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1.
At the Lower Rhine, three congeneric, invasive Gobiidae constitute the major part of local fish assemblages. Reproduction strategies play an important role in biological invasion processes and help to predict future population development. Up to now, studies on the reproductive traits of the three species have led to contradictory findings in invaded areas, and studies on native populations are scarce, especially when regarding a co-occurrence of the three species. This study provides an overview over already existing knowledge of reproductive traits in native as well as invaded regions and describes the reproduction modes of all three species at the Lower Rhine. Based on intensive beach seine sampling in nursery habitats for three consecutive years, detailed information about the spawning habits of the bighead goby Ponticola kessleri, the round goby Neogobius melanostomus, and the monkey goby N. fluviatilis is provided. Spawning onset as well as length and intensity of the spawning season were determined by estimating growth rates and subsequent back-calculations of hatching and spawning events. Gonadosomatic index (GSI) analyses of adult gobies were used to cross-validate results and to assess validity of the used method. Growth rates for the first three months of all three species were highly variable and ranged between 0.3 mm and 0.95 mm total length per day. All three Gobiidae displayed different reproductive modes with bighead gobies spawning first in March/April, but probably only once a season. In contrast, round and monkey gobies started reproducing slightly later, but then continued spawning throughout the season until September, exhibiting temporal separation with varying intensities. High variation in spawning onset, length and reproductive output for all three species was observed over the years and no relationship between spawning onset and temperature could be detected. For the first time, highly resolvent information on spawning onset and spawning season length is obtained and the fine-tuned differences between the three species are highlighted.  相似文献   

2.
Tilapia rendalli is a predominately macrophagous fish. However, it was able to colonise an oligotrophic dam (Flag Boshielo) with limited macrophytes. Therefore, the diet of T. rendalli in this dam was investigated; its stomach contents were examined over 12 months. A size related dietary shift was evident. Juveniles fed mainly on zooplankton while sub-adult and adult fish grazed on both macrophytes and marginal vegetation. T. rendalli’s ability to strive in an environment with limited food resources led to a subsequent study to determine its aquaculture potential. Its growth performance was compared to that of the commonly cultured Oreochromis mossambicus. Juveniles of both species were fed a commercial tilapia diet for 60 days. Specific growth rate and protein efficiency ratio was comparable to that of O. mossambicus (P > 0.05, ANOVA). Feed conversion ratio was significantly higher (P < 0.05) in T. rendalli (1.43) than in O. mossambicus (1.25) indicating a better efficiency in feed utilisation by O. mossambicus. At a physiological level, protease, lipase and cellulase activities did not differ significantly between the two fish species (P > 0.05). Amylase activities were significantly higher (P < 0.05) in T. rendalli than in O. mossambicus. The highest amylase activities were recorded in the proximal intestines as 26.34 and 22.00 μmol/min/mg protein in T. rendalli and O. mossambicus respectively. This may be an indicator that T. rendalli is better equipped to digest plant diets. T. rendalli may be the aquaculture species of choice for emerging fish farmers who cannot afford the highly priced fishmeal as a protein source in fish diets.  相似文献   

3.
Under laboratory conditions, we assessed by video-recorder whether or not the interference competition of invading racer goby Babka gymnotrachelus affects the habitat (refuge) use and behaviour of European bullhead Cottus gobio, a native nest-guarding species, during and after the spawning season, spring and autumn respectively, when available refuge (nesting) habitat is limited. A bullhead was placed in a tank for 24 h and then an intruder, either a goby or another bullhead was added for the next 24 h. We assessed: (1) the time spent by the two fish in the shelter; and (2) acts of aggression and attempts to enter the shelter. In spring, the amount of time spent in the shelter by the bullhead was considerably lower in the presence of a racer goby of larger or equal size than when the bullhead was alone. In autumn, bullheads occupied shelters at a similar frequency with and without a goby present. Smaller gobies were not able to replace bullhead residents in the shelter. Racer gobies were much more aggressive than bullheads of equal or smaller size, particularly in spring. Bullheads kept outside of the shelter by a resident goby made more attempts to enter the refuge during spring. In summary, under experimental conditions, racer gobies modified the typical behaviour of bullheads by reducing the use of shelter and potential nesting places.  相似文献   

