Kevin D. McKeegan

Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments.

No microorganism capable of anaerobic growth on methane as the sole carbon source has yet been cultivated. Consequently, information about these microbes has been inferred from geochemical and microbiological observations of field samples. Stable isotope analysis of lipid biomarkers and rRNA gene surveys have implicated specific microbes in the anaerobic oxidation of methane (AOM). Here we use combined fluorescent in situ hybridization and secondary ion mass spectrometry analyses, to identify anaerobic methanotrophs in marine methane-seep sediments. The results provide direct evidence for the involvement of at least two distinct archaeal groups (ANME-1 and ANME-2) in AOM at methane seeps. Although both archaeal groups often occurred in direct physical association with bacteria, they also were observed as monospecific aggregations and as single cells. The ANME-1 archaeal group more frequently existed in monospecific aggregations or as single filaments, apparently without a bacterial partner. Bacteria associated with both archaeal groups included, but were not limited to, close relatives of Desulfosarcina species. Isotopic analyses suggest that monospecific archaeal cells and cell aggregates were active in anaerobic methanotrophy, as were multispecies consortia. In total, the data indicate that the microbial species and biotic interactions mediating anaerobic methanotrophy are diverse and complex. The data also clearly show that highly structured ANME-2/Desulfosarcina consortia are not the sole entities responsible for AOM at marine methane seeps. Other microbial groups, including ANME-1 archaea, are capable of anaerobic methane consumption either as single cells, in monospecific aggregates, or in multispecies consortia.

The oxygen isotopic composition of the sun inferred from captured solar wind

The Sun is highly enriched in the most abundant isotope of oxygen, oxygen-16, relative to most other planetary materials. All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, 16O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in 16O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in 17O and 18O, relative to 16O, by ~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

Mass-independent isotope effects in Archean (2.5 to 3.8 Ga) sedimentary sulfides determined by ion microprobe analysis

We report sulfur isotope anomalies with 33 S, the deviation from a mass-dependent fractionation line for the three-isotope system ( 34 S/ 32 S vs. 33 S/ 32 S), ranging up to 2‰ within individual Archean sedimentary sulfides from a variety of localities. Our measurements, which are made in situ by multicollector secondary ion mass spectrometry, unequivocally corroborate prior bulk measurements of mass-independent fractionations (MIF) in sulfur and provide additional evidence for an anoxic atmosphere on the Earth before 2 Ga. This technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record. The presence of MIF sulfur in sulfides from a 3.8-Ga Fe-rich quartzite from Akilia (island), West Greenland, is consistent with a marine sedimentary origin for this rock. Copyright

THE OXYGEN ISOTOPIC COMPOSITION OF OLIVINE AND PYROXENE FROM CI CHONDRITES

The CI chondrites are taken to represent average solar system material based on the similarity of their elemental compositions to that of the solar photosphere. However, their oxygen isotope geochemistry is dominated by secondary minerals that formed during aqueous alteration on the CI parent body. Precursors to this alteration, namely olivine and pyroxene, are extremely rare in CI chondrites, precluding previous measurements of their oxygen isotopic composition. We report ion microprobe analyses of oxygen isotopes in single olivine and pyroxene grains separated from CI chondrites Orgueil and Ivuna. The CI chondrite olivine and pyroxene grains most likely represent liberated chondrule phenocrysts, based on petrographic, chemical, and isotopic evidence consistent with crystallization from a melt. The oxygen isotope data form an array that falls nearly along the carbonaceous chondrite 16O mixing line with δ18O values ranging from −9.3%, to + 12.3%o and δ17O from −11.3%o to +7.8%o, consistent with nebular processes being the source of the oxygen isotopic compositions. The degree of 16O-enrichment in Orgueil olivines is negatively correlated with FeO content, but the exact nature and timing of the process that introduced this variation remains unknown. The pyroxene oxygen isotopic compositions are similar to those of olivines with >5 mol% fayalite. The oxygen isotopic analyses of the olivine and pyroxene in CI chondrites have been used to revise previous models for the isotopic evolution of CI materials. Our data require more complete gas-solid equilibration in the nebula and constrain the initial aqueous fluids on the CI parent body to have lower Δ17O values than previously postulated. The refined model indicates that the temperature of aqueous activity on the CI parent body was no higher than ∼50°C, and the fluid:rock ratio was significantly less than previously estimated. Even prior to alteration and formation of secondary minerals, the CI chondrites were the most 16O-depleted carbonaceous chondrites and thus the solids originally contained in the CI chondrites are the most equilibrated nebular materials represented in the carbonaceous chondrites. The data suggest the oxygen isotopic composition of average solar system to be approximately equivalent to average terrestrial oxygen as recorded in the compositions of terrestrial and lunar basalts.

