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排序方式: 共有1013条查询结果,搜索用时 718 毫秒
931.
Leah H Joseph David K Rea Ben A van der Pluijm James D Gleason 《Earth and Planetary Science Letters》2002,201(1):127-142
Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then. 相似文献
932.
The scyphomedusa, Chrysaora quinquecirrha, preys on a variety of estuarine organisms. A series of experiments were conducted to measure the feeding rate of adult medusae on zooplankton prey. Artemia was used as a substitute for the endemic and abundant copepod, Acartia tonsa. The feeding rates ranged from three Artemia per hour per ml of medusae volume to 107 per hour per ml in 51 trials. The consumption was also linearly related to the initial prey concentration. Certain results suggested that a toxic factor might be exuded by the medusae, affecting the observed feeding rates. The results verify the possibility that Acartia may be a major prey organism in estuarine environments. 相似文献
933.
Robert O. Rye Roelof D. Schuiling Danny M. Rye J.Ben H. Jansen 《Geochimica et cosmochimica acta》1976,40(9):1031-1049
At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C.The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ18O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ18OH2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite.The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δDH2O values for the metamorphic fluid decrease from ?5‰ in the glaucophane zone to an average of about ?70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend.Theδ18O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism.Detailed data on 20 marble units show systematic variations of δ18O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ18O gradients at the margins and large δ18O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ18O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates.Almost all the δ13C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ13C values of most marble units show a general correlation with δ18O values.The mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ13C values of the CO2 range from ?8.0 to 3.6‰ and indicate that at some localities CO2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles. 相似文献
934.
U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed. 相似文献
935.
Early in 1976 benthic studies were initiated in a 20 kilometer long portion of the Western Sacramento-San Joaquin River Estuary. Water quality determinations indicated little vertical or horizontal differences in pH, temperature, or dissolved oxygen concentration within the study area. Low river outflows allowed the encroachment of seawater into the study area, an area normally exposed to fresh or slightly brackish water. The sediment composition changed dramatically at most stations during the year, being dominated by sands early in the year but by silts and clays in late summer. The shift in sediment composition was accompanied by an increase in grease and oilland metals content. The benthic community of the study area was generally dominated by the Asiatic clam (Corbicula manilensis), Macoma balthica, oligochaetes, the amphipods Corophium stimpsoni and C. spinicorne, nematodes, and a spionid polychaete, Boccardia ligerica. These taxa comprised 98% on average of the total benthic macroinvertebrates collected at each study site. The benthic assemblages of each of the stations were generally very similar to one another. Faunal similarities and changes in benthos composition were related to substrate composition and salinity incursion. In general, the upstream-channel stations had higher abundance of benthos than the other stations in the study area. Total benthic abundance was lowest at the downstream end of the study area. Total standing crop peaked in June and was lowest in November. Our studies indicate that the most important factors controlling the size and species composition of the benthos of the study area are salinity and sediment composition. 相似文献
936.
A synthetic approach integrating surface and subsurface data for prospecting deep aquifers: the Southeast Tunisia 总被引:2,自引:0,他引:2
The Southeast Tunisia has remarkable thermal springs that have been used for bathing since many years. Currently, studies
are being carried out to investigate the possible use of the hot water for heating buildings and for greenhouses. Springs
with discharge temperatures between 26.8 and 58°C exist in the area. The flow rates range between 7 and 66 l/s with total
dissolved solids (TDS) varying between 2,020 and 4,030 mg/l. Both geological–hydrogeological setting and the similarity in
chemical characteristics between the Zarat spring and nearby water wells suggest that the hot water from this spring may be
related to the Senonian aquifer. However, the hydrogeothermal approach shows that the depth of the reservoir that supplies
the Zarat spring is greater than that of the Senonian aquifer. In fact, geothermometry approach indicates that fluids emerging
from the Lower Cretaceous sandstones reach temperatures of approximately 78°C at depth, while the fluid temperature from the
nearby water wells is 32°C. Through the integration of various types of data we found that the thermal water of Southeast
Tunisia is of meteoric origin. This water infiltrates deeply into the Lower Cretaceous sandstones and reaches a high temperature
before rising upwards to supply the hot springs and well discharge zones. It appears that the upward movement of hot water
from the deep aquifers to shallow ones is probably due to the abundant fractures in the area. 相似文献
937.
Abdelilah Tahayt Taoufik Mourabit Alexis Rigo Kurt L. Feigl Abdelali Fadil Simon McClusky Robert Reilinger Mostafa Serroukh Abdelouahed Ouazzani-Touhami Driss Ben Sari Philippe Vernant 《Comptes Rendus Geoscience》2008,340(6):400-413
GPS velocities and seismicity across the Betic–Rif Arc structural domains (Morocco and Iberia) provide a basis to evaluate present-day seismotectonic processes between different deformation belts. The results show asymmetric movements in the complex Alboran system accommodating the convergence between the African (Nubian) and Eurasian plates. While the Betic Mountains are attached to Iberia, moving toward the southeast with respect to Africa, the Rif is divided into three blocks with distinct displacements relative to Nubia: (1) the Tangier block moving southeastward, (2) the Central Rif block moving SSW, and (3) the Oriental Rif block undergoing clockwise rotation. GPS-derived motions decrease in rate from the Rif nappes complex to the foreland. 相似文献
938.
939.
Postglacial relative sea‐level observations from Ireland and their role in glacial rebound modelling
Anthony J. Brooks Sarah L. Bradley Robin J. Edwards Glenn A. Milne Ben Horton Ian Shennan 《第四纪科学杂志》2008,23(2):175-192
The British Isles have been the focus of a number of recent modelling studies owing to the existence of a high‐quality sea‐level dataset for this region and the suitability of these data for constraining shallow earth viscosity structure, local to regional ice sheet histories and the magnitude/timing of global meltwater signals. Until recently, the paucity of both glaciological and relative sea‐level (RSL) data from Ireland has meant that the majority of these glacial isostatic adjustment (GIA) modelling studies of the British Isles region have tended to concentrate on reconstructing ice cover over Britain. However, the recent development of a sea‐level database for Ireland along with emergence of new glaciological data on the spatial extent, thickness and deglacial chronology of the Irish Ice Sheet means it is now possible to revisit this region of the British Isles. Here, we employ these new data to constrain the evolution of the Irish Ice Sheet. We find that in order to reconcile differences between model predictions and RSL evidence, a thick, spatially extensive ice sheet of ~600–700 m over much of north and central Ireland is required at the LGM with very rapid deglaciation after 21 k cal. yr BP. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
940.
Paolo Paruolo Ben Murphy Greet Janssens-Maenhout 《Stochastic Environmental Research and Risk Assessment (SERRA)》2015,29(1):93-107
This paper analyzes the relation between income and emissions in the period 1970–2008, for all world countries. We consider time-series of CO2, SO2 and GWP100, and use Vector Autoregressive models that allow for nonstationarity and cointegration. At 5 % significance level, income and emissions are found to be driven by unrelated random walks with drift (respectively by a common random walk with drift) in about 70 % (respectively 25 %) of cases; in the remaining cases the variables are trend-stationary. Tests of Granger-causality show evidence of both directions of causality. For the case of unrelated stochastic trends, we almost never find income driving emissions, as predicted by a consumption-function interpretation. These causality results and the absence of a common trend challenge the main implications of the Environmental Kuznets Curve, namely that the dominant direction of causality should be from income to emissions, and that for increasing levels of income, emissions should tend to decrease. 相似文献