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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Compound-specific Isotope Analysis of Cyanobacterial Pure cultures and Microbial Mats: Effects of Photorespiration?

Microbial mats are considered modern homologs of Precambrian stromatolites. The carbon isotopic compositions of organic matter and biomarker lipids provide clues to the depositional environments of ancient mat ecosystems. As the source of primary carbon fixation for over two billion years, an understanding of cyanobacterial lipid biosynthesis, associated isotopic discriminations, and the influence of physiological factors on growth and isotope expression is essential to help us compare modern microbial ecosystems to their ancient counterparts. Here, we report on the effects of photorespiration (PR) on the isotopic composition of cyanobacteria and biomarker lipids, and on potential PR effects associated with the composition of various microbial mats. The high light, high O2 and limiting CO2 conditions often present at the surface of microbial mats are known to support PR in cyanobacteria. The oxygenase function of ribulose bisphosphate carboxylase/oxygenase can result in photoexcretion of glycolate and subsequent degration by heterotrophic bacteria. We have found evidence which supports an isotopic depletion (increased apparent E) scaled to O2 level associated with growth of Phormidium luridum at low CO2 concentrations (less than 0.04%). Similar to previous studies, isotopic differences between biomass and lipid biomarkers, and between lipid classes were positively correlated with overall fractionation, and should provide a means of estimating the influence of PR on overall isotopic composition of microbial mats. Several examples of microbial mats growing in the hydrothermal waters of Yellowstone National Park and the hypersaline marine evaporation ponds at Guerrero Negro, Baja Sur Mexico will be compared with a view to PR as a possible explanation of the relatively heavy C-isotope composition of hypersaline mats.

Jahnke, L. L.↗
NASA NTRS1991

The biogeochemistry of microbial mats, stromatolites and the ancient biosphere

Stromatolites offer an unparalleled geologic record of early life, because they constitute the oldest and most abundant recognizable remains of microbial ecosystems. Microbial mats are living homologs of stromatolites; thus, the physiology of the microbiota as well as the processes which create those features of mats (e.g., biomarker organic compounds, elemental and stable isotopic compositions) which are preserved in the ancient record. Observations of the carbon isotopic composition (delta C-13) of stromatolites and microbial mats were made and are consistent with the hypothesis that atmospheric CO2 concentrations have declined by at least one to two orders of magnitude during the past 2.5 Ga. Whereas delta C-13 values of carbonate carbon average about 0 permil during both the early and mid-Proterozoic, the delta C-13 values of stromatolitic organic matter increase from an average of -35 between 2.0 and 2.6 Ga ago to an average of about -28 about 1.0 Ga ago. Modern microbial mats in hypersaline environments have delta C-13 values typically in the range of -5 to -9, relative to an inorganic bicarbonate source at 0 permil. Both microbial mats and pur cultures of cyanobacteria grown in waters in near equilibrium with current atmospheric CO2 levels exhibit minimal discrimination against C-13. In contrast, hot spring cyanobacterial mats or cyanobacterial cultures grown under higher CO2 levels exhibit substantially greater discrimination. If care is taken to compare modern mats with stromatolites from comparable environments, it might be possible to estimate ancient levels of atmospheric CO2. In modern microbial mats, a tight coupling exists between photosynthetic organic carbon production and subsequent carbon oxidation, mostly by sulfate reduction. The rate of one process fuels a high rate of the other, with much of the sulfate reduction occurring within the same depth interval as oxygenic photosynthesis. Other aspects of this study are presented.

Desmarais, D. J.↗
NASA NTRS1987

Isotopic compositions and probable origins of organic molecules in the Eocene Messel shale

It is shown here that the carbon isotopic compositions of biomarkers from the Eocene Messel shale, accumulated 47 + or - 2 million years ago in anaerobic waters at the bottom of a lake, allow identification of specific sources for some materials and reconstruction of carbon flows within the lake and its sediments. The C-13 content of organic matter synthesized by lacustrine primary producers can be estimated from the observed C-13 content of the geoporphyrins derived from their chlorophylls. Total organic material in the shale is depleted in C-13 by six parts per thousand relative to that input. This difference cannot be explained by selective loss of components enriched in C-13, nor, as shown by isotopic compositions of other biomarkers, by inputs from land plants surrounding the lake or from methanogenic bacteria.

