首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We use statistical correlation of palaeomagnetic secular variation (PSV) curves from a varved Holocene lake sediment sequence in west central Sweden (Lake Kälksjön) against those of a Fennoscandian master stack (FENNOSTACK) to correct for an apparent error in the varve chronology. Additional correlation between a lead pollution-derived chronology for the last 2000 years corroborates the PSV results. Use of the FENNOSTACK palaeomagnetic master curve reveals no significant difference in duration between large-scale features from ～2500 to ～8000 cal. yrs BP. Statistical correlation, however, implies that 270 years are missing from the younger part (<1000 cal. yrs BP) of the varve chronology, and that there is an overestimation by approximately 230 years in the number of varves counted in the early Holocene (>8000 cal. yrs BP). A similar comparison between the PSV-determined ages and calibrated bulk radiocarbon ages suggests that the sediments of mid-Holocene age contain substantial amounts of old carbon, probably of soil origin, which causes bulk sediment-calibrated mean 14C ages to be up to 850 years older than the corrected varve chronology, which extends to 9193 ± 186 cal. yrs BP. This study highlights both the use of statistical correlation as a technique for detecting errors between chronologies, and the importance of validating incremental chronologies with more than one independent method.  相似文献   

2.
Qinghai Lake is situated in the northeast of the Qinghai-Tibetan Plateau (QTP). Its size and proximity to the junction of three major climate systems make it sensitive to climate changes. Some investigations on shorelines of Qinghai Lake suggested highstands during MIS 3, but to what extent the lake level was higher than today is yet undetermined. Others proposed that the maximum highstands probably dated to MIS 5. It has also been shown that the lake level 120 m higher than today occurred at around 12 ka. Most of these previous ages were obtained using 14C dating or multiple-aliquot IRSL/OSL dating. For 14C dating, because of the dating limit (<40 ka) and the lack of suitable dating materials in this arid area, it is difficult to establish reliable chronological control. In the present study, seven samples collected from lacustrine deposits (five samples) and sand wedges (two samples) were dated using quartz optically stimulated luminescence (OSL) with the single aliquot regenerative-dose (SAR) protocol. OSL dating results showed that (1) the lake had experienced two high lake levels, one was in MIS 5 and another in early to middle MIS 3; (2) no evidence of high lake levels in MIS 4 has been found; (3) the alluvial gravels, whose surface is at an elevation of ～3246 m, were formed at least 28.8 ± 2.3 ka ago, and the widespread sand wedges within the alluvial gravels were formed during the period of 15.1–28.8 ka, which implied that the lake level had not reached an elevation of ～3240 m after 28.8 ± 2.3 ka.  相似文献   

3.
The Qujialing site is a representative Neolithic archaeological site in the middle reaches of the Yangtze River, China. Absence of suitable material for radiocarbon dating in this region makes the timing of the similar sites difficult. Here we applied optically stimulated luminescence (OSL–SAR) and thermoluminescence (TL–SAR) techniques to date the archaeological and natural deposits from the Qujialing site with known age, testing the techniques on samples at archaeological sites in this region. The results showed that the luminescence properties of quartz from sediment and baked earth samples are very similar. The quartz OSL ages obtained for a sediment sample and a baked earth sample from the cultural layer are 5.4 ± 0.3 and 5.1 ± 0.3 ka, respectively. The quartz TL age of the baked earth sample is 5.6 ± 0.5 ka. These dates are consistent with the calibrated radiocarbon ages (4.9 ± 0.1 and 5.1 ± 0.1 ka cal BP (±1σ)) of the two charcoal samples from the cultural layer at a nearby locality, and are also in agreement with the age of Qujialing culture period. The results indicate that the OSL dating techniques can be applied to date similar archaeological sites in the middle reaches of the Yangtze River, China.  相似文献   

