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Biostratigraphic, carbon isotope, and U-Pb zircon geochronological data from the Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542.0 ± 0.3 Ma, consistent with previous age constraints from Siberia and Namibia. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary.

Integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal Proterozoic to Cambrian Witvlei and Nama Groups of Namibia. Coupled with precise U-Pb zircon age constraints, these correlations make it possible to construct a composite reference section for use in calibrating terminal Proterozoic chronostratigraphy. The Namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the Proterozoic-Cambrian boundary. The two glacial horizons may represent each of two distinct Varanger-age glaciations better known from the North Atlantic region. From the higher of the two glacial horizons up, the composite stratigraphy preserves one of the thickest and most complete available records of carbon-isotope variability in post-Varanger terminal Proterozoic seawater. Four carbon-isotope chemostratigraphic intervals are recognized: (1) a postglacial negative delta 13C excursion (Npg interval); (2) a rising interval (Pr interval) of increasing positive delta 13C values; (3) a falling interval (Pf interval) characterized by decreasing positive delta 13C and culminating in near zero or negative values; and (4) an interval of moderately positive, relatively invariant delta 13C values (I interval) that extends up to the unconformity that contains the Proterozoic-Cambrian boundary. Each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world. By comparing sediment accumulation rate in the radiometrically calibrated Namibian stratigraphy with sediment accumulation rates in correlative sections in Arctic Canada and Oman, a maximum age of 564 Ma is estimated for the end of the younger Varanger glaciation, 25 m.y. younger than previous estimates.

Discovery of fossils of the Ediacara biota near the top of the Spitzkopf Member at farm Swartpunt extends the known range of these remains in Namibia more than 600 m to near the sub-Cambrian unconformity. The fossiliferous beds occur approximately 100 m above a volcanic ash dated at 543 ± 1 Ma, and thus may be the youngest Proterozoic Ediacara-type fossils reported anywhere in the world. Fossils are preserved within and on the tops of dm-thick beds of storm-deposited sandstone at two stratigraphic levels; the environment is interpreted as open marine, generally calm but with episodic disruptions by storm waves, and probably within the euphotic zone. The presence of Pteridinium carolinaense (St. Jean), which is also known from the classic sections in Ediacara and the White Sea among others, reinforces evidence from geochronology and chemostratigraphy that the Swartpunt section is terminal Neoproterozoic in age. The new genus and species Swartpuntia germsi is a large, multifoliate frond that exhibits at least three quilted petaloids. Macroscopically, Swartpuntia resembles Pteridinium and Ediacara-type fronds such as Charniodiscus traditionally interpreted as Cnidaria, whereas microscopically it exhibits segmentation that is remarkably similar to that of the putative worm Dickinsonia. Combination of diagnostic characters of these supposedly disparate groups in a single species suggests that many species of quilted Ediacaran organisms were more similar to each other than they were to any modern groups, and provides support for the concept of the “Vendobionta” as a late Neoproterozoic group of mainly multifoliate organisms with a distinctive quilted segmentation.

In this work, we test an amplitude-independent method of seisimic data analysis designed to extract lithologic information about stratigraphic horizons. We apply the method of singularity characterization in an attempt to determine the sharpness of lithologic boundaries. We infer the sharpness of the boundary based upon a fractional integration of the seismic trace. The order of fractional integration is taken to represent the abruptness of the lithologic transition responsible for a given reflector. We find that the method output behaves in a geologically reasonable manner which suggests that our method is responding to lithologic variations along boundaries responsible for prominent reflectors in the data.

Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.

THE secular cooling of the Earth since its formation, together with the law of radioactive decay, imply that the Earth's internal heat generation and heat loss must have been two to three times higher in Archaean and Proterozoic times than today, but this inference is difficult to test directly. Here we present a flexural analysis of the Proterozoic Kilohigok basin, in the Archaean Slave craton of Canada, which shows that 1.9 Gyr ago the craton had an effective elastic thickness of only 12 ±4 km. Because the present effective elastic thickness of the region is ∼ 100 ±25 km, these results imply a two- to fourfold decrease in the thermal gradient beneath the Slave craton since 1.9 Gyr, and indicate that at 1.9 Gyr—700 Myr after the accretion of the continental lithosphere forming the craton—the thick continental root beneath this part of the Canadian shield had not yet formed.