Hayes, J. M.↗
NASA NTRS1994

Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C [is equivalent to epsilon(p)] in a central equatorial Pacific sediment core that spans the last approximately 255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic composition of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon(p), derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 [is equivalent to c(e)] based on a new, site-specific calibration of the relationship between epsilon(p) and c(e). The calibration was based on reassessment of existing epsilon(p) versus c(e) data, which support a physiologically based model in which epsilon(p) is inversely related to c(e). Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone-unsaturation index U(K/37). Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon(p) and 1/c(e). These are discussed in detail and it is concluded that the observed record of epsilon(p) most probably reflects significant variations in delta pCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from approximately 110 microatmospheres during glacial intervals (ocean > atmosphere) to approximately 60 microatmospheres during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO2 differences and other parameters revealed significant (p<0.01) inverse correlations of delta pCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.

NASA Discipline Exobiology↗
NASA NTRS2014

Alteration of the Carbon and Nitrogen Isotopic Composition in the Martian Surface Rocks Due to Cosmic Ray Exposure

C-13/C-12 and N-15/N-14 isotopic ratios are pivotal for our understanding of the Martian carbon cycle, history of the Martian atmospheric escape, and origin of the organic compounds on Mars. Here we demonstrate that the carbon and nitrogen isotopic composition of the surface rocks on Mars can be significantly altered by the continuous exposure of Martian surface to cosmic rays. Cosmic rays can effectively produce C-13 and N-15 isotopes via spallation nuclear reactions on oxygen atoms in various Martian rocks. We calculate that in the top meter of the Martian rocks, the rates of production of both C-13 and N-15 due to galactic cosmic rays (GCRs) exposure can vary within 1.5-6 atoms/cm3/s depending on rocks' depth and chemical composition. We also find that the average solar cosmic rays can produce carbon and nitrogen isotopes at a rate comparable to GCRs in the top 5-10 cm of the Martian rocks. We demonstrate that if the total carbon content in a surface Martian rock is <10 ppm, then the "light," potentially "biological" C-13/C-12 ratio would be effectively erased by cosmic rays over 3.5 billion years of exposure. We found that for the rocks with relatively short exposure ages (e.g., 100 million years), cosmogenic changes in N-15/N-14 ratio are still very significant. We also show that a short exposure to cosmic rays of Allan Hills 84001 while on Mars can explain its high-temperature heavy nitrogen isotopic composition (N-15/N-14). Applications to Martian meteorites and the current Mars Science Laboratory mission are discussed.

cosmic ray exposure↗
NASA NTRS1993

Stable isotopic biogeochemistry of carbon and nitrogen in a perennially ice-covered Antarctic lake

Lake Hoare (77 degrees 38' S, 162 degrees 53' E) is an amictic, oligotrophic, 34-m-deep, closed-basin lake in Taylor Valley, Antarctica. Its perennial ice cover minimizes wind-generated currents and reduces light penetration, as well as restricts sediment deposition into the lake and the exchange of atmospheric gases between the water column and the atmosphere. The biological community of Lake Hoare consists solely of microorganisms -- both planktonic populations and benthic microbial mats. Lake Hoare is one of several perennially ice-covered lakes in the McMurdo Dry Valleys that represent the end-member conditions of cold desert and saline lakes. The dry valley lakes provide a unique opportunity to examine lacustrine processes that operate at all latitudes, but under an extreme set of environmental conditions. The dry valley lakes may also offer a valuable record of catchment and global changes in the past and present. Furthermore, these lakes are modern-day equivalents of periglacial lakes that are likely to have been common during periods of glacial maxima at temperate latitudes. We have analyzed the dissolved inorganic carbon (DIC) of Lake Hoare for delta 13C and the organic matter of the sediments and sediment-trap material for delta 13C and delta 15N. The delta 13C of the DIC indicates that 12C is differentially removed in the shallow, oxic portions of the lake via photosynthesis. In the anoxic portions of the lake (27-34 m) a net addition of 12C to the DIC pool occurs via organic matter decomposition. The dissolution of CaCO3 at depth also contributes to the DIC pool. Except near the Canada Glacier where a substantial amount of allochthonous organic matter enters the lake, the organic carbon being deposited on the lake bottom at different sites is isotopically similar, suggesting an autochthonous source for the organic carbon. Preliminary inorganic carbon flux calculations suggest that a high percentage of the organic carbon fixed in the water column is remineralized as it falls through the water column. At nearby Lake Fryxell, the substantial (relative to Lake Hoare) glacial meltstream input overprints Fryxell's shallow-water biological delta 13C signal with delta 13C-depleted DIC. In contrast, Lake Hoare is not significantly affected by surface-water input and mixing, and therefore the delta 13C patterns observed arise primarily from biological dynamics within the lake. Organic matter in Lake Hoare is depleted in 15N, which we suggest is partially the result of the addition of relatively light inorganic nitrogen into the lake system from terrestrial sources.

