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1.
We examined spatial distribution characteristics of extreme hydrological events in Xinjiang, China, using district data from 1901 to 2010. Frequency distribution showed a general symmetry along the Tianshan Mountains, with even distribution in Junggar Basin and Tarim Basin. Frequency was more in the north-west than in the south-east. The maximum incidence was in west Tianshan Mountains and generally decreased south-eastward. There were significant regional variations in type distribution. Rainstorm floods were more common in central Xinjiang. Hailstorms mainly occurred in the central Junggar Basin, the southern slope of the western Tianshan Mountains and north-west of Tarim Basin. Debris flow was mainly distributed in Ili Valley and the central northern Tianshan Mountains. Glacier lake outburst floods were more common in the Karakorum Mountains and southern slopes of the western Tianshan Mountains. Ice floods were mainly distributed in the western Tianshan Mountains. Snow hazards were mainly distributed in the wide northern areas, especially the Altai Mountains and Hamilton Basin. Snowmelt floods were mainly distributed in the Tacheng Basin and Ili Valley. The incidence of extreme hydrological events was greatly affected by weather systems and terrain features. 相似文献
2.
Christel van den Bogaard Walter Drfler Rainer Glos Marie-Jose Nadeau Pieter M. Grootes Helmut Erlenkeuser 《Quaternary Research》2002,57(3):314
Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry 14C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog. 相似文献
3.
Magnetic susceptibility (MS) of surface sediment varies systematically across the Loess Plateau in central China, decreasing exponentially from >200×10−8 m3/kg at the northern margin of the Qinling Shan to ≤30×10−8 m3/kg near the southern margin of the Mu Us Desert. MS correlates highly with loess median grain size (r2=0.79), which decreases south-southeastward across the plateau. It also correlates with mean annual temperature (MAT) and mean annual precipitation (MAP) (r2=0.58 and 0.60, respectively), and with their product MAT×MAP (r2=0.83), which is considered a measure of potential pedogenic activity. Because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel, it is difficult to determine their relative influence on MS. A simple MS model, based on the observed spatial variation in loess thickness, permits quantitative assessment of the effect of the dust accumulation rate on the MS signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals. The model suggests that 84% of the loess MS variance is dictated by the diluting effect of dust and 10–11% is associated with meteorological factors, primarily precipitation. The observed and modeled relationships support hypotheses that attribute variations in MS in the loess-paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils, both of which are controlled by monsoon climate. 相似文献
4.
Bryan G. Mark Geoffrey O. Seltzer Donald T. Rodbell Adam Y. Goodman 《Quaternary Research》2002,57(3):287
Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10−5 to 114×10−5 km3 yr−1 and 112×10−5 to 247×10−5 km3 yr−1 for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10−5 to 220×10−5 km3 yr−1, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent. 相似文献
5.
Oxygen isotope stage 3 (OIS 3) climate and its variations are the focus of the Stage 3 Project. The objective of the OIS 3 modeling effort is twofold: (1) to explore the importance of different boundary conditions on the climate of Europe and (2) to develop climate simulations that best reproduce the wealth of OIS 3 observations. Given the complexity of the topography and coastlines, the modeling effort is based on a “nested” General Circulation Model (GCM) and mesoscale model (RegCM2) with climate simulations for Europe on a 60-km grid spacing. The key conclusions are as follows: (1) The mesoscale model, driven by GCM output, does a reasonable job of reproducing the modern European climate. (2) OIS 3 variations in orbit, CO2, and ice-sheet size are of little significance in explaining the observed climate variability. (3) The model results focus attention on North Atlantic sea-surface temperatures (SST) as a major factor in explaining OIS 3 climates. (4) Experiments for different SST values capture a number of systematic changes in sea-level pressure and precipitation. (5) Climate models simulate substantial European cooling and significant changes in precipitation, but they do not explain large differences between OIS 3 warm and cold episodes. 相似文献
6.
乌鲁木齐河流域的黄土研究 总被引：8，自引：1，他引：8
本文较详细地论述了乌鲁木齐河流域的黄土,对部分黄土样品进行了粒度分析和矿物鉴定,利用14C和热释光方法做了年龄测定,指出这个地区的黄土大部分是在晚更新世中晚期干旱、寒冷的气候条件下形成的,与马兰黄土相当,有一些后来受流水改造成为次生黄土。分布在河源末次冰期冰碛上的黄土主要形成于全新世,属于天山“托木尔型”黄土。 相似文献
7.