Arising from: T. M. McCollom & B. M. Hynek Nature 438, 1129–1131 (2005); McCollom & Hynek reply. The Mars Exploration Rover Opportunity discovered sulphate-rich sedimentary rocks at Meridiani Planum on Mars, which are interpreted by McCollom and Hynek1 as altered volcanic rocks. However, their conclusions are derived from an incorrect representation of our depositional model2, 3, which is upheld by more recent Rover data4, 5, 6, 7. We contend that all the available data still support an aeolian and aqueous sedimentary origin for Meridiani bedrock.

[Curiosity Special Section Index][1] The 6 August 2012 arrival of the curiosity rover on the surface of Mars delivered the most technically advanced geochemistry laboratory ever sent to the surface of another planet. Its 10 instruments ( 1 )[*][2] were commissioned for operations and were tested on a diverse set of materials, including rocks, soils, and the atmosphere, during the first 100 martian days (sols) of the mission. The five articles presented in full in the online edition of Science ([www.sciencemag.org/extra/curiosity][3]), with abstracts in print (pp. 1476–1477), describe the mission's initial results, in which Curiosity's full laboratory capability was used. Curiosity was sent to explore a site located in Gale crater, where a broad diversity of materials was observed from orbit. Materials representing interactions with aqueous environments were targeted for study because of the emphasis on understanding habitable environments. In addition, the mission's science objectives also include characterizing the geologic diversity of the landing site at all scales, including loose surface materials such as impact ejecta, soils, and windblown accumulations of fine sediments. In certain cases, such characterization may even provide constraints on the evolution of the planet as a whole. Two notable points along Curiosity's initial 500-m traverse included Jake_M, a loose rock sitting on the plains, and Rocknest, an accumulation of windblown sand, silt, and dust that formed in the lee of some rocky outcrops. Sparse outcrops of lithified fluvial conglomerate were also encountered ( 2 ). ![Figure][4] Curiosity used its scoop to collect two samples of a small aeolian deposit. The deposit's upper surface is armored by sand grains 0.5 to 1.5 mm in size. These coarse grains are coated with fine dust, giving the deposit an overall light brownish red color. Beneath the coarse sand crust is finer sand, dark brown in color. This Mars Hand Lens Imager image was acquired on sol 84. CREDIT: NASA As described by Stolper et al. , Jake\_M was encountered ∼282 m away from the landing site and is a dark, macroscopically homogeneous igneous rock representing a previously unknown martian magma type. In contrast to the relatively unfractionated Fe-rich and Al-poor tholeiitic basalts typical of martian igneous rocks, it is highly alkaline and fractionated. No other known martian rock is as compositionally similar to terrestrial igneous rocks; Jake\_M compares very closely with an uncommon terrestrial rock type known as a mugearite, typically found on ocean islands and in rift zones. It probably originates from magmas generated by low degrees of partial melting at high pressure of possibly water-rich, chemically altered martian mantle that is different from the sources of other known martian basalts. Over the first 100 sols of the mission, the ChemCam instrument returned >10,000 laser-induced breakdown spectra, helping to characterize surface material diversity. ChemCam's laser acts effectively as a microprobe, distinguishing between fine soil grains and coarser ∼1-mm grains. Based on these data, Meslin et al. report that the coarse soil fraction contains felsic (Si- and Al-rich) grains, mimicking the composition of larger felsic rock fragments found during the traverse and showing that these larger components probably break apart to form part of the soil. In contrast, the fine-grained soil component is mafic, similar to soils observed by the Pathfinder and Mars Exploration Rover missions. Curiosity scooped, processed, and analyzed a small deposit of windblown sand/silt/dust at Rocknest that has similar morphology and bulk elemental composition to other aeolian deposits studied at other Mars landing sites. Based solely on analysis of CheMin x-ray diffraction (XRD) data from Mars, calibrated with terrestrial standards, Bish et al. estimate the Rocknest deposit to be composed of ∼71% crystalline material of basaltic origin, in addition to ∼29% x-ray–amorphous materials. In an independent approach, Blake et al. used Alpha Particle X-ray Spectrometer data to constrain the bulk composition of the deposit and XRD data and phase stoichiometry to constrain the chemistry of the crystalline component, with the difference being attributed to the amorphous component, resulting in estimates of ∼55% crystalline material of basaltic origin and ∼45% x-ray–amorphous materials. The amorphous component may contain nanophase iron oxide similar to what was observed by earlier rovers. The similarity between basaltic soils observed at Rocknest and other Mars sites implies either global-scale mixing of basaltic material or similar regional-scale basaltic source material or some combination of both. No hydrated phases were detected. However, as shown by Leshin et al. , pyrolysis of Rocknest fines using the Sample Analysis at Mars (SAM) instrument suite revealed volatile species, probably in the amorphous component, including H2O, SO2, CO2, and O2, in order of decreasing abundance. ChemCam measurements of these materials also revealed the presence of H. It is likely that H2O is contained in the amorphous component and CO2 was liberated via the decomposition of Fe/Mg carbonates present below the XRD detection limit of 1 to 2%. Isotopic data from SAM indicate that this H2O, and possibly the CO2, were derived from the atmosphere. SAM analysis also revealed oxychloride compounds similar to those found by earlier missions, suggesting that their accumulation reflects global planetary processes. The evolution of CO2 during pyrolysis and the observation of simple chlorohydrocarbons during SAM gas chromatograph mass spectrometer analyses could be consistent with organic carbon derived from a terrestrial instrument background source, or a martian source, either exogenous or indigenous. [Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars][5] [X-ray Diffraction Results from Mars Science Laboratory: Mineralogy of Rocknest at Gale Crater][6] [Volatile, Isotope, and Organic Analysis of Martian Fines with the Mars Curiosity Rover][7] [The Petrochemistry of Jake_M: A Martian Mugearite][8] [Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow][9] Additional Curiosity research, news, and multimedia can be found at [www.sciencemag.org/extra/curiosity][10] [1]: http://www.sciencemag.org/site/extra/curiosity/index.xhtml [2]: #fn-1 [3]: http://www.sciencemag.org/extra/curiosity [4]: pending:yes [5]: http://www.sciencemag.org/content/341/6153/1238670.abstract [6]: http://www.sciencemag.org/content/341/6153/1238932.abstract [7]: http://www.sciencemag.org/content/341/6153/1238937.abstract [8]: http://www.sciencemag.org/content/341/6153/1239463.abstract [9]: http://www.sciencemag.org/content/341/6153/1239505.abstract [10]: http://www.sciencemag.org/extra/curiosity/index.xhtml

