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Paleoenvironments during Younger Dryas‐Early Holocene retreat of the Greenland Ice Sheet from outer Disko Trough,central west Greenland
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34.
Multivariate statistical analyses of geomorphic variables from 23 forest stream reaches in southeast Alaska result in successful discrimination between pristine streams and those disturbed by land management, specifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model discriminates 10 disturbed from 13 undisturbed reaches with 90 per cent and 92 per cent correct classification respectively. These variables are the total number of pools per reach, the ratio of mean residual pool depth to mean bankfull depth, and the ratio of critical shear stress of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classification; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, can also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. However, channel unit classification and inventory can be subject to considerable error and observer bias. Abundance of pool-related large woody debris is highly correlated with pool frequency and is an important factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and disturbed channel conditions, providing a reference standard for channel assessment and restoration efforts. 相似文献
35.
Late Pleistocene sequences around Dundalk Bay, eastern Ireland, record glaciomarine sedimentation near the margin of a grounded ice lobe around 15 ka BP. A coastal exposure at Cooley Point consists of four major facies deposited outside this ice limit. (1) A basal mud facies deposited from sediment plumes accumulated following the initial ice marginal retreat inland. It contains a well-preserved Arctic microfauna dominated by the foram Elphidium clavatum and the ostracod Roundstonia globulifera. (2) A flat to undulating boulder facies (pavement), mostly one clast thick, is found pressed into the mud and is characterized by bevelled and striated upper clast surfaces. Pavement attributes are a result of intertidal activity in a cold climate. The boulder source is due to rafting by ice floes from glacigenic debris deposited during an ice advance. (3) Laminated sand facies drape the pavement and are a result of variable current activity. (4) The overlying gravel facies is separated from the sand facies by a marine erosion surface. The gravel facies is subaqueous, channelized and is overlain by late glacial raised beach ridges. Locally the gravels have been deformed by ice pressure from partially floating ice floes. Facies changes record terrestrial submergence and provide evidence for changes in relative sea level during part of the last deglacial cycle. The boulder pavement and deformed gravel facies suggest that ice floes and sea ice effects may be more common within stratigraphies along emergent coasts than previously thought, though they have a low preservation potential. Extreme conditions during the deglacial favoured opportunistic microfaunas during mud deposition. This event may be related to a major meltwater event within the Irish Sea Basin. 相似文献
36.
Published accounts of outburst floods from glacier-dammed lakes show that a significant number of such floods are associated not with drainage through a tunnel incised into the basal ice—the process generally assumed—but rather with ice-marginal drainage, mechanical failure of part of the ice dam, or both. Non-tunnel floods are strongly correlated with formation of an ice dam by a glacier advancing from a tributary drainage into either a main river valley or a pre-existing body of water (lake or fiord). For a given lake volume, non-tunnel floods tend to have significantly higher peak discharges than tunnel-drainage floods. Statistical analysis of data for floods associated with subglacial tunnels yields the following empirical relation between lake volume V and peak discharge Qp : Qp = 46V0.66 (r2 = 0.70), when Qp is expressed in metres per second and V in millions of cubic metres. This updates the so-called Clague–Mathews relation. For non-tunnel floods, the analogous relation is Qp = 1100V0.44 (r2 = 0.58). The latter relation is close to one found by Costa (1988) for failure of constructed earthen dams. This closeness is probably not coincidental but rather reflects similarities in modes of dam failure and lake drainage. We develop a simple physical model of the breach-widening process for non-tunnel floods, assuming that (1) the rate of breach widening is controlled by melting of the ice, (2) outflow from the lake is regulated by the hydraulic condition of critical flow where water enters the breach, and (3) the effect of lake temperature may be dealt with as done by Clarke (1982). Calculations based on the model simulate quite well outbursts from Lake George, Alaska. Dimensional analysis leads to two approximations of the form Qp ∝ Vqf(hi, θ0), where q = 0.5 to 0.6, hi is initial lake depth, θ0 is lake temperature, and the form of f (hi, θ0) depends on the relative importance of viscous dissipation and the lake's thermal energy in determining the rate of breach opening. These expressions, along with the regression relations, should prove useful for assessing the probable magnitude of breach-type outburst floods. 相似文献
37.
