Archaeomagnetic investigations in Bulgaria: Field-intensity determinations     

Mary Kovachieva 《Physics of the Earth and Planetary Interiors》1977,13(4):355-359

Geomagnetic paleointensity determination have been made by the Thellier method using samples from 27 sites in Bulgaria. The samples include bricks, specimens taken from historic kilns, from prehistoric hearths and the sites of ancient fires. The ages of the samples, which range from about 4500 B.C. to the 19th century A.D., have been determined partly by the ¹⁴C method and partly from archaeological evidence. The (residual NMR)-(induced TRM) diagrams tend to be less linear for the prehistoric samples either due to weathering or because the NRM is not a total TRM.  相似文献   

The economic impact of climate change on Kenyan crop agriculture: A Ricardian approach   总被引：3，自引：2，他引：1  

Jane Kabubo-Mariara  Fredrick K. Karanja 《Global and Planetary Change》2007,57(3-4):319-330

This paper measures the economic impact of climate on crops in Kenya. We use cross-sectional data on climate, hydrological, soil and household level data for a sample of 816 households. We estimate a seasonal Ricardian model to assess the impact of climate on net crop revenue per acre. The results show that climate affects crop productivity. There is a non-linear relationship between temperature and revenue on one hand and between precipitation and revenue on the other. Estimated marginal impacts suggest that global warming is harmful for crop productivity. Predictions from global circulation models confirm that global warming will have a substantial impact on net crop revenue in Kenya. The results also show that the temperature component of global warming is much more important than precipitation. Findings call for monitoring of climate change and dissemination of information to farmers to encourage adaptations to climate change. Improved management and conservation of available water resources, water harvesting and recycling of wastewater could generate water for irrigation purposes especially in the arid and semi-arid areas.  相似文献   

Environmental archaeology. Theoretical and practical approaches. N. Branch,M. Canti,P. Clark and C. Turney (eds). Publisher: Hodder Arnold,London, 2005 (240 pp.) ISBN: 0‐340‐80871‐3     

Jane Sidell 《第四纪科学杂志》2007,22(7):745-746

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Reforestation Programs in Southwest China： Reported Success, Observed Failure, and the Reasons Why   总被引：4，自引：2，他引：2  

Christine Jane Trac Stevan Harrell Thomas M. Hinckley Amanda C. Henck 《山地科学学报》2007,4(4):275-292

Ever since the disastrous floods of 1998, the Chinese government has used the Natural Forest Protection and Sloping Land Conversion Programs to promote afforestation and reforestation as means to reduce runoff, control erosion, and stabilize local livelihoods. These two ambitious programs have been reported as large-scale successes, contributing to an overall increase in China’s forest cover and to the stated goals of environmental stabilization. A small-scale field study at the project level of the implementation of these two programs in Baiwu Township, Yanyuan County, Sichuan, casts doubt upon the accuracy and reliability of these claims of success; ground observations revealed utter failure in some sites and only marginal success in others. Reasons for this discrepancy are posited as involving ecological, economic, and bureaucratic factors. Further research is suggested to determine whether these discrepancies are merely local aberrations or represent larger-scale failures in reforestation programs.  相似文献   

Currents and sediment transport in the Mississippi Canyon and effects of Hurricane Georges     

Cheryl Burden Ross  Wilford D. Gardner  Mary Jo Richardson  Vernon L. Asper 《Continental Shelf Research》2009,29(11-12):1384-1396

The temporal variability in currents, temperature, and particulate matter concentration were measured in the Mississippi Canyon axis where the thalweg was 300 m deep from May–July and August–November 1998 using current meters, thermographs, a light-scattering sensor, and sediment traps. Canyon sediments were sampled by coring and observed using an ROV video camera. Currents in the upper Mississippi Canyon generally oscillated up/down canyon with diurnal periodicity and were bottom-intensified. Mean current speed at 3.5 mab was approximately 8 cm s^?1 during both deployments, reaching maximum speeds of over 50 cm s^?1 under normal conditions. Based on current velocities, critical bed shear stress for resuspension of canyon-floor sediments was exceeded about 30% of the time during both deployments. In late September, Hurricane Georges passed 150 km NE of the study site, significantly intensifying current velocities, bed shear stress, resuspension, trap fluxes and temperature fluctuations. As the hurricane passed, maximum current speed reached 68 cm^?s and temperature decreased ～7 °C in less than two hours. Critical bed shear stress for sediment resuspension was exceeded approximately 50% of the time during the five days of hurricane influence. Further evidence for sediment resuspension was the five-fold (and perhaps 70–130 fold) increase in trap fluxes and compositional similarities between canyon surface sediment and material collected by traps.  相似文献   

