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931.
23 layers of altered volcanic ash (bentonites) originating from the North Atlantic Igneous Province have been recorded in early Eocene deposits of the Austrian Alps, about 1,900 km away from the source area. The Austrian bentonites are distal equivalents of the “main ash-phase” in Denmark and the North Sea basin. We have calculated the total eruption volume of this series as 21,000 km3, which occurred in 600,000 years. The most powerful single eruption of this series took place 54.0 million years ago (Ma) and ejected ca. 1,200 km3 of ash material, which makes it one of the largest basaltic pyroclastic eruptions in geological history. The clustering of eruptions must have significantly affected the incoming solar radiation in the early Eocene by the continuous production of stratospheric dust and aerosol clouds. This hypothesis is corroborated by oxygen isotope values, which indicate a global decrease of sea surface temperatures between 1 and 2°C during this major phase of explosive volcanism. 相似文献
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934.
The existence of the upper mantle low velocity layer (LVL) below 100 km depth in cratonic areas is tested with surface waves dispersion curves. Given the ambient noise we find that a pronounced LVL (80 km thick and 2% velocity reduction or 40 km thick and 5% velocity reduction) can be distinguished from a constant velocity model by comparison of the fundamental mode group velocities, whereas a thin LVL (less than 40 km thick) with small velocity contrast (less than 2%) cannot be resolved. The fundamental modes of Love and Rayleigh waves have similar properties and, in general, the phase velocity differences are smaller than the standard error. Phase velocity alone cannot discriminate between the models, and the group velocity is in general more sensitive to the velocity structure than the phase velocity. The higher modes at short periods could potentially determine a LVL but in reality it is difficult to obtain sufficiently accurate measurements. We invert the synthetic dispersion curves by the non-linear Hedgehog inversion method. A pronounced LVL (more than 40 km thick and with a strong velocity contrast of about 5%) is detectable by the non-linear inversion but for a thin LVL with a strong velocity contrast it is not possible to resolve both velocity and thickness. In the inversions all solutions include a LVL for models with a pronounced LVL, whereas the solution space includes models with and without a LVL for models with a zero or positive gradient velocity–depth structure.We invert also real data with travel path across the Siberian craton with the Hedgehog method. Almost all solutions include a LVL in the depth range of 80–150 km with a velocity contrast up to 2% to the surrounding intervals. Hence, the LVL appears to be a common feature of the Siberian upper mantle, although a constant velocity at the same depth range cannot be totally excluded. Despite low resolution at large depth, a pronounced asthenospheric LVL below a depth of about 225 km is a constant characteristic of the set of solutions. 相似文献
935.
Martin Hovland Christine Fichler Hkon Ruesltten Hans Konrad Johnsen 《Journal of Geochemical Exploration》2006,89(1-3):157
Deep-rooted enigmatic piercement structures in sedimentary basins, including ‘mud volcanoes’, ‘shale diapirs’, ‘salt diapirs’, and ‘asphalt volcanoes’, range in size from less than 1 km2, surface area, up to 64 km2, and have often an unknown depth of penetration due to incomplete imaging. We propose that they form a family associated with fluid flow. Our argument is based partly on their inferred location (above deep faults) and on the chemical analysis of emitted products, which includes liquid clays, brines and other substances from salt diapirs, and asphalt and light oils from the asphalt volcanoes. We explain these compositions by chemical alteration caused partly by supercritical water, a phase of water existent at high pressure and temperature, locally and temporarily achieved at depths generally beyond 10 km below surface, i.e., at the sediment–crust boundary. Our hypothesis overcomes some of the problems with interpreting fluid flow products, which are otherwise very difficult to explain. In case this hypothesis can be further verified, the family could perhaps be called ‘hydrothermally associated piercement structures’. 相似文献
936.
