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Bottom currents are key processes that contribute to the shaping of submarine slopes, with the redistribution of sediments in contourite systems. Despite numerous recent studies on contourite systems, the complexity and diversity of these sedimentary systems are still not fully understood and often underestimated. Their understanding requires comprehensive works integrating all scales from seismic architecture to microfacies. This paper focuses on a contouritic ridge located between 2000 m and 2500 m water depth on the Mozambique margin. Bathymetry, seismic data and piston cores collected during the PAMELA-MOZ3 cruise allow a multi-scale study from large depositional geometries to sedimentary facies. At the seismic scale, the contouritic ridge show three stages of evolution with: (i) initiation and development of the drift/moat system; (ii) an intermediate stage with successive incisions and aggradations; and (iii) moat infill and drift erosion during the Plio-Quaternary. Plio-Quaternary deposits are composed of hemipelagic, turbiditic and contouritic facies filling the moat. Coarse-grained contouritic facies, dominated by planktonic foraminifera, are identified on the western flank of the ridge between the moat infill and the erosional area at the top of the ridge. They consist of condensed deposits, with sedimentation rate about 0.3 cm/ka, indicating a strong and stable bottom current that winnows away the fine-grained component. This facies could be present more generally in intermediate position between erosion and depositional areas in contourite systems. At present, the contourite system is located at the transition depth between North Atlantic Deep Water and Antarctic Intermediate Water. Trajectories of bottom currents are complex and interact with sporadic turbidity currents and anti-clockwise eddies that participate in reshaping the sea floor morphology. Although Plio-Quaternary depositional geometries indicate the end of drift/moat development, the moat filling and drift erosion are also related to bottom currents and constitute atypical contouritic sedimentation.

International audience; Distribution patterns of selected heavy metals content in sediments from the Bay of Quiberon and Gulf of Morbihan were studied to understand the current heavy metals contamination due to urbanization and mariculture activities in the coastal area. Therefore, a survey was conducted and 196 sediments collected were characterized for heavy metals content using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) after mix acid digestion process. The distribution maps of the concentrations of the heavy metals studied were produced as an isopleth map based on data interpolation by the ArcGIS software application. The association with the adverse effects on aquatic organisms was determined by the classification of the sediment according to the sediment quality guidelines. Therefore, two approaches were employed namely: direct comparison with Sediment Quality Guidelines (SQGs) by USEPA (United States Environmental Protection Agency) and comparison with other numerical SQGs, threshold effect level/probable effect level, and effect range low/effect range medium. In order to estimate the effect of multiple contaminations of heavy metals, the mean-ERM-quotient was calculated at each sampling point.

Marine mud belts represent potential continuous high-resolution climatic, environmental and anthropogenic archives. In this study, a geochemical record of the Gulf of Lions mud belt, which receives sediments from the Rhône watershed and to a lesser extent from the Languedoc region, is reported from Core KSGC-31. The effects of natural climatic changes and possible anthropogenic disturbances on Holocene sedimentation were ascertained by analysing sedimentation rates, chemical weathering (Al2O3/K2O) and sediment-source shifts (neodymium isotopic ratios; εNd). Measurements of elemental and isotopic lead were used to trace the source and determine the potential vectors of anthropogenic contaminations over the Holocene. High εNd values, recorded from 9000 to 3000 calibrated annum before present (cal. a BP) and around 1500 and 600 cal. a BP, are interpreted as an increase in sediment transport from the Alpine crystalline massifs to the sea induced by enhanced hydro-sedimentary conditions upstream. During the early and middle Holocene, low and stable weathering conditions were persistent, while the late Holocene was characterized by higher and more fluctuating weathering conditions. Sudden changes in the 206Pb/207Pb ratio observed during the Roman and Medieval periods suggest clear shifts in lead source from a natural Holocene background to late Holocene anthropogenic contaminations. Even though those shifts are coeval with atmospheric lead contaminations from Spain and Germany recorded in several sediment and ice archives, the local origin (the Cévennes) and the fluvial contamination is more likely in these cases. Those findings are contemporaneous with historical mining records in the Cévennes and point to an intensification of the merchant shipping.

During the Quaternary, global sea level was characterized by large fluctuations in amplitude and increased frequency. However, the eastern marginal seas of China, especially the northern East China Sea and Yellow Sea did not experience large-scale transgression until the middle Pleistocene. The Zhe-Min (Zhejiang-Fujian) Uplift was an important barrier in the East China Sea preventing transgression of sea water into the northern marginal seas of China, but its Quaternary evolution is still poorly constrained. Here we present a thick sand layer record (the top age is ~416 ka) of the Integrated Ocean Drilling Program (IODP) Site U1428 from the northern Okinawa Trough to reconstruct the history of the Zhe-Min Uplift since the middle Pleistocene. Provenance analysis indicates that sediments from both sand and imbedded fine-grained calcareous-rich clay layers at the study site were mainly derived from the Yellow River, which for the first time indicates that the Yellow River sediments could influence the northern Okinawa Trough since at least the middle Pleistocene. Significant change of lithology at IODP Site U1428 at ~416 ka was attributed to the large-scale subsidence of the Zhe-Min Uplift, which is probably due to the rapid uplifting of Tibet during 0.52–0.38 Ma. Before ~416 ka, obstacle of the emersed Zhe-Min Uplift induced extremely severe reworking and promoted sand ridge formation in the shelf edge of southeastern Yellow Sea, and supplied sand sediments to the northern Okinawa Trough during transgression and high sea-level stages. After the subsidence of the Zhe-Min Uplift at ~416 ka, westward of tidal sand ridge and Yellow River mouth with the process of shelf transgressions favored fine-grained sediments input to the northern Okinawa Trough. Even during low sea-level stages, the flat terrain of the East Asian marginal sea shelf and low rainfall amount resulted in low discharge of Yellow River, and supplied fine-grained particles to the northern Okinawa Trough.