4.
Erosion and sediment yield are a significant problem in the Guadalquivir River basin. Such phenomena are largely driven by a land use devoted to intensive cultivation of olive trees, with a large socioeconomic influence in Andalusia. This sediment overload in rivers causes serious impacts on all fluvial ecosystem components.In this study we assess the chronic effect of sediment yield on fish communities at 104 river sites located in two different sub-catchments – the Bembézar and Guadajoz rivers – both with different lithological composition and erosion rates. Sediment yield was estimated using a semi-quantitative Factorial Score Model (FSM), developed specifically for Spanish rivers. The fish populations of both basins were evaluated in composition and abundances by the study of Fernández-Delgado et al., 2014. The influence of sediment yield on the fish community was analyzed using General Additive Models.The sediment yield was higher in the Guadajoz basin (921 T/Km2 per year) than in Bembézar (701 T/Km2 per year). In the former, fish communities were poorer in both fish density and diversity, with Luciobarbus sclateri as the only substantially present species and a significant relationship between sediment yield and load, and fish density. In contrast, in the Bembézar basin, sediment yield was correlated with total fish density, including Luciobarbus sclateri, Pseudochondrostoma willkommii, Cobitis paludica, Iberochondrostoma lemmingii, Anaecypris hispanica, and Cyprinus carpio. Intermediate values of sediment yield led to maximum densities, while those higher decreased the density of these species.  相似文献   

5.
This study tested the hypothesis that the flood pulse affects the diet composition and the niche breadth of Moenkhausia forestii, a small characid fish inhabiting the littoral zone of lakes. To this end, we compared the diet composition (at the population and individual levels) between hydrological periods (high and low water phases) in a floodplain lake of the Upper Paraná River. PERMANOVA revealed differences in the diet between periods (pseudoF1,38 = 8.5; p < 0.001), with predominant consumption of chironomid larvae and Ephemeroptera (aquatic resources) in the low-water period and an increase in the contribution of terrestrial resources (Hymenoptera, Coleoptera, and Orthoptera) during the high-water period. Based on the PERMDISP results, inter-individual variability in M. forestii diet also differed between periods (F1,38 = 5.80; p = 0.02), with higher values during the high-water period resulting in a dietary niche expansion. During the low-water period, we observed the dominance of chironomid larvae in the diets of most individuals, resulting in lower inter-individual variability and thus narrower niche breadth. The diet of M. forestii was affected by the flood pulse at both the population and individual levels. The most important difference was found in the origin of food items; during the low-water period, the diet consisted mainly of aquatic resources, and during the high-water period, there was a large contribution of terrestrial resources. This variation is related to the increased availability of allochthonous resources in the high period, when terrestrial areas are flooded by the overflow of the river, thereby increasing the input of resources into the aquatic environment. The increased availability of food resources during this period allowed the expansion of the trophic niche of M. forestii, accompanied by the highest richness (19 items) and the highest evenness of food items. Our findings demonstrated that the flood pulse affected the composition of the M. forestii diet at both the population and individual levels. These results support the importance of the flood pulse, which connects aquatic and terrestrial ecosystems, in providing food resources for fish.  相似文献   

6.
《Marine pollution bulletin》2009,58(6-12):889-894
As basic research for the effect of heavy oil on the fish immune system, in this study, the number of leukocyte was counted in Japanese flounder Paralichthys olivaceus, after exposure to heavy oil at a concentration of 30 g/8 L for 3 days. To compare the numbers of bacteria in the skin mucus between oil-exposed and control fish, viable bacteria were enumerated by counting colony forming unit (CFU). Compared with 5.79 ± 1.88 × 107 leukocytes/mL in the controls, the exposed fish demonstrated higher counts, averaging 1.45 ± 0.45 × 108 cells/mL. The bacterial numbers of control fish were 4.27 ± 3.68 × 104 CFU/g, whereas they were 4.58 ± 1.63 × 105 CFU/g in the exposed fish. The results suggest that immune suppression of the fish occurred due to heavy oil stressor, and bacteria could invade in the mucus, resulting in the increasing leukocyte number to prevent infectious disease.  相似文献   

