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NASA NTRS1992

Aqueous geochemistry on Mars: Possible clues from salts and clays in SNC meteorites

All subgroups of the shergottite, nakhlite, and chassignite (SNC) meteorites contain traces of water precipitated minerals that include various combinations of carbonates, sulfates, halides, ferric oxides, and aluminosilicate clays of preterrestrial origin. Oxygen three-isotope analysis of thermally extracted bulk water has confirmed that at least some of the water in SNC's is, indeed, extraterrestrial. A mixture of aqueous precipitates found in the SNC's, comprising smectite, illite, and gypsum (with minor halite +/- calcite and hematite), provides a self-consistent, though not unique, model for the bulk elemental composition of surface sediments at the Viking Lander sites. Therefore, if the salts and clays in SNC's are truly linked to aqueous alteration and soil formation on Mars, then the suite of SNC secondary minerals might provide the best currently available insight into near-surface martian chemistry.

Gooding, James L.↗

NASA NTRS1992

Soil mineralogy and chemistry on Mars - Possible clues from salts and clays in SNC meteorites

If the shergottite, nakhlite, and chassignite (SNC) meteorites' parent planet is Mars, then the aqueous precipitates found in them imply that oxidizing, water-based solutions may have been chemically active on that planet over the past 200-1300 million yrs. It is suggested that the mixture of aqueous precipitates found in the SNCs furnish a self-consistent model for the bulk elemental composition of surface sediments at the Viking Lander sites. Further mineralogical and stable-isotope studies of the secondary minerals may establish the limits for biological activity over the last 1300 million years of Mars' water-based chemistry.

Gooding, James L.↗

NASA NTRS1988

Trace element and isotope geochemistry of Cretaceous-Tertiary boundary sediments: identification of extra-terrestrial and volcanic components

Trace element and stable isotope analyses were performed on a series of sediment samples crossing the Cretaceous-Tertiary (K-T) boundary from critical sections at Aumaya and Sopelano, Spain. The aim is to possibly distinguish extraterrestrial vs. volcanic or authigenic concentration of platinum group and other elements in K-T boundary transitional sediments. These sediments also have been shown to contain evidence for step-wise extinction of several groups of marine invertebrates, associated with negative oxygen and carbon isotope excursions occurring during the last million years of the Cretaceous. These isotope excursions have been interpreted to indicate major changes in ocean thermal regime, circulation, and ecosystems that may be related to multiple events during latest Cretaceous time. Results to date on the petrographic and geochemical analyses of the Late Cretaceous and Early Paleocene sediments indicate that diagenesis has obviously affected the trace element geochemistry and stable isotope compositions at Zumaya. Mineralogical and geochemical analysis of K-T boundary sediments at Zumaya suggest that a substantial fraction of anomalous trace elements in the boundary marl are present in specific mineral phases. Platinum and nickel grains perhaps represent the first direct evidence of siderophile-rich minerals at the boundary. The presence of spinels and Ni-rich particles as inclusions in aluminosilicate spherules from Zumaya suggests an original, non-diagenetic origin for the spherules. Similar spherules from southern Spain (Caravaca), show a strong marine authigenic overprint. This research represents a new approach in trying to directly identify the sedimentary mineral components that are responsible for the trace element concentrations associated with the K-T boundary.

Margolis, S. V.↗

NASA NTRS1979

The isotopic composition of Nd in different ocean masses

The paper examines the isotopic composition of Nd in marine environments. The Sm-Nd data for authigenic ferromanganese sediments indicate that the Atlantic, Pacific, and Indian Oceans have a distinct range in Nd isotopic composition characteristics of each ocean basin and reflect the dissolved load of Nd in the water mass. Measurements of the Nd isotopic seawater composition of seawater indicate that the rare earth elements (REE) in ferromanganese sediments are derived by direct precipitation of these elements out of seawater. It is believed that the Nd isotopic variations in these sediments represent true variations in the dissolved Nd isotopic composition which reflect the age and (Sm-147)/(Nd-144) ratios of the continental masses sampled believed to be the major source of REE in seawater.