The external Alps record a whole Wilson cycle that began at early Mesozoïc times by an extensional phase leading to the deposition of thick marine deposits upon an upper Triassic basement including a thick salt layer. Several diapiric structures (e.g. Astoin, the Barre de Chine ; Célini et al., 2020) are the witnesses of this important salt activity during deposition and the subsequent deformation through the Lower Jurassic. Otherwise, Triassic salt allowed thrusting on several decollement levels and emplacement of major thrusted units, such as the “Nappe de Digne” or the Authon thrust sheet, during the alpine phase s.s, initiated at the Oligocene-Miocene boundary. Between these two periods, the external Alps story is more uncertain and none salt activity has been clearly demonstrated except westwards in the Vocontian basin. In the whole South-East basin, only few clues, as bipyramidal quartz found in Priabonian deposits in the western Baronnies suggest a potential salt activity at surface during the Paleogene. However, in the St-Geniez areas, some Oligocene sediments, located at the vicinity of salt structures suggest a potential diapiric growth during this period. Indeed, some stratigraphic gypsum beds are found in an Oligocene lacustrine series, directly thrusted by the Authon thrust sheet. None evaporite environments are described in the whole region at Oligocene times, which suggest a possible recycling of Triassic evaporites.In order to determine if theses deposits are related to a Paleogene salt activity, a multi-analytical approach was used. First, a field study allowed characterizing the facies and the sedimentary filling and defining the stress regime during the deposit, by kinematic inversion on fractures which indicates a constant N-S compression during the Oligocene. The presence of halophilic fauna at the base of the lacustrine series of the St-Geniez area attests for saline influences during deposit. Moreover, 4km to the SW, a wedge in the conglomerates of the alpine continental molasse (so called red molasse) resting directly on Sorine’s Triassic diapir was put forward. Cargneules and dolomites from the Triassic constitute an important part of the reworked material. These observations indicate that the Sorine's diapir was active during the deposition of the Oligocene series. Then, a precise chemostratigraphic framework was determined by use of δ13C and δ18O isotopic data on the lacustrine limestones. 87Sr/86Sr isotopic ratio on gypsum beds of the lacustrine series aimed at determining their ages and a possible Triassic evaporite sourcing. Our results gave an age ranging from 6 to 23 Ma, which does not correspond with the Oligocene age of the overlying and underlying sediments. Moreover, the large variation in isotope ratios suggests that this gypsum did not come from primary precipitation but from leaching of a pre-existing evaporite source. In conclusion, field observations, together with geochemical analyses, made it possible to highlight the relationships between tectonics, salt tectonics and sedimentation and also to reconstruct the paleogeography of the region at the end of the Paleogene. ReferencesCélini, N., Callot, J.P., Ringenbach, J.C., Graham, R. (2020,). Jurassic salt tectonics in the SW sub-Alpine fold and thrust belt. Tectonics

Distribution patterns of selected heavy metals content in sediments from the Bay of Quiberon and Gulf of Morbihan were studied to understand the current heavy metals contamination due to urbanization and mariculture activities in the coastal area. Therefore, a survey was conducted and 196 sediments collected were characterized for heavy metals content using Inductively Coupled Plasma – Mass Spectrometry (ICP–MS) after mix acid digestion process. The distribution maps of the concentrations of the heavy metals studied were produced as an isopleth map based on data interpolation by the ArcGIS software application. The association with the adverse effects on aquatic organisms was determined by the classification of the sediment according to the sediment quality guidelines. Therefore, two approaches were employed namely; direct comparison with Sediment Quality Guidelines (SQGs) by USEPA (United States Environmental Protection Agency) and comparison with other numerical SQGs, threshold effect level/probable effect level, and effect range low /effect range medium. In order to estimate the effect of multiple contaminations of heavy metals, the mean–ERM–quotient was calculated at each sampling point.

(IF 7.96; Q1); International audience; Anthropogenic impacts on rivers have increased significantly over the past ~150 years, particularly at the beginning of the industrial revolution. Among other signs, this impact is manifested through the addition of trace metals and metalloid elements to rivers. The Eure River watershed in France covers an area of 6017 km2 and is a major tributary of the Seine estuary. It is not exempt from anthropogenic pressures and has been exposed to significant metal discharges over the last 80 years. The average concentrations of metals (i.e., Cr, Co, Ni, Cu, Zn, Ag, Cd, Sb, and Pb), in suspended particulate matter currently transported by the river are high compared to the local geochemical background. Moreover, the lack of correlation between concentration variations and the hydrosedimentary behaviour of the Eure River suggests that the river is currently under anthropogenic pressure. Analysis of sediment cores indicate strong As contamination during the 1940s, Cr, Co, Ni, Cu, Zn, Ag, and Cd contamination during the 1960s and 1970s, and Sb and Pb contamination during the 1990s and 2000s. The enrichment factors calculation suggests that total anthropogenic pressure within the Eure River watershed since the 1940s was comparable or higher than those in many other French watersheds. An estimation of particulate metal flux in 2017 shows that the Eure River watershed contributed to 7, 8, 9, 10 and 16% of total inputs to the Seine estuary in Cr, Cu, Zn, Cd and Pb respectively. Moreover, the estimation of past theoretical flux indicates that during the 1990s the Eure River watershed was the main contributor of particulate Pb to the estuary. The use of Pb isotopes has revealed that this contamination was primarily of industrial origin.

International audience; Reconstructing the Cenozoic sedimentary history of the Bay of Bengal (BoB) is significant for understanding the evolutionary history of South Asian river systems and the links between river development, tectonic deformation and global climate change. Here, we present the first long-term clay mineral record combined with Sr-Nd isotopic compositions from a 200-m-long sediment core from Ocean Drilling Program (ODP) Site 758 in the southern BoB to establish past variations in sediment sources and river evolution that have occurred since the late Eocene. Provenance analysis suggests that the contribution of the Himalayan source materials from the Irrawaddy and Brahmaputra Rivers to the study site became more important and stable since 22 Ma and 8 Ma, respectively. This new evidence indicates that the Brahmaputra and Irrawaddy Rivers were possibly initiated in the early Miocene (~22 Ma). Considering the timing of major tectonic and climatic evolution in South Asia, we conclude that the two phases of tectonic uplift of the Himalayan orogen that occurred in the early and late Miocene were the primary controls on river development in South Asia and provenance changes in the southern BoB. Global cooling and Indian drying after the late Miocene could also have strengthened Himalayan erosion and contributed more illite and chlorite to the South Asian margin.

International audience; The Meillon (Callovo-Oxfordian) and Mano (Tithonian) Formations are dolomitized carbonate reservoirs that actively produce oil and gas (Aquitaine Basin, France). In this study, the dolomitization conditions of their counterparts exhumed in the northwestern Pyrenees are detailed using a combination of field observations, petrography, fluid inclusion microthermometry, elemental and isotopic geochemistry, and carbonate U-Pb geochronology. Dolomitization occurred in several stages spanning from the Neocomian (pre-rift) to the Albian (syn-rift, associated with mantle exhumation and active salt tectonics). Both formations were first massively dolomitized in near-surface to shallow burial conditions during the Berriasian-Valanginian, likely triggered by the influx of marine-derived waters. Between the Barremian and the Albian, the Early Cretaceous rifting caused the upward influx of hot fluids associated with the partial to complete recrystallization of the initial dolomites. During the Albian, subsequent dolomites precipitated in both formations as high-temperature (T > 160 • C) veinand pore-filling cement. Distinct fluid inclusion chlorinities and rare earth element patterns between the Meillon and Mano Formations point to fluid compartmentalization during this stage. Whereas dolomite cements indicate the involvement of evaporite-derived brines in the Meillon Formation, precipitation was likely related to clayderived water in the Mano Formation. Lastly, a final episode of dolomite cementation occurred only in the vicinity of faults and volcanic intrusions during the Albian when the highest temperatures were recorded in both formations (T > 250 • C). These saddle dolomites precipitated from hydrothermal water with a mixture of mantle-, crustal-, and evaporite-derived waters channeled by faults and active diapirs. Subsequent quartz and calcite cement precipitation reveals a temperature decrease in a post-rift to inversion setting (post-Cenomanian) and indicates fluid compartmentalization between both formations. This study highlights the major control exerted by rifting, combined with the presence of diapiric salt, on dolomitization, making carbonate platforms of modern salt-rich passive margins potential targets for exploration.

