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101.
John Houston 《水文研究》2002,16(15):3019-3035
The Chacarilla fan in the Atacama Desert is one of several formed in the Late Miocene at the foot of the Pre‐Andean Cordillera overlying the large, complex, Pampa Tamarugal aquifer contained in the continental clastic sediments of the fore‐arc basin. The Pampa Tamarugal aquifer is a strategic source of water for northern Chile but there is continuing doubt over the resource magnitude and recharge. During January 2000 a 1 in 4 year storm in the Andes delivered a 34 million m3 flash flood to the fan apex where c. 70% percolated to the underlying aquifers. Groundwater recharge through the fan is calculated to be a minimum of 200 l/s or 6% of the long‐term catchment rainfall. These figures are supported by hydrochemical data that suggest that recharge may be 9% of long‐term rainfall. Isotopic data suggest groundwater less than 50 years old is transmitted westward through the permeable sheetflood sediments of the fan overlying the main aquifer. Analysis of this and other events shows that the hydrological system is non‐linear with positive feedback. The magnitude of groundwater recharge is dependent on climatic variations, antecedent soil moisture storage and changes in channel characteristics. Long‐term declines in groundwater level may partly result from climatic fluctuations and the causes of such fluctuations are discussed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
102.
C. F. M. Lewis C. W. HeilJr. J. B. Hubeny J. W. King T. C. MooreJr. D. K. Rea 《Journal of Paleolimnology》2007,37(3):435-452
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern
Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was
inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from
the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence
rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the
Lake Stanley phase.
A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates
the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated
lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is
attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in
the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley
lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows
that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest
of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows
that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The
late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time.
A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand,
late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes.
This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004),
held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special
issue. 相似文献
103.
Rodrigo Riquelme Grard Hrail Joseph Martinod Reynaldo Charrier Jos Darrozes 《Geomorphology》2007,86(3-4):283-306
We analyze remarkable examples of the large ( 10,000 km2) and local-scale ( 100 km2) landscape forms related to Late Cenozoic geomorphologic evolution of the Andean forearc region in the Southern Atacama Desert. We also consider the continental sedimentary deposits, so-called “Atacama Gravels”, which are related to the degradation of the landscape during the Neogene. Our analysis integrates 1:50,000 field cartography, Landsat TM images observations, 1:1000 sedimentary logging data, and 50 m horizontal resolution topographic data to reconstruct the Late Cenozoic geomorphologic evolution of this region and discuss the factors that control it, i.e., Miocene aridification of the climate and Neogene Central Andean uplift. We determine that the Precordillera was already formed in the Oligocene and most of the present-day altitude of the Precordillera was reached before that time. Afterward, five episodes of geomorphologic evolution can be differentiated: (1) the development of an Oligocene deep incised drainage system cutting the uplifted Precordillera (up to 2000 m of vertical incision) and connecting it to the Ocean; followed by (2) the infilling of deep incised valleys by up to 400 m of Atacama Gravels. This infill started in the Early Miocene with the development of fluvial deposition and finished in the Middle Miocene with playa and playa lake depositions. We propose that playa-related deposition occurs in an endorheic context related to tectonic activity of the Atacama Fault System and Coastal Cordillera uplift. However, the upward sedimentologic variation in the Atacama Gravels evidences a progressive aridification of the climate. Subsequently, we have identified the effects of the Middle–Upper Miocene slow tectonic deformation: the Neogene Andean uplift is accommodated by a tilting or flexuring of the inner-forearc (Central Depression and Precordillera) related to some hundreds of meters of uplift in the Precordillera. This tilting or flexuring results in (3) the Middle Miocene re-opening of the valley network to the Pacific Ocean. Upper Miocene aridification, from arid to hyperarid, induces alluvial fans backfilling in the Central Depression (4) resulting in up to 50 m of Atacama Gravel deposition. Finally, in response to an increase in the rate of tilting, a new phase of vertical incision (up to 800 m in the Precordillera) allows the development of the canyon that crosses the forearc (5). 相似文献
104.
A climatic-limnologic model of diatom succession for paleolimnological interpretation of varved sediments at Elk Lake,Minnesota 总被引:1,自引:0,他引:1
J. Platt Bradbury 《Journal of Paleolimnology》1988,1(2):115-131
Sediment traps placed in the profundal region of Elk Lake, north central Minnesota during the 1979 spring and 1983–84 fall and spring seasons monitored seasonal diatom production for two climatically distinctive periods. The spring of 1979 was one of the coldest and wettest on record. Ice out at Elk Lake was 10 days later than average, and spring circulation was short. Fragilaria crotonensis dominated the late spring and early summer diatom production in association with Synedra and Cyclotella species, perhaps because rates of phosphorus supply were low compared to silica. The winter of 1983–84 was drier than usual, and the early but cold spring of 1984 caused ice out at Elk Lake to be about 1 week earlier than normal. Spring storms promoted a long and full circulation that allowed Stephanodiscus minulutus to bloom, presumably in response to increased phosphorus loading related to deep and vigorous circulation. The two dominant diatoms in Elk Lake, F. crotonensis and S. minutulus may reflect climatic patterns that control lake circulation. The climatically regulated occurrence of these diatoms is generally, but not specifically, comparable to their distribution in lake surface sediments throughout Minnesota. A combination of regional and lake-specific studies on the controls of diatom succession and distribution provides climatic insights for interpreting paleolimnogical records of diatoms. 相似文献
105.
