Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced Be in river-borne quartz     

Erik Thorson Brown  Robert F. Stallard  Matthew C. Larsen  Didier L. Bourls  Grant M. Raisbeck  Franoise Yiou 《Earth and Planetary Science Letters》1998,160(3-4):723-728

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.  相似文献   

Solute Changes During Aquifer Storage Recovery Testing in a Limestone/Clastic Aquifer   总被引：1，自引：0，他引：1  

June E. Mirecki  Bruce G. Campbell  Kevin J. Conlon  Matthew D. Petkewich 《Ground water》1998,36(3):394-403

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Exposed cross-sections through the continental crust: implications for crustal structure,petrology, and evolution     

David M. Fountain  Matthew H. Salisbury 《Earth and Planetary Science Letters》1981

The continental crust is exposed in cross-section at numerous sites on the earth's surface. These exposures, which appear to have formed by obduction along great faults during continental collision, may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive Bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle. From an examination of five complexes which meet these criteria, it is concluded that the most prominent layering in the crust is not compositional but metamorphic. The lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks, respectively. Post-metamorphic granitic intrusions are common at intermediate to shallow levels. Position of discontinuities in refraction velocity, where present, commonly correspond to changes in composition or metamorphic grade with depth. The continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by COCORP. Field observations, coupled with geochemical data, indicate a complex evolution of the lower crust which can include anatexis, multiple deformation, polymetamorphism and reworking of older crustal material. The complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments.  相似文献   

Grassroots social innovations and food localisation: An investigation of the Local Food programme in England     

James Kirwan  Brian Ilbery  Damian Maye  Joy Carey 《Global Environmental Change》2013,23(5):830-837

This paper seeks to orientate research on local food networks more firmly towards ideas of grassroots and social niche innovations. Drawing on recent conceptual ideas from strategic niche management, this paper provides an exploratory analysis of attempts to spread grassroots social innovations through the Big Lottery Local Food programme run by the Royal Society of Wildlife Trusts in England. This £ 59.8 million programme aims to distribute grants to a variety of food-related projects and to make locally grown food more accessible and affordable to local communities. Insights into 29 funded projects, of varying length and scale of operation, are provided through over 150 telephone and personal interviews. While the Local Food programme is undoubtedly about bringing small, often neglected pieces of land into production and increasing access to affordable food, results show that the programme is also very much seen as a vehicle for building community capacity through facilitating community cohesion, healthy eating, educational enhancement and integrating disadvantaged groups into mainstream society and economy. The paper concludes with some reflections on the extent to which the concept of grassroots social innovations, as a form of niche innovation, can help understand the ability of local food networks to develop the capacity of communities to respond to locally identified problems and to effect more widespread, sustainable change.  相似文献   

Electrostatic Potentials in Supernova Remnant Shocks     

Matthew G. Baring  Errol J. Summerlin 《Astrophysics and Space Science》2007,307(1-3):165-168

Recent advances in the understanding of the properties of supernova remnant shocks have been precipitated by theChandra and XMM X-ray Observatories, and the HESS Atmospheric Čerenkov Telescope in the TeV band. A critical problem for this field is the understanding of the relative degree of dissipative heating/energization of electrons and ions in the shock layer. This impacts the interpretation of X-ray observations, and moreover influences the efficiency of injection into the acceleration process, which in turn feeds back into the thermal shock layer energetics and dynamics. This paper outlines the first stages of our exploration of the role of charge separation potentials in non-relativistic electron-ion shocks where the inertial gyro-scales are widely disparate, using results from a Monte Carlo simulation. Charge density spatial profiles were obtained in the linear regime, sampling the inertial scales for both ions and electrons, for different magnetic field obliquities. These were readily integrated to acquire electric field profiles in the absence of self-consistent, spatial readjustments between the electrons and the ions. It was found that while diffusion plays little role in modulating the linear field structure in highly oblique and perpendicular shocks, in quasi-parallel shocks, where charge separations induced by gyrations are small, and shock-layer electric fields are predominantly generated on diffusive scales.  相似文献   

Bluewater shopping centre,Kent: a tale of wasted opportunity?     