Piepgras, D. J.↗

NASA NTRS1997

Geochemistry of approximately 1.9 Ga sedimentary rocks from northeastern Labrador, Canada

Fifty-eight rock chips from fifteen samples of sedimentary rocks from the Ramah Group (approximately 1.9 Ga) in northeastern Labrador, Canada, were analyzed for major and minor elements, including C and S, to elucidate weathering processes on the Earth's surface about 1.9 Ga ago. The samples come from the Rowsell Harbour, Reddick Bight, and Nullataktok Formations. Two rock series, graywackes-gray shales of the Rowsell Harbour, Reddick Bight and Nullataktok Formations, and black shales of the Nullataktok Formation, are distinguishable on the basis of lithology, mineralogy, and major and trace element chemistry. The black shales show lower concentrations than the graywackes-gray shales in TiO2 (0.3-0.7 wt% vs. 0.7-1.8 wt%), Al2O3 (9.5-20.1 wt% vs. 13.0-25.0 wt%), and sigma Fe (<1 wt% vs. 3.8-13.9 wt% as FeO). Contents of Zr, Th, U, Nb, Ce, Y, Rb, Y, Co, and Ni are also lower in the black shales. The source rocks for the Ramah Group sediments were probably Archean gneisses with compositions similar to those in Labrador and western Greenland. The major element chemistry of source rocks for the Ramah Group sedimentary rocks was estimated from the Al2O3/TiO2 ratios of the sedimentary rocks and the relationship between the major element contents (e.g., SiO2 wt%) and Al2O3/TiO2 ratios of the Archean gneisses. This approach is justified, because the Al/Ti ratios of shales generally retain their source rock values; however, the Zr/Al, Zr/Ti, and Cr/Ni ratios fractionate during the transport of sediments. The measured SiO2 contents of shales in the Ramah Group are generally higher than the estimated SiO2 contents of source rocks by approximately 5 wt%. This correction may also have to be applied when estimating average crustal compositions from shales. Two provenances were recognized for the Ramah Group sediments. Provenance I was comprised mostly of rocks of bimodal compositions, one with SiO2 contents approximately 45 wt% and the other approximately 65 wt%, and was the source for most sedimentary rocks of the Ramah Group, except for black shales of the Nullataktok Formation. The black shales were apparently derived from Provenance II that was comprised mostly of felsic rocks with SiO2 contents approximately 65 wt%. Comparing the compositions of the Ramah Group sedimentary rocks and their source rocks, we have recognized that several major elements, especially Ca and Mg, were lost almost entirely from the source rocks during weathering and sedimentation. Sodium and potassium were also leached almost entirely during the weathering of the source rocks. However, significant amounts of Na were added to the black shales and K to all the rock types during diagenesis and/or regional metamorphism. The intensity of weathering of source rocks for the Ramah Group sediments was much higher than that of typical Phanerozoic sediments, possibly because of a higher PCO2 in the Proterozoic atmosphere. Compared to the source rock values, the Fe3+/Ti ratios of many of the graywackes and gray shales of the Ramah Group are higher, the Fe2+/Ti ratios are lower, and the sigma Fe/Ti ratios are the same. Such characteristics of the Fe geochemistry indicate that these sedimentary rocks are comprised of soils formed by weathering of source rocks under an oxygen-rich atmosphere. The atmosphere about 1.9 Ga was, therefore, oxygen rich. Typical black shales of Phanerozoic age exhibit positive correlations between the organic C contents and the concentrations of S, U, and Mo, because these elements are enriched in oxygenated seawater and are removed from seawater by organic matter in sediments. However, such correlations are not found in the Ramah Group sediments. Black shales of the Ramah Group contain 1.7-2.8 wt% organic C, but are extremely depleted in sigma Fe (<1 wt% as FeO), S (<0.3 wt%), U (approximately l ppm), Mo (<5 ppm), Ni (<2 ppm), and Co (approximately 0 ppm). This lack of correlation, however, does not imply that the approximately 1.9 Ga atmosphere-ocean system was anoxic. Depletion of these elements from the Ramah Group sediments may have occurred during diagenesis.