7.
Using over 20 years of ground-based magnetometer data from the CANOPUS/CARISMA magnetometer array, we present a statistical characterisation of Pc5 ultra-low frequency (ULF) power in the 2–10 mHz band as a function of magnetic local time (MLT), L-shell, and solar wind speed. We examine the power across L-shells between 4.2 and 7.9, using data from the PINA, ISLL, GILL and FCHU stations, and demonstrate that there is a significant MLT dependence in both the H- and D-component median 2–10 mHz power during both fast (>500 km/s) and slow (<500 km/s) solar wind speeds. The H-component power consistently dominates over D-component power at all MLTs and during both fast and slow solar wind. At the higher-L stations (L>5.4), there are strong MLT power peaks in the morning and midnight local time sectors; the morning sector dominating midnight during fast solar wind events. At lower L-shells, there is no evidence of the midnight peak and the 2–10 mHz power is more symmetric with respect to MLT except during the fastest solar wind speeds. There is little evidence in the ground-based power of a localised MLT peak in ULF power at dusk, except at the lowest L-shell station, predominantly in the H-component. The median 2–10 mHz power increases with an approximate power law dependence on solar wind speed, at all local times across the L-shell domain studied in both components. The H-component power peaks at the latitude of the GILL station, with significantly lower power at both higher and lower L-shells. Conversely, the D-component power increases monotonically. We believe that this is evidence for 2–10 mHz power accumulating at auroral latitudes in field line resonances. Finally, we discuss how such ULF wave power characterisation might be used to derive empirical radiation belt radial diffusion coefficients based on, and driven by, the solar wind speed dependence of ULF wave power.  相似文献   

8.
Acid deposition during the 20th century led to the gradual elimination of fish in Brooktrout Lake (Adirondack Mountains, New York State). Thereafter, the lake was colonized by Chaoborus americanus, a dipteran with an aquatic larval stage that typically resides in the pelagic zone in fishless lakes. During subsequent chemical recovery from acidification, the lake was stocked with Brook Trout (Salvelinus fontinalis). For seven years following this reintroduction we examined the re-adaptation of the food chain. The C. americanus abundance and distribution was quantified utilizing a combination of hydroacoustics, traditional vertical net tows and Schindler-Patalas trap collections. Hydroacoustic backscattering signals were repeatable and correlated (r = 0.86, p = 0.003) with C. americanus abundance. Backscattering, depth, month and year were used to develop a random forest model that predicted the C. americanus density (r2 = 0.67,  p< 0.05). The hydroacoustic signal revealed a clear but limited diurnal vertical migration of C. americanus. The signal continued in the presence of the fish population beginning with reintroduction in 2005 and extending through 2011. In 2012, the hydroacoustic signal no longer was present in the lake, suggesting that the fish had eliminated the C. americanus population, which was verified with net tows. Using novel and traditional survey methods, we demonstrate that the reintroduction of fish can alter the lake community structure significantly through the extirpation of the major component of the pelagic zone.  相似文献   

9.
《Marine pollution bulletin》2012,64(5-12):516-522
Surface and core sediments collected from six mariculture farms in the Pearl River Delta (PRD) were analyzed to evaluate contamination levels of polychlorinated biphenyls (PCBs). The ∑PCBs (37 congeners) concentrations ranged from 5.10 to 11.0 ng g−1 (mean 7.96 ng g−1) in surface and 3.19 to 22.1 ng g−1 (mean 7.75 ng g−1) in core sediments, respectively. The concentrations were significantly higher than that measured in the sediments of their corresponding reference sites, whereby the average enrichment percentages were 62.0% and 42.7% in surface and core sediments, respectively. Significant correlations (R2 = 0.77, p < 0.05) of PCB homologue group proportions between fish feeds and surface mariculture sediments suggested that fish feed input was probably the main source for the enrichment of PCBs. Due to the fact that PCBs could be transferred along food chains, PCB contamination in fish feeds and mariculture sediments should not be overlooked.  相似文献   