NASA Program Exobiology↗
NASA NTRS1989

Stable carbon and sulfur isotopes as records of the early biosphere

The abundance ratios of the stable isotopes of light elements such as carbon and sulfur can differ between various naturally-occurring chemical compounds. If coexisting compounds have achieved mutual chemical and isotopic equilibrium, then the relative isotopic composition can record the conditions at which equilibrium was last maintained. If coexisting chemical compounds indeed formed simultaneously but had not achieved mutual equilibrium, then their relative isotopic compositions often reflect the conditions and mechanisms associated with the kinetically controlled reactions responsible for their production. In the context of Mars, the stable isotopic compositions of various minerals might record not only the earlier environmental conditions of the planet, but also whether or not the chemistry of life ever occurred there. Two major geochemical reservoirs occur in Earth's crust, both for carbon and sulfur. In rocks formed in low temperature sedimentary environments, the oxidized forms of these elements tend to be enriched in the isotope having the larger mass, relative to the reduced forms. In sediments where the organics and sulfides were formed by biological processes, these isotopic contrasts were caused by the processes of biological CO2 fixation and dissimilatory sulfate reduction. Such isotopic contrasts between oxidized and reduced forms of carbon and sulfur are permitted by thermodynamics at ambient temperatures. However, nonbiological chemical reactions associated with the production of organic matter and the reduction of organics and sulfides are extremely slow at ambient temperatures. Thus the synthesis of organics and sulfides under ambient conditions illustrates life's profound role as a chemical catalyst that has altered the chemistry of Earth's crust. Because the stable isotopes of carbon and sulfur can reflect their chemistry, they are useful probes of the Martian surface.

Desmarais, David J.↗
NASA NTRS1997

Isotopic Signature of the Ancient Biosphere

The age distribution of 261 field localities, sampled for their well-preserved Archean and Proterozoic sedimentary rocks, revealed a 500-700 Ma episodicity. Assuming that the numbers of sites are a proxy for mass of sediments, the record of well-preserved sediments is more abundant in the intervals 3.5-3.3, 2.8-2.5, 2.1-1.8, 1.5-1.3, and 1.0-0.54 Ga than in the intervening intervals. It is proposed that the crustal inventory of photosynthetic organic carbon was modulated by the volume of sedimentation in sites favorable for the burial and long-term preservation of organic carbon. Tectonic processes controlled this sediment volume. Episodic increases in the organic inventory led to stepwise increases in oxidized reservoirs (e.g., O2, SO4(2-), Fe(3+). The interval 2.9-2.5 Ga recorded a large rise in seawater Sr-87/Sr-86, the oldest-known extensive banded iron formations, and the first evidence (C-13-depleted kerogens) of O2 use by methylotrophic bacteria. The interval 2.2-1.8 Ga has both carbon isotopic evidence for a stepwise increase in the organic reservoir and also paleosol evidence for an O2 increase. The interval 1.1-0.6 Ga shows isotopic evidence for another organic carbon increase. The interval 1.5-1.3 Ga revealed no such increases as yet, perhaps because incomplete rifting of the mid-Proterozoic supercontinent was associated with extensive sedimentation in oxidized continental basins, producing redbeds, coarse clastics, etc. Such sedimentation did not promote the burial of reduced carbon.

DesMarais, D. J.↗
NASA NTRS1986

Spectroscopic observations of the planets

The first phase of a program to determine D/H in the atmospheres of all of the major planets and Titan was completed. D/H on Titan and Uranus is significantly higher than on Jupiter and Saturn. Titan has the highest value, consistent with the possibility for hydrogen escape. These measurements were all made in terms of CH3D/CH4. They suggest that at least two reservoirs for deuterium exist in the outer solar system - a large one in hydrogen and a much smaller one in hydrogen-containing compounds that have not isotopically equilibrated with the hydrogen. This result also demonstrates that a large fraction of the carbon in the outer solar nebula was in the form of methane and it supports the nucleation model for the formation of the outer planets. Other studies included an investigation of the phosphine abundance over the Great Red Spot (GRS) of Jupiter, pre-Voyager determination of the amount of acetylene on Uranus, and the carbon isotope ratio in Halley's Comet. UV spectra of the GRS obtained with the IUE show no evidence of the enhancement of phosphine that would be expected if the red color of this object is caused by P4.