Non-NASA Center↗

NASA NTRS1984

Minor elements in relict olivine grains of deep-sea spheres: Match with Mg-rich olivines from C2 meteorites

The bulk composition and relict minerals of meteoroid ablation spheres from deep sea sediments can be related to the parental material, and bulk compositions and elemental ratios favor a CI/CM affinity for most spheres. Although largely melted, some deep sea spheres (DSS) have retained rare grains apparently unmodified chemically by ablation heating or seawater alteration. Minor elements in relict olivines for comparison with compositions of olivines in known meteorites were analyzed. All relict olivines are very Mg rich. No terrestrial olivines match the chemical features which reinforces other evidence for an extraterrestrial origin. There is no match with achondritic olivines. Mg rich olivines occur in all types of carbonaceous meteorites, but the minor elements of most DSS olivines do not match with those for Allende (C3) olivines, and fit poorly with those of Murchison (C2) olivines. There is a good fit for Fe and Cr with those of the olivines in the unusual Belgica 7904 (C2) meteorite (3). It seems likely that the relict olivines of at least many deep sea spheres are chemically related to olivines in at least one C2 meteorite.

Smith, J. V.↗

NASA NTRS2014

Aqueous Alteration of Endeavour Crater Rim Apron Rocks

Mars Exploration Rover Opportunity is exploring Noachian age rocks of the rim of 22 km diameter Endeavour crater. Overlying the pre-impact lithologies and rim breccias is a thin apron of fine-grained sediments, the Grasberg fm, forming annuli on the lower slopes of rim segments. Hesperian Burns fm sandstones overly the Grasberg fm. Grasberg rocks have major element compositions that are distinct from Burns fm sandstones, especially when comparing interior compositions exposed by the Rock Abrasion Tool. Grasberg rocks are also different from Endeavour rim breccias, but have general compositional similarities to them. Grasberg sediments are plausibly fine-grained materials derived from the impact breccias. Veins of CaSO4 transect Grasberg fm rocks demonstrating post-formation aqueous alteration. Minor/trace elements show variations consistent with mobilization by aqueous fluids. Grasberg fm rocks have low Mn and high Fe/Mn ratios compared to the other lithologies. Manganese likely was mobilized and removed from the Grasberg host rock by redox reactions. We posit that Fe2+ from acidic solutions associated with formation of the Burns sulfate-rich sandstones acted as an electron donor to reduce more oxidized Mn to Mn2+. The Fe contents of Grasberg rocks are slightly higher than in other rocks suggesting precipitation of Fe phases in Grasberg materials. Pancam spectra show that Grasberg rocks have a higher fraction of ferric oxide minerals than other Endeavour rim rocks. Solutions transported Mn2+ into the Endeavour rim materials and oxidized and/or precipitated it in them. Grasberg has higher contents of the mobile elements K, Zn, Cl, and Br compared to the rim materials. Similar enrichments of mobile elements were measured by the Spirit APXS on West Spur and around Home Plate in Gusev crater. Enhancements in these elements are attributed to interactions of hydrothermal acidic fluids with the host rocks. Interactions of fluids with the Grasberg fm postdate the genesis of the Endeavour rim phyllosilicates. The aqueous alteration history of Endeavour rim rocks is complicated by different styles of alteration that have spanned the Noachian and Hesperian. Late stage acidic aqueous alteration of Grasberg fm materials is likely penecontemporaneous with the diagenesis of the sulfate-rich sediments of Meridiani Planum.

Mittlefehldt, David W.↗

NASA NTRS1993

Fabrication and properties of functionally graded NiAl/Al2O3 composites

A modified sedimentation process was used in the production of a functionally gradient material (FGM), NiAl/Al2O3. A simple finite element model was used to guide our design and fabrication efforts by estimating residual stress states as a function of composite structure. This approach could lead to tailored designs that enhance or avoid specific residual stress states. Thermal cycling tests were factored into the model to predict time dependent or steady-state internal temperature and stress profiles. Four-point bend tests were conducted to establish the mechanical load-displacement behavior of a single interlayer FGM at room temperature, 800 and 1000 K. Room temperature bend strength of the FGM was 3-4 times that of the base NiAl. At elevated temperatures, composite fracture occurred in a gradual, noncatastrophic mode involving NiAl retardation of a succession of cracks originating in the alumina face.

