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951.
António A. Martins Pedro P. Cunha Jan-Pieter Buylaert Sébastien Huot Andrew S. Murray Pedro Dinis Martin Stokes 《Quaternary Geochronology》2010,5(2-3):176-180
We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected De values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected De of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (~Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (~MIS9-6); T5 is 136–75 ka (~MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2). 相似文献
952.
The mechanism of cation and oxygen isotope exchange in alkali feldspars under hydrothermal conditions 总被引:3,自引:1,他引:2
Dominik R. D. Niedermeier Andrew Putnis Thorsten Geisler Ute Golla-Schindler Christine V. Putnis 《Contributions to Mineralogy and Petrology》2009,157(1):65-76
The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite
crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on
a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction
contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered
Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration
from ~476 cm−1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm−1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the
replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as
well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation
mechanism. 相似文献
953.
Zhiguo Rao Zhaoyu Zhu Guodong Jia Andrew C.G. Henderson Qian Xue Suping Wang 《Organic Geochemistry》2009,40(8):922-930
The stable carbon and hydrogen isotope composition of higher plant-derived long chain n-alkanes (δ13Cn-alkanes and δDn-alkanes) from 45 surface soil samples (within well characterized vegetation zones) from eastern China (18°N–50°N) are reported. The weighted average δDn-alkanes value for n-C27, n-C29 and n-C31 in the samples and the annual average δD of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations. The δD of n-alkanes shows a gradual depletion in value with increasing latitude. The results demonstrate that, on a large spatial scale, the δD values of long chain n-alkanes derived from higher plants have the potential to record the δD of meteoric water, although many other factors can also influence the isotope values. There appears to be no apparent relationship between the δD of the n-alkanes extracted from the surface soil and the overlying vegetation type (i.e. forest/grassland or C3/C4 composition). Therefore, palaeoenvironmental studies utilizing δDn-alkanes from higher plant-derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle. 相似文献
954.
955.
956.
Kristen M. Lester Cynthia A. Heil Merry B. Neely Danylle N. Spence Susan Murasko Thomas L. Hopkins Tracey T. Sutton Scott E. Burghart Richard N. Bohrer Andrew W. Remsen Gabriel A. Vargo John J. Walsh 《Continental Shelf Research》2008
Blooms of the toxic dinoflagellate Karenia brevis are common in the Gulf of Mexico, yet no in situ studies of zooplankton and K. brevis have been conducted there. Zooplankton abundance and taxonomic composition at non-bloom and K. brevis bloom stations within the Ecology of Harmful Algal Blooms (ECOHAB) study area were compared. At non-bloom stations, the most abundant species of zooplankton were Parvocalanus crassirostris, Oithona colcarva, and Paracalanus quasimodo at the 5-m isobath and P. quasimodo, O. colcarva, and Oikopleura dioica at the 25-m isobath. There was considerable overlap in dominance of zooplankton species between the 5 and 25-m isobaths, with nine species contributing to 90% of abundance at both isobaths. At stations within K. brevis blooms however, Acartia tonsa, Centropages velificatus, Temora turbinata, Evadne tergestina, O. colcarva, O. dioica, and P. crassirostris were dominant. Variations in abundance between non-bloom and bloom assemblages were evident, including the reduction in abundance of three key species within K. brevis blooms. 相似文献
957.
Shelley M. Blackwell Mark A. Moline Andrew Schaffner Thomas Garrison Grace Chang 《Continental Shelf Research》2008
Patchiness or spatial variability is ubiquitous in marine systems. With increasing anthropogenic impacts to coastal resources and coastal systems being disproportionately large contributors to ocean productivity, identifying the spatial scales of this patchiness, particularly in coastal waters, is of critical importance to understand coastal ecosystem dynamics. The current work focuses on fine scale structure in three coastal regions. More specifically, we utilize variogram analyses to identify sub-kilometer scales of variability in biological and physical parameters measured by an autonomous underwater vehicle (AUV) in the Mid-Atlantic Bight, Monterey Bay, and in San Luis Obispo Bay between 2001 and 2004. Critical scales of variability in density, turbidity, fluorescence, and bioluminescence are examined as a function of depth and distance offshore. Furthermore, the effects of undersampling are assessed using predictive error analysis. Results indicate the presence of scales of variability ranging from 10s to 100s of meters and provide valuable insight for sampling design and resource allocation for future studies. 相似文献
958.
959.
The marginal zone of the Mount Lowe Intrusion (MLI) is composed of a mineralogically and geochemically distinctive suite of quartz poor, alkali-enriched monzodiorites and quartz monzodiorites. Low initial 87Sr/86Sr, Rb, and Rb/Sr coupled with high Ab/Or, K/Rb, Ba and Sr suggest that feldspathic continental crust did not contribute significantly to the magma. The inferred original magma composition is most consistent with experimental data for about 20% batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition. The high absolute Ba and Sr abundances require in addition an enriched component, perhaps derived from dehydration processes within a subducted slab. At the level of exposure, the range of observed compositions resulted from fractionation of hornblende, plagioclase, sphene, apatite and magnetite. Fractionation resulted in formation of a peraluminous, Mn — enriched residual liquid which crystallized garnet. The grandite — enriched composition of the garnet is inferred to be due to high pressure (ca. 6.7 kb), high
crystallization, relative to typical plutonic garnets. The composition of the marginal zone of MLI has many chemical similarities to early Mesozoic plutonism in the Cordilleran continental margin magmatic arc of western North America. 相似文献
960.
Matthew?S.?Van Den BroekeEmail authorView authors OrcID profile Andrew?Kalin Jose?Abraham?Torres?Alavez Robert?Oglesby Qi?Hu 《Theoretical and Applied Climatology》2018,134(3-4):801-816
In climate modeling studies, there is a need to choose a suitable land surface model (LSM) while adhering to available resources. In this study, the viability of three LSM options (Community Land Model version 4.0 [CLM4.0], Noah-MP, and the five-layer thermal diffusion [Bucket] scheme) in the Weather Research and Forecasting model version 3.6 (WRF3.6) was examined for the warm season in a domain centered on the central USA. Model output was compared to Parameter-elevation Relationships on Independent Slopes Model (PRISM) data, a gridded observational dataset including mean monthly temperature and total monthly precipitation. Model output temperature, precipitation, latent heat (LH) flux, sensible heat (SH) flux, and soil water content (SWC) were compared to observations from sites in the Central and Southern Great Plains region. An overall warm bias was found in CLM4.0 and Noah-MP, with a cool bias of larger magnitude in the Bucket model. These three LSMs produced similar patterns of wet and dry biases. Model output of SWC and LH/SH fluxes were compared to observations, and did not show a consistent bias. Both sophisticated LSMs appear to be viable options for simulating the effects of land use change in the central USA. 相似文献