Pb–Nd isotopic constraints on sedimentary input into the Lesser Antilles arc system

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north–south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions.Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd–Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. [White, W.M., Dupré, B. and Vidal, P., 1985. Isotope and trace element geochemistry of sediments from the Barbados Ridge–Demerara Plain region, Atlantic Ocean. Geochimica et Cosmochimica Acta, 49: 1875–1886.] and confirm their relatively uniform isotopic compositions (i.e., ²⁰⁶Pb/²⁰⁴Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (²⁰⁶Pb/²⁰⁴Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3.The north–south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Δ8/4 values that characterize the Lesser Antilles arc.     

An alternative measure of discharge effectiveness

Since 1960, geomorphic effectiveness has been quantified by analytically combining measures of the frequency and the magnitude of geomorphic processes, most often for the case of sediment yield in rivers via the calculation of effective discharge, Q_eff. Here we leverage the Q_eff analysis approach to develop an alternative metric, the functional‐equivalent discharge Q_fed, which is the discharge that will reproduce the magnitude of the sediment load generated by the full hydrologic distribution. Q_fed is intended to be a more representative metric of hydrologic variability than Q_eff: whereas Q_eff provides a measure of the effectiveness of individual flow rates, Q_fed incorporates a measure of the average effectiveness of the entire flow distribution. We develop an analytical approximation of Q_fed based on relatively few parameters, apply the approach to a set of suspended sediment load data and describe how Q_fed varies broadly with sediment and hydrologic conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

The South Eastern Durance fault permanent network: Preliminary results

The Durance fault area is located in South EasternFrance. This fault system is characterized byhistorical earthquakes (one every century, since 1509,with a magnitude between 5.0 and 5.3). This is theonly fault in France with such a periodic historicalseismic activity. In order to study an active fault ina moderate seismic context, the IPSN (Institute forNuclear Safety and Protection) decided to install apermanent network in 1992, surrounding the fault area.Such a permanent seismic network has been installed inthe french Pyrenees in the Arette area (Gagnepain etal., 1980). While the Arette network covers a regionaffected by several major faults, our network isdevoted to the study of the specific Durance fault.Major historical earthquakes are clearly associatedwith this structure. From an instrumental point ofview, few earthquakes have been recorded since 1962with the national network. Our network shows a smallseismic activity, with the epicenters well alignedalong the fault direction. Moreover, focal mechanismscomputed for two events agree with the regionalmicrostructural studies (Cushing et al., 1997).Finally, a study of the shear wave splittingunderlines preferential S wave polarization for twostations. The H/V ratio on noise microtremors has beencomputed for each station in order to check theirpositions in term of site effects. It does not exhibitany amplification effect (except for two stations).The comparisons with H/V ratio on earthquake datasetshow the important biases we can obtain with realearthquakes.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

Diel cycles of hydrogen peroxide in marine bathing waters in Southern California, USA: In situ surf zone measurements

Hydrogen peroxide is photochemically produced in natural waters. It has been implicated in the oxidative-induced mortality of fecal indicator bacteria (FIB), a microbial water quality measure. To assess levels and cycling of peroxide in beach waters monitored for FIB, diel studies were carried out in surf zone waters in July 2009 at Crystal Cove State Beach, Southern California, USA. Maximum concentrations of 160–200 nM were obtained within 1 h of solar noon. Levels dropped at night to 20–40 nM, consistent with photochemical production from sunlight. Day-time production and night-time dark loss rates averaged 16 ± 3 nM h⁻¹ and 12 ± 4 nM h⁻¹ respectively. Apparent quantum yields averaged 0.07 ± 0.02. Production was largely dominated by sunlight, with some dependence on chromophoric dissolved organic matter (CDOM) levels in waters with high absorption coefficients. Peroxide levels measured here are sufficient to cause oxidative-stress-induced mortality of bacteria, affect FIB diel cycling and impact microbial water quality in marine bathing waters.     

