Influence of sediment grain size and mineralogy on testate amoebae test construction     

Eric Armynot du Châtelet  François GuillotPhilippe Recourt  Sandra VentalonNicolas Tribovillard 《Comptes Rendus Geoscience》2010,342(9):710-717

Testate amoebae are increasingly used for environmental monitoring as well as paleoenvironmental reconstructions. Paleoecological interpretations of testate amoebae assemblages depend on the understanding of the ecological processes operating today. We then ask the question of the link between testate structure and its environment. This study analyses both the grain size and mineralogical assemblage of tests of common species belonging to the genus Centropyxis and Difflugia. It is concluded that grain size is a limiting factor for test construction, whereas mineral composition is not. Hence, when analyzing agglutinated testate amoebae for paleoenvironmental reconstructions, it should be taken into account the mean grain size of the sediment. A non-appropriate grain-size probably inhibits the development of a testate amoebae specific assemblage.  相似文献   

Caracas, Venezuela, Site Effect Determination with Microtremors   总被引：1，自引：0，他引：1  

A.-M. Duval  S. Vidal  J.-P. Méneroud  A. Singer  F. De Santis  C. Ramos  G. Romero  R. Rodriguez  A. Pernia  N. Reyes  C. Griman 《Pure and Applied Geophysics》2001,158(12):2513-2523

—?Caracas 1967 earthquake caused heavy damage to multi-story buildings. In 1995, 184 microtremor measurement points were performed over the city. The measurement grid was more or less dense and covered the main part of the alluvial basin as well as surrounding rock basement. For each point, the horizontal record spectrum was divided by the vertical one (H/V ratio). Subsequently, the strongest value (Ao) of this ratio in a given frequency band was kept, as well as the frequency (Fo) where it occurred. Spatial interpolations of Ao and Fo were performed among all points of Palos Grandes district. A map was plotted representing a single surface where Ao is relief and Fo is represented by grey gradation. An alluvion thickness (H) map can be compared with this result. Damaged buildings are located on the same map. Fo decreases until 0.6?Hz when alluvion thickness (H) increases. Fo values fit with frequencies previously predicted from computation and with 1967 earthquake observations. Moreover, interpolation surfaces show that amplification (Ao) of H/V on microtremor is quite low above rock but is high on the south part of the basin. The maximum occurs over the non-urbanized zone. However the other area, where Ao is higher than 5, roughly corresponds to the location of the four collapsed buildings. Furthermore, the estimated natural frequency of these buildings was around Fo. Graphics showing H, Fo and Ao evolution through the basin were composed. Links between all these values are noticeable. Fo is claimed to be very similar to the resonance frequency of soil. As for Ao, it could be a fairly relevant sign of damage seriousness. Microtremor technique is an economic tool and it allows measurement grid as dense as desired. H/V ratio processing followed by interpolation of maximum values provides precise and useful information about expected site effect.  相似文献   

Late quaternary evolution of a subantarctic paleofjord, Tierra del Fuego     

Sandra Gordillo  Andrea M.J. Coronato  Jorge O. Rabassa 《Quaternary Science Reviews》1993,12(10)

Lago Roca-Lapataia valley (54°50′S, 68°34′W) is a paleofjord that was occupied by a valley-glacier system during the glacial maximum of the late Pleistocene (estimated ca. 18–20 ka BP). Deglaciation began before 10,080 ± 270 BP. The marine fauna in several marine terraces found in the area shows that early-middle Holocene climatic conditions were basically the same as at present. Species found are characteristic of cold and shallow waters, although minor temperature fluctuations cannot be ruled out for this period. A recent radiocarbon date of 7518 ± 58 BP on Chlamys patagonica (NZ # 7730) confirms that Lago Roca was transformed into a fjord ca. 7500–8000 BP. The sea reached its maximum level of 8–10 m a.s.l. around 6000 BP and at 4000–4500 BP was at least above 6 ± 1 m a.s.l. Later, when sea level fell, Lago Roca was occupied by fresh water and was no longer tidal. The relative land-sea positions during this period are a consequence of combined eustatic and neotectonic processes.  相似文献   