The Chlorine Isotope Composition of the Moon and Implications for an Anhydrous Mantle

Over the Moon Based on recent analyses of lunar rocks, it has been argued that the lunar interior contained much more water than previously thought. Sharp et al. (p. 1050, published online 5 August) measured the chlorine isotope content of lunar samples returned by the Apollo missions and found that the spread in their chlorine isotope composition is 25-fold greater than for rocks and minerals that have been measured from Earth and meteorites. This result implies that the hydrogen content of the Moon (and therefore its water content) is much lower than suggested by recent studies. The range of chlorine isotope values of the Moon is distinct from those of Earth and meteorites, indicating that the Moon is dry. Arguably, the most striking geochemical distinction between Earth and the Moon has been the virtual lack of water (hydrogen) in the latter. This conclusion was recently challenged on the basis of geochemical data from lunar materials that suggest that the Moon’s water content might be far higher than previously believed. We measured the chlorine isotope composition of Apollo basalts and glasses and found that the range of isotopic values [from –1 to +24 per mil (‰) versus standard mean ocean chloride] is 25 times the range for Earth. The huge isotopic spread is explained by volatilization of metal halides during basalt eruption—a process that could only occur if the Moon had hydrogen concentrations lower than those of Earth by a factor of ~104 to 105, implying that the lunar interior is essentially anhydrous.

Oxygen Isotopic Constraints on the Genesis of Carbonates from Martian Meteorite ALH84001

Abstract Ion microprobe oxygen isotopic measurements of a chemically diverse suite of carbonates from Martian meteorite ALH84001 are reported. The δ 18 O values are highly variable, ranging from +5.4 to +25.3‰, and are correlated with major element compositions of the carbonate. The earliest-forming (Ca-rich) carbonates have the lowest δ 18 O values and the late-forming (Mg-rich) carbonates have the highest δ 18 O values. Two models are presented which can explain the isotopic variations. The carbonates could have formed in a water rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is ∼125°C, with fluctuations of over 250°C possible within the constraints of the model. Alternatively, the data can be explained by a closed-system model in which the carbonates precipitated from a limited amount of CO 2 -rich fluid. This scenario can reproduce the isotopic variations observed at a range of temperatures, including relatively high temperatures (> 500°C). Thus the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Although more information is needed in order to distinguish between the models, neither of the implied environments is consistent with biological activity. Thus, we suggest that features associated with the carbonates which have been interpreted to be the result of biological activity were most probably formed by inorganic processes.