4.
The study analyses the long-term biophysical and demographic changes in Dal lake, located in the heart of Srinagar city, Kashmir India, using a repository of historical, remote sensing, socio-economic and water quality data supported by the extensive field observations. The lake faces multiple pressures from the unplanned urbanization, high population growth, nutrient load from intensive agriculture and tourism. The data showed that the lake has shrunk from 31 km2 in 1859–24 km2 in 2013. Significant changes were observed in the land use and land cover (LULC) within the lake (1859–2013) and in the vicinity of the lake (1962–2013). Analysis of the demographic data indicates that the human population within the lake has shown more than double the national growth rate. Additionally, 7 important water quality parameters from 82 well distributed sites across the lake were analyzed and compared with the past data to determine the historical changes in the water quality from 1971 to 2014. The changes in the LULC and demography have adversely affected the pollution status of this pristine lake. Ortho-phosphate phosphorous concentration has increased from 16.75 μg L−1 in 1977–45.78 μg L−1 in 2014 and that of the nitrate-nitrogen from 365 μg L−1 to 557 μg L−1, indicating nutrient enrichment of the lake over the years. Built-up area within the lake has increased 40 times since 1859, which, together with the changes in the population and settlements, have led to the high discharge of untreated nutrient-rich sewage into the lake. Similarly the expansion of floating gardens within the lake and agriculture lands in the catchment has contributed to the increased nutrient load into the lake due to the increasing use of fertilizers. The information about the existing land cover, demography and water quality was integrated and analyzed in GIS environment to identify the trophic status of the lake. The analysis indicated that 32% of the lake falls under sever degradation, 48% under medium degradation while as 20% of the lake waters are relatively clean. It is believed that the results provide improved knowledge and insights about the lake health and causal factors of its degradation necessary for effectively restoring its ecological and hydrological functionality.  相似文献   

5.
Tuff layers are vital stratigraphic tools that allow correlations to be made between widely dispersed exposures. Despite their widespread occurrence in the central Andes, tuffs from both natural exposures and sedimentary cores extracted from the region's extensive salars (salt pans) are relatively unstudied. Here we lay the foundation for a tephrostratigraphic framework in the central Andes (14–28°S) by chemically and morphologically characterizing ash shards, and in some cases dating 36 Neogene distal tuffs. These tuffs occur in lacustrine and alluvial deposits from the southern Bolivian Altiplano and adjacent Atacama Desert. All tuffs are calc-akaline rhyolites, consistent with their setting in the Central Andean Volcanic Zone. Five of the older tuffs were 40Ar/39Ar dated and yield an age range of 6.63–0.75 Ma. Organic material associated with tuffs deposited into paleolake sediments, paleowetland deposits, or urine-encrusted rodent middens provide constraints on the age of several Late Pleistocene and Holocene tuffs.These tuffs provide key stratigraphic markers and ages for lake cycles and archeological sites on the Bolivian Altiplano and for assessing rates of surficial processes and archeology in both the Atacama and Altiplano. While modern climate, and consequently questions about geomorphic processes and climate change, differs in the hyperarid Atacama and the semi-arid Altiplano, the most extensive air-fall tuffs covered both regions, placing the Atacama and the Bolivian Altiplano in the same tephrostratigraphic province. For example, the Escara B tuff (～1.85 Ma), can be securely identified in both the Altiplano and Atacama. On the Altiplano, dates from the Escara B and E tuffs securely establish the age of the Escara Formation—representing the oldest expansive lake documented on the Bolivian Altiplano. By contrast, the presence of the Escara B tuff below ～6 m of alluvial sediment at the Blanco Encalado site in the Atacama desert yields information about sedimentation rates in this hyperarid region. Indeed, most tuffs from the Atacama Desert are older than 600,000 years, even though they occur within fluvial terraces immediately adjacent to the alluvial fans that are still active. Most of these geomorphic surfaces in the Atacama also possess well-developed saline soils that, when combined with the radiometric ages of the distal tuffs, suggest slow rates of geomorphic change and exceptional landscape stability for this area during the Quaternary.In contrast, younger tuffs are more abundant in the more recent lake records of the Altiplano. The Chita tuff was deposited at ～15,650 cal yr B.P., during the regressive phase of the region's deepest late Quaternary lake cycle—the “Tauca lake cycle”—which spanned 18.1–14.1 cal yr B.P. Two Holocene tuffs, the Sajsi tuff and the Cruzani Cocha tuff, are widespread. The Sajsi tuff was deposited just before 1700 cal yr B.P., whereas the Cruzani Cocha tuff appears to be mid-Holocene in age and shows some chemical affinities to a Holocene tuff (202B) deposited between 4420 and 5460 cal yr B.P. in a urine-encased rodent midden in the Atacama Desert.  相似文献   

