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961.
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
962.
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
963.
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
964.
965.
966.
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
967.
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
968.
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
969.
Epiphytic diatoms as flood indicators 总被引:1,自引:0,他引:1
Johan A. Wiklund Natalie Bozinovski Roland I. Hall Brent B. Wolfe 《Journal of Paleolimnology》2010,44(1):25-42
The hydroecology of floodplain lakes is strongly regulated by flood events. The threat of climate warming and increasing human
activities requires development of scientific methods to quantify changes in the frequency of short-lived flood events, because
they remain difficult to identify using conventional paleolimnological and monitoring approaches. We developed an approach
to detect floods in sediment records by comparing the abundance and composition of epiphytic diatom communities in flooded
and non-flooded ponds of the Peace-Athabasca Delta (PAD), Canada, that grew on submerged macrophytes (Potamogeton zosteriformis, P.
perfoliatus) and an artificial substrate (polypropylene sheets) during the open-water season of 2005. Analysis of similarity tests showed
that epiphytic diatom community composition differs significantly between flooded and non-flooded ponds. After accounting
for the “pond effect,” paired comparisons of the three substrates determined that variation in community composition between
the artificial substrate and macrophytes was similar to that between the macrophyte taxa. Similarity percentage analysis identified
diatom taxa that discriminate between flooded and non-flooded ponds. The relative abundance of ‘strong flood indicator taxa’
was used to construct an event-scale flood record spanning the past 180 years using analyses of sedimentary diatom assemblages
from a closed-drainage pond (PAD 5). Results were verified by close agreement with an independent paleoflood record from a
nearby flood-prone oxbow pond (PAD 54) and historical records. Comparison of epiphytic diatoms in flooded and non-flooded
lakes in this study provides a promising approach to detect changes in flood frequency, and may have applications for reconstructing
other pulse-type disturbances such as hurricanes and pollutant spills. 相似文献
970.
The Geology and Petroleum Resources of Kansas: A Review From Alpha to Omega or From the Pleistocene to the Precambrian 总被引:1,自引:0,他引:1
Daniel F. Merriam 《Natural Resources Research》2010,19(4):293-316
The geological story of Kansas is told through the rocks that are present. It is a simple story in generalities but complex
in detail. Knowing the story, gives insight into understanding the occurrence and location of possible economic valuable minerals,
such as petroleum. This is a brief review of Kansas geology with respect to the known occurrence of oil and gas. Kansas is
part of the Midcontinent oil province with oil having been discovered 150 years ago and commercial production commencing in
1873. Although many prospects remain in Kansas, the state has gone from the number 1 producer in the U.S. in 1916 to 8th today.
Exploration for new oil and gas production therefore is going to have to be more imaginative and utilize new approaches and
techniques to find the elusive petroleum. There are possibilities however for the prospector who can search diligently. Although
the big fields probably have been discovered, the prospects today are deeper, in more undetectable traps, and in essentially
untested places. 相似文献