The stratigraphy of marine Plio-Pleistocene sediments from northeast Rhodes (Dodecanese islands, Greece) is revised in the light of facies mapping and the recognition of three major transgression–regression cycles. Before late Pliocene submergence, metamorphosed Mesozoic limestones formed a high-relief karstic landscape. During subsequent transgressions, subsiding basins with spectacular cliffed and bioeroded margins were infilled by a mosaic of carbonate-dominated sediments, sourced by high rates of carbonate productivity on narrow shelves rimming the basins and their drowning islands. Periods of relative sea-level fall superimposed a complex series of ‘fossil’ coastal geomorphological features such as cliffs, abrasion platforms, surf caves, notches, boulder beaches and palaeokarsts. These created the rugged present day topography of the island's northeast coast. Three main lithostratigraphic units, the Kritika, Rhodes and Lindos Acropolis Formations, are defined. The Rhodes and Lindos Acropolis Formations are subdivided into mappable facies groups rather than members, as the lithofacies are strongly diachronous. The Kritika and Rhodes Formations were each subaerially exposed and partly eroded before re-transgression. Although there is a general pattern of lithological succession, details vary across the study area and some facies groups are restricted in development, indicating differences in tectonic behaviour and palaeo-geomorphology between neighbouring basins. A preliminary process-response model is presented for the sedimentation history of the Rhodes and Lindos Acropolis Formations, using sedimentological, palaeontological, palaeoecological and ichnological data. 相似文献
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ANNE E. BJUNE H. JOHN B. BIRKS SYLVIA M. PEGLAR ARVID ODLAND 《Boreas: An International Journal of Quaternary Research》2010,39(4):674-688
Bjune, A. E., Birks, H. J. B., Peglar, S. M. & Odland, A. 2010: Developing a modern pollen–climate calibration data set for Norway. Boreas, Vol. 39, pp. 674–688. 10.1111/j.1502‐3885.2010.00158.x. ISSN 0300‐9483. Modern pollen–climate data sets consisting of modern pollen assemblages and modern climate data (mean July temperature and mean annual precipitation) have been developed for Norway based on 191 lakes and 321 lakes. The original 191‐lake data set was designed to optimize the distribution of the lakes sampled along the mean July temperature gradient, thereby fulfilling one of the most critical assumptions of weighted‐averaging regression and calibration and its relative, weighted‐averaging partial least‐squares regression. A further 130 surface samples of comparable taphonomy, taxonomic detail and analyst became available as a result of other projects. These 130 samples, all from new lakes, were added to the 191‐lake data set to create the 321‐lake data set. The collection and construction of these data sets are outlined. Numerical analyses involving generalized linear modelling, constrained ordination techniques, weighted‐averaging partial least‐squares regression, and two different cross‐validation procedures are used to asses the effects of increasing the size of the calibration data set from 191 to 321 lakes. The two data sets are used to reconstruct mean July temperature and mean annual precipitation for a Holocene site in northwest Norway and a Lateglacial site in west‐central Norway. Overall, little is to be gained by increasing the modern data set beyond about 200 lakes in terms of modern model performance statistics, but the down‐core reconstructions show less between‐sample variability and are thus potentially more plausible and realistic when based on the 321‐lake data set. 相似文献
40.
Abstract The Bundelkhand area of central India consists of weathered granitic bedrock. The aim of the present study was to identify the effects of surface water irrigation on groundwater quality in shallow aquifers used for domestic supply. A set of 37, 58 and 17 samples were collected, respectively, from the non-irrigated area, the irrigated area and surface waters. Concentrations of major ions (including NO3 ?), PO4 = and Fe++ were determined. A statistical comparison of the data groups was carried out assuming that the only difference between the two areas was in agricultural practice. It was discovered that, despite the better quality of the surface waters, the irrigated area groundwaters were higher in concentration than those of the non-irrigated area. The main causes for that appear to be evaporation/transpiration and fertilizer application, modified by calcite precipitation and ion exchange. The proposed approach is simple enough to be used routinely in monitoring. 相似文献