We agree with Caldeira that the mechanism we proposed (1) to account for large decreases in the C isotopic composition of surface sea water during Neoproterozoic and Permo-Triassic can work only if the negative excursions lasted for less than 10^6 years. It is likely that the Permo-Triassic boundary excursion persisted no longer than the 10^5-year time scale permitted in Kump's model (2). At the proposed Permo- Triassic boundary stratotype at Meishan, China, the stratigraphic interval containing the excursion is 1 meter thick (3); an earlier negative excursion described by Jin (4) from a Capitanian-Wujiapingian boundary section in southwestern China is confined to a comparably thin stratigraphic interval. For the one Neoproterozoic negative excursion closely constrained by radiometric data, uranium-lead zircon dates on volcanic rocks just below and just above the event are the same to within the measurement error of one million years (5).

Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds.

Herringbone calcite is a previously undescribed carbonate cement and sea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks. It is abundant in the 2520 Ma Campbellrand-Malmani platform, South Africa, where field relationships, such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones, demonstrate that it precipitated from ambient marine water. This interpretation is supported by depositional relationships in the >= 2.6 Ga Huntsman Limestone of the Bulawayo greenstone belt, Zimbabwe; the 2.6 Ga Carawine Dolomite, Australia; the 1.90 Ga Rocknest Formation and the 1.8-1.2 Ga Dismal Lakes Group, Canada; the Ordovician Porterfi ld carbonate buildup, Virginia; and various Silurian carbonate buildups in the Midcontinent, United States. Each of these occurrences is associated with anaerobic depositional environments or organic-rich sediments. Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick. Microscopically, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement. The bases of the crystals are optically unoriented, but upwards in each crystal, the optical c axis rotates until it is perpendicular to crystal elongation. The tops of the elongate crystal are thus optically aligned and length slow. The light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals, whereas the dark bands correspond to the optically unoriented, lower parts of the elongate crystals. Microspar crystals are also present in some dark ba ds. A Mg-calcite precursor for herringbone calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of micro-dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low-Mg calcite. Precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface. Since herringbone calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water. Thus, the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time.