Miller, D. P.↗

NASA NTRS1982

Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from the old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites. Sr model ages are variable within each group, reflecting Rb-Sr fractionation, and, in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Unlike tektites, which are dense homogeneous objects, sanidine spherules are porous, fine grained inhomogeneous objects. The leaching experiment employed by the present study shows that the sanidine spherules could have been formed by an oceanic impact involving basaltic crust and overlying sediments or seawater.

Shaw, H. F.↗

NASA NTRS2003

The Disposition of Pt, Pd, Ir, Os, and Ru in Marine Sediments and the K/T Boundary

The marine record of platinum group elements (PGEs) and Os isotopic compositions provides information on different inputs of PGEs into the oceans. Some studies based on a smaller subset of the PGEs suggest that the PGEs may suffer post-depositional mobility during diagenesis. In some K/T boundary clays, Kyte and others showed that the relative abundances of Pt, Pd, Ir, and Os can differ significantly from chondritic, which is the signature expected from fallout of the meteorite impact. In some K/T boundary sections, elevated Ir concentrations are observed as far as 1 meter from the cm-thick boundary clay containing the meteoritic ejecta. The purpose of this study was to characterize Pt, Pd, Ir, Os, and Ru abundances in zones including the K/T boundary. We determined PGE abundances of boundary clays at two hemipelagic sites (Stevns Klint, Denmark and Caravaca, Spain) in which previous studies by Kyte and others showed that the Ir anomaly is confined to within a few cm. We also analyzed two pelagic Pacific sites: a boundary clay from the north Pacific (Hole 465A) characterized by a 0.5 m thick Ir anomaly and a transect across the K/T boundary from the south Pacific (Hole 596) where the Ir anomaly spans 2 m. The Stevns Klint, Caravaca, and north Pacific sites are characterized by abundant marls and limestones in the section, whereas the south Pacific site is dominated by clays. Samples were spiked with isotopic tracers, mixed with a flux, S and Ni, and equilibrated by fusion. PGEs were extracted from the Ni and analyzed on a Finnigan Element ICP-MS. We find that the narrow Caravaca and Stevns Klint boundary clays have relative PGE abundance patterns indistinguishable from chondritic values. The two Pacific sites were found to have nearly identical PGE patterns but have ratios at the peak, which differ from chondritic values as found earlier by Evans et al. The Pacific sites were found to have nearly identical PGE patterns but are extremely depleted in OS (Os/Ir = 0.07-0.15) and slightly enriched in Pd and Pt relative to Ir.

Lee, Cin-Ty↗

NASA NTRS2006

NanoSIMS Reveals New Structural and Elemental Signatures of Early Life

The young technology of NanoSIMS is unlocking new information from organic matter in ancient sediments. We have used this technique to characterize sub-micron scale element composition of Proterozoic organics that are clearly biogenic as a guide for interpreting problematic structures in terrestrial or extraterrestrial samples. We used the NanoSIMS 50 of the National Museum of Natural History in Paris to map carbon, nitrogen (as CN), and sulfur in organic structures from the approximately 0.8 Ga Bitter Springs Formation. We analyzed spheroidal and filamentous microfossils as well as organic laminae that appeared amorphous by optical and scanning electron microscopy. In clear-cut microfossils, a coincidence between optical images and NanoSIMS element maps suggests a biological origin for the mapped carbon, sulfur, and nitrogen; this conclusion is supported by high resolution NanoSIMS maps showing identical spatial distributions of C, CN and S. High resolution images also demonstrate distinctive nano structure of the filaments and spheroids. In the amorphous laminae, NanoSIMS reveals morphologies reminiscent of compressed microfossils. Distinct CN/C ratios of the spheroids, filaments, and laminae may reflect their biological precursors (cell walls, cyanobacterial sheaths, and microbial communities/biofilms, respectively). Similar amorphous laminae comprise a preponderance of the organic matter in many Precambrian deposits. Thus it is possible that NanoSIMS will provide fresh insight into a large body of previously uninterpretable material. Additionally, NanoSIMS analysis may establish new biosignatures that will be helpful for assessing the origin and biogenicity of controversial Archean structures and any organic materials that may occur in Martian or other extraterrestrial samples.

