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21.
The Late Archean Blake River Group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the Abitibi greenstone belt. It contains a number of silicic volcanic centers of different size, including the large Noranda volcanic complex, which is host to 17 past-producing volcanogenic massive sulfide deposits. The Noranda complex consists of a 7- to 9-km-thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism. Massive sulfide formation coincided with a period of intense magmatic activity (cycle III) and the formation of the Noranda cauldron. Hydrothermal alteration in these rocks is interpreted to reflect large-scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex. The alteration includes abundant albite, chlorite, epidote and quartz (silicification), which exhibit broad stratigraphic and structural control and correlate with previously mapped whole-rock oxygen isotope zonation. The Mine Sequence volcanic rocks are characterized by abundant iron-rich chlorite (Fe/Fe+Mg >0.5), hydrothermal amphibole (ferroactinolite) and coarse-grained epidote of clinozoisite composition (<10 wt% Fe 2O 3). Volcanic rocks of the pre-cauldron sequences, which contain only subeconomic stringer mineralization, are characterized by less abundant chlorite and mainly fine-grained epidote (>10 wt% Fe 2O 3) lacking the clinozoisite solid solution. Alteration in the Mine Sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits (as far as several tens of kilometers) and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale. The lack of similar alteration in the pre-cauldron sequences is consistent with limited 18O-depletion and suggests that the early history of the volcanic complex did not support large-scale, high-temperature fluid flow in these rocks. Comparisons with a much smaller, barren volcanic complex in nearby Ben Nevis township reveal important differences in the alteration mineralogy between volcanoes of different size, with implications for area selection during regional-scale mineral exploration. The Ben Nevis Complex consists of a 3- to 4-km-thick succession of mafic, intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion. Alteration of the volcanic rocks comprises mainly low-temperature assemblages of prehnite, pumpellyite, magnesium-rich chlorite (Fe/Fe+Mg <0.5), iron-rich epidote (>10 wt% Fe 2O 3) and calcite. Actinolite ± magnetite alteration occurs proximal to the intrusive core of the complex, but the limited extent of this alteration indicates only local high-temperature fluid circulation adjacent to the intrusion. A distal zone of carbonate alteration is located 4–6 km from the center of the volcano. Although iron-bearing carbonates are present locally within this zone, the absence of siderite argues against a high-temperature origin for this alteration. These observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit.  相似文献   
22.
Effects of nitrate on phosphorus release: comparison of two Berlin lakes   总被引:1,自引:0,他引:1  
After installation of phosphorus elimination plants at the inflows of the eutrophic Lake Tegel and Schlachtensee, phosphorus (P) loading declined by a factor of 40 and 100, respectively. This resulted in a pronounced reduction of phosphorus concentrations in the lake water, followed by a decline of chlorophyll‐a concentrations. However, for many years P release from sediments due to mineralization and desorption of sedimentary P continued. In Schlachtensee, the presence of nitrate above the sediment suppresses P release, because the Fe/P ratio is sufficient to provide enough aerobic sorption capacity. In Lake Tegel, some P release occurs even under aerobic conditions because of the low aerobic P sorption capacity of the sediments. There, nitrate could moderate the P release peaks which occur when the Fe‐P cycle at the sediment water interface is disturbed by precipitation of iron sulfide after reduction of sulfate during times of high mineralization intensity. The potentially mobile P pool in the sediments of both lakes is rather small, indicating that the P release could subside after sufficient reduction of the external P load in Lake Tegel and the disruption of the internal P cycle in Schlachtensee.  相似文献   
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24.
