Climate and environmental change at the end of the Holocene Humid Period: A pollen record off Pakistan     

Sarah J. Ivory  Anne-Marie Lézine 《Comptes Rendus Geoscience》2009,341(8-9):760-769

Pollen studies from core SO90-56KA recovered from the Arabian Sea off the Makran Coast (24° 509N, 65° 559E; 695 m depth) show that the end of the Holocene Humid Period, linked to the weakening of Indian monsoon fluxes, took place between 4700 and 4200 BP. Two periods of strong summer monsoon activity are identified between 5400–4200 BP and 2000–1000 BP during which the montane pollen taxa coming from the Himalayas reached the Makran coast due to increased fluvial activity of the Indus River. A contrasting period, dominated by the winter monsoon between 4200 and 2000 BP, is identified based on the presence of pollen taxa from the Baluchistan plateaus. The regional vegetation of the low- and midaltitudes, arid and semiarid, are remarkably stable from 4500 BP to the present.  相似文献   

The crystal and melt structure of spinel and alumina at high temperature: An in-situ XANES study at the Al and Mg K-edge     

Daniel R. Neuville  Dominique de Ligny  Grant S. Henderson  Anne-Marie Flank 《Geochimica et cosmochimica acta》2009,73(11):3410-2801

We present structural information obtained on spinel and alumina at high temperature (298-2400 K) using in-situ XANES at the Mg and Al K-edges. For spinel, ^[4](Al_x,Mg_1−x)^[6](Al_2−x,Mg_x)O_4, with increasing temperature, a substitution of Mg by Al and Al by Mg in their respective sites is observed. This substitution corresponds to an inversion of the Mg and Al sites. There is a significant change in the Al K-edge spectra between crystal and liquid, which can be attributed to a change of the ^[6]Al normally observed in corundum at room temperature, to a mixture of ^[6]Al-^[4]Al in the liquid state. This conclusion is in good agreement with previous ²⁷Al NMR experiments. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.  相似文献   

Upcoming International Events     

Anne-Marie de Grosbois 《Environmental Geology》2007,52(8):1667-1672

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Textural evidence for recent co-seismic circulation of fluids in the Nojima fault zone, Awaji island, Japan     

Anne-Marie Boullier  Koichiro Fujimoto  Tomoyuki Ohtani  Gabriela Roman-Ross  ric Lewin  Hisao Ito  Philippe Pezard  Benoît Ildefonse 《Tectonophysics》2004,378(3-4):165

The Hirabayashi borehole (Awaji Island, Japan) was drilled by the Geological Survey of Japan (GSJ) 1 year after the Hyogo-ken Nanbu (Kobe) earthquake (1995, M_JMA=7.2). This has enabled scientists to study the complete sequence of deformation across the active Nojima fault, from undeformed granodiorite to the fault core. In the fault core, different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation. Above the fault core and within the hanging wall, compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5–50 μm euhedral and zoned siderite and ankerite crystals. These carbonate-filled fractures have been observed within a 5.5-m-wide zone above the fault, but are especially abundant in the vicinity (1 m) of the fault. The log-normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying-rate nucleation accompanied by surface-controlled growth in a fluid saturated with respect to these carbonates. These carbonate-filled fractures are interpreted as the result of co-seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault, with the fast nucleation of carbonates attributed to a sudden fluid or CO₂ partial pressure drop due to fracturing. The fractures cut almost all visible structures at a thin section scale, although in some places, the original idiomorphic shape of carbonates is modified by a pressure-solution mechanism or the carbonate-filled fractures are cut and brecciated by very thin gouge zones; these features are attributed to low and high strain-rate mechanisms, respectively. The composition of the present-day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite, calcite, dolomite and rhodochrosite. Taken together, this suggests that these fractures formed very late in the evolution of the fault zone, and may be induced by co-seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present-day groundwater. They are therefore potentially related to recent earthquake activity (<1.2 Ma) on the Nojima fault.  相似文献   

Early Cretaceous megaspore assemblages from southeastern Australia     

Anne-Marie P. Tosolini  Stephen McLoughlin  Andrew N. Drinnan 《Cretaceous Research》2002,23(6):807

Twenty-nine megaspore species including six new taxa (Bacutriletes otwayensis sp. nov.,Erlansonisporites cerebrus sp. nov., Erlansonisporites decisum sp. nov., Hughesisporites coronatus sp. nov., Hughesisporites dettmanniae sp. nov., and Verrutriletes depressus sp. nov.) are documented from Aptian and Albian strata of the Gippsland and Otway basins, southeastern Australia. Together with six taxa known only from underlying Neocomian strata, these megaspores are used to establish four provisional biozones for the Lower Cretaceous that complement existing biostratigraphic schemes based on miospores and plant macrofossils. Megaspores are best represented in silty floodbasin facies and it is likely that the parent plants predominantly occupied moist understorey to fully aquatic habitats on the floodplain. Megaspores are sparsely represented in most other fluvial facies chiefly due to reworking of floodbasin sediments into higher energy channel and crevasse deposits. The relatively high diversity of lycophyte and fern megaspores contrasts with the scarcity of these plant groups in macrofossil assemblages. The megaspore record suggests that heterosporous cryptogams may have been significantly more prominent in the vegetation of this region than previously suggested. Several megaspores from southeastern Australia are closely comparable to forms from India and Argentina indicating broad similarities between Early Cretaceous heterosporous fern and lycophyte communities across Gondwana. These similarities also suggest that megaspores may be useful for inter-continental biostratigraphic correlation.  相似文献   

