Engineering topics

Des Marais, D. J.

Publications and source records attributed to Des Marais, D. J..

68 records · Page 4

NASA NTRS1986

Carbon abundance measurements in oceanic basalts - The need for a consensus

Large systematic discrepancies currently exist among the carbon data reported for oceanic basalts by different laboratories. These discrepancies are likely attributable both to nonuniform criteria for the removal of carbon contamination from the samples, and to systematic errors in analytical procedures. In order to solve these problems, investigators must agree upon effective criteria for the removal of carbon contamination, and they must create an accurately calibrated set of basaltic glass carbon standards.

Des Marais, D. J.↗

NASA NTRS1986

Competition for sulfide among colorless and purple sulfur bacteria in cyanobacterial mats

The vertical zonation of light, O2, H2S, pH, and sulfur bacteria was studied in two benthic cyanobacterial mats from hypersaline ponds at Guerrero Negro, Baja California, Mexico. The physical-chemical gradients were analyzed in the upper few mm at < or = 100 micrometers spatial resolution by microelectrodes and by a fiber optic microprobe. In mats, where oxygen produced by photosynthesis diffused far below the depth of the photic zone, colorless sulfur bacteria (Beggiatoa sp.) were the dominant sulfide oxidizing organisms. In a mat, where the O2-H2S interface was close to the photic zone, but yet received no significant visible light, purple sulfur bacteria (Chromatium sp.) were the dominant sulfide oxidizers. Analysis of the spectral light distribution here showed that the penetration of only 1% of the incident near-IR light (800-900 nm) into the sulfide zone was sufficient for the mass development of Chromatium in a narrow band of 300 micromoles thickness. The balance between O2 and light penetration down into the sulfide zone thus determined in micro-scale which type of sulfur bacteria became dominant.

NASA Center ARC↗

NASA NTRS1986

A simple fiber-optic microprobe for high resolution light measurements: application in marine sediment

A fiber-optic microphobe is described which is inexpensive and simple to build and use. It consists of an 80-micrometers optical fiber which at the end is tapered down to a rounded sensing tip of 20-30-micrometers diameter. The detector is a hybrid photodiode/amplifier. The probe has a sensitivity of 0.01 microEinst m-2 s-1 and a spectral range of 300-1,100 nm. Spectral light gradients were measured in fine-grained San Francisco Bay sediment that had an undisturbed diatom coating on the surface. The photic zone of the mud was only 0.4 mm deep. Measured in situ spectra showed extinction maxima at 430-520, 620-630, 670, and 825-850 nm due to absorption by chlorophyll a, carotenoids, phycocyanin, and bacterio-chlorophyll a. Maximum light penetration in the visible range was found in both the violet and the red < or = 400 and > or = 700 nm.

NASA Center ARC↗

NASA NTRS1985

Carbon exchange between the mantle and the crust, and its effect upon the atmosphere - Today compared to archean time

Estimates of the midocean ridge mantle carbon flux, whose exchange of carbon with the earth's surface has over geological time influenced atmospheric CO2, can be made by estimating concentration ratios of carbon to helium in hydrothermal fluids and tholeiitic glasses, followed by their multiplication by the oceanic primordial He-3 flux. A net carbon flux from the mantle of 1 billion moles/yr would require less than 700 million years to generate the present day crustal carbon inventory. It is deduced that the atmosphere of 3 billion years ago contained at least two orders of magnitude more CO2 than it does today.

Des Marais, D. J.↗

NASA NTRS1984

Concentrations and isotope ratios of carbon, nitrogen and sulfur in ocean-floor basalts

Ocean floor basalts studied from the Galapagos Ridge, FAMOUS area, Cayman Trough and Kilauea east rift contain 20-200 ppm carbon and 0.3-2.8 ppn nitrogen as sums of the vesicle-filling gases CO2 and N2 and dissolved species. The wide range of carbon contents found is due partly to the different extent of outgassing of vesicle-filling gases and partly to depth dependency of dissolved CO2 in the basalts. Sulfate commonly exists with sulfide in these basalts, and the sulfate/sulfide ratio increases with increasing water content, perhaps reflecting the higher oxidation potential in basalt melt of the higher water content.