International audience; We present high-resolution analyses of clay mineral assemblages combined with analysis of Sr–Nd–Pb isotopic compositions of the

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100 km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geo- physical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that sub- marine fl at-top seamounts correspond to karsti fi ed and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well- constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical envi- ronments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typi fi ed by the establishment of wide lava fl ow complexes, and (2) to extensional tectonic deformation associat- ed with high-offset normal faults dividing the fl at-top seamounts into distinctive structural blocks. Explosive vol- canic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long-term evolution of shallow-water car- bonate platforms

International audience; In this study we use bulk sediment organic matter stable isotopes (δ13Corg and δ15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (δ13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ∼3‰ values in δ15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represent 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel deposition.

An extensive compilation of published neodymium (Nd) concentrations and isotopic compositions (Nd IC) was realized in order to establish a new database and a map (using a high-resolution geological map of the area) of the distribution of these parameters for all the Mediterranean margins. Data were extracted from different kinds of samples: river solid discharge deposited on the shelf, sedimentary material collected on the margin or geological material outcropping above or close to a margin. Additional analyses of surface sediments were done in order to improve this data set in key areas (e.g. Sicilian strait). The Mediterranean margin Nd isotopic signatures vary from non-radiogenic values around the Gulf of Lion, (εNd values ∼ −11) to radiogenic values around the Aegean and the Levantine sub-basins up to +6. Using a high-resolution regional oceanic model (1/12° of horizontal-resolution), εNd distribution was simulated for the first time in the Mediterranean Sea. The high resolution of the model provides a unique opportunity to represent a realistic thermohaline circulation in the basin and thus apprehend the processes governing the Nd isotope distribution in the marine environment. Results are consistent with the preceding conclusions on boundary exchange (BE) as an important process in the Nd oceanic cycle. Nevertheless this approach simulates a too-radiogenic value in the Mediterranean Sea; this bias will likely be corrected once the dust and river inputs will be included in the model. This work highlights that a significant interannual variability of εNd distribution in seawater could occur. In particular, important hydrological events such as the Eastern Mediterranean Transient (EMT), associated with deep water formed in the Aegean sub-basin, could induce a shift in εNd at deep/intermediate depths that could be noticeable in the eastern part of the basin. This underlines that the temporal and geographical variations of εNd could represent an interesting insight of Nd as tracer of the Mediterranean Sea circulation, in particular in the context of palaeo-oceanographic applications.

(IF 3.55; Q1); International audience; Little is known about the geological history of the Glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the Somali Basin. This study focused on describing and reconstructing the long-term stratigraphic evolution of the Glorieuses seamount (SW Indian Ocean) to identify the mechanisms that have occurred through time to finally shape the emerged modern islands. Distinct terrace levels, currently submerged along the flanks of the seamount and surrounding seamounts, have already been interpreted as resulting from successive carbonate development and back-stepping episodes over the last 62 Myr. New isotopic and biostratigraphic dating on the flanks of the seamount, coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin, provide new constraints for the Late Cretaceous and Cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin. Even if starved steep slopes prevent a straightforward source-to-sink continuity between the platform and the basin domains, our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin, and discuss a long-term geological model that includes the main driving factors behind deposition (volcanic events, subsidence vs uplift phases, climate and hydro-dynamism changes) and their quantitative impact on the evolution of the isolated carbonate sedimentary system. Our results show that: (i) the Glorieuses volcanic seamount emerged from two successive Late Cretaceous magmatic pulses, firstly during the Turonian, then during the Maastrichtian (ii) at least two potential uplift phases are recognized during the Tertiary (Paleogene and/or the Eocene and Tortonian); (iii) basinal sedimentation recorded an abrupt change probably related to major regional hydro-dynamical changes in Late Eocene times in the Western Indian Ocean; (iv) the export of sediments from the platform towards the basin (numerous gravity flow processes) is strongly enhanced after the Mid Miocene, and is probably linked to the onset of the Asian monsoon winds and bipolar circulation. Finally, the Glorieuses seamount, although located in the vicinity of the Comoros islands, appears to have a much longer history and is geologically more comparable to the nearby Seychelles. This long-term study has enabled us to associate the Glorieuses seamount with the SSE-NNW Madagascar-Seychelles alignment rather than with the Comoro hot spot evolution.

WOS: 000423619700006, The isotopic composition of evaporites can shed light on their environment of precipitation and their subsequent recycling processes. In this study, we performed Sr, O and S isotopic analyses on evaporitic sulphates in the halokinetic Sivas Basin. The main objectives were to decipher the age and origin of the evaporites responsible for the salt tectonics, and to test whether diapir dissolution acts as the source of younger evaporitic layers in continental mini-basins. The Sr isotopes demonstrate that the first evaporites precipitated from seawater during the Middle-Late Eocene. The similar isotopic values measured in the halokinetic domain confirm that the Eocene evaporites triggered the salt tectonics and were continuously recycled in Oligo-Miocene mini-basins as lacustrine to sabkha evaporites. Modern halite precipitates suggest that the dissolution and recycling of diapiric halite is ongoing. This study demonstrates the efficiency of isotopic analyses in constraining evaporite recycling processes in continental halokinetic domains., Total SA, The authors thank Total SA for financial support and C. Kergaravat, C. Ribes and G. Calinak for field work assistance. C. Helvaci, B. Brunner and J.P. Girard are thanked for fruitful discussions. C. Taberner, T.M. Peryt, M. Coleman and three anonymous reviewers helped to improve the manuscript.

The offshore branch of the East African Rift System (EARS) has developed during late Cenozoic time along the eastern Africa continental margin. While Neogene–Pleistocene extensional tectonic deformation has been evidenced along the northern segment of the Davie Ridge, the spatial extent of deformation further south remains poorly documented. Based on recent and various oceanographic data sets (bathymetric surveys, dredge samples and seismic profiles), our study highlights active normal faulting, modern east–west extensional tectonic deformation and Late Cenozoic alkaline volcanism at the Sakalaves Seamounts (18°S, Davie Ridge) that seem tightly linked to the offshore EARS development. In parallel, rift‐related tectonic subsidence appears responsible for the drowning of the Sakalaves Miocene shallow‐water carbonate platform. Our findings bring new insights regarding the development of the EARS offshore branch and support recent kinematic models proposing the existence of a plate boundary across the Mozambique Channel.

