Field observations of alongshore runup variability under dissipative conditions in presence of a shoreline sandwave
Video measurements of runup were collected at low tide along several profiles covering an alongshore distance of 500 m. The morphology displayed a complex shape with a shoreline sandwave in the lower beach face of about 250 m long mirrored in the inner sandbar. Wave conditions were stationary and moderate (offshore height of 2 m and peak period of nearly 13 s) but yet dissipative. Runup energy was dominated by infragravity frequencies. Alongshore variations in runup (by a factor up to 3) observed both in the incident and infragravity bands were much higher than reported previously (e.g., Guedes et al., 2012, https://doi.org/10.1016/j.csr.2012.08.022; Ruggiero et al., 2004, https://doi.org/10.1029/2003JC002160) while the alongshore variations in other environmental parameters (e.g., foreshore beach slope) appear to be much lower. Our data suggest that the beach morphology in the inner surf zone plays a crucial role by inducing rapid and significant modification in the incident wave pattern and the alongshore coherence length scales were consistent with the typical alongshore length scale of the morphology.
(Journal of Geophysical Research. Oceans. vol. 123, n° 2169-9275, pp. 6800-6817, 28/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Microplastic contamination in the Seine River estuary
(23/08/2018)
LEESU, ENPC, UPEC UP12, LERN, COAST, IFREMER, IFREMER, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Benefactor facilitation and beneficiary feedback effects drive shrub-dominated community succession in a semi-arid dune ecosystem
Questions (a) How do the facilitative effects of benefactors on understorey species change along a successional gradient; (b) what are the feedback effects of the beneficiary species on the benefactor; and (c) how do these bidirectional interactions drive community dynamics of a shrub‐dominated ecosystem. Location A sand‐fixing community, Mu Us Desert, China. Methods We quantified the effects of the shrub Artemisia ordosica on understorey herb abundance, biomass and richness using an observational procedure at four positions along a successional gradient, i.e., semi‐fixed dunes (D1 stage), fixed dunes (D2 stage), fixed dunes covered with biological crusts (D3 stage) and fixed dunes with herbs (D4 stage). The relative interaction index (RII) was used to calculate the effect of the shrub on the herbs. Additionally, we conducted a removal experiment at the latest successional stage to assess the beneficiary feedback effects on the growth of the shrub's vegetative and reproductive twigs. Results The response of herbs to the effect of the benefactor shrub showed a humped‐back curve for abundance and biomass along the successional gradient, supporting the collapse of the interaction model, but a linear curve for richness, thus supporting the stress‐gradient hypothesis. This difference could likely be explained by a turnover of life‐history groups of understorey species from annuals to perennials along the successional gradient, with the growth of the latter group being more susceptible to shrub facilitation than the former. The herbs had a negative feedback effect on the growth of the shrub's reproductive twigs. Conclusions Our results suggest that the relationship between the shrub A. ordosica and the associated herbs was antagonistic, and the absence of shrub seedling recruitment at the end of succession suggests that this antagonistic relationship was a crucial driver of a successional shift from a shrub‐dominated to a herb‐dominated community.
(Applied Vegetation Science. vol. 21, n° 1402-2001, pp. 595-606, 08/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Deformation bands in volcaniclastic rocks – Insights from the Shihtiping tuffs, Coastal Range of Taiwan
Deformation bands have been extensively studied in sandstones, whereas far less is known about bands occurring in porous volcaniclastic rocks. Here we investigate spectacular outcrop exposures of late Miocene tuffaceous rocks in the Coastal Range of Taiwan, which host several deformation band types: (i) disaggregation-dominated, layer-bound, sub-vertically-dipping pure compaction bands (PCB); (ii) cataclastic, layer-bound, reverse-sense compactional shear bands (RCSB); and (iii) non-layer-bound and intensely cataclastic transverse-sense compactional shear bands (TCSB). RCSBs and TCSBs host discrete slip surfaces on individual bands. The bands formed in an overall compressive stress regime related to convergence of the Eurasian and Philippine Sea plates. PCBs and RCSBs formed first, whereas progressive burial caused a shift to a transverse stress state and formation of TCSBs. The occurrence of cataclasis in RCSBs but not PCBs is inferred to be shear-driven, rather than confining-pressure-driven. Our findings suggest that cataclasis in deformation bands in volcaniclastic rocks is bimodal. Shear localization preferentially affects weak glass shards, causing intense comminution of volcanic glass. Feldspar, pyroxene and amphibole phenocrysts are comparatively less crushed, and cataclasis is strongly controlled by mineralogic cleavage planes. We conclude that increasing glass content reduces shear resistance, and that deformation bands in volcaniclastic rocks effectively exhibit a strain-weakening behavior.