Lateral and vertical changes within a 40-m-thick interval of peritidal carbonates (Middle Cambrian Jangle Limestone Member, Carrara Formation, SE California) produce a mosaic pattern of laterally continuous and discontinuous facies, shallowing-upward cycles 2-20 m thick (parasequences), and surfaces bounding parasequences. This mosaic pattern was traced, bed by bed, across 1.8 km of continuously exposed outcrop, and then correlated to sections in nearby ranges. Within parasequences, contacts between successive subtidal facies tend to be sharp but may pass laterally into gradational contacts. Most subtidal facies extend laterally for less than 300 m, passing gradationally (over tens of centimeters to meters) into other, laterally adjacent subtidal facies. Lateral facies contacts between subtidal nodular ribbon rock and intertidal fenestral limestone are gradual over a few meters, whereas contacts between vertically juxtaposed facies generally are sharp. Parasequence boundaries generally are sharp and locally show several tens of centimeters of erosional relief over lateral distances of several meters. Most parasequences and their bounding surfaces extend for the 1.8 km of outcrop studied and correlate to nearby ranges with varying degrees of confidence. Several parasequences, however, do not extend laterally for more than a few hundred meters and are not distinguishable in a vertical profile from those that extend laterally for tens of kilometers. The stratigraphic relationships at Eagle Mountain (southeastern California) support a model in which the lateral continuity of parasequence surfaces is principally determined by accommodation variations, and the lateral continuity of facies is most strongly influenced by the intrinsic dynamics of spatially and temporally heterogeneous sediment production. The presence of parasequence boundaries of highly variable lateral extent suggests that the magnitude of accommodation variations was comparatively small, consistent with the independent evidence for passive-margin subsidence and transitional greenhouse conditions for the Middle Cambrian.

Recent field observations of the statistical distribution of turbidite and debris flow deposits are discussed. In some cases one finds a good fit over 1.5-2 orders of magnitude to the scaling law N(h) α h-B, where N(h) is the number of layers thicker than h. Observations show that the scaling exponent B varies widely from deposit to deposit, ranging from about 1/2 to 2. Moreover, one case is characterized by a sharp crossover in which B increases by a factor of two as h increases past a critical thickness. We propose that the variations in B, either regional or within the same deposit, are indicative of the geometry of the sedimentary basin and the rheological properties of the original gravity-driven flow. The origin of the power-law distribution remains an open question.

We propose that the distribution of layer thicknesses of turbidite deposits that show minimal erosional truncation and amalgamation should obey the power law N(h) h-B, where N(h) is the number of layers of thickness greater than h and B 1. We support this proposal with two sets of observations, one from formation-microscanner images obtained in offshore wells that penetrate Tertiary fore-arc turbidites (Hiscott et al. 1992) and the other from our own field measurements of turbidites in the Neoproterozoic Kingston Peak Formation, deposited in a glacially influenced rift basin. Both sets of observations show roughly the same power-law distribution above a small-h cutoff. otivated by the possible generality of these results, and given strong geological and sedimentological contrasts between the two data sets, we consider the available theoretical and experimental evidence that could support or deny these observations. We tentatively conclude that the power law is generic in data sets characterized by minimal erosional truncation and amalgamation but emphasize that further study is required for a definitive statement. Proceeding from the assumption that the scaling law is valid for arbitrarily thin layers, we derive an upper bound for B. We then detail simple and plausible assumptions that provide a theoretical estimate of B. We discuss possible ramifications of this analysis for the interpretation of further data.

[1] Samuel Bowring and Hans Keppler each received the 2010 N. L. Bowen Award at the 2010 AGU Fall Meeting, held 13–17 December in San Francisco, Calif. The award recognizes “outstanding contributions to volcanology, geochemistry, or petrology.”

An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian rocks located in northeast Siberia indicate that the Cambrian period began at approximately 544 million years ago and that its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both the faunal diversification and subsequent Cambrian turnover.

Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numerical models are used to evaluate the effects of several processes influencing ooid size. The first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth. Several Neoproterozoic oolites contain unusually large ooids, some reaching diameters of up to 16 mm. While lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher growth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during Neoproterozoic time.

