Presumed magnetic biosignatures observed in magnetite derived from abiotic reductive alteration of nanogoethite     

Jessica L. Till  Yohan Guyodo  France Lagroix  Guillaume Morin  Nicolas Menguy  Georges Ona-Nguema 《Comptes Rendus Geoscience》2017,349(2):63-70

The oriented chains of nanoscale Fe-oxide particles produced by magnetotactic bacteria are a striking example of biomineralization. Several distinguishing features of magnetite particles that comprise bacterial magnetosomes have been proposed to collectively constitute a biosignature of magnetotactic bacteria (Thomas-Keprta et al., 2001). These features include high crystallinity, chemical purity, a single-domain magnetic structure, well-defined crystal morphology, and arrangement of particles in chain structures. Here, we show that magnetite derived from the inorganic breakdown of nanocrystalline goethite exhibits magnetic properties and morphologies remarkably similar to those of biogenic magnetite from magnetosomes. During heating in reducing conditions, oriented nanogoethite aggregates undergo dehydroxylation and transform into stoichiometric magnetite. We demonstrate that highly crystalline single-domain magnetite with euhedral grain morphologies produced abiogenically from goethite meets several of the biogenicity criteria commonly used for the identi?cation of magnetofossils. Furthermore, the suboxic conditions necessary for magnetofossil preservation in sediments are conducive to the reductive alteration of nanogoethite, as well as the preservation of detrital magnetite originally formed from goethite. The findings of this study have potential implications for the identification of biogenic magnetite, particularly in older sediments where diagenesis commonly disrupts the chain structure of magnetosomes. Our results indicate that isolated magnetofossils cannot be positively distinguished from inorganic magnetite on the basis of their magnetic properties and morphology, and that intact chain structures remain the only reliable distinguishing feature of fossil magnetosomes.  相似文献   

Biotite Reference Materials for Secondary Ion Mass Spectrometry 18O/16O Measurements          下载免费PDF全文

Guillaume Siron  Lukas Baumgartner  Anne‐Sophie Bouvier  Benita Putlitz  Torsten Vennemann 《Geostandards and Geoanalytical Research》2017,41(2):243-253

Five new biotite reference materials were calibrated at the SwissSIMS laboratory (University of Lausanne) for oxygen isotope determination by secondary ion mass spectrometry (SIMS) and are available to the scientific community. The oxygen isotope composition of the biotites, UNIL_B1 to B5, was determined by laser‐heating fluorination to be 11.4 ± 0.11‰, 8.6 ± 0.15‰, 6.1 ± 0.04‰, 7.1 ± 0.05‰ and 7.6 ± 0.04‰, respectively. SIMS analyses on spots smaller than 20 μm gave a measurement repeatability of 0.3‰ (2 standard deviation, 2s). The matrix effect due to solid solution in natural biotite could be expressed as a linear function of X_Mg and X_F for biotite. No effect was found for different crystallographic orientations. SIMS analysis allows the oxygen isotope composition of biotite to be measured with a measurement uncertainty of 0.3–0.4‰ (2s) for biotites with similar major element compositions. A measurement uncertainty of 0.5‰ (2s) is realistic when F poor biotites (lower than 0.2% m/m oxides) within the compositional range of X_Mg of 0.3–0.9 were compared from different sessions. The linear correlation with F content offers a reasonable working curve for F‐rich biotites, but additional reference materials are needed to confirm the model.  相似文献   

Shallow-water longshore drift-fed submarine fan deposition (Moisie River Delta, Eastern Canada)     

Alexandre Normandeau  Patrick Lajeunesse  Guillaume St-Onge 《Geo-Marine Letters》2013,33(5):391-403

Submarine canyons and associated submarine fans are in some cases located at the end of a littoral cell where they act as conduits for the transfer of eroded terrigenous sediments to the marine environment. Such fans are generally found in deep-water settings at >500 m water depth. Offshore the Moisie River Delta (NW Gulf of St. Lawrence, Eastern Canada), high-resolution multibeam bathymetry and seismic data led to the discovery of an unusually shallow submarine fan (≤60 m) located at the end of a littoral cell. Sediment is transported westward on the shallow coastal shelf, as demonstrated by the downcurrent displacement of oblique nearshore sandbars where the shelf narrows to less than 1 km. The steep slope near the end of the littoral cell is incised by a channel that feeds a submarine fan composed of smaller channels and depositional lobes. According to existing Holocene evolution models for the region, the fan formed within the last 5,000 years. Its evolution is largely due to the transport of sediment by longshore drift. Multibeam echosounder and seismic data also reveal that the gravity-driven accretion of the submarine fan is characterized mainly by two processes, i.e., frequent small-scale, downslope migration of sandwaves on the slope, and more episodic slumping/turbidity-current activity in the deeper part of the fan. This study documents that, besides their common deep-water location, smaller-scale submarine fans can occur also in very shallow water, implying that they could be more frequent than previously thought both in modern environments and in the rock record.  相似文献   

