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991.
Understanding the formation of laminated, organic-rich sediments is an essential topic for researchers interested in fossil fuels, biogeochemical cycles, Earth's environmental history and global change. Biologists have very recently demonstrated that some marine phytoplankton blooms actively govern their own sedimentation by the formation of sticky transparent gels that facilitate rapid aggregation, accelerated sinking and efficient export flux. Here we present fossil evidence of unfragmented, low-diversity phytoplankton assemblages preserved as sedimentary laminae and irregular flocs that are attributable to a similar phytoplankton-driven sedimentary mechanism we term ‘self-sedimentation’. The geological evidence suggests that self-sedimentation precludes significant heterotrophic grazing, propels the formation of some conspicuous hemipelagic sedimentary laminae and results in efficient carbon and opal flux to the sediments. We suggest that the self-sedimentation phenomenon may have broad implications for the geological history of biogeochemical cycling, oceanic ecological dynamics, and abrupt atmospheric/environmental change. Broader recognition of the self-sedimentation phenomenon as explicitly defined here is a prerequisite to testing these unconventional hypotheses. 相似文献
992.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
993.
S. Meiners 《GeoJournal》1997,42(2-3):259-302
The historical to post-glacial glacier positions have beenreconstructed from the recent glaciation history of selectedexamples. The chronological sequence of the ice margin positionscan be deduced from the increasing ages of the structures, whichare proportional to the distance from the recent terminus of theglacial tongue, or to the difference between the elevation of theice surface and that of the crest of the lateral moraine. Achronological classification of the glacial events in `phases'can be defined by calculating the snow-line depression aftertaking the relief factor into account. On a small scale, theyoungest glacial positions may vary depending on the localrelief. In both areas under investigation, the post-glacial iceextents are similar in so far as they are not more than 6 km fromthe Recent ice margin, and the historical and Neoglacial phasesare spatially close to one another. A decrease in the elevationof the glacial surface is a more important factor, compared tothe oscillations of the glacial tongue, influencing thedevelopment of the multiphase, lateral moraines that characterisethe Holocene glacial fluctuations. 相似文献
994.
995.
K. S. Voudouris N. J. Lambrakis G. Papatheothorou P. Daskalaki 《Mathematical Geology》1997,29(1):43-59
R- andQ-mode factor analysis is applied to 51 groundwater samples collected from wells drilled in the Plio-Pleistocene aquifers of
NW Achaia, Greece. The purpose ofR- andQ-mode factor analysis application is to identify (i) the regional groundwater flow pattern, and (ii) the deterioration of
groundwater quality. Sixteen hydrogeological parameters are used in order to examine their importance and to provide significant
insight into their correlations. In theR-mode factor analysis, a six-factor model is suggested which can explain more than 77.5% of the total variance. The contribution
of each factor at every site (factor scores) also is computed. Maps are constructed showing the geographical distribution
of the factor scores. From these maps, the high salinity areas are delineated (seawater intrusion, possible appearance of
halite layers) and the areas with elevated contribution of karastic-water are defined. Using theQ-mode correspondence analysis the meaning of the electrical conductivity as the most important variable in groundwater quality
characterization is demonstrated. 相似文献
996.
997.
Detailed study of the Aljustrel volcanic rocks, through petrography, mineral chemistry, whole-rock geochemistry, and previously
published stable isotope data, show that the rocks experienced extreme metasomatism. The metasomatic phenomena presented are
explained accurately by a sea-water convective model. The evolution of the sea-water-derived fluid as it progresses downwards
through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn.
Several lines of evidence (feldspar mineralogy, paragenetic relations between regional and ore-zone alteration, and stable
isotope data) show that the temperature of the hydrothermal circulation increased through time, and that the water-to-rock
ratio evolved from very high to low values. According to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the
Aljustrel Volcanics. This implies, for both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained
in the massive sulphide deposits of the Aljustrel area. With regard to Fe, about 1% was on average removed from the rocks,
producing enough metal to account for all the Fe in the MS deposits and overlying chemical sediments. This study documents
semi-quantitatively the provenance of the main metals in major massive sulphide deposits via sea water leaching of the immediate
host rocks.
