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71.
Acta Geotechnica - The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand-fines mixtures under cyclic... 相似文献
72.
The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in 18O of more than 3‰ relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in 18O for these species of 1.7 to 2.3‰. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens’ δ13C values are mainly controlled by seasonal variations in δ13CDIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope ‘vital effects’ in ostracods. Two mechanisms that might enrich the 18O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation. 相似文献
73.
Susanne Seitz Lukas P. Baumgartner Anne‐Sophie Bouvier Benita Putlitz Torsten Vennemann 《Geostandards and Geoanalytical Research》2017,41(1):69-75
Two quartz samples of igneous origin, UNIL‐Q1 (Torres del Paine Intrusion, Chile) and BGI‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL‐Q1 and BGI‐Q1 were evaluated for homogeneity using SIMS. Their reference δ18O values were determined by CO2 laser fluorination. The average δ18O value found for UNIL‐Q1 is 9.8 ± 0.06‰ and that for BGI‐Q1 is 7.7 ± 0.11‰ (1s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2s; UNIL‐Q1) and 0.40–0.48‰ (2s; BGI‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI‐Q1 is also suitable for SIMS trace element measurements. 相似文献
74.
Rare earth elements (REE), while not essential for the physiologic functions of animals, are ingested and incorporated in ppb concentrations in bones and teeth. Nd isotope compositions of modern bones of animals from isotopically distinct habitats demonstrate that the 143Nd/144Nd of the apatite can be used as a fingerprint for bedrock geology or ambient water mass. This potentially allows the provenance and migration of extant vertebrates to be traced, similar to the use of Sr isotopes. Although REE may be enriched by up to 5 orders of magnitude during diagenesis and recrystallization of bone apatite, in vivo143Nd/144Nd may be preserved in the inner cortex of fossil bones or enamel. However, tracking the provenance of ancient or extinct vertebrates is possible only for well-preserved archeological and paleontological skeletal remains with in vivo-like Nd contents at the ppb-level. Intra-bone and -tooth REE analysis can be used to screen for appropriate areas. Large intra-bone Nd concentration gradients of 101-103 are often measured. Nd concentrations in the inner bone cortex increase over timescales of millions of years, while bone rims may be enriched over millenial timescales. Nevertheless, εNd values are often similar within one εNd unit within a single bone. Larger intra-bone differences in specimens may either reflect a partial preservation of in vivo values or changing εNd values of the diagenetic fluid during fossilization. However, most fossil specimens and the outer rims of bones will record taphonomic 143Nd/144Nd incorporated post mortem during diagenesis. Unlike REE patterns, 143Nd/144Nd are not biased by fractionation processes during REE-uptake into the apatite crystal lattice, hence the εNd value is an important tracer for taphonomy and reworking. Bones and teeth from autochthonous fossil assemblages have small variations of ±1 εNd unit only. In contrast, fossil bones and teeth from over 20 different marine and terrestrial fossil sites have a total range of εNd values from -13.0 to 4.9 (n = 80), often matching the composition of the embedding sediment. This implies that the surrounding sediment is the source of Nd in the fossil bones and that the specimens of this study seem not to have been reworked. Differences in εNd values between skeletal remains and embedding sediment may either indicate reworking of fossils and/or a REE-uptake from a diagenetic fluid with non-sediment derived εNd values. The latter often applies to fossil shark teeth, which may preserve paleo-seawater values. Complementary to εNd values, 87Sr/86Sr can help to further constrain the fossil provenance and reworking. 相似文献
75.
Torsten?HaberzettlEmail author Michael?Fey Andreas?Lücke Nora?Maidana Christoph?Mayr Christian?Ohlendorf Frank?Sch?bitz Gerhard?H.?Schleser Michael?Wille Bernd?Zolitschka 《Journal of Paleolimnology》2005,33(3):283-302
The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The total inorganic carbon content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the Medieval Climate Anomaly in southeastern Patagonia. At the beginning of the Little Ice Age the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century. 相似文献
76.
Jens Jasche Benedetta Ciardi Torsten A. Enßlin 《Monthly notices of the Royal Astronomical Society》2007,380(2):417-429
One of the most-outstanding problems in the gravitational collapse scenario of early structure formation is the cooling of primordial gas to allow for small-mass objects to form. As the neutral primordial gas is a poor radiator at temperatures T ≤ 104 K , molecular hydrogen is needed for further cooling down to temperatures T ∼ 100 K . The formation of molecular hydrogen is catalyzed by the presence of free electrons, which could be provided by the ionization due to an early population of cosmic rays (CRs). In order to investigate this possibility, we developed a code to study the effects of ionizing CRs on the thermal and chemical evolution of primordial gas. We found that CRs can provide enough free electrons needed for the formation of molecular hydrogen, and therefore can increase the cooling ability of such primordial gas under following conditions. A dissociating photon flux with F < 10−18 erg cm−2 Hz−1 s−1 , initial temperature of the gas ∼103 K , total gas number densities n ≥ 1 cm−3 , and cosmic-ray sources with . 相似文献
77.
