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排序方式: 共有117条查询结果,搜索用时 187 毫秒
81.
Komatiites of the Karasjok Greenstone Belt, northern Norway, show two unusual features: they have certain compositional differences compared with other komatiites, and they are largely volcaniclastic in origin. Their geological setting suggests that the komatiites were crupted into shallow water, thus permitting phreatomagmatic eruption, in a small ocean basin that opened in the Baltic Shield. The major oxides (except for TiO2), the trace elements Y, Sc, V, heavy rare earth elements (HREE), Cr, Co, Ni and the platinum group elements (PGE) cover similar ranges to those observed in other komatiites, but TiO2, Sm, Zr and Hf (Ti-associated elements, TAE) are enriched compared with abundances commonly reported for komatiites. Thus, the Karasjok komatiites have interelement ratios 2 to 3 times greater than chondritic between the TAE and the HREE, PGE, Sc, V, Y, Al (HRE-associated elements, HAE). The light rare earth elements (LREE), Ta and Th are enriched in some samples relative to Ti, Sm, Zr, and Hf, but are depleted in others. One group of rocks that is similar to the Karasjok komatiites both in terms of geological setting and geochemistry is the Baffin Bay picrites. The reason for the high concentrations of TAE in the Karasjok komatiites could be that they formed at lower degrees of partial melting than most komatiites. The greater-than-chondritic TAE/HAE ratios indicate that garnet was a residual phase during their formation, requiring that the melt formed at a pressure greater than 40 kb. A model involving decompression melting of a mantle plume rising in a rifting environment, can explain the main features of the Karasjok komatiites. 相似文献
82.
Optically stimulated luminescence dating of a Holocene beach ridge plain in Northern Jutland, Denmark 总被引:1,自引:0,他引:1
Asger Nielsen Andrew S. Murray Morten Pejrup Bo Elberling 《Quaternary Geochronology》2006,1(4):305-312
This study presents results from 20 optically stimulated luminescence (OSL) ages from one of the world's largest beach ridge plains, the Jerup beach ridge plain at the base of the Skagen Spit in the northernmost part of Jutland, Denmark. The OSL ages were obtained using quartz and a SAR protocol, and used to establish a chronology for the beach ridge plain and for the underlying coastal plain. The accuracy of the chronology is tested both by laboratory tests (recuperation, recycling ratio and dose recovery) and by comparison with independent age controls, e.g. previously reported radiocarbon dates, map sources, anecdotal evidence and settlement names. It is concluded that the OSL signals are internally consistent, and that the derived OSL ages are in good agreement with a large range of independent age controls. The ridge plain is shown to cover a time span from 1000 to 2700 years ago; this chronology is more detailed and precise than those previously available, and gives an average beach ridge formation rate of 15 yr/ridge and an average lateral migration rate of 2.0 m/yr. This study adds to the growing knowledge that OSL dating has a large potential for establishing detailed and precise chronologies in coastal marine sediments, including beach ridges. 相似文献
83.
84.
Tephra abundance data and geochemistry in Late‐glacial and Holocene sediments on the East Greenland shelf are presented. Two well‐known tephras were identified from electron microprobe analysis of tephra shards picked from ash peaks in the cores. These are the Vedde Ash and Saksunarvatn Ash, which probably were deposited on the shelf after transport on drifting ice. The radiocarbon dates (marine reservoir corrected by −550 yr) that constrain the timing of deposition of the tephra layers compare well with the terrestrial and ice‐core ages of the tephras without requiring additional reservoir correction to align them with the known tephra ages. Several prominent tephra layers with a composition of Ash Zone 2 tephra punctuate the deglacial sediments. These tephra peaks coincide with significant light stable isotope events (signifying glacial meltwater) and fine‐grained sediments poor in ice‐rafted detritus. We interpret the Ash Zone 2 tephra peaks as sediment released from the Greenland Ice Sheet during strong melting pulses of the deglaciation. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
85.
The use of functional endpoints in risk assessment of pollutants for marine pelagic communities is scarce, especially for the function of zooplankton communities. This work presents a rapid, inexpensive and ecologically relevant technique to assess the effect of toxic compounds on the grazing rates of zooplankton communities. The combination of a (14)C-tracer labelling technique with a fast (< 3 h) and representative sub-sampling and handling makes it possible to measure changes in food uptake of freshly collected natural zooplankton communities in a short-term assay. The methodology is described and its validity is determined by a full account of the fate of isotope tracers (up to 15% taken up by zooplankton). Egg production of Acartia tonsa cultures exposed to DCOI yielded EC(50) values of 118+/-33 nM DCOI after three days, where EC(50) values from grazing of natural zooplankton communities was 136+/-29 nM DCOI, indicating that grazing was the more sensitive variable. Finally the method was tested in a mesocosm experiment designed to evaluate the toxicity of the antifouling compound zinc pyrithion (ZPT) with an EC(50) value of 17 nM ZPT (SD=3.3 nM, n=3). The method was found to be reproducible both at the community (CV=8-39%) and individual level (CV=23-53%), and could detect small changes in response to a toxicant in a natural pelagic system. The use of natural communities in combination with the fast and cost-effective procedure makes this technique a powerful tool in risk assessment, where lack of ecological relevance or reproducible results often results in unwanted high uncertainties. 相似文献
86.
Morten B. Andersen Claudine H. Stirling Emma-Kate Potter Alex N. Halliday Steven G. Blake Malcolm T. McCulloch Bridget F. Ayling Michael O'Leary 《Earth and Planetary Science Letters》2008,265(1-2):229-245
Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ~ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the 238U–234U–230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model. 相似文献
87.
Holmer M Argyrou M Dalsgaard T Danovaro R Diaz-Almela E Duarte CM Frederiksen M Grau A Karakassis I Marbà N Mirto S Pérez M Pusceddu A Tsapakis M 《Marine pollution bulletin》2008,56(9):1618-1629
This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows. 相似文献
88.
Accelerating the T‐matrix approach to seismic full‐waveform inversion by domain decomposition
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A seismic variant of the distorted Born iterative inversion method, which is commonly used in electromagnetic and acoustic (medical) imaging, has been recently developed on the basis of the T‐matrix approach of multiple scattering theory. The distorted Born iterative method is consistent with the Gauss–Newton method, but its implementation is different, and there are potentially significant computational advantages of using the T‐matrix approach in this context. It has been shown that the computational cost associated with the updating of the background medium Green functions after each iteration can be reduced via the use of various linearisation or quasi‐linearisation techniques. However, these techniques for reducing the computational cost may not work well in the presence of strong contrasts. To deal with this, we have now developed a domain decomposition method, which allows one to decompose the seismic velocity model into an arbitrary number of heterogeneous domains that can be treated separately and in parallel. The new domain decomposition method is based on the concept of a scattering‐path matrix, which is well known in solid‐state physics. If the seismic model consists of different domains that are well separated (e.g., different reservoirs within a sedimentary basin), then the scattering‐path matrix formulation can be used to derive approximations that are sufficiently accurate but far more speedy and much less memory demanding because they ignore the interaction between different domains. However, we show here that one can also use the scattering‐path matrix formulation to calculate the overall T‐matrix for a large model exactly without any approximations at a computational cost that is significantly smaller than the cost associated with an exact formal matrix inversion solution. This is because we have derived exact analytical results for the special case of two interacting domains and combined them with Strassen's formulas for fast recursive matrix inversion. To illustrate the fact that we have accelerated the T‐matrix approach to full‐waveform inversion by domain decomposition, we perform a series of numerical experiments based on synthetic data associated with a complex salt model and a simpler two‐dimensional model that can be naturally decomposed into separate upper and lower domains. If the domain decomposition method is combined with an additional layer of multi‐scale regularisation (based on spatial smoothing of the sensitivity matrix and the data residual vector along the receiver line) beyond standard sequential frequency inversion, then one apparently can also obtain stable inversion results in the absence of ultra‐low frequencies and reduced computation times. 相似文献
89.
Morten Sparre Andersen Tove Nielsen Aage Bach Srensen Lars Ole Boldreel Anton Kuijpers 《Global and Planetary Change》2000,24(3-4)
A complex history of Cenozoic vertical movements in the Faroe region has been revealed from interpretation of geophysical and geological data, mainly offshore reflection seismic data, side-scan images, shallow cores, and onshore mapping. The history comprises several phases of tectonic disturbances observed at different scales. On the eastern margin of the Faroe Platform a late Eocene–early Oligocene phase of doming of the Faroe Platform has caused a postdepositional tilting of Eocene strata along the southern margin of the platform; a mid-Miocene phase of compressional tectonics is evidenced on seismic transects as gentle anticlines and associated reverse faults; and possible Pliocene uplift of the Faroe Islands is indicated by a progradational wedge of sediments deposited on the eastern Faroe Platform. At the continental margin/slope north of the Faroe Platform, reflection seismic data imaging the postbasalt sedimentary strata indicate three distinct tectonic events phases in the Eocene–Oligocene, Miocene and Pliocene, respectively. In contrast to the Faroe Platform the Faroe–Shetland Channel was characterised by more or less continuous subsidence dominated throughout the Cenozoic. During the Eocene, sediments deposited in the Faroe–Shetland Channel was mostly derived from a source area on the British shelf. 相似文献
90.
Microbial availability and degradation of dissolved organic carbon and nitrogen in two coastal areas 总被引:3,自引:1,他引:2
Microbial availability and degradation rates of dissolved organic carbon (DOC) and nitrogen (DON) were estimated at two coastal stations (Horsens Fjord and Darss Sill) in Denmark, by measuring the decrease in DOC and DON concentrations during long-term laboratory incubations (150 days). The experiments used two different treatments: one only receiving a microbial inoculum and another additionally to the inoculum, carbon and phosphate to ensure nitrogen limitation. The additions of carbon and phosphate led to increased DON bioavailability in all experiments. The incubations showed that bioavailable DOC (BDOC) accounted for 22 ± 13% of total DOC in Horsens Fjord and 14 ± 5% at Darss Sill. Bioavailable DON (BDON) accounted for 43 ± 10% (Horsens Fjord) and 28 ± 12% of DON (Darss Sill). The linear relations between BDOM and DOM suggested that the DOC variations in Horsens Fjord were controlled by the available fraction, while this was only partly the case for DOC at Darss Sill and DON (both stations), showing that the refractory pool to some degree controlled the seasonal variations in DOM at these coastal stations. Additionally we found that DOC and DON were cycled at approximately the same speed, probably due to a high carbon demand of the microbial community. Calculating the amounts of DON degraded within the two areas using the obtained decay rates showed that compared with the ambient inorganic nitrogen levels BDON contained a large proportion (52 ± 37%, Horsens Fjord and 74 ± 19%, Darss Sill) of the bioavailable nitrogen (BDON + DIN). These calculations further suggested that bioavailable DOM was washed out of the respective areas and could contribute to heterotrophic growth in adjacent waters. 相似文献