Multi-annual and multi-decadal evolution of sediment accretion in a saltmarsh of the French Atlantic coast: Implications for carbon sequestration

Coastal marshes offer natural solutions for adapting to and mitigating the effects of climate change and sea level rise. However, the resilience of the marsh physical system and, with it, the ecosystem services that it provides, is largely site specific. This calls for the increase in the spatial cover of coastal marsh studies in order to assess the controlling factors of marsh evolution, and their long-term carbon storage capacities. Here, we study the spatio-temporal variations in sedimentation rates and organic carbon (OC) sequestration capacity of the macrotidal minerogenic saltmarshes in Aiguillon Bay, belonging to one of the largest French coastal marshes. Supported by aerial photographs and satellite image analysis, we first show that saltmarshes of the Aiguillon Bay have prograded at very high rates, up to 14 m yr−1 since 1950. Sediment accumulation rates (SAR) were estimated at both multi-annual to multi-decadal scales based on two approaches: (i) LiDAR-based digital elevation models from multiple acquisition dates (2010–2021); and (ii) depth profiles of 210Pb in excess and 137Cs in sediment cores collected along cross-shore transects in the saltmarshes. Long-term SAR range from 0.8 to 2.2 cm yr−1 and are among the highest reported worldwide for equivalent systems. The positive accretion balance (accretion rate minus local sea-level rise rate) provides important clues on marsh resilience suggesting that the Aiguillon Bay is currently able to adapt to rising sea level. Despite relatively low organic carbon content (1.3–6.0%), high SAR leads to high carbon sequestration rates (99–345 gC m−2 yr−1; or a mean value of 2.5 Mg C ha−1 yr−1). The isotopic signature of sediment OC reveals a significant and rapid decomposition of organic material in surface cores, while allochthonous sediment of marine origin dominates the signature of chemically-stable OC of marsh sediments. This implies that the carbon sequestration capacity of minerogenic saltmarshes, such as those of the Pertuis Charentais, also depends upon the wealth of adjacent coastal environments through high sediment supply and primary productivity.Evolution de l'identité patrimoniale des marais des Pertuis Charentais en réponse à l'aléa de submersion marin

Pre-collisional geodynamics of the Mediterranean Sea: the Mediterranean Ridge and the Tyrrhenian Sea

Today the Mediterranean Sea consists of a series of small-sized and almost geographically disconnected oceanic or continental crust rooted marine basins. It is also an area almost totally surrounded by mountain ranges, which chiefly belong to the alpine realm. This overall geodynamic setting results from a long term convergence between the two major, African and European, plates. Previous collisions have led to the edification of surrounding chains, while subduction and new-collisional processes tend to create new extensional back-arc basins and wide tectonized accretionary prisms. In this paper we briefly outline the most recent and almost land-locked back-arc basin that has developed in the Mediterranean,i.e., the Tyrrhenian Sea, and the Mediterranean Ridge, which may be regarded as a collisional sedimentary wedge predating a future mountain chain

Pre-collisional geodynamics of the Mediterranean Sea: the Mediterranean Ridge and the Tyrrhenian Sea

Today the Mediterranean Sea consists of a series of small-sized and almost geographically disconnected oceanic or continental crust rooted marine basins. It is also an area almost totally surrounded by mountain ranges, which chiefly belong to the alpine realm. This overall geodynamic setting results from a long term convergence between the two major, African and European, plates. Previous collisions have led to the edification of surrounding chains, while subduction and new-collisional processes tend to create new extensional back-arc basins and wide tectonized accretionary prisms. In this paper we briefly outline the most recent and almost land-locked back-arc basin that has developed in the Mediterranean,i.e., the Tyrrhenian Sea, and the Mediterranean Ridge, which may be regarded as a collisional sedimentary wedge predating a future mountain chain

Changement climatique et risques littoraux : apports scientifiques pour une adaptation durable et juste

