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Gale crater on Mars was once a lake fed by rivers and groundwater. Hurowitz et al . analyzed 3.5 years of data from the Curiosity rover’s exploration of Gale crater to determine the chemical conditions in the ancient lake. Close to the surface, there were plenty of oxidizing agents and rocks formed from large, dense grains, whereas the deeper layers had more reducing agents and were formed from finer material. This redox stratification led to very different environments in different layers, which provides evidence for Martian climate change. The results will aid our understanding of where and when Mars was once habitable. Science , this issue p. eaah6849 Gale crater on Mars was once a lake that separated into layers with differing chemical conditions. In 2012, NASA’s Curiosity rover landed on Mars to assess its potential as a habitat for past life and investigate the paleoclimate record preserved by sedimentary rocks inside the ~150-kilometer-diameter Gale impact crater. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment fed by rivers draining into the crater. We synthesize geochemical and mineralogical data from lake-bed mudstones collected during the first 1300 martian solar days of rover operations in Gale. We present evidence for lake redox stratification, established by depth-dependent variations in atmospheric oxidant and dissolved-solute concentrations. Paleoclimate proxy data indicate that a transition from colder to warmer climate conditions is preserved in the stratigraphy. Finally, a late phase of geochemical modification by saline fluids is recognized.

Tridymite, a low-pressure, high-temperature (>870 °C) SiO₂ polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO₂ (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO₂ glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides–perchlorates–chlorates), and has minor TiO₂ and Fe₂O₃T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin’s tridymite, cristobalite, feldspar, and any residual high-SiO₂ glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO₂ glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

BackgroundGallstone disease is a complex condition which accounts for the majority of elective surgery requests and poses a considerable morbidity and financial cost to health systems worldwide. Previous studies have linked gallstone formation to a variety of causes which include genetics, medical history, physico-chemical imbalance and most recently, bacterial processes. A definitive model is yet to be made that can account for the composition, morphology and growth mechanisms behind all gallstone disease reports and findings. This is mostly due to the increasingly observed complexity of the disease and a history of incongruent data. The majority of studies have concentrated on multiple small-sized (0.5–1 cm) cholesterol and pigmented stones as they are considered to be the prevailing presentation types. Few studies, however, have investigated large (2–4 cm), solitary or unusual stones in considerable detail. Further, several researchers have begun to explore the possible connection between gallstone lithification and biomineralisation events observed in nature which has been shown to exhibit similar morphologies to gallstones.MethodsIn this study, we explore five large gallstones in greater detail using state-of-the-art scanning electron microscopic techniques to elucidate possible formation mechanisms that may be obscured in smaller stones. The observed stone morphology is compared to known biomineralisation models in nature, and a proposed theoretical model for gallstone formation is provided. We compliment this with compound-specific isotopic analysis of cholesterol in the rings of a particularly large solitary stone to ascertain possible environmental factors that may be at play in stone formation.ResultsThe current study suggests that biliary sludge and stone development is due to bile imbalance in which cholesterol is able to precipitate into crystals, and aggregate via successive biofilm coatings produced by prolific biofilm forming and bile resistant pathogenic bacteria. This formation model provides a framework for future work in this area, initiating a greater focus on the importance of bacterial processes, and how they may influence and manipulate environmental and human systems in similar ways.ConclusionsThis study introduces a new model of gallstone formation informed by detailed investigations into biomineralisation events in nature.

The sedimentary cycle, including the processes of erosion, transport, and lithification, is a key part of how planets evolve over time. Early images of Venus’s vast volcanic plains, numerous volcanoes, and rugged tectonic regions led to the interpretation that Venus is a volcanic planet with little sediment cover and perhaps few processes for generating sedimentary rocks. However, in the years since the Magellan mission in the 1990s we have developed a better understanding of sedimentary process on Venus. Impact craters are the largest present-day source of sediments, with estimates from the current crater population suggesting an average sediment layer 8–63 cm in thickness if distributed globally. There is clear evidence of fine-grained material in volcanic summit regions that is likely produced through volcanism, and dune fields and yardangs indicate transport of sediments and erosion of rocks through wind. Landslides and fine-grained materials in highland tessera regions demonstrate erosive processes that move sediment downhill. It is clear that sediments are an important part of Venus’s geology, and it is especially important to realize that they mantle features that may be of interest to future landed or low-altitude imaging missions. The sinks of sediments are less well known, as it has been difficult to identify sedimentary rocks with current data. Layering observed in Venera images and in Magellan images of some tessera regions, as well as calculated rock densities, suggest that sedimentary rocks are present on Venus. New data is needed to fully understand and quantify the present-day sedimentary cycle and establish with certainty whether sedimentary rock packages do, in fact, exist on Venus. These data sets will need to include higher-resolution optical and radar imaging, experimental and geochemical measurements to determine how chemical weathering and lithification can occur, and topography to better model mesospheric winds. Sediments and sedimentary rocks are critical to understanding how Venus works today, but are also extremely important for determining how Venus’s climate has changed through time and whether it was once a habitable planet.

Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s −1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution. Collisions between primordial planetesimals led to the formation of our asteroids and meteorites. Here, the authors use modelling to explore the compaction of planetsimals, tracking how pressure, temperature and porosity may have varied during the impacts, helping interpret early Solar System processes.

Trypanites is a common boring in Ordovician hardgrounds of Estonia (Baltica). The depth of the sedimentary basin and sedimentation rates controlled the distribution of Trypanites. The trace-makers’ community was diverse and changing over time. Three ichnospecies of Trypanites can be distinguished: T. sozialis, T. weisei and Trypanites isp. All three morphotypes can be recognized in the same hardground. It is impossible to distinguish between the different ichnospecies based only on the size of the boring aperture. The depth of early lithification of the seafloor determines the morphological variability seen in T. sozialis. The occurrence of elongated borings, such as T. weisei and Trypanites isp., is related to tropical environments, and their trace-makers strongly preferred substrates with a homogeneous and dense texture. The texture and available volume of hard substrate controls the ichnodiversity of Trypanites ichnospecies.

Tepee structures are upward‐buckling fracture rims that form a reticulated network of polygonal boundaries when viewed from an aerial perspective. These structures are thought to result from crystallisation forces of cement, causing lateral expansion of consolidating sedimentary materials. Tepee structures are indicative of subaerial exposure and can serve as stratigraphic markers in ancient carbonate sequences. While tepee structures are common in ancient carbonate sequences, in modern settings, they have only been described in the Arabian Gulf. The discovery of fields of polygonal structures, on satellite images of the Red Sea Island of Sheybarah, Saudi Arabia, motivated the authors to assess their genesis and distribution in Arabia. The objectives are to describe in detail tepees and their arrangement in polygonal patterns, investigate the timing of their formation and constrain the relative timing of tepee generation. Furthermore, this study investigates, based on a detailed satellite image survey, whether similar polygonal features can be identified around the Arabian Peninsula. The polygonal crusts on Sheybarah Island are found in intertidal to supratidal settings overlying a well‐lithified ravinement surface. They are composed of locally derived poorly sorted sand‐to‐pebble‐sized coral, mollusc and foraminifera debris of predominantly aragonite and high Mg‐calcite mineralogy with minor admixtures of siliciclastics. Based on petrographic analysis, microbial‐induced cement precipitation is the major lithification agent. Lithification occurs in the upper intertidal zone based on the ubiquitous presence of keystone vugs. Radiocarbon ( 14 C) dates reveal elevation‐aligned ages between approximately 1.5 and 3.0 ka BP, allowing the reconstruction of late Holocene minor sea‐level changes aligned with major global climate variations. Hence, polygonal tepee structures may serve as proxies for sea‐level changes. Of 126 occurrences of polygonal fields identified from satellite images, 89 in the Red Sea are probably composed of polygonal reefs. The authors hypothesise that polygonal coral reefs originated on tepee crust.

Three nano-resolution petrological microtextures were discovered in the siliceous shale at the bottom of the Longmaxi Formation in the Zigong area, Sichuan Basin. Based on observations of the occurrences of the minerals, organic matter, and organic matter pores in the different microtextures and analysis of their relationships by means of nano-resolution petrological image datasets obtained using the Modular Automated Processing System (MAPS 3.18), the formation mechanism of the siliceous shale was studied. The results show that the strong modification of clay-rich sediments by a deep-water traction current was the basis for the formation of the siliceous shale. The clay-rich sediments were converted into flocculent sediments rich in oxygen and nutrients via agitation and transport by the deep-water traction current, providing space and a material basis for microbes to flourish. Under the continuous activity of the deep-water traction current, the clay-rich sediments were transformed into microbial mats, in which in situ terrigenous detrital quartz and feldspar, endogenous detrital calcite, authigenic dolomite, and dolomite ringed by ferrodolomite were scattered. During the burial stage, the microbial mats were lithified into the siliceous shale composed of three petrological microtextures. Microtexture I was mainly transformed by microbes. Microtexture II was formed via lithification of the residual clay-rich sediments. Microtexture III was composed of migratory organic matter filling hydrocarbon-generating pressurized fractures. Due to the universality of deep-water traction flow and the diversity of microbes in deep-water sediments, we firmly believe that more and more deep-water microbialites will be discovered worldwide through systematic characterization of nano-resolution petrology with the booming development of the shale gas industry.