R. Scott Anderson Mitchell J. Power Susan J. Smith Kathleen Springer Eric Scott 《Quaternary Research》2002,58(3):310
Analysis of a buried deposit in the Diamond Valley of southern California has revealed well-preserved pollen, wood, and diatom remains. Accelerator mass spectrometry dates of 41,200±2100 and 41,490±1380 14C yr B.P. place this deposit in marine isotope stage 3. Diatoms suggest a shallow lacustrine environment. Pollen data suggest that several plant communities were present near the site, with grassland, scrub, chaparral, forest, and riparian communities represented. Comparison with modern pollen suggests similarities with montane forests in the nearby San Bernardino and San Jacinto ranges, indicating vegetation lowering by at least 900 m elevation and temperatures 4°–5°C cooler than today. An increase in high-elevation conifer pollen documents climatic cooling near the profile top. Early-profile diatoms are typical of warm water with high alkalinity and conductivity, whereas later diatoms suggest a higher flow regime and input of cooler water into the system. We suggest that the sequence is part of the cooling phase of an interstadial Dansgaard–Oeschger cycle. Records of the middle Wisconsin period are rare in southern California, but the Diamond Valley site is similar to records from Tulare Lake in the San Joaquin Valley and the ODP Site 893A record from Santa Barbara Basin. It is probable that the Diamond Valley assemblage is a local expression of a vegetation type widespread in the ranges and basins of southwestern California during the middle Wisconsin. 相似文献
8.
Denis-Didier Rousseau Natalia Gerasimenko Zhanna Matviischina George Kukla 《Quaternary Research》2001,56(3):349
The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess–steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe. 相似文献
9.
10.
Glenn D. Thackray 《Quaternary Research》2001,55(3):257
Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,000 14C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,000 14C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,300 14C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,300 14C yr B.P.; the Twin Creeks 1 advance, 19,100–18,300 14C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events. 相似文献
11.
Stephan Steinke Markus Kienast Uwe Pflaumann Mara Weinelt Karl Stattegger 《Quaternary Research》2001,55(3):352
The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, 14C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (Uk′37) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the Uk′37 SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the Uk′37 and the FP-12E SST estimates, as well as the planktonic foraminiferal δ18O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the Uk′37 SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These Uk′37 SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period. 相似文献
12.
Donald T. Rodbell Stefan Bagnato Jeffrey C. Nebolini Geoffrey O. Seltzer Mark B. Abbott 《Quaternary Research》2002,57(3):343
Despite the presence of numerous active volcanoes in the northern half of Ecuador, few, if any, distal tephras have been previously recognized in the southern one third of the country. In this article, we document the presence of thin (0.1–1.0-cm-thick) distal tephras comprising glass and/or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in Las Cajas National Park (2°40′–3°00′S, 79°00′–79°25′W). The lake cores contain from 5 to 7 tephras, and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of 710 glass shards and 440 phenocrysts of feldspar and hornblende. The loss of sodium with exposure to the electron microbeam causes a 10±7 wt.% (±1σ) reduction in Na content, which we empirically determined and corrected for before correlating tephras among the sediment cores. We use a similarity coefficient to correlate among the sediment cores; pair-wise comparison of all tephras generally yields an unambiguous correlation among the cores. Six tephras can be traced among all or most of the cores, and several tephras are present in only one or two of the cores. Twenty-six accelerator mass spectrometry 14C dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology. The widespread tephras were deposited 9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P. The oldest tephras were deposited 15,500 and 15,100 cal yr B.P., but these are not found in all cores. Two of the tephras appear correlative with volcaniclastic strata on the flanks of Volcán Cotopaxi and one tephra may correlate with strata on the flanks of Volcán Ninahuilca; both volcanoes are in central Ecuador. The absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions, which did deposit tephras in the lakes, must have been either especially voluminous, or southerly winds must have prevailed at the time of the eruption, or both. 相似文献
13.