A mass mortality of subtropical intertidal populations of the sea urchin Paracentrotus lividus: analysis of potential links with environmental conditions     

Dominique Girard  Sabrina Clemente  Kilian Toledo-Guedes  Alberto Brito  José Carlos Hernández 《Marine Ecology》2012,33(3):377-385

An outbreak of bald sea urchin disease was detected affecting intertidal populations of the edible temperate species Paracentrotus lividus at its southernmost geographical limit. The mortality event was detected in three sites (Güímar, Palmar and Alcalá) off Tenerife, Canary Islands (Northeastern Atlantic), in October 2003, coinciding with the highest sea surface temperatures (SST) and the lowest wave heights of the year. The prevalence of infection reached 100% but did not affect equally all studied sites or the different size classes. The disease was more prevalent in large sea urchins (>40 mm). The prevalence appeared to be positively correlated with the environmental variable SST and negatively correlated with wave height. In October 2004, only two infected urchins were found (prevalence 1%) at one of the studied sites. However, during 2004 no such extreme values of temperature and wave height were observed. We conclude that high SSTs and low wave heights such as those observed during 2003 are conditions that may trigger outbreaks of urchin disease.  相似文献   

Modeling coastal tsunami hazard from submarine mass failures: effect of slide rheology,experimental validation,and case studies off the US East Coast     

Grilli  Stéphan T.  Shelby  Mike  Kimmoun  Olivier  Dupont  Guillaume  Nicolsky  Dmitry  Ma  Gangfeng  Kirby  James T.  Shi  Fengyan 《Natural Hazards》2017,86(1):353-391

Natural Hazards - We perform numerical simulations to assess how coastal tsunami hazard from submarine mass failures (SMFs) is affected by slide kinematics and rheology. Two types of two-layer SMF...  相似文献   

Tracing sediment redistribution across a break in slope using rare earth elements   总被引：1，自引：0，他引：1  

Katerina Michaelides  Ioana Ibraim  Guillaume Nord  Michel Esteves 《地球表面变化过程与地形》2010,35(5):575-587

Experimentally determined spatial patterns of soil redistribution across a break in slope derived using 10 rare earth element (REE) oxides as sediment tracers are presented. An erosion experiment was conducted using simulated rainfall within a laboratory slope model measuring 2·5 m wide by 6 m long with a gradient of 15° declining to 2°. Soil was tagged with multiple REE and placed in different locations over the slope and at the end of the experiment REE concentrations were measured in samples collected spatially. A new method was developed to quantify the erosion and deposition depths spatially, the relative source contributions to deposited sediment and the sediment transport distances. Particle‐size selectivity over an area of net deposition was also investigated, by combining downslope changes in particle‐size distributions with changes in sediment REE composition within a flow pathway. During the experiment, the surface morphology evolved through upslope propagation of rill headcuts, which gradually incised the different REE‐tagged zones and led to sediment deposition at the break in slope and the development of a fan extending over the shallow slope segment. The spatial patterns in REE concentrations, the derived erosion and deposition depths, the relative source contributions to deposition zones and the sediment transport distances, corroborate the morphological observations and demonstrate the potential of using REE for quantifying sediment transport processes. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview     

Marie Boucher  Guillaume Favreau  Marc Descloitres  Jean-Michel Vouillamoz  Sylvain Massuel  Yahaya Nazoumou  Bernard Cappelaere  Anatoli Legchenko 《Comptes Rendus Geoscience》2009,341(10-11):800-809

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.  相似文献   

A seasonal dipolar eddy near Ras Al Hamra (Sea of Oman)     

Pierre L’Hégaret  Léo Lacour  Xavier Carton  Guillaume Roullet  Rémy Baraille  Stéphanie Corréard 《Ocean Dynamics》2013,63(6):633-659

