Juvenile Chemical Sediments and the Long Term Persistence of Water at the Surface of Mars

Chemical sediments and the aqueous alteration products of volcanic rocks clearly indicate the presence of water, at least episodically, at the Martian surface. Compared to similar materials formed on the early Earth, however, Martian deposits are juvenile, or diagenetically under-developed. Here we examine the role of water in facilitating various diagenetic reactions and evaluate the predicted effects of time and temperature for aqueous diagenesis on Mars. Using kinetic formulations based on terrestrial sedimentary geology, we quantify the integrated effects of time and temperature for a range of possible burial and thermal histories of precipitated minerals on Mars. From this, we estimate thresholds beyond which these precipitates should have been converted to the point of non-detection in the presence of water. Surface water has been shown to be at least episodically present in recent times. Nonetheless, the integrated duration of aqueous activity recorded over geologically long intervals by hydrated amorphous silica, smectite clays and Fe-sulfate minerals suggests that where these minerals occur water did not persist much beyond their initial deposition. This geochemical conclusion converges with geomorphologic studies that suggest water limitation during the late Noachian–Hesperian peak of valley formation and a still more limited footprint of water since that time. In addition to documenting the presence of water and its chemical properties, a complete assessment of potentially habitable environments on Mars should address the timescales on which liquid water has persisted and the timing of aqueous episodes relative to major planetary events.Organismic and Evolutionary Biolog

Physicochemical properties of concentrated Martian surface waters

Understanding the processes controlling chemical sedimentation is an important step in deciphering paleoclimatic conditions from the rock records preserved on both Earth and Mars. Clear evidence for subaqueous sedimentation at Meridiani Planum, widespread saline mineral deposits in the Valles Marineris region, and the possible role of saline waters in forming recent geomorphologic features all underscore the need to understand the physical properties of highly concentrated solutions on Mars in addition to, and as a function of, their distinct chemistry. Using thermodynamic models predicting saline mineral solubility, we generate likely brine compositions ranging from bicarbonate-dominated to sulfate-dominated and predict their saline mineralogy. For each brine composition, we then estimate a number of thermal, transport, and colligative properties using established models that have been developed for highly concentrated multicomponent electrolyte solutions. The available experimental data and theoretical models that allow estimation of these physicochemical properties encompass, for the most part, much of the anticipated variation in chemistry for likely Martian brines. These estimates allow significant progress in building a detailed analysis of physical sedimentation at the ancient Martian surface and allow more accurate predictions of thermal behavior and the diffusive transport of matter through chemically distinct solutions under comparatively nonstandard conditions

Small Shelly Fossil Preservation and the Role of Early Diagenetic Redox in the Early Triassic

Minute fossils from a variety of different metazoan clades, collectively referred to as small shelly fossils, represent a distinctive taphonomic mode that is most commonly reported from the Cambrian Period. Lower Triassic successions of the western United States, deposited in the aftermath of the end-Permian mass extinction, provide an example of small shelly style preservation that significantly post-dates Cambrian occurrences. Glauconitized and phosphatized echinoderms and gastropods are preserved in the insoluble residues of carbonates from the Virgin Limestone Member of the Moenkopi Formation. Echinoderm plates, spines and other skeletal elements are preserved as stereomic molds; gastropods are preserved as steinkerns. All small shelly style fossils are preserved in the small size fractions of the residues (177 to 420 lm), which is consistent with the size selection of small shelly fossils in the Cambrian. Energy-dispersive X-ray spectra of individual fossils coupled with X-ray diffraction of residues confirm that the fossils are dominantly preserved by apatite and glauconite, and sometimes a combination of the two minerals. The nucleation of both of these minerals requires that pore water redox oscillated between oxic and anoxic conditions, which, in turn, implies that Lower Triassic carbonates periodically experienced oxygen depletion after deposition and during early diagenesis. Long-term oxygen depletion persisted through the Early Triassic, creating diagenetic conditions that were instrumental in the preservation of small shelly fossils in Triassic and, likely, Paleozoic examples

Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7?lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was?16.6 $\pm$ 1.7?at 20?C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are?21.5$\pm$ 1.1?,?0.2 $\pm$ 1.9?and 15.0 $\pm$ 12.3?for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes

Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7‰lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was�16.6 ± 1.7‰at 20�C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are�21.5± 1.1‰,�0.2 ± 1.9‰and 15.0 ± 12.3‰for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes

Microstructures in Metasedimentary Rocks from the Neoproterozoic Bonahaven Formation, Scotland: Microconcretions, Impact Spherules, or Microfossils?

