Subaquatic canyons are an important pathway for sediment transport into oceanic and lacustrine basins. Understanding the mechanisms governing their geomorphological evolution is a key to predict the ...sediment distribution patterns through these sediment conduits as well as to implement geo-hazard assessments. Submerged channels developed in large lacustrine basins offer a small-scale natural laboratory to understand the sedimentological processes operating in submarine channels. For this reason, a multidisciplinary research initiative -including time-lapse, high-resolution bathymetric surveys, innovative coring using submersibles, in situ geotechnical tests, and geophysical and sedimentological analyses-was applied to unravel the factors controlling the geomorphological evolution of the Rhone delta channels in Lake Geneva during the last decades. The morphology of the lacustrine Rhone Delta consists of a freshwater delta system deeply incised by nine canyons (C1C9). Geotechnical measurements in proximal areas and sediment cores retrieved in the distal fans at the end of each canyon revealed complex sediment dynamics. No turbidity current events have occurred in the easternmost canyons (C1C4) during the last decades while the western canyons sediment record (C5C9) indicated repeated flushing events during the 20th century. The main “active” canyon C8 has been dominated by turbidite activity on the canyon floor with frequent overspill events along the levees. A large 6.2 × 106 m3 Mass-Transport Deposit (MTD) that resembles a debrite in its upper section was found in the distal area of the active channel. The MTD was dated at 1998–2000 CE and most likely originated from proximal delta areas affected by frequent slope failures of the steep channel walls. In situ geotechnical tests on the modern proximal channel floor showed an unconsolidated soft top-layer that might have served as a low-friction surface favouring the MTD long run-out distance to the distal part of the channel. The MTD has had a major effect morphological evolution of the distal channel by filling the existing conduit, indirectly promoting the formation of a new channel. The role of MTD emplacement in subaquatic channels has important implications for hydrocarbon exploration as they control channel avulsion processes and the location of sand-prone deposits. This study gives a detailed insight on poorly investigated short-term sedimentological dynamics that affect the long-term evolution of turbidite systems and channel migration processes. This detailed model of a river-dominated deep-lacustrine depositional system can be used as an analog for similar modern and ancient deep-water systems.
Schematic depositional model displaying the main sedimentological processes in active (C8), intermittently active (C5) and inactive sublacustrine canyons. -Mass Transport Deposit (MTD); Sedimentation Rate (SR). Display omitted
•We provide clastic depositional models for lacustrine channel-levee complexes.•The thicknesses of unconsolidated surficial sediment layers reveal channel’s activity.•Mass-Transport Deposits (MTD) blocking distal channels influence avulsion processes.•Short-lived MTDs control the long-term morphological evolution of deepwater channels.
Suspended sediment export from large Alpine catchments (> 1000 km2) over decadal timescales is sensitive to a number of factors, including long-term variations in climate, the activation–deactivation ...of different sediment sources (proglacial areas, hillslopes, etc.), transport through the fluvial system, and potential anthropogenic impacts on the sediment flux (e.g. through impoundments and flow regulation). Here, we report on a marked increase in suspended sediment concentrations observed near the outlet of the upper Rhône River Basin in the mid-1980s. This increase coincides with a statistically significant step-like increase in basin-wide mean air temperature. We explore the possible explanations of the suspended sediment rise in terms of changes in water discharge (transport capacity), and the activation of different potential sources of fine sediment (sediment supply) in the catchment by hydroclimatic forcing. Time series of precipitation and temperature-driven snowmelt, snow cover, and ice melt simulated with a spatially distributed degree-day model, together with erosive rainfall on snow-free surfaces, are tested to explore possible reasons for the rise in suspended sediment concentration. We show that the abrupt change in air temperature reduced snow cover and the contribution of snowmelt, and enhanced ice melt. The results of statistical tests show that the onset of increased ice melt was likely to play a dominant role in the suspended sediment concentration rise in the mid-1980s. Temperature-driven enhanced melting of glaciers, which cover about 10 % of the catchment surface, can increase suspended sediment yields through an increased contribution of sediment-rich glacial meltwater, increased sediment availability due to glacier recession, and increased runoff from sediment-rich proglacial areas. The reduced extent and duration of snow cover in the catchment are also potential contributors to the rise in suspended sediment concentration through hillslope erosion by rainfall on snow-free surfaces, and increased meltwater production on snow-free glacier surfaces. Despite the rise in air temperature, changes in mean discharge in the mid-1980s were not statistically significant, and their interpretation is complicated by hydropower reservoir management and the flushing operations at intakes. Overall, the results show that to explain changes in suspended sediment transport from large Alpine catchments it is necessary to include an understanding of the multitude of sediment sources involved together with the hydroclimatic conditioning of their activation (e.g. changes in precipitation, runoff, air temperature). In addition, this study points out that climate signals in suspended sediment dynamics may be visible even in highly regulated and human-impacted systems. This is particularly relevant for quantifying climate change and hydropower impacts on streamflow and sediment budgets in Alpine catchments.