Owen, T.↗
NASA NTRS2015

Investigation of Benthic Foraminiferal Non-Traditional Stable Isotopes to Reconstruct Methane Fluxes in Sedimentary Environments

Methane (CH4) is an important greenhouse gas, with a global warming potential much higher than carbon dioxide (CO2) on a short time scale. Even if the residence time of CH4 in the atmosphere is relatively short (tens of years), one of the products of CH4 oxidation is CO2, a greenhouse gas with a much longer residence time in the atmosphere (tens to hundreds of years). CH4 has been proposed as one of the trigger mechanisms for rapid global climate change today and in the geological past. With regards to the geological past, numerous studies proposed the benthic foraminiferal carbon isotope ratio (Delta13C) as a tool to reconstruct the impact of marine CH4 on rapid climate changes; however, the investigation of modern benthic foraminiferal Delta13C have produced inconclusive results. CH4 has a distinctive hydrogen isotope (Delta(D)) and Delta13C signature compared to seawater, and sulfate reduction, often coupled to CH4 anaerobic oxidation in sediments, changes the sulfur isotope signature (Delta34S) of the remaining sulfate in porewater. Therefore, we hypothesize that the Delta(D) and Delta34S signature of infaunal benthic foraminiferal species can provide a complementary approach to Delta13C to study CH4 dynamics in sedimentary environments. Here, we present the preliminary results obtained analyzing Uvigerina peregrina Delta(D) and Delta34S from three different locations at Hydrate Ridge, offshore Oregon. Unfortunately, the lack of chemical data related to the moment of foraminiferal calcification makes difficult to build a robust relationship among the U. peregrina stable isotopes and the CH4 fluxes at the sampling sites. However, our results look very promising, as each site is characterized by a different Delta(D) and Delta34S signature. We emphasize that this study represents the first step in the development of new proxies (Delta(D)) and Delta34S), which may complement the more traditional benthic foraminiferal Delta13C values, to reconstruct marine CH4 fluxes in the geological past.

Borrelli, C.↗
NASA NTRS1977

On the origin of biological chirality via natural beta-decay

An hypothesis to account for the chirality (handedness) of some biological molecules is given. Experimental evidence suggests that longitudinally polarized electrons having the chirality of terrestrial beta-decay electrons remove dextro-leucine from a racemic mixture. If, by a similar mechanism, the terrestrial environment provided more levo- than dextro-amino acids, that would account for the chirality now observed in organic molecules. An isotope of potassium has been proposed as the natural beta-emitter responsible for biomolecular chirality; however, Carbon 14 may be an even more plausible candidate. Ready availability of the carbon isotope in the terrestrial environment of 4.5 aeons ago, and the role of leucine in protein synthesis indicate that these two agents may have been chief factors in the evolution of biomolecular chirality. Suggestions for further research in this area are made.

Noyes, H. P.↗
NASA NTRS1996

Sulfur and Hydrogen Isotope Anomalies in Organic Compounds from the Murchison Meteorite

