Lotic communities of two small temporary karstic stream systems (East Westphalia, Germany) along a longitudinal gradient of hydrological intermittency   总被引：1，自引：0，他引：1  

Alexander Meyer  Elisabeth I. Meyer  Christina Meyer 《Limnologica》2003,33(4):271-279

The spatial and temporal discharge regime and its effects on benthic communities were studied in two small temporary karstic stream systems of the Paderborner Hochfläche (East Westphalia, Germany). Both streams are characterized by very small perennial springbrook sections. Benthic invertebrates were sampled three times at 12 sites and discharge was measured monthly from March to September 2000. The spatial extension of streamflow was observed regularly to evaluate the duration of drought. Both streams showed a longitudinal gradient of hydrological intermittency from very small perennial reaches with low intermittency to an extremely harsh hydrological situation. The benthic communities displayed a decreasing species richness at increasing intermittency. The different hydrological stream sections were colonized by different lotic communities, characterized by typical species of temporary streams and by several typical species of springs. Particularly the perennial springs and springbrooks are very important for the species richness in these karstic stream systems.  相似文献   

Decoupled elastic prestack depth migration   总被引：1，自引：0，他引：1  

Alexander Druzhinin   《Journal of Applied Geophysics》2003,54(3-4):369

This paper presents a new decoupled form of the formula for common-shot or common-receiver amplitude-preserving elastic prestack depth migration (PreSDM), which can be used for estimating angle-dependent elastic reflection coefficients in laterally inhomogeneous anisotropic media. The multi-shot or multi-receiver extension of this formula is suitable for automated prestack amplitude-versus-angle (AVA) elastic inversion of ocean-bottom cable (OBC), walkaway VSP (WVSP) or standard towed-cable data at any subsurface location. The essence of the theory is a systematic application of the stationary-phase principle and high-frequency approximations to the basic elastic Green's theorem. This leads to nonheuristic explicit wave mode decoupling and scalarization of vector PreSDM. Used in combination, ray-trace and finite-difference (FD) eikonal solvers create a useful tool to calculate accurate Green's function travel time and amplitude maps. Examples of synthetic OBC data and applications to field WVSP data show that the new imaging technique can produce a clear multi-mode elastic image.  相似文献   

Momentum Transfer By A Mountain Meadow Canopy: A Simulation Analysis Based On Massman's (1997) Model     

Georg Wohlfahrt  Alexander Cernusca 《Boundary-Layer Meteorology》2002,103(3):391-407

Using a mountain meadow as a case study it is the objective of the present paper todevelop a simple parameterisation for the within-canopy variation of the phytoelementdrag (C_d) and sheltering (P_m) coefficients required for Massman's model of momentum transfer by vegetation. A constant ratio between C_d and P_m is found to overestimate wind speed in the upper canopy and underestimate it in the lower canopy.Two simple parameterisations of C_d/P_m as a function of the plant area density and the cumulative plant area index are developed, using values optimised by least-squares regression between measured and predicted within-canopy wind speeds. A validation with independently measured data indicates that both parameterisations work reliably for simulating wind speed in the investigated meadow. Model predictions of the normalised zero-plane displacement height and the momentum roughness length fall only partly within the range of values given in literature, which may be explained by the accumulation of plantmatter close to the soil surface specific for the investigated canopies. The seasonal course of the normalised zero-plane displacement height and the momentum roughness length are discussed in terms of the seasonal variation of the amount and density of plant matter.  相似文献   

Remobilization of authigenic uranium in marine sediments by bioturbation   总被引：1，自引：0，他引：1  

Yan Zheng  Robert F. Anderson  Alexander van GeenMartin Q. Fleisher 《Geochimica et cosmochimica acta》2002,66(10):1759-1772

