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Through Earth history, atmospheric oxygen has increased from initial values near zero to its present day level of about 21 % by volume; concomitantly, changes in ocean redox conditions have fundamentally altered global biogeochemical cycles. While there is a reasonable understanding of where oxygen history begins and ends, the quantitative timetable of oxygenation that links the endpoints has proven contentious. Equilibrium between marine surface environments and the overlying atmosphere suggests that carbonate-based redox proxies could refine palaeoredox records in time and space. Here we explore the use of Zn/Fe ratios to infer the evolution of atmospheric O2 through time, based on marine carbonate rocks that are well characterised in terms of depositional age, environmental setting, and diagenetic history. While Fe and Zn in the shallow ocean are mainly sourced from hydrothermal inputs, their redox sensitivities differ significantly, so that geological intervals with higher O2 would be characterised by stepped increases in Zn/Fe as preserved in shallow marine carbonates. Therefore, Zn/Fe analyses of ancient carbonates allow us to constrain past atmospheric pO2 levels, providing a secular record of atmospheric O2 over the past 3.5 billion years. In particular, we corroborate an earlier proposal that for much of the Proterozoic Eon, O2 levels were as low as 0.1-1 % of present atmospheric level. We conclude that Zn/Fe in shallow marine carbonate rocks has potential to provide a quantitative tracer for the long-term redox evolution of the oceans and the rise of atmospheric O2.

We assessed dust coverage on the Mars Science Laboratory Mars Hand Lens Imager (MAHLI) and Alpha Particle X-Ray Spectrometer (APXS) calibration targets from 14 MAHLI images acquired at

Marine sulfate ( SO 4 2 − ) is intimately connected to the global carbon and oxygen cycles through its important role as an electron acceptor for the microbial respiration of organic carbon. The biogeochemical feedbacks within the sulfur, carbon, and oxygen cycles may have changed through time, reflecting changes in the concentration of sulfate in the oceans. Unfortunately, there is much uncertainty about the size of the marine sulfate reservoir throughout Earth history. In particular, conflicting estimates for marine sulfate exist during the latest Neoproterozoic, an interval of time associated with striking changes in Earth system evolution and oxidation: published interpretations of fluid inclusion chemistry place sulfate greater than 16 mmol/kg, whereas other interpretations of carbonate-associated sulfate data suggest concentrations less than 2 mmol/kg. Calcium isotope ratios in evaporite successions provide an independent method for deriving semi-quantitative constraints on sulfate concentrations, as well as other properties of seawater chemistry. Here, the calcium isotope behavior of bedded sulfate evaporites from ∼1050 Ma (Baffin and Bylot Islands, Nunavut, Canada), ∼830 Ma (Officer Basin, Western Australia), and ∼545 Ma (South Oman Salt Basin, Sultanate of Oman) are examined. In combination with other geological observations, the results suggest relatively low, millimolal-level sulfate in the latest Mesoproterozoic and a more specific range of 6–10 mmol/kg sulfate during the latest Neoproterozoic. These new constraints suggest that previous interpretations of sulfate concentrations and seawater chemistry need to be revised, opening up new possible solution spaces for the major ion composition of seawater.

Photic zone euxinia (PZE) is a condition where anoxic, H(2)S-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by (i) photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and (ii) enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impact on the evolution of life.

Environmental fluctuations are recorded in a variety of sedimentary archives of lacustrine depositional systems. Geochemical signals recovered from bottom sediments in closed-basin lakes are among the most sensitive paleoenvironmental indicators and are commonly used in reconstructing lake evolution. Microbialites (i.e., organosedimentary deposits accreted through microbial trapping and binding of detrital sediment or in situ mineral precipitation on organics [Palaios, 2, 1987, 241]), however, have been largely overlooked as paleoenvironmental repositories. Here, we investigate concentrically laminated mineralized microbialites from Laguna Negra, a high-altitude (4,100 m above sea level) hypersaline, closed-basin lake in northwestern Argentina, and explore the potential for recovery of environmental signals from these unique sedimentary archives. Spatial heterogeneity in hydrological regime helps define zones inside Laguna Negra, each with their own morphologically distinct microbialite type. Most notably, platey microbialites (in Zone 3A) are precipitated by evaporative concentration processes, while discoidal oncolites (in Zone 3C) are interpreted result from fluid mixing and biologically mediated nucleation. This spatial heterogeneity is reflected in petrographically distinct carbonate fabrics: micritic, botryoidal, and isopachous. Fabric type is interpreted to reflect a combination of physical and biological influences during mineralization, and paired C-isotope measurement of carbonate and organic matter supports ecological differences as a dominant control on C-isotopic evolution between zones. Laminae of Laguna Negra microbialites preserve a range of δ13 Ccarb from +5.75‰ to +18.25‰ and δ18 Ocarb from -2.04‰ to +9.28‰. Temporal trends of lower carbon and oxygen isotopic compositions suggest that the influence of CO2 degassing associated with evaporation has decreased over time. Combined, these results indicate that microbialite archives can provide data that aid in interpretation of both lake paleohydrology and paleoenvironmental change.

Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism—can provide insight into potentially changeable depositional conditions of early Mars.

International audience; The Yellowknife Bay formation represents a similar to 5m thick stratigraphic section of lithified fluvial and lacustrine sediments analyzed by the Curiosity rover in Gale crater, Mars. Previous works have mainly focused on the mudstones that were drilled by the rover at two locations. The present study focuses on the sedimentary rocks stratigraphically above the mudstones by studying their chemical variations in parallel with rock textures. Results show that differences in composition correlate with textures and both manifest subtle but significant variations through the stratigraphic column. Though the chemistry of the sediments does not vary much in the lower part of the stratigraphy, the variations in alkali elements indicate variations in the source material and/or physical sorting, as shown by the identification of alkali feldspars. The sandstones contain similar relative proportions of hydrogen to the mudstones below, suggesting the presence of hydrous minerals that may have contributed to their cementation. Slight variations in magnesium correlate with changes in textures suggesting that diagenesis through cementation and dissolution modified the initial rock composition and texture simultaneously. The upper part of the stratigraphy (similar to 1m thick) displays rocks with different compositions suggesting a strong change in the depositional system. The presence of float rocks with similar compositions found along the rover traverse suggests that some of these outcrops extend further away in the nearby hummocky plains.

Oxygenation of Earth's oceans in the Proterozoic was a protracted process that began prior to the Great Oxidation Event (GOE) and culminated in deep ocean ventilation following Neoproterozoic glacial episodes, with both of these major thresholds accompanied by profound changes in the evolution of Earth's biosphere. Our understanding of the redox structure of Mesoproterozoic oceans, however, is considerably less certain. Limited proxy investigations of oceanic redox in the Mesoproterozoic have focused largely on early Mesoproterozoic shelf to basinal settings, with comparatively little attention paid to epeiric sea environments that dominate the late Mesoproterozoic sedimentary record. Here we report multi-proxy shale redox data from epicratonic and pericratonic environments of the 1.1 Ga Atar and El Mreiti Groups, Taoudeni Basin, Mauritania. These strata were deposited across the West African craton during global sea level highstand, and provide insight into the redox structure of Mesoproterozoic epeiric seas. Iron speciation, pyrite sulfur isotope, and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions, with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments. We suggest that a protracted increase in biospheric oxygenation in the late Mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow, nearshore areas, contributing to the fragmentary pattern of early eukaryotic evolution and diversification.

The Mars Science Laboratory Mast camera and Descent Imager investigations were designed, built, and operated by Malin Space Science Systems of San Diego, CA. They share common electronics and focal plane designs but have different optics. There are two Mastcams of dissimilar focal length. The Mastcam‐34 has an f/8, 34 mm focal length lens, and the M‐100 an f/10, 100 mm focal length lens. The M‐34 field of view is about 20° × 15° with an instantaneous field of view (IFOV) of 218 μrad; the M‐100 field of view (FOV) is 6.8° × 5.1° with an IFOV of 74 μrad. The M‐34 can focus from 0.5 m to infinity, and the M‐100 from ~1.6 m to infinity. All three cameras can acquire color images through a Bayer color filter array, and the Mastcams can also acquire images through seven science filters. Images are ≤1600 pixels wide by 1200 pixels tall. The Mastcams, mounted on the ~2 m tall Remote Sensing Mast, have a 360° azimuth and ~180° elevation field of regard. Mars Descent Imager is fixed‐mounted to the bottom left front side of the rover at ~66 cm above the surface. Its fixed focus lens is in focus from ~2 m to infinity, but out of focus at 66 cm. The f/3 lens has a FOV of ~70° by 52° across and along the direction of motion, with an IFOV of 0.76 mrad. All cameras can acquire video at 4 frames/second for full frames or 720p HD at 6 fps. Images can be processed using lossy Joint Photographic Experts Group and predictive lossless compression., Key Points The Mars Descent Imager, an f/3 9.7 mm, 2 M pixel color camera operated autonomously during landing taking a descent video at 4 frames/secondMastcam‐34 f/8, 34 mm camera takes