6.
The eruptions of Mt Ruapehu in the North Island of New Zealand in 1995 and 1996 caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys were carried out on a number of lines along and across it in order to test whether the extent of the seepage could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent resistivity data showed that between the initial measurements, made in January 2005, and February 2006, there was a gradual decrease in resistivity above the old outlet from ~ 50–60 Ωm to ~ 30 Ωm. This gave the first indication that lake water was seeping into the barrier. Between October and December 2006 there was a rapid rise in lake level to only 2 m below the top of the barrier, and a further resistivity survey in January 2007 showed that there had been a further decrease in resistivity throughout the entire barrier with values dropping to < 10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this stage lake water was penetrating the barrier and starting to cause erosion on its downstream side. Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the observed changes in measured resistivity were directly related to the progress of seepage of lake water into the barrier.  相似文献   

7.
The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the 14C calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event.  相似文献   

8.
Understanding climate change is an active topic of research. Much of the observed increase in global surface temperature over the past 150 years occurred prior to the 1940s and after the 1980s. The main causes invoked are solar variability, changes in atmospheric greenhouse gas content or sulfur due to natural or anthropogenic action, or internal variability of the coupled ocean–atmosphere system. Magnetism has seldom been invoked, and evidence for connections between climate and magnetic field variations have received little attention. We review evidence for correlations which could suggest such (causal or non-causal) connections at various time scales (recent secular variation ∼ 10–100 yr, historical and archeomagnetic change ∼ 100–5000 yr, and excursions and reversals ∼ 103–106 yr), and attempt to suggest mechanisms. Evidence for correlations, which invoke Milankovic forcing in the core, either directly or through changes in ice distribution and moments of inertia of the Earth, is still tenuous. Correlation between decadal changes in amplitude of geomagnetic variations of external origin, solar irradiance and global temperature is stronger. It suggests that solar irradiance could have been a major forcing function of climate until the mid-1980s, when “anomalous” warming becomes apparent. The most intriguing feature may be the recently proposed archeomagnetic jerks, i.e. fairly abrupt (∼ 100 yr long) geomagnetic field variations found at irregular intervals over the past few millennia, using the archeological record from Europe to the Middle East. These seem to correlate with significant climatic events in the eastern North Atlantic region. A proposed mechanism involves variations in the geometry of the geomagnetic field (f.i. tilt of the dipole to lower latitudes), resulting in enhanced cosmic-ray induced nucleation of clouds. No forcing factor, be it changes in CO2 concentration in the atmosphere or changes in cosmic ray flux modulated by solar activity and geomagnetism, or possibly other factors, can at present be neglected or shown to be the overwhelming single driver of climate change in past centuries. Intensive data acquisition is required to further probe indications that the Earth's and Sun's magnetic fields may have significant bearing on climate change at certain time scales.  相似文献   

9.
《Journal of Geodynamics》2007,43(1):170-186
Stratigraphic and sedimentological studies indicate that Iceland has experienced over 20 glaciations during the last 4–5 Myr, in reasonable agreement with the number of glaciations reconstructed from the ∂18O record in deep-sea sediment. The pattern of glacial erosion was to a large part controlled by constructive volcanic processes resulting in increased topographic relief after 2.5 Myr. Between 2.5 and 0.5 Ma valleys up to 400 m deep were excavated into the Tertiary basalts of eastern and south Iceland with an average erosion rate of 10–20 cm ka−1. During the last 0.5 million years rates of erosion increased to 50–175 cm ka−1, with an additional 200 to over 1000 m of valley excavation. Previous estimates of the rate of landscape erosion during the Holocene vary widely, from 5 to 70,000 cm ka−1. We present new studies that define the rates of landscape denudation during the major part of the Holocene (the last 10,200 years): one based on the Iceland shelf sediment record, the other from the sediment record in the glacier-fed lake, Hvítárvatn. Both studies indicate average Holocene erosion rates of about 5 cm ka−1 similar to our erosion rate estimate for 4–5 Ma old strata that has not been subjected to regional glaciation.  相似文献   