10.
Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (?0.64‰ < δ30Si < ?0.01‰ and + 8.6‰ < δ18O < + 11.9‰) to silicified basalts (δ30Si and δ18O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ30Si < + 1.05‰ and + 10.9 < δ18O < + 17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to + 19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.  相似文献   

11.
《Marine pollution bulletin》2012,65(12):2857-2859
Baseline Hg concentration in bycatch fish from the SE Gulf of California were determined in muscle and liver of 19 species. Levels of Hg in muscle were compared with legal limits of this element in national and international legislation. Considering all fish species, mean concentrations in liver (2.458 ± 1.997 μg g−1) were significantly higher (p < 0.05) than in muscle (0.993 ± 0.670 μg g−1). The sequence of averaged Hg concentrations in most ichthyofauna was liver > muscle. Highest level of Hg in muscle (2.556 μg g−1) and liver (7.515 μg g−1) corresponded to Diapterus peruvianus and Ophioscion strabo, respectively. Considering muscle samples, none of the species had levels of Hg above the limit (1.0 μg g−1 wet weight) in the Mexican legislation; with respect to the Japanese (0.4 μg g−1 wet weight) and British (0.3 μg g−1 wet weight) legislations, 26.3% and 31.6% of the species respectively, were above the corresponding limits.  相似文献   

12.
《Continental Shelf Research》2007,27(3-4):296-308
This paper focuses on the delivery of water and sediment to the northern Adriatic to better understand the short-term evolution of continental margin sedimentation under natural and human impact. For that reason, the Po and six Apennine rivers (Metauro, Musone, Potenza, Tronto, Chienti and Pescara) are investigated. The climate-driven hydrological model HydroTrend is used to simulate discharge and sediment loads where observational data are limited. The northern Apennine hinterland has a significant impact on the sediment flux leaving the Po River, contributing 56% of the sediment it delivers to the Adriatic Sea. The Po River experienced a strong decrease in its sediment load (17.2–6.4 Mt/yr) across 1933–1987, in contrast to a small increase in its water discharge. The rivers draining the southern Apennine hinterland contribute more than 50% of the sediment load entering the Adriatic Sea, and this is in spite of human modification of their discharge through numerous small reservoirs that invariably reduce a river's sediment load. As a result, hyperpycnal flows, which historically carried 20–40% of the sediment flux from these Apennine rivers, become rare. Sediment load reduction is also reflected by retreat of the Apennine coastline. Based on the ART model (used in HydroTrend), the total sediment load to the northern Adriatic is 43 MT/yr where the northern Alpine rivers contribute 8 MT/yr, the Po River 13 MT/yr and the Apennine rivers contribute 22 MT/yr.  相似文献   

13.
A double-spike method was used to obtain Mo isotope data for sediments and waters of the seasonally anoxic Chesapeake Bay, and its primary tributary, the Susquehanna River. The dissolved Mo distribution in the estuary is non-conservative, reflecting minor Mo loss to the sediments, although removal of Mo to the sediments does not have a large influence on the isotopic composition of the water column. The δ98Mo of dissolved Mo in most of the estuary is dominated by seawater. Six samples with salinity > 15 have an average δ98Mo = + 2.17‰ (± 0.12), which agrees well with a δ98Mo value for the CASS-4 seawater standard of + 2.23‰. A single sample of Susquehanna River water has a δ98Mo of + 1.02‰, consistent with recent findings of positive δ98Mo in rivers worldwide. Susquehanna river sediments, in contrast, have δ98Mo ～ ? 0.1‰. The difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin. The δ98Mo values for estuarine sediments are offset from values in the overlying water. Most samples deposited before 1925 have δ98Mo less than 0‰, similar to the Susquehanna sediments. Subsequently, there is an increase in the variability of δ98Mo, with values ranging up to + 0.8‰. The transition to increased variability coincides with the onset of authigenic Mo deposition, which was previously attributed to escalating summertime anoxia. Authigenic Mo concentrations correlate poorly with δ98Mo in core samples, suggesting that independent mechanisms influence the two parameters. Authigenic Mo concentrations may be controlled by shifting pore water H2S levels, while δ98Mo may be primarily affected by annual variations in Mn refluxing.  相似文献   

