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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.

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.

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; 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.

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.

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.

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.

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.

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