Controls on Cyclic Formation of Quaternary Early Diagenetic Dolomite

The origin of sedimentary dolomite and the factors that control its formation within the geological record remain speculative. In most models, dolomite formation is linked to evaporative conditions, high water temperature, increasing Mg/Ca ratio, increasing alkalinity, and high amounts of biomass. Here we challenge these archetypal views, by documenting a case example of Quaternary dolomite which formed in Lake Van at constantly low temperature (<4°C) and without direct control of the latter conditions. Dolomite occurs within highstand sediments related to suborbital climate variability (Dansgaard‐Oeschger cycles). We propose that dolomite precipitation is a product of a microbially influenced process, triggered by ecological stress, resulting from reventilation of the water‐sediment interface. Independently from the validity of this hypothesis, our results call for a reevaluation of the paleoenvironmental conditions often invoked for early diagenetic dolomite‐rich intervals within sedimentary sequences and for caution when interpreting time series of subrecent lacustrine carbonates

Similarities and differences in the dolomitization history of two coeval Middle Triassic carbonate platforms, Balaton Highland, Hungary

Dolomitization of platform carbonates is commonly the result of multiphase processes. Documentation of the complex dolomitization history is difficult if completely dolomitized sections are studied. Two Middle Anisian sections representing two coeval carbonate platforms were investigated and compared in the present study. Both sections are made up of meter-scale peritidal–lagoonal cycles with significant pedogenic overprint. One of the sections contains non-dolomitized, partially dolomitized, and completely dolomitized intervals, whereas the other is completely dolomitized. Based on investigations of the partially dolomitized section, penecontemporaneous dolomite formation and/or very early post-depositional dolomitization were identified in various lithofacies types. In shallow subtidal facies, porphyrotopic dolomite was found preferentially in microbial micritic fabrics. Microbially induced dolomite precipitation and/or progressive replacement of carbonate sediments could be interpreted for stromatolites. Cryptocrystalline to very finely crystalline dolomite, probably of pedogenic origin, was encountered in paleosoil horizons. Fabric-destructive dolomite commonly found below these horizons was likely formed via reflux of evaporated seawater. As a result of the different paleogeographic settings of the two platforms, their shallow-burial conditions were significantly different. One of the studied sections was located at the basinward platform margin where pervasive fabric-retentive dolomitization took place in a shallow-burial setting, probably via thermal convection. In contrast, in the area of the other, smaller platform shallow-water carbonates were covered by basinal deposits, preventing fluid circulation and accordingly pervasive shallow-burial dolomitization. In the intermediate to deep burial zone, recrystallization of partially dolomitized limestone and occlusion of newly opened fractures and pores by coarsely crystalline dolomite took place

Re-Assessing the Need for Apatite- and Dolomite-Specific Calibrations of the Carbonate Clumped Isotope Thermometer

International audienceA few key methodological uncertainties remain for the carbonate clumped isotope community. One is how to compare data among published data sets that are not anchored to the InterCarb Carbon Dioxide Equilibrium Scale (I-CDES). A second is how temperature calibrations of calcite compare to those of other carbonate minerals in the I-CDES—particularly dolomite and apatite—which can elucidate several Earth system dynamics. Previous calibrations of the clumped isotope thermometer for dolomite are discrepant from one another and variably (dis)agree with calibrations developed for calcite; apatite calibrations have not yet been compared between laboratories using carbonate-based standardization. Here we report I-CDES standardized values for a suite of 11 carbonates that are commonly measured by the clumped isotope community to aid future comparisons of non-I-CDES data sets. In addition, 17 dolomite samples (25-1,200°C) and five apatite samples (1-38°C) of known precipitation temperature were measured using carbonate-based standardization. Excellent agreement between calcites and dolomites heated to similar temperatures (1,100-1,200°C) suggests no mineral-specific differences in absolute acid fractionation factor. We show that calcite and dolomite regressions largely agree but are sensitive to sample characteristics, regression method, and how equations are statistically compared. We suggest that there is no need for a dolomite-specific clumped isotope calibration, although our results suggest that further work is necessary to determine the influence of sample characteristics on this relationship. The apatite calibration equation defined in this study is statistically indistinguishable from calcite-based calibrations; we corroborate previous findings that an apatite-specific calibration is unnecessary

Synthetic topography from the decameter to the centimeter scale on Mars for scientific and rover operations of the ESA-Roscosmos ExoMars mission

