The Experts below are selected from a list of 252 Experts worldwide ranked by ideXlab platform
Esther Arning - One of the best experts on this subject based on the ideXlab platform.
-
Microscopy evidence of bacterial microfossils in Phosphorite crusts of the Peruvian shelf: Implications for phosphogenesis mechanisms
Chemical Geology, 2013Co-Authors: Julie Cosmidis, Karim Benzerara, N. Menguy, Esther ArningAbstract:Phosphorites are sedimentary formations enriched in Ca-phosphate minerals. The precipitation of these minerals is thought to be partly mediated by the activity of microorganisms. The vast majority of studies on Phosphorites have focused on a petrological and geochemical characterization of these rocks. However, detailed descriptions are needed at the sub-micrometer scale atwhich crucial information can be retrieved about traces of past ormodern microbial activities. Here, scanning electron microscopy (SEM) analyses of a recent Phosphorite crust from the upwelling-style phosphogenesis area off Peru revealed that it contained a great number of rod-like and coccus-like shaped micrometer-sized (~1.1 μm and 0.5 μm, respectively) objects, referred to as biomorphs. Some of these biomorphs were filled with carbonate fluoroapatite (CFA, a calcium-phosphate phase common in Phosphorites); some were empty; some were surrounded by one or two layers of pyrite. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDXS) analyses were performed on focused ion beam(FIB) milled ultrathin foils to characterize the texture of CFA and pyrite in these biomorphs at the fewnanometer scale. Non-pyritized phosphatic biomorphs were surrounded by a thin (5-15 nmthick) rimappearing as a void on TEM images. Bundles of CFA crystals sharing the same crystallographic orientations (aligned along their c-axis) were found in the interior of some biomorphs. Pyrite formed a thick (~35-115 nm) layer with closely packed crystals surrounding the pyritized biomorphs, whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely. Scanning transmission X-ray microscopy (STXM) analyses performed at the C K-edge provided maps of organic and inorganic carbon in the samples. Inorganic C, mainly present as carbonate groups in the CFA lattice, was homogeneously distributed, whereas organic C was concentrated in the rims of the phosphatic biomorphs. Finally, STXM analyses at the Fe L2,3-edges together with TEMEDXS analyses, revealed that somepyritized biomorphs experienced partial oxidation. The mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra- and extracellularly in laboratory experiments. Similarly, pyritized biomorphs resemble bacteria encrusted by pyrite. We therefore interpret phosphatic and pyritized biomorphs present in the Peruvian Phosphorite crust as microorganisms fossilized near the boundary of zones of sulfate reduction. The implications of these observations are then discussed in the light of the different possible and non-exclusive microbiallydriven phosphogenesis mechanisms that have been proposed in the past: (i) Organic matter mineralization, in particular mediated by iron reducing bacteria and/or sulfate-reducing bacteria (SRB), (ii) reduction of iron- (oxyhydr)oxides by iron-reducing bacteria and/or SRB, and (iii) polyphosphate metabolism in sulfide-oxidizing bacteria, possibly associated with SRB.
