High‐frequency sediment transport responses on a vegetated foredune     

R. G. D. Davidson‐Arnott  B. O. Bauer  I. J. Walker  P. A. Hesp  J. Ollerhead  C. Chapman 《地球表面变化过程与地形》2012,37(11):1227-1241

Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Engineering rodents create key habitat for lizards     

A.D. Davidson  D.C. Lightfoot  J.L. McIntyre 《Journal of Arid Environments》2008,72(12):2142-2149

There is growing recognition among ecologists that ecosystem engineers play important roles in creating habitat for other species, but the comparative and combined effects of co-existing engineers are not well known. Here, we evaluated the separate and interactive effects of two burrowing rodents, Gunnison's prairie dogs (Cynomys gunnisoni) and banner-tailed kangaroo rats (Dipodomys spectabilis), on lizards in the Chihuahuan Desert grassland (USA). We found that the mounds and burrow systems of both rodent species provided important habitat for lizards, with lizard abundance being 2 to 4-fold higher on mounds than in adjacent areas without mounds. Kangaroo rat mounds supported greater numbers of lizards than prairie dog mounds, but the prairie dog colony with kangaroo rats supported 2-times more lizards than the landscape with only kangaroo rats. A greater number of mound habitats were available for lizards where prairie dogs and kangaroo rats co-occurred, and the rodents created unique structural mound types with different spatial distributions on the landscape. Our results demonstrate the importance of burrowing rodents in creating habitat for other animals, and that the combined effect of ecosystem engineers, especially those with large ecological roles, can be complementary and additive in areas where they co-occur.  相似文献   

Predicting survival of western rock lobsters Panulirus cygnus using discriminant analysis of haemolymph parameters taken immediately following simulated handling treatments     

B. D. Paterson  P. T. Spanoghe  G. W. Davidson  W. Hosking  S. Nottingham  J. Jussila 《新西兰海洋与淡水研究杂志》2013,47(5):1129-1143

Instances of morbidity amongst rock lobsters (Panulirus cygnus) arriving at factories in Western Australia (WA) have been attributed to stress during post‐harvest handling. This study used discriminant analysis to determine whether physiological correlates of stress following a period of simulated post‐harvest handling had any validity as predictors of future rejection or morbidity of western rock lobsters. Groups of 230 western rock lobsters were stored for 6 h in five environments (submerged/flowing sea water, submerged/recirculating sea water, humid air, flowing seawater spray, and recirculated seawater spray). The experiment was conducted in late spring (ambient sea water 22°C), and repeated again in early autumn (ambient sea water 26°C). After 6 h treatment, each lobster was graded for acceptability for live export, numbered, and its haemolymph was sampled. The samples were analysed for a number of physiological and health status parameters. The lobsters were then stored for a week in tanks in the live lobster factory to record mortality. The mortality of lobsters in the factory was associated with earlier deviations in haemolymph parameters as they emerged from the storage treatments. Discriminant analysis (DA) of the haemolymph assays enabled the fate of 80–90% of the lobsters to be correctly categorised within each experiment. However, functions derived from one experiment were less accurate at predicting mortality when applied to the other experiments. One of the reasons for this was the higher mortality and the more severe patho‐physiological changes observed in lobsters stored in humid air or sprays at the higher temperature. The analysis identified lactate accumulation during emersion and associated physiological and haemocyte‐related effects as a major correlate of mortality. Reducing these deviations, for example by submerged transport, is expected to ensure high levels of survival. None of the indicators tested predicted mortality with total accuracy. The simplest and most accurate means of comparing emersed treatments was to count the mortality afterwards.  相似文献   

Palaeoenvironmental evolution of Cenote Aktun Ha (Carwash) on the Yucatan Peninsula,Mexico and its response to Holocene sea-level rise     

Jeremy J. Gabriel  Eduard G. Reinhardt  Matthew C. Peros  Dawn E. Davidson  Peter J. van Hengstum  Patricia A. Beddows 《Journal of Paleolimnology》2009,42(2):199-213

A 61-cm core was obtained from 4 m below the water table in Cenote Aktun Ha, on the Yucatan Peninsula, Mexico. The cenote is 8.6 km from the Caribbean coast and its formation and evolution have been largely affected by sea-level change. The base of the core dates to 6,940–6,740 cal year BP and overlying sediments were deposited rapidly over the subsequent ~200 years. The pollen record shows that the cenote evolved from a marsh dominated by red mangrove (Rhizophora mangle) and fern (Polypodiaceae) to an open-water system. These vegetation changes were controlled by water level and salinity and are thus useful indicators of past sea level. At the base, the δ¹³C_org isotopic ratios reveal the influence of terrestrial vegetation (−29‰ VPDB), but shift to more negative values up-core (−33‰), indicating an influence from particulate matter in the flooded cenote pool. Although microfossil populations were nearly absent through most of the core, the microfossil assemblage in the upper 6 cm of the core is dominated by the juvenile foraminifer Ammonia tepida and the thecamoebian genus Centropyxis. These populations indicate open-water conditions in the cenote and a major environmental shift around 6,600 cal year BP, which is related to sea-level rise in the Caribbean basin. These data fit well with previously established sea-level curves for the Caribbean Sea. Our reconstruction of the environmental history of Cenote Aktun Ha helps elucidate the floral and hydrological history of the region, and highlights the utility of cenote sediments for studying the Holocene sea-level history of the Caribbean Sea.  相似文献   

