Preconditioning effects of intermittent stream flow on leaf litter decomposition     

D. Dieter  D. von Schiller  E. M. García-Roger  M. M. Sánchez-Montoya  R. Gómez  J. Mora-Gómez  F. Sangiorgio  J. Gelbrecht  K. Tockner 《Aquatic Sciences - Research Across Boundaries》2011,73(4):599-609

Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV–VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change.  相似文献   

Ordinary chondrite-related giant (>800 μm) cosmic spherules from the Transantarctic Mountains, Antarctica     

Clément Suavet  Carole Cordier  Luigi Folco  Corinne Sonzogni 《Geochimica et cosmochimica acta》2011,75(20):6200-6210

In order to identify the parent bodies of cosmic spherules (melted micrometeorites) with porphyritic olivine (PO) and cryptocrystalline (CC) textures, we measured the oxygen isotopic composition of 15 giant (>800 μm) cosmic spherules recovered in the Transantarctic Mountains, Antarctica, with IR-laser fluorination/mass spectrometry, and we conducted a characterization of their petrographic and magnetic properties. Samples include 6, 8 and 1 spherules of PO, CC and barred olivine (BO) textural types, respectively. Eleven spherules (∼70% of the total: 4/6 PO and 6/8 CC, and the BO spherule) are related to ordinary chondrites based on oxygen isotopic compositions. Olivines in ordinary chondrite-related spherules have compositions Fa_8.5-11.8, they are Ni-poor to Ni-rich (0.04-1.12 wt.%), and tend to be richer in CaO than other spherules (0.10-0.17 wt.%). Ordinary-chondrite related spherules also have high magnetite contents (∼2-12 wt.%). One PO and one CC spherules are related to previously identified ¹⁷O-enriched cosmic spherules for which the parent body is unknown. One CC spherule has an oxygen isotopic signature relating it to CM/CR carbonaceous chondrites. The majority of PO/CC cosmic spherules derive from ordinary chondrites; this result exemplifies how the texture of cosmic spherules is not only controlled by atmospheric entry heating conditions but also depends on the parent body, whether be it through orbital parameters (entry angle and velocity), or chemistry, mineralogy, or grain size of the precursor.  相似文献   

Geochemical evidence of hydrothermal recharge in Lake Baringo,central Kenya Rift Valley     

Corinne Tarits  Robin W. Renaut  Jean‐Jacques Tiercelin  Alain Le Hriss  Jo Cotten  Jean‐Yves Cabon 《水文研究》2006,20(9):2027-2055

Lake Baringo, a freshwater lake in the central Kenya Rift Valley, is fed by perennial and ephemeral rivers, direct rainfall, and hot springs on Ol Kokwe Island near the centre of the lake. The lake has no surface outlet, but despite high evaporation rates it maintains dilute waters by subsurface seepage through permeable sediments and faulted lavas. New geochemical analyses (major ions, trace elements) of the river, lake, and hot spring waters and the suspended sediments have been made to determine the main controls of lake water quality. The results show that evaporative concentration and the binary mixing between two end members (rivers and thermal waters) can explain the hydrochemistry of the lake waters. Two zones are recognized from water composition. The southern part of the lake near sites of perennial river inflow is weakly influenced by evaporation, has low total dissolved species (TDS), and has a seasonally variable load of mainly detrital suspended sediments. In contrast, waters of the northern part of the lake show evidence for strong evaporation (TDS of up to eight times inflow). Authigenic clay minerals and calcite may be precipitating from those more concentrated fluids. The subaerial hot‐spring waters have a distinctive chemistry and are enriched in some elements that are also present in the lake water. Comparison of the chemical composition of the inflowing surface waters and lake water shows (1) an enrichment of some species (HCO₃^?, Cl, SO₄^2?, F, Na, B, V, Cr, As, Mo, Ba and U) in the lake, (2) a depletion in SiO₂ in the lake, and (3) a possible hydrothermal origin for most F. The rare earth element distribution and the F/Cl and Na/Cl ratios give valuable information on the rate of mixing of the river and hydrothermal fluids in the lake water. Calculations imply that thermal fluids may be seeping upward locally into the lake through grid‐faulted lavas, particularly south of Ol Kokwe Island. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Vulkanischer Aschenregen im La Plata-Gebiete am 11./12. April 1932     

Walther Schiller 《International Journal of Earth Sciences》1932,23(3-4):237-238

Ohne Zusammenfassung  相似文献   

Die meerischen Grenzschichten der Kreide und des Tertiärs von Roca in Nordpatagonien     

Walther Schiller 《International Journal of Earth Sciences》1922,13(3):193-216

Ohne Zusammenfassung  相似文献   

Überschiebungen in der Tandil-Gebirgsgruppe der argentinischen Provinz Buenos Aires     

Walther Schiller 《International Journal of Earth Sciences》1928,19(4):257-263

Ohne Zusammenfassung  相似文献   

Long-term fluctuations in volcanic activity: implications for future environmental impact     