The Mars Science Laboratory Curiosity rover has been traversing strata at the base of Aeolis Mons (informally known as Mount Sharp) since September 2014. The Murray formation makes up the lowest exposed strata of the Mount Sharp group and is composed primarily of finely laminated lacustrine mudstone intercalated with rare crossbedded sandstone that is prodeltaic or fluvial in origin. We report on the first three drilled samples from the Murray formation, measured in the Pahrump Hills section. Rietveld refinements and FULLPAT full pattern fitting analyses of X-ray diffraction patterns measured by the MSL CheMin instrument provide mineral abundances, refined unit-cell parameters for major phases giving crystal chemistry, and abundances of X-ray amorphous materials. Our results from the samples measured at the Pahrump Hills and previously published results on the Buckskin sample measured from the Marias Pass section stratigraphically above Pahrump Hills show stratigraphic variations in the mineralogy; phyllosilicates, hematite, jarosite, and pyroxene are most abundant at the base of the Pahrump Hills, and crystalline and amorphous silica and magnetite become prevalent higher in the succession. Some trace element abundances measured by APXS also show stratigraphic trends; Zn and Ni are highly enriched with respect to average Mars crust at the base of the Pahrump Hills (by 7.7 and 3.7 times, respectively), and gradually decrease in abundance in stratigraphically higher regions near Marias Pass, where they are depleted with respect to average Mars crust (by more than an order of magnitude in some targets). The Mn stratigraphic trend is analogous to Zn and Ni, however, Mn abundances are close to those of average Mars crust at the base of Pahrump Hills, rather than being enriched, and Mn becomes increasingly depleted moving upsection. Minerals at the base of the Pahrump Hills, in particular jarosite and hematite, as well as enrichments in Zn, Ni, and Mn, are products of acid-sulfate alteration on Earth. We hypothesize that multiple influxes of mildly to moderately acidic pore fluids resulted in diagenesis of the Murray formation and the observed mineralogical and geochemical variations. The preservation of some minerals that are highly susceptible to dissolution at low pH (e.g., mafic minerals and fluorapatite) suggests that acidic events were not long-lived and that fluids may not have been extremely acidic (pH>2pH>2). Alternatively, the observed mineralogical variations within the succession may be explained by deposition in lake waters with variable Eh and/or pH, where the lowermost sediments were deposited in an oxidizing, perhaps acidic lake setting, and sediments deposited in the upper Pahrump Hills and Marias Pass were deposited lake waters with lower Eh and higher pH.

Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high-relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge-like vertical supports draped by concave-upward, subhorizontal elements, resulting in tent-shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late-stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice-covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non-motile communities under nutrient-limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O2 conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic--across a variety of depositional environments--may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.

Microbialites comprise the mineralized record of early life on Earth and preserve a spectrum of fabrics that reflect complex physical, chemical, and biological interactions. The relatively rarity of microbialites in modern environments, however, challenges our interpretation of ancient structures. Here we report the occurrence of microbial mats, mineral precipitates, and stromatolitic oncolites in the Laguna Negra, a high-altitude Andean lake in Catamarca Province,Argentina. Extreme environmental conditions, including high UV-radiation, salinity, and temperature extremes, restrict multicellular life so that mineralization reflects a combination of local hydrologic conditions, lake geochemistry, and microbial activity. The resulting carbonate microtextures are strikingly similar to those observed in Proterozoic stromatolites, providing insight into potential mechanisms of mineralization. Here, increased saturation in carbonate minerals due to mixing of spring-fed inlets and lake waters favors microbialite formation and preservation. This highlights the importance of hydrological mixing zones in microbialites formation and as taphonomic windows to record microbial activity. Recent discoveries of minerals related to evaporating playa-lake systems on Mars further highlights the potential of Laguna Negra to provide critical insight into biosignature preservation in both terrestrial and extraterrestrial settings. Fil: Gomez, Fernando Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Kah, Linda C.. University of Tennessee; Estados Unidos Fil: Bartley, Julie K.. Gustavus Adolphus College; Estados Unidos Fil: Astini, Ricardo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina

Sedimentological investigation of the late Paleoproterozoic (Orosirian) Vempalle Formation of the Cuddapah Basin, Dharwar craton, India, reveals three facies association that range from supratidal to deep subtidal. Sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone, intraformational carbonate conglomerate, and a variety of columnar, domal, and stratiform microbialite facies. Deposition occurred in an extensional regime during development of a low-gradient ramp, where the distribution of microbialite facies is distinctly depth-partitioned. A gradual increase in synoptic relief of columnar stromatolites through the section, and the upward transition from stratiform to columnar microbialites, record a prolonged marine transgression with little or no influx of terrigenous detritus. Siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity. Redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory. Earthquake-induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep-seated mantle-plume that resulted in thermal doming of the Dharwar crust prior to the onset of Cuddapah deposition. Isotopic and elemental chemistry of a selection of Vempalle Formation carbonate rocks record elevated Mn2+ and Fe2+ concentrations and depleted carbon isotope values in inner ramp lagoonal facies, relative to more open marine stromatolitic facies. Patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses—either within the water column or within substrate pore fluids—that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments. These data suggest that, even in the aftermath of Early Paleoproterozoic oxygenation, that ocean chemistry was heterogeneous and strongly affected by local basin conditions.

The Curiosity rover has analyzed abundant light-toned fracture-fill material within the Yellowknife Bay sedimentary deposits. The ChemCam instrument, coupled with Mastcam and ChemCam/Remote Micro Imager images, was able to demonstrate that these fracture fills consist of calcium sulfate veins, many of which appear to be hydrated at a level expected for gypsum and bassanite. Anhydrite is locally present and is found in a location characterized by a nodular texture. An intricate assemblage of veins crosses the sediments, which were likely formed by precipitation from fluids circulating through fractures. The presence of veins throughout the entire similar to 5 m thick Yellowknife Bay sediments suggests that this process occurred well after sedimentation and cementation/lithification of those sediments. The sulfur-rich fluids may have originated in previously precipitated sulfate-rich layers, either before the deposition of the Sheepbed mudstones or from unrelated units such as the sulfates at the base of Mount Sharp. The occurrence of these veins after the episodes of deposition of fluvial sediments at the surface suggests persistent aqueous activity in relatively nonacidic conditions.

A suite of eight rocks analyzed by the Curiosity Rover while it was stopped at the Rocknest sand ripple shows the greatest chemical divergence of any potentially sedimentary rocks analyzed in the early part of the mission. Relative to average Martian soil and to the stratigraphically lower units encountered as part of the Yellowknife Bay formation, these rocks are significantly depleted in MgO, with a mean of 1.3 wt %, and high in Fe, averaging over 20 wt % FeO_T, with values between 15 and 26 wt % FeO_T. The variable iron and low magnesium and rock texture make it unlikely that these are igneous rocks. Rock surface textures range from rough to smooth, can be pitted or grooved, and show various degrees of wind erosion. Some rocks display poorly defined layering while others seem to show possible fractures. Narrow vertical voids are present in Rocknest 3, one of the rocks showing the strongest layering. Rocks in the vicinity of Rocknest may have undergone some diagenesis similar to other rocks in the Yellowknife Bay Formation as indicated by the presence of soluble calcium phases. The most reasonable scenario is that fine-grained sediments, potentially a mixture of feldspar-rich rocks from Bradbury Rise and normal Martian soil, were lithified together by an iron-rich cement.

The Sheepbed member of the Yellowknife Bay formation in Gale crater contains millimeter-scale nodules that represent an array of morphologies unlike those previously observed in sedimentary deposits on Mars. Three types of nodules have been identified in the Sheepbed member in order of decreasing abundance: solid nodules, hollow nodules, and filled nodules, a variant of hollow nodules whose voids have been filled with sulfate minerals. This study uses Mast Camera (Mastcam) and Mars Hand Lens Imager (MAHLI) images from the Mars Science Laboratory Curiosity rover to determine the size, shape, and spatial distribution of the Sheepbed nodules. The Alpha Particle X-Ray Spectrometer (APXS) and ChemCam instruments provide geochemical data to help interpret nodule origins. Based on their physical characteristics, spatial distribution, and composition, the nodules are interpreted as concretions formed during early diagenesis. Several hypotheses are considered for hollow nodule formation including origins as primary or secondary voids. The occurrence of concretions interpreted in the Sheepbed mudstone and in several other sedimentary sequences on Mars suggests that active groundwater systems play an important role in the diagenesis of Martian sedimentary rocks. When concretions are formed during early diagenetic cementation, as interpreted for the Sheepbed nodules, they have the potential to create a taphonomic window favorable for the preservation of Martian organics.