Sensitivity of UT1 determined by single-baseline VLBI to atmospheric delay model, terrestrial and celestial reference frames     

Jan Hefty  Anne-Marie Gontier 《Journal of Geodesy》1997,71(5):253-261

 At the present time, the daily VLBI observations on the Westford-Wettzell baseline is the only continually running VLBI project for studies of high-frequency Earth rotation variations. An analysis of this experiment with regard to the potential errors in the atmospheric delay model and in adopted celestial and terrestrial reference frames is presented in the paper. A new VLBI geometric delay model is applied and an algorithm for global adjustment for this specific single-baseline VLBI developed. The results over three years show discrepancies at the milliarcsecond level between the daily observations and the adopted atmospheric model as well as the combined celestial reference frame. A significant number of these discrepancies are removed by the global adjustment. Received: 19 August 1996; Accepted: 13 September 1996  相似文献   

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays     

Norbert Clauer  Michel Hoffert  Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.  相似文献   

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry   总被引：9，自引：1，他引：9  

Françoise Chalot-Prat  Anne-Marie Boullier 《Contributions to Mineralogy and Petrology》1997,129(4):284-307

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997  相似文献   

Resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing,Adirondack Mountains,New York     

Ian Cartwright  John W. Valley  Anne-Marie Hazelwood 《Contributions to Mineralogy and Petrology》1993,113(2):208-225

The petrography, petrology, and oxygenisotope geochemistry of granulite-facies granitic and syenitic orthogneisses of the Diana and Stark complexes, Adirondack Mountains, New York, show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable. There is a strong correlation between retrogression and shearing, and the rocks may be divided texturally into: (1) unsheared lithologies that preserve little-retrogressed pyroxene-or hornblendebearing peak-metamorphic mineralogies; and (2) sheared rocks that underwent retrogression, marked by the growth of late biotite, in centimetre-to metre-wide shear zones after the peak of metamorphism. Oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite (Mt-Ilm) and ferrosilite-magnetic-quartz (Fs-Mt-Qtz) equilibria. Mt-Ilm yields logfO_2Mt-Ilm values of-15.9 to-17.6 (0.6 to 1.3 log units below the fayalite-magnetite-quartz buffer, FMQ) and temperatures of 670–745°C that agree with those from other geothermometry and phase equilibria studies. These data suggest that, aside from oxyexsolution of ilmenite from magnetite, the Fe-Ti system underwent only minor resetting during cooling, and the Fe-Ti oxides yield good estimates of peak-metamorphic temperatures and fO₂. In unsheared ilmenite + magnetite + orthopyroxene + quartz assemblages, values of logfO_2Mt-Ilm are lower than logfO_2Fs-Mt-Qtz by an average of 0.6 when the orthopyroxene activity model of Sack and Ghiorso is used. Minor resetting of the Fe-Ti oxides, analytical errors, and errors in the placement of end-member reactions probably account for this relatively small difference in fO₂ values. Whole-rock ¹⁸O values of unsheared Diana and Stark lithologies range from 4.0 to 10.3 reflecting pre-regional metamorphic oxygen-isotope ratios. Peak-metamorphic minerals preserve high-temperature oxygen-isotope fractionations, and, in many samples, the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data. These data suggest that the rocks did not contain an aqueous fluid phase during cooling. The combination of petrologic, isotopic, and textural data also permits a detailed study of shearing and retrogression. Ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks, resulting in logfO_2Mt-Ilm values as low as-24.1 (3.1 log units below FMQ) and Mt-Ilm temperatures that are up to 175°C below regional estimates. Sheared rocks also have higher ¹⁸O values (up to 13.3). During shearing, ¹⁸O values of biotite, K-feldspar, and magnetite reset readily, while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization.This paper is a contribution to IGCP Project 304, Lower Crustal Processes  相似文献   

Oligotrophication from wetland epuration alters the riverine trophic network and carrying capacity for fish     

Christiane Hudon  Antonella Cattaneo  Anne-Marie Tourville Poirier  Philippe Brodeur  Pierre Dumont  Yves Mailhot  Jean-Pierre Amyot  Simon-Pierre Despatie  Yves de Lafontaine 《Aquatic Sciences - Research Across Boundaries》2012,74(3):495-511

Submerged aquatic vegetation (SAV) constitutes a major component of fish habitat, providing support for epiphytes and invertebrates as well as shelter from predators. The effects of wetland epuration from a mesotrophic to a nearly oligotrophic state were examined over a 15?km long reach of the St. Lawrence River under the direct influence of major farmland tributaries. We hypothesized that the nutrient-enriched zone would support a higher biomass of SAV, epiphytes, macroinvertebrates and fish than the nitrogen-deficient epurated zone located downstream of the wetland. Predictions included that the enriched habitat would support a richer fish assemblage, with higher biomass and growth of juvenile yellow perch than found in the epurated zone. Results supported these hypotheses, demonstrating the chain of effects of nutrient reduction on the biomass of SAV (fourfold drop), invertebrate prey (ninefold), small (threefold) and large (1.5-fold) fish between the two zones. In addition to the reduction in SAV biomass, the replacement of filamentous chlorophytes by benthic mats of filamentous cyanobacteria in the epurated zone resulted in a less complex 3-D habitat structure and a low invertebrate availability for fish. Oligotrophication by wetland epuration exerted negative effects on fish habitat quality, food quantity and availability, with an impairment of juvenile perch growth and recruitment. A generalized model of the changes in habitat carrying capacity occurring with epuration (oligotrophication) or eutrophication is presented, with examples of other aquatic systems in which strong linkages between trophic status, SAV, invertebrates and fish productivity were also demonstrated.  相似文献