International audience; The Marquesas Island chain in Polynesia is quite unusual because the alignment of the islands on the Pacific oceanic plate (N40°W) does not follow the plate motion in the region (N65°W). The exact location of the active hotspot is unknown but has been predicted to underlie the Marquesas Fracture Zone Ridge. Nevertheless, no concrete evidence exists. Here, we document the occurrence on this ridge of fresh tephrites dated at ~ 92 ka by the 40Ar39Ar method. The lavas dredged on a small seamount have trace element contents and Sr, Nd, Pb isotopic compositions typical of the southwest Marquesas Islands, the Fatu Hiva group. This discovery demonstrates that the Marquesas plume is still active and it puts new constraints on its present location. It also supports McNutt et al.’s (1989) interpretation of the Marquesas Fracture Zone Ridge as a very young volcanic construction underlain by a hotspot. We suggest that the present location of the Marquesas plume is under the ridge, at its intersection with the isotopic divide known along the Marquesas chain. We attribute the presence of young volcanic products 190 km southwest of this location to preferential magma flow along the Marquesas Fracture lithospheric weakness zone. We also suggest that the puzzling general direction of the archipelago is the consequence of a persistent low magma flux over the past 5 Ma that could only find its way to the surface through multiple weak zones in the Pacific plate.

High-resolution multi-proxy records, including clay minerals and Sr-Nd-Pb isotopes of the clay-sized silicate fraction of sediments from IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for distinct control mechanism on fine-grained sediments input from the Yellow River and the southern Japanese Islands to the northern Okinawa Trough since 34 ka BP. Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu. Since the last glacial, clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation. During low sea-level stage (∼34‒14 ka BP), the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough, favoring large fluvial discharge or even direct input of detrital sediments, which resulted about four times more flux of clay-sized sediments supply to the study area as during the relatively high sea-level stage (∼14‒0 ka BP). The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current and Tsushima Warm Current intensity, with increased input in phase with weakened Kuroshio Current/Tsushima Warm Current. Our study suggests that the Kuroshio Current was very likely flowed into the Okinawa Trough and thus influenced the fine-grained sediment transport in the area throughout the last glacial and deglacial. During ∼34‒11 ka BP, the Kyushu clay-sized sediment input was mainly controlled by the Kuroshio Current. Since ∼11 ka BP, the occurrence of Tsushima Warm Current became important in influencing the Kyushu fine-grained sediment input to the northern Okinawa Trough.

International audience; The sulfur biogeochemical cycle plays a key role in regulating Earth’s surface redox through diverse abiotic and biological reactions that have distinctive stable isotopic fractionations. As such, variations in the sulfur isotopic composition (δ34S) of sedimentary sulfate and sulfide phases over Earth history can be used to infer substantive changes to the Earth’s surface environment, including the rise of atmospheric oxygen. Such inferences assume that individual δ34S records reflect temporal changes in the global sulfur cycle; this assumption may be well grounded for sulfate-bearing minerals but is less well established for pyrite-based records. Here, we investigate alternative controls on the sedimentary sulfur isotopic composition of marine pyrite by examining a 300-m drill core of Mediterranean sediments deposited over the past 500,000 y and spanning the last five glacial−interglacial periods. Because this interval is far shorter than the residence time of marine sulfate, any change in the sulfur isotopic record preserved in pyrite (δ34Spyr) necessarily corresponds to local environmental changes. The stratigraphic variations (>76‰) in the isotopic data reported here are among the largest ever observed in pyrite, and are in phase with glacial−interglacial sea level and temperature changes. In this case, the dominant control appears to be glacial−interglacial variations in sedimentation rates. These results suggest that there exist important but previously overlooked depositional controls on sedimentary sulfur isotope records, especially associated with intervals of substantial sea level change. This work provides an important perspective on the origin of variability in such records and suggests meaningful paleoenvironmental information can be derived from pyrite δ34S records.

Arnaud Fabien, Giguet-Covex Charline, Wilhelm Bruno, Jenny Jean-Philippe, Sabatier Pierre, Poulenard Jerôme, Desmet Marc, Enters Dirk, Revillon Sidonie, Millet Laurent, Debret Maxime, Pignol Cécile, Malet Emmanuel, Fanget Bernard. L'objet emblématique Lacs de Montagne. L'étude des sédiments lacustres au laboratoire EDYTEM. In: Collection EDYTEM. Cahiers de géographie, numéro 16, 2014. Environnements, dynamiques et territoires de la montagne : Dix ans de recherches au laboratoire EDYTEM. pp. 107-118.

International audience; Although long-term evolutions of isolated shallow-water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades, their driving processes are still the subject of active debates. The Mozambique Channel (SW Indian Ocean) is characterized by several flat-topped seamounts ranging from 11°S to 21°S in latitudes. Based on a comprehensive geomorphologic study and on dredged samples analysis, we show that these features correspond to tropical isolated shallow-water carbonate platforms. Coupling strontium isotopy and foraminifera biostratigraphy, well-constrained chronostratigraphy results indicate that shallow-water carbonate production started in the Mozambique Channel during distinct Cenozoic periods ranging from Paleocene to Early Miocene. Our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism. While some of them kept developed until present days, forming modern carbonate systems, some others were drowned during Late Neogene and subsided to form guyots. Although different factors can be discussed, tectonic and volcanism appear as good potential triggers for demise episodes during Late Miocene-Early Pliocene times. Chronology and location of this geodynamical activity tend to emphasize influence of East African rift system until southern Mozambique Channel.

An interdisciplinary study was conducted to evaluate the relationship between geological and paleoenvironmental parameters and the bacterial and archaeal community structure of two contrasting subseafloor sites in the Western Mediterranean Sea (Ligurian Sea and Gulf of Lion). Both depositional environments in this area are well-documented from paleoclimatic and paleooceanographic point of views. Available data sets allowed us to calibrate the investigated cores with reference and dated cores previously collected in the same area, and notably correlated to Quaternary climate variations. DNA-based fingerprints showed that the archaeal diversity was composed by one group, Miscellaneous Crenarchaeotic Group (MCG), within the Gulf of Lion sediments and of nine different lineages (dominated by MCG, South African Gold Mine Euryarchaeotal Group (SAGMEG) and Halobacteria) within the Ligurian Sea sediments. Bacterial molecular diversity at both sites revealed mostly the presence of the classes Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria within Proteobacteria phylum, and also members of Bacteroidetes phylum. The second most abundant lineages were Actinobacteria and Firmicutes at the Gulf of Lion site and Chloroflexi at the Ligurian Sea site. Various substrates and cultivation conditions allowed us to isolate 75 strains belonging to four lineages: Alpha-, Gammaproteobacteria, Firmicutes and Actinobacteria. In molecular surveys, the Betaproteobacteria group was consistently detected in the Ligurian Sea sediments, characterized by a heterolithic facies with numerous turbidites from a deep-sea levee. Analysis of relative betaproteobacterial abundances and turbidite frequency suggested that the microbial diversity was a result of main climatic changes occurring during the last 20 ka. Statistical direct multivariate canonical correspondence analyses (CCA) showed that the availability of electron acceptors and the quality of electron donors (indicated by age) strongly influenced the community structure. In contrast, within the Gulf of Lion core, characterized by a homogeneous lithological structure of upper-slope environment, most detected groups were Bacteroidetes and, to a lesser extent, Betaproteobacteria. At both site, the detection of Betaproteobacteria coincided with increased terrestrial inputs, as confirmed by the geochemical measurements (Si, Sr, Ti and Ca). In the Gulf of Lion, geochemical parameters were also found to drive microbial community composition. Taken together, our data suggest that the palaeoenvironmental history of erosion and deposition recorded in the Western Mediterranean Sea sediments has left its imprint on the sedimentological context for microbial habitability, and then indirectly on structure and composition of the microbial communities during the late Quaternary.