The Burnside Formation records the shift from marine-shelf to largely alluvial conditions in the Paleoproterozoic ( 1.9 Ga) Kilohigok foreland basin in the northwest Canadian Shield. The Burnside is a thick (up to 3.5 km), northwest-tapering siliciclastic wedge representing a sandy braid-delta and braided-river system that prograded northwest across the Archean Slave craton. Criteria are developed to distinguish shallow-marine facies from alluvial facies allowing construction of a stratigraphic framework for this unfossiliferous, alluvially dominated succession. The sandy alluvial system prograded over a storm-influenced fine-grained siliciclastic shelf. Shelf associations can be divided into siltstone-dominated and mixed siltstone-sandstone. The siltstone-dominated association comprises facies ranging from mudstones deposited below storm-wave influence to siltstones deposited under storm and fair-weather wave conditions. Mixed siltstone-sandstone facies form coarsening-upward sequences on the scale of several meters to a few tens of meters that appear to represent prograding marine sand bars. Initial alluvial influence is manifested by associations of mixed siltstone and sandstone with shallow-water wave-formed and current-formed structures, abundant soft-sediment deformation, and isolated meter-size channels filled with trough-cross-bedded andstone and associated rip-up clasts. Thick (tens to hundreds of meters) tabular-bedded fine sandstones represent a delta-platform environment between coarser-grained trough-cross-bedded braid-delta distributary channels. Unidirectional paleocurrents in the transitional alluvial facies indicate predominantly west to southwest transport. The bulk of the Burnside comprises several associations representing sandy alluvial braid-plain facies. Conglomeratic lithologies are subordinate to sandy ones and are restricted largely to the proximal (southeastern) part of the basin. Braid-plain facies, occupying hundreds of meters of section, consist mainly of hierarchies of trough cross-bedding on scales of a few meters to many tens of meters. Unidirectional paleocurrents from the main alluvial associations cluster with a single narrow west-northwest mode. Associations of sedimentary facies and paleocurrents indicate that initially the alluvial system was restricted to the proximal part of the basin and drained longitudinally between the orogenic hinterland and a synsedimentary flexural arch created by thrust loading. Contemporaneous shallow-marine-shelf facies were present to the northwest over and beyond the flexural arch. An abrupt shift to transverse drainage and the superposition of medial-braid-plain facies directly on marine-shelf and deltaic facies in the foreland mark an abrupt, unconformable contact representing the progradation of the alluvial system over the cratonic foreland. The area of maximum disparity between marine and nonmarine facies coincides with the area of greatest stratigraphic thinning and erosion, and marks th crest of the flexural arch. Only the transverse paleocurrent mode is recorded over the flexural arch and in the distal foreland. Stratigraphic relations suggest that marginal-marine and braid-delta facies tracts in the distal foreland shifted abruptly over large distances and were sensitive to changes in relative sea level. In contrast, braid-plain facies tracts shifted very little, and more gradually. The large lateral extent of alluvial facies suggests that the source area had a humid or monsoon climate.

Introduction Modern microbial mats are structurally coherent macroscopic accumulations of microorganisms. Mats are widely distributed on earth. They are found in a surprisingly large number of diverse environments from the equatorial zones to both polar regions. They vary in size from extensive terrestrial and hypersaline mats that cover areas several square kilometers in extent to minute mats only a few square centimeters in area found in small thermal springs. They vary in thickness from massive accumulations measured in meters, such as those in the Persian Gulf and the Red Sea region, to thin films less than a few millimeters in thickness. In addition to being highly varied in size, modern microbial mats are also very diverse in morphology, community structure, and physiological characteristics. What do such mats have in common? Under what conditions do they form? What is the basis of their diversity? What insight do they provide, if any, to the interpretation of the widespread stromatolites of the Proterozoic? A Terminology Microbial mats are accretionary cohesive microbial communities which are often laminated and found growing at the sediment-water (occasionally sediment-air) interface. Most mats stabilize unconsolidated sediment. The mats are comprised of the various microorganisms that accumulate along with their metabolic products. The most conspicuous of these products is usually a copious amount of extracellular polysaccharide which helps hold the cells together to form a cohesive structure.

High spatial and spectral resolution reflectance data acquired by the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument reveal the presence of H_2O- and SiOH-bearing phases on the Martian surface. The spectra are most consistent with opaline silica and glass altered to various degrees, confirming predictions based on geochernicall experiments and models that amorphous silica should be a common weathering product of the basaltic Martian crust. These materials are associated with hydrated Fe sulfates, including H_3O-bearing jarosite, and are found in finely stratified deposits exposed on the floor of and on the plains surrounding the Valles Marineris canyon system. Stratigraphic relationships place the formation age of these deposits in the late Hesperian or possibly the Amazonian, implying that aqueous alteration continued to be an important and regionally extensive process on Mars during that time.