中国风成黄土及其形成下限研究 总被引：8，自引：1，他引：8
“中国岩漠·砾漠·沙漠·黄土分布与主风向关系图”显示西北地区的第四纪黄土堆积是以风力为主的综合因素所形成，即就地起沙，近沙成土，一定地域的黄土在物源上隶属于一定地区的沙漠。黄土高原及新疆天山、昆仑山北麓的黄土地层结构的综合对比研究表明，典型风成黄土在不同地域和不同地貌单元上最早开始堆积的层位基本上是一致的，即均始于所有黄土剖面中的下粉砂层（Ｌ＿（１５））。该层以下的第四纪沉积类型因地而异，为非黄土沉积（湖相、冲洪积相及火山岩相等）。由黄土高原各代表性剖面的磁性地层学研究及新疆于田黄土下伏的安山玄武岩的同位素年龄测定获得黄土层开始堆积的时代为早更新世晚期，约１．２ＭａＢ．Ｐ。风成黄土的开始堆积是与黄土南缘各造山带（昆仑山、天山、祁连山及秦岭等）强烈的新构造隆升并产生区域环境效应有成因联系。这一更新世早期的事件层位得到了古生物学、新构造学、地貌学及地外物体高速撞击地球表面所产生的玻璃陨石等事件的佐证。 相似文献
14.
Shmuel Belitzky 《Quaternary Research》2002,58(3):372
Geological, geomorphological, and morphotectonic studies at the Gesher Benot Ya'aqov Acheulean site, Northern Jordan Valley, provide: 1) a unique opportunity to establish the ecological background of hominid behavior during the early stages of widespread human occupation; 2) crucial data for understanding the paleo-environment and the various processes that affected the area near the Gesher Benot Ya'aqov site; and 3) a basis for further comprehensive research in the region.Quaternary tectonic activity near the Acheulean archeological site has produced a complex local setting that controls the surrounding landscape. Tectonic activity formed the Benot Ya'aqov embayment, which attracted hominids and a variety of other fauna. Sediments of the Benot Ya'akov formation deposited in the embayment facilitated excellent preservation of the remnants of Acheulean hominid activities and of abundant floral and faunal remains. The formation was subsequently affected by faulting and folding. Local uplift ended the deposition of lacustrine sediments in the embayment of Hula Valley Basin and caused erosion and vertical incision of the Jordan River that resulted in the capture of the Rosh Pinna River and the exposure of the Benot Ya'akov formation. These deposits are found in different structural positions along the north–south oriented morphotectonic embayment. The outcrops contain numerous archeological sites, giving the study area an unusually high potential for future hominid discoveries. 相似文献
15.
The muskrat (Ondatra zibethicus) is presumed to have undergone a rapid phyletic size decrease near the end of the Pleistocene. Evolutionary changes in the size of middle to late Wisconsinan (ca. 32,000–12,300 14C yr B.P.) muskrats from the Aucilla River, Jefferson County, Florida, were reconstructed by examining length and width of the lower first molar (m1). Body mass, estimated from m1 length, was relatively stable from 32,000 to 16,000 14C yr B.P. and decreased only slightly by 12,300 14C yr B.P. If the size trend found in the Aucilla River material is characteristic of the southeastern United States, a body size decrease after 12,300 14C yr B.P. is needed to explain the smaller size of modern populations. It was previously thought that the length/width (l/w) ratio of the muskrat m1 was a paleoenvironmental indicator based on its presumed correlation with latitude in modern populations. We examined the length and width of modern muskrats from several geographic regions and found only a very weak trend in the size of the m1 between northern and southern populations; however, highly significant differences were found between regions of similar latitude. Our data indicate that chronoclines in the m1 of the Aucilla muskrat material and other such documented trends among fossil muskrats have paleoenvironmental significance, but it is not yet clear which environmental variables can best be predicted from them. 相似文献
16.
The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandlines cut by still-stands of pluvial lakes correlative with oxygen isotope stage (OIS) 5e/6 (120,000–186,000 yr B.P.) and OIS-2 (10,000–35,000 yr B.P.). This study presents geologic mapping and geomorphic analyses (Gilbert's criteria, longitudinal profiles), which indicate that only the highest bench at Mormon Point (90 m above mean sea level (msl)) is a lake strandline. The other prominent benches on the north-descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform. The faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the Mormon Point turtleback fault, implying late Quaternary slip on this low-angle normal fault. Our geomorphic reinterpretation implies that the OIS-5e/6 lake receded rapidly enough not to cut strandlines and was 90 m deep. Consistent with independent core studies of the salt pan, no evidence of OIS-2 lake strandlines was found at Mormon Point, which indicates that the maximum elevation of the OIS-2 lake surface was −30 m msl. Thus, as measured by pluvial lake depth, the OIS-2 effective precipitation was significantly less than during OIS-5e/6, a finding that is more consistent with other studies in the region. The changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at Mormon Point are uninterpretable with respect to lake history. 相似文献
17.
Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 10Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 10Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada. 相似文献
18.
Sheryl K. Filby Sharon M. Locke Mark A. Person Thomas C. Winter Donald O. Rosenberry John L. Nieber William J. Gutowski Emi Ito 《Quaternary Research》2002,58(3):246
A hydrologic model of the Shingobee Watershed in north-central Minnesota was developed to reconstruct mid-Holocene paleo-lake levels for Williams Lake, a surface-water body located in the southern portion of the watershed. Hydrologic parameters for the model were first estimated in a calibration exercise using a 9-yr historical record (1990–1998) of climatic and hydrologic stresses. The model reproduced observed temporal and spatial trends in surface/groundwater levels across the watershed. Mid-Holocene aquifer and lake levels were then reconstructed using two paleoclimatic data sets: CCM1 atmospheric general circulation model output and pollen-transfer functions using sediment core data from Williams Lake.Calculated paleo-lake levels based on pollen-derived paleoclimatic reconstructions indicated a 3.5-m drop in simulated lake levels and were in good agreement with the position of mid-Holocene beach sands observed in a Williams Lake sediment core transect. However, calculated paleolake levels based on CCM1 climate forcing produced only a 0.05-m drop in lake levels. We found that decreases in winter precipitation rather than temperature increases had the largest effect on simulated mid-Holocene lake levels. The study illustrates how watershed models can be used to critically evaluate paleoclimatic reconstructions by integrating geologic, climatic, limnologic, and hydrogeologic data sets. 相似文献
19.
新疆伊犁黄土磁化率增强机制差异性分析 总被引：2，自引：0，他引：2
本文对处于西风区的新疆伊犁昭苏黄土剖面进行了磁化率和粒度分析,初步探讨了伊犁黄土磁化率增强机制的差异性。研究表明,伊犁黄土虽然粘土含量较黄土高原黄土高,但细粒组分对磁化率的贡献非常有限,粗颗粒组分对磁化率贡献较大,说明磁性载体主要存于粗颗粒组分中。同时研究发现地形、沉积环境、源区矿物组分和气候等多种因素对磁化率产生不同程度的影响。伊犁黄土磁化率成因机制既具有阿拉斯加风速论模式的特点又叠加了黄土高原成壤作用的模式,即使在同一剖面的不同时段两种模式对磁化率增强的贡献也不同,伊犁黄土磁化率增强机制存在着时空差异性。 相似文献
20.
The 36Cl dating method is increasingly being used to determine the surface-exposure history of Quaternary landforms. Production rates for the 36Cl isotopic system, a critical component of the dating method, have now been refined using the well-constrained radiocarbon-based deglaciation history of Whidbey and Fidalgo Islands, Washington. The calculated total production rates due to calcium and potassium are 91±5 atoms 36Cl (g Ca)−1 yr−1 and are 228±18 atoms 36Cl (g K)−1 yr−1, respectively. The calculated ground-level secondary neutron production rate in air, Pf(0), inferred from thermal neutron absorption by 35Cl is 762±28 neutrons (g air)−1 yr−1 for samples with low water content (1–2 wt.%). Neutron absorption by serpentinized harzburgite samples of the same exposure age, having higher water content (8–12 wt.%), is 40% greater relative to that for dry samples. These data suggest that existing models do not adequately describe thermalization and capture of neutrons for hydrous rock samples. Calculated 36Cl ages of samples collected from the surfaces of a well-dated dacite flow (10,600–12,800 cal yr B.P.) and three disparate deglaciated localities are consistent with close limiting calibrated 14C ages, thereby supporting the validity of our 36Cl production rates integrated over the last 15,500 cal yr between latitudes of 46.5° and 51°N. Although our production rates are internally consistent and yield reasonable exposure ages for other localities, there nevertheless are significant differences between these production rates and those of other investigators. 相似文献