Recent climatic warming and associated glacial retreat may have a large impact on sediment release and transfer in Alpine river basins. Concurrently, the sediment transport capacity of many European ...Alpine streams is affected by hydropower exploitation, notably where flow is ed but the sediment supply downstream is maintained. Here, we investigate the combined effects of climate change and flow ion on morphodynamics and sediment transfer in the Borgne River, Switzerland. From photogrammetrically derived historical Digital Elevation Models (DEMs), we find considerable net aggradation of the braided river bed (up to 5 m) since the onset of flow ion in 1963. Reaches responded through bed level steepening which was strongest in the upper most reach. Widespread aggradation however did not commence until the onset of glacier retreat in the late 1980s and the dry and warm years of the early 1990s. Upstream flow intake data shows that this aggradation coincided with an increase in sediment supply, although aggradation accounts for no more than 25% of supplied material. The remainder was transferred through the studied reaches. Estimations of bed load transport capacity indicate that flow ion reduces transport capacity by 1–2 orders of magnitude. While residual transport rates vary with morphological evolution, they are in the same order of magnitude as the sediment supply rates, which is why significant transport remains. However, the reduction in transport capacity makes the system more sensitive to short‐term (annual) changes in climate‐driven hydrological variability and climate‐induced changes in intake management and sediment delivery rates.
Key Points
Hydropower‐related flow ion may drastically reduce sediment transport capacity, but only to rates that are of similar magnitude as sediment supply
This causes downstream river bed aggradation and morphodynamics to be very sensitive to external forcing mechanisms related to flow management or climate change
Climate‐driven sediment supply may propagate through Alpine streams despite large‐scale flow ion
Freshwater biofilms can be useful indicators of water quality and offer the possibility to assess contaminant effects at the community level. The present field study examines the effects of ...chlor-alkali plant effluents on the community composition of biofilms grown in the Olt River (Romania) reservoirs. The relationship between ambient water quality variables and community composition alterations was explored. Amplicon sequencing revealed a significant modification of the composition of microalgal, bacterial and fungal communities in the biofilms collected in the impacted reservoirs in comparison with those living in the uncontaminated control reservoir. The abundance corrected Simpson index showed lower richness and diversity in biofilms collected in the impacted reservoirs than in the control reservoir. The biofilm bacterial communities of the impacted reservoirs were characterized by the contaminant-tolerant Cyanobacteria and Bacteroidetes, whereas microalgal communities were predominantly composed of Bacillariophyta and fungal communities of Lecanoromycetes and Paraglomycetes. A principal component analysis revealed that major contaminants present in the waste water of the chlor-alkali production plant, i.e. Na
, Ca
, Cl
and Hg, were correlated with the alteration of biofilm community composition in the impacted reservoirs. However, the biofilm composition was also influenced by water quality variables such as NO
, SO
, DOC and Zn from unknown sources. The results of the present study imply that, even when below the environmental quality standards, typical contaminants of chlor-alkali plant releases may affect biofilm composition and that their impacts on the microbial biodiversity might be currently overlooked.
OMEGA/Mars Express has discovered large outcrops rich in phyllosilicates in the region of Mawrth Vallis, Mars (around 20°W, 25°N). The region is located in Noachian highly cratered terrains, close to ...the limit of the Martian dichotomy, where the outflow channel Mawrth Vallis cuts the highlands. We have examined this region using OMEGA spectra of the surface from 0.9 μm to 2.6 μm, with spatial sampling from 500 m to 3 km, offering a full coverage of the region. OMEGA spectra show two broad bands centered at 1 μm and 2.2 μm, revealing the presence of clinopyroxene on dark surfaces. Phyllosilicates have been identified by absorption bands at 1.4 μm, 1.9 μm, and 2.2 or 2.3 μm. Comparison with laboratory spectra reveals similarities with Al‐OH smectites such as montmorillonites, or Fe‐ or Mg‐OH smectites such as nontronite. A precise location of the phyllosilicate‐rich areas on visible HRSC images indicates that they are placed exclusively on bright outcrops, mostly on the plateaus, dated to the Noachian period. On HRSC and MOC images the phyllosilicate‐rich outcrops reveal strong erosional features such as numerous residual buttes composed of layers a few meters thick. The phyllosilicate‐rich unit corresponds to a geological unit more than 100 m thick, over a horizontal extension approximately of 300 km × 400 km. This unit implies a large volume of altered rocks, either in situ or after transport and deposition, in Noachian terrains, revealing a different climatic and geologic environment from the present one.
We present the distribution of olivine on Mars, derived from spectral parameters based on the 1 µm olivine absorption band. The olivine can be defined with respect to two spectral end‐members: type 1 ...corresponds to olivine with low iron content and/or small grain size and/or small abundance, and type 2, which corresponds to olivine with higher iron content and/or larger grain size and/or larger abundance. The spatial and statistical analysis of the global olivine distribution points out five major geological settings where olivine is detected: (1) Early Hesperian olivine‐bearing smooth crater floors and flat intercrater plains throughout the southern highlands; (2) olivine deposits around the three main basins Argyre, Hellas, and Isidis; (3) olivine in intercrater dunes, crater ejecta, or extended deposits in the northern plains; (4) olivine associated with outcrops and sand in the floor of Valles Marineris; and (5) olivine‐bearing butte outcrops in the vicinity of Hellas. The geological context, the age, and the composition of the olivine detections associated with these five major geological settings are detailed. Their origin and the implication of their occurrence on the composition of the Martian mantle and crust, as well as on the evolution of Mars volcanism are discussed.
Key Points
Global mapping and analysis of olivine bearing deposits
Identification of five major geological settings
Implications on volcanic and internal evolution of Mars
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