International audience

International audience; During the Holocene there has been a gradual increase in the influence of humans on Earth systems. High-resolution sedimentary records can help us to assess how erosion and weathering have evolved in response to recent climatic and anthropogenic disturbances. Here we present data from a high-resolution (∼75 cm/k.y.) sedimentary archive from the South China Sea. Provenance data indicate that the sediment was derived from the Red River, and can be used to reconstruct the erosion and/or weathering history in this river basin. Accelerator mass spectrometry 14C dating provides direct age control and reveals coherent variations in clay mineralogy, geochemistry, and terrigenous flux, indicative of strong chemical weathering and physical erosion during the mid-Holocene warm period (6400–4000 cal [calibrated] yr B.P.), followed by weakening from ca. 4000–1800 cal yr B.P., and renewed intensification since 1800 cal yr B.P.. Comparison with climatic records from China indicates that precipitation and temperature controlled both physical erosion and chemical weathering intensity before 1800 cal yr B.P.. However, weathering proxies in the offshore sediment indicate recent increased soil erosion. We suggest that enhanced human activity (deforestation, cultivation, and mining) since the end of the Chinese Han Dynasty (220 CE) has overwhelmed the natural climatic controls on erosion in the Red River.

The presence of coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some mantle-derived materials reflects longterm elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186 Os and 187 Os, with 186 Os/ 188 Os ranging between 0.1198397 7 22 and 0.1198470 7 38, and with QOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186 Os/ 188 Os of approximately 0.119870 and QOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth’s interior that mixes with a variety of different mantle compositions distinguished by variations in QOs. The 187 Os- and 186 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186 Os/ 188 Os and QOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal^liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.

[1] The goal of the subduction factory project is to obtain a better understanding of processes occurring at convergent margins and performs mass balance calculations for tracer elements. As part of this project, a major objective of ODP Leg 185 is to determine the chemical composition of the inputs to the West Pacific Subduction Factory for use in calculations of elemental mass balance across the subduction zone. To understand such elemental budgets, it is critical to know the lithological diversity and chemical characteristics of the down going oceanic plate. Hole 801C, located ocean-ward of the Mariana island arc system, is the most complete section of old oceanic crust sampled to date but, core recovery in basement holes is often poor (e.g.

The composition of the mantle plumes that created large oceanic plateaus such as Ontong Java or the Caribbean is still poorly known. Geochemical and isotopic studies on accreted portions of the Caribbean plateau have shown that the plume source was heterogeneous and contained isotopically depleted and relatively enriched portions. A distinctive feature of samples from the Caribbean plateau is their unusual Sr isotopic compositions, which, at a given Nd isotopic ratio, are far higher than in samples from other oceanic plateaus. Sr, O and He isotopic compositions of whole rocks and magmatic minerals (clinopyroxene or olivine) separated from komatiites, gabbros and peridotites from Gorgona Island in Colombia were determined to investigate the origin of these anomalously radiogenic compositions. Sequentially leached clinopyroxenes have Sr isotopic compositions in the range 87Sr/86Sr=0.70271–0.70352, systematically lower than those of leached and unleached whole rocks. Oxygen isotopic ratios of clinopyroxene vary within the range ?18 O=5.18–5.35‰, similar to that recorded in oceanic island basalts. He isotopic ratios are high (R/Ra=8–19). The lower 87Sr/86Sr ratios of most of the clinopyroxenes shift the field of the Caribbean plateau in Nd–Sr isotope diagrams toward more `normal' values, i.e. a position closer to the field defined by mid-ocean ridge basalts and oceanic-island basalts. Three clinopyroxenes have slightly higher 87Sr/86Sr ratios that cannot be explained by an assimilation model. The high 87Sr/86Sr and variations of 143Nd/144Nd are interpreted as a source characteristic. Trace-element ratios, however, are controlled mainly by fractionation during partial melting. We combine these isotopic data in a heterogeneous plume source model that accounts for the diversity of isotopic signatures recorded on Gorgona Island and throughout the Caribbean plateau. The heterogeneities are related to old recycled oceanic lithosphere in the plume source; the high 3He/4He ratios may indicate that the source material once resided in the lower mantle.

[1] Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large-scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core-log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log-based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six-fold from that reported using core description alone, demonstrating convincingly that core-log integration is essential to provide an accurate representation of the input flux. The log-based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast-spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.

The Beata Ridge is a prominent SSW‐trending topographic structure in the central Caribbean basin. It is characterized by unusually thick oceanic crust (up to 20 km) and is believed to form part of the Caribbean oceanic plateau. Samples recovered by submersible during the Nautica‐Beata cruise show the ridge to be composed mainly of gabbros, dolerites, and rare pillow basalts. Textures, which vary significantly, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. Major‐element compositions of gabbros and dolerites plot on simple trends that correspond to fractional crystallization of olivine, clinopyroxene, and plagioclase. Trace‐element ratios are close to chondritic [(Nb/Zr)N 0.85–1.1] and rare earth element patterns (REE) are almost flat [(La/Yb)N 0.63–1.02]. The source, however, was isotopically depleted (eNd +7.4 to +9.5). To explain these geochemical features, we propose that the magmas formed through pooling of fractional melts of spinel peridotite. The rar...

The only known post-Archaean komatiites are found on Gorgona, INTRODUCTION a small island off the Colombian coast that forms part of the Surprisingly little is known about the internal structure Caribbean oceanic plateau. Mafic and ultramafic intrusions are and magmatic evolution of oceanic plateaux. This is due located in the interior of the island. To establish the relationship in part to the relative inaccessibility of type examples between intrusive and extrusive phases of ultramafic magmatism, such as Ontong Java and other plateaux in the Pacific, and to help understand how an oceanic plateau is constructed, we which are located far from continents, in deep waters, undertook the first petrological and geochemical study of the intrusive and often beneath a deep layer of sediments. Samples rocks. Rare earth element patterns in gabbros range from almost flat obtained by dredging or drilling the submerged portions, to moderately depleted; in dunites and wehrlites, the depletion is or through field work on the few sections exposed on more pronounced. These patterns fall midway in the range measured islands around the Ontong Java plateau consist prein Gorgona volcanics, whose compositions vary from slightly enriched dominantly of tholeiitic basalt with relatively uniform, to extremely depleted. Nd isotope compositions indicate two distinct magmatically evolved compositions (Mahoney et al., 1993; mantle sources, one highly depleted, the other less depleted. MgO Neal et al., 1997). These compositions are far removed contents of parental liquids are estimated from olivine compositions from those of the primary picritic magmas that formed at 20–25% in ultramafic lavas, and 12–13% in the intrusives. through partial melting of the mantle source at subPetrographic observations and similarities in trace-element contents lithosphere depths (Cox, 1980). The generation of basalt indicate that the two magma types are comagmatic, related through from picrite involves crystallization of large volumes olivine fractionation. Modelling of major and trace elements indicates of olivine, which presumably accumulated in magma that the primary ultramafic magmas formed by advanced critical chambers at deeper levels in the plateau. It is commonly melting at high pressure in a rising mantle plume. The plumbing assumed that these chambers are located at the base of system that fed the Gorgona plateau was complex, being characterized the crust where magma would pool as it passes from by a series of magma chambers at different crustal levels. Mantlederived ultramafic liquids either travelled directly to the surface to dense mantle peridotite to less dense basaltic rocks. This erupt as komatiite flows, or were trapped in magma chambers where assumption is supported by geophysical data—layers with they differentiated into basaltic liquid and mafic to ultramafic high seismic velocities in the lower parts of oceanic cumulates. Gorgona gabbros and peridotites formed in shallow-level plateaux are commonly equated with olivine-rich cuexamples of these intrusions. mulates (Coffin & Eldholm, 1994; Leroy & Mauffret, 1996; Mauffret & Leroy, 1997)—and from numerical modelling. Farnetani et al. (1996) have shown that the