Lida Ajer and Ngalau Gupin are karstic caves situated in the Padang Highlands, western Sumatra, Indonesia. Lida Ajer is best known for yielding fossil evidence that places the arrival of Homo sapiens in Southeast Asia during Marine Isotope Stage 4, one of the earliest records for the region. Ngalau Gupin recently produced the first record of hippopotamid Hexaprotodon on the island, representing the only globally extinct taxon in Pleistocene deposits from Sumatra. Microstratigraphic (micromorphological) analyses were applied to unconsolidated fossil-bearing cave sediments from these two sites. We use micromorphology as part of a micro-contextualised taphonomic approach to identify the diagenetic processes affecting fossils and sediments within these caves, through phases of their depositional history. The fossil-bearing sediments in Lida Ajer have been subjected to a suite of natural sedimentation processes ranging from water action to carnivore occupation, which would indicate the fossils underwent significant reworking prior to lithification of the deposit. The results demonstrate that the base of the unconsolidated fossil-bearing sediments in Ngalau Gupin were derived from the interior of the cave, where the matrix was partially phosphatized as a result of guano-driven diagenesis. These observations can be used to test hypotheses about the integrity of incorporated vertebrate remains and to aid in local palaeoenvironmental reconstructions. The methods employed in this research have not previously been applied to cave sediments from sites in the Padang Highlands and provide key new insights into the palaeontological and natural history of the western region of Sumatra.

Cherts are used to reconstruct the evolution of seawater δ18O and temperature over geological time. However, given the influence of marine diagenesis, reconstructing seawater from the isotope composition of cherts is not straightforward, resulting in ambiguity of interpretation. Here, we present a detailed isotope and petrographic investigation of deep‐sea drilled 135–40 Ma cherts with focus on the effects of marine diagenesis. We combined triple O‐isotope data with in‐situ δ′18O‐16OH/16O measurements using secondary ion mass spectrometry (SIMS). We also provide electron microprobe maps, traditional δ′18O measurements from petrographically diverse domains, and δD and H2O wt.% values. The bulk δ′18O values range between 29‰ and 38‰ in our collection, while SIMS δ′18O data reveal significant intra‐sample heterogeneities up to 6‰ related to distinct petrographic features (e.g., filled radiolarian tests) and to micrometer‐scale variations in silica forms. Further, the δ′18O—Δ′17O values of these seafloor‐drilled cherts plot near and under equilibrium curve. Both triple‐O and SIMS δ′18O results reflect diagenesis in presence of marine pore waters at temperatures higher than ambient seawater, which is especially appreciable in cherts deposited on young oceanic crust. Despite the relatively constant δ18O seawater values over last 135 Ma, the marine silica spanning between 0 and 135 Ma occupies a wide compositional space in the δ′18O—Δ′17O rather than an equilibrium curve. The δ′18O values of cherts from modern‐seafloor positively correlate with the oceanic crustal age at the time of deposition, hinting at the importance of the heat flux in the diagenetic recrystallization of marine silica. Plain Language Summary Chemical sediments extracted from seafloor represent an archive of ocean temperatures and solutes. Chemical resilience of siliceous sediments with composition of >90 wt.% SiO2 presents a promising avenue to constrain temperature of ancient oceans from distant geological epochs. However, such rocks termed cherts undergo complex recrystallization during compaction and lithification below the seafloor, preventing direct measurements of ocean temperatures. To isolate the effect of diagenesis, we carefully investigated several Mesozoic and Cenozoic seafloor‐drilled cherts. We used oxygen isotope ratios as a common proxy for temperature to test for the effects of recrystallization of these cherts that were buried at depths between 80 and 960 m. We combined two state‐of‐the‐art types of isotope measurements: (a) non‐destructive in‐situ measurements from microscopic domains and (b) bulk high‐precision triple O‐isotope ratios. We observe complex mineralogical and isotope compositions on the scale of 10–100 μm. The triple O‐isotope ratios depict the combined effects of original precipitation from seawater column and the thermal regime of recrystallization in subseafloor conditions. The compiled O‐isotope ratios of silica extracted from seafloor correlates with the age of oceanic crust. Since the age of oceanic crust and the heat flux are related, these observations hint at the importance of the sediment geothermal evolution during diagenesis. Key Points Mesozoic and Cenozoic seafloor‐drilled cherts were measured for triple O‐isotope and in situ secondary ion mass spectrometry δ′18O‐16OH/16O values In situ measurements display up to 6‰ range in the δ′18O values of seafloor‐drilled cherts The δ′18O‐Δ′17O values of cherts plot near and under the silica‐seawater equilibrium curve (20°C–40°C). Together with the in‐situ variability, these data are best explained by a combination of original precipitation and recrystallization in sub‐seafloor conditions