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991.
An extensive survey and topographic analysis of five watersheds draining the Luquillo Mountains in north‐eastern Puerto Rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS‐based topographic analysis was used to examine channel profiles, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial‐alluvial transitions. Non‐fluvial processes, such as landslides, deliver coarse boulder‐sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fining in the downstream reaches; a pattern associated with a mid‐basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid‐basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self‐forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach‐scale channel morphology, strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin‐scale patterns. Copyright © 2010 John Wiley and Sons, Ltd. 相似文献
992.
This article reports on the special issue of Earth Surface Processes and Landforms dedicated to remote sensing of rivers. This emerging subdiscipline of river science has grown at a rapid rate in recent years because of: (a) the growing desire and need for data to document and explore the full range of spatial and temporal variations in river systems; (b) evolving technologies that enable lower cost data acquisition, processing and analysis at reach to catchment to continental scales; and (c) the increasing engagement of river scientists with GIScience. The convergence of these factors and the ever growing number of practitioners speaks to the need for more communication among researchers, a major reason for creating this special issue. The 12 articles in the volume cover a broad spectrum of applications that use a variety of platforms and sensors, ranging from photogrammetric mapping of riffle‐pool morphology beneath forest canopy using a camera mounted on a hand held pole to satellite‐based synthetic radar mapping of subcontinental scale hydrology of large rivers. In this overview each of the 12 articles is briefly summarized. Based on these works and other research, it is concluded that the time for more widespread application of river remote sensing techniques is now. To promote more widespread use of remote sensing techniques for river science and management, the following are advocated: (a) developing stand alone or plug‐in software products that enable non‐expert users to implement these new methods, (b) incorporating remote sensing of rivers training into classes, workshops, and on‐line tutorials; and (c) promoting more intentional and formal collaboration among members of the river remote sensing community. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
993.
Eldo E. Ávila Rodrigo E. Bürgesser Nesvit E. Castellano Andrew B. Collier Rosa H. Compagnucci Arthur R.W. Hughes 《Journal of Atmospheric and Solar》2010,72(14-15):1114-1121
Satellite observations of cloud top temperature and lightning flash distribution are used to examine the relationship between deep convection and lightning activity over the tropical regions of the northern and southern hemispheres. In agreement with previous work, the analysis of the results shows that, in the summer of both hemispheres, the lightning activity in continental deep convective storms is more intense than that in marine deep convective storms by a factor of between 7 and 10. Furthermore, it was observed that on average the daily lightning rate per 1°×1° grid cell for the southern hemisphere (SH) is about 20% greater than that of the northern hemisphere (NH), which can be attributed to a larger fractional cover by deep convective clouds in the SH. By using a set of independent indicators, it is shown that deep convection and lightning activity over land are well correlated (with correlation coefficients of 0.8 and 0.6 for NH and SH, respectively). This suggests the capacity for observations to act as a possible method of monitoring continental deep convective clouds, which play a key role in regulating the Earth’s climate. Since lightning can be monitored easily from ground networks and satellites, it could be a useful tool for validating the performance of model convective schemes and for monitoring changes in climate parameters. 相似文献
994.
A review of terrestrial radar interferometry for measuring surface change in the geosciences
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This paper presents a review of the current state of the art in the use of terrestrial radar interferometry for the detection of surface changes related to mass movement. Different hardware‐types and acquisition concepts are described, which use either real or synthetic aperture for radar image formation. We present approaches for data processing procedures, paying special attention to the separation of high resolution displacement information from atmospheric phase variations. Recent case studies are used to illustrate applications in terrestrial radar interferometry for change detection. Applications range from detection and quantification of very slow moving (millimeters to centimeters per year) displacements in rock walls from repeat monitoring, to rapid processes resulting in fast displacements (~50 m/yr) acquired during single measurement campaigns with durations of only a few hours. Fast and episodic acting processes such as rockfall and snow avalanches can be assessed qualitatively in the spatial domain by mapping decorrelation caused by those processes. A concluding guide to best practice outlines the necessary preconditions that have to be fulfilled for successful application of the technique, as well as in areas characterized by rapid decorrelation. Empirical data from a Ku‐band sensor show the range of temporal decorrelation of different surfaces after more than two years for rock‐surfaces and after a few seconds to minutes in vegetated areas during windy conditions. The examples show that the displacement field can be measured for landslides in dense grassland, ice surfaces on flowing glaciers and snowpack creep. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
995.
Mangrove‐forest evolution in a sediment‐rich estuarine system: opportunists or agents of geomorphic change?
