Seasonal dynamics of aggregates and their typical biocoenosis in the Elbe Estuary     

Heike Zimmermann-Timm  Henry Holst  Stefan Müller 《Estuaries and Coasts》1998,21(4):613-621

In 1995, an extensive investigation was carried out in the Elbe Estuary in Germany. Special attention was paid to organisms, including bacteria, amoebae, ciliates, flagellates, rotifers, larvae ofDreissena polymorpha, and nematodes, dispersed in the water column and associated firmly or loosely with different types of aggregates. The abundance, size, and colonization by microorganisms of the aggregates varied during the seasons. The largest aggregates were found during spring and summer, when diatoms, rotifers, and crustaceans were present. The most aggregates were encountered in spring and from summer to autumn. Most of the species observed during the study were more common in pelagic habitats than in benthic ones. However, the presence of ciliates in the groups Hypotrichida and Sessilida as well as as flagellates in the groups Biosoecida, Cercomonadida, Choanoflagellida, Chrysomonadida, Kinetoplastida, and amoebae and some nematodes in the open water depends, upon the availability of surfaces, because they are sessile or poor swimnters, and some flagellates and amoebae need to attach themselves to an object to feed. Most organisms were much more abundant in or on aggregates than in the surrounding water during spring and summer, which indicates that aggregates enhance the habitat and promote the development of the organisms. From spring through autumn, the structure of the community associated with the aggregates is influenced by the pelagic environment. The presence of the species in the benthic community was detected only during summer.  相似文献   

High-redshift galaxies in the Hubble Deep Field— II. Colours and number counts     

Lucia Pozzetti  Piero Madau  Gianni Zamorani  Henry C. Ferguson  & Gustavo Bruzual A. 《Monthly notices of the Royal Astronomical Society》1998,298(4):1133-1144

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Optical, infrared and radio observations of the Seyfert galaxy Markarian 315     

M. Nonino  J. P. Henry  C. Fanti  R. Fanti  & J. Davies 《Monthly notices of the Royal Astronomical Society》1998,299(2):332-340

We report on new optical, infrared and radio observations of the Seyfert galaxy Markarian 315. We confirm the detection at all wavelengths of a secondary peak of emission, which lies ≈ 2 arcsec east of the Seyfert nucleus. Moreover, we detect a chain-like structure which surrounds the active nucleus, with peculiar behaviour westward of the nucleus. We consider different interpretations for the origin of the secondary peak emission.  相似文献   

Current Convenience,Desperate Deforestation: Ghana's AdjustmentProgram and the Forestry Sector     

J. Henry Owusu 《The Professional geographer》1998,50(4):418-436

Since the early 1980s, most Sub-Saharan African countries have resorted to structural adjustment programs to reform their ailing economies. Adjustment, however, may provide a convenient means for governments of the adjusting economies and international capital to meet their current economic and political interests, usually by sacrificing the physical environment, with huge environmental costs for the adjusting country. This paper uses Ghana's forestry sector, which has been characterized by a dramatic increase in woodexports since adjustment, to demonstrate a direct link between adjustment and environmental destruction. Ghana's dramatic increase in wood exports, involving a rapid and extensive deforestation, results from the government's need to meet its increasing external debt service obligations, and is exacerbated by the series of massive local currency devaluation required under adjustment “to get prices right.” The systematic reduction in government revenue from devaluation to amortize the increasing debts, keeps the government and indebted wood processing firms on a treadmill of export-based extraction/deforestation.  相似文献   

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E     

Anton P le Roex  Henry J B Dick  Ronald T Watkins 《Contributions to Mineralogy and Petrology》1992,110(2-3):253-268

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K₂O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)_n ratios are particularly high (3.4–7.8). ⁸⁷Sr/⁸⁶Sr (0.70290–0.70368), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51284) and ²⁰⁶Pb/²⁰⁴Pb (18.708–19.564) isotopic ratios further indicate the geochemically enriched nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.  相似文献   