Received: 1 February 1997 / Accepted: 30 July 1997 相似文献
Resumen (translated by E. Pascual) Un estudio detallado de las rocas volcánicas de Aljustrel, mediante petrografia, química mineral, geoquímica de roca total y datos de isótopos estables previamente publicados muestra que estas rocas han sufrido metasomatismo en grado extremo. Los fenómenos metasomáticos presentados se explican con precisión mediante un modelo convectivo con agua de mar. Es evidente la evolución del fluido derivado del agua de mar conforme progresa en profundidad a través de las rocas, desde un fluido frío, oxidante y con magnesio a otro reductor, sin magnesio y enriquecido en Si, Fe, Cu y Zn. Varias líneas de evidencia (mineralogía de los feldespatos, relaciones paragenéticas entre la alteración regional y de las zonas mineralizadas, isótopos estables) muestran que la temperatura y la circulación hidrotermal se incrementaron con el tiempo y que la relación agua-roca evolucionó de valores muy altos a bajos. De acuerdo con nuestros datos, de las rocas volcánicas de Aljustrel se han extraído unas 30 ppm de Cu y unas 100 ppm de Zn. Esto implica, para ambos metales, que la misma roca lixiviada ha producido los 2 Mt de Cu y los 6 Mt de Zn contenidos en los depósitos de sulfuros masivos del área de Aljustrel. En cuanto al Fe, una media de alrededor de 1% fue extraída de las rocas, produciendo metal suficiente para explicar todo el Fe de los depósitos de sulfuros masivos y de los sedimentos químicos suprayacentes. Este estudio documenta de forma semicuantitativa la procedencia de los principales metales en depósitos mayores de sulfuros masivos, mediante lixiviado por agua de mar de sus rocas huéspedes inmediatas.
Received: 1 February 1997 / Accepted: 30 July 1997 相似文献
998.
999.
Ore lead and sulphur isotope pattern from the low-temperature veins of the Catalonian Coastal Ranges (NE Spain) 总被引:4,自引:0,他引:4
The lead isotopic composition of galenas from low-temperature veins of the Catalonian Coastal Ranges (NE Spain) displays
a small but distinct range of values: 18.318 ≤206Pb/204Pb ≤ 18.678; 15.678 ≤207Pb/204Pb ≤ 15.767 and 38.534 ≤ 208Pb/204Pb ≤ 39.189. The data define a linear trend in the 208Pb/204Pb versus 206Pb/204Pb diagram that can be interpreted in terms of mixing of lead from different reservoirs. The lead appears to be derived from
a solution that preferentially leached granites of Late-Hercynian age. Sulphur isotopic compositions of the galenas range
from −8.5 to + 7.3‰ but variations in a single deposit are small (1–2‰), pointing to different sources of sulphur. In a δ34S versus 206Pb/204Pb plot no clear correlation trends appear. Lead isotope data from deposits of similar characteristics situated within the
Hercynian orogenic belt as the Linares-La Carolina district (S Spain) and the Inglesiente-Sulcis area (Sardinia) show slight
differences indicating a source area of similar lead isotopic composition. In the CCR and Linares-La Carolina districts,
the main source of lead is related to the Hercynian granites of each area, although small contributions from metasediments
cannot be ruled out. The contribution of lead from metasedimentary rocks is particularly seen in the Sardinian (Hercynian
vein-type) ores.
Received: 8 August 1996 / Accepted: 5 December 1996 相似文献
1000.
Assessing rapid environmental change using geoindicators 总被引:1,自引:1,他引:1
A. R. Berger 《Environmental Geology》1997,32(1):36-44
Geoindicators are high-resolution measures of short-term (<100 years) surface or near-surface changes in earth processes,
and phenomena that are significant for environmental monitoring and assessment. They are based on standard concepts and procedures
and can be used to track changes in fluvial, coastal, desert, mountain, permafrost and other terrestrial areas. Geoindicators
assess both catastrophic events and those that are more gradual, but evident within a human lifespan. Most deal with changes
on the landscape (0.1–10 km) and meso-scales (10–100 km), but some, such as relative sea level and volcanic unrest, have regional
and global dimensions. Some are complex and costly to measure, others are relatively simple and easy to apply. Geoindicators
can also be used to unravel trends over the past few centuries and longer through paleoenvironmental research, thus providing
the important baselines against which human-induced and natural stresses can be better understood. Geoindicators have been
designed by the International Union of Geological Sciences as an aid to state-of-the-environment reporting and long-term ecological
monitoring.
Received: 26 July 1996 / Accepted: 1 November 1996 相似文献