Sakai, H.↗

NASA NTRS1984

Carbon and its isotopes in mid-oceanic basaltic glasses

Sample surface carbon, mantle carbon dioxide in vesicles, and mantle carbon dissolved in glasses, are the three carbon components evident in the 11 mid-oceanic basalts presently analyzed. The total carbon content may be controlled by the depth of the shallowest ridge magma chamber, and carbon isotopic fractionation accompanies magma degassing. Using He-3 and carbon data for submarine hydrothermal fluids, the present day midoceanic ridge carbon flux is approximately estimated to be 1.0 x 10 to the 13th g C/yr, requiring 8 Gyr to accumulate the earth's present crustal carbon inventory.

Des Marais, D. J.↗

NASA NTRS1984

Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison meteorite

Carbon isotopic compositions have been measured for individual hydrocarbons and monocarboxylic acids from the Murchison meteorite, a C2 carbonaceous chondrite which fell in Australia in 1969. With few exceptions, notably benzene, the volatile products are substantially isotopically heavier than their terrestrial counterparts, signifying their extraterrestrial origin. For both classes of compounds, the ratio of C-13 to C-12 decreases with increasing carbon number in a roughly parallel manner, and each carboxylic acid exhibits a higher isotopic ratio than the hydrocarbon containing the same number of carbon atoms. These trends are consistent with the kinetically controlled synthesis of higher homologues from lower ones. The results suggest the possibility that the production mechanisms for hydrocarbons and carboxylic acids may be similar, and impose constraints on the identity of the reactant species.

Yuen, G.↗

NASA NTRS1983

Distribution, abundance and carbon isotopic composition of gaseous hydrocarbons in Big Soda Lake, Nevada - An alkaline, meromictic lake

The study of the distribution and isotopic composition of low molecular weight hydrocarbon gases at the Big Soda Lake, Nevada, has shown that while neither ethylene nor propylene were found in the lake, ethane, propane, isobutane and n-butane concentrations all increased with water column depth. It is concluded that methane has a biogenic origin in both the sediments and the anoxic water column, and that C2-C4 alkanes have biogenic origins in the monimolimnion water and shallow sediments. The changes observed in delta C-13/CH4/ and CH4/(C2H6 + C3H8) with depth in the water column and sedimeents are probably due to bacterial processes, which may include anaerobic methane oxidation and different rates of methanogenesis, and C2-to-C4 alkane production by microorganisms.

Oremland, R. S.↗

NASA NTRS1983

Light element geochemistry and spallogenesis in lunar rocks

Abundance and isotopic compositions are measured for the very volatile elements carbon, nitrogen and sulfur in 11 lunar rocks representing a wide spectrum of textures and compositions. Samples were combusted sequentially at three temperatures in order to remove terrestrial contaminants before melting the lunar rock and liberating lunar volatiles. The combustion results indicate very little terrestrial sulfur contamination, with sulfur contents correlated with the TiO2 contents of the basalts analyzed. Sulfur isotopic compositions are remarkably uniform and similar to the Canon Diablo meteorite standard. Nitrogen levels are found to be no greater than those obtained with procedural blanks, corresponding to abundances less than 0.1 microg/g. Stable nitrogen isotope measurements indicate a spallogenic N-15 production rate of 4.1 x 10 to the -6th microg N-15/g sample/million years, in agreement with previous estimates. No indigenous carbon in excess of procedural blank levels of about 0.7 microg/g is found in lunar basalts. Levels of 1 to 5 microg/g found in highland rocks may derive from meteoritic or terrestrial sources. The average measured spallogenic C-13 production rate is 4.1 x 10 to the -6th microg C-13/g sample/million years.