Trajectories and hydrological data from two Argo floats indicate that warm and salty water at 200–300-m depths was ejected from the coast of Oman, near Ras al Hamra, in spring 2008, 2011, and 2012. This warm and salty water, Persian Gulf Water (PGW), once ejected from the coast, recirculated cyclonically in the western Sea of Oman, but also flowed eastward along the Iranian and Pakistani coasts. There, it was expelled seaward by mesoscale eddies as shown by other float data. Seasonal maps of salinity were computed from all available Argo floats; they showed that, in spring, PGW is present in the middle and north of the Sea of Oman, contrary to fall, when the salinity maxima lie southeast of Ras al Hadd. The ejection of PGW from Ras al Hamra is related here to the influence of a mesoscale dipolar eddy which often appears near this cape in spring. The time-averaged and empirical orthogonal functions of altimetric maps over 11 years for this season confirm the frequent presence and the persistence of this feature. From surface currents and hydrology, deep currents were computed via thermal wind balance, and the associated shear and strain fields were obtained. This deformation field is intense near Ras al Hamra, with an offshore direction. This flow structure associated with the mesoscale dipole explains PGW ejection from the coast. This observation suggests that PGW distribution in the Northern Arabian Sea can be strongly influenced by seasonal mesoscale eddies.  相似文献   

Geodynamic evolution of a forearc rift in the southernmost Mariana Arc     

Julia M. Ribeiro  Robert J. Stern  Fernando Martinez  Osamu Ishizuka  Susan G. Merle  Katherine Kelley  Elizabeth Y. Anthony  Minghua Ren  Yasuhiko Ohara  Mark Reagan  Guillaume Girard  Sherman Bloomer 《Island Arc》2013,22(4):453-476

The southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough backarc basin in late Neogene time, erupting basalts at 3.7–2.7 Ma that are now exposed in the Southeast Mariana Forearc Rift (SEMFR). Today, SEMFR is a broad zone of extension that formed on hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth ≤ 30–50 km). It comprises NW–SE trending subparallel deeps, 3–16 km wide, that can be traced ≥ ∼30 km from the trench almost to the backarc spreading center, the Malaguana‐Gadao Ridge (MGR). While forearcs are usually underlain by serpentinized harzburgites too cold to melt, SEMFR crust is mostly composed of Pliocene, low‐K basaltic to basaltic andesite lavas that are compositionally similar to arc lavas and backarc basin (BAB) lavas, and thus defines a forearc region that recently witnessed abundant igneous activity in the form of seafloor spreading. SEMFR igneous rocks have low Na₈, Ti₈, and Fe₈, consistent with extensive melting, at ∼23 ± 6.6 km depth and 1239 ± 40°C, by adiabatic decompression of depleted asthenospheric mantle metasomatized by slab‐derived fluids. Stretching of pre‐existing forearc lithosphere allowed BAB‐like mantle to flow along the SEMFR and melt, forming new oceanic crust. Melts interacted with pre‐existing forearc lithosphere during ascent. The SEMFR is no longer magmatically active and post‐magmatic tectonic activity dominates the rift.  相似文献   

LES of turbulent surface shear stress and pressure-gradient-driven flow on shallow continental shelves     

Guillaume Martinat  Ying Xu  Chester E. Grosch  Andrés E. Tejada-Martínez 《Ocean Dynamics》2011,61(9):1369-1390

Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom is important) are of great importance because tide- and wind-driven flows on the shelf are drivers of the transfer of momentum, heat, and mass (gas) across the air–sea interface. These turbulent flows play an important role because vertical mixing and current are vectors for the transport of sediment and bioactive material on continental shelves. Understanding the dynamics of this class of flows presents complications because of the presence of a free surface and also because the flow can be driven by a pressure gradient (a tidal current), a stress at the free surface (a wind-driven current), or a combination of both. In addition, the flow can be modified by the presence of a wave field that can induce Langmuir circulation (Langmuir, Science 87:119–123, 1938). Large eddy simulation is used to quantify the effects of pressure gradient and wind shear on the distinctive structures of the turbulent flow. From these computations, an understanding of the physics governing the turbulence of pressure-driven and wind-driven flows, how they can interact in a normal or a tangential direction, and the effect of wave forcing on these flows is obtained.  相似文献