The Tenoumer impact structure is a small, well-preserved crater within Archean to Paleoproterozoic amphibolite, gneiss, and granite of the Reguibat Shield, north-central Mauritania. The structure is surrounded by a thin ejecta blanket of crystalline blocks (granitic gneiss, granite, and amphibolite) and impact-melt rocks. Evidence of shock metamorphism of quartz, most notably planar deformation features (PDFs), occurs exclusively in granitic clasts entrained within small bodies of polymict, glass-rich breccia. Impact-related deformation features in oligoclase and microcline grains, on the other hand, occur both within clasts in melt-breccia deposits, where they co-occur with quartz PDFs, and also within melt-free crystalline ejecta, in the absence of co-occurring quartz PDFs. Feldspar deformation features include multiple orientations of PDFs, enhanced optical relief of grain components, selective disordering of alternate twins, inclined lamellae within alternate twins, and combinations of these individual textures. The distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures, starting below that required for shock deformation of quartz. We suggest that experimental analysis of feldspar behavior, combined with detailed mapping of shock metamorphism of feldspar in natural systems, may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures.

[1] We have assessed the characteristics of clasts along Curiosity's traverse to shed light on the processes important in the genesis, modification, and transportation of surface materials. Pebble- to cobble-sized clasts at Bradbury Landing, and subsequently along Curiosity's traverse to Yellowknife Bay, reflect a mixing of two end-member transport mechanisms. The general clast population likely represents material deposited via impact processes, including meteorite fragments, ejecta from distant craters, and impactites consisting of shocked and shock-melted materials from within Gale Crater, which resulted predominantly in larger, angular clasts. A subset of rounded pebble-sized clasts has likely been modified by intermittent alluvial or fluvial processes. The morphology of this rounded clast population indicates that water was a more important transporting agent here than at other Mars sites that have been studied in situ. Finally, we identified populations of basalt clasts and porphyritic clasts of undetermined composition by their morphologic and textural characteristics; basalts are confirmed by geochemical data provided by ChemCam.

[1] During Martian solar days 57–100, the Mars Science Laboratory Curiosity rover acquired and processed a solid (sediment) sample and analyzed its mineralogy and geochemistry with the Chemistry and Mineralogy and Sample Analysis at Mars instruments. An aeolian deposit—herein referred to as the Rocknest sand shadow—was inferred to represent a global average soil composition and selected for study to facilitate integration of analytical results with observations from earlier missions. During first-time activities, the Mars Hand Lens Imager (MAHLI) was used to support both science and engineering activities related to sample assessment, collection, and delivery. Here we report on MAHLI activities that directly supported sample analysis and provide MAHLI observations regarding the grain-scale characteristics of the Rocknest sand shadow. MAHLI imaging confirms that the Rocknest sand shadow is one of a family of bimodal aeolian accumulations on Mars—similar to the coarse-grained ripples interrogated by the Mars Exploration Rovers Spirit and Opportunity—in which a surface veneer of coarse-grained sediment stabilizes predominantly fine-grained sediment of the deposit interior. The similarity in grain size distribution of these geographically disparate deposits support the widespread occurrence of bimodal aeolian transport on Mars. We suggest that preservation of bimodal aeolian deposits may be characteristic of regions of active deflation, where winnowing of the fine-sediment fraction results in a relatively low sediment load and a preferential increase in the coarse-grained fraction of the sediment load. The compositional similarity of Martian aeolian deposits supports the potential for global redistribution of fine-grained components, combined with potential local contributions.