Hazel Rymer  Corinne A. Locke  rea Borgia  Maria Martinez  Jorge Brenes  Rodolfo Van der Laat  Glyn Williams-Jones 《地学学报》2009,21(4):304-309

Acidic crater lakes at persistently active volcanoes act as both an index and a moderator of volcanic processes. A catastrophic drop in lake level can therefore lead to serious local environmental damage. In the early 1990s, the crater lake at Poás volcano, Costa Rica diminished, and acid aerosols erupted with devastating consequences for local health, environment and economy. The first indications of this event can be  retrospectively identified to have started from 1985, on the basis of our unique 20-year data time series, which provides evidence for the shallow intrusion of magma. New data presented in this article show similar trends and we conclude that Poás has now entered another active period with renewed intrusion. Severe environmental damage in this region is expected within the next few years if the current trend continues.  相似文献   

The stability of the North Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model     

A. Schiller  U. Mikolajewicz  R. Voss 《Climate Dynamics》1997,13(5):325-347

 The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the Northern Hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface water in high-northern latitudes, which allows them to accumulate more precipitation and runoff from the continents. As a consequence the stratification in the North Atlantic becomes more stable. This effect is further amplified by an enhanced northward atmospheric water vapour transport, which increases the freshwater input into the North Atlantic. The reduced northward oceanic heat transport leads to colder sea-surface temperatures and an intensification of the atmospheric cyclonic circulation over the Norwegian Sea. The associated Ekman transports cause increased upwelling and increased freshwater export with the East Greenland Current. Both the cooling and the wind-driven circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback destabilizes modes without deep water formation in the North Atlantic, but has been neglected in almost all studies so far. After the meltwater input stops, the North Atlantic deepwater formation resumed in all experiments and the meridional overturning returned within 200 years to a conveyor belt pattern. This happened although the formation of North Atlantic deep water was suppressed in one experiment for more than 300 years and the Atlantic overturning had settled into a circulation pattern with Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude that the conveyor belt-type thermohaline circulation seems to be much more stable than hitherto assumed from experiments with simpler models. Received 31 January 1996/Accepted 22 August 1996  相似文献   

Ecology of the Symbiotic Coral Cladocora caespitosa (L.) (Faviidae, Scleractinia) in the Bay of Piran (Adriatic Sea): I. Distribution and Biometry     

Christian  Schiller 《Marine Ecology》1993,14(3):205-219

Abstract. Cladocora caespitosa is a relatively common, colonial, symbiotic, scleractinian coral in the Mediterranean Sea. It inhabits a wide ecological range: hard and soft bottoms, shallow waters or depths to about 50m, calm water or sites exposed to strong currents and wave action. The present study compares the distribution of this coral at two sites, one shallow (3m — wave exposed) and one deeper (12m — soft bottom).
Different parameters including dry weight, volume, and skeletal growth rates were investigated. The polyp biomass, carbon content, and overall area of fully expanded polyps were also measured. Equations are given for the relationship of skeleton diameter to polyp tissue dry weight as well as skeleton diameter to the possible filter area of fully expanded polyps.
C. caespitosa harbors a rich community of vagile and sessile organisms within the skeleton. The cryptofauna was sorted to higher taxonomic levels and their biomass determined. Several highly significant linear relationship were found between skeleton dry weight, volume and cryptofauna biomass. Thus, the determination of colony volume allows the prediction of cryptofauna biomass.
The interstitial space is dominated by sponges - many of them borers - which erode the corallites close to the living tissue until they break off from the colony. At the deeper sampling site, 26% of all colonies are smaller than 2 cm and 48% are smaller than 5 cm. The fragmentation process is discussed with respect to reproduction.  相似文献   

Geomorphology of desert sand dunes: A review of recent progress   总被引：2，自引：0，他引：2  

Ian Livingstone  Giles F.S. Wiggs  Corinne M. Weaver 《Earth》2007,80(3-4):239-257

Through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes. Alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes. As developments with equipment allowed, field measurements became more sophisticated. However, by the mid-1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported. Most importantly, the attempt to measure the stresses imposed by the wind on the sand surface proved impossible, and the use of shear (or friction) velocity as a surrogate for shear stress also failed to deliver. At the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux. In a development paralleled in fluvial geomorphology, aeolian geomorphologists have attempted to measure and model turbulent structures over dunes. Progress has recently been made through the use of more complex numerical models based on computational fluid dynamics (CFD). Some of the modelling work has also suggested that notions of dune ‘equilibrium’ form may not be particularly helpful. This range of recent developments has not meant that field studies are now redundant. For linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground-penetrating radar images of the internal structure of the dunes.

The paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form. At least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies. More recently, attention has shifted–although not completely–to modelling approaches, and very considerable progress has been made in developing models of dune development. It is clear, however, that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology.  相似文献