Oehler, Dorothy Z.↗

NASA NTRS1989

Geochemical and isotopic evidence for paleoredox conditions during deposition of the Devonian-Mississippian New Albany Shale, southern Indiana

The upper part of the New Albany Shale is divided into three members. In ascending order, these are (1) the Morgan Trail Member, a laminated brownish-black shale; (2) the Camp Run Member, an interbedded brownish-black and greenish-gray shale; and (3) the Clegg Creek Member, also a laminated brownish-black shale. The Morgan Trail and Camp Run Members contain 5% to 6% total organic carbon (TOC) and 2% sulfide sulfur. Isotopic composition of sulfide in these members ranges from -5.0% to -20.0%. C/S plots indicate linear relationships between abundances of these elements, with a zero intercept characteristic of sediments deposited in a non-euxinic marine environment. Formation of diagenetic pyrite was carbon limited in these members. The Clegg Creek Member contains 10% to 15% TOC and 2% to 6% sulfide sulfur. Isotopic compositions of sulfide range from -5.0% to -40%. The most negative values occur in the uppermost Clegg Creek Member and are characteristic of syngenetic pyrite, formed within an anoxic water column. Abundances of carbon and sulfur are greater and uncorrelated in this member, consistent with deposition in as euxinic environment. In addition, DOP (degree of pyritization) values suggest that formation of pyrite was generally iron limited throughout Clegg Creek deposition, but sulfur isotopes indicate that syngenetic (water-column) pyrite becomes an important component in the sediment only in the upper part of the member. At the top of the Clegg Creek Member, a zone of phosphate nodules and trace-metal enrichment coincides with maximal TOC values. During euxinic deposition, phosphate and trace metals accumulated below the chemocline because of limited vertical circulation in the water column. Increased productivity would have resulted in an increased flux of particulate organic matter to the sediment, providing an effective sink for trace metals in the water column. Phosphate and trace metals released from organic matter during early diagenesis resulted in precipitation of metal-rich phosphate nodules.

NASA Discipline Exobiology↗

NASA NTRS1997

Seventh Annual V. M. Goldschmidt Conference

Topics considered include: Subduction of the Aseismic Cocos Ridge Displaced Magma Sources Beneath the Cordillera de Talamanca, Costa Rica; Topography of Transition Zone Discontinuities: A Measure of 'Olivine' Content and Evidence for Deep Cratonic Roots; Uranium Enrichment in Lithospheric Mantle: Case Studies from French Massif Central; Rare-Earth-Element Anomalies in the Decollement Zone of the nankai Accretionary Prism, Japan: Evidence of Fluid Flow?; Rare Earth Elements in Japanese Mudrocks: The Influence of Provenance; The Evolution of Seawater Strontium Isotopes in the Last Hundred Million Years: Reinterpretation and Consequences for Erosion and Climate Models; From Pat to Tats: The Lead Isotope Legacy in the Studies of the Continental Crust-Upper Mantle System; Geochronology of the Jack Hills Detrital Zircons by Precise Uranium-Lead Isotope-Dilution Analysis of Crystal Fragments; Iridium in the Oceans; The Helium-Heat-Lead Paradox; Control of Distribution Patterns of Heavy Metals in Ganga Plain Around Kanpur Region, India, by Fluvial Geomorphic Domains; Geochemical and Isotopic Features of Ferrar Magmatic Provience (Victoria Land, Antarctica); Rare Earth Elements in Marine Fine-Grained Sediments from the Northwestern Portuguese Shelf (Atlantic); Aspects of Arc Fluxes; General Kinetic Model for Dolomite Precipitation Rate with Application to the Secular History of Seawater Composition; High-Precision Uranium-series Chronology from Speleothems; Trace-Element Modeling of Aqueous Fluid-Peridotite Interaction in the Mantle Wedge of Subduction Zones; Rainfall Variations in Southeastern Australia over the Last 500,000 Years from Speleothem Deposition; The Role of Water in High-Pressure Fluids; The Kinetic Conditions of Metamorphic Minearogenesis: Evidence from Minerals and Assemblages.