106.
Although highly branched from the base, all shrubs have short main axes linking canopies to root systems. Main axes become increasingly segmented into independent canopy/stem/root segments as aridity increases across continents. The resulting hydraulic modularity has been proposed as an adaption to low soil moisture that prevents runaway embolism and minimizes risk of hydraulic failure. Here we test the hypotheses that (1) at a regional scale, the importance of axis-splitting species in communities declines with increasing elevation, as a proxy for precipitation, and (2) that this decline is explained by lower occurrence of low-elevation dominant species. We evaluated all species for axis splitting and determined importance values in plots along an elevational transect in the Mojave Desert. As predicted, as elevation increased, the total importance of axis-splitting species declined from 100% at low-elevation sites to 75% at the highest elevation site. However, this decline was not due solely to the decline of the lower elevation dominant species. At the high elevation site, the influx of new species resulted in a six-fold increase in species richness and almost all of the new high elevation woody eudicotyledonous species exhibited axis splitting; non-splitting species were represented by other growth forms. 相似文献
107.
库布齐沙漠典型地区沙漠化动态分析 总被引:5,自引:0,他引:5
以库布齐沙漠东部达拉特旗的农牧交错区为研究区域,以达拉特旗1987年、1995年、2000年和2003年四期遥感影像为信息源,在3S技术支持下,对遥感影像进行解译,获得1987—2003年来沙漠化动态变化数据,分析了该地区20世纪80年代中期到21世纪初的沙漠化变化情况,从土地流向角度探讨了沙漠化发展演变的动力机制。结果表明:该地区近20 a来,四个时期的沙漠化土地面积处于减少—增加—再增加的过程中,而沙漠化程度呈现减少—减少—增加的趋势,且2000—2003年间,是该地区沙漠化程度动态变化最大的时期。从土地流向看,各种土地类型向沙漠化土地之间的转化是导致沙漠化的一个重要原因之一,而土地利用类型之间的转化主要是由人类不合理的土地利用方式造成的。 相似文献
108.
109.
Gehad M. Saleh 《中国地球化学学报》2006,25(4):307-317
1 Introduction The association of massive Fe-Ni-Cu sulfides andchromite is a very unusual feature of podiformchromitites occurring in mantle tectonites of ophioliticcomplexes. It has only been described in theSoutheastern Desert, Egypt, where sulfides a… 相似文献
110.
A study on two closed salt lake basins, Tal Chapar and Parihara in the eastern margin of the Thar Desert, Rajasthan, was carried
out to unravel late Quaternary geomorphic evolution of these saline lakes. Both lakes are elliptical in shape bordered by
stabilised dunes, and are oriented in a NE-SW direction, i.e., in the direction of the prevailing summer monsoon wind. Both
lakes have been formed in the wind-shadow zones of isolated hills of Precambrian quartzite. Our study indicates that the late
Quaternary sediments in the lakes began with the cyclic deposition of laminated fine silt layers (0.5 m thick), rich in organic
matter, alternating with ripple cross-bedded sand layers (each ∼1.5–2 m thick). Sand layers that are moderately sorted are
separated by laminated silt-clay layers with gypsum/calcite and this unit occurs in the upper most 4 m sequence in deeper
sections. The presence of gypsum crystals within the laminated sediments suggests a high concentration of Ca in the inflowing
water.
At Parihara Lake the organic carbon-rich sediments at 95 cm depth was dated to 7,375 + 155/−150 year BP. At Tal Chapar radiocarbon
dates of 7,190 + 155/−150 and 9,903 + 360/−350 was obtained from the sediments rich in organic carbon occurring at a depth
of 1.35 m and 1.80 m, respectively. The study reveals strong hydrologic oscillations during the past ∼14,000 year BP (13,090 + 310/−300 year
BP).
Quaternary geomorphic processes, especially the strong aeolian processes during dry climatic phases, played a major role in
the formation of the lake basins, as well as the fringing linear dunes. Geochemical and mineralogical analyses of the lacustrine
sediments, supported by radiocarbon dates indicate the existence of an ephemeral lake earlier than ∼13,000 year BP as sediments
began to be deposited in a lacustrine environment implying sustained runoff in the catchments. A freshwater lake formed between
9,000 year and 7,000 year BP. The lake dried periodically and this strong fluctuating regime continued until about ∼7,000 year
BP. Mid-Holocene was wet and this was possibly due to higher winter rains A saline lake existed between 6,000 year and 1,300 year
BP and finally present day semi arid conditions set in since 1,200 year BP. Remnants of a habitation site (hearth and charred
bones) on stabilised dune at Devani near Tal Chapar were dated to 240 ± 120 year, while that at Gopalpura was dated to 335 ± 90 year.
These historical sites on stabilised dunes were, according to the local accounts, settlements of people who used the lake
brine for manufacturing salt. 相似文献