Isotopic measurements have been made on organic sulfur and phosphorus compounds recently discovered in the Murchison meteorite. Carbon, hydrogen and sulfur measurements were performed on individual members of the organic sulfur compounds, alkyl sulfonates; and carbon and hydrogen measurements were made on bulk alkyl phosphonates. Cooper and Chang reported the first carbon isotopic measurements of Murchison organic sulfonates, providing insight into the potential synthetic mechanisms of these and, possibly, other organic species. Hydrogen isotopic measurements of the sulforiates now reveal deuterium excesses ranging from +660 to +2730 %. The deuterium enrichments indicate formation of the hydrocarbon portion of these compounds in a low temperature astrophysical environment consistent with that of dense molecular clouds. Measurements of the sulfur isotopes provide further constraints on the origin and mechanism of formation of these organic molecules. Recently, there has been growing documentation of sulfur isotopic anomalies in meteoritic material. Thiemens and Jackson have shown that some bulk ureilites possess excess S-33 and Thiemens et al. have reported excess S-33 in an oldhamite separate from the Norton County meteorite. Rees and Thode reported a large S-33 excess in an Allende acid residue, however, attempts to verify this measurements have been unsuccessful, possibly due to the heterogeneous nature of the carrier phase. With the recognition that sulfur isotopes may reflect chemistry in the protosolar nebula or the precursor molecular cloud, identification of potential carriers is of considerable interest. In the present study, the stable isotopes of sulfur were measured in methane sulfonic acid extracted from the Murchison meteorite. The isotopic composition was found to be: (delta)S-33 = 2.48 %, (delta)S-34 = 2.49 % and (delta)S-36 = 6.76 %. Based upon analysis of more than 60 meteoritic and numerous terrestrial samples, the mass fractionation lines are defined by Delta-33 = (delta)S-33 -0.50(delta)S-34 and Delta-36 = (delta)s-36 - 1.97 (delta)S-34. From these relationships Delta-33 = 1.24 % and Delta-36 = 0.89 % are observed. These anomalies, particularly the Delta-33, lie well outside the range of analytical uncertainty. They are the largest observed in any meteoritic component and the first found in an organosulfur compound. As discussed by Thiemens and Jackson, due to it's position on the periodic chart, sulfur undergoes chemically induced mass independent isotopic fractionations as does oxygen. Experiments by Mauersberger et. al. show that in such processes, the magnitude of fractionation for the different isotopically substituted species varies with mass and angular momentum; thus, anomalies are expected for both S-33 and S-36, but not necessarily of the same magnitude. Laboratory experiments have also confirmed that chemically produced, mass independent fractionations are mediated by molecular symmetry factors. A chemical source of fractionation requires that the sulfur isotopic anomaly was established in the gas phase, probably in reactions involving symmetric CS2. The discovery of an anomalous sulfur isotopic composition in a specific molecule containing excess deuterium is an important advance in the understanding of the cosmochemistry of sulfur. This evidence suggests that methanesulfonic acid was synthesized by interstellar processes. Further measurements and details of possible synthesis and fractionation mechanisms will be presented.

Cooper, G. W.↗
NASA NTRS2010

Parent Body Influences on Amino Acids in the Tagish Lake Meteorite

The Tagish Lake meteorite is a primitive C2 carbonaceous chondrite with a mineralogy, oxygen isotope, and bulk chemical. However, in contrast to many CI and CM carbonaceous chondrites, the Tagish Lake meteorite was reported to have only trace levels of indigenous amino acids, with evidence for terrestrial L-amino acid contamination from the Tagish Lake meltwater. The lack of indigenous amino acids in Tagish Lake suggested that they were either destroyed during parent body alteration processes and/or the Tagish Lake meteorite originated on a chemically distinct parent body from CI and CM meteorites where formation of amino acids was less favorable. We recently measured the amino acid composition of three different lithologies (11h, 5b, and 11i) of pristine Tagish Lake meteorite fragments that represent a range of progressive aqueous alteration in order 11h < 5b < 11i as inferred from the mineralogy, petrology, bulk isotopes, and insoluble organic matter structure. The distribution and enantiomeric abundances of the one- to six-carbon aliphatic amino acids found in hot-water extracts of the Tagish Lake fragments were determined by ultra performance liquid chromatography fluorescence detection and time of flight mass spectrometry coupled with OPA/NAC derivatization. Stable carbon isotope analyses of the most abundant amino acids in 11h were measured with gas chromatography coupled with quadrupole mass spectrometry and isotope ratio mass spectrometry.

Glavin, D. P.↗
NASA NTRS2019

Forearc Carbon Sink Reduces Long-Term Volatile Recycling Into the Mantle

Carbon and other volatiles are transported from Earth’s surface into the mantle at subduction margins. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep (mantle) and shallow (crustal) reservoirs, as well as Earth’s oxidation state. However, the proportion of volatiles released in the forearc and backarc are not well constrained compared to fluxes from the volcanic front. Here, we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that ~91% of carbon released from the slab/mantle beneath the Costa Rica forearc is sequestered within the crust by calcite deposition, and an additional ~3% is incorporated into biomass through microbial chemolithoautotrophy. We estimate that ~1.2 × 10(exp 8) to 1.3 × 10(exp 10) mol CO2/yr are released from the slab beneath the forearc, resulting in up to ~19% less carbon being transferred to Earth’s deep mantle than previously estimated.

Carbon cycle↗
NASA NTRS1985

Isotopic Biogeochemistry

An overview is provided of the biogeochemical research. The funding, productivity, personnel and facilities are reviewed. Some of the technical areas covered are: carbon isotopic records; isotopic studies of banded iron formations; isotope effects in microbial systems; studies of organic compounds in ancient sediments; and development in isotopic geochemistry and analysis.