10.
A 5.6-m-long lake sediment core from Bear Lake, Alaska, located 22 km southeast of Redoubt Volcano, contains 67 tephra layers deposited over the last 8750 cal yr, comprising 15% of the total thickness of recovered sediment. Using 12 AMS 14C ages, along with the 137Cs and 210Pb activities of recent sediment, we evaluated different models to determine the age–depth relation of the core, and to determine the age of each tephra deposit. The selected age model is based on a mixed-effect regression that was passed through the adjusted tephra-free depth of each dated layer. The estimated age uncertainty of the 67 tephras averages ±105 yr (95% confidence intervals). Tephra-fall frequency at Bear Lake was among the highest during the past 500 yr, with eight tephras deposited compared to an average of 3.7/500 yr over the last 8500 yr. Other periods of increased tephra fall occurred 2500–3500, 4500–5000, and 7000–7500 cal yr. Our record suggests that Bear Lake experienced extended periods (1000–2000 yr) of increased tephra fall separated by shorter periods (500–1000 yr) of apparent quiescence. The Bear Lake sediment core affords the most comprehensive tephrochronology from the base of the Redoubt Volcano to date, with an average tephra-fall frequency of one every 130 yr.  相似文献   

11.
《Continental Shelf Research》2006,26(17-18):2050-2072
A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr−1 over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s−1. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l−1. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods.  相似文献   

12.
《Marine pollution bulletin》2014,85(1-2):373-378
The surficial coastal sediments in Kendari Bay are sampled in the field to determine the concentration and pollution level of three heavy metals (Pb, Cd and Cr). Twenty-five sampling points ranging from the inner (Wanggu River) to the outer area of the bay have been chosen. The physicochemical properties, such as temperature, pH, salinity and TDS of the overlying water, as well as the sediment type and TOC of the surficial sediments, are also measured. The total concentrations of the Pb, Cd and Cr in the sediment samples are quantified using inductively-coupled plasma mass spectrometry (ICP-MS). The concentrations of the heavy metals (Pb, Cd and Cr) ranged from 0.84 to 17.02 μg/g, 0.02 to 0.17 μg/g and 1.92 to 40.11 μg/g (dry weight), respectively, following the Cr > Pb > Cd sequence. To assess the degree of contamination, a geoaccumulation index (Igeo) is measured. Kendari Bay is not a contaminated area regarding Pb, Cd and Cr.  相似文献   

13.
Contamination with As, Cd and Hg, their spatial and temporal distribution are reported from the coastal wetland sediments of the northern Beibu Gulf, South China Sea. The content of As, Cd, Hg and TOC in surface sediments is 8.1 ± 5.8 μg g?1, 0.08 ± 0.14 μg g?1, 0.034 ± 0.028 μg g?1 and 0.45 ± 0.39%, respectively. The mean sedimentation rates are 0.93–1.37 cm year?1 during 1920s to 2008 determined by 210Pb and 137Cs dating in three cores. The vertical profiles of As, Cd and Hg content in the cores retrieved from Qin and Nanliu River estuaries show increasing trends during 1985–2008 due to anthropogenic impact caused by local economic development. Locally the surface sediments have potential ecological risk of As to benthos according to the NOAA sediment quality guidelines.  相似文献   