Late Quaternary paleoclimate of western Alaska inferred from fossil chironomids and its relation to vegetation histories     

Joshua Kurek  Les C. Cwynar  Thomas A. Ager  Mark B. Abbott  Mary E. Edwards 《Quaternary Science Reviews》2009,28(9-10):799-811

Fossil Chironomidae assemblages (with a few Chaoboridae and Ceratopogonidae) from Zagoskin and Burial Lakes in western Alaska provide quantitative reconstructions of mean July air temperatures for periods of the late-middle Wisconsin (～39,000–34,000 cal yr B.P.) to the present. Inferred temperatures are compared with previously analyzed pollen data from each site summarized here by indirect ordination. Paleotemperature trends reveal substantial differences in the timing of climatic warming following the late Wisconsin at each site, although chronological uncertainty exists. Zagoskin Lake shows early warming beginning at about 21,000 cal yr B.P., whereas warming at Burial Lake begins ～4000 years later. Summer climates during the last glacial maximum (LGM) were on average ～3.5 °C below the modern temperatures at each site. Major shifts in vegetation occurred from ～19,000 to 10,000 cal yr B.P. at Zagoskin Lake and from ～17,000 to 10,000 cal yr B.P. at Burial Lake. Vegetation shifts followed climatic warming, when temperatures neared modern values. Both sites provide evidence of an early postglacial thermal maximum at ～12,300 cal yr B.P. These chironomid records, combined with other insect-based climatic reconstructions from Beringia, indicate that during the LGM: (1) greater continentality likely influenced regions adjacent to the Bering Land Bridge and (2) summer climates were, at times, not dominated by severe cold.  相似文献   

Editor's note 1976     

Mary Swallow 《Progress in Oceanography》1976,7(1):1

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Trends and correlations in annual extreme precipitation indices for mainland Portugal, 1941–2007     

M. Isabel P. de Lima  Fátima Espírito Santo  Alexandre M. Ramos  Ricardo M. Trigo 《Theoretical and Applied Climatology》2015,119(1-2):55-75

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Compressed sensing for next generation instruments     

A. Asensio Ramos 《Astronomische Nachrichten》2010,331(6):652-654

This document discusses the possibility of using compressed sensing techniques for measuring 2D spectro‐polarimetric information using only one etalon and a broad prefilter. Instead of using an etalon and an extremely narrow prefilter (with all the subsequent problems of alignment), the idea is to use multiplexing techniques to include in the observations all the secondary peaks of the etalon. The reconstruction of the signal is done under the assumption that it can be efficiently reproduced in an orthogonal basis set (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Barchan dune asymmetry: Observations from Mars and Earth     

Mary C. Bourke 《Icarus》2010,205(1):183-197

Barchan dune asymmetry refers to the extension of one barchan limb downwind. It is a common dune form on Earth and also occurs on Mars and Titan. A new classification of barchan limbs is presented where three types of limb morphology are identified: linear, kinked and beaded. These, along with other dune-scale morphological signatures, are used to identify three of the causes of barchan asymmetry on Mars: bi-directional winds, dune collision and the influence of inclined topography.The potential for specific dune asymmetric morphologies to indicate aspects of the formative wind regime on planetary surfaces is shown. For example, the placement of dune limbs can indicate the general direction and relative strength of formative oblique winds; an extreme barchan limb length may indicate a long duration oblique wind; a kinked limb may be evidence of the passage of a storm; beaded limbs may represent surface-wave instabilities caused by an increase in wind energy parallel to the dune. A preliminary application of these signatures finds evidence for bi-modal winds on Mars. However, these and other morphological signatures of wind direction and relative strength should be applied to planetary landforms with caution as more than one process (e.g., bi-modal winds and collision) may be operating together or sequentially on the dunefield. In addition, analysis should be undertaken at the dunefield scale and not on individual dunes. Finally, morphological data should be acquired from similar-scale dunes within a dunefield.In addition to bi-modal wind regimes on Mars, the frequent parallel alignment of the extended barchan limb to the dune suggests that dune collision is also an important cause of asymmetry on Mars. Some of the more complex dunefield patterns result from a combination of dune collision, limb extension and merging with downwind dunes.Dune asymmetric form does not inhibit dune migration in the Namib Desert or on Mars. Data from the Namib suggest that dune migration rates are similar for symmetric and asymmetric dunes. Further modeling and field studies are needed to refine our understanding of the potential range of limb and dune morphologies that can result from specific asymmetry causes.  相似文献