Microscopic spherules in relatively undeformed mudstones of the Neoproterozoic Bonahaven Formation, Islay, Scotland, are differentiated from their matrix by a sharp micron-scale, smoothly rounded boundary. These elongate spherules were earlier interpreted as hollow bodies filled penecontemporaneously by glauconite and subsequently metamorphosed to phengite, but their origin remains a matter of debate. Spherules observed in thin section are predominantly rounded (∼74%) but can exhibit a flat edge or protrusion at one end. In 11% of a sample population, two or more spherules are conjoined. X-ray diffraction indicates that spherule-bearing mudstones consist mainly of muscovite, with variable amounts of kaolin-group minerals and minor iron-chlorites. A range of physical origins for the spherules – including microconcretions or metamorphic microstructures; deposition from the sky as micrometeorites, microtektites/microkrystites, or accretionary volcanic ash particles; and detrital grains – is considered but rejected on distributional, morphological, and mineralogical evidence. Biological origins are considered most likely, especially protistan tests similar to the vase-shaped microfossils found in somewhat older Neoproterozoic rocks. If correct, this provides the first report of eukaryotic life in the Dalradian succession that passes critical tests for biogenicity and new evidence for testate microfossils in post-Sturtian but pre-Marinoan aged rocks.Earth and Planetary SciencesOrganismic and Evolutionary Biolog

The Sedimentary Cycle on Early Mars

Two decades of intensive research have demonstrated that early Mars (2 Gyr) had an active sedimentary cycle, including well-preserved stratigraphic records, understandable within a source-to-sink framework with remarkable fidelity. This early cycle exhibits first-order similarities to (e.g., facies relationships, groundwater diagenesis, recycling) and first-order differences from (e.g., greater aeolian versus subaqueous processes, basaltic versus granitic provenance, absence of plate tectonics) Earth's record. Mars’ sedimentary record preserves evidence for progressive desiccation and oxidation of the surface over time, but simple models for the nature and evolution of paleoenvironments (e.g., acid Mars, early warm and wet versus late cold and dry) have given way to the view that, similar to Earth, different climate regimes on Mars coexisted on regional scales and evolved on variable timescales, and redox chemistry played a pivotal role. A major accomplishment of Mars exploration has been to demonstrate that surface and subsurface sedimentary environments were both habitable and capable of preserving any biological record

Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

he identification of sediment sources to the ocean is a prerequisite to using marine sediment cores to extract information on past climate and ocean circulation. Sr and Nd isotopes are classical tools with which to trace source provenance. Despite considerable interest in the Arctic Ocean, the circum-Arctic source regions are poorly characterised in terms of their Sr and Nd isotopic compositions. In this study we present Sr and Nd isotope data from the Paleogene Central Basin sediments of Svalbard, including the first published data of stream suspended sediments from Svalbard. The stream suspended sediments exhibit considerable isotopic variation (εNd = −20.6 to −13.4; 87Sr ∕ 86Sr = 0.73421 to 0.74704) which can be related to the depositional history of the sedimentary formations from which they are derived. In combination with analysis of the clay mineralogy of catchment rocks and sediments, we suggest that the Central Basin sedimentary rocks were derived from two sources. One source is Proterozoic sediments derived from Greenlandic basement rocks which are rich in illite and have high 87Sr ∕ 86Sr and low εNd values. The second source is Carboniferous to Jurassic sediments derived from Siberian basalts which are rich in smectite and have low 87Sr ∕ 86Sr and high εNd values. Due to a change in depositional conditions throughout the Paleogene (from deep sea to continental) the relative proportions of these two sources vary in the Central Basin formations. The modern stream suspended sediment isotopic composition is then controlled by modern processes, in particular glaciation, which determines the present-day exposure of the formations and therefore the relative contribution of each formation to the stream suspended sediment load. This study demonstrates that the Nd isotopic composition of stream suspended sediments exhibits seasonal variation, which likely mirrors longer-term hydrological changes, with implications for source provenance studies based on fixed end-members through time

Sedimentary talc in Neoproterozoic carbonate successions

Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800–700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO4 synthetic seawater solutions at varying pH, Mg2+ and SiO2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg2+ and/or SiO2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg2+, elevated SiO2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.Organismic and Evolutionary Biolog

Fossilisation processes and our reading of animal antiquity

Estimates for animal antiquity exhibit a significant disconnect between those from molecular clocks, which indicate crown animals evolved ∼800 million years ago (Ma), and those from the fossil record, which extends only ∼574 Ma. Taphonomy is often held culpable: early animals were too small/soft/fragile to fossilise, or the circumstances that preserve them were uncommon in the early Neoproterozoic. We assess this idea by comparing Neoproterozoic fossilisation processes with those of the Cambrian and its abundant animal fossils. Cambrian Burgess Shale-type (BST) preservation captures animals in mudstones showing a narrow range of mineralogies; yet, fossiliferous Neoproterozoic mudstones rarely share the same mineralogy. Animal fossils are absent where BST preservation occurs in deposits ≥789 Ma, suggesting a soft maximum constraint on animal antiquity