Hayes, J. M.↗
NASA NTRS1995

Stable isotope analysis at the molecular level: A new approach for determining the origins of amino acids in the Murchison meteorite

A combined gas chromatography/isotope ratio mass spectrometry (GC/IRMS) method has been developed that permits the direct stable carbon isotope analysis of N(O)-trifluoroacetyl-isopropyl esters of individual amino acids and their respective enantiomers at nanomole abundances. Calculation of the original delta C-13 values of the amino acids is accomplished via a correction for the carbon introduced during the derivatization process. Previous GC/IRMS analyses of individual amino acids in the non-hydrolyzed water extract of an interior sample of a Murchison meteorite stone revealed an enrichment in C-13 relative to terrestrial organic matter, in agreement with previous findings for bulk extracts. The range of amino acid delta C-13 values (+5 to +30 per mill, PDB) suggests possible kinetic effects during synthesis. In this study, an apparent kinetic isotope effect was also observed for the amino acid products of a spark discharge experiment. These preliminary resutls are supportive of a similar mechanism for the abiotic synthesis of amino acids in the Murchison meteorite.

Engel, M. H.↗
NASA NTRS1990

Probing the possibility of a C-12/C-13 galactic abundance gradient

High S/N (equal to or greater than 500) observations of interstellar CH+ with the 3.0 m telescope were performed at Lick Observatory and with the 4.0 m telescope at CTIO, of the reddened, early-type stars HD 183143, HD 24432, and HD 157038 in an effort to probe the existence of a C-12/C-13 abundance gradient in our Galaxy. Previous very high quality optical observations of interstellar CH+ toward five stars within 1 kpc of the Sun have yielded a precise weighted mean C-12/C-13 isotope ratio of 43 plus or minus 4 (1 sigma) (Hawkins, Jura, and Meyer 1985; Hawkins and Jura 1987). The isotope ratios derived toward four lines of sight in the local ISM are uniform within 12 percent. The similarity among these carbon isotope ratios determined in diffuse clouds possessing different physical conditions precludes the possibility that the CH+ molecule is suffering from isotope selective effects in these regions. The precise C-12/C-13 derivable from high quality observations of (12)CH+ and (13)CH+ provide the unique opportunity to probe the homogeneity of the ISM in a large scale and the history of nucleosynthesis in our Galaxy. Since CH+ seems to be the most sensitive probe of C-12/C-13 in the diffuse ISM, observations toward more distant stars located up to 2.5 kpc from the Sun are the best way to study the possibility of a Galactic (C-12/C-13) abundance gradient. The researcher obtained 4232 angstrom data toward all three of the stars mentioned above, 3957 angstrom data toward HD 183143 and HD 157038, and 3745 angstrom data toward HD 157038. Because of the poorer quality of the HD 24432 spectrum, and its weaker CH+ absorption lines, the satellite (13)CH+ line was not detected, and thus only a lower limit on the C-12/C-13 ratio toward this star was obtained. The results obtained from careful reduction and analysis of the data toward these stars in the Northern and Southern Hemispheres are presented.

Hawkins, Isabel↗
NASA NTRS2013

A Search for Amino Acids and Nucleobases in the Martian Meteorite Roberts Massif 04262 Using Liquid Chromatography-Mass Spectrometry

The investigation into whether Mars contains signatures of past or present life is of great interest to science and society. Amino acids and nucleobases are compounds that are essential for all known life on Earth and are excellent target molecules in the search for potential Martian biomarkers or prebiotic chemistry. Martian meteorites represent the only samples from Mars that can be studied directly in the laboratory on Earth. Here, we analyzed the amino acid and nucleobase content of the shergottite Roberts Massif (RBT) 04262 using liquid chromatography-mass spectrometry. We did not detect any nucleobases above our detection limit in formic acid extracts; however, we did measure a suite of protein and nonprotein amino acids in hot-water extracts with high relative abundances of beta-alanine and gamma-amino-eta-butyric acid. The presence of only low (to absent) levels of several proteinogenic amino acids and a lack of nucleobases suggest that this meteorite fragment is fairly uncontaminated with respect to these common biological compounds. The distribution of straight-chained amine-terminal eta-omega-amino acids in RBT 04262 resembled those previously measured in thermally altered carbonaceous meteorites. A carbon isotope ratio of -24(0/00) +/- 6(0/00) for beta-alanine in RBT 04262 is in the range of reduced organic carbon previously measured in Martian meteorites (Steele et al. 2012). The presence of eta-omega-amino acids may be due to a high temperature Fischer-Tropschtype synthesis during igneous processing on Mars or impact ejection of the meteorites from Mars, but more experimental data are needed to support these hypotheses.

Laboratory analysis↗