This contribution aims to be a summary and a presentation of knowledge from research conducted by the laboratories of Nouvelle-Aquitaine on the physical risks of coastal areas, their developments as a result of climate change and socio-economic and political dynamics at work with regard to the measures and strategies for adaptation. It is the result of collaboration between approximately 30 researchers from the following laboratories: BRGM, Criham (Université de Poitiers), EPOC (UMR Université de Bordeaux - CNRS), ETBX (Irstea), GREThA (UMR Université de Bordeaux - CNRS), LIENSs (UMR Université de La Rochelle - CNRS), ONF, SIAME (Université de Pau et des Pays de l'Adour).La présente contribution se veut une synthèse et un porter à connaissance des recherches menées par les laboratoires de Nouvelle-Aquitaine sur les risques physiques en zones côtières, leurs évolutions sous l'effet du changement climatique, ainsi que les dynamiques socioéconomiques et politiques à l'oeuvre concernant les mesures et stratégies d'adaptation. Elle résulte d'une collaboration d'environ 30 chercheur.e.s issu.e.s des laboratoires suivants : BRGM, Criham (Université de Poitiers), EPOC (UMR Université de Bordeaux - CNRS), ETBX (Irstea), GREThA (UMR Université de Bordeaux - CNRS), LIENSs (UMR Université de La Rochelle - CNRS), ONF, SIAME (Université de Pau et des Pays de l'Adour)

Assessment of static flood modeling techniques: application to contrasting marshes flooded during Xynthia (western France)

This study aims to assess the performance of raster-based flood modeling methods on a wide diversity of coastal marshes. These methods are applied to the flooding associated with the storm Xynthia, which severely hit the western coast of France in February 2010. Static and semi-dynamic methods are assessed using a combination of LiDAR data, post-storm delineation of flooded areas and sea levels originating from both tide gauge measurements and storm surge modeling. Static methods are applied to 27 marshes showing a wide geomorphological diversity. It appears that these methods are suitable for marshes with a small distance between the coastline and the landward boundary of the marsh, which causes these marshes to flood rapidly. On the contrary, these methods overpredict flooded areas for large marshes where the distance between the coastline and the landward boundary of the marsh is large, because the flooding cannot be considered as instantaneous. In this case, semi-dynamic methods based on surge overflowing volume calculations can improve the flooding prediction significantly. This study suggests that static and semi-dynamic flood modeling methods can be attractive and quickly deployed to rapidly produce predictive flood maps of vulnerable areas under certain conditions, particularly for small distances between the coastline and the landward boundary of the low-lying coastal area

Structural setting and tectonic control of mud volcanous from the central Mediterranean Ridge (Eastern Mediterranean)

Marine Geology, v. 209, n. 1-4, p. 245-263, 2004. http://dx.doi.org/10.1016/j.margeo.2004.05.002International audienceBased on a recent marine geophysical data set, including swath bathymetry, acoustic imagery and six-channel seismics, recorded over a large area of the Mediterranean Ridge (MR) in early 1998 during the Prismed 2 survey, this paper presents a study of the various relationships observed between tectonic features cutting across the Central Mediterranean Ridge accretionary wedge and massive mud expulsions (known as mud volcanoes), identified over large areas of the ridge.Regional mapping of two of the mud volcano fields previously only partly investigated (Olimpi and United Nations Rise) revealed the presence of many new mud expulsion centres and a third new mud volcano field has been identified. All the mud features show great variability in morphology, size and backscatter strength of their surrounding mud flows. Based on their contrasting morpho-acoustic characteristics, we propose a classification into three main groups of mud construction: (1) “mud volcanoes”, these consist of subcircular and prominent reliefs, often associated with high backscatter mud flows; (2) “mud domes”, similar to mud volcano, but smaller, these occurrences correspond to weakly reflective mud constructions; (3) “mud plateaus” represent a third category which appears as wide, often highly reflective and rather flat mud extrusions. From all available data, an attempt to explain the different mud ascent processes and driving forces is discussed, in relation to the initial collision structural setting of the Central Mediterranean Ridge. Within this area, most of the mud constructions have been observed to be associated with tectonic features and, in particular, with strike-slip faulting for the first time. As a hypothesis, we propose in this paper two different ascent processes to explain the contrasting mud constructions: (1) “extrusion” for the mud volcanoes and plateaus and (2) “intrusion” for the mud domes, connected to two different mud levels controlled by the crustal geometry of this pre-collision area and especially the southwards extension of the Cretan continental crust below the Mediterranean Ridge accreted sediments

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seiemic reflection profiling

Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north-south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean-Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area. ©2001 Elsevier Science B.V. All rights reserved