Formation of weathering-derived magnesite deposits in the New England Orogen, New South Wales, Australia: Implications from mineralogy, geochemistry and genesis of the Attunga magnesite deposit

Nodular, cryptocrystalline, weathering-derived magnesite deposits in the New England Orogen, Australia, provide a significant source of high-purity magnesite. Common textural features and related isotopic fingerprints indicate a close genetic relationship between weathering-derived magnesite deposits hosted by ultramafic rocks at Attunga and by sediments at Kunwarara while silica-carbonate rock alteration and rare hydrothermal magnesite vein deposits reflect contrasting conditions of formation. Localised weathering of carbonates in a soil environment shifts stable isotopic composition towards low δ ¹³C and high δ ¹⁸O typical for weathering-derived magnesites while intrusion-related fluids do not significantly change the isotopic composition of affected carbonates. At Attunga, magnesite consists of irregular, nodular veins and masses filling faults and cracks in the weathered serpentinite host rock as well as soft powdery magnesite in pervasive serpentinite alteration zones. The high-grade magnesite at Attunga can be contaminated by amorphous silica and serpentine relicts but does not contain dolomite or ferroan magnesite as observed for its hydrothermal equivalent, the Piedmont magnesite deposit, or other widespread deposits of silica-carbonate rock in the Great Serpentinite Belt. Heavy δ ¹⁸O values are compatible with a supergene formation from meteoric waters while low δ ¹³C suggests C3-photosynthetic plants as the predominant source of carbon for the Attunga magnesites. We infer that weathering-derived, nodular magnesite deposits hosted in ultramafic rocks like the Attunga magnesite deposit have formed in a two-step process involving the hypogene formation of a pre-cursor magnesite deposit and complete supergene overprinting by meteoric waters that acquired carbon from percolation through soil.     

Tropical cyclone risk perceptions in Darwin,Australia: a comparison of different residential groups

Different individuals and groups perceive risk differently. This can significantly affect risk management and mitigation practices and requirements. This paper presents findings from a study of tropical cyclone risk perceptions in the city of Darwin in the Northern Territory of Australia. Primary in-depth interview data and other secondary data are analysed, focussing in particular on wind damage, storm surge and life safety risk perceptions of residents since Cyclone Tracy, which impacted in 1974, and perceptions of future climate change as it relates to tropical cyclone risk. The analysis reveals that a number of perceptions prevail. In particular, the study reveals a wide difference of perceptions between short-term residents (Group 1) and long-term and expert residents (Group 2) in relation to wind damage, storm surge and life safety risk. It also reveals a large division between laypersons (Group 3) and expert residents’ (Group 4) perceptions of climate change risk as it relates to tropical cyclone risk. The author recommends that flexible, multiple and integrative management and mitigation approaches are required to deal with such different perceptions and divisions in the resident population.     

Notes on the origin of inertinite macerals in coals: Observations on the importance of fungi in the origin of macrinite

Macrinite is a, generally, rare inertinite maceral, often incorporating remnants and fragments of other macerals, including vitrinite, liptinite, and other inertinite. The associated inertinites include multiple forms of funginite. Funginite is also commonly found in association with vitrinite of slightly elevated reflectance and with degraded varieties of vitrinite. Together with the highly degraded macrinite, the latter two associations are here inferred to be part of a continuum of fungal and microbial degradation of peat. In any case, the origin of some macrinite is potentially distinct from that of inertinite generated by fire.     

Submarine hydrothermal brine seeps off Milos, Greece. Observations and geochemistry

A shallow hydrothermal brine seep located off the Greek island of Milos in the Aegean Sea was studied. The brine fluid outcropped as a pool of water in a seabed depression and was detected in the surrounding pore-waters of sediments colonised by the sulphur bacterium Achromatium volutans. The seep fluid was highly saline and sulphidic, depleted in Mg²⁺ and SO₄²⁻, but enriched over seawater in Na⁺, Ca²⁺, K⁺, Cl⁻, SiO₂, reduced species and dissolved gases. The high concentrations of Na⁺, Ca²⁺ and K⁺ were consistent with the Milos tectonic setting. Na-K and Na-K-Ca geothermometers predicted a reservoir temperature of 300–325 °C for the most concentrated seep samples. The deep geothermal reservoir within the metamorphic basement of Milos island has already been located and studied and may represent the source of the seep fluid. Faunal diversity was lowest in seep-influenced sediments, but a sulphide-intolerant species was found in areas of the bacterial mat where salinity and temperature were much lower. Pressure-induced variations in the vertical depth of the brine interface may be occurring in the sediment.     