(Journal of Structural Geology. vol. 113, n° 0191-8141, pp. 155-175, 01/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Hydro- and sediment dynamics in the Gironde estuary (France): Sensitivity to seasonal variations in river inflow and sea level rise
Understanding estuarine hydrodynamics and sediment dynamics is of key importance to provide the foundation for sound management of these coastal systems. Turbidity maxima, which are zones of elevated suspended sediment concentration (SSC), are of particular interest as they control biogeochemical cycling and affect the overall environmental quality of the estuary. These turbidity maxima, however, are complex dynamic features that respond to changes in forcing conditions. In this study we use a 3D numerical model to investigate the response of hydrosedimentary dynamics to variations in river inflow and sea level rise in the Gironde estuary, which is one of the largest estuarine systems in Europe. Yearly simulations and comparisons with satellite data and measurements of salinity and SSC show that the model reproduces variations in salinity intrusion and the migration of the turbidity maximum driven by seasonal fluctuations in river inflow. Numerical experiments indicate that the formation of this dynamic turbidity maximum is mainly driven by tidal asymmetry. Density gradients play a secondary role by maintaining the stability of the suspended sediment mass. The model also simulates the presence of a secondary turbidity maximum which is more stable, consistent with observations. Evaluation of the sediment budget shows that sediment export mainly occurs during spring tides and when river discharge is high. Simulations including sea level rise suggest that salinity levels in the middle estuary will increase and rising water levels cause tidal amplification, strengthening of tidal currents and enhanced SSC levels in the upper estuary. On the other hand, the locations of the salinity front and the turbidity maximum remain relatively stable under rising water levels. Overall, our simulations suggest that decadal changes in river inflow can potentially have a larger effect on turbidity maximum dynamics than sea level rise.
(Continental Shelf Research. vol. 165, n° 0278-4343, pp. 37-50, 01/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Influence of the bioturbator Upogebia cf. pusilla on trace metal remobilization: Does parasitism matter?
(Marine Environmental Research. vol. 139, n° 0141-1136, pp. 87-98, 01/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LEFE, INEE-CNRS, CNRS, UT3, Comue de Toulouse, CNRS, Toulouse INP, Comue de Toulouse, LOBB, OOB, UPMC, CNRS, UPMC, INSU - CNRS, CNRS
Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events
Floods carry sediments to river deltas and the coastal zone, but little is known about the geochemical evolution of this particulate material deposited over a short period of time. Here, we studied two recent contrasting flood deposits in the Rhône River prodelta area (northwestern Mediterranean Sea). We monitored the porewater and solid-phase chemistry over periods ranging from a few days to 6 months after deposition. Non-steady state diagenetic processes associated with episodic deposition promote a wide spectrum of transient redox conditions in the shallow prodelta region of the Rhône. Specific attributes of diagenetic responses depend on the sources of flood material and scale (thickness) of deposition. The first flood unit of 20-30 cm was composed of light gray mud, poor in organic carbon and rich in reactive manganese oxides. The short-term responses of early diagenetic processes contrasted with a rapid consumption of O2 and NO3-over a few hours just after the deposition event, accompanied by a slower build-up of Mn2+ concentration, and a lagged response in Fe2+ concentration over a few days or weeks. This difference was due to the redox capacity of the sediment, evolving from oxidized, during the flood layer deposition, to more reducing conditions, after a few days or weeks, allowing Fe2+ to build up and remain in solution. Sulfate reduction may have started within a few days within the flood deposit and was greatly enhanced just below the former redox front due to a fresh input of organic matter (OM). This large production of H2S probably led to the precipitation of sulfide minerals in close vicinity to the former redox front, limiting the accumulation of Fe2+ and H2S. The unit was sampled repeatedly three times during the six months following the flood event, and showed that manganese oxides were reduced at a rate of 1.8 mmol m−2 d−1, whereas the iron oxide concentration did not vary substantially.