-
Microscopy evidence of bacterialmicrofossils in Phosphorite crusts of the Peruvian shelf: Implications for phosphogenesis mechanisms
Chemical Geology, 2013Co-Authors: Julie Cosmidis, Karim Benzerara, N. Menguy, Esther ArningAbstract:Phosphorites are sedimentary formations enriched in Ca-phosphate minerals. The precipitation of these minerals is thought to be partlymediated by the activity of microorganisms. The vast majority of studies on Phosphorites have focused on a petrological and geochemical characterization of these rocks. However, detailed descriptions are needed at the sub-micrometer scale atwhich crucial information can be retrieved about traces of past ormodern microbial activities. Here, scanning electron microscopy (SEM) analyses of a recent Phosphorite crust from the upwelling-style phosphogenesis area off Peru revealed that it contained a great number of rod-like and coccus-like shaped micrometer-sized (~1.1 μm and 0.5 μm, respectively) objects, referred to as biomorphs. Some of these biomorphs were filled with carbonate fluoroapatite (CFA, a calcium-phosphate phase common in Phosphorites); some were empty; some were surrounded by one or two layers of pyrite. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDXS) analyses were performed on focused ion beam(FIB)milled ultrathin foils to characterize the texture of CFA and pyrite in these biomorphs at the fewnanometer scale. Non-pyritized phosphatic biomorphswere surrounded by a thin (5-15 nmthick) rimappearing as a void on TEM images. Bundles of CFA crystals sharing the same crystallographic orientations (aligned along their c-axis) were found in the interior of some biomorphs. Pyrite formed a thick (~35-115 nm) layer with closely packed crystals surrounding the pyritized biomorphs, whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely. Scanning transmission X-ray microscopy (STXM) analyses performed at the C K-edge provided maps of organic and inorganic carbon in the samples. Inorganic C, mainly present as carbonate groups in the CFA lattice, was homogeneously distributed, whereas organic C was concentrated in the rims of the phosphatic biomorphs. Finally, STXM analyses at the Fe L2,3-edges together with TEMEDXS analyses, revealed that some pyritized biomorphs experienced partial oxidation. The mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra- and extracellularly in laboratory experiments. Similarly, pyritized biomorphs resemble bacteria encrusted by pyrite. We therefore interpret phosphatic and pyritized biomorphs present in the Peruvian Phosphorite crust as microorganisms fossilized near the boundary of zones of sulfate reduction. The implications of these observations are then discussed in the light of the different possible and non-exclusive microbiallydriven phosphogenesis mechanisms that have been proposed in the past: (i) Organic matter mineralization, in particular mediated by iron reducing bacteria and/or sulfate-reducing bacteria (SRB), (ii) reduction of iron- (oxyhydr)oxides by iron-reducing bacteria and/or SRB, and (iii) polyphosphate metabolism in sulfide-oxidizing bacteria, possibly associated with SRB
Ibrahim H. Khalifa - One of the best experts on this subject based on the ideXlab platform.
-
Application of multivariate statistical analyses in the interpretation of geochemical behaviour of uranium in phosphatic rocks in the Red Sea, Nile Valley and Western Desert, Egypt
Journal of environmental radioactivity, 2002Co-Authors: Abd El-gabar M. El-arabi, Ibrahim H. KhalifaAbstract:Factor and cluster analyses as well as the Pearson correlation coefficient have been applied to geochemical data obtained from Phosphorite and phosphatic rocks of Duwi Formation exposed at the Red Sea coast. Nile Valley and Western Desert. Sixty-six samples from a total of 71 collected samples were analysed for SiO2, TiO2, Al203, Fe2O3, CaO, MgO, Na2O, K2O, P2O5, Sr, U and Pb by XRF and their mineral constituents were determined by the use of XRD techniques. In addition, the natural radioactivity of the phosphatic samples due to their uranium, thorium and potassium contents was measured by gamma-spectrometry. The uranium content in the phosphate rocks with P2O5 > 15% (average of 106.6 ppm) is higher than in rocks with P2O5 < 15% (average of 35.5 ppm). Uranium distribution is essentially controlled by the variations of P2O5 and CaO, whereas it is not related to changes in SiO2, TiO2, Al2O3, Fe2O3, MgO, Na2O and K2O concentrations.-Factor analysis and the Pearson correlation coefficient revealed that uranium belaves geochemically in different ways in the phosphatic sediments and Phosphorites in the Red Sea, Nile Valley and Western Desert. In the Red Sea and Western Desert Phosphorites, uranium occurs mainly in oxidized U6+ state where it seems to be fixed by the phosphate ion, forming secondary uranium phosphate minerals such as phosphuranylite. In the Nile Valley Phosphorites, ionic substitution of Ca2+ by U4 is the main controlling factor in the concentration of uranium in phosphate rocks. Moreover, fixation of U6- by phosphate ion and adsorption of uranium on phosphate minerals play subordinate roles.