Uranium behaves as a nearly conservative element in oxygenated seawater, but it is precipitated under chemically reducing conditions that occur in sediments underlying low-oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon. Sites characterized by a range of bottom-water oxygen (BWO) and organic carbon flux (OCF) were studied to better understand the conditions that determine formation and preservation of authigenic U in marine sediments. Our study areas are located in the mid latitudes of the northeast Pacific and the northwest Atlantic Oceans, and all sites receive moderate (0.5 g/cm² kyr) to high (2.8 g/cm² kyr) OCF to the sediments. BWO concentrations vary substantially among the sites, ranging from <3 to ∼270 μM. A mass balance approach was used to evaluate authigenic U remobilization at each site. Within each region studied, the supply of particulate nonlithogenic U associated with sinking particles was evaluated by means of sediment traps. The diffusive flux of U into sediments was calculated from pore-water U concentration profiles. These combined sources were compared with the burial rate of authigenic U to assess the efficiency of its preservation. A large fraction (one-third to two-thirds) of the authigenic U precipitated in these sediments via diffusion supply is later regenerated, even under very low BWO concentrations (∼15 μM). Bioturbating organisms periodically mix authigenic U-containing sediment upward toward the sediment-water interface, where more oxidizing conditions lead to the remobilization of authigenic U and its loss to bottom waters.  相似文献   

The preservation of high-pressure rocks during exhumation: metagranites and metapelites     

Alexander Proyer 《Lithos》2003,70(3-4):183-194

Metagranites in the NKFMASH system require external hydration during prograde high-pressure metamorphism in order to equilibrate to ambient HP conditions by producing more siliceous muscovite. Any lack of external fluid or the disappearance of biotite stops re-equilibration and thus prevents recording of high-pressure conditions. The same hydration reactions cause dehydration during exhumation. Orthogneiss from shear zones or adjacent to metapelites and metabasites will take up external fluid during subduction and record the highest P–T conditions, but will also be the first to dehydrate upon exhumation, now hydrating other lithologies and probably refuelling shearzones.

The (de)hydration behavior of Ca-bearing metagranitoids is similar to that in the Ca-free system. However, the anorthite component of plagioclase decomposes with increasing pressure to form either (clino)zoisite or a grossular-rich garnet. Both reactions are fluid-consuming. If H₂O is supplied from an external source, the garnet-bearing assemblage can record P–T conditions up to very high pressures, but dehydrates again during heating and/or decompression to form a more Fe-rich garnet and Al-rich mica(s). The garnet compositions observed in natural HP-metagranites are mostly too Fe-rich to be formed in the presence of an H₂O-rich fluid.

N(C)KFMASH metapelites generally have a more complex mineralogy and succession of mineral assemblages along a P–T path. The H₂O contained in hydrous silicates like chlorite and chloritoid is only partly released, but partly transferred to other minerals like paragonite, glaucophane or phengite during subduction and further dehydration during exhumation is common. The mineral assemblage preserved by the rock may then record P–T conditions way below those of the actual pressure and temperature peak of the path. Contouring of the P–T pseudosection of a specific metapelite composition with mode isopleths for H₂O shows which P–T conditions along a given path are the ones most likely recorded by the rock.  相似文献   

Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model   总被引：19，自引：0，他引：19  

Alexander Yakubchuk 《Journal of Asian Earth Sciences》2004,23(5):761

The Altaids are an orogenic collage of Neoproterozoic–Paleozoic rocks located in the center of Eurasia. This collage consists of only three oroclinally bent Neoproterozoic–Early Paleozoic magmatic arcs (Kipchak, Tuva–Mongol, and Mugodzhar–Rudny Altai), separated by sutures of their former backarc basins, which were stitched by new generations of overlapping magmatic arcs. In addition, the Altaids host accreted fragments of the Neoproterozoic to Early Paleozoic oceanic island chains and Neoproterozoic to Cenozoic plume-related magmatic rocks superimposed on the accreted fragments. All these assemblages host important, many world-class, Late Proterozoic to Early Mesozoic gold, copper–molybdenum, lead–zinc, nickel and other deposits of various types.In the Late Proterozoic, during breakup of the supercontinent Rodinia, the Kipchak and Tuva–Mongol magmatic arcs were rifted off Eastern Europe–Siberia and Laurentia to produce oceanic backarc basins. In the Late Ordovician, the Siberian craton began its clockwise rotation with respect to Eastern Europe and this coincides with the beginning of formation of the Mugodzhar–Rudny Altai arc behind the Kipchak arc. These earlier arcs produced mostly Cu–Pb–Zn VMS deposits, although some important intrusion-related orogenic Au deposits formed during arc–arc collision events in the Middle Cambrian and Late Ordovician.The clockwise rotation of Siberia continued through the Paleozoic until the Early Permian producing several episodes of oroclinal bending, strike–slip duplication and reorganization of the magmatic arcs to produce the overlapping Kazakh–Mongol and Zharma-Saur–Valerianov–Beltau-Kurama arcs that welded the extinct Kipchak and Tuva–Mongol arcs. This resulted in amalgamation of the western portion of the Altaid orogenic collage in the Late Paleozoic. Its eastern portion amalgamated only in the early Mesozoic and was overlapped by the Transbaikal magmatic arc, which developed in response to subduction of the oceanic crust of the Paleo-Pacific Ocean. Several world-class Cu–(Mo)-porphyry, Cu–Pb–Zn VMS and intrusion-related Au mineral camps, which formed in the Altaids at this stage, coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs. Major Pb–Zn and Cu sedimentary rock-hosted deposits of Kazakhstan and Central Asia formed in backarc rifts, which developed on the earlier amalgamated fragments. Major orogenic gold deposits are intrusion-related deposits, often occurring within black shale-bearing sutured backarc basins with oceanic crust.After amalgamation of the western Altaids, this part of the collage and adjacent cratons were affected by the Siberian superplume, which ascended at the Permian–Triassic transition. This plume-related magmatism produced various deposits, such as famous Ni–Cu–PGE deposits of Norilsk in the northwest of the Siberian craton.In the early Mesozoic, the eastern Altaids were oroclinally bent together with the overlapping Transbaikal magmatic arc in response to the northward migration and anti-clockwise rotation of the North China craton. The following collision of the eastern portion of the Altaid collage with the Siberian craton formed the Mongol–Okhotsk suture zone, which still links the accretionary wedges of central Mongolia and Circum-Pacific belts. In the late Mesozoic, a system of continent-scale conjugate northwest-trending and northeast-trending strike–slip faults developed in response to the southward propagation of the Siberian craton with subsequent post-mineral offset of some metallogenic belts for as much as 70–400 km, possibly in response to spreading in the Canadian basin. India–Asia collision rejuvenated some of these faults and generated a system of impact rifts.  相似文献   

Amoeboid olivine aggregates with low-Ca pyroxenes: a genetic link between refractory inclusions and chondrules?     

Alexander N. Krot  Michail I. Petaev 《Geochimica et cosmochimica acta》2004,68(8):1923-1941