A simple photometer for precise determination of dissolved oxygen concentration by the Winkler method with recommendations for improving respiration rate measurements in aquatic organisms     

Sandra L. Vargo  R. Ken Forcé 《Estuaries and Coasts》1981,4(1):70-74

A simple inexpensive photometer designed for Winkler titration end-point detection is described. The precision of replicate dissolved oxygen measurements using this instrument was 0.06–0.22%. This high precision is needed to measure the small changes in dissolved oxygen concentration for determining the respiration rate of small aquatic organisms. Other recommendations for improving precision are also presented.  相似文献   

The stable isotopic composition of coral skeletons: control by environmental variables   总被引：1，自引：0，他引：1  

Sandra M. Weil  Robert W. Buddemeier  Stephen V. Smith  Peter M. Kroopnick 《Geochimica et cosmochimica acta》1981,45(7):1147-1153

The reef corals Pocillopora damicornis and Montipora verrucosa were cultured under various controlled temperature and light conditions. The corals were analyzed for growth rate, tissue pigment content and skeletal ¹³C and ¹⁸O. Coral skeletal δ¹³C values varied with light dose and correlated with changes in zooxanthellar pigment. The δ¹³C values of skeletal aragonite seem to be modified by oxidation of photosynthetically produced organic matter.Functionally significant relationships between coral skeletal δ¹⁸O values and temperature have been determined. The temperature coefficients of the δ¹⁸O values [?4.4°C (%.)^?1] are similar to the first order coefficient in the equilibrium paleotemperature equation, but the δ¹⁸O values have taxonomically consistent offsets from equilibrium. The offsets may be attributed to the coral metabolism with slight but statistically significant differences between the two genera. Environmental and metabolic variables other than temperature have little or no effect on skeletal δ¹⁸O.  相似文献   

Diffusion of ions in sea water and in deep-sea sediments   总被引：3，自引：0，他引：3  

Sandra Gregory 《Geochimica et cosmochimica acta》1974,38(5):703-714

The tracer-diffusion coefficient of ions in water, D_j⁰, and in sea water, $\text{D}{}_{\mathrm{j}}{}^1$, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective $\text{D}{}_{\mathrm{j}}{}^1$ or D_j⁰ values. The tracer diffusion coefficients of ions in deep-sea sediments, D_j,sed., can be related to $\text{D}{}_{\mathrm{j}}{}^1$ by $\text{D}{}_{\mathrm{j,sed.}}\text{= D}{}_{\mathrm{j}}{}^1\text{·}\text{α}\text{θ}{}^2$, where θ is the tortuosity of the bulk sediment and a a constant close to one.  相似文献   

Bimodal early proterozoic volcanism in the Grythytte field and associated volcano-plutonic complexes,Bergslagen, Central Sweden     

Willem van der Velden  James Baker  Sandra de Maesschalck  Thea van Meerten 《International Journal of Earth Sciences》1982,71(1):171-181

The Grythytte field sequence forms a NNW belt of acid-basic metavolcanic and metasedimentary rocks. The geology shows characteristics of a dynamic process of trough-forming and infilling which to a large extent post-dates the Hjulsjö volcanoplutonic complex situated to the east. In the latter complex acid metapyroclastics strike around a centre of granite and granophyre, which cut and are cut by basic sills and dykes. Field and petrochemical evidence points to a distinct bimodal magmatism that ist manifested during one short phase of a rifting period.

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Zusammenfassung Die Grythytte Schichtenfolge bildet einen NNW verlaufenden Gürtel aus sauren und basischen Metavulkaniten und Metasedimenten. Die geologischen Befunde weisen auf einen dynamischen Entstehungsprozeß von Grabenbildung und Füllung hin. Im weiter östlich gelegenen vulkanisch-plutonischen Hjulsjö-Komplex streichen älteren metamorphen pyroklastischen Gesteine rund ein Granit-Granophyr Zentrum. Dieser Komplex durchschneidet und wird durchschnitten von basischen Sills und Gängen. Feldgeologische und geochemische Untersuchungen weisen deutlich auf einen bimodalen Magmatismus hin, der während einer kurzen Phase der Grabenbildung stattfand.