Julie Cosmidis - One of the best experts on this subject based on the ideXlab platform.
-
Microscopy evidence of bacterial microfossils in Phosphorite crusts of the Peruvian shelf: Implications for phosphogenesis mechanisms
Chemical Geology, 2013Co-Authors: Julie Cosmidis, Karim Benzerara, N. Menguy, Esther ArningAbstract:Phosphorites are sedimentary formations enriched in Ca-phosphate minerals. The precipitation of these minerals is thought to be partly mediated by the activity of microorganisms. The vast majority of studies on Phosphorites have focused on a petrological and geochemical characterization of these rocks. However, detailed descriptions are needed at the sub-micrometer scale atwhich crucial information can be retrieved about traces of past ormodern microbial activities. Here, scanning electron microscopy (SEM) analyses of a recent Phosphorite crust from the upwelling-style phosphogenesis area off Peru revealed that it contained a great number of rod-like and coccus-like shaped micrometer-sized (~1.1 μm and 0.5 μm, respectively) objects, referred to as biomorphs. Some of these biomorphs were filled with carbonate fluoroapatite (CFA, a calcium-phosphate phase common in Phosphorites); some were empty; some were surrounded by one or two layers of pyrite. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDXS) analyses were performed on focused ion beam(FIB) milled ultrathin foils to characterize the texture of CFA and pyrite in these biomorphs at the fewnanometer scale. Non-pyritized phosphatic biomorphs were surrounded by a thin (5-15 nmthick) rimappearing as a void on TEM images. Bundles of CFA crystals sharing the same crystallographic orientations (aligned along their c-axis) were found in the interior of some biomorphs. Pyrite formed a thick (~35-115 nm) layer with closely packed crystals surrounding the pyritized biomorphs, whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely. Scanning transmission X-ray microscopy (STXM) analyses performed at the C K-edge provided maps of organic and inorganic carbon in the samples. Inorganic C, mainly present as carbonate groups in the CFA lattice, was homogeneously distributed, whereas organic C was concentrated in the rims of the phosphatic biomorphs. Finally, STXM analyses at the Fe L2,3-edges together with TEMEDXS analyses, revealed that somepyritized biomorphs experienced partial oxidation. The mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra- and extracellularly in laboratory experiments. Similarly, pyritized biomorphs resemble bacteria encrusted by pyrite. We therefore interpret phosphatic and pyritized biomorphs present in the Peruvian Phosphorite crust as microorganisms fossilized near the boundary of zones of sulfate reduction. The implications of these observations are then discussed in the light of the different possible and non-exclusive microbiallydriven phosphogenesis mechanisms that have been proposed in the past: (i) Organic matter mineralization, in particular mediated by iron reducing bacteria and/or sulfate-reducing bacteria (SRB), (ii) reduction of iron- (oxyhydr)oxides by iron-reducing bacteria and/or SRB, and (iii) polyphosphate metabolism in sulfide-oxidizing bacteria, possibly associated with SRB.