Hans J. Rick Silke Rick Urban Tillmann Uwe Brockmann Uwe Gärtner Claus Dürselen Jürgen Sündermann 《Estuaries and Coasts》2006,29(1):4-23
Within the KUSTOS program (Coastal Mass and Energy Fluxes-the Land-Sea Transition in the Southeastern North Sea) 28 to 36
German Bight stations were seasonally surveyed (summer 1994, spring 1995, winter 1995–1996) for light conditions, dissolved
inorganic nutrient concentrations, chlorophylla (chla), and photosynthesis versus light intensity (P:E) parameters. Combining P:E curve characteristics with irradiance, attenuation,
and chlorophyll data resulted in seasonal estimates of the spatial distribution of total primary production. These data were
used for an annual estimate of the total primary production in the Bight. In winter 1996 the water throughout the German Bight
was well mixed. Dissolved inorganic nutrient concentrations were relatively high (nitrogen [DIN], soluble reactive phosphorus
[SRP], and silicate [Si]: 23, 1, and 10 μM, respectively). Chla levels generally were low (< 2 μg l−1) with higher concentrations (4–16 μg l−1) in North Frisian coastal waters. Phytoplankton was limited by light. Total primary production averaged 0.2 g C m−2 d−1. Two surveys in April and May 1995 captured the buildup of a strong seasonal thermo-cline accompained by the development
of a typical spring diatom bloom. High nutrient levels in the mixed layer during the first survey (DIN, SRP, and Si: 46, 0.45,
and 11 μM, respectively) decreased towards the second survey (DIN, SRP, and Si: 30.5, 0.12, and 1.5 μM, respectively) and
average nutrient ratios shifted further towards highly imbalanced values (DIN:SRP: 136 in survey 1, 580 in survey 2; DIN:Si:
13.5 in survey 1, 96 in survey 2). Chla ranged from 2 to 16 μg l−1 for the first survey and rose to 12–50 μg l−1 in the second survey. Phytoplankton in nearshore areas continued to be light limited during the second survey, while data
from the stratified regions in the open German Bight indicates SRP and Si limitation. Total primary production ranged from
4.0 to 6.3 g C m−2 d−1. During summer 1994 a strong thermal stratification was present in the German Bight proper and shallow coastal areas showed
unusually warm (up to 22°C), mixed waters. Chla concentrations ranged from 2 to 18 μg l−1. P:E characteristics were relatively high despite the low nutrient regime (DIN, SRP, and Si: 2, 0.2, and 1.5 μM, respectively),
resulting in overall high total primary production values with an average of 7.7 g C m−2 d−1. Based on the seasonal primary production estimates of the described surveys a budget calculation yielded a total annual
production of 430 g C m−2 yr−1 for the German Bight. 相似文献
937.
European whitefish (Coregonus lavaretus) is a dominant zooplanktivorous fish in many praealpine lakes and of great commercial interest. Estimation of prey consumed by European whitefish is essential for the prediction of top down effects in the pelagic zone. Whitefish is known to feed selectively on cladocerans. Quantitative estimates of prey consumed by whitefish are often not easy to get because stomachs frequently contain only degraded fragments of already digested prey. In this study stomach contents of European whitefish which were composed of digested, degraded prey were analyzed. Daphnid prey could be quantified by the number of mandibles divided by two, the number of head capsules and abdomina present. Predatory cladocerans could be quantified by the number of mandibles divided by two and the number of tail appendages present. For stomachs which contained heavily digested fragments counting the mandibles proved to be a useful and sometimes the only applicable means to quantify prey items. Other body parts, like structured legs which were broken into many pieces, were unsuited to quantify prey items. 相似文献
938.
Chris Kennedy Hans Ericsson P. L. R. Wong 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(1-2):119-125
A technique for modeling contaminant transport based on Markov process theory is developed. Transport is quantified by summing
the first two moments of independent random displacements and applying the central limit theorem (CLT) to obtain solute distributions
of a Gaussian nature. For non-uniform flow fields the CLT is applied in a streamfunction/equi-travel time space and transforms
are used to give concentrations in Cartesian coordinates. Simulations in uniform, radially converging and circular flow fields
show the method to be two to three orders of magnitude faster than modeling with the advection-dispersion equation, using
a control volume technique. 相似文献
939.
940.