Redetermination of the transformation enthalpies of the xanthoconite–proustite, pyrostilpnite–pyrargyrite and trechmannite–smithite phase transitions

 We report here refined values of the transformation enthalpy for xanthoconite to proustite (54.81 kJ/mol) and pyrostilpnite to pyrargyrite (40.32 kJ/mol). Additionally, the enthalpy for the transformation of trechmannite to smithite (5.82 kJ/mol) has been determined. The refinement was possible by taking into account a previously unknown dependence of electrochemical signals on the amount of substance undergoing the reduction process. Received December 21, 1995/Revised, accepted June 29, 1996     

Sampling Fauna in Stream Sediments as well as Groundwater Using One Net Sampler

Groundwater bores act as traps. Net samplers are regularly used for sampling this type of trap for fauna. To enable direct comparisons of faunal communities in groundwater bores and stream sediments, stream sediment tubes were built similar to groundwater bores and were sampled with net samplers for fauna. These stream sediment tubes consisted of a tube anchored in the stream sediment, also called interstitial space. To test the efficacy of this trap method in stream sediments, it was compared to another type of trap, Hahn's trap. Faunal communities sampled by a net in the stream sediment tubes did not differ hugely from fauna in Hahn's trap samples. Physical and chemical factors of sampled water in both the stream sediment tubes, the surrounding interstitial sediments and the second type of traps, Hahn's traps, showed that water in both the tubes and Hahn's traps was closely related to interstitial water. The net sampler is inexpensive and easy to handle. It is suggested that sampling stream tubes with nets may be an appropriate method for long‐term monitoring studies.     

Rivers turned to rock: Late Quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the Australian monsoon tropics

Late Quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching Gilbert River in the monsoon tropics of the Gulf of Carpentaria. The Gilbert, one of a number of rivers in this region, has contributed to an extensive system of coalescing low-gradient and partly indurated riverine plains. Extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope (OI) Stage 5. Subsequent flow declined, probably associated with increased aridity, however, enhanced runoff recurred again in OI Stages 4–3 (65–50 ka). Aridity then capped these plains with 4–7 m of mud. A widespread network of sandy distributary channels was incised into this muddy surface from sometime after the Last Glacial Maximum (LGM) to the mid Holocene during a fluvial episode more active than the present but less so than those of OI Stages 5 and 3. This network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main Gilbert flow path.A tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids, waterfalls and inset gorges, features characteristic more of bedrock than alluvial systems. Thermoluminescence (TL) and comparative optically stimulated luminescence (OSL) ages of the sediments are presented along with U/Th ages of pedogenic calcrete and Fe/Mn oxyhydroxide/ oxide accumulations. They show that calcrete precipitated during the Late Quaternary at times similar to those that favoured ferricrete formation, possibly because of an alternating wet–dry climate. Intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the Late Quaternary but, probably due to exceptional dryness, not during the LGM. The result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload. Consequent avulsions have caused local stream powers to increase by an order of magnitude, inducing knickpoint erosion, local incision and the sudden influx of additional bedload that has triggered further avulsions. The Gilbert River, while less energetic than its Pleistocene ancestors, is clearly an avulsive system, and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks, generating nickpoints, reworking sediment and thereby developing and maintaining an indurated and anabranching river style.     

Mise au point d'un milieu de culture pour l'étude de l'altération de silicates en présence de Pseudomonas aeruginosa

The influence of microorganisms on mineral alteration is not easy to determine in environmental conditions, because of the difficulty to raise for comparison purposes an identical but abiotic system. Another problem in this context is the choice of reliable tracers to evaluate the alteration rate of materials during in vitro experiments. To face such difficulties, we elaborated a defined medium allowing both the growth of Pseudomonas aeruginosa and a precise measurement of the elements solubilized from the minerals. Thanks to this medium, we were able to quantitatively determine the amounts of major elements solubilized from the materials in the presence of bacterial growth, compared to a sterile system. Moreover, the analysis by ICP-MS of trace elements was possible after a chromatographic treatment, which selectively eliminated 99% of the sodium content of the medium. To cite this article: G. Aouad et al., C. R. Geoscience 337 (2005).     

