturity, total surface-correlated carbon increases due to increasing total soil surface area. The role of agglutinates in the incorporation of surface-correlated carbon into aggregate grains is examined; agglutinates contain a major percentage of the carbon found in mature soil, and the volume-correlated carbon component in agglutinates apparently continues to increase after the surface-correlated carbon concentrations have reached a constant value. Constraints that may limit the carbon concentration in lunar soils to a value not greater than 300 micrograms/g are considered.

Desmarais, D. J.↗
NASA NTRS2020

Carbon isotope evidence for the substrates and mechanisms of prebiotic synthesis in the early solar system

Meteorites contain prebiotic, bio-relevant organic compounds including amino acids. Their syntheses could result from diverse sources and mechanisms and provide a window on the conditions and materials present in the early solar system. Here we constrain alanine’s synthetic history in the Murchison meteorite using site-specific 13C/12C measurements, reported relative to the VPDB standard. The δ13CVPDB values of −29 ± 10‰, 142 ± 20‰, and −36 ± 20‰ for the carboxyl, amine-bound, and methyl carbons, respectively, are consistent with Strecker synthesis of interstellar-medium-derived aldehydes, ammonia, and low-δ13C nebular or interstellar-medium-derived CN. We report experimentally measured isotope effects associated with Strecker synthesis, and use them to constrain the δ13C values of the alanine precursors, which we then use to construct a model that predicts the molecular-average δ13C values of 19 other organic compounds of prebiotic significance found in Murchison if they were made by our proposed synthetic network. Most of these predictions agree with previous measurements, suggesting that interstellar-medium-derived aldehydes and nebular and/or pre-solar CN could have served as substrates for synthesis of a wide range of prebiotic compounds in the early solar system.

L.Chimiak↗
DOE OSTI2020

Carbon isotope evidence for the substrates and mechanisms of prebiotic synthesis in the early solar system

Meteorites contain prebiotic, bio-relevant organic compounds including amino acids. Their syntheses could result from diverse sources and mechanisms and provide a window on the conditions and materials present in the early solar system. Here we constrain alanine’s synthetic history in the Murchison meteorite using site-specific 13 C/ 12 C measurements, reported relative to the VPDB standard. The δ 13 C VPDB values of -29 ± 10‰, 142 ± 20‰, and -36 ± 20‰ for the carboxyl, amine-bound, and methyl carbons, respectively, are consistent with Strecker synthesis of interstellar-medium-derived aldehydes, ammonia, and low-δ 13 C nebular or interstellar-medium-derived CN. We report experimentally measured isotope effects associated with Strecker synthesis, and use them to constrain the δ 13 C values of the alanine precursors, which we then use to construct a model that predicts the molecular-average δ 13 C values of 19 other organic compounds of prebiotic significance found in Murchison if they were made by our proposed synthetic network. Finally, most of these predictions agree with previous measurements, suggesting that interstellar-medium-derived aldehydes and nebular and/or pre-solar CN could have served as substrates for synthesis of a wide range of prebiotic compounds in the early solar system.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1986

Stable carbon isotope ratios in atmospheric methane and some of its sources

Ratios of C-13/C-12 have been measured in atmospheric methane and in methane collected from sites and biota that represent potentially large sources of atmospheric methane. These include temperate marshes (about -48 percent to about -54 percent), landfills (about -51 percent to about -55 percent), and the first reported values for any species of termite (-72.8 + or - 3.1 percent for Reticulitermes tibialis and -57.3 + or - 1.6 percent for Zootermopsis angusticollis). Numbers in parentheses are delta C-13 values with respect to PDB (Peedee belemnite) carbonate. Most methane sources reported thus far are depleted in C-13 with respect to atmospheric methane (-47.0 + or - 0.3 percent). Individual sources of methane should have C-13/C-12 ratios characteristic of mechanisms of CH4 formation and consumption prior to release to the atmosphere. The mass-weighted average isotopic composition of all sources should equal the mean C-13 of atmospheric methane, corrected for a kinetic isotope effect in the OH attack of CH4. Assuming the kinetic isotope effect to be small (about -3.0 percent correction to -47.0), as in the literature, the new values given here for termite methane do not help to explain the apparent discrepancy between C-13/C-12 ratios of the known CH4 sources and that of atmospheric CH4.

Tyler, Stanley C.↗
NASA NTRS1992

The carbon isotope biogeochemistry of acetate from a methanogenic marine sediment

The delta C-13 value of porewater acetate isolated from the anoxic sediments of Cape Lookout Bight (North Carolina) ranged from -17.6 percent in the sulfate reduction zone to -2.8 percent in the underlying methanogenic zone. The large C-13 enrichment in the sulfate-depleted sediments appears to be associated with the dissimilation of acetate to CH4 and CO2. Fractionation factors for that process were estimated to be 1.032 +/- 0.014 and 1.036 +/- 0.019 for the methyl and carboxyl groups. A subsurface maximum in delta C-13 of the total acetate molecule, as well as the methyl and carboxyl carbons at 10-15 cm depth within the sediment column, indicate that changes in the relative rates of acetate cycling pathways occur in the methanogenic zone. The methyl group of the acetate was depleted in C-13 by 7-14 percent relative to the carboxyl moiety. The intramolecular heterogeneity may be the result of both synthetic and catabolic isotope effects.

Blair, N. E.↗
NASA NTRS1977

Variation in stable carbon isotopes in organic matter from the Gunflint Iron Formation

Results are presented for an isotopic analysis of the kerogen separated from 15 samples of the Gunflint Iron Formation, Ontario, and the conformably overlying Rove Formation. Reasons for which the Gunflint Iron Formation is suitable for such a study of a single Precambrian formation are identified. The general geology of the formation is outlined along with sample selection, description, and preparation. Major conclusions are that the basal Gunflint algal chert and shale facies are depleted in C-13 relative to the chert-carbonate and taconite facies, that differences in the delta C-13 values between Gunflint facies correlate with marked differences in their biological source materials as evidenced by their respective microbiotas, that the anthraxolites are anomalously depleted in C-13 relative to the kerogen of their encompassing cherts and shales, and that the effects of igneous intrusion and concomitant thermal alteration are shown by a marked loss of C-12 at the contact. The demonstration that not all kerogens are isotopically alike stresses the importance of facies data to the interpretation of C-13/C-12 ratios of ancient organic matter.

Barghoorn, E. S.↗