Large‐scale erosion and overbank deposition caused by the July 2013 flood of the Abu River,Yamaguchi City,Japan

This paper reports on the erosion, transport, and deposition processes associated with an overbank deposit formed by the flooding of the Abu River on July 28, 2013, in Yamaguchi City, Japan. At the study site, river flows overtopped the levee revetment upstream of a meander bend cutting it off and flowing back into the main channel downstream. In this sequential process, it deposited large amounts of sediments, ranging from mud to cobbles, on the floodplain. The surface of paddy fields adjacent to a railway line, located at the center of the affected floodplain, was severely eroded by the flood flows. Overbank deposits composed of both upstream finer sediments and eroded coarser terrestrial sediments are laid down in the affected area. Large amounts of pebbles and cobbles originating from the eroded terrestrial area formed a gravelly pile on top of the sand and gravel sediments derived from the river. This finding indicates that sands and gravels were deposited prior to the formation of the gravelly pile, probably before and during peak flood flows. An inverse grading structure is evident in the lower to middle part of these comparatively thick deposits, most likely due to differences in transport pattern between entrained terrestrial gravels and upstream finer sediments.     

Toward a High-Resolution Monitoring of Continental Surface Water Extent and Dynamics,at Global Scale: from GIEMS (Global Inundation Extent from Multi-Satellites) to SWOT (Surface Water Ocean Topography)

Up to now, high-resolution mapping of surface water extent from satellites has only been available for a few regions, over limited time periods. The extension of the temporal and spatial coverage was difficult, due to the limitation of the remote sensing technique [e.g., the interaction of the radiation with vegetation or cloud for visible observations or the temporal sampling with the synthetic aperture radar (SAR)]. The advantages and the limitations of the various satellite techniques are reviewed. The need to have a global and consistent estimate of the water surfaces over long time periods triggered the development of a multi-satellite methodology to obtain consistent surface water all over the globe, regardless of the environments. The Global Inundation Extent from Multi-satellites (GIEMS) combines the complementary strengths of satellite observations from the visible to the microwave, to produce a low-resolution monthly dataset (\(0.25^\circ \,\times \,0.25^\circ\)) of surface water extent and dynamics. Downscaling algorithms are now developed and applied to GIEMS, using high-spatial-resolution information from visible, near-infrared, and synthetic aperture radar (SAR) satellite images, or from digital elevation models. Preliminary products are available down to 500-m spatial resolution. This work bridges the gaps and prepares for the future NASA/CNES Surface Water Ocean Topography (SWOT) mission to be launched in 2020. SWOT will delineate surface water extent estimates and their water storage with an unprecedented spatial resolution and accuracy, thanks to a SAR in an interferometry mode. When available, the SWOT data will be adopted to downscale GIEMS, to produce a long time series of water surfaces at global scale, consistent with the SWOT observations.     

Yam Cultivation on the East Coast of New Caledonia: adaptation of agriculture to social and economic changes

Agriculture represents less than 2 per cent of the GDP of New Caledonia where the economy (and wage-earning activities) is dominated by the service sector and the nickel industry. However, family farming remains strong, especially for the Indigenous Melanesian tribes of the east coast of New Caledonia. Yams (Dioscorea spp.) are still one of the pillars of traditional Kanak society despite the social and economic changes that followed colonisation by France. This study of Kanak yam cultivation on the north-eastern coast of New Caledonia underscores the fact that yam cultivation systems have greatly changed since the 1950s, when they were last described. In addition to customary gift exchanges and subsistence consumption, yams are increasingly being grown for market sales. Five cropping systems, three yam production systems, and three Kanak social activity systems are described and their linkages to each other are explained. There are three levels of Kanak market participation: none, transitional, and fully engaged, and the extent of market participation and associated technical choices defines farmers’ levels of cultural activity and the nature and characteristics of yam cultivation.