The relation of Holocene fluvial terraces to changes in climate and sediment supply,South Fork Payette River,Idaho

Well-preserved Holocene terraces along the South Fork Payette River in central Idaho provide a record of fluvial system behavior in a steep mountain watershed characterized by weathered and erodible Idaho Batholith granitic rocks. Terrace deposit ages were provided by ¹⁴C dating of charcoal fragments and optically stimulated luminescence (OSL) dating of sandy sediments. Along with pairing of many terrace tread heights, these data indicate episodic downcutting during the Holocene, with a mean incision rate of ～0.9 m/ka from ～7 ka to present. Prior to 7 ka, the river incised to within～3 m of current bankfull, but then aggraded by ～5 m over at least a ～10 km-long reach in an episode centered ～7–6 ka. Aggradation may relate to (1) increased hillslope sediment input from landslides and debris flows in steep tributary basins with abundant grussified granitic bedrock, (2) possible local landslide-damming of the channel, (3) decreased peak discharge, or (4) a combination of these factors. Middle Holocene channel aggradation ca. 7–6 ka corresponds with a period of prolonged and widespread aridity in the northern Rocky Mountains. Between ～5 and 1.3 ka, the river aggraded slightly and then remained stable, forming a prominent terrace tread at ～3 m above current bankfull. Modest aggradation to vertical stability of the South Fork Payette River at the 1.5 m terrace level ～1.0–0.7 ka corresponds with large fire-related debris flows in tributaries during Medieval droughts. Three intervals of incision (～5.5–5 ka, 1.3–1.0 ka and 0.5 ka) correspond with frequent but small fire-related sedimentation events and generally cooler, wetter conditions suggesting increased snowmelt runoff discharges. Other possible drivers of channel incision include an increase in stochastic or climate-modulated large storms and floods and a reduction in delivery of hillslope sediment to the channel. Aggradation is more confidently tied to climate through increases in hillslope sediment delivery and (or) decreased stream power, both likely related to warmer, drier conditions (including high-severity fires) that reduce snowmelt and decrease vegetation cover on steep slopes. Thus, the Holocene terraces of the South Fork Payette River do not reflect simple stepwise incision with periods of vertical stability and lateral migration, but record substantial episodes of aggradation as well. We infer that increases in hillslope erosion and mass movements combined with reduced discharges during prolonged droughts episodically reverse the post-glacial trend of downcutting, in particular during the middle Holocene. The present bedrock-dominated channel implies a strong tendency toward incision in the late Holocene.     

Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola

We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a^?1 has remained stable during the entire Quaternary.     

Landsat-8 vs. Sentinel-2: examining the added value of sentinel-2’s red-edge bands to land-use and land-cover mapping in Burkina Faso

The availability of freely available moderate-to-high spatial resolution (10–30 m) satellite imagery received a major boost with the recent launch of the Sentinel-2 sensor by the European Space Agency. Together with Landsat, these sensors provide the scientific community with a wide range of spatial, spectral, and temporal properties. This study compared and explored the synergistic use of Landsat-8 and Sentinel-2 data in mapping land use and land cover (LULC) in rural Burkina Faso. Specifically, contribution of the red-edge bands of Sentinel-2 in improving LULC mapping was examined. Three machine-learning algorithms – random forest, stochastic gradient boosting, and support vector machines – were employed to classify different data configurations. Classification of all Sentinel-2 bands as well as Sentinel-2 bands common to Landsat-8 produced an overall accuracy, that is 5% and 4% better than Landsat-8. The combination of Landsat-8 and Sentinel-2 red-edge bands resulted in a 4% accuracy improvement over that of Landsat-8. It was found that classification of the Sentinel-2 red-edge bands alone produced better and comparable results to Landsat-8 and the other Sentinel-2 bands, respectively. Results of this study demonstrate the added value of the Sentinel-2 red-edge bands and encourage multi-sensoral approaches to LULC mapping in West Africa.     

Resource Model Updating For Compositional Geometallurgical Variables

In the field of mineral resources extraction, one main challenge is to meet production targets in terms of geometallurgical properties. These properties influence the processing of the ore and are often represented in resource modeling by coregionalized variables with a complex relationship between them. Valuable data are available about geometalurgical properties and their interaction with the beneficiation process given sensor technologies during production monitoring. The aim of this research is to update resource models as new observations become available. A popular method for updating is the ensemble Kalman filter. This method relies on Gaussian assumptions and uses a set of realizations of the simulated models to derive sample covariances that can propagate the uncertainty between real observations and simulated ones. Hence, the relationship among variables has a compositional nature, such that updating these models while keeping the compositional constraints is a practical requirement in order to improve the accuracy of the updated models. This paper presents an updating framework for compositional data based on ensemble Kalman filter which allows us to work with compositions that are transformed into a multivariate Gaussian space by log-ratio transformation and flow anamorphosis. This flow anamorphosis, transforms the distribution of the variables to joint normality while reasonably keeping the dependencies between components. Furthermore, the positiveness of those variables, after updating the simulated models, is satisfied. The method is implemented in a bauxite deposit, demonstrating the performance of the proposed approach.