High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, resolve impact megabreccia unconformably overlain by sediments deposited during two Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying darker-toned and often blocky upper unit drapes the sequence and was emplaced during later high-magnitude flooding as an impounded Uzboi Vallis lake overtopped the crater rim. The stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early Mars.

Replying to: K. Grey & C. R. Calver , doi: 10.1038/nature06360 (2007). Calver and Grey1 point out the difficulties of relating the Australian acritarch record to the global record of environmental change. This results from the lack of documented stratigraphic sections that have overlapping chronologies of both acritarch evolution and global chemostratigraphic proxies, such as δ13Ccarb, and we agree that coupled records are required to confirm our hypothesis2. Until such records exist, any correlations between Australian acritarch-bearing strata and strata containing records of global secular variations in δ13Ccarb are necessarily tenuous. This uncertainty, however, does not affect the principal conclusion of our paper, which emphasizes the evidence for sequential chemical oxidation of the Ediacaran ocean. Our inferred correlation between chemical oxidation and biological evolution is supported by subsequent results on strata containing strong geochronologic tie-points and a robust biostratigraphic and chemostratigraphic record3,4.

Abundant and well-preserved molecular biomarkers are prevalent in sediments and oils of Neoproterozoic-Cambrian age from the Huqf Supergroup, South Oman Salt Basin. Amongst the diverse compound classes present, distinctive free (extractable) and kerogen-bound C_(26) and C_(30) steranes (McCaffrey et al., 1994) produced by marine demosponges provide evidence for the first appearance of multicellular animals (metazoa) in the geological record sometime between the Sturtian (711 Myr) and Marinoan (635 Myr) glaciations. The timing of the sponge biomarker appearance corresponds remarkably well to divergence ages for Demospongiae based on the most recent molecular clocks (minimum evolution estimates) using protein sequences ( Peterson and Butterfield, 2005). These demosponge steranes are potentially the first evidence for the appearance of animals in the fossil record since they predate, by at least 50 Myr, the fossil fauna from the Doushantuo Fm. (Li et al., 1998 and Xiao et al., 1998) now considered younger than 580 Myr (Condon et al., 2005). The sterane biomarkers are found in all formations of the Nafun and Ara Groups and constitute an apparently continuous 100 Myr record of demosponges through the Terminal Neoproterozoic and into the Early Cambrian. The application of lipid biomarkers for constraining the divergence ages of basal metazoa circumvents problems associated with uneven preservation of soft-bodied metazoa and their poor capacity for leaving trace or body fossils.

High-resolution paired carbon (δ^(13)C_(carb) and δ^(13)C_(org)) and sulfur (δ^(34)S_(SO4) and δ^(34)S_(pyrite)) isotope records of the Ediacaran strata of the Huqf Supergroup, Sultanate of Oman record a three-stage oxidation of the Ediacaran ocean. Following the Marinoan glaciation, stage I oxidation lasts from ∼635 to 610 million years ago (Myr) and records an increase in marine sulfate concentrations above ∼200 μM, likely due to a rise in atmospheric oxygen. Stage II oxidation, from ∼575 to 550 Myr, coincides with the Shuram excursion, ɑ > 13‰ negative excursion in δ^(13)C_(carb)Burns and Matter, 1993 created by the oxidation of a large reservoir of deep ocean dissolved organic carbon (DOC). The onset of stage II oxidation coincides with the evolution of acanthomorph acritarchs in Australia and China Grey, 2005, and the appearance of the first Ediacaran metazoa, Charnia-type segmented fronds Narbonne, 2005, while the first motile macroscopic metazoa (e.g., Kimberella) appear at ∼555 Myr Narbonne, 2005 as the Shuram excursion draws to a close. Stage III oxidation, from ∼550 to 547 Myr, is marked by presence of sulfur disproportionating metabolisms and the onset of co-variation in δ^(13)C_(carb) and δ^(13)C_(org), which is absent in older strata. Coincident with the onset of stage III oxidation, we see the evolution of macroscopic multicellular algae Xiao, 2004, an increase in the diversity of the acanthomorph acritarchs Burns and Matter, 1993, and the appearance of the weakly calcifying metazoa Cloudina and Namacalathus Narbonne, 2005. Following stage III oxidation, there is a period of quiescence characterized by little perturbation in the carbon cycle and an absence of significant evolutionary events. At ∼542 Myr, anoxia at the Ediacaran–Cambrian boundary causes the extinction of many Ediacaran organisms, setting the stage for the Cambrian radiation.