The Foundation hotspot-Pacific-Antarctic Ridge (PAR) system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it. The morphology, crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the PAR show evidence of the influence of the hotspot, that is presently located roughly 35 km west of the spreading ridge axis. Along-axis variation in the Mantle Bouguer anomaly is about 28 mGal, corresponding to a crustal thickening of 1.5 km where the hotspot is nearer to the PAR. Anomalous ridge elevation is 650 m and the along-axis width of the chemical anomaly is 200 km. A comparison of these axial parameters with those derived for other ridge-hotspot systems, suggests that the amount of plume material reaching the ridge axis is smaller for the Foundation-PAR system. This implies a weaker connection between the plume and the ridge. Cumulative effects of a fast spreading rate and of a fast ridge-hotspot relative motion can be responsible for this weak plume-ridge flow. The flow from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot.

We studied the petrography, mineralogy and geochemistry of picrites from three different regions: the island of Curacao which forms part of a Cretaceous oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which formed during the rifting that created the Atlantic ocean. Using the compositions of olivine phenocrysts and relations between MgO and FeO, Al2O3 and Ni, we estimated compositions of parental liquids and the proportion of accumulated olivine in each rock. Picrites from Curacao formed mainly from a liquid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the form of phenocrysts. A small proportion of more forsterite-rich olivine grains are xenocrysts from a more magnesian source. Picrites from Iceland formed from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblage. Picrites from the Greenland volcanic margin formed from a liquid that was distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-rock composition is roughly equivalent to the liquid composition. The picrites from the three areas formed under different conditions and through contrasting melting processes. The Curacao picrites derive from pooled liquids formed through moderate degrees of melting at moderate depths beneath a relatively old and thick oceanic lithosphere. Iceland picrites, on the other hand, formed through advanced fractional melting of mantle that ascended almost to the base of the crust at the mid-ocean spreading center. An unusual combination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively low degrees of melting at greater depths in the mantle.

The study of different magmatic provinces between the Resolution fracture zone (33°S-131°W) and the Pacific-Antarctic Ridge (PAR) axis (37°S-111°W) suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20-25 Ma and 0-5 Ma ago. There is evidence of this process from the changes in composition observed in the lavas erupted near 400-300 km between the present day PAR axis (37°S-111'W) and the eastern tip of the Foundation Seamount (FS) hotspot near 36°20'S-114°W where the last alkali enriched volcanics [K/Ti>0.30, Zr/Y>6 and (Ce/Yb)N>4] have erupted. This transitional province between the PAR and the FS consists of volcanic cones built on several volcanic ridges ( 20 Ma) also built on volcanic ridges are identified west (>1200 km from the PAR axis) of a Failed Rift Propagator (FRP) forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made of alkali basalts were built during the initiation of the FS hotspot 20-23 Ma ago. The interaction and the influence (thermal) of mantle plume magmatism with the ancient spreading ridge of the Farallon-Pacific plates was responsible for the eruptions of the T-MORBs and andesitic lavas. This situation is comparable to that presently observed on the PAR axis where silicic lavas are also erupted in association with T-MORBs.

An interdisciplinary study was conducted to evaluate the relationship between geological and paleontological parameters and the bacterial and archaeal community structure of two contrasted subseafloor sites in the Western Mediterranean Sea (Ligurian Sea and Gulf of Lions). Since both depositional environments were well-documented in this area, large data-sets were available and allowed to calibrate the investigated cores with several reference and dated cores previously collected in the same area, and notably correlated to Quaternary climate variations. Molecular-based fingerprints showed that the Ligurian Sea sediments, characterized by an heterolithic facies with numerous turbidites from a deep-sea levee, were unexpectedly dominated by Betaproteobacteria (more than 70 %), at the base of the core mainly below five meters in the sediment. Analysis of relative betaproteobacterial abundances and turbidites frequency indicated that the microbial diversity was controlled by the important climatic changes occurring during the last 20 ka. This result was supported by statistical direct multivariate canonical correspondence analyses (CCA). In contrast, the Gulf of Lions core, characterized by a homogeneous lithology of upper-slope environment, was dominated by the Bacteroidetes group and in a lesser extent, by the Betaproteobacteria group. At both sites, the dominance of Betaproteobacteria coincided with increased terrestrial inputs, as confirmed by the geochemical measurements (Si, Sr, Ti and Ca). In the Gulf of Lions, geochemical parameters were also found to drive microbial community composition. Taken together, our data suggest that the palaeoenvironmental history of erosion and deposition recorded in the Western-Mediterranean Sea sediments has left its imprint on the structure/composition of the microbial communities during the late Quaternary.

International audience; Flood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19th century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170 ma.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (137Cs and 241Am), historic lead contamination and the correlation of major flood- and earthquaketriggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270 years (AD 1740-AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20th century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments.

Two well-dated ca Holocene-long sedimentary sequences from deepest parts of Lake Bourget provide new insights onto the evolution of erosion patterns at a regional scale in NW European Alps. The combination of high resolution geochemistry - XRF core scanning, calibrated by 150 punctual measurements - and isotope geochemistry (epsilon Nd) of the terrigenous fraction permitted the reconstruction not only of the intensity, but also the type (physical erosion vs. chemical weathering) and the location (Prealpine massifs vs. High Crystalline massifs) of dominant erosion processes. Those data point the persistency of weak erosion fluxes from 9600 to 5500 cal. BP due both to a dry climate and the growing sheltering effect of soils that rapidly progressed between 9600 and 8000 cal. BR Soils then reached a steady state before being destabilised around 4400 cal. BP, probably in response to human impact. The human impact then reached a sufficient intensity to change erosion patterns at a regional scale, but did not result in a significant increase of the regional terrigenous flux. The following enhancement of erosion processes occurred around 2700 cal. BP. It was first paced by changing climatic conditions, but probably reinforced by human impact during Late Iron Age - Antiquity period. Over the long-term trend, the Lake Bourget record pinpoints an evolution of paleohydrological conditions in the Alps dominated by dry conditions from 9500 to 4400 cal. BP and a subsequent drift toward wetter conditions that culminated during the so-called Little Ice Age (ca 1350-1900 AD). In such a context the current dry conditions in European Alps appear out-of-trend. At high resolution, 17 periods of enhanced hydrological activity highlight the rapid climatic changes that are typical of the Holocene. (C) 2012 Elsevier Ltd. All rights reserved.

