Interplay of S and As in Mekong Delta sediments during redox oscillations

The cumulative effects of periodic redox cycling on the mobility of As, Fe, and S from alluvial sediment to groundwater were investigated in bioreactor experiments. Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated: Matrix A (14 m deep) had a higher pyrite concentration than matrix B (7 m deep) sediments. Gypsum was present in matrix B but absent in matrix A. In the reactors, the sediment suspensions were supplemented with As(III) and SO42-, and were subjected to three full-redox cycles entailing phases of nitrogen/CO2, compressed air sparging, and cellobiose addition. Major differences in As concentration and speciation were observed upon redox cycling. Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles. Indeed, a high pyrite content associated with a low SO42- content resulted in an increase in dissolved As concentrations, mainly in the form of As(III), after anoxic half-cycles; whereas a decrease in As concentrations mainly in the form of As(V), was instead observed after oxic half-cycles. In addition, oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation, increasing the As pool available for mobilization. The same processes seem to occur in sediment with the presence of gypsum, but, in this case, dissolved As were sequestered by biotic or abiotic redox reactions occurring in the Fe-S system, and by specific physico-chemical condition (e.g. pH). The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work, and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta (Vietnam). (C) 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.

Triggering mechanisms and geomorphological implications of debris flows in subaquatic canyons: The Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

Subaquatic canyons in deltas are major pathways for the transport of particles from rivers / the upper delta slops to the deep basins. They represent active environments with frequent deltaic failures and massmovements potentially bearing hazard-related (tsunami waves) and economic (infrastructure damages) implications. Understanding sedimentary processes and mass-movement triggering mechanisms is crucial to assess related consequences and to carry out geological risk assessments. The Rhone River delta in Lake Geneva (Switzerland, France) is a complex underwater structure with several active and inactive subaquatic canyons, similar to marine deltas but at a smaller scale. The difference between two bathymetric surveys in 1986 and 2002 revealed an inversion in the topography of the distal active canyon, as a former distal canyon was transformed into a mound-like structure. A 12 m-thick layer was deposited in the canyon and modified the sediment transfer conduit. Sediment cores from this deposit were retrieved in-situ in 2002 and 2011 via the “F.-A. Forel” and Russian MIR submersibles, respectively. These cores contained a homogeneous, sandy material. Its sediment texture, grain-size, high density and shear strength, and low water content suggests that it corresponds to a debris-flow deposit that possibly took place after the initiation of a mass movement due to a scarp failure in proximal areas of the canyon. In addition, in situ geotechnical tests on the modern canyon floor have shown a soft top layer above a stiffer substratum. This soft layer, which increases in thickness towards distal areas, may act as a basal surface for hydroplaning, and might have allowed the debrite to be transported 9 km away from the source of the scarp failure. Similarities in textures and grain-size of the debris flow and levee deposits hint at the proximal northern levee as the source of this material. Rapid sediment loading in this area, at the rate of >2.5cm/yr, steep slopes in the canyon walls and increased pore pressure due to high methane concentrations may have reduced the stability of the canyon wall in this area. Discrete sandy intervals show very high methane concentrations and thus could correspond to potentially weak layers prone to scarp failures. Nevertheless, the probable cause for the 2000 AD Rhone delta event was an exceptional flood in October 2000 which undercut the slope, and subsequently decreased the stability by increasing the shear stress and triggered the mass failure in the already unstable canyon walls. Besides economic and hazard-related implications, such mass failures represent significant and underestimated causes in morphological evolution of underwater canyons by damming the channel and, eventually, forming short-term meanders susceptible to further erosion, as seen in a recent multibeam bathymetric map obtained in 2012.

2012

Sedimentary processes and triggering mechanisms of debris flows in subaquatic canyons in Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