Source record↗

NASA NTRS2012

Biogeochemical and Optical Analysis of Coastal DOM for Satellite Retrieval of Terrigenous DOM in the U.S. Middle Atlantic Bight

Estuaries and coastal ocean waters experience a high degree of variability in the composition and concentration of particulate and dissolved organic matter (DOM) as a consequence of riverine/estuarine fluxes of terrigenous DOM, sediments, detritus and nutrients into coastal waters and associated phytoplankton blooms. Our approach integrates biogeochemical measurements (elemental content, molecular analyses), optical properties (absorption) and remote sensing to examine terrestrial DOM contributions into the U.S. Middle Atlantic Bight (MAB). We measured lignin phenol composition, DOC and CDOM absorption within the Chesapeake and Delaware Bay mouths, plumes and adjacent coastal ocean waters to derive empirical relationships between CDOM and biogeochemical measurements for satellite remote sensing application. Lignin ranged from 0.03 to 6.6 ug/L between estuarine and outer shelf waters. Our results demonstrate that satellite-derived CDOM is useful as a tracer of terrigenous DOM in the coastal ocean

Mannino, A.↗

NASA NTRS1990

Geochemistry of Darwin impact glass and target rocks

Results are presented from major and trace element composition analyses of 18 Darwin (Australia) glass samples and seven target rocks from the Darwin crater area. It is found that the chemical composition of the glass samples is in agreement with an origin from terrestrial sediments during an impact. Analyses of the target rocks from the Darwin crater and mixing calculations show that the Darwin glass can be formed from the local target rocks. Evidence is presented for a loss of the volatile elements Ga, Zn, F, and B during the impact event. The elements Na, K, Rb, and Cs show lower abundance in the glass than in the target rocks. This suggests selective volatilization from the impact melt.

Meisel, Thomas↗

NASA NTRS2012

Initial Observations by the Mars Exploration Rover Opportunity at Cape York, Meridiani Planum

Since the beginning of its mission, the MER rover Opportunity has visited a sequence of progressively larger impact craters in order to characterize rocks that represent an ever broader stratigraphic range. Endeavour Crater is by far the largest crater that this rover has visited, therefore the crater rim provides materials from strata that are much deeper and older than any materials yet sampled. Indeed, the Mars Reconnaissance Orbiter CRISM instrument detected spectral signatures of hydrated phyllosilicates in several rim segments, consistent with a Noachian provenance. In addition, the crater rim might have recorded the effects of the original impact as well as the thermal processes that occurred in its aftermath. This report summarizes initial observations of geologic structures and the fabrics, chemistry and mineralogy of rocks. Cape York is an N/NE - S/SW-trending segment of crater rim that is bounded by rim segments that are more deeply eroded, forming embayments. Opportunity drove onto the southwest margin of Cape York and has traversed across the Cape to a ridge near its northern end. The rim of Endeavour Crater was severely degraded and then on lapped by Meridiani sulfate-rich deposits, indicating that Cape York deposits experienced extensive erosion prior to the deposition of Meridiani Planum sulfate-rich sediments. Cape York consists of impact-uplifted rocks that are surrounded by a younger bedrock bench that might represent sediments shed from Cape York. The uplifted rock "Tisdale-2," located near the southern end of Cape York, is a polymict lithic breccia. Its elemental composition resembles a mixture of basalt plus bedrock encountered previously at Meridiani Planum, and it is relatively enriched in Ni, Zn, P, characteristic of hydrothermal fluids. "Chester Lake" and "Greeley Haven" are bedrock surfaces located near the southern and northern ends of Cape York, respectively, and they exhibit fabrics reminiscent of suevites. The rocks that were analyzed so far are quite different from any rocks previously encountered by Opportunity and Spirit. Several light-toned vein deposits (approx 1 cm) were discovered in the bench that borders the western flanks of Cape York. The vein "Homestake" is rich in Ca and S, and it exhibits a spectral feature near 1000 run caused by hydration, indicating that "Homestake" consists substantially of gypsum. The relationship(s) between these veins and the sulfate-rich layered deposits at Meridiani Planum remains uncertain. The veins might have been deposited by sulfate-rich fluids perhaps either in the immediate aftermath of the impact that formed Endeavour Crater or at some later time. Current uncertainties notwithstanding, the novel lithologies at Cape York are significantly older than the sulfate-rich sedimentary rocks at Meridiani Planum, and these lithologies have recorded episodes of aqueous activity from earlier epochs, perhaps during the Noachian. The author acknowledges the contributions of the Athena Science Team, the MER engineering team, JPL and NASA.