Species belonging to the same ecological guild can coexist only if there are differences in their responses to limiting resources. Thus, competitor introductions may modify the niches of the resident competitors, but empirical studies are rare. The extent of niche modification might further depend on the species’ trophic specialisation along the specialists-to-generalists axis. In this study, we performed a semi-natural experiment by introducing a trophically specialised piscivorous fish (pikeperch) to the resident predator guild of a trophic generalist (perch) and a trophic specialist (pike) in a temperate freshwater lake. Individual diet data were used to calculate total niche hypervolume and inter- and intraspecific dietary niche overlap to evaluate strength of competitive interactions before and after pikeperch introduction. In both pike and perch, there was a moderate niche compression in response to pikeperch stocking. Furthermore, whereas perch diet hypervolume did not overlap at all with that of pikeperch, the intersection of niche space between the two trophic specialists was highly asymmetrical in favour of pike due to its less specialised diet composition in comparison with pikeperch. The intraspecific dietary niche overlap between perch size classes reflected an enhanced partitioning, whereas that of pike size classes was found almost unchanged in response to pikeperch stocking. These empirical results corroborate that competitive interactions and the degree of specialisation may strongly shape the success of introductions of top-predators in freshwater ecosystems. The most specialised species may persist only if the competitors belong rather to the generalists along the specialisation continuum. 相似文献
78.
Syee Weldeab Kay-Christian EmeisChristoph Hemleben Torsten W VennemannHartmut Schulz 《Geochimica et cosmochimica acta》2002,66(20):3585-3598
Isotopic ratios of Sr and Nd from lithogenic components of three isochronous core sections recovered from an east-west transect in the Eastern Mediterranean Sea (EMS) have been analyzed. The data are used for a quantitative estimate of the temporal and spatial variation of detrital flux to the EMS, assuming Saharan dust and Aegean/Nile particulate matter as dominant end members. It was established that the carbonate-free Saharan dust flux during deposition of the nonsapropel layers of marine oxygen isotope stage 5.4 (MIS 5.4) was similar to the present flux. During the deposition of sapropels S5 and S6, however, the Saharan dust input was drastically reduced and was not balanced by a change in the riverine influx at this time. Denser vegetation cover during more humid conditions may have reduced physical erosion and sediment removal in the source area. During marine oxygen isotope stage 6.2 (MIS 6.2) a pronounced increase of Saharan dust and detrital influx from the Aegean region is evident and implies more arid conditions in the southern and northern catchment areas. During this period, intersite variations are interpreted in terms of their geographic location relative to the seaways connecting the Aegean Sea and EMS. The width of the straits and hence the amount of sediment entering the eastern basins may have been affected by a low sea level that impeded interbasin sediment dispersal. 相似文献
79.
80.
Sebastian Wagner Martin Widmann Julie Jones Torsten Haberzettl Andreas Lücke Christoph Mayr Christian Ohlendorf Frank Schäbitz Bernd Zolitschka 《Climate Dynamics》2007,29(4):333-355
This study investigates the atmospheric circulation in transient climate simulations with a coupled atmosphere–ocean general
circulation model (GCM) for the mid-Holocene (MH) period 7–4.5 ka BP driven with combinations of orbital, solar and greenhouse
gas forcings. The focus is on southern South America. Statistical downscaling models are derived from observational data and
applied to the simulations to estimate precipitation in south-eastern Patagonia during the MH. These estimates are compared
with lake level estimates for Laguna Potrok Aike (LPA) from sediments. Relative to pre-industrial conditions (i.e. 1550–1850),
which show extraordinarily high lake levels, the proxy-based reconstructed lake levels during the MH are lower. The downscaled
simulated circulation differences indicate higher LPA precipitation during the MH from March to August, higher annual means,
and reduced precipitation from September to February. Thus the reconstructed lower LPA lake levels can not be explained solely
by the simulated precipitation changes. Possible reasons for this discrepancy are discussed. Based on proxy data from southern
South America hypotheses have also been proposed on the latitudinal position of the southern hemispheric westerlies (SHWs).
In agreement with some of these hypotheses our simulations show an increased seasonal cycle of the latitudinal position of
the SHWs during the MH, which can be explained by the orbital forcing. The simulations also show stronger SHWs over southern
Patagonia during austral summer and weaker SHWs during winter. The downscaling model associates weaker SHWs with increased
precipitation in the LPA region. However, this relationship is only moderate, and therefore the downscaling model does not
support the assumption of a strong link between mean SHWs and precipitation over south-eastern Patagonia, which is the basis
of many proxy-based hypotheses about the SHWs. 相似文献