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 656 毫秒
1.
Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200?cal?yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake??s general vicinity that have 18 associated AMS 14C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200?cal?yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500?cal?yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700?cal?yr BP. After 10,500?cal?yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in ??13C values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000?cal?yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7???) decrease in the ??18O values from Pisidiidae shells at ~3,000?cal?yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6?km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000?cal?yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the ??18OPisidiidae and ??13CPisidiidae data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. This evidence of a progressively stronger evaporative influence on the lake??s closed hydrology after ~3,000?cal?yr BP is consistent with our modern ??18O and ??D water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers.  相似文献   

2.
The stratigraphic, geochemical, and organic matter study of the late Holocene San Nicolás succession of the Cauca paleolake (Santa Fé–Sopetrán pull-apart basin) in the middle Cauca Valley, northern Colombia, suggests that it was deposited in a ria lake environment, at sedimentary accumulation rates in excess of 600 cm ky?1 between ~3500 and ~500 yr BP. Laminated deposition occurred, first under igapo (black-water), and then under varzea (white-water) conditions. The transition occurred ca. 3000 yr BP, a time of major change in El Niño/Southern Oscillation (ENSO) behavior in the Cariaco Basin, thus reflecting the southern migration of the intertropical convergence zone and intensified rain upstream the Cauca Valley. A second, but less conspicuous change occurred ca. 2000 yr BP, which apparently corresponds