How reliable is the210Pb dating method? Old and new results from Switzerland     

H. R. von Gunten  R. N. Moser 《Journal of Paleolimnology》1993,9(2):161-178

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.  相似文献   

Sediment transport by burrowing mammals,Marin County,California     

Thomas A. Black  David R. Montgomery 《地球表面变化过程与地形》1991,16(2):163-172

Field observations suggest that burrowing activity is the primary mode of sediment transport currently active in a small grassland drainage basin in Marin County, California. Spatial concentrations of the 1150 gopher mounds surveyed vary from 0-16 mounds m^?2 on interfluves to 0.32 mounds m^?2 on sideslopes and in the topographic hollow, with localized concentrations of up to 2.88 mounds m^?2 on the margins of the colluvial deposit. Simple models of sediment transport by burrowing activity yield estimates of between 0.91 and 2.33 cm³ cm^?1 yr^?1 for the basin as a whole, with absolute minimum and maximum rates of 0.48 and 631 cm³ cm^?1 yr^?1. These values are similar to those previously estimated for this area (Lehre, 1982) and are nearly an order-of-magnitude less than average long-term sediment transport rates at the same site (Reneau, 1988).  相似文献   

Remeasurement of weathering rates,St. Paul's Cathedral,London     

S. T. Trudgill  H. A. Viles  R. J. Inkpen  R. U. Cooke 《地球表面变化过程与地形》1989,14(3):175-196

Remeasurements of weathering rates at six sites on the base of the balustrade of St. Paul's Cathedral gave substantially lower mean results (0.06 mm a^?1, 1980–1985) than for earlier results (0.14 mm a^?1, 1980–1981). Statistical analysis of the data showed that for five of the sites there were no significant differences between the measurement periods. The only significant spatial difference was for the southwest (exposed) site which was greater than the other sites for 1980–1981, but not at later times. This site also showed the only significant decrease over time. If this site is included in the statistical analysis, the whole set shows a significant decrease over time; omitting this site from the data removes any significant differences over time. The results for this one site for 1980–1981 thus have significant bias on the data body as a whole. Methodological problems, such as rock erosion by the probe and temperature effects, are thought to have minimal effect on the interpretation of the data. Calibrations showed that measurements were accurate to ±0.006 mm and that only results of a change ≥ 0.02 mm were reliably interpretable as different. Remeasurements of the Lead Plug Index (LPI) for the top of the balustrade in 1987 gave a mean rate for 1718–1985 of 0.081 mm a^?1. This is not significantly different from the 1718–1980 mean rate of 0.078 mm a^?1 but the data are not directly comparable, the 1987 sample being smaller than the 1980 set. For the 1987 data, the only statistically significant differences are for the southeast rate ≧ northeast and for a rise in rate in the northeast quadrant. The LPI data are comparable to most of the 1980–1985 MEM data. The conclusion is that most erosion rates have neither increased nor decreased significantly in the measurement time, except for a significant decrease at one (southwest) site from 1980–1981 to 1981–1982. Such a sustained level of erosion gives cause for concern in the context of stone conservation. Causative factors of air quality are discussed and while SO₂ levels have decreased, NO₂ and smoke levels show increases. An overall decrease in stone decay in association with the decrease in SO₂ levels is thus not in evidence.  相似文献   

Human influence upon sedimentation in Llangorse Lake,Wales     

R Jones  K. Benson-Evans  F. M. Chambers 《地球表面变化过程与地形》1985,10(3):227-235

Palaeolimnological studies at Llangorse Lake, Wales, constitute part of the British contribution to IGCP Project 158 (The Palaeohydrology of the Temperate Zone in the Last 15000 Years). The lake has been adopted as the Reference Site for the South Wales region under Subproject 158B (Lake and Mire Environments) and is located in the Reference Area for Britain: the Severn Basin. Results of biological, physical, and multielement chemical analyses on a 12.35 m Livingstone core are presented and compared with previous analyses on short Mackereth cores taken from two troughs in the lake basin. Human impact upon post-glacial sedimentation is assessed with reference to sediment chemistry, physical properties, and sedimentation rates, based on relative pollen, ¹⁴C, and ²¹⁰Pb dating. Vegetational history of the catchment is inferred from a pollen diagram for a section of the core from c. 8 to 6.75 m depth in which evidence for human impact is first apparent. Water content, loss on ignition, and carbonate content are presented for the whole core; density, water content, loss on ignition, carbonates, and elemental analyses of P, Ca, Mg, K, Ti, Al, Fe, and Mn for the interval 6 75 and 8.25 m depth. Sedimentation rates were low during Boreal and Atlantic times (c. 9000- 5000 BP), rose significantly from the time of the elm decline (estimated at 5000 BP) and increased further at c. 1800 BP when nekron muds were succeeded by deposition of silty clay. Inferred rates of erosion of soils from the catchment are discussed and related to sediment chemistry.  相似文献   

Slip rates variability and sediment mobilization on a shallow landslide in the northern Swiss Alps     

Marco Schwab  Christine Läderach  Dirk Rieke-Zapp  Fritz Schlunegger 《Swiss Journal of Geoscience》2007,100(2):281-292

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基于博士生培养视角看中国人文地理学的未来——以中山大学为例     

保继刚  朱竑 《世界地理研究》2007,16(4):36-40

以中山大学人文地理学博士点为例，通过对博士生培养过程中存在的学术贡献有限、研究空洞、缺乏一手数据、重复型明显等相关问题的分析，从学术贡献的角度探讨了中国人文地理学的未来发展。  相似文献   

陈代光教授对岭南历史地理研究的贡献     

许桂灵  司徒尚纪 《地域研究与开发》2007,26(2):124-128

陈代光教授是我国一位著名历史地理学者.文章主要论述其在岭南广州、港澳、雷州等城市、南海海上丝绸之路交通、区域历史地理、广东历史地图集编纂、地名、中国历史地理教材建设、历史地理人才培养,以及历史地理为社会经济发展服务等方面的学术成就和重要贡献,以作为对陈代光教授的一种纪念,同时唤起社会各界对这门又古又今、既要看书、又要走路的学科的关注.  相似文献   

COOLING RATES, SKY VIEW FACTORS AND THE DEVELOPMENT OF INTRA-URBAN AIR TEMPERATURE DIFFERENCES     

BJÖRN HOLMER  SOFIA THORSSON  INGEGÄRD ELIASSON 《Geografiska Annaler: Series A, Physical Geography》2007,89(4):237-248

Intra‐urban cooling rates were monitored for a year in the centre of Göteborg on the Swedish west coast. Five sites with different building geometry ‐ from dense canyon to open space ‐ within a radius of 300 m were analysed. Results showed two modes of cooling during the night. In the first mode, the cooling was site‐dependent ‐the open space cooled the most and the dense canyon the least. In the second mode, which began about 3–4 hours after sunset, all sites cooled at the same rate. Our hypothesis is that in the early part of the evening both radiative divergence and sensible heat dominate the cooling. However, as the evening progresses, a spatially homogeneous inversion is established which controls the radiative cooling, making the cooling independent of both surface geometry and surface type. From April to November the sites cooled rapidly around sunset and the cooling slowly decreased during the rest of the night. However, between December and March, the cooling was less intense, with an almost constant rate during the entire night. It has been suggested that this might be the result of the draining of stored solar heat during the summer and a continuous flux of anthropogenic heating during the winter.  相似文献   

Geographic variations in shell growth rates of the mussel Diplodon chilensis from temperate lakes of Chile: Implications for biodiversity conservation     

Claudio Valdovinos  Pablo Pedreros 《Limnologica》2007,37(1):63-75

The Chilean lake district includes diverse lentic ecosystems along ca. 700 km of the country (36°–43°S), including the “Nahuelbutan lakes”, “Araucanian lakes” and “Chiloe lakes”. This area is recognized as an important “hot spot” of benthic freshwater biodiversity in Southern South America. In Chilean temperate lakes, increased nutrient loads of P and N caused eutrophication, particularly in the Nahuelbutan Lakes. The freshwater Hyriidae mussel Diplodon chilensis (Gray, 1828) which is one of the most abundant species in Chilean temperate lakes, is known to be very susceptible to eutrophication. This species presents a clear reduction in its geographic ranges and is considered to be a threatened species in many Chilean lakes. In this study, we used a correlative approach to determine how eutrophication-driven changes in the food supply and in geographical parameters of different Chilean lakes affected the shell growth rates of D. chilensis. The results obtained from sclerochronological analyses of the mussel shells suggest an association with a group of environmental variables, including geographical types (negative), such as latitude and altitude, and limnological types (positive), especially phosphorous and turbidity. However, the D. chilensis populations under extreme conditions of turbidity in eutrophic and hypertrophic lakes are extinct or nearly so. The high positive correlation of the mean D. chilensis growth rates with orthophosphate (R=0.76; P<0.05), in relation to dissolved inorganic nitrogen, suggests that P is the major limiting factor of the primary productivity in Chilean temperate lakes. We discuss some implications of our results in terms of the conservation of biodiversity in temperate lake ecosystems at different taxonomic levels.  相似文献   

A  3.5 Ga record of water-limited, acidic weathering conditions on Mars     

Joel A. Hurowitz  Scott M. McLennan 《Earth and Planetary Science Letters》2007,260(3-4):432-443

The secondary mineral budget on Earth is dominated by clay minerals, Al-hydroxides, and Fe-oxides, which are formed under the moderate pH, high water-to-rock ratio conditions typical of Earth's near-surface environment. In contrast, geochemical analyses of rocks and soils from landed missions to Mars indicate that secondary mineralogy is dominated by Mg (± Fe, Ca)-sulfates and Fe-oxides. This discrepancy can be explained as resulting from differences in the chemical weathering environment of Earth and Mars. We suggest that chemical weathering processes on Mars are dominated by: (1) a low-pH, sulfuric acid-rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite (± Fe–Ti oxides) is promoted; and (2) relatively low water-to-rock ratio, such that other silicate phases with slower dissolution rates (e.g., plagioclase, pyroxene) do not contribute substantially to the secondary mineral budget at the Martian surface. Under these conditions, Al-mobilization is limited, and the formation of significant Al-bearing secondary phases (e.g., clays, Al-hydroxides, Al-sulfates) is inhibited. The antiquity of rock samples analyzed in-situ on Mars suggest that water-limited acidic weathering conditions have more than likely been the defining characteristic of the Martian aqueous environment for billions of years.  相似文献