Peter Doyle  & Matthew R. Bennett 《Geology Today》1999,15(4):143-144

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Coastal marsh response to historical and future sea-level acceleration     

Matthew Kirwan  Stijn Temmerman 《Quaternary Science Reviews》2009,28(17-18):1801-1808

We consider the response of marshland to accelerations in the rate of sea-level rise by utilizing two previously described numerical models of marsh elevation. In a model designed for the Scheldt Estuary (Belgium–SW Netherlands), a feedback between inundation depth and suspended sediment concentrations allows marshes to quickly adjust their elevation to a change in sea-level rise rate. In a model designed for the North Inlet Estuary (South Carolina), a feedback between inundation and vegetation growth allows similar adjustment. Although the models differ in their approach, we find that they predict surprisingly similar responses to sea-level change. Marsh elevations adjust to a step change in the rate of sea-level rise in about 100 years. In the case of a continuous acceleration in the rate of sea-level rise, modeled accretion rates lag behind sea-level rise rates by about 20 years, and never obtain equilibrium. Regardless of the style of acceleration, the models predict approximately 6–14 cm of marsh submergence in response to historical sea-level acceleration, and 3–4 cm of marsh submergence in response to a projected scenario of sea-level rise over the next century. While marshes already low in the tidal frame would be susceptible to these depth changes, our modeling results suggest that factors other than historical sea-level acceleration are more important for observations of degradation in most marshes today.  相似文献   

Cassini imaging of Saturn's rings: II. A wavelet technique for analysis of density waves and other radial structure in the rings     

Matthew S. Tiscareno  Joseph A. Burns  Philip D. Nicholson  Carolyn C. Porco 《Icarus》2007,189(1):14-34

We describe a powerful signal processing method, the continuous wavelet transform, and use it to analyze radial structure in Cassini ISS images of Saturn's rings. Wavelet analysis locally separates signal components in frequency space, causing many structures to become evident that are difficult to observe with the naked eye. Density waves, generated at resonances with saturnian satellites orbiting outside (or within) the rings, are particularly amenable to such analysis. We identify a number of previously unobserved weak waves, and demonstrate the wavelet transform's ability to isolate multiple waves superimposed on top of one another. We also present two wave-like structures that we are unable to conclusively identify. In a multi-step semi-automated process, we recover four parameters from clearly observed weak spiral density waves: the local ring surface density, the local ring viscosity, the precise resonance location (useful for pointing images, and potentially for refining saturnian astrometry), and the wave amplitude (potentially providing new constraints upon the masses of the perturbing moons). Our derived surface densities have less scatter than previous measurements that were derived from stronger non-linear waves, and suggest a gentle linear increase in surface density from the inner to the mid-A Ring. We show that ring viscosity consistently increases from the Cassini Division outward to the Encke Gap. Meaningful upper limits on ring thickness can be placed on the Cassini Division (3.0 m at r∼118,800 km, 4.5 m at r∼120,700 km) and the inner A Ring (10-15 m for r<127,000 km).  相似文献   

Climatic and environmental controls on speleothem oxygen-isotope values     

Matthew S. Lachniet 《Quaternary Science Reviews》2009,28(5-6):412-432

Variations in speleothem oxygen-isotope values (δ¹⁸O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem δ¹⁸O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem δ¹⁸O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on δ¹⁸O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of δ¹⁸O in the atmosphere include temperature and relative humidity through their role in the multiple isotope “effects”. Variability and modifications of water δ¹⁸O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO₃, resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO₃, and have implications for the ‘Hendy’ test. To assist interpretation of speleothem δ¹⁸O time series, quantitative and semi-quantitative δ¹⁸O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of δ¹⁸O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics.  相似文献   

Late Quaternary moisture export across Central America and to Greenland: evidence for tropical rainfall variability from Costa Rican stalagmites     

Matthew S. Lachniet  Leah Johnson  Yemane Asmerom  Stephen J. Burns  Victor Polyak  William P. Patterson  Lindsay Burt  April Azouz 《Quaternary Science Reviews》2009,28(27-28):3348-3360

We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ¹⁸O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ¹⁸O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland.  相似文献