Marais, David J.↗

NASA NTRS2000

Mineralogical and Geochemical Analyses of Antarctic Lake Sediments: A Reflectance and Moessbauer Spectroscopy Study with Applications for Remote Sensing on Mars

Lakebottom sediments from the Dry Valleys region of Antarctica have been analyzed here in order to study the influence of water chemistry on the mineralogy and geochemistry of these sediments, as well as to evaluate techniques for remote spectral identification of potential biomarker minerals on Mars. Lakes from the Dry Valleys region of Antarctica have been investigated as possible analogs for extinct lake environments on early Mars. Sediment cores were collected in the present study from perennially ice-covered Lake Hoare in the Taylor Valley. These sediments were taken from a core in an oxic region of the lake and another core in an anoxic zone. Differences between the two cores were observed in the sediment color, Fe(II)/Fe(III) ratio, the presence of pyrite, the abundance of Fe, S and some trace elements, and the C, N and S isotope fractionation patterns. The results of visible-infrared reflectance spectroscopy (0.3-25 microns) Mossbauer spectroscopy (77 and 4 K) and X-ray diffraction are combined to determine the mineralogy and composition of these samples. The sediments are dominated by plagioclase, K-feldspar, quartz and pyroxene. Algal mats grow on the bottom of the lake and organic material has been found throughout the cores. Calcite is abundant in some layers of the aerobic core (shallow region) and pyrite is abundant in some layers of the anaerobic core (deep region). Analysis of the spectroscopic features due to organics and carbonates with respect to the abundance of organic C and carbonate contents was performed in order to select optimal spectral bands for remote identification of these components in planetary regoliths. Carbonate bands near 4 and 6.8 microns (approx.2500 and 1500/cm) were detected for carbonate abundances as low as 0.1 wt.% CO2. Organic features at 3.38, 3.42 and 3.51 microns (2960, 2925 and 2850/cm) were detected for organic C abundances as low as 0.06 wt.% C. The d13C trends show a more complex organic history for the anaerobic sediments than for the aerobic sediments. The biogenic pyrite found in the anaerobic core is associated with lighter d34S values and high organic C levels and could be used as a biomarker mineral for paleolakes on Mars.

Froeschl, Heinz↗

NASA NTRS2015

Predicting the Mineral Composition of Dust Aerosols: Representing Key Processes - Part 1

Soil dust aerosols created by wind erosion are typically assigned globally uniform physical and chemical properties within Earth system models, despite known regional variations in the mineral content of the parent soil. Mineral composition of the aerosol particles is important to their interaction with climate, including shortwave absorption and radiative forcing, nucleation of cloud droplets and ice crystals, heterogeneous formation of sulfates and nitrates, and atmospheric processing of iron into bioavailable forms that increase the productivity of marine phytoplankton. Here, aerosol mineral composition is derived by extending a method that provides the composition of a wet-sieved soil. The extension accounts for measurements showing significant differences between the mineral fractions of the wetsieved soil and the emitted aerosol concentration. For example, some phyllosilicate aerosols are more prevalent at silt sizes, even though they are nearly absent at these diameters in a soil whose aggregates are dispersed by wet sieving. We calculate the emitted mass of each mineral with respect to size by accounting for the disintegration of soil aggregates during wet sieving. These aggregates are emitted during mobilization and fragmentation of the original undispersed soil that is subject to wind erosion. The emitted aggregates are carried far downwind from their parent soil. The soil mineral fractions used to calculate the aggregates also include larger particles that are suspended only in the vicinity of the source. We calculate the emitted size distribution of these particles using a normalized distribution derived from aerosol measurements. In addition, a method is proposed for mixing minerals with small impurities composed of iron oxides. These mixtures are important for transporting iron far from the dust source, because pure iron oxides are more dense and vulnerable to gravitational removal than most minerals comprising dust aerosols. A limited comparison to measurements from North Africa shows that the model extensions result in better agreement, consistent with a more extensive comparison to global observations as well as measurements of elemental composition downwind of the Sahara, as described in companion articles.

aerosols↗