New observations at Erebus crater (Olympia outcrop) by the Mars Exploration Rover Opportunity between sols 671 and 735 (a sol is a martian day) indicate that a diverse suite of primary and penecontemporaneous sedimentary structures is preserved in sulfate-rich bedrock. Centimeter-scale trough (festoon) cross-lamination is abundant, and is better expressed and thicker than previously described examples. Postdepositional shrinkage cracks in the same outcrop are interpreted to have formed in response to desiccation. Considered collectively, this suite of sedimentary structures provides strong support for the involvement of liquid water during accumulation of sedimentary rocks at Meridiani Planum.

Integrated sedimentologic, stratigraphic, and geochemical (δ 13 C, δ 18 O, Fe, Mn, and Sr) data from Vendian-Cambrian carbonate-ramp deposits of the northeast Siberian platform (Olenek uplift and Kharaulakh Mountains region) are used to constrain primary, time-dependent oscillations in the carbon isotope record and to evaluate δ 13 C chemostratigraphy for high-resolution intrabasinal correlation. The Vendian δ 13 C record of northeast Siberia reflects global variations seen elsewhere by displaying (in ascending order) a strong positive isotopic shift to values near +6‰ (herein named the P-interval), an intermediate interval of relatively little isotopic change (I-interval), which shows, in some sections, a monotonic decrease in δ13C from +2‰ at the base to near 0‰ at the top, and a negative excursion to ≈−4‰ (N-interval) just beneath the Vendian-Cambrian boundary. In addition to secular isotopic shifts, these strata exhibit local small-scale signals related to intrabasinal variations in subsidence, erosion, and diagenetic alteration (e.g., degradation of organic matter, dolomitization, and hydrothermal and burial effects). These local, intrabasinal processes, in some cases, have modified both the magnitude and form of the primary isotopic excursions, which are used as the basis for correlation in all studies. Carbon isotope profiles provide an important method to evaluate intrabasinal variations in subsidence, erosion, and stratigraphic completeness. These profiles reflect increasing subsidence along the platform-to-basin transition, whereas progressive truncation of isotopic profiles along this trend illustrate pronounced uplift and erosion of ramp strata along unconformity surfaces.

Late Permian reefs of the Capitan complex, west Texas; the Magnesian Limestone, England; Chuenmuping reef, south China; and elsewhere contain anomalously large volumes of aragonite and calcite marine cements and seafloor crusts, as well as abundant microbial precipitates. These components strongly influenced reef growth and may have been responsible for the construction of rigid, open reefal frames in which bryozoans and sponges became encrusted and structurally reinforced. In some cases, such as the upper biostrome of the Magnesian Limestone, precipitated microbialites and inorganic crusts were the primary constituents of the reef core. These microbial and inorganic reefs do not have modern marine counterparts; on the contrary, their textures and genesis are best understood through comparison with the older rock record, particularly that of the early Precambrian. Early Precambrian reefal facies are interpreted to have formed in a stratified ocean with anoxic deep waters enriched in carbonate alkalinity. Upwelling mixed deep and surface waters, resulting in massive seafloor precipitation of aragonite and calcite. During Mesoproterozoic and early Neoproterozoic time, the ocean became more fully oxidized, and seafloor carbonate precipitation was significantly reduced. However, during the late Neoproterozoic, sizeable volumes of deep ocean water once again became anoxic for protracted intervals; the distinctive "cap carbonates" found above Neoproterozoic tillites attest to renewed upwelling of anoxic bottom water enriched in carbonate alkalinity and ^(12)C. Anomalous late Permian seafloor precipitates are interpreted as the product, at least in part, of similar processes. Massive carbonate precipitation was favored by: 1) reduced shelf space for carbonate precipitation, 2) increased flux of Ca to the oceans during increased continental erosion, 3) deep basinal anoxia that generated upwelling waters with elevated alkalinities, and 4) further evolution of ocean water in the restricted Delaware, Zechstein, and other basins. Temporal coincidence of these processes resulted in surface seawater that was greatly supersaturated by Phanerozoic standards and whose only precedents occurred in Precambrian oceans.

Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary. Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust&-Yudoma Formation in southeastern Siberia. In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionarily important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.