Molybdenum is a bioessential micronutrient whose abundance in the global oceans may have played a primary role in evolution of the Earth's nitrogen cycle and, ultimately, in the timing of the ecologic expansion of early eukaryotes. Because molybdenum (Mo) is delivered to the ocean under conditions of oxic weathering and is removed in suboxic and euxinic environments, the concentration of Mo in the oceans reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation. Recent compilations of Mo within euxinic shales ( Scott et al., 2008 , Och and Shields-Zhou, 2012 , Sahoo et al., 2012 ) indicate that the oceanic reservoir of Mo did not rise substantially above crustal values until the Great Oxidation Event (~ 2.3 Ga), and a modern Mo cycle did not develop until the contraction of ocean euxinia in the latest Proterozoic. At present, a paucity of data from euxinic shales of the late Mesoproterozoic (1.3 to 1.0 Ga) limits our ability to relate marine redox evolution to biological innovation during a critical interval of eukaryotic development. Here we present data from marine shales of the 1.1 Ga Atar and El Mreiti groups, Mauritania, which were deposited during sea level highstand when shallow, epeiric seas covered much of the West African craton. Epicratonic strata of the El Mreiti Group were deposited under fluctuating redox conditions near a shallow chemocline (as evidenced by iron speciation), and record generally low Mo concentrations (

Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.

Carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in Earth's biogeochemical cycles. Chemostratigraphic interpretations, however, often rely on the assumption that the isotopic composition of dissolved inorganic carbon (DIC) in the surface oceans is broadly homogeneous. At times of globally high sea level, when epicratonic (epeiric) environments dominate our sedimentary record, carbon isotope records reveal substantial lateral variability. Here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the Mesoproterozoic (∼1.1 Ga) Atar/El Mreiti Groups of the Taoudeni Basin, West Africa. Restricted-marine epicratonic environments are consistently 2–4‰ lighter than open-marine pericratonic environments, suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments. In contrast to epicratonic environments in the Paleozoic, where input of isotopically light carbon is generally attributed to input of terrestrial organic matter, we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability. Within epicratonic El Mreiti Group strata, the extent of organic matter remineralization (and thus degree of 13 C depletion) is correlated with water depth, and associated with distinct differences in both total organic carbon (TOC) and pyrite concentration, suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments. In such settings, the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition, or the intermittent oxidation of an enhanced dissolved organic carbon (DOC) pool.

The discovery of the extreme antiquity of specific minerals through radiometric dating (e.g., Strutt 1910), coupled with Norman L. Bowen’s recognition of a deterministic evolutionary sequence of silicate minerals in igneous rocks (Bowen 1915, 1928), implies that Earth’s crustal mineralogy has changed dramatically through more than 4.5 billion years of planetary history. Detailed examination of the mineralogical record has led to a growing realization that varied physical, chemical, and biological processes have resulted in a sequential increase in diversification of the mineral kingdom. This diversification has been accompanied by significant changes in the near-surface distribution, compositional range (including minor and trace elements), size, and morphology of minerals (Ronov et al. 1969; Nash et al. 1981; Zhabin 1981; Meyer 1985; Wenk and Bulakh 2004; Hazen et al. 2008, 2009, 2011). Variation in Earth’s mineralogical character thus reflects the tectonic, geochemical, and biological evolution of Earth’s near-surface environment (Bartley and Kah 2004; Hazen et al. 2009, 2012; Grew and Hazen 2010a,b; McMillan et al. 2010; Krivovichev 2010; Grew et al. 2011; Tkachev 2011). The mineral kingdom’s evolutionary narrative shares many features with the increased complexity inherent within other evolving systems, including the nucleosynthesis of elements and isotopes, the prebiotic synthesis of organic molecules, biological evolution through Dar-winian natural selection, and the evolution of social and material culture (Hazen and Eldredge 2010). In particular, well-known biological phenomena such as diversification, punctuation, and extinction appear to be common traits within a wide range of complex, evolving systems. Perhaps more than any other element, carbon exemplifies these processes of “mineral evolution.” Four episodes outline major events in the mineral evolution of carbon: (1) the synthesis of the first mineral, likely diamond, and perhaps a dozen other “ur-minerals” …