14.
Culvert styles are being replaced on many road-stream crossings to provide long-term (>2 years) benefits, but these projects may result in short-term (0–2 years) biological impacts. We quantified the short-term effects of replacing steel-pipe culverts with open-arch structures on the benthic-macroinvertebrate communities of 6 streams in the Klamath National Forest of Northern California USA. Physical habitat showed notable site-specific effects in channel form and sedimentation, but no significant change among sites. In contrast, we observed small though significant impacts of the culvert style replacement on benthic macroinvertebrates among sites, including a statistically significant reduction in both taxa richness (p = 0.012) and abundance of intolerant taxa (p = 0.004). Moreover, there was also modest evidence of slightly elevated variability in the benthic-macroinvertebrate communities downstream following the replacement of culvert style. The long-term benefits of culvert style replacement that have been observed in other studies may outweigh the minor, short-term biological impacts observed in these streams.  相似文献   

15.
Our study summarizes data from six small water reservoirs in West Slovakia and analyzes the occurrence of zooplankton groups in relation to physico-chemical and catchment variables. The reservoirs are in two different catchments – of the Morava and Váh rivers. A total of 103 species were identified; 64 crustaceans (in both the pelagic and littoral zones) and 39 planktonic Rotifera in the pelagic zones. Significant differences were observed in species richness, abundance and biomass of planktonic crustaceans: 48 species were characteristic of the Váh catchment, while 53 were found in the Morava catchment. The density of zooplankton in the three reservoirs of the Váh River catchment ranged from 102 ind L?1 to 21,488 ind L?1 and the zooplankton biomass ranged from 0.12 mg L?1 to 103.29 mg L?1. The density of zooplankton in three Morava River catchment reservoirs ranged from 2 ind L?1 to 3928 ind L?1 and the zooplankton biomass ranged from 0.1 mg L?1 to 27.3 mg L?1. The differences were found to be related to catchment (altitude and catchment affiliation), chemical (BOD5, DO) and biological (Chromophyta, Chlorophyta) factors. Eutrophication of reservoirs in the Váh catchment was mainly due to agriculture and fish management, resulting in high nutrient concentrations. Species richness showed an unimodal response to BOD5 and N-NH4 with near optimum low values, 4.6 and 0.19 respectively. The relationship to oxygen content reflects preferences for less eutrophic waters and species richness tended to decrease with increasing DO and to decrease with increasing nutrient content.  相似文献   

16.
We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.  相似文献   

17.
In the frame of the R&D activities performed on the Boom Clay for assessing the suitability of deep clayey formations for radioactive waste disposal, the transferability of the scientific results obtained on the Boom Clay in Mol to the whole Campine Basin is investigated. Boreholes were drilled at different locations (e.g. Mol, Doel, Essen) and cores were sampled over the entire thickness of the Boom Clay formation on which the migration parameters for iodide and tritiated water (HTO) are determined.At Essen, the transport parameters in the Boom Clay can be considered as homogeneous in the range from 159 m to 241 m Below Drilling Table. The average hydraulic conductivity is (5.4 ± 1.7) × 10−12 m/s. The average ηR value for iodide is 0.25 ± 0.03 and 0.42 ± 0.05 for HTO. For HTO, this high value is mainly due to a higher value in the Putte Member (0.46 ± 0.03) compared to the other members (0.39 ± 0.02). The apparent diffusion coefficient is (1.3 ± 0.1) × 10−10 m2/s for HTO and (1.1 ± 0.2) × 10−10 m2/s for iodide. The expected effect of ionic strength (increasing with depth) on the ηR value of iodide is of the same size as the measurement error, what might explain why it was not observed.On a lateral (horizontal) level, the hydraulic conductivity at the Essen-1 borehole (5.4 × 10−12 m/s) lies between that of Boom Clay in Mol-1 (2.5 × 10−12 m/s) and that of Boom Clay in Doel-2b (1.4 × 10−11 m/s). For iodide, the higher ηR value in Essen-1 and Doel-2b (ηR ≈ 0.25) than in Mol-1 (ηR ≈ 0.16) can partly be explained by a higher ionic strength of the pore water. Apart from the Putte Member at Essen-1, the HTO porosities of the Terhagen Member and the Transition zone in Essen are in the range of the average porosities observed in Mol and Doel (ηR ≈ 0.37–0.39). For both iodide and HTO, the value of the apparent diffusion coefficient Dapp is similar in Mol-1 and in Doel-2b, with a clearly higher value for HTO than for iodide. In Essen-1, the apparent diffusion coefficients for iodide and HTO are nearly equal, and slightly smaller than the iodide value in Mol-1/Doel-2b. Accordingly, the HTO apparent diffusion coefficient is considerably smaller in Essen-1 than in Mol-1/Doel-2b.  相似文献   