High-resolution modeling of the mean flow and meso-scale eddy variability around the Grand Banks of Newfoundland     

Li Zhai  Youyu Lu  Simon Higginson  Fraser Davidson  Frédéric Dupont  François Roy  Jérôme Chanut  Gregory C. Smith 《Ocean Dynamics》2015,65(6):877-887

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The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach   总被引：1，自引：0，他引：1  

Robin G. D. Davidson‐Arnott  Yanqi Yang  Jeff Ollerhead  Patrick A. Hesp  Ian J. Walker 《地球表面变化过程与地形》2008,33(1):55-74

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Measuring the level of interstellar inheritance in the solar protoplanetary disk          下载免费PDF全文

Conel M. O'D. Alexander  Larry R. Nittler  Jemma Davidson  Fred J. Ciesla 《Meteoritics & planetary science》2017,52(9):1797-1821

The timing and extent to which the initial interstellar material was thermally processed provide fundamental constraints for models of the formation and early evolution of the solar protoplanetary disk. We argue that the nonsolar (solar Δ¹⁷O ≈ ?29‰) and near‐terrestrial (Δ¹⁷O ≈ 0‰) O‐isotopic compositions of the Earth and most extraterrestrial materials (Moon, Mars, asteroids, and comet dust) were established very early by heating of regions of the disk that were modestly enriched (dust/gas ≥ 5–10 times solar) in primordial silicates (Δ¹⁷O ≈ ?29‰) and water‐dominated ice (Δ¹⁷O ≈ 24‰) relative to the gas. Such modest enrichments could be achieved by grain growth and settling of dust to the midplane in regions where the levels of turbulence were modest. The episodic heating of the disk associated with FU Orionis outbursts were the likely causes of this early thermal processing of dust. We also estimate that at the time of accretion the CI chondrite and interplanetary dust particle parent bodies were composed of ~5–10% of pristine interstellar material. The matrices of all chondrites included roughly similar interstellar fractions. Whether this interstellar material avoided the thermal processing experienced by most dust during FU Orionis outbursts or was accreted by the disk after the outbursts ceased to be important remains to be established.  相似文献   

Debt Wish: entrepreneurial cities,US federalism,and economic development     

Mark Davidson  Kevin Ward 《Urban geography》2017,38(8):1282-1289

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Pulsars: Observational parameters and a discussion on dispersion measures     

Yervant Terzian  Kris Davidson 《Astrophysics and Space Science》1976,44(2):479-500

The relevant data for the known 147 pulsars are presented in graphical and tabular forms. Various data correlations are discussed, and a detailed analysis of pulsar dispersion measures and distances is given. The range of the electron densities in the diffuse interstellar medium is found to be 0.01 cm^–3n _e0.1 cm^–3, and n _e0.03 cm^–3. The dispersion scale height for pulsars is found to be 5.9±0.7 pc cm^–3 implying a linear scale height of 200 pc, which is much smaller than the electron scale height of our Galaxy.Astrophysics and Space Science Review Paper.  相似文献   

Eutrophication erodes inter-basin variation in macrophytes and co-occurring invertebrates in a shallow lake: combining ecology and palaeoecology     

Jorge Salgado  Carl D. Sayer  Stephen J. Brooks  Thomas A. Davidson  Beth Okamura 《Journal of Paleolimnology》2018,60(2):311-328

Aquatic biodiversity is commonly linked with environmental variation in lake networks, but less is known about how local factors may influence within-lake biological heterogeneity. Using a combined ecological and multi-proxy palaeoecological approach we investigated long-term changes in the pathways and processes that underlie eutrophication and water depth effects on lake macrophyte and invertebrate communities across three basins in a shallow lake—Castle Lough, Northern Ireland, UK. Contemporary data allow us to assess how macrophyte assemblages vary in composition and heterogeneity according to basin-specific factors (e.g. variation in water depth), while palaeoecological data (macrophytes and co-occurring invertebrates) enable us to infer basin-specific impacts and susceptibilities to nutrient-enrichment. Results indicate that variability in water depth promotes assemblage variation amongst the lake basins, stimulating within-lake macrophyte assemblage heterogeneity and hence higher lake biodiversity. The palaeo-data indicate that eutrophication has acted as a strong homogenising agent of macrophyte and invertebrate diversities and abundances over time at the whole-lake scale. This novel finding strongly suggests that, as eutrophication advances, the influence of water depth on community heterogeneity is gradually eroded and that ultimately a limited set of eutrophication-tolerant species will become homogeneously distributed across the entire lake.  相似文献