Subaquatic canyons in deltas are major pathways for the transport of particles from land to the deep basins. They represent active environments with frequent deltaic failures and mass-movement deposits potentially leading to hazardous (tsunami waves) and economic (infrastructure damages) implications. Understanding sedimentary processes and mass-movement triggering mechanisms is crucial to assess related consequences and to carry out geological risk assessments. The Rhone River delta in Lake Geneva (Switzerland, France) is a complex underwater structure with several active and inactive subaquatic canyons, similar to marine deltas but at a smaller scale. The difference between two bathymetric surveys in 1986 and 2002 revealed an inversion in the topography of the distal active canyon, as a former distal canyon was transformed into a mound-like structure. A 12 m-thick layer was deposited in the canyon and modified the sediment transfer conduit. Sediment cores from this deposit were retrieved in-situ in 2002 and 2011 via the "F.-A. Forel" and Russian MIR submersibles, respectively. These cores contained a homogeneous, sandy material. Its sediment texture, grain-size, high density and shear strength, and low water content suggests that it corresponds to a debris-flow deposit that possibly took place after the initiation of a mass movement due to a scarp failure in proximal areas of the canyon. In addition, in-situ geotechnical tests on the modern canyon floor have shown a soft top layer above a stiffer substratum. This soft layer, which increases in thickness towards distal areas, may act as a basal surface for hydroplaning, and might have allowed the debrite to be transported ~9 km away from the source of the scarp failure. Similarities in textures and grain-size of the debris flow and levee deposits hints at proximal northern levee as the source of this material. Rapid sediment loading in this area, at the rate of >3cm/yr, steep slopes in the canyon walls and increased pore pressure due to high methane concentrations may have reduced the stability of the canyon wall in this area. Discrete sandy intervals show very high methane concentrations and thus could correspond to potentially weak layers prone to scarp failures. Nevertheless, the probable cause for the 2000 AD Rhone delta event was an exceptional flood in October 2000 which undercut the slope, and subsequently decreased the stability by increasing the shear stress and triggered the mass failure in the already unstable canyon walls. Besides economic and hazardous implications, such mass failures represent significant and underestimated causes in morphological evolution of underwater canyons by damming the channel and, eventually, forming short-term meanders susceptible to further erosion.

2012

Internal carbon and nutrient cycling in Lake Baikal: sedimentation, upwelling, and early diagenesis

Bernhard Wehrli, Alfred Johny Wüest

The internal cycles of carbon, silica, nitrogen, and phosphorus in the South and North Basins of Lake Baikal were quantified in the frame of a multidisciplinary collaboration. Fluxes of particulate organic matter from the epilimnion to the deep water were quantified with integrating sediment traps deployed at 200- to 250-m water depth and compared with fluxes measured in near-bottom traps to reveal mineralization in the water column. Sedimentation rates were determined with dated sediment cores to calculate mass accumulation rates of elements in the sediment. Advective and turbulent transport of dissolved nutrients in the water column was based on a set of monitoring data, which included temperature and current data, as well as hydrochemical data of the water column. Diffusive fluxes from the sediment to the overlying water column were determined by applying different porewater sampling techniques. The combination of these data resulted in consistent internal budgets for carbon, nitrogen, and phosphorus in Lake Baikal: the new production in the South Basin was 1730 mmol C m(-2) year(-1) and the mass accumulation rate in the sediment 220 mmol C m(-2) year(-1), whereas in the more secluded North Basin, new production was only 1220 mmol C m(-2) year(-1) and mass accumulation rate 125 mmol C m(-2) year(-1). Fluxes of particle-bound nitrogen, phosphorus, and biogenic silica were by about 30% smaller in the North Basin than in the South Basin. Export fluxes of nitrogen from the surface zone to the deep water were 150 mmol N and 100 mmol N m(-2) year(-1). Denitrification rates in the sediment were estimated from mass-loss calculation to 38 and 53 mmol N m(-2) year(-1) for the South and North Basin, respectively, corresponding to 25% and 52% of the total nitrogen input to the hypolimnion. Nitrogen (19 and 13 mmol m(-2) year(-1)) was finally buried in the sediments of the South and North Basins; 10.1 and 3.5 mmol P m(-2) year(-1), and 1830 and 1400 mmol Si m(-2) year(-1) were transferred to the deep water in the South and North Basin where 28% and 70% P, and 64% and 54% Si were retained in the sediments. A diatom bloom occurred during our sampling period of 2 years, which usually occurs only every 3 to 5 years. Accordingly, Si flux data from sediment traps were increased by an estimated 50% compared with the long-term average illustrating the necessity of several years of field measurements to compensate for the natural dynamics of Lake Baikal. (C) 2004 Elsevier B.V. All rights reserved.

Elsevier2005

Triggering mechanisms and geomorphological implications of debris flows in subaquatic canyons: The Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

2012

Sedimentary processes and triggering mechanisms of debris flows in subaquatic canyons in Rhone delta (Lake Geneva, Switzerland, France)

Nicolas Le Dantec

2012

Internal carbon and nutrient cycling in Lake Baikal: sedimentation, upwelling, and early diagenesis

Bernhard Wehrli, Alfred Johny Wüest

Elsevier2005