14.
The Late Jurassic Kimmeridge Clay Formation (KCF) is an economically important, organic-rich source rock of Kimmeridgian–Early Tithonian age. The main rock types of the KCF in Dorset, UK, include grey to black laminated shale, marl, coccolithic limestone, and dolostone, which occur with an obvious cyclicity at astronomical timescales. In this study, we examine two high-resolution borehole records (Swanworth Quarry 1 and Metherhills 1) obtained as part of a Rapid Global Geological Events (RGGE) sediment drilling project. Datasets examined were total organic carbon (TOC), and borehole wall microconductivity by Formation Microscanner (FMS). Our intent is to assess the rhythmicity of the KCF with respect to the astronomical timescale, and to discuss the results with respect to other key Late Jurassic geological processes. Power spectra of the untuned data reveal a hierarchy of cycles throughout the KCF with ～ 167 m, ～ 40 m, 9.1 m, 3.8 m and 1.6 m wavelengths. Tuning the ～ 40 m cycles to the 405-kyr eccentricity cycle shows the presence of all the astronomical parameters: eccentricity, obliquity, and precession index. In particular, ～ 100-kyr and 405-kyr eccentricity cycles are strongly expressed in both records. The 405-kyr eccentricity cycle corresponds to relative sea-level changes inferred from sequence stratigraphy. Intervals with elevated TOC are associated with strong obliquity forcing. The 405-kyr-tuned duration of the lower KCF (Kimmeridgian Stage) is 3.47 Myr, and the upper KCF (early part of the Tithonian Stage, elegans to fittoni ammonite zones) is 3.32 Myr. Two other chronologies test the consistency of this age model by tuning ～ 8–10 m cycles to 100-kyr (short eccentricity), and ～ 3–5 m cycles to 36-kyr (Jurassic obliquity). The ‘obliquity-tuned’ chronology resolves an accumulation history for the KCF with a variation that strongly resembles that of Earth's orbital eccentricity predicted for 147.2 Ma to 153.8 Ma. There is evidence for significant non-deposition (up to 1 million years) in the lowermost KCF (baylei–mutabilis zones), which would indicate a Kimmeridgian/Oxfordian boundary age of 154.8 Ma. This absolute calibration allows assignment of precise numerical ages to zonal boundaries, sequence surfaces, and polarity chrons of the lower M-sequence.  相似文献   

15.
《Journal of Hydrology》2006,316(1-4):213-232
The Magdalena River, a major fluvial system draining most of the Colombian Andes, has the highest sediment yield of any medium-sized or large river in South America. We examined sediment yield and its response to control variables in the Magdalena drainage basin based on a multi-year dataset of sediment loads from 32 tributary catchments. Various morphometric, hydrologic, and climatic variables were estimated in order to understand and predict the variation in sediment yield. Sediment yield varies from 128 to 2200 t km−2 yr−1 for catchments ranging from 320 to 59,600 km2. The mean sediment yield for 32 sub-basins within the Magdalena basin is ∼690 t km−2 yr−1. Mean annual runoff is the dominant control and explains 51% of the observed variance in sediment yield. A multiple regression model, including two control variables, runoff and maximum water discharge, explains 58% of the variance. This model is efficient (ME=0.89) and is a valuable tool for predicting total sediment yield from tributary catchments in the Magdalena basin. Multiple correlations for those basins corresponding to the upper Magdalena, middle basin, Eastern Cordillera, and catchment areas greater than 2000 km2, explain 75, 77, 89, and 78% of the variance in sediment yield, respectively. Although more variance is explained when dataset are grouped into categories, the models are less efficient (ME<0.72). Within the spatially distributed models, six catchment variables predict sediment yield, including runoff, precipitation, precipitation peakedness, mean elevation, mean water discharge, and relief. These estimators are related to the relative importance of climate and weathering, hillslope erosion, and fluvial transport processes. Time series analysis indicates that significant increases in sediment load have occurred over 68% of the catchment area, while 31% have experienced a decreasing trend in sediment load and thus yield. Land use analysis and increasing sediment load trends indicate that erosion within the catchment has increased over the last 10–20 years.  相似文献   