Economic Globalisation and the Tropical World in the New Millennium: An Introduction     

Henry Wai-chung Yeung  & Peter Dicken 《Singapore journal of tropical geography》2000,21(3):225-232

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Barringer Medal Citation for Boris A. Ivanov: 1998 July 29, Dublin,Ireland     

Henry Jay Melosh 《Meteoritics & planetary science》1999,34(Z4):A7-A7

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Modern and ancient geotherms beneath southern Africa     

Sanford Ballard  Henry N. Pollack 《Earth and Planetary Science Letters》1988,88(1-2)

Estimates of subsurface temperatures in the Archean craton of southern Africa during the Archean derived from diamond thermobarometry studies are remarkably similar to temperatures estimated for the same depths today, even though heat production in the earth and the mean global heat flow were probably substantially higher in the Archean. We present multi-dimensional numerical models for the thermal environment of the Archean craton in southern Africa during the Archean in which deep mantle heat is diverted away from the craton toward the surrounding oceanic lithosphere by a lithospheric root beneath the craton. Extrapolation of present-day models to thermal conditions appropriate for the Archean is inadequate to explain the similarity of present-day and Archean temperatures in the cratonic root. Reconciliation of the modern and ancient temperature estimates requires either relaxation of the constraints that the cratonic crustal heat production and/or the earth's mean mantle temperature were higher in the Archean than they are today, or that substantial “erosion” of the lithosphere comprising the cratonic root has occurred since the Archean. The latter possibility could perhaps result from revolatilization of the cratonic root in association with thermal perturbations in the mantle, for which there is evidence in southern Africa in the form of post-Archean tectonic and igneous activity.  相似文献   

Acidic deposition impacts mediated by sulfur cycling in a coastal plain forest ecosystem     

Mark D. Morgan  Ralph E. Good  Henry G. Spratt Jr. Dr. 《GeoJournal》1988,17(2):183-187

Deposition in the New Jersey Pinelands was very acidic (pH=4.17) and contained high levels of SO₂ ⁻² based on bulk deposition measurementsfrom July 1984–July 1986. Streamwater over the same interval in undisturbed watersheds was less acidic (pH =4.52) and had proportionately less SO₄ ⁻². A preliminary alkalinity budget for undisturbed watersheds suggested that SO₄ ⁻² retention within Pinelands watersheds accounted for a large portion of the total alkalinity generated and thereby lessened the impact of acidic deposition on surface waters. The only process capable of explaining the retention of SO₄ ⁻² was microbial sulfate reduction in the extensive wetlands surrounding Pinelands streams which occurred at high rates.  相似文献   

The surface of labradorite feldspar after acid hydrolysis     

William H. Casey  Henry R. Westrich  Thomas Massis  Jillian F. Banfield  George W. Arnold 《Chemical Geology》1989,78(3-4):205-218

After reaction with a pH < 4, the surface of labradorite is extensively enriched in Si and H, and depleted in Al, Ca and Na relative to an unreacted crystal. However, the amount of hydrogen measured in the reacted surface is less than that predicted on the basis of exchange of hydrogen (or hydronium) ions for cations in the feldspar. By analogy with studies of silicate glass, this low concentration of hydrogen suggests that some silanol groups in the reacted surface repolymerize subsequent to ion-exchange and depolymerization reactions. The net result is a relatively porous, Si-rich leached layer which is amorphous to electron diffraction, and which allows rapid diffusion of unreactive solutes.

Both the surface area of the reacted feldspar and the porosity increase with acid hydrolysis. Modeling of nitrogen sorption onto the surface suggests that the pores have a nominal radius of 20–80 Å or less. This distribution of pore sizes resembles other acid-reacted silicate materials, such as glass, chrysotile and kaolinite. Although the mineral surface clearly becomes more porous during acid hydrolysis, the increase in powder area also does not coincide with an increase in the flux of dissolved Si from the powder. We thus attribute most of this increase in area to spallation of the silica-rich surface from the feldspar upon drying.  相似文献