We investigated the effect of CO2 and primary production on the carbon isotopic fractionation of alkenones and particulate organic matter (POC) during a natural phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi. In nine semi-closed mesocosms (∼11 m3 each), three different CO2 partial pressures (pCO2) in triplicate represented glacial (∼180 ppmv CO2), present (∼380 ppmv CO2), and year 2100 (∼710 ppmv CO2) CO2 conditions. The largest shift in alkenone isotopic composition (4-5‰) occurred during the exponential growth phase, regardless of the CO2 concentration in the respective treatment. Despite the difference of ∼500 ppmv, the influence of pCO2 on isotopic fractionation was marginal (1-2‰). During the stationary phase, E. huxleyi continued to produce alkenones, accumulating cellular concentrations almost four times higher than those of exponentially dividing cells. Our isotope data indicate that, while alkenone production was maintained, the interaction of carbon source and cellular uptake dynamics by E. huxleyi reached a steady state. During stationary phase, we further observed a remarkable increase in the difference between δ13C of bulk organic matter and of alkenones spanning 7-12‰. We suggest that this phenomenon is caused mainly by a combination of extracellular release of 13C-enriched polysaccharides and subsequent particle aggregation induced by the production of transparent exopolymer particles (TEP).  相似文献   
25.
Sedimentological studies of a 30 m thick coastal cliff section within the Middle Proterozoic Eriksfjord Formation in western South Greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams (Type I), semi-perennial streams (Type II), and ephemeral flash floods (Type III). Perennial river sand sheets are characterised by co-sets of medium-scale trough cross-beds, interbedded with isolated medium- and large-scale, high-angle, tabular cross-beds. Indications of desiccation or subaerial exposure are absent. Semi-perennial fluvial sand sheets consist predominantly of low-angle cross-beds, interbedded with isolated sets of high-angle tabular cross-beds with common reactivation surfaces. Horizontal lamination and climbing ripple lamination form subordinate structures. Associated with the sand sheets are adhesion structures and 0.05–0.4 m thick sets of wind ripple-lamination indicating periods of subaerial exposure and aeolian reworking. High-energy ephemeral flash flood sand sheets consist almost exclusively of planar-parallel lamination and climbing ripple lamination with some isolated sets of low-angle cross-bedding. Scouring and internal truncation surfaces are common. The three types of sand sheets are considered to reflect deposition under changing climatic conditions, varying from humid to arid or semi-arid. Aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation (Type II), while sand sheets showing features typical of arid and or semi-arid flow conditions (Type III) contain no preserved aeolian deposits. This selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime. The deposits show that during pre-vegetational times the preservation of aeolian deposits, under certain conditions, may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi-arid or arid circumstances. The occurrence of aeolian deposits within a Precambrian succession of fluvial deposits therefore, need not be an indication of the most arid environmental conditions.  相似文献   
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27.
Investigations of the zodiacal dust cloud give evidence for a significant contribution of asteroidal dust to the interplanetary dust cloud, a result which can now be compared to measurements of the ULYSSES dust detector during its passage of the asteroid belt. Especially we discuss the ULYSSES data with respect to the IRAS dust bands and consider geometric selection effects for the detector. From calculations of radiation pressure forces, we conclude that particles in the IRAS dust bands with massesm≥ 10−12g will stay in bound orbits after their release from asteroid fragmentation. This is already in the mass range (10−16–10−7g) of particles detectable with the dust detector onboard ULYSSES. The absence of these particles in the ULYSSES data cannot be explained exclusively in terms of their small detection probability. Thus we conclude that the size distribution of particles in the IRAS dust bands most probably cannot be continued to the submicrometer range. Concerning the global structure of the inner zodiacal cloud (i.e., about solar distancer< 3.5 AU) the ULYSSES data are not inconsistent with present models. Recent estimates of the total mass of the interplanetary cloud require a dust production rate of about 1014g/year of which a significant amount is assumed to result from the asteroids. Our estimate for the production of dust particles in an IRAS dust band, based on the assumption that the dust band results from a single destruction of an asteroid of 100 km size, yields a production rate of 1010g/year. Other models of the IRAS dust bands suggest production rates up to 1012g/year and also cannot provide a significant source of the dust cloud.  相似文献   
28.
