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941.
K. P. Kodama J. C. Lyons Peter A. Siver Anne-Marie Lott 《Journal of Paleolimnology》1997,17(2):173-189
A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. 210Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawacs southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the 210Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change. 相似文献
942.
Bowser Lake, a fiord lake in the northern Coast Mountains of British Columbia, contains a thick Holocene fill consisting mainly of silt and clay varves. These sediments were carried into the lake by proglacial Bowser River which drains a high-energy, heavily glacierized basin. Sedimentation in the lake is controlled by seasonal snow and ice melt, by autumn rainstorms, and by rare, but very large jökulhlaups from glacier-dammed lakes in the upper Bowser River basin which complicate environmental inferences from the sedimentary record. Sediment is dispersed through the deep western part of the lake by energetic turbidity currents. The turbidity currents apparently do not overtop a sill that separates the western basin from much shallower areas to the east. Large amounts of silt and clay are deposited from suspension in the eastern part of the lake, but sediment accumulation rates there are much lower than to the west. Several strong acoustic reflectors punctuate the varved fill in the western basin; these may be thick or relatively coarse beds deposited during jökulhlaups or exceptionally large storms. The contemporary sediment yield to Bowser Lake, estimated from sediments in the lake, is about 360 t km-2a-1. This is a relatively high value, but it is less than yields insome other, similar montane basins with extensive snow and ice cover.The most likely explanation for the difference is that large amounts of sediment have been, and continue to be, stored on the Bowser delta andin small proglacial lakes. 相似文献
943.
A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district 总被引:1,自引:0,他引:1
K. D. van der Post F. Oldfield E. Y. Haworth P. R. J. Crooks P. G. Appleby 《Journal of Paleolimnology》1997,18(2):103-120
Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing. 相似文献
944.
The existence of a large subglacial lake beneath the antarctic Ice Sheet at Terre Adélie indicates the presence of basal ice at its pressure-melting temperature. A numerical model of the ice-sheet thermal regime is employed using the balance velocity of the ice sheet as an initial model input in order to calculate ice-sheet basal temperatures. However, the results from this model show the Terre Adélie area to be characterised by basal freezing. Heat in addition to that accounted for in the model is thus required at the ice-sheet base in order for pressure melting temperatures to be attained. The sources for such heat are (1) an enhanced geothermal heat flux and (2) an increase in frictional heating caused by the flow of ice. In this paper the latter possibility is expanded by hypothesising that subglacial topography induces convergent ice flow around Terre Adélie, causing enhanced basal ice velocities. Model experiments indicate that an increase in ice velocity (from 7 to at least 42 m yr−1 ) is required to raise the temperature of the basal ice to the pressure melting value. Increased ice velocity, and consequent frictional heat production due to convergent ice flow, may therefore be important in explaining the location of the subglacial lake in this region. These results allow the process of convergent ice flow within a contemporary ice sheet to be quantified. A verification (or otherwise) of the model results may be possible if ice surface velocity measurements from modem GPS methods are made. 相似文献
945.
MARTIN J. PASQUALETTI 《Geographical review》1997,87(1):73-91
ABSTRACT. From the perspective of a human lifetime, the hazards of some nuclear wastes are permanent, so the warnings we place at contaminated nuclear sites must be permanent too. I address questions of how best to provide one hundred centuries of public warning at the first facility for permanent disposal, the Waste Isolation Pilot Plant in New Mexico. Scenarios of intrusion developed to guide the design of warning markers predicted that most of the changes in the area will be social and cultural. Because blatant and permanent markers will increase, not reduce, the probability of inadvertent intrusion, the most appropriate warning is a “landscape of illusion.” Such a landscape needs not permanent surface markers but underground warning devices beneath a soft surface marker. No warning can guarantee deterrence for 10,000 years, however. 相似文献
946.
947.
Hannah L. Hilbert‐Wolf Edward L. Simpson Wendy S. Simpson Sarah E. Tindall Michael C. Wizevich 《Basin Research》2009,21(6):856-871
The Upper Cretaceous Wahweap Formation accumulated in the active Cordilleran foreland basin of Utah. Soft‐sediment deformation structures are abundant in the capping sandstone member of the Wahweap Formation. By comparing with well‐established criteria, a seismogenic origin was determined for the majority of structures, which places these soft‐sediment deformation features in a class of sedimentary features referred to as seismites. A systematic study of the seismite trends included their vertical and horizontal distribution and a semi‐quantitative intensity analysis using a scale from 1 to 5 that is based on magnitude, sedimentary structure type, and the predominance of inferred process of hydroplastic deformation, liquefaction or fluidization. In addition, orientations of soft‐sediment fold axes were recorded. Construction of a northwest‐to‐southeast stratigraphic and seismite intensity cross‐section demonstrates: (1) reduction in stratigraphic thickness and percentage of conglomerates to the southeast, (2) the presence of lower seismite, middle nonseismite, and upper seismite zones within the capping sandstone (permitting subdivision of the capping sandstone member), and (3) elimination of the nonseismite zone and amalgamation of the lower and upper seismite zones to the southeast. Regional isoseismal contour maps generated from the semi‐quantitative analysis indicate a decrease in overall intensity from northwest to southeast in the upper and lower seismic zones and in sandstone within 5 m stratigraphically of the contact between the upper and capping sandstone members. In addition, cumulative seismite fold orientations support a west–northwest direction towards regional epicentres. Isoseismal maps are used to distinguish the effects of intrabasinal normal faulting from those of regional orogenic thrusting. Thus, this study demonstrates the utility of mapping seismites to separate the importance of regional vs. local tectonic activity influencing foreland basin sedimentation by identifying patterns that delineate palaeoepicentres associated with specific local intrabasinal normal faults vs. regional trends in soft‐sediment deformation related to Sevier belt earthquakes. 相似文献
948.