Carbon, element 6, displays remarkable chemical flexibility and thus is unique in the diversity of its mineralogical roles. Carbon has the ability to bond to itself and to more than 80 other elements in a variety of bonding topologies, most commonly in 2-, 3-, and 4-coordination. With oxidation numbers ranging from −4 to +4, carbon is observed to behave as a cation, as an anion, and as a neutral species in phases with an astonishing range of crystal structures, chemical bonding, and physical and chemical properties. This versatile element concentrates in dozens of different Earth repositories, from the atmosphere and oceans to the crust, mantle, and core, including solids, liquids, and gases as both a major and trace element (Holland 1984; Berner 2004; Hazen et al. 2012). Therefore, any comprehensive survey of carbon in Earth must consider the broad range of carbon-bearing phases. The objective of this chapter is to review the mineralogy and crystal chemistry of carbon, with a focus primarily on phases in which carbon is an essential element: most notably the polymorphs of carbon, the carbides, and the carbonates. The possible role of trace carbon in nominally acarbonaceous silicates and oxides, though potentially a large and undocumented reservoir of the mantle and core (Wood 1993; Jana and Walker 1997; Freund et al. 2001; McDonough 2003; Keppler et al. 2003; Shcheka et al. 2006; Dasgupta 2013; Ni and Keppler 2013; Wood et al. 2013), is not considered here. Non-mineralogical carbon-bearing phases treated elsewhere, including in this volume, include C-O-H-N aqueous fluids (Javoy 1997; Zhang and Duan 2009; Jones et al. 2013; Manning et al. 2013); silicate melts (Dasgupta et al. 2007; Dasgupta 2013; Manning et al. 2013); carbonate melts (Cox 1980; Kramers …

a b s t r a c t Analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. Earlier Mesoproterozoic successions, however, have received comparatively little attention due to the relatively subdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profiles from three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of the early Mesoproterozoic (1600-1400 Ma, Calymmian period) Jixian Group. High-resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con- strain both the pattern and origin of isotopic variation in the Mesoproterozoic. Marine carbonate rocks of the Jixian Group show generally muted isotopic variation with average values near 0‰, consistent with previous observations from the early Mesoproterozoic. Data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long-term decrease in pCO2 and, conse- quently, in the isotopic buffering capacity of marine dissolved inorganic carbon (DIC). By contrast, the isotopic composition of marine organic matter suggests facies-dependent differences in carbon cycling. Organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom- position in shallow-water environments, and increased remineralization by anaerobic heterotrophs in deeper-water environments. Correlation between organic carbon composition and depositional environ- ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters.

The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater-derived brines, and mixed seawater-freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

International audience; The Curiosity rover has analyzed various detrital sedimentary rocks at Gale Crater, among which fluvial and lacustrine rocks are predominant. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale Crater rims. In this study, we report the results of six conglomerate targets analyzed by Alpha-Particle X-ray Spectrometer and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end-members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali-rich composition, with a Na2O/K2O\textgreater5. The second group (Kimberley type) is typical of conglomerates analyzed between sols 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali-rich potassic composition with Na2O/K2O\textless2. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali-rich float rock compositions analyzed in the hummocky plains. The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale Crater. Differences between the composition of conglomerates and that of finer-grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg enrichments in fine-grained rocks), physical sorting, and mixing with finer-grained material of different composition.

The Mars Science Laboratory (MSL) Mars Hand Lens Imager (MAHLI) investigation will use a 2-megapixel color camera with a focusable macro lens aboard the rover, Curiosity, to investigate the stratigraphy and grain-scale texture, structure, mineralogy, and morphology of geologic materials in northwestern Gale crater. Of particular interest is the stratigraphic record of a ∼5 km thick layered rock sequence exposed on the slopes of Aeolis Mons (also known as Mount Sharp). The instrument consists of three parts, a camera head mounted on the turret at the end of a robotic arm, an electronics and data storage assembly located inside the rover body, and a calibration target mounted on the robotic arm shoulder azimuth actuator housing. MAHLI can acquire in-focus images at working distances from ∼2.1 cm to infinity. At the minimum working distance, image pixel scale is ∼14 μm per pixel and very coarse silt grains can be resolved. At the working distance of the Mars Exploration Rover Microscopic Imager cameras aboard Spirit and Opportunity, MAHLI’s resolution is comparable at ∼30 μm per pixel. Onboard capabilities include autofocus, auto-exposure, sub-framing, video imaging, Bayer pattern color interpolation, lossy and lossless compression, focus merging of up to 8 focus stack images, white light and longwave ultraviolet (365 nm) illumination of nearby subjects, and 8 gigabytes of non-volatile memory data storage.