In this study, we undertook a reconnaissance study of sediments provenance in the Gulf of Lions focusing over the last 16 ka. We used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing. Sediments samples were selected both onshore and offshore from the western, eastern, and central part of the Gulf of Lions. We analyzed bulk sediments, coarse and fine silt, and clay fractions. Elemental and Nd isotope compositions appeared to differ from one grain size fraction to another one. These are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the Nd isotope compositions. Our results indicate that sediments in the Gulf of Lions mainly originated from the Rhone River watershed although a contribution of Saharan dust is seen in one sample. Influence of Pyrenean small rivers is minor in these samples. Some Sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions.

International audience; Multiproxy analyses were performed on core MS27PT recovered in hemipelagic sediments deposited on the Nile margin in order to reconstruct Nile River palaeohydrological fluctuations during the last 100,000 years. The strontium and neodymium isotope composition of the terrigenous fraction and the major element distribution reveal large and abrupt changes in source, oscillating between a dominant aeolian Saharan contribution during arid periods and a dominant Nile River contribution during pluvial periods. Iron content shows a strong correlation with strontium and neodymium isotopes. This allows the use of a high-resolution continuous Fe record as a proxy of Blue Nile sediment input over the last 100,000 years. The detailed Fe record, with approximately 10 years resolution during pluvial periods, is consistent with subtropical African records of well-dated lake level fluctuations and thus constitutes a first continuous high resolution record of the East African monsoon regime intensity over Ethiopia.;The detailed Fe record shows the two main known pluvial periods attributed to strengthening of the African monsoon over Ethiopia, the Nabtian period from 14 to 8 ka cal BP and the Saharan period from 98 to 72 ka BR For the first time, the last glacial period (Marine Isotope Stage (MIS) 2,3 and 4) is documented with a continuous record showing large oscillations between high and low East African palaeo-monsoon regimes.;The end of the Nabtian period occurred at 8 ka in core MS27PT, i.e. much earlier than on the East Equatorial African region where it ended around 5.5 ka. We interpret this as evidence that the southward shift of the rain belt occurred 3000 years earlier over the Eastern Ethiopian Highland and propose that the gradual southward migration of the rain belt was associated with highly variable intensity and longer rainy seasons from 8 to 5 ka. During the last glacial period, two wet periods are present around 60-50 ka BP and 38-30 ka BP. These two humid periods are in phase with the rise of atmospheric CH4 concentrations suggesting that wetland tropical African area was one of the sources of the atmospheric CH4 during the MIS 3. During the Last Glacial Maximum and MIS 4, high Saharan aeolian influxes in phase with records of aeolian dust deposited in East Antarctica are documented. This study highlights the importance of reconstructions of monsoon rainfall fluctuation at high temporal resolution to better understand the link between low- and high-latitude climate variability at millennial timescales.

Total dissolution is a critical step in geochemical analysis. Despite the number of published protocols, this issue still draws attention for sediment samples, which are particularly difficult to dissolve due to the common occurrence and high abundance of refractory phases such as zircon. We present tests of different chemical digestion procedures carried out on reference materials (RM) of stream (JSd-1, JSd-2 and JSd-3) and lake (JLk-1 and LKSD-1) sediments from the Geological Survey of Japan (GSJ) and the Canadian Certified Reference Material Programme (CCRPM). We demonstrate that the fusion technique is not appropriate for our studies as not all elements of interest were recovered and blank levels were too high to permit further Sr and Pb isotopic composition measurements. Similarly, conventional HF+HNO3 dissolution methods were not efficient enough for detrital samples. Our preferred method involved using high pressure Teflon (R) vessel bombs in association with HClO4. This protocol ensured a complete dissolution of the powder, as well as a complete recovery of trace elements. Moreover, blank levels were sufficiently low that Sr or Pb isotope compositions could be measured from the same mother solution. We also tested the homogeneity of RM powders by performing tests on various amount of powder.La dissolution totale d'un echantillon est une phase critique pour toute analyse geochimique. Malgre le nombre de protocoles publies, la mise en solution d'echantillons de sediments reste problematique en raison de la presence de phases refractaires comme les zircons. Nous presentons des tests de differentes procedures chimiques realises sur des Materiaux de Reference (RM) de sediments de ruisseau (JSd-1, JSd-2 et JSd-3) et de lac (JLk-1 et LKSD-1) du Service Geologique de Japon (GSJ) et de Canadian Certified Reference Material Programme (CCRPM). Nous montrons que la technique de fusion utilisee n'est pas appropriee nos etudes car tous les elements d'interet ne sont pas recuperes quantitativement et le niveau des blancs est trop eleve pour permettre l'analyse des compositions isotopiques de Sr et de Pb partir des memes solutions. De meme, les techniques de mise en solution conventionnelles par attaque acide HF+HNO3 sur plaque chauffante ne sont pas suffisamment efficaces pour une mise en solution complete de sediments detritiques. Notre methode preferee implique l'utilisation de bombes en teflon et de HClO4. Ce protocole permet la mise en solution totale des echantillons et la recuperation de tous les elements trace interessants. De plus, les blancs de chimie sont suffisamment bas pour que les compositions isotopiques, e.g., Sr, Pb puissent etre mesurees partir du meme aliquot. Par ailleurs, nous avons aussi teste l'homogeneite des RM utilises en appliquant notre protocole d'attaque sur des quantites de poudre variables., La dissolution totale d'un échantillon est une phase critique pour toute analyse géochimique. Malgré le nombre de protocoles publiés, la mise en solution d'échantillons de sédiments reste problématique en raison de la présence de phases réfractaires comme les zircons. Nous présentons des tests de différentes procédures chimiques réalisés sur des Matériaux de Référence (RM) de sédiments de ruisseau (JSd-1, JSd-2 et JSd-3) et de lac (JLk-1 et LKSD-1) du Service Geologique de Japon (GSJ) et de Canadian Certified Reference Material Programme (CCRPM). Nous montrons que la technique de fusion utilisée n'est pas appropriée à nos études car tous les éléments d'intérêt ne sont pas récupérés quantitativement et le niveau des blancs est trop élevé pour permettre l'analyse des compositions isotopiques de Sr et de Pb à partir des mêmes solutions. De même, les techniques de mise en solution conventionnelles par attaque acide HF+HNO3 sur plaque chauffante ne sont pas suffisamment efficaces pour une mise en solution complète de sédiments détritiques. Notre méthode préférée implique l'utilisation de bombes en téflon et de HClO4. Ce protocole permet la mise en solution totale des échantillons et la récupération de tous les éléments trace intéressants. De plus, les blancs de chimie sont suffisamment bas pour que les compositions isotopiques, e.g., Sr, Pb puissent être mesurées à partir du même aliquot. Par ailleurs, nous avons aussi testé l'homogénéité des RM utilisés en appliquant notre protocole d'attaque sur des quantités de poudre variables.