Comparison of flood hazard assessments on desert piedmonts and playas: A case study in Ivanpah Valley, Nevada 总被引:1,自引:0,他引:1
Colin R. Robins Brenda J. Buck Amanda J. Williams Janice L. Morton P. Kyle House Michael S. Howell Maureen L. Yonovitz 《Geomorphology》2009,103(4):520-532
Accurate and realistic characterizations of flood hazards on desert piedmonts and playas are increasingly important given the rapid urbanization of arid regions. Flood behavior in arid fluvial systems differs greatly from that of the perennial rivers upon which most conventional flood hazard assessment methods are based. Additionally, hazard assessments may vary widely between studies or even contradict other maps. This study's chief objective was to compare and evaluate landscape interpretation and hazard assessment between types of maps depicting assessments of flood risk in Ivanpah Valley, NV, as a case study. As a secondary goal, we explain likely causes of discrepancy between data sets to ameliorate confusion for map users. Four maps, including three different flood hazard assessments of Ivanpah Valley, NV, were compared: (i) a regulatory map prepared by FEMA, (ii) a soil survey map prepared by NRCS, (iii) a surficial geologic map, and (iv) a flood hazard map derived from the surficial geologic map, both of which were prepared by NBMG. GIS comparisons revealed that only 3.4% (33.9 km2) of Ivanpah Valley was found to lie within a FEMA floodplain, while the geologic flood hazard map indicated that ~ 44% of Ivanpah Valley runs some risk of flooding (Fig. 2D). Due to differences in mapping methodology and scale, NRCS data could not be quantitatively compared, and other comparisons were complicated by differences in flood hazard class criteria and terminology between maps. Owing to its scale and scope of attribute data, the surficial geologic map provides the most useful information on flood hazards for land-use planning. This research has implications for future soil geomorphic mapping and flood risk mitigation on desert piedmonts and playas. The Ivanpah Valley study area also includes the location of a planned new international airport, thus this study has immediate implications for urban development and land-use planning near Las Vegas, NV. 相似文献
949.
Despite low growth rates, plants in arid areas have a strong ability to modify soil surface properties affecting ecosystem processes and community dynamics. But our knowledge on species effects on soil properties in these areas comes largely from observational studies, increasing the risk of confounding factors and precluding estimations of rates of change. We evaluated changes in soil surface properties underneath Acacia salicina, Pinus halepensis and Eucalyptus occidentalis in a 10-year-old common garden experiment established on a degraded Stipa tenacissima steppe in southern Tunisia. The three species tested improved soil properties compared to those of open areas. Acacia salicina ranked first as soil modifier as the soil underneath this species showed higher total organic carbon, total nitrogen, available phosphorus, soil CO2 efflux and infiltration rate, and lower soil hydrophobicity than soil in open areas. The richness of vascular plants was higher under A. salicina than under the other types of cover. This species showed higher ability to improve microsite conditions and foster succession. Short rotations of A. salicina could thus be employed for the restoration of degraded S. tenacissima steppes provided that other aspects of its ecology are controlled. Pinus halepensis represents a good alternative when native species are a priority, albeit facilitative ability is lower. 相似文献
950.
The Gray Fossil Site (GFS) includes a small (<2 ha) paleosinkhole lake fill with an exceptionally well-preserved record of
sedimentation and fossils from the latest Miocene to earliest Pliocene. The uppermost lacustrine stratigraphy is characterized
by rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty clay (B)
laminae. Both the A and B components are almost exclusively comprised of exogenic sediment (including organic matter). Periodicities
of 24 and 4.4 are recorded within a continuous 96 interpreted year sequence of rhythmite sediment. In a small lake with a
poorly oxygenated bottom, the presence of laterally continuous laminated sediment that includes well-known periodicities in
rhythmite thickness is interpreted as representing annually generated varves that correspond to seasonal variations in sedimentation.
The distinctly larger fraction of medium sand-size quartz grains present within the A laminae, as well as the abrupt transitions
between A and B components suggest that the rhythmites represent deposition during alternating high-energy and lower-energy
seasons, which is consistent with a monsoonal precipitation pattern. The seasonal climate may relate to changes in the ocean
circulation pattern prior to 4.6 Ma that resulted in an increased temperature and atmospheric pressure gradient between the
east coast of North America and the Atlantic Ocean, but this climate phase seems to be only a temporary condition, as underlying
and overlying sediment are both consistent with drier conditions. The periodicity at 24 interpreted years is consistent with
the well-known Hale solar cycle. The 4.4 interpreted-year periodicity occurs within the ENSO frequency band, and if this documentation
of ENSO-like interannual climate change is correct, then it suggests that ENSO operated at times during the warm Earth conditions
characterizing the late Tertiary. 相似文献