排序方式： 共有19条查询结果，搜索用时 593 毫秒
1.
Seismotectonics of the Cyprian Arc 总被引：1，自引：0，他引：1
2.
Tidal marsh degradation has been attributed to a number of different causes, but few studies have examined multiple potential factors at the same sites. Differentiating the diverse drivers of marsh loss is critical to prescribing successful interventions for conservation and restoration of this important habitat. We evaluated two hypotheses for vegetation loss at two marshes in Long Island Sound (LIS): (1) marsh submergence, caused by an imbalance between sea-level rise and marsh accretion, and (2) defoliation associated with herbivory by the purple marsh crab, Sesarma reticulatum. At our western LIS site, we found no evidence of herbivory: crabs were scarce, and crab-exclusion cages provided no benefit. We attribute degradation at that site to submergence, a conclusion supported by topographic and hydrologic data showing that loss of vegetation occurred only in wetter parts of the marsh. In contrast, at our central LIS site, our observations were consistent with herbivory as a driving force: There were substantial populations of Sesarma, crab-exclusion cages allowed plants to thrive, and vegetation loss took place across a variety of elevations. We also analyzed soil conditions at both sites, in order to determine the signatures of different degradation processes and assess the potential for restoration. At the submergence site, unvegetated soils exhibited high bulk density, low organic content, and low soil strength, posing significant biogeochemical challenges to re-colonization by vegetation. At the herbivory site, unvegetated soils had a characteristic “riddled-peat” appearance, resulting from expansion and erosion of Sesarma burrow networks. The high redox potential and organic content of those soils suggested that revegetation at the herbivory site would be likely if Sesarma populations could be controlled before erosion leads to elevation loss. 相似文献
3.
High-rate GPS measurements of earthquake-induced strong crustal movements reveal important information on large amplitude displacements, which cannot be obtained by other seismic monitoring equipment. However, obtaining accurate measurements of these strong movements can be challenging, because large magnitude earthquakes (M > 7) affect a wide area surrounding the epicenter. As a result, the GPS recorded movements are calculated with respect to distant sites (relative positioning), or with satellite parameters estimated from distant sites (precise point positioning). In order to improve the accuracy of the strong motion GPS measurements, we developed a new method, based on a spatial filtering technique. The method calculates the displacement of a high-rate monitoring network with respect to a moving near field site and uses a stacking technique to remove the movements of the reference site from all the time series. We applied the new method to the analysis of 5 Hz data acquired by the Nicoya Peninsula network, which recorded strong crustal movements induced by the 2012, M = 7.6 Costa Rica earthquake. The results were successfully tested with respect to 1 Hz time series calculated with a far field reference site. The spatial filtering method also removes other systematic common noise from the time series, possibly due to atmospheric delay or orbital errors and, hence, produces more accurate solutions that those based on far fields sites, or on near field site experiencing earthquake-induced action. 相似文献
4.
M. Shimon Y. Rephaeli B. W. O'Shea M. L. Norman 《Monthly notices of the Royal Astronomical Society》2006,368(2):511-511
Scattering of the cosmic microwave background (CMB) in clusters of galaxies polarizes the radiation. We explore several polarization components which have their origin in the kinematic quadrupole moments induced by the motion of the scattering electrons, either directed or random. Polarization levels and patterns are determined in a cluster simulated by the hydrodynamical enzo code. We find that polarization signals can be as high as ∼1 μK , a level that may be detectable by upcoming CMB experiments. 相似文献
5.
M. Shimon Y. Rephaeli S. Sadeh B. Keating 《Monthly notices of the Royal Astronomical Society》2009,399(4):2088-2098
Mapping cosmic microwave background (CMB) polarization is an essential ingredient of current cosmological research. Particularly challenging is the measurement of an extremely weak B-mode polarization that can potentially yield unique insight on inflation. Achieving this objective requires very precise measurements of the secondary polarization components on both large and small angular scales. Scattering of the CMB in galaxy clusters induces several polarization effects whose measurements can probe cluster properties. Perhaps more important are levels of the statistical polarization signals from the population of clusters. Power spectra of five of these polarization components are calculated and compared with the primary polarization spectra. These spectra peak at multipoles ℓ≥ 3000 , and attain levels that are unlikely to appreciably contaminate the primordial polarization signals. 相似文献
6.
