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Calcareous green algae (CGA) are an artificially united but highly heterogeneous group of large unicellular benthic algae with one character in common: all have the capability of secreting a calcareous coating on the outer side of the cytoplasmic envelope. Today, they are a major contributor to carbonate sedimentation at all scales from clay-sized particles (aragonitic needles) to coarser grains (sand and gravel) and even to plurimetric sedimentary structures. There are fossil analogues to the features listed above. Phycologists know best Halimeda, Penicillus, Acetabularia and Cymopolia; micropaleontologists and carbonate sedimentologists are most knowledgeable about Acicularia, Clypeina, Neoteutloporella, Salpingoporella, Anthracoporella, Boueina, and Eugonophyllum. The CaCO 3 precipitated to form the coating is generally aragonite (the orthorhombic form) but there are short periods in the geologic record during which its calcite variant (the rhombohedric form) existed contemporaneously in discrete species. Recent studies on Halimeda have shown that some of the Bryopsidales have the capability to calcify strongly in the lower portion of the euphotic zone (where respiration becomes more important than photosynthesis in the process of mineralization) and to produce positive sedimentary reliefs (bioherms) in situ below the fair-weather wave base. Previous models of paleoenvironments considered the presence of Dasycladales or Bryopsidales to indicate shallow-water, that is the upper euphotic zone (from the sea surface down to -25 m), and predominantly low-energy, protected, lagoonal environments. When the algal remains were found in grain-supported facies, they were taken to have been subjected to dynamic transport and therefore indicative of high-energy environments of deposition. The new deeper-water finds have changed interpretations of the environments ascribed fossil algae. A current conception is that ancestral inarticulated Bryopsidales could have grown at depths as great as -120 m (near the base of the lower euphotic zone). This preliminary review concludes with suggestions about fields for continuing investigations. © Publications Scientifiques du Muséum national d'Histoire naturelle, Paris. Source

Pringle E.A.,Washington University in St. Louis | Savage P.S.,Washington University in St. Louis | Badro J.,CNRS Paris Institute of Global Physics | Barrat J.-A.,CNRS Oceanic Domains Laboratory | Moynier F.,Washington University in St. Louis
Earth and Planetary Science Letters | Year: 2013

Core formation is the main differentiation event in the history of a planet. However, the chemical composition of planetary cores and the physicochemical conditions prevailing during core formation remain poorly understood. The asteroid 4-Vesta is the smallest extant planetary body known to have differentiated a metallic core. Howardite, Eucrite, Diogenite (HED) meteorites, which are thought to sample 4-Vesta, provide us with an opportunity to study core formation in planetary embryos.Partitioning of elements between the core and mantle of a planet fractionates their isotopes according to formation conditions. One such element, silicon, shows large isotopic fractionation between metal and silicate, and its partitioning into a metallic core is only possible under very distinctive conditions of pressure, oxygen fugacity and temperature. Therefore, the silicon isotope system is a powerful tracer with which to study core formation in planetary bodies. Here we show through high-precision measurement of Si stable isotopes that HED meteorites are significantly enriched in the heavier isotopes compared to chondrites. This is consistent with the core of 4-Vesta containing at least 1. wt% of Si, which in turn suggests that 4-Vesta's differentiation occurred under more reducing conditions (δIW~-4) than those previously suggested from analysis of the distribution of moderately siderophile elements in HEDs. © 2013 Elsevier B.V. Source

Rooney T.O.,Michigan State University | Mohr P.,Tonagharraun | Dosso L.,CNRS Oceanic Domains Laboratory | Hall C.,University of Michigan
Geochimica et Cosmochimica Acta | Year: 2013

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ∼31Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31Ma and 22Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22Ma to 12Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension. © 2012 Elsevier Ltd. Source

Booth A.M.,California Institute of Technology | Lamb M.P.,California Institute of Technology | Avouac J.-P.,California Institute of Technology | Delacourt C.,CNRS Oceanic Domains Laboratory
Geophysical Research Letters | Year: 2013

Quantifying the velocity, volume, and rheology of deep, slow-moving landslides is essential for hazard prediction and understanding landscape evolution, but existing field-based methods are difficult or impossible to implement at remote sites. Here we present a novel and widely applicable method for constraining landslide 3-D deformation and thickness by inverting surface change data from repeat stereo imagery. Our analysis of La Clapière, an ~1 km2 bedrock landslide, reveals a concave-up failure surface with considerable roughness over length scales of tens of meters. Calibrating the thickness model with independent, local thickness measurements, we find a maximum thickness of 163 m and a rheology consistent with distributed deformation of the highly fractured landslide material, rather than sliding of an intact, rigid block. The technique is generally applicable to any mass movements that can be monitored by active or historic remote sensing. Key Points We invert landslide velocity and elevation change data for the 3D slip surface La Clapiere landslide has a maximum thickness of 163m and volume of 38million m3 Distributed deformation, rather than block sliding, best fits observations. © 2013. American Geophysical Union. All Rights Reserved. Source

Moynier F.,Washington University in St. Louis | Agranier A.,CNRS Oceanic Domains Laboratory | Hezel D.C.,Natural History Museum in London | Bouvier A.,Arizona State University
Earth and Planetary Science Letters | Year: 2010

High-precision stable Sr isotopic variations (88Sr/86Sr) are reported in a variety of terrestrial samples, martian and lunar meteorites, HED, undifferentiated primitive meteorites, chondrules and refractory inclusions. Almost all the whole-rock samples are isotopically indistinguishable at a 50parts per million (ppm) level. The exceptions are CV and CO chondrites which are isotopically light and for which we believe that their isotopic composition is controlled by the proportion of refractory material. Five separated chondrules and one refractory inclusion from Allende are isotopically light, with δ88/86Sr fractionations up to δ1.73‰, whereas the matrix is enriched in the heavy isotopes (δ88/86Sr=+0.66‰). The depletion in heavy isotopes observed in chondrules and refractory inclusions could be attributed to the condensation of a material already depleted in Sr, however, in that case more than 60% of the original material would be unaccounted. We propose instead that isotopic fractionation by electromagnetic sorting of ionized heavy Sr from neutral Sr in the early solar system for the origin of the fractionation observed in refractory inclusions and redistribution of Sr by aqueous alteration for the origin of the fractionation observed in chondrules and matrix. We conclude that CV and CO chondrites are not the primary building blocks for Earth and Mars. © 2010 Elsevier B.V. Source

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