Des Marais, D. J.↗

NASA NTRS1981

Molecular carbon isotopic evidence for the origin of geothermal hydrocarbons

Isotopic measurements of individual geothermal hydrocarbons that are, as a group, of higher molecular weight than methane are reported. It is believed in light of this data that the principal source of hydrocarbons in four geothermal areas in western North America is the thermal decomposition of sedimentary or groundwater organic matter.

Des Marais, D. J.↗

NASA NTRS1980

The carbon isotope biogeochemistry of the individual hydrocarbons in bat guano and the ecology of the insectivorous bats in the region of Carlsbad, New Mexico

The structures and C-13 contents of individual hydrocarbons extracted from bat guano found in the Carlsbad region of New Mexico are analyzed in order to elucidate details of the carbon flow in the plant-insect-bat ecosystem. Carbon isotopic analyses indicate that equivalent numbers of plants with C3 and C4 photosynthetic pathways occupy the feeding area of the bats, which supports alfalfa and cotton as well as native plants. The molecular composition of the guano is consistent with an origin in two distinct populations of insects with different feeding habits, one of which may graze predominantly on crops. It is also pointed out that isotopic analyses of more ancient guano deposits may be useful in characterizing prevalent vegetation and climate of earlier periods.

Des Marais, D. J.↗

NASA NTRS1978

Variable-temperature cryogenic trap for the separation of gas mixtures

The paper describes a continuous variable-temperature U-shaped cold trap which can both purify vacuum-line combustion products for subsequent stable isotopic analysis and isolate the methane and ethane constituents of natural gases. The canister containing the trap is submerged in liquid nitrogen, and, as the gas cools, the gas mixture components condense sequentially according to their relative vapor pressures. After the about 12 min required for the bottom of the trap to reach the liquid-nitrogen temperature, passage of electric current through the resistance wire wrapped around the tubing covering the U-trap permits distillation of successive gas components at optimal temperatures. Data on the separation achieved for two mixtures, the first being typical vacuum-line combustion products of geochemical samples such as rocks and the second being natural gas, are presented, and the thermal behavior and power consumption are reported.

Des Marais, D. J.↗

NASA NTRS1978

Carbon, nitrogen and sulfur in Apollo 15, 16 and 17 rocks

Carbon, nitrogen and sulfur contents, as well as carbon isotopic compositions, were determined for seven lunar rocks from the Apollo 15, 16 and 17 missions. Sequential combustion at three temperatures was used to resolve terrestrial contamination from indigenous lunar volatiles. Nitrogen abundances averaged 0.4 micrograms/gram in the samples. The results of the sequential combustion analysis suggested that all the samples contained variable amounts of terrestrial carbon contamination prior to examination. Indigenous lunar carbon abundances ranging from 2.5 to 6 micrograms/gram were found. Thus the moon appears to be substantially depleted in nitrogen and carbon relative to the earth and C and H meteorites.

Des Marais, D. J.↗

NASA NTRS1975

Integrated investigation of the mixed origin of lunar sample 72161,11

The comminution-agglutination model and the solar-wind implantation-retention model are used to postulate the origins of the particulate components of lunar sample (72161,11), a submillimeter fraction of a surface sample for the dark mantle regolith at LRV-3. Grain-size analysis was performed by wet sieving with liquid argon, and analyses for CO2, CO, CH4, and H2 were carried out by stepwise pyrolysis in a helium atmosphere. The results indicate that the present sample is from a mature regolith, but the agglutinate content is only 30% in the particle-size range between 90 and 177 microns, indicating an apparent departure from steady state. Analyses of the carbon, methane, and hydrogen concentrations in size fractions larger than 149 microns show that the volume-correlated component of these species increases with increased grain size. It is suggested that the observed increase can be explained in terms of mixing of a dominant local population of coarser agglutinates having high carbon and hydrogen concentrations with an imported population of finer agglutinates relatively poor in carbon and hydrogen.

Basu, A.↗