Shear faults in Upper Cretaceous limestones of the central Negev desert adjacent to the Dead Sea Transform (DST) feature extensive ferruginous mineralization and dolomitization. This has been related to topographically driven flow of metalliferous groundwaters through an underlying clastic (Nubian Sandstone) aquifer and rise of the fluids up the fault zones. The present study combines Pb and Sr isotope measurements with detailed sampling and petrography at the eastern end of the Paran fault (Menuha Ridge) in order to identify the types of groundwater and the sources of enriched elements in this regional-scale sedimentary mineralization. Ferroan and non-ferroan dolomitization along the Paran fault caused significant enrichment of several elements (Mg, Cu, Mn, Ni, V, Zn, Pb, and U) and 87Sr/86Sr values that are significantly higher than the Upper Cretaceous limestone country rock. The non-ferroan dolomite and the ferroan dolomite sampled at three sites along the Menuha Ridge have similar 87Sr/86Sr values 0.7076-0.7089, and 0.7077-0.7086, respectively. Additionally, there is a positive correlation between Mg-content of the dolomites and their 87Sr/86Sr values. The isotopic composition of Sr and Pb of dolomite corresponds to the mineralogical type identified in the mineralized rock (non-ferroan dolomite, simple-zoned ferroan dolomite, and complex-zoned ferroan dolomite). The 207Pb/204Pb and 206Pb/204Pb ratios of Fe oxides and dolomites from the three sites plot on a straight line, where the simple-zoned ferroan dolomite values are at the non-radiogenic end of the line and the complex-zoned ferroan dolomites at the radiogenic end. Both 206Pb/204Pb and 207Pb/204Pb ratios in dolomites and to a lesser degree in Fe-oxides suggest that a mixing between two end-members controls the behavior of Pb in the mineralization products along the Paran fault. Rather than a single fluid source, the study indicates that two types of metalliferous groundwaters were involved in the dolomitization and mineralization along the Paran fault. The first, and hitherto undocumented, fluid source is the Mg-rich Dead Sea Rift brine, migrating in the sub-surface before dolomitizing the carbonate bedrock. Migration of the brines took a deep path to the site of mineralization, with temperatures reaching 75 °C. Based on the geological history of the region, this probably took place in the Late Miocene-Early Pliocene interval. The second type of groundwater acquired its high solute concentrations from leaching igneous rocks and clastic sediments in the sub-surface, and infiltrated along the Paran fault, precipitating Fe-rich minerals and caused the first stage of dolomitization. This groundwater flowed at shallower depth than the DSR brines, and at lower temperatures (T ? 50 °C). The study shows that sedimentary mineralization in faults adjacent to active transform fault zones may arise from the combination of several different fluid flow regimes. 相似文献
7.
8.
Analysis of a 1.15 km deep apatite fission track (AFT) thermochronology profile at the Underground Research Laboratory (URL), in the southwestern Canadian Shield suggests two Phanerozoic heating and cooling episodes indicating significant, previously unsuspected, Phanerozoic heat flow variations. Phanerozoic temperature and heat flow variations are temporally associated with burial and erosion of the Precambrian crystalline shield and its overlying Phanerozoic successions, which are now eroded completely. Maximum Phanerozoic temperatures occurred in the late Paleozoic when the geothermal gradient is estimated to have been ~ 40-50 °C/km (compared to a present day gradient of ~ 14 ± 2 °C/km) and the sedimentary cover was ~ 800-1100 m thick. Our thermal history models, confirm regional stratigraphic relationships that suggest that the Paleozoic succession was completely eroded prior to beginning of Mesozoic sedimentation. A second heating phase occurred during Late Cretaceous-Paleogene burial when the geothermal gradient is estimated to have been ~ 20-25 °C/km and the Mesozoic and Cenozoic succession was ~ 1200 to 1400 m thick. The Phanerozoic thermal history at the URL site shows a pattern similar to that inferred previously for the epicratonic Williston Basin, the centre of which lies several 100 km to the west. This implies a common regional thermal history for cratonic rocks underlying both the basin and the currently exposed shield. It is suggested that the morphotectonic differences between the Williston Basin and the exposed shield at the URL are due to a dissimilar thermomechanical response to a common, but more complicated than previously inferred, Phanerozoic geodynamic history. The two Phanerozoic periods of variations in geothermal gradient (heat flow) were coeval with epeirogenic movements related to the deposition and erosion of sediments. These paleogeodynamic variations are tentatively attributed to far-field effects of orogenic processes occurring at the plate margin (i.e. the Antler and the Cordilleran orogenies) and the associated accumulation of cratonic seaway sedimentary sequences (Kaskaskia and Zuni sequences). 相似文献
9.
Many salt marshes in densely populated areas have been subjected to a reduction in tidal flow. In order to assess the impact of tidal flow restriction on marsh sedimentation processes, sediment cores were collected from flow-restricted restricted salt marshes along the Connecticut coast of Long Island Sound. Cores were also collected from unrestricted reference marshes and from a marsh that had been previously restricted but was restored to fuller tidal flushing in the 1970's. High bulk densities and low C and N concentrations were found at depth in the restricted marsh cores, which we attribute to a period of organic matter oxidation, sediment compaction, and marsh surface subsidence upon installation of flow restrictions (between 100 and 200 years before the present, depending on the marsh). Recent sedimentation rates at the restricted marshes (as determined by137Cs and210Pb dating) were positive and averaged 78% (137Cs) and 50% (210Pb) of reference marsh sedimentation rates. The accumulation of inorganic sediment was similar at the restricted and reference marshes, perhaps because of the seasonal operation of the tide gates, while organic sediment accretion (and pore space) was significantly lower in the restricted marshes, perhaps because of higher decomposition rates. Sedimentation rates at the restored marsh were significantly higher than at the reference marshes. This marsh has responded to the higher water levels resulting from restoration by a rapid increase in marsh surface elevation. 相似文献
10.
We report the results of a 5-year fertilization experiment in a central Long Island Sound salt marsh, aimed at understanding
the impacts of high nutrient loads on marsh elevational processes. Fertilization with nitrogen led to some significant changes
in marsh processes, specifically increases in aboveground primary production and in CO2 fluxes from the soil. However, neither nitrogen nor phosphorus fertilization led to elevation loss (relative to controls),
reduced soil carbon, or a decrease in belowground primary production, all of which have been proposed as links between elevated
nutrient loads and marsh drowning. Our data suggest that high nutrient levels increase gross carbon loss from the sediment,
but that this is compensated for by other processes, leading to no net deleterious effect of nutrient loading on carbon storage
or on marsh stability with respect to sea level rise. 相似文献