排序方式： 共有6条查询结果，搜索用时 15 毫秒
1
1.
2.
Dmitry A.Ruban 《地学前缘(英文版)》2013,4(1):87-93
Many gastropod taxa went extinct during the Late Cretaceous.The stratigraphic ranges of 268 genera permit to establish the longevity of extinction victims for each stage of this epoch."Young" taxa (originated within 3 epochs before the extinction) prevailed among victims of the extinctions in all stages.The proportion of "old" taxa(originated before the Cretaceous) that went extinct was the highest in the Cenomanian,and it was the lowest in the Coniacian and the Maastrichtian.It appears that the end-Cretaceous mass extinction affected chiefly "young" taxa.However,the comparison with the earlier time intervals suggests that this pattern of selectivity by generic longevity was not specific for the noted catastrophe,but,in contrast,it was typical for the entire Late Cretaceous.The latest Cenomanian environmental perturbation(OAE2) caused a stronger extinction of "old" taxa,and thus,this biotic crisis was less selective by generic longevity.This hypothesis,however,is not proven by the statistical test. 相似文献
3.
Various abiotic and biotic factors determine the natural fluctuations of Daphnia spec. populations; food quality and dissolved humic substances (HSs) being among these factors. In this contribution, we try to disentangle the relative impact of food quality and simultaneous HSs exposure on the fertility and longevity of D. magna. It is understood that HS-mediated stress leads to reduced fecundity in well-fed D. magna females; hence, it was expected that poor food, as a second stressor, would aggravate the HS-mediated effects. Three diets were tested: the green algae Pseudokirchneriella subcapitata, baker’s yeast alone, and baker’s yeast plus dissolved ascorbic acid, and exposed D. magna to a HS preparation which has been shown effective in previous bioassays. It was hypothesized that the lifespan and fertility of D. magna would be best when fed green algae, and worst when fed only baker’s yeast. However, contrary to these expectations, any addition of HSs reduced the stress caused by poor food quality and increased lifespan and fecundity. In the yeast series, asexually produced diapausing eggs occurred via a so far unknown pathway. With yeast diet, the expanded lifespans were slightly above, whereas the increased offspring numbers lay below, the corresponding data of the algae-fed individuals. The potential of HSs as an additional food source and as a means to extend the lifespan is discussed. These findings open the innovative perspective that under low quality food conditions, additional stressors at certain intensities may even be beneficial to individuals and populations. 相似文献
4.
5.
Recently emerged indications imply that particular concentrations and qualities of dissolved organic carbon (DOC) have the potential to reduce species diversity in zooplankton. In this contribution, we tested the impact of a major component of DOC, the dissolved humic substances (HSs), on two life history traits of a clone of Daphnia magna, namely lifespan and reproduction. We chose reproduction as a variable since it guarantees the persistence of the population in its habitat, and lifespan was selected as the most integrative individual variable. We found that lifespan was extended by exposure to HSs, but lifetime reproduction was clearly reduced. On the other hand, the mineral and biochemical composition of food also significantly modulated these life history traits. By feeding coccal green algae that contained linolenic acid, arginine (Arg), and histidine (His), but that lacked polyunsaturated fatty acids (PUFAs) with C-back bones >18, we examined how short-chained PUFAs, either alone or in concert with Arg and His, are able to modulate life history traits. Our expectation was that the additional challenge by HSs would shift the energy allocation in favor of reproduction. We found that the carbon and amino acid content may account for longevity, even when challenged by HSs. Reproduction appeared to be triggered by the C:P and N:P ratios of the food, particularly given the rather low P-content, which was within the range of severe growth limitation. Consequently, exposure to HSs reduced offspring numbers. Resting eggs without males occurred only in Desmodesmus armatus fed Daphnia that were simultaneously exposed to HSs. Under these conditions, the total amino acid (AA) content seemed to trigger reproduction. In summary, exposure to HSs appeared to have the potential to change the competitive ability of zooplankters by modulating the energy allocation between body maintenance and reproduction. 相似文献
6.
Anthony C. Harris W. James Dunlap Peter W. Reiners Charlotte M. Allen David R. Cooke Noel C. White Ian H. Campbell Suzanne D. Golding 《Mineralium Deposita》2008,43(3):295-314
Application of multiple chronometers (including U–Pb and 40Ar/39Ar geochronology and zircon and apatite (U–Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry
copper–gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U–Pb geochronology,
combined with our new 40Ar/39Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first
at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries).
Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, 40Ar/39Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12 ± 0.13 Ma; biotite) to
the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated
hydrothermal alkali feldspar (6.82 ± 0.05 Ma and 6.64 ± 0.09 Ma). The temperature-sensitive Ar data also record an unexpected
prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent
protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (~100–200°C) reheating
events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part
by our new 40Ar/39Ar data (including multidomain diffusion modeling) and (U–Th)/He ages. Single-grain (U–Th)/He ages (n = 5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization
of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent
with the deposit cooling rapidly from magmatic temperatures to below about 300°C, with a more protracted history down to 150°C.
We conclude that the late-stage low-temperature (150 to 200°C) thermal anomaly localized at Bajo de la Alumbrera resulted
from radiation of heat and/or fluids sourced from deeper-seated magma bodies, emplaced beneath the deposit. To produce the
observed thermal longevity of the porphyry system, magma bodies underlying the Bajo de la Alumbrera deposit must have been
repeatedly replenished by new magma batches. Without replenishment, crystallization of the source magma will occur, and heat
release will stop, leading to rapid cooling (in less than ten thousand years). The influx of deep-seated magma may have caused
the development of late low-temperature hydrothermal alteration assemblages at Bajo de la Alumbrera, at the same time that
mineralization formed at Agua Rica, some 25 km away. All available chronologic data for the Bajo de la Alumbrera deposit suggest
that the hydrothermal system was active episodically over at least a three-million and possibly up to a four-million-year
period.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
1