-
Microscopy evidence of bacterialmicrofossils in Phosphorite crusts of the Peruvian shelf: Implications for phosphogenesis mechanisms
Chemical Geology, 2013Co-Authors: Julie Cosmidis, Karim Benzerara, N. Menguy, Esther ArningAbstract:Phosphorites are sedimentary formations enriched in Ca-phosphate minerals. The precipitation of these minerals is thought to be partlymediated by the activity of microorganisms. The vast majority of studies on Phosphorites have focused on a petrological and geochemical characterization of these rocks. However, detailed descriptions are needed at the sub-micrometer scale atwhich crucial information can be retrieved about traces of past ormodern microbial activities. Here, scanning electron microscopy (SEM) analyses of a recent Phosphorite crust from the upwelling-style phosphogenesis area off Peru revealed that it contained a great number of rod-like and coccus-like shaped micrometer-sized (~1.1 μm and 0.5 μm, respectively) objects, referred to as biomorphs. Some of these biomorphs were filled with carbonate fluoroapatite (CFA, a calcium-phosphate phase common in Phosphorites); some were empty; some were surrounded by one or two layers of pyrite. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDXS) analyses were performed on focused ion beam(FIB)milled ultrathin foils to characterize the texture of CFA and pyrite in these biomorphs at the fewnanometer scale. Non-pyritized phosphatic biomorphswere surrounded by a thin (5-15 nmthick) rimappearing as a void on TEM images. Bundles of CFA crystals sharing the same crystallographic orientations (aligned along their c-axis) were found in the interior of some biomorphs. Pyrite formed a thick (~35-115 nm) layer with closely packed crystals surrounding the pyritized biomorphs, whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely. Scanning transmission X-ray microscopy (STXM) analyses performed at the C K-edge provided maps of organic and inorganic carbon in the samples. Inorganic C, mainly present as carbonate groups in the CFA lattice, was homogeneously distributed, whereas organic C was concentrated in the rims of the phosphatic biomorphs. Finally, STXM analyses at the Fe L2,3-edges together with TEMEDXS analyses, revealed that some pyritized biomorphs experienced partial oxidation. The mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra- and extracellularly in laboratory experiments. Similarly, pyritized biomorphs resemble bacteria encrusted by pyrite. We therefore interpret phosphatic and pyritized biomorphs present in the Peruvian Phosphorite crust as microorganisms fossilized near the boundary of zones of sulfate reduction. The implications of these observations are then discussed in the light of the different possible and non-exclusive microbiallydriven phosphogenesis mechanisms that have been proposed in the past: (i) Organic matter mineralization, in particular mediated by iron reducing bacteria and/or sulfate-reducing bacteria (SRB), (ii) reduction of iron- (oxyhydr)oxides by iron-reducing bacteria and/or SRB, and (iii) polyphosphate metabolism in sulfide-oxidizing bacteria, possibly associated with SRB
Briggs Deg - One of the best experts on this subject based on the ideXlab platform.
-
Palaeobiology of latest Ediacaran Phosphorites from the upper Khesen Formation, Khuvsgul Group, northern Mongolia
'Informa UK Limited', 2018Co-Authors: Rp Anderson, Fa Macdonald, Ds Jones, Mcmahon S, Briggs DegAbstract:Microfossil assemblages that include large acritarchs with complex processes, known as Doushantuo-Pertatataka-type acritarchs, are recovered from early Ediacaran successions globally. They are commonly found in shale and chert lithologies, but their diversity and palaeobiological significance is greatest when they are phosphatized. The best-known examples are from the Doushantuo Formation, South China, which preserves over 60 taxa including possible embryonic forms which may represent the oldest fossil animals. Fossils have only been recorded in four Ediacaran Phosphorite deposits. Here we report the fifth such occurrence, from Phosphorites of the upper Khesen Formation, Khuvsgul Group, northern Mongolia, where preservation rivals that in the Doushantuo Formation. The assemblage includes the likely cyanobacteria Obruchevella delicata, O. magna, O. parvissima and O. valdaica, as well as various Siphonophycus filaments, the possible alga Archaeophycus yunnanensis, and the Doushantuo-Pertatataka-type acritarchs Appendisphaera grandis, A. fragilis, A. tenuis, Cavaspina basiconica, Variomargosphaeridium gracile