18.
During the past two-three decades the water chemistry in southern Norway has recovered considerably from acidification. The biological response, however, has been more variable. Thus, it has become increasingly apparent that decades of acidification may have masked other restrictors to fish populations in this area. The current study compiles data on young brown trout (Salmo trutta) density and water chemistry from 16 sites in River Sira in southwestern Norway during 2003–2014. The water chemistry during late snowmelt period was highly dilute, having a median conductivity and Ca of 10.5 μS/cm and 0.42 mg/l, respectively (n = 208). The corresponding minimum values were 5.0 μS/cm and 0.15 mg/l. With a pH median value of 5.89, the water was only slightly acidic. No effect of pH on the density of young brown trout was found, suggesting that acidification is no longer a limiting factor. However, both conductivity in year n-1 and Ca in year n correlated significantly with the density of 1+ brown trout. Al in year n, and surprisingly Ca in year n-1, correlated negatively on their densities. We conclude that very dilute water chemistry during late snowmelt is a limiting factor for the recruitment of brown trout in the study river. This effect has probably existed in the past as well, but has been less apparent due to decades of severe acidification.  相似文献   

19.
Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation.For this study, we have measured the Zn isotopic composition of 20 tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites (δ66/64Zn = 0.61 ± 0.30‰); North American bediasites (δ66/64Zn = 1.61 ± 0.49‰); Ivory Coast tektites (δ66/64Zn = 1.66 ± 0.18‰); the Australasian tektites (others than the Muong Nong-type indochinites) (δ66/64Zn = 1.84 ± 0.42‰); and Central European moldavites (δ66/64Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ66/64Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials.The elemental abundance of Zn is negatively correlated with δ66/64Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion-limited regime.  相似文献   

20.
《Marine pollution bulletin》2010,60(8-12):221-233
This study assessed foraminiferal assemblages in Biscayne Bay, Florida, a heavily utilized estuary, interpreting changes over the past 65 years and providing a baseline for future comparisons. Analyses of foraminiferal data at the genus level revealed three distinct biotopes. The assemblage from the northern bay was characterized by stress-tolerant taxa, especially Ammonia, present in low abundances (∼2.0 × 103 foraminifers/gram) though relatively high diversity (∼19 genera/sample). The southwestern margin of the bay was dominated by Ammonia and Quinqueloculina, an assemblage characterized by the lowest diversities (∼12 genera/sample) and highest abundances (∼1.1 × 104 foraminifers/gram), influenced by both reduced salinity and elevated organic-carbon concentrations. A diverse assemblage of smaller miliolids and rotaliids (∼26 genera/sample) characterized the open-bay assemblage, which also had a significant component (∼10%) of taxa that host algal endosymbionts. In the past 65 years, populations of symbiont-bearing taxa, which are indicators of normal-marine conditions, have decreased while stress-tolerant taxa, especially Ammonia spp., have increased in predominance.  相似文献   

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