(Continental Shelf Research. vol. 166, n° 0278-4343, pp. 65-76, 01/08/2018)
IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, OCEANIS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LPG-ANGERS, LPG, UA, UN UFR ST, UN, INSU - CNRS, CNRS, IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Application of a multidisciplinary and integrative weight-of-evidence approach to a 1-year monitoring survey of the Seine River
Quality assessment of environments under high anthropogenic pressures such as the Seine Basin, subjected to complex and chronic inputs, can only be based on combined chemical and biological analyses. The present study integrates and summarizes a multidisciplinary dataset acquired throughout a 1-year monitoring survey conducted at three workshop sites along the Seine River (PIREN-Seine program), upstream and downstream of the Paris conurbation, during four seasonal campaigns using a weight-of-evidence approach. Sediment and water column chemical analyses, bioaccumulation levels and biomarker responses in caged gammarids, and laboratory (eco)toxicity bioassays were integrated into four lines of evidence (LOEs). Results from each LOE clearly reflected an anthropogenic gradient, with contamination levels and biological effects increasing from upstream to downstream of Paris, in good agreement with the variations in the structure and composition of bacterial communities from the water column. Based on annual average data, the global hazard was summarized as “moderate” at the upstream station and as “major” at the two downstream ones. Seasonal variability was also highlighted; the winter campaign was least impacted. The model was notably improved using previously established reference and threshold values from national-scale studies. It undoubtedly represents a powerful practical tool to facilitate the decision-making processes of environment managers within the framework of an environmental risk assessment strategy.
(Environmental Science and Pollution Research. vol. 25, n° 0944-1344, pp. 23404-23429, 01/08/2018)
SEBIO, INERIS, URCA, ULH, NU, URCA, CNRS, UR HBAN, IRSTEA, ULB, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, GEDI, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UR MALY, IRSTEA, IRSTEA, ESE, UP11, CNRS
A nonlinear weakly dispersive method for recovering the elevation of irrotational surface waves from pressure measurements
We present the derivation of a nonlinear weakly dispersive formula to reconstruct , from pressure measurements, the surface elevation of nonlinear waves propagating in shallow water. The formula is simple and easy to use as it is local in time and only involves first and second order time derivatives of the measured pressure. This novel approach is evaluated on laboratory and field data of shoaling waves near the breaking point. Unlike linear methods, the nonlinear formula is able to reproduce at the individual wave scale the peaked and skewed shape of nonlinear waves close to the breaking point. Improvements in the frequency domain are also observed as the new method is able to accurately predict surface wave elevation spectra over four harmonics. The nonlinear weakly dispersive formula derived in this paper represents an economic and easy to use alternative to direct wave elevation measurement methods (e.g. acoustic surface tracking and LiDAR scanning)..
(Coastal Engineering. vol. 138, n° 0378-3839, pp. 1 - 8, 01/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, IMB, UB, Bordeaux INP, CNRS, LIENSs, INSU - CNRS, ULR, CNRS, LEGI, Grenoble INP, CNRS, UGA [2016-2019], CNRS, UGA [2016-2019], Grenoble INP ENSGI, Grenoble INP, UGA [2016-2019]
A two-million-year-long hydroclimatic context for hominin evolution in southeastern Africa
The past two million years of eastern African climate variability is currently poorly constrained, despite interest in understanding its assumed role in early human evolution. Rare palaeoclimate records from northeastern Africa suggest progressively drier conditions or a stable hydroclimate. By contrast, records from Lake Malawi in tropical southeastern Africa reveal a trend of a progressively wetter climate over the past 1.3 million years. The climatic forcings that controlled these past hydrological changes are also a matter of debate. Some studies suggest a dominant local insolation forcing on hydrological changes whereas others infer a potential influence of sea surface temperature changes in the Indian Ocean. Here we show that the hydroclimate in southeastern Africa (20–25° S) is controlled by interplay between low-latitude insolation forcing (precession and eccentricity) and changes in ice volume at high latitudes. Our results are based on a multiple-proxy reconstruction of hydrological changes in the Limpopo River catchment, combined with a reconstruction of sea surface temperature in the southwestern Indian Ocean for the past 2.14 million years. We find a long-term aridification in the Limpopo catchment between around 1 and 0.6 million years ago, opposite to the hydroclimatic evolution suggested by records from Lake Malawi. Our results, together with evidence of wetting at Lake Malawi, imply that the rainbelt contracted toward the Equator in response to increased ice volume at high latitudes. By reducing the extent of woodland or wetlands in terrestrial ecosystems, the observed changes in the hydroclimate of southeastern Africa—both in terms of its long-term state and marked precessional variability—could have had a role in the evolution of early hominins, particularly in the extinction of Paranthropus robustus.
(Nature. vol. 560, n° 0028-0836, pp. 76 - 79, 01/08/2018)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, GFZ, MARUM, PACEA, UB, CNRS, UNIVERSITé DE DURHAM, JAMSTEC, CLIM, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, VUB, GM, IFREMER