The ESA-Roscosmos ExoMars platform and rover mission will have complex interactions with the martian surface. In order to plan and perform landing, roving and scientific operations, the morphological characteristics of the terrain surrounding the rover need to be characterized from the decameter to the centimeter spatial scale. The smallest possible features currently identifiable are typically >0.75 m in size, corresponding to three times the ground sampling resolution of the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO). We have developed a synthetic topography with a ground sampling resolution of 0.01 m by integrating 1) a modeled topography of small-scale topographic features (e.g., rocks) into 2) a 0.25 m resolution digital elevation model (DTM) built by applying stereo and shape-from-shading techniques to HiRISE data. The modeled topography of small-scale topographic features is based on the extrapolation of the abundance and spatial distribution pattern of geological features measurable in HiRISE data. We determined that the cumulative fractional area covered by relief, i.e., float blocks and ridged outcrops, over the entire Oxia Planum landing site spans the range 0-30% with a mean abundance of relief of 7 +/- 5%. The synthetic topography can be used for highly realistic environment simulations and rover drive planning. Artificial rover camera images produced with the synthetic topography incorporate orbital-based geomorphological units and can be used for improved planning and analyses of actual rover images

Speleothems in a north Cuban cave register sea-level changes and Pleistocene uplift rates

A flight of marine terraces along the Cuban coast records Quaternary sea‐level highstands and a general slowly uplifting trend during the Pleistocene. U/Th dating of these limestone terraces is difficult because fossil reef corals have been affected by open system conditions. Terrace ages are thus often based on geological and geomorphological observations. In contrast, the minimum age of the terraces can be constrained by dating speleothems from coastal mixing (flank margin) caves formed during past sea‐level highstands and carving the marine limestones. Speleothems in Santa Catalina Cave have ages >360 ka and show various cycles of subaerial–subaqueous corrosion and speleothem growth. This suggests that the cave was carved during the MIS 11 sea‐level highstand or earlier. Some stalagmites grew during MIS 11 through MIS 8 and were submerged twice, once at the end of MIS 11 and then during MIS 9. Phreatic overgrowths (POS) covering the speleothems suggest anchialine conditions in the cave during MIS 5e. Their altitude at 16 m above present sea level indicates a late Pleistocene uplift rate of <0.1 mm/ka, but modelling also shows uplift to have been insignificant over a long timespan during the middle Pleistocene since the cave was carved. Our study shows that some flank margin caves in the region of Matanzas are older than commonly believed (i.e. MIS 11 rather than MIS 5). These caves not only can be preserved but are good markers of interglacial sea‐level highstands, more reliable than marine abrasion surfaces

Coastal uplift rate at Matanzas (Cuba) inferred from MIS5e phreatic overgrowths on speleothems

Many morphological elements in Cuba's landscape (e.g. marine terraces, tidal notches) demonstrate that coastal uplift has taken place, but the rate at which this occurs is not known. Carbonate phreatic overgrowths on speleothems have been found in a cave in Central North Cuba, ~1\ua0km from the present coastline at 16\ua0m asl. They form exceptional and unique mushroom-shaped speleothems and balconies decorating the walls of the rooms. These phreatic overgrowths on speleothems (POS) formed at the oscillating air\u2013water interface in sea-level controlled anchialine lakes. U/Th dating of these overgrowths suggests ages that are compatible with the Marine Isotope Stage 5e (i.e. 130\u2013115\ua0ka). These POS have fixed this sea-level highstand and demonstrate that this part of Cuba has been subjected to a much lower uplift rate than previously reported, that is, less than 0.1\ua0mm/year since the last interglacial

Prevalence of skin lesions in familial adenomatous polyposis : a marker for presymptomatic diagnosis?

BACKGROUND AND AIMS: Benign skin tumors such as lipomas, fibromas, and epidermal cysts are among the extracolonic manifestations of familial adenomatous polyposis (FAP). Readily detectable by inspection, they could serve as presymptomatic diagnostic markers to identify FAP patients. We therefore prospectively determined the prevalence of cutaneous lesions in genetically confirmed adenomatous polyposis coli (APC) mutation carriers and assessed their potential usefulness in the identification of FAP patients. METHODS: Whole-skin examination was performed in 56 adult APC mutation carriers, compared with a control group (n = 116). In addition, FAP patients were investigated for the presence of congenital hypertrophy of the retinal pigment epithelium (CHRPE), an established clinical marker for FAP, and a detailed review of medical records was performed. RESULTS: Nearly half of all FAP patients (48.2%) had at least one FAP-associated skin lesion, compared with one third (34.5%) of controls. Only multiple lipomas and combined skin lesions were significantly more prevalent in APC mutation carriers. CHRPE was observed in 22 (43.1%) of 51 FAP patients, including 14 (37.8%) of 37 individuals with APC mutations outside the CHRPE-associated region between codons 311 and 1465. CONCLUSIONS: Despite a significantly higher prevalence of multiple lipomas, occurring at younger age, and combined skin lesions in APC mutation carriers, the low diagnostic sensitivity of FAP-associated skin lesions precludes their use as markers for FAP in clinical practice. Based on our findings, the common CHRPE-associated region should be extended to APC codons 148-2043

ExoFiT: ExoMars-Like Field Trials – a Mission Simulation.

International audienceHere, we focus on the ExoMars-Like Field Trial (ExoFiT), a series of realistic mission simulations based on the ESA ExoMars rover mission slated to be launched in 2020. The mission will carry a suite of instruments to carry out its mission objectives focused on geological and exobiological researc