16.
This work presents the first synthesis of secular to millenary morphological evolutions and stratigraphy of a wave-dominated estuary, the Arcachon lagoon, from a combination of unpublished bathymetric maps (1865 and 2001), core results and high-resolution seismic profiles recorded for the first time in this lagoon. The Arcachon lagoon is located on the Atlantic coast of France, facing the wave-dominated shelf of the Bay of Biscay. It is a mesotidal semi-enclosed environment of about 160 km2.The sediment budget of the Arcachon lagoon was computed by subtracting the 1865 bathymetric map from that of 2001. The computed volume difference is low (?9.9±35×106 m3 in 136 yrs) and is the result of the balance between erosion and accretion that occurs within tidal channels and tidal flats, respectively. This morphological evolution pattern is explained by low sediment supply and also by the tidal distortion resulting from the morphology of the lagoon. Deep channels connected to the inlet are dominated by ebb currents inducing erosion. Tidal flats and transverse channels display weak or flood-dominated tidal currents leading to the deposition of silts. The areas of tidal flat siltation locally correlate with the presence of oyster farms, suggesting the influence of Man on the lagoon sediment-fill. Transverse channel-infill is related to weak tidal currents resulting from the hydraulically inefficient orientation of these channels which served as an ancient drainage network.Evidence for tidal channel-infill and channel abandonment are also provided by seismic profiling and cores. The upper stratigraphic succession of the lagoon (about 10 m thick) includes four main stratigraphic units dominated by channel-fills. The two lower units (around 7500–2800 yrs BP) display tabular-shape sandy channels interpreted to be records of the open estuarine phase of the Arcachon lagoon. The two upper units (around 2800 yrs BP to present-day) display U-shaped mixed sand-and-mud channel-fills interpreted to be records of the closure of the lagoon. Given that the basal estuarine units are transgressive and the upper lagoonal units are regressive, the main stratigraphic change at around 2800 yrs BP is interpreted as being the maximum flooding surface (MFS). This late MFS is explained by the low sediment supply. It is proposed that the transition from the estuarine to the lagoonal phase is related to the development of the Cap-Ferret spit in response to an increase in the ratio between wave power to tide power. This change in wave-to-tide ratio may be triggered by wave power increase following the Subboreal/Subatlantic climate instability or a decrease in tide power following a decrease in tidal prism related to the lagoon sediment-fill.Thus, the evolution of the Arcachon lagoon over the last millenaries was mainly controlled by its spit development, leading to a wave-dominated estuary in terms of its geomorphology. Once it was partially closed, extensive mud flats developed in the lagoon which became ebb-dominated.  相似文献   

17.
蒋庆丰  钱鹏  周侗  洪佳  范华  刘静峰 《湖泊科学》2016,28(2):444-454
通过对现代乌伦古湖附近出露的古湖相沉积剖面的AMS~(14)C测年,粒度、总有机碳、总有机氮以及碳酸盐等环境代用指标的分析及其与全新世钻孔沉积记录的对比研究,结果发现:乌伦古湖在MIS-3晚期的33600-22500 cal a BP以及冰后期至早中全新世的16500-6500 cal a BP期间,维持着湖相沉积环境,湖面约比现在湖面高40 m.33600-22500 cal a BP的MIS-3晚期,气候相对温暖,乌伦古湖呈现高湖面特征,湖泊沉积物来源以流水搬运为主;22500-16500 cal a BP的末次冰期冰盛期,气候寒冷干燥,湖泊沉积物来源以风力搬运为主;16500-6500 cal a BP的冰后期以及早、中全新世期间,气候回暖,湖泊沉积物主要来源于河流径流作用.6500-5500 cal a BP,受高温干旱事件的影响,湖面收缩、水位剧降,除沉积中心外的其它钻孔位置出现沉积中断.5500 cal a BP后气候转冷变湿,湖泊重新恢复到现在的状态.乌伦古湖MIS-3晚期以来的古湖相沉积环境变化及其反映的古气候万年尺度上的干湿变化与周边区域气候环境变化记录有很好的一致性,响应了区域环境变化和全球气候突变事件.季风和西风的强度消长变化及其引起的环流条件改变以及温度变化引起的蒸发效应可能是区域气候环境变化的主要原因.这一古湖相沉积记录的研究可为MIS-3晚期以来北疆地区的古湖泊演化以及长时间尺度上西风和季风环流相互关系及其影响区的气候环境演化提供地质证据.  相似文献   

18.
The Qaidam Basin in the northeastern Qinghai–Tibetan Plateau (QTP) is one of the largest hyper-arid intermontane basins in the northern hemisphere, and has abundant records for the study on palaeo-lake level fluctuations and palaeoclimatic changes. Significant efforts have been invested to define the timing of shoreline deposits using radiocarbon dating. However, due to the dating limit, the absence of organic materials and carbon reservoir effects for radiocarbon dating in arid areas, it is difficult to establish a reliable chronology for shoreline deposits. Therefore, controversy exists regarding the chronology for the high lake level in the Qaidam Basin, as well in the QTP. Some proposed that high lake levels occurred during late Marine Isotope Stage (MIS) 3, while others recently argued that the highest lake level in the QTP and adjacent regions existed in MIS 5. In Gahai Lake (now a salt lake), we investigated a section comprising lacustrine and shoreline deposits, which was about 25 m above the present lake level. Seven samples were collected for quartz optically stimulated luminescence (OSL) dating. A sample collected from a fine sand layer (the bottom of the section, and 12 m above the present lake level), which was assumed to have been deposited underwater, gave an OSL age of 82 ± 8 ka. It suggested that the lake level was at least 12 m higher than present in late MIS 5. The high lake level could maintain till about 73 ± 6 ka, and then decreased. This lake level decrease resulted in a gravel layer deposit between 73 ± 6 and 63 ± 6 ka (roughly during MIS 4). The lake level rose again (about 24 m above the present lake level) between 63 ± 6 and 55 ± 5 ka (roughly in early MIS 3). No lacustrine or shoreline deposits higher than the top of the current section were found around Gahai Lake. Thus, higher than present lake levels in Gahai Lake occurred in both late MIS 5 and early MIS 3.  相似文献   