Abstract

Ice floes along the Labrador Coast were tracked using visible NOAA satellite images on two consecutive days (26 and 27 April, 1984) when the ice‐pack extended beyond the Labrador Current, and winds were weak. The resulting “snapshot” of the velocity field reveals strong topographic steering of the Labrador Current, such that the current speed and width in different areas are dependent on the steepness of the continental slope, and the current deflects into and out of Hopedale Saddle. Between 55 and 58°N, the main core of the current is 60–90 km wide, with speeds of 30–55 cm s?1. The overall circulation pattern is in good agreement with historical water mass analyses over the shelf and slope, and with estimates of the speed of the Labrador Current obtained by other methods.  相似文献   
29.
Abstract The 65 Ma Chicxulub impact crater formed in the shallow coastal marine shelf of the Yucatán Platform in Mexico. Impacts into water‐rich environments provide heat and geological structures that generate and focus sub‐seafloor convective hydrothermal systems. Core from the Yaxcopoil‐1 (Yax‐1) hole, drilled by the Chicxulub Scientific Drilling Project (CSDP), allowed testing for the presence of an impact‐induced hydrothermal system by: a) characterizing the secondary alteration of the 100 m‐thick impactite sequence; and b) testing for a chemical input into the lower Tertiary sediments that would reflect aquagene hydrothermal plume deposition. Interaction of the Yax‐1 impactites with seawater is evident through redeposition of the suevites (unit 1), secondary alteration mineral assemblages, and the subaqueous depositional environment for the lower Tertiary carbonates immediately overlying the impactites. The least‐altered silicate melt composition intersected in Yax‐1 is that of a calc‐alkaline basaltic andesite with 53.4–56 wt% SiO2(volatile‐free). The primary mineralogy consists of fine microlites of diopside, plagioclase (mainly Ab 47), ternary feldspar (Ab 37 to 77), and trace apatite, titanite, and zircon. The overprinting alteration mineral assemblage is characterized by Mg‐saponite, K‐montmorillonite, celadonite, K‐feldspar, albite, Fe‐oxides, and late Ca and Mg carbonates. Mg and K metasomatism resulted from seawater interaction with the suevitic rocks producing smectite‐K‐feldspar assemblages in the absence of any mixed layer clay minerals, illite, or chlorite. Rare pyrite, sphalerite, galena, and chalcopyrite occur near the base of the impactites. These secondary alteration minerals formed by low temperature (0–150°C) oxidation and fixation of alkalis due to the interaction of glass‐rich suevite with down‐welling seawater in the outer annular trough intersected at Yax‐1. The alteration represents a cold, Mg‐K‐rich seawater recharge zone, possibly recharging higher temperature hydrothermal activity proposed in the central impact basin. Hydrothermal metal input into the Tertiary ocean is shown by elevated Ni, Ag, Au, Bi, and Te concentrations in marcasite and Cd and Ga in sphalerite in the basal 25 m of the Tertiary carbonates in Yax‐1. The lower Tertiary trace element signature reflects hydrothermal metal remobilization from a mafic source rock and is indicative of hydrothermal venting of evolved seawater into the Tertiary ocean from an impact‐generated hydrothermal convective system.  相似文献   
30.
The late Quaternary evolution of central-eastern Brazil has been under-researched. Questions remain as to the origin of the Cerrado, a highly endangered biome, and other types of vegetation, such as the Capões – small vegetation islands of semi-deciduous and mountain forests. We investigated the factors that influenced the expansion and contraction of the Cerrado and Capões during the late Quaternary (last ~35 ka), using a multi-proxy approach: stable isotopes (δ13C, δ15N), geochemistry, pollen and multivariate statistics derived from a peat core (Pinheiro mire, Serra do Espinhaço Meridional). Five major shifts in precipitation, temperature, vegetation and landscape stability occurred at different timescales. Our study revealed that changes in the South Atlantic Convergence Zone (SACZ) seem to have been coeval with these shifts: from the Late Glacial Maximum to mid-Holocene the SACZ remained near (~29.6 to ~16.5k cal a bp ) and over (~16.5 to ~6.1 k cal a bp ) the study area, providing humidity to the region. This challenges previous research which suggested that climate was drier for this time period. At present, the Capões are likely to be a remnant of a more humid climate; meanwhile, the Cerrado biome seems to have stablished in the late Holocene, after ~3.1 k cal a bp .  相似文献   
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