Evolution of Oxygen Isotopic Composition in the Inner Solar Nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR-group carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula—the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26 Al (t1=2 � 0:73 Myr) indicates that the inner solar nebula gas was 16 O-rich when refractory inclusions formed, but less than 0.8 Myr later, gas in the inner solar nebula became 16 O-poor, and this state persisted at least until CR chondrules formed � 1‐2 Myr later. We suggest that the inner solar nebula became 16 O-poor because meter-sized icy bodies, which were enriched in 17 Oa nd 18 O as a result of isotopic self-shielding during the ultraviolet photodissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snow line, drifted rapidly toward the Sun, and evaporated at the snow line. This led to significant enrichment in 16 O-depleted water,whichthenspreadthroughtheinnersolarsystem.Astronomicalstudiesofthespatialandtemporalvariations of water abundance in protoplanetary disks may clarify these processes. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — planetary systems: formation — solar system: formation

Constraints on the Formation Age of Cometary Material from the NASA Stardust Mission

Sun Stuff Comets are thought to be remnants of the Suns protoplanetary disk; hence, they hold important clues to the processes that originated the solar system. Matzel et al. (p. 483, published online 25 February) present Al-Mg isotope data on a refractory particle recovered from comet Wild 2 by the NASA Stardust mission. The lack of evidence for the extinct radiogenic isotope 26Al implies that this particle crystallized 1.7 million years after the formation of the oldest solar system solids. This observation, in turn, requires that material formed near the Sun was transported to the outer reaches of the solar system and incorporated into comets over a period of at least two million years. Transport of inner solar system material to the Kuiper Belt and incorporation into comets took at least 2 million years. We measured the 26Al-26Mg isotope systematics of a ~5-micrometer refractory particle, Coki, returned from comet 81P/Wild 2 in order to relate the time scales of formation of cometary inclusions to their meteoritic counterparts. The data show no evidence of radiogenic 26Mg and define an upper limit to the abundance of 26Al at the time of particle formation: 26Al/27Al < 1 × 10−5. The absence of 26Al indicates that Coki formed >1.7 million years after the oldest solids in the solar system, calcium- and aluminum-rich inclusions (CAIs). The data suggest that high-temperature inner solar system material formed, was subsequently transferred to the Kuiper Belt, and was incorporated into comets several million years after CAI formation.

Oxygen isotopic compositions of individual minerals in Antarctic micrometeorites: Further links to carbonaceous chondrites

Abstract We report in situ measurements of oxygen isotopic abundances in individual silicate and oxide minerals from 16 Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of 10 olivine and 11 pyroxene grains are enriched in 16 O relative to terrestrial minerals, and on an oxygen three-isotope diagram they plot on the low δ 18 O side of the 16 O mixing line defined by calcium-aluminum-rich inclusions (CAI) from chondritic meteorites. AMM olivine and pyroxene δ 18 O values range from –9.9‰ to +8.0‰ and δ 17 O ranges from –11.3‰ to +5.5‰, similar to values measured in individual olivine grains and whole chondrules from carbonaceous chondrites. These data indicate that the mineral grains preserve their pre-terrestrial oxygen isotopic compositions, and provide another link between AMMs and carbonaceous chondrites. However, no clear relationship with one single subgroup of carbonaceous chondrite can be established. Based on their textures, crystal chemistries, and oxygen isotopes, some coarse-grained crystalline AMMs could originate from chondrule fragmentation. Whether the remaining mineral grains were formed by igneous or condensation processes is unclear. No clear correlation is observed between isotopic compositions and mineral compositions of AMM olivine grains, suggesting that the FeO- and 16 O-enrichment processes are not coupled in a simple way. Nor are any relatively large 16 O enrichments measured in any of the olivine grains, however two Mg-Al spinels and a melilite grain are 16 O enriched at the level of δ 18 O ∼ δ 17 O ∼ –40‰. The discovery of an 16 O-enriched melilite grain in AMMs supports the hypothesis that refractory minerals throughout the solar nebula formed from a relatively uniformly 16 O-enriched reservoir. This unique 16 O-rich signature of refractory minerals in primitive solar system materials suggests that they either formed from a widespread 16 O-rich reservoir in the solar nebula, or that an efficient mechanism (such as bipolar outflows) was acting to spread them from a highly localized 16 O-rich region over the early solar nebula.

The Genesis solar-wind collector materials

Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector arrays and elsewhere targeted for the analyses of specific solar-wind components.Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community throughout the 21st Century.