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Résumé La séquence de Grythytte forme une ceinture de roches métavolcaniques et métasédimentaires de direction NNO. La géologie fait apparaître les caractéristiques d'un processus dynamique de formation de fosse et de remplissage, qui s'est produit, pour la plus grande partie, après la formation du complexe volcano-plutonique de Hjulsjö situé à l'Est. Dans ce dernier complexe des produits pyroclastiques acides sont disposés autour d'un centre granitoïde et granophyrique, qui à son tour recoupe et est recoupé par des sills et des dykes basiques. Les observations de terrain et les faits pétrochimiques indiquent un procèssus magmatique distinctement bimodal qui s'est manifesté pendant une phase courte dans une période de «rifting».

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Grythytte , NNW . . Hjulsj -- . . , , .

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Statistical analysis of the largest possible earthquake magnitudes on the Ecuadorian coast for selected return periods     

Sandra García-Bustos  Jimmy Landín  Ricardo Moreno  A.S.E. Chong  Maurizio Mulas  Mónica Mite 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2020,14(1):56-68

ABSTRACT

In this work, we have studied the largest earthquake magnitudes on the Ecuadorian coast by using the principles of Extreme Value Analysis based on its two approaches: Block Maxima and Peaks-over-Threshold. First, before modelling the recorded earthquakes, the K-means clustering technique was applied to determine a classification according to the level of magnitude of the earthquakes. Then, models based on the Extreme Value theory of earthquake magnitudes were developed for each of the four clusters that were found, and finally, the best-fitted models were those known as Fréchet and Gumbel ones. The zone with the greatest earthquake magnitudes on the Ecuadorian coast is located between the north of the province of Manabí and the south of the province of Esmeraldas, with a return period of 50 years for an earthquake with magnitude greater than 7.7 M_W.  相似文献   

The carbon isotopic distribution of Murchison amino acids     

Sandra Pizzarello  Yongsong Huang 《Geochimica et cosmochimica acta》2004,68(23):4963-4969

The δ¹³C values of thirty-four individual amino acids and two pyridine carboxylic acids have been obtained fromthe Murchison meteorite. They were found to range from +4.9 to +52.8‰, with statistically significant differences observed both within and between amino acid subgroups. The ¹³C content of α-amino acids declines with increasing chain length, a trend similar to the ones previously observed for carboxylic acids and alkanes. Also 2-methyl-2-amino acids were found to be heavier in ¹³C than the corresponding 2-H homologues. The3-, 4-, and 5-amino acids do not show a comparable declining trend in δ¹³C values and neither do the amino dicarboxylic acids. This variability in δ ¹³C values can be interpreted as to indicate that the synthetic histories of soluble organics in meteorites may have been diverse even within groups of compounds with very similar functional group composition.  相似文献   

Generalized thermostatistics and wavelet analysis of solar wind and proton density variability     

Maurício Jos Alves Bolzan  Reinaldo Roberto Rosa  Fernando Manuel Ramos  Paulo Roberto Fagundes  Yogeshwar Sahai 《Journal of Atmospheric and Solar》2005,67(17-18):1843

In this paper, we analyze the probability density function (PDF) of solar wind velocity and proton density, based on generalized thermostatistics (GT) approach, comparing theoretical results with observational data. The time-series analyzed were obtained from the SOHO satellite mission where measurements were sampled every hour. We present in the investigations data for two years of different solar activity: (a) moderate activity (MA) period (1997) and (b) high activity (HA) period (2000). For the MA period, the results show good agreement between experimental data and GT model. For the HA period, the agreement between experimental and theoretical PDFs was fairly good, but some distortions were observed, probably due to intermittent characteristics of turbulent processes. As a complementary analysis, the global wavelet spectrum (GWS) was obtained, allowing the characterization of the predominant temporal variability scales for both periods and the stochastic aspects of the nonlinear solar wind variability are discussed.  相似文献