and V. aculeiparvum, sp. nov. The Phosphorites also preserve the multicellular embryo-like taxon Megasphaera, which is represented by M. minuscula sp. nov. and potentially by M. puncticulosa. Geological and chemostratigraphical data suggest a latest Ediacaran age for the Khesen assemblage, immediately prior to the Proterozoic–Phanerozoic boundary. Thus, this is the youngest Doushantuo-Pertatataka-type microfossil assemblage yet described. It extends the range of Appendisphaera, Cavaspina, Megasphaera and Variomargosphaeridium upward by tens of millions of years. The assemblage adds to a growing database of Ediacaran fossils and emphasizes the importance of Mongolian strata to understanding the transition from a broadly microbial Proterozoic Eon to a Phanerozoic Eon where macroscopic animals acted as geobiological agents
-
Doushantuo-type microfossils from latest Ediacaran Phosphorites of northern Mongolia
'Geological Society of America', 2017Co-Authors: Rp Anderson, Fa Macdonald, Ds Jones, Mcmahon S, Briggs DegAbstract:Phosphorites of the latest Ediacaran upper Khesen Formation in the Khuvsgul Group of northern Mongolia preserve a newly discovered, three-dimensionally phosphatized Doushantuo-type microfossil assemblage. Eight genera include the second occurrence of the putative multicellular fossil animal embryo Megasphaera outside South China, the Doushantuo-Pertatataka–type acanthomorphic acritarchs Appendisphaera, Cavaspina, and Variomargosphaeridium, and the possible alga Archaeophycus yunnanensis. The assemblage occurs in the lowermost Phosphorite horizon in foreland basin deposits on the Khuvsgul terrane; lithostratigraphic and δ13C correlation with the Zavkhan terrane of southwestern Mongolia establishes a latest Ediacaran age for the fossiliferous Phosphorites. Thus, this is the youngest Doushantuo-type assemblage yet reported. It extends the range of Megasphaera, filling a gap in the record of phosphatized embryo-like forms between the ca. 600 Ma Doushantuo Weng’an biota and Cambrian examples. The Khesen fossil assemblage emphasizes the potential of Mongolian Phosphorites to provide new paleontological data on the Ediacaran-Cambrian transition, and to resolve the phylogenetic debate surrounding Megasphaera embryo-like taxa
Haiyan Guo - One of the best experts on this subject based on the ideXlab platform.
-
origin of the ediacaran weng an and kaiyang Phosphorite deposits in the nanhua basin sw china
Journal of Asian Earth Sciences, 2019Co-Authors: Haiying Yang, Jiafei Xiao, Yong Xia, Zhuojun Xie, Qinping Tan, Haiyan GuoAbstract:Abstract The Weng’an and Kaiyang Phosphorite deposits are located in Central Guizhou, southwestern China, and formed within the Sinian (Ediacaran) Doushantuo Formation. A systematic investigation of the geology, mineralogy, and geochemistry of these Phosphorites was conducted to constrain the redox environment, origin, and genetic mechanisms of Ediacaran Phosphorite. In the lower ore layer (layer A), the phosphate minerals are mainly globular phosphate intraclasts, which are characterized by special sedimentary and reworking textures, including microgranular sedimentary, isopachous cement, shrinkage crack, and spongy texture. The phosphate minerals from the upper layer (layer B) have microbial phosphate components, namely embryo- and algae-like microfossils. These phosphatic microfossils have biological structures, and both consist of interior structure and isopachous wall. The P2O5 concentration of Phosphorites in layer A reach 32.31%, somewhat higher than that in layer B (28.7%), whereas the P content in globular phosphate intraclasts (40.05%) resemble that of the microbial phosphate component (41.19%). Rocks from layer A have “left-inclining” post-Archean Australian shale (PAAS)–normalized rare earth element (REE) distributions, with higher Ce anomalies (Ceanom) ranging from –0.12 to –0.01 (average –0.07). In contrast, rocks from layer B have “hat-shaped” PAAS–normalized REE distributions, with lower Ceanom ranging from –0.32 to –0.23 (average –0.28). These geochemical characteristics suggest that hydrothermal fluids mixed with normal seawater might have contributed to metallogenesis, and the redox environment transitioned from anoxic to oxic from layer A to B. We conclude that the lower Doushantuo globular intraclasts formed by the mechanical reworking of previous phosphatic sediments, which were dominated by the strong mechanical power of seawater. The upper Doushantuo microbial Phosphorite, on the other hand, formed by microbially mediated accretionary growth.