19.
Three shallow basins in Huizhou West Lake, China, were compared with respect to phosphorus (P) cycling between sediment and water, binding forms of P in sediment, and macrophyte biomass. The basins had similar sediments and similar depths, but two of the basins were restored by carp fish removal and macrophyte transplantation. These two basins have had clear water, low Chl.a and high macrophyte coverage for seven and ten years, whilst the unrestored control basin had turbid water and higher Chl.a. Judged by diffusive ammonium efflux, sediments in restored basins had higher mineralization rates than the unrestored basin, but the release of total dissolved P were more similar. However, sediments of restored basins released primarily dissolved organic P, while the sediment from the unrestored basin only released dissolved inorganic P. One third of the P release in the unrestored basin occurred from resuspended sediment, while this pathway contributed less than 3% in restored basins where resuspension rates were 10 times lower and the surface sediments affinity for phosphate higher. Besides from the presence of carps in the unrestored basin, the main differences were a large pool of P (700–850 mg P m−2) in macrophyte biomass and a smaller pool (∼150 mg m−2) as loosely adsorbed P in the sediment of restored basins than in the unrestored (0 in macrophytes and 350 mg P m−2 as loosely adsorbed). Also, a tendency of higher concentrations of oxidized iron was observed in the surface sediment from restored basins. The study underlines the potential of trophic structure changes to alter internal nutrient cycling in shallow lakes.  相似文献   

20.
In the past decades, archaeologists have found evidences for prehistorical human activity in the Qinghai–Tibetan Plateau (QTP). In 1982, some Paleolithic stone tools were found in a section from a terrace of the Xiao Qaidam Lake in the Qaidam Basin, NE of the QTP. The age of this Paleolithic site has remained unknown by far. Some believed that the age of human inhabitation in this Paleolithic site was about 30 ka. In this study, quartz optically stimulated luminescence was used to date 10 samples collected from four sections in the Xiao Qaidam Lake, using the single-aliquot regeneration-dose protocol. The two samples from section XCDH2, which is from a lake terrace about ～7–8 m above the present lake level and in which the top gravel layer contains stone tools, were not well-bleached before deposition. Their ages (>101 and >159 ka) determined by SAR should be considered minimums. OSL dating results of six samples from two sections (XCDH1 and XCDH3) of an adjacent lake terrace, which is ～12 m above the present lake level, suggest two possibilities for the age of the tool-bearing gravel layer: (1) younger than ～3 ka if the lake terrace of XCDH2 is younger than the terrace represented by XCDH1 and XCDH3; or (2) between ～3 and 11 ka if these two terraces are part of the deposit of the same time period. In either case, the age of the archaeological layer should be much younger than the previously proposed ～30 ka. As the climate in the early Holocene after 11 ka was increasingly warm and the Xiao Qaidam Lake area could be suitable for human inhabitation then, we deduce that the age range of ～3–11 ka is more likely the time frame for this archaeological site. The age of 3.1 ± 0.3 ka for the surface of terrace XCDH1/XCDH3 suggests a significant lake level decrease after this time and a corresponding arid event at ～3 ka; the lake level did not reach this level again after that time. Section XCDH4 is more than 40 m above the present lake level, and two samples gave ages of 37 ± 4 and 51 ± 4 ka. These two dates and the dates from the other sections demonstrate that two lake levels higher than present existed for Xiao Qaidam Lake, one at ～12 m and dated ～3–11 ka and the another at >40 m and dated ～37–51 ka.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号