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81.
Joris Gieskes Chris Mahn Shelley Day Jonathan B. Martin Jens Greinert Tony Rathburn Brian McAdoo 《Chemical Geology》2005,220(3-4):329-345
Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ13C (DIC) values are low (isotopically light), whereas in the Eel River area ( 350–500 m), δ13C (DIC) values are much higher (isotopically heavier). In all cases, the δ13C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ13C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ13C (up to 23‰;) and δ18O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin. 相似文献
82.
Joris de Vente Jean Poesen Paolo Bazzoffi Anton Van Rompaey Gert Verstraeten 《地球表面变化过程与地形》2006,31(8):1017-1034
Predicting sediment yield at the catchment scale is one of the main challenges in geomorphologic research. The application of both physics‐based models and regression models has until now not provided very satisfying results for prediction of sediment yield for medium to large sized catchments (c. >50 km2). The explanation for this lies in a combination of the large data requirements of most models and a lack of knowledge to describe all processes and process interactions at the catchment scale. In particular, point sources of sediment (e.g. gullies, mass movements), connectivity and sediment transport remain difficult to describe in most models. From reservoir sedimentation data of 44 Italian catchments, it appeared that there was a (non‐significant) positive relation between catchment area and sediment yield. This is in contrast to what is generally expected from the theory of decreasing sediment delivery rates with increasing catchment area. Furthermore, this positive relation suggests that processes other than upland erosion are responsible for catchment sediment yield. Here we explore the potential of the Factorial Scoring Model (FSM) and the Pacific Southwest Interagency Committee (PSIAC) model to predict sediment yield, and indicate the most important sediment sources. In these models different factors are used to characterize a drainage basin in terms of sensitivity to erosion and connectivity. In both models an index is calculated that is related to sediment yield. The FSM explained between 36 and 61 per cent of the variation in sediment yield, and the PSIAC model between 57 and 62 per cent, depending on the factors used to characterize the catchments. The FSM model performed best based on a factor to describe gullies, lithology, landslides, catchment shape and vegetation. Topography and catchment area did not explain additional variance. In particular, the addition of the landslide factor resulted in a significantly increased model performance. The FSM and PSIAC model both performed better than a spatially distributed model describing water erosion and sediment transport, which was applied to the same catchments but explained only between 20 and 51 per cent of the variation in sediment yield. Model results confirmed the hypothesis that processes other than upland erosion are probably responsible for sediment yield in the Italian catchments. A promising future development of the models is by the use of detailed spatially distributed data to determine the scores, decrease model subjectivity and provide spatially distributed output. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
83.
T. Arlt T. Armbruster R. Miletich P. Ulmer T. Peters 《Physics and Chemistry of Minerals》1998,26(2):100-106
Single crystals of the garnet Mn2+ 3Mn3+ 2[SiO4]3 and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn3+ 2[GeO4]3 (space group I41/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn3+-O=1.995 (4) Å (octahedron), Mn2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn2+ 3Mn3+ 2[SiO4]3 were assigned based on a calderite Mn2+ 3Fe3+ 2[SiO4]3 model extrapolated from andradite and grossular literature data. 相似文献
84.
Deep-sea mineral dust deposits are a powerful tool allowing to deduct atmospheric flow patterns. Here deep-sea sediments from the eastern tropical Atlantic were used to derive climatic conditions over the African continent where in summer the dust is lifted in the transition zone of the Sahara and the Sahel. The meteorological process causing this aeolian dust injection are the squall lines, the rain bearing disturbances of the Sahel. They produce a high surface wind velocity area in their northern vicinity. The deep-sea deposits of sediments and their spatial patterns allowed to conclude that the dust raising mechanism connected with the squall lines were similar for present day conditions and for the time of the last glacial maximum. The Saharan trades and the African monsoon had about the same extension in either time slice coupled with an identical travel belt of the squall lines. However, the intensity and the frequency of the squall lines were lower in glacial times.
Zusammenfassung Tiefseesedimente liefern interessante und wertvolle Aufschlüsse über atmosphärische Strömungsbedingungen. Hier werden die Sedimente aus dem Gebiet des östlichen tropischen Nordatlantiks verwendet, um klimatische Angaben über die Strömungsverhältnisse über dem afrikanischen Kontinent zu gewinnen, da die Quelle des Staubes auf dem Kontinent im Übergangsbereich zwischen der Sahara und dem Sahel zu suchen ist. Die meteorologische Ursache des systematischen Staubeintrages in die Atmosphäre sind die regenbringenden sommerlichen Störungen des Sahel («>squall lines»<), die in ihrem nördlichen Randbereich ein Gebiet hoher Windgeschwindigkeit am Boden aufweisen, so daß es hier regelmäßig zu Staubaufwirbelungen kommt. Mit Hilfe der räumlichen und zeitlichen Verteilung der Tiefseesedimente konnte schon früher gezeigt werden, daß während der Periode des letzten Hochglazials die Lage der Grenzzone zwischen Passat und afrikanischem Monsun sich im Vergleich zur Gegenwart nicht verändert hat. Hier wird nun zusätzlich deutlich, daß sowohl die Intensität als auch die Frequenz der «>squall lines»< zum Höhepunkt der Vereisung geringer was als heute.