Chronology,stratigraphy and palaeoenvironmental interpretation of a Late Pleistocene to mid‐Holocene cave accumulation on Kangaroo Island,South Australia

Chronological, sedimentological and geochemical analyses of a clastic infill from Kelly Hill Cave (5K1), Kangaroo Island, document a palaeoenvironmental record that spans from the Late Pleistocene to the middle Holocene. We AMS radiocarbon‐dated bone collagen and U–Th‐dated speleothem to determine that fossiliferous sediments were deposited between >20 ka and 7 ka ago. Most of the 15 sedimentary layers are dominated by sand‐ and silt‐sized quartz that is physically and geochemically comparable with surface soils in the Kelly Hill area. Late Pleistocene and Last Glacial Maximum strata are represented primarily by homogeneous, poorly sorted quartz‐rich sediments that contain little organic matter, but include a thin layer composed largely of silt‐sized clay pellets that resemble sediments deflated from playa lakes. Microstructures observed in petrographic slides indicate that, with the exception of one layer, all sediments experienced little reworking once deposited in the cave. Some layers display pedogenic microstructures such as redeposited clays and opaline silica infilling that indicate postdepositional modification; that is, cave‐floor soil development. Overlying Holocene‐aged sediments also consist mainly of quartz but have much greater organic matter content. Some of these sediments have been strongly influenced by re‐precipitated organic matter that appears to have been transported into the cave via vadose drip water. The presence of dissolved organic matter in soil/vadose waters suggests a high vegetation density and acidic soils, which are congruent with the more equitable climatic conditions characteristic of the Holocene. The sediments described here provide a valuable palaeoenvironmental record that will facilitate future interpretation of associated vertebrate fossils.     

Extended survival of several organisms and amino acids under simulated martian surface conditions

Recent orbital and landed missions have provided substantial evidence for ancient liquid water on the martian surface as well as evidence of more recent sedimentary deposits formed by water and/or ice. These observations raise serious questions regarding an independent origin and evolution of life on Mars. Future missions seek to identify signs of extinct martian biota in the form of biomarkers or morphological characteristics, but the inherent danger of spacecraft-borne terrestrial life makes the possibility of forward contamination a serious threat not only to the life detection experiments, but also to any extant martian ecosystem. A variety of cold and desiccation-tolerant organisms were exposed to 40 days of simulated martian surface conditions while embedded within several centimeters of regolith simulant in order to ascertain the plausibility of such organisms’ survival as a function of environmental parameters and burial depth. Relevant amino acid biomarkers associated with terrestrial life were also analyzed in order to understand the feasibility of detecting chemical evidence for previous biological activity. Results indicate that stresses due to desiccation and oxidation were the primary deterrent to organism survival, and that the effects of UV-associated damage, diurnal temperature variations, and reactive atmospheric species were minimal. Organisms with resistance to desiccation and radiation environments showed increased levels of survival after the experiment compared to organisms characterized as psychrotolerant. Amino acid analysis indicated the presence of an oxidation mechanism that migrated downward through the samples during the course of the experiment and likely represents the formation of various oxidizing species at mineral surfaces as water vapor diffused through the regolith. Current sterilization protocols may specifically select for organisms best adapted to survival at the martian surface, namely species that show tolerance to radical-induced oxidative damage and low water activity environments. Additionally, any hypothetical martian ecosystems may have evolved similar physiological traits that allow sporadic metabolism during periods of increased water activity.     

A 421-d activity cycle in the BeX recurrent transient A0538–66 from MACHO monitoring

We present a ∼5-yr optical light curve of the recurrent Be/X-ray transient A0538–66 obtained as a by-product of the MACHO Project. These data reveal both a long-term modulation at P =420.8±0.8 d and a short-term modulation at 16.6510±0.0022 d which, within errors, confirms the previously found orbital period. Furthermore, the orbital activity is only seen at certain phases of the 421-d cycle, suggesting that the long-term modulation is related to variations in the Be star envelope.     

Intercomparison of soil pore water extraction methods for stable isotope analysis and interpretation of hillslope runoff sources

Intercomparison of soil pore water extraction methods for stable isotope analysis has been a focus of recent studies in relation to plant source waters, which found a wide isotopic variance depending on the extraction method. Few studies have yet explored extraction effects for mobile pore waters that relate to hillslope runoff. This is because it is extremely difficult in natural systems to control the boundary conditions in order to assess and compare impacts of pore water extraction on resulting hillslope flow. With our new semicontrolled experiments on outdoor mini‐hillslopes, we studied mixing and runoff processes by means of stable isotopes of water and quantified relations between pore water extraction methods. We tested the null hypothesis that nondestructive and destructive pore water sampling methods sample the same soil water pool. Three hillslopes were mounted on load cells, filled with loamy sand textured soils from the Landscape Evolution Observatoryat Biosphere 2, equipped with soil moisture and temperature sensors, a bottom outflow, and a surface runoff gauge for isotope sampling. We followed the precipitation isotopic composition over and through the soil profile. One hillslope was instrumented with suction cups, on the second we installed sampling ports for in‐situ soil water vapour measurements, and the third hillslope was sampled destructively for applying the centrifugation and vapour equilibrium methods. All hillslopes were sampled at four depths (0–10, 10–20, 20–30, and 30–40 cm) at three different downslope positions. ²H and ¹⁸O analyses were performed via laser spectroscopy. We found no isotopic differences between rainfall, surface runoff, and bottom outflow. The in situ vapour ports' soil isotope data showed the widest spread over all hillslope positions and depths. Centrifugation's and suction cups' isotope results plotted closest to the local meteoric water line and within the range of hillslope runoff and bottom outflow data. Hillslope position did not influence the soil isotope results. These results suggest caution be used in the field when selecting an extraction technique for matching soil waters to runoff waters. Soil suction lysimeters and centrifugation appeared to be the most appropriate tools in this regard.