We present a revised method for the determination of concentrations of rare earth (REE) and other trace elements (Y, Sc, Zr, Ba, Hf, Th) in geological samples. Our analytical procedure involves sample digestion using alkaline fusion (NaOH-Na2O2) after addition of a Tm spike, co-precipitation on iron hydroxides, and measurement by sector field-inductively coupled plasma-mass spectrometry (SF-ICP-MS). The procedure was tested successfully for various rock types (i.e., basalt, ultramafic rock, sediment, soil, granite), including rocks with low trace element abundances (sub ng g(-1)). Results obtained for a series of nine geological reference materials (BIR-1, BCR-2, UB-N, JP-1, AC-E, MA-N, MAG-1, GSMS-2, GSS-4) are in reasonable agreement with published working values.Nous prEsentons une mEthode rEvisEe pour la dEtermination des teneurs en Terres Rares et autres ElEments traces (Y, Sc, Zr, Ba, Hf, Th) dans les Echantillons gEologiques. Notre procEdure analytique implique la digestion des Echantillons par fusion alcaline (NaOH-Na2O2) apres ajout d'un spike de Tm, la co-prEcipitation avec des oxydes de Fer, et la mesure par spectromEtrie de masse a secteur magnEtique couplEe a une source a plasma induit (SF-ICP-MS). La procEdure a EtE testEe avec succes pour plusieurs types de roches (basalte, roche ultramafique, sEdiment, sol, granite), y compris des roches caractErisEes par de faibles teneurs en ElEments traces (sub ng g(-1)). Les rEsultats obtenus pour une sErie de neuf matEriaux de rEfErences gEologiques (BIR-1, BCR-2, UB-N, JP-1, AC-E, MA-N, MAG-1, GSMS-2, GSS-4) sont en accord avec les valeurs de travail publiEes dans la littErature.

Hole 1205A was drilled on Nintoku Seamount, which lies in the midportion of the Emperor Seamount Chain. This seamount was emergent ∼56 Myr ago but was submerged by 54 Ma, so the lavas have endured weathering in both subaerial and submarine environments. We have studied the petrology, mineralogy, and geochemistry of intercalated altered basalts, breccias, and soil samples recovered at Hole 1205A to quantify the chemical exchanges between the seamount and seawater and/or meteoric fluids. The secondary mineralogy is relatively uniform throughout the section and comprises smectite, Fe-oxyhydroxides, iddingsite, and Ca-carbonates. Soils are composed of variably altered basaltic clasts in a matrix of kaolinite, smectite, and vermiculite with minor goethite, hematite, and magnetite. Throughout the basement section, altered basalts, breccias, and soils are depleted in Si, Mg, Ca, Na, Sr, Rb, and Ba and enriched in Fe. Fe3+/FeT (up to ∼1), δ18O (up to ∼+20‰), and 87Sr/86Sr ratios are strongly elevated relative to primary igneous values. Differences in the 87Sr/86Sr ratios define an Upper Alteration Zone with 87Sr/86Sr close to 56 Ma seawater (∼0.7077) from a Lower Alteration Zone where 87Sr/86Sr are less elevated (∼0.704). The Lower Alteration Zone likely reflects interaction with a subaerial oxidizing fluid at low temperature. This zone probably retained most of the original subaerial weathering signature. The Upper Alteration Zone was altered through circulation of large quantities of cold oxidizing seawater that partially overprinted the subaerial weathering chemical characteristics. Altered samples were compared to estimated protolith compositions to calculate chemical gains and losses. Global chemical fluxes are calculated for the entire basement section using different lithological proportions models and different rates of oceanic island emplacement. Although the global construction rate of ocean islands is small compared to igneous accretion at mid-ocean ridges, the magnitude of the chemical changes indicates that ocean islands and seamounts may be a significant contributor to the chemical budget of the oceans.

International audience; In this paper we review the scientific efforts that were led over the last decades to reconstruct geomorphic patterns from continuous alpine lake sediment records. Whereas our results point a growing importance of humans as erosion forcing factors, we will focus here on climate-related processes. Our main dataset is made of a regional approach which was led without any a priori regarding erosion forcing factors. We hence integrated a set of sediment sequences from various environment along an altitudinal gradient from 200 up to 2400m asl in Northern French Alps. Altogether our data point climate change as one of the main factor of erosion variability. In particular, the last two cold spells that occurred during the early middle age (Dark Age) and between the 14th and the 20th century AD (Little Ice Age) appear to be outstanding compared to any other periods of enhanced erosion along the Holocene. The climatic forcing of those erosion phases is supported by an increase in the contribution of glacier-eroded material at a regional scale. At local scales, our data also point the growing importance, since at least the mid Bronze Age (ca. 3500 cal. BP) of human activities as a major erosion factor. This influence peaked during the late Iron Age and Antiquity periods (200 BC - 400 AD) when we record a regional generalised period of enhanced erosion in response to the development of pasturing activities. Thanks to provenance and weathering markers, we evidenced a strong relationship between the changes in ecosystems, soil development and erosion patterns. We hence showed the vegetal colonisation of bared soil led to a period of intense weathering while new soils were under formation between 11,000 and 8,000 cal. BP. Soils then knew an optimum until the onset of the Neoglacial at ca. 4,500 cal. BP prior to decline under both climate and human pressures. Altogether our data point the complexity of processes that affected the Earth critical zone along the Holocene. However, we highlight the interest of leading spatialized paleo-investigation in order to reconstruct those dynamics through and thus better understand the processes in play in critical zone dynamics over long time periods.

Summary Although contourites and mixed turbidite/contourite depositional systems are significant to the oil industry, they are still relatively poorly known by this sector, and there is no present-day accepted sedimentological model for sandy contourites. Our goal here is to better understand the possibility of occurrence of contouritic sands susceptible to form potential reservoirs for hydrocarbons, and therefore possible new plays. After the Gulf of Cadiz ( Hernandez-Molina et al., 2013 ) and the Brazilian margin ( Mutti et al., 2014 ), the discovery of coarse granulometry contourite facies in Plio-pleistocene deposits offshore Mozambique is a great step toward the proof of concept of reservoir potential for these objects. The results presented derive from the Pamela (PAssive Margin Exploration Laboratories) project, led by Ifremer and TOTAL in collaboration with Universites de Bretagne Occidentale, Rennes-1, P&M Curie, CNRS and IFPEN. First class outcome, the PAMELA-MOZ-3 cruise has revealed occurrence of coarse-grained contourite facies deposited in a condensed layer wedging on a contourite terrace. Sandy contourite facies (siliciclastic or carbonate) is now proved to occur where favorable conditions gather. Consolidated with state-of-the art techniques of subsurface data interpretation, the new model integrating geometrical and amplitude characters should give keys to help interpreters predict the reservoir potential of contourites.

International audience