Résumé Les sédiments amenés par le vent dans les mers profondes fournissent de précieuses indications sur la distribution des courants atmosphériques. Dans cette note, les sédiments de la partie orientale de l'Atlantique tropical sont examinés en vue de déduire les conditions climatiques régnant sur le continent africain, où la source de la poussière est située entre le Sahara et le Sahel. En effet, l'origine météorologique des apports éoliens sont les pertubations pluvieuses (lignes de grains) qui sont accompagnées sur leur bordure nord de vents de grande vitesse au sol. La distribution des sédiments sur le fond océanique a déjà permis de conclure que le mécanisme de transport était, lors de la dernière glaciation (18.000 ans), semblable à ce qu'il est aujourd'hui. Toutefois, l'intensité et la frequence des perturbations étaient plus faibles qu'aujourd'hui.
. , , , . ("squall lines"), , , . , . , , squall lines .相似文献
85.
The transport of water vapour and dust in the Sahel can be assigned to identical atmospheric flow systems, the African Easterly Jet, the Easterly Waves and the Squall Lines. The yield of the rainy season in the Sahel depends on the conditions in the preceding rains of the season and is closely coupled to the surface heat budget both in the monsoonal air in the S and the trades in the N. The heat budget, in particular the evaporation after the passage of a Squall Line, is influenced not only by the total amount of preceding rains but also by the plant cover pumping the water to the surface and smoothing the magnitude of the evaporation in time. This feed-back mechanism may be made responsible for the persistence of the Sahel drought as experienced in recent years. The dust export from the African continent towards the Atlantic can only be so effective, because the Squall Lines and the African Easterly Jet are meshed in a way that produces large amounts of suspended material and lifts it to great heights where the transport velocity is very fast. 相似文献
86.
W. E. Schaaf D. S. Peters D. S. Vaughan L. Coston-Clements C. W. Krouse 《Estuaries and Coasts》1987,10(3):267-275
We have developed a simulation model to estimate pollution effects on economically important estuarine-dependent fish populations. Traditionally, pollution studies have focused upon impacts on individual organisms; however, wise management of marine resources depends upon an understanding of dynamics at the population level. As a required first step toward conducting relevant pollution studies, we have compiled available life history data on eight species (14 spatial-temporal stocks), concentrating on age-specific rates of growth, survival, and fecundity. Leslie matrix models of species population dynamics were used to predict pollutant impacts—mediate through changes in 1st-year survival. On average, and without compensation, these modelled stocks respond to a one-time-50% reduction in first-year survival by taking ten years to equilibrate at 88% of their preimpact abundance. Our synthesis of the data included a search for derived (standardized) population parameters to evaluate differences in susceptibility among and within fish populations to pollutant stresses. We demonstrated that knowledge of a species’ age-specific fecundity pattern provides additional predictive power of its response to pollution perturbation. 相似文献
87.
Alan M. Shiller Joris M. Gieskes N. Brian Price 《Geochimica et cosmochimica acta》1985,49(5):1239-1249
Particulate Fe and Mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes. A study has been made of suspended particulate composition in the Santa Barbara Basin, a shallow near-shore basin off southern California with sub-oxic conditions below sill depth. Observations have revealed several interesting phenomena relating to the geochemistry of Fe and Mn. Most striking is a profound enrichment of particulate Fe in samples from the bottom two hundred meters. These particulates have a constant Fe/P mole ratio of about three and may originate at the sediment-water interface or may be transported to the basin from local marshes. For particulate Mn, enrichments are observed both in the sub-sill waters and near the base of the euphotic zone. A consideration of particle removal rates suggests that the sub-photic zone enrichment has a biogenic origin. In the sub-sill waters, enrichment in Mn is apparently due to the precipitation of dissolved Mn diffusing from the anoxic basin sediments. A simple mass balance suggests that most of the Mn lost from the sediments is transported from the Santa Barbara Basin in dissolved form. 相似文献
88.
Tiltmeter data from an array of three boreholes at the Charlevoix observatory in the Charlevoix seismic region of Que´bec have been analysed for evidence of tectonically related signals. The secular tilt is dominated by water table induced effects which can be substantially removed by linear regression of the water level on the tilt. Short-term (days to months) anomalies are shown to be detectable at the 0.3 μrad to 1 μrad level depending on the depth of the measurement. Long-term changes in the linear drift as small as 0.1 μrad/yr would be detectable in all of the boreholes.Large spatial anomalies in the mean tidal admittance among boreholes preclude its use in refining either models of the regional crustal structure or the adjacent marine tide distribution. Strongly coherent time variations in the tidal admittance among the observations of all the major tidal constituents are shown to be generated by corresponding variations in marine tidal loading in the St. Lawrence estuary. Diurnal band variations are closely correlated with the tide gauge data. The semi-diurnal constituents show a weaker correlation because of the complex spatial pattern within the estuary of time variations in this band. Inspection of the residual admittance variations for the M2 and O1 constituents demonstrates that the thresholds for detecting tectonic tidal tilt anomalies are ±2% and ±5–8%, respectively.The level of earthquake activity in the Charlevoix seismic zone throughout the period of the borehole tiltmeter experiment was sufficiently low that no significant tilt anomalies were expected or were undeniably detected. 相似文献
89.