Amoeboid olivine aggregates (AOAs) in primitive (unmetamorphosed and unaltered) carbonaceous chondrites are uniformly ¹⁶O-enriched (Δ¹⁷O ∼ −20‰) and consist of forsterite (Fa_<2), FeNi-metal, and a refractory component (individual CAIs and fine-grained minerals interspersed with forsterite grains) composed of Al-diopside, anorthite, ±spinel, and exceptionally rare melilite (Åk_<15); some CAIs in AOAs have compact, igneous textures. Melilite in AOAs is replaced by a fine-grained mixture of spinel, Al-diopside, and anorthite. Spinel is corroded by anorthite or by Al-diopside. In ∼10% of > 500 AOAs studied in the CR, CV, CM, CO, CH, CB, and ungrouped carbonaceous chondrites Acfer 094, Adelaide, and LEW85332, forsterite is replaced to a various degree by low-Ca pyroxene. There are three major textural occurrences of low-Ca pyroxene in AOAs: (i) thin (<10 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) haloes and subhedral grains around FeNi-metal nodules in AOA peripheries, and (iii) thick (up to 70 μm) continuous layers with abundant tiny inclusions of FeNi-metal grains around AOAs. AOAs with low-Ca pyroxene appear to have experienced melting of various degrees. In the most extensively melted AOA in the CV chondrite Leoville, only spinel grains are relict; forsterite, anorthite and Al-diopside were melted. This AOA has an igneous rim of low-Ca pyroxene with abundant FeNi-metal nodules and is texturally similar to Type I chondrules.Based on these observations and thermodynamic analysis, we conclude that AOAs are aggregates of relatively low temperature solar nebular condensates originated in ¹⁶O-rich gaseous reservoir(s), probably CAI-forming region(s). Some of the CAIs were melted before aggregation into AOAs. Many AOAs must have also experienced melting, but of a much smaller degree than chondrules. Before and possibly after aggregation, melilite and spinel reacted with the gaseous SiO and Mg to form Ca-Tschermakite (CaAl₂SiO₆)-diopside (CaMgSi₂O₆) solid solution and anorthite. Solid or incipiently melted olivine in some AOAs reacted with gaseous SiO in the CAI- or chondrule-forming regions to form low-Ca pyroxene: Mg₂SiO₄ + SiO_(g) + H₂O_(g) = Mg₂Si₂O₆ + H_2(g). Some low-Ca pyroxenes in AOAs may have formed by oxidation of Si-bearing FeNi-metal: Mg₂SiO₄ + Si_{(in FeNi)} + 2H₂O_(g) = Mg₂Si₂O₆ + 2H_2(g) and by direct gas-solid condensation: Mg_(g) + SiO_(g) +H₂O_(g) = Mg₂Si₂O_6(s) + H_2(g) from fractionated (Mg/Si ratio < solar) nebular gas.Although bulk compositions of AOAs are rather similar to those of Type I chondrules, on the projection from spinel onto the plane Ca₂SiO₄-Mg₂SiO₄-Al₂O₃, these objects plot on different sides of the anorthite-forsterite thermal divide, suggesting that Type I chondrules cannot be produced from AOAs by an igneous fractionation. Formation of low-Ca pyroxene by reaction of AOAs with gaseous SiO and by melting of silica-rich dust accreted around AOAs moves bulk compositions of the AOAs towards chondrules, and provide possible mechanisms of transformation of refractory materials into chondrules or chondrule precursors. The rare occurrences of low-Ca pyroxene in AOAs may indicate that either AOAs were isolated from the hot nebular gas before condensation of low-Ca pyroxene or that condensation of low-Ca pyroxene by reaction between forsterite and gaseous SiO was kinetically inhibited. If the latter is correct, then the common occurrences of pyroxene-rich Type I chondrules may require either direct condensation of low-Ca pyroxenes or SiO₂ from fractionated nebular gas or condensation of gaseous SiO into chondrule melts.  相似文献   

NMR studies of chemical structural variation of insoluble organic matter from different carbonaceous chondrite groups   总被引：1，自引：0，他引：1  

George D. Cody  Conel M.O.’D. Alexander 《Geochimica et cosmochimica acta》2005,69(4):1085-1097

Solid-state ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectroscopic experiments have been performed on isolated meteoritic Insoluble Organic Matter (IOM) spanning four different carbonaceous chondrite meteorite groups; a CR2 (EET92042), a CI1 (Orgueil), a CM2 (Murchison), and the unique C2 meteorite, Tagish Lake. These solid state NMR experiments reveal considerable variation in bulk organic composition across the different meteorite group’s IOM. The fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake. The increases in aromatic carbon are offset by reductions in aliphatic (sp³) carbon moieties, e.g., “CH_x,” and “CH_x(O,N).” Oxidized sp² bonded carbon, e.g., carboxyls and ketones grouped as “CO,” are largely conservative across these meteorite groups. Single pulse (SP) ¹³C magic angle spinning (MAS) NMR experiments reveal the presence of nanodiamonds with an apparent concentration ranking in the IOM of CR2 < CI1 < CM2 < Tagish Lake. A pair of independent NMR experiments reveals that, on average, the aromatic moieties in the IOM of all four meteoritic IOM fractions are highly substituted. Fast spinning SP ¹H MAS NMR spectral data combined with other NMR experimental data reveal that the average hydrogen content of sp³ bonded carbon functional groups is low, requiring a high degree of aliphatic chain branching in each IOM fraction. The variation in chemistry across the meteorite groups is consistent with alteration by low temperature chemical oxidation. It is concluded that such chemistry principally affected the aliphatic moieties whereas the aromatic moieties and nanodiamonds may have been largely unaffected.  相似文献   

Isotopic fractionation of iron, potassium, and oxygen in stony cosmic spherules: Implications for heating histories and sources     

S. Taylor  C.M.O.’D. Alexander  P. Ma  G.F. Herzog 《Geochimica et cosmochimica acta》2005,69(10):2647-2662

Atmospheric heating alters the compositions and textures of micrometeorites. To understand the changes and to test a proposed relationship between a micrometeorite’s petrographic texture and its degree of heating, we made elemental and multiple isotope analyses of stony cosmic spherules (sCS) collected from the South Pole Water well. Specifically, we analyzed the elemental compositions of 94 sCS and the isotopic ratios of Fe, K and O, on 43, 12 and 8 of these sCS, respectively.Our results show that sCS classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements. Of the 43 spherules analyzed for Fe isotopes, only 5 have δ⁵⁷Fe >5‰. In contrast, enrichment of ⁴¹K is pervasive (δ⁴¹K >0 in all 12 spherules analyzed) and large (up to 183‰). The determination of K isotope abundances in sCS may therefore be useful in deciphering thermal histories. Three of the eight sCS analyzed for O isotopes are mass fractionated with δ¹⁸O >30‰. We attribute two of these three δ¹⁸O enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase, perhaps a sulfate or a carbonate. The K isotope and O isotope data are broadly consistent with the proposed textural classification.Because most spherules were not heated enough to fractionate Al, Mg, or Si, we compared the measured Mg/Al and Si/Al ratios directly to those of conventional meteorites and their matrices. ∼30% of the sCS have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene- and olivine-rich materials and may be samples of chondrules. The other 70% have Mg/Al and Si/Al ratios similar to those of CI, CM, CO, and CV chondrites. Natural variability of the Mg/Al and Si/Al ratios precludes the assignment of an individual sCS to a particular class of C-chondrite.  相似文献   

Longwave Measurements for the Coast of British Columbia and Improvements to the Tsunami Warning Capability   总被引：1，自引：0，他引：1  

Rabinovich  Alexander B.  Stephenson  Fred E. 《Natural Hazards》2004,32(3):313-343

A few years ago the Canadian Hydrographic Service initiated a major upgrade toall tide gauges and tsunami stations on the coast of British Columbia (B.C.). Thisprogram was undertaken to address shortcomings of the earlier digital systems andwas driven by concerns about emergency response continuity in the year 2000. By1999, thirteen tide gauge stations had been installed and were operational. Three ofthese stations (Tofino, Winter Harbour, and Langara) were selected for use as tsunamiwarning stations. Several years of continuous, high quality data have now been collectedat these stations and used for analysis of long waves in the tsunami frequency band.Careful examination of these data revealed two weak tsunamis recorded by severalB.C. stations: a distant tsunami of June 23, 2001 generated by the Peru Earthquake(M_w = 8.4), and a local tsunami of October 12, 2001 induced by the Queen Charlotte Earthquake (M_w = 6.3$). Spectral characteristics of these two tsunamis are compared with the spectral characteristics of long waves generated by a strong storm (October, 2000) and of ordinary background oscillations. The topographic admittance functions (frequency responses) constructed for all stations showed that most of them (in particular, Winter Harbour, Tofino, Bamfield, Port Hardy, and Victoria) have strong resonance at periods from 2.5 to 20 min, indicating that these locations are vulnerable to relatively high-frequency tsunamis. The Winter Harbour station also has two strong resonant peaks with periods of 30 and 47 min and with amplification factors of about 7. The estimated source functions show very clear differences between long waves associated with the seismic source (typical periods 10–30 min) and those generated by a storm, which typically have shorter periods and strong energy pumping from high-frequencies due to non-linear interaction of wind waves.  相似文献