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Abstract— Alteration of surficial suevites at Ries crater, Germany was studied by means of X‐ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post‐impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al‐Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post‐impact cooling, then the alteration was of limited character and is completely masked by later weathering.  相似文献   
2.
During the last decade the frequency of landslides at river valley slopes eroding into the glaciolacustrine plain in western Estonia has grown considerably. We studied in detail nine recent landslides out of 25 known and recorded sliding events in the area. All landslides occurred at the river banks in otherwise almost entirely flat areas of proglacial deposits capped with marine sands. Glaciolacustrine varved clay is the weakest soil type in the area and holds the largest landslides. Slope stability modelling shows that critical slope gradient for the clay is ≥ 10° and for the marine sand ≥ 20°. Fluvial erosion is the main process in decreasing slope stability at the outer bends of the river meanders. An extra shear stress generated by groundwater flow following the high stand of the groundwater level or rapid water level drawdown in the river channels are responsible for triggering the landslides. Consecutive occurrence of small-scale slides has a direct effect in triggering the large, retrogressive complexes of slides in the glaciolacustrine clay. A landslide hazard zonation map was composed based on digital elevation model and the data on spatial distribution of glaciolacustrine clays and marine sands, and on existing and critical slope angles of these deposits.  相似文献   
3.
Peeter Maandi 《Geoforum》2009,40(3):454-39
With an explicit focus on the spatial articulation of landownership, this paper explores various ways in which pre-Soviet land rights were expressed in rural Estonia during the Soviet period (1940-1941, 1944-1991). Drawing on cartographical analyses and interviews made in the rural district of Muhu, the paper demonstrates that people who owned land before the Soviet occupation kept track of the officially annulled pre-Soviet land rights, by relating to inertial landscape elements as memory-aids. To local inhabitants the landscape, in which past and present structures always merge, provided substantial evidence in support of the idea of legal continuity of pre-Soviet land rights. Hence, the post-Soviet land restitution reform often implied a re-discovery or re-expression of property rights that had been silenced, but not lost.  相似文献   
4.
A device was developed to make fine-scale in situ measurements of formation and ground water conductivity at depths up to 10 meters in unconsolidated shallow aquifers. It consists of a casing that is left in place ("disposable") and a probe that slides down the center of the casing making readings or taking water samples through screened slots. The device is inexpensive, designed to be jetted into place, and intended for use in monitoring situations where drilling rigs and wireline logs are impractical. It offers, in principle, 10cm resolution of the conductivities and the formation factor as a function of time.
Results from three of these units installed across a landfill plume near North Bay, Ontario, demonstrate the usefulness of these kinds of measurements in a monitoring study, and the problems that can be encountered in a highly contaminated environment.  相似文献   
5.
Hydrogeologic investigations of fractured rock are evolving toward increasing spatial and temporal resolution with increasing use of multilevel systems with 10 or more intervals in a single borehole, each with auto‐sampling sensors monitoring pressure, temperature or chemistry for weeks or months, creating large quantities of densely sampled data (time and space). These data are typically displayed as hydrographs for analysis of site‐specific controls on groundwater flow. We present a method for presentation of high density pressure head data from multilevel installations referred to as time‐elevation head (TEH) sections that improves visualization of spatial and temporal responses of the hydrogeologic system to external stresses. Data collected from two multilevel installations, each with 13 functioning pressure transducers monitoring the upper 40 m of a dolostone aquifer, over a period of 83 d, prior to, during and after a pumping test are used to present TEH sections and examples of data processing. TEH sections are produced using commercially available software designed for geophysical data collected at closely spaced intervals along sub‐parallel lines. These algorithms perform calculations orthogonally either in time (“X” axis) or elevation (“Y” axis) to interpolate a regular grid of head and subsequently when filtering is used to identify subtle trends within the data. The base and filtered TEH sections are used to interpret response of the system to transients and infer hydrogeologic characteristics of the site. The utility of the process is dependent on the precision and accuracy of the head data as well as an informed user to avoid introducing spurious features into the sections.  相似文献   
6.
We studied high-resolution stratigraphy of phosphorus (P) forms in the Holocene sediments of large shallow Lake Peipsi (Estonia/Russia) in order to evaluate the lake ecosystem response to environmental changes and track the lake’s eutrophication history. We distinguished four main periods in the history of Lake Peipsi, each having likely different factors responsible for the distribution pattern of P fractions in the sediment record. We suggest that in the oldest period, from ca. 10.4 up to 7.3 cal ka BP, the sediment composition was mainly determined by rising water level, the second period dated 7.3–2.3 cal ka BP was governed mainly by stable hydrology and P loading, while the third period between 2.3 and 0 cal ka BP was primarily influenced by emerging anthropogenic impact. The sediments from the last period since 1950 are subject of ongoing diagenetic processes but still reflect rapid eutrophication of the lake. Comparison of the results with periods derived from other sediment proxies proved the usability of P fractions stratigraphy in reconstruction of the development of lakes.  相似文献   
7.
: Phytoplankton primary production measurements and sediment trapping in the large (270 km2) shallow (mean depth 2.8 m) Lake Võrtsjärv were performed during ice-free periods in 1995-1996. Sedimentation rate varied from 26 to 700 g DWm-2d-1 (DW=dry weight), with a mean value of 170 g DWm-2d-1. Under the influence of prevailing westerly and south-westerly winds, the sedimentation rate was higher in the northern part, and lower in the western and southern parts of the lake. An increasing trend towards autumn could be followed in both years. The mean sedimentation rates in the dry year 1996 with a low lake water level exceeded those of the “normal” 1995 by a factor of more than two. New sedimentation formed 10-15 % of the downward sediment flux in 1995 and 4-6 % in 1996, while the major part of entrapped matter originated from resuspended bottom sediments. The dynamics of gross sedimentation showed no relation to the primary production but correlated significantly with the concentration of suspended solids, with the average wind speed and with the mean depth of the lake during trap exposition.¶The composition of entrapped matter did not show seasonal variation but differed between the sampling stations. The amount of particulate organic matter (POM) per DW varied from 11 % at places where the bottom sediment consisted of moraine or sand to 28 % at places with muddy bottom. N:POM was rather stable (45-61 mgg-1, Cvar = 6 %). The variability of P:POM (3.5-7.1, Cvar = 20 %) exceeded that of P:DW (0.75-1.31 mgg-1, Cvar = 10 %) showing the essential role of mineral-bound P in the settling material. In these conditions, the N:P ratio was determined by the POM content of settling matter. POM:DW in settling material was generally higher than in the 0-2 cm layer of bottom sediments.  相似文献   
8.
Carbon storage in lakes can have huge implications for the global carbon cycle, as lakes annually accumulate up to one half the amount of organic carbon buried in marine sediments. Yet little is known of the effect of recent climate change on carbon storage in lakes. We analyzed century-scale time series of climate variables (precipitation, temperature, NAO winter index) and profiles of sediment characteristics in a dated sediment core from shallow, eutrophic Lake Võrtsjärv, south Estonia. We used path analysis to evaluate the effect of climate conditions on phytoplankton biomass in the lake and accumulation of organic and inorganic carbon in the sediment. Changes in winter and spring climate influenced the lake’s phytoplankton growth significantly. Carbon pathways in hard-water Lake Võrtsjärv were influenced by both hydrological (most significant in colder periods) and biogeochemical processes. Increased nutrient and water input to Lake Võrtsjärv, anticipated with projected climate warming, favours greater in-lake productivity, larger accumulation of inorganic carbon in sediments, and an increase in organic carbon mineralisation, which fuels atmospheric greenhouse gas emissions from the lake.  相似文献   
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