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The majority of the world's mangrove forests occur on mostly mineral sediments of fluvial origin. Two perspectives exist on the biogeomorphic development of these forests, i.e. that mangroves are opportunistic, with forest development primarily driven by physical processes, or alternatively that biophysical feedbacks strongly influence sedimentation and resulting geomorphology. On the Firth of Thames coast, New Zealand, we evaluate these two possible scenarios for sediment accumulation and forest development using high‐resolution sedimentary records and a detailed chronology of mangrove‐forest (Avicennia marina) development since the 1950s. Cores were collected along a shore‐normal transect of known elevation relative to mean sea level (MSL). Activities for lead‐210 (210Pb), caesium‐137 (137Cs) and beryllium‐7 (7Be), and sediment properties were analysed, with 210Pb sediment accumulation rates (SARs), compensated for deep subsidence (~8 mm yr?1) used as a proxy for elevation gain. At least four phases of forest development since the 1950s are recognized. An old‐growth forest developed by the late‐1970s with more recent seaward forest expansion thereafter. Excess 210Pb profiles from the old‐growth forest exhibit relatively low SARs near the top (7–12 mm yr?1) and bottom (10–22 mm yr?1) of cores, separated by an interval of higher SARs (33–100 mm yr?1). A general trend of increasing SAR over time characterizes the recent forest. Biogeomorphic evolution of the system is more complex than simple mudflat accretion/progradation and mangrove‐forest expansion. Surface‐elevation gain in the old‐growth forest displays an asymptotic trajectory, with a secondary depocentre developing on the seaward mudflat from the mid‐1970s. Two‐ to ten‐fold increases in 210Pb SARs are unambiguously large and occurred years to decades before seedling recruitment, demonstrating that mangroves do not measurably enhance sedimentation over annual to decadal timescales. This suggests that mangrove‐forest development is largely dependent on physical processes, with forests occupying mudflats once they reach a suitable elevation in the intertidal. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
996.
Andrew S. Carr Arnoud Boom Brian M. Chase David L. Roberts Zoë E. Roberts 《Journal of Paleolimnology》2010,44(4):947-961
Pyrolysis–gas chromatography mass spectrometry (py-GC/MS) allows the characterisation of complex macromolecular organic matter.
In lakes and wetlands this can potentially be used to assess the preservation/diagenesis and provenance of sediment organic
matter. It can complement palaeoenvironmental investigations utilising ‘bulk’ sediment variables such as total organic carbon
(TOC) and TOC/total nitrogen ratios. We applied py-GC/MS analyses to a ~32,000-year sediment record from the southern Cape
coastline of South Africa. We used the results to evaluate the sources and extent of degradation of organic matter in this
semi-arid environment. Marked down-core changes in the relative abundance of multiple pyrolysis products were observed. Correspondence
analysis revealed that the major driver of this down-core variability in OM composition was selective preservation/degradation.
Samples comprising highly degraded OM are primarily confined to the lower half of the core, older than ~12,000 years, and
are characterised by suites of low-molecular-weight aromatic pyrolysis products. Samples rich in organic matter, e.g. surface
sediments, are characterised by products derived from fresh emergent or terrestrial vegetation, which include lignin monomers,
plant-derived fatty acids and long-chain n-alkanes. Pyrolysates from the late glacial-early Holocene period, approximately mid-way down the core are characterised by
distinct suites of long-chain n-alkene/n-alkane doublets, which may reflect the selective preservation of recalcitrant aliphatic macromolecules and/or enhanced inputs
of the algal macromolecule algaenan/polymerised algal lipids. Increased TOC, lower δ13C and increased abundance of more labile lignin and fatty acid products at the same depths suggest this period was associated
with increased lake primary productivity and enhanced inputs of terrestrial OM. TOC is the only ‘bulk’ parameter correlated
with the correspondence analysis axes extracted from the py-GC/MS data. Distinct fluctuations in TOC/total nitrogen ratio
are not explained by variation in organo-nitrogen pyrolysis products. Notwithstanding, the study suggests that py-GC/MS has
potential to complement palaeolimnological investigations, particularly in regions such as southern Africa, where other paleoenvironmental
proxy variables in sediments may be lacking or equivocal. 相似文献
997.
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
998.
Alan D. WanamakerJr. Karl J. Kreutz Bernd R. Schöne Kirk A. Maasch Andrew J. Pershing Harold W. Borns Douglas S. Introne Scott Feindel 《International Journal of Earth Sciences》2009,98(1):19-29
To investigate environmental variability during the late Holocene in the western Gulf of Maine, USA, we collected a 142-year-old
living bivalve (Arctica islandica) in 2004, and three fossil A. islandica shells of the Medieval Warm Period (MWP) and late MWP / Little Ice Age (LIA) period (corrected 14CAMS = 1030 ± 78 ad; 1320 ± 45 ad; 1357 ± 40 ad) in 1996. We compared the growth record of the modern shell with continuous plankton recorder (CPR) time-series (1961–2003)
from the Gulf of Maine. A significant correlation (r
2 = 0.55; p < 0.0001) exists between the standardized annual growth index (SGI) of the modern shell and the relative abundance of zooplankton
species Calanus finmarchicus. We therefore propose that SGI data from A. islandica is a valid proxy for paleo-productivity of at least one major zooplankton taxa. SGIs from these shells reveal significant
periods of 2–6 years (NAO-like) based on wavelet analysis, multitaper method (MTM) analysis and singular spectrum analysis
(SSA) during the late Holocene. Based on established physical oceanographic observation in the Gulf of Maine, we suggest that
slope water variability coupled with North Atlantic Oscillation (NAO) dynamics is primarily responsible for the observed SGI
variability.
Special Issue: AGU OS06 special issue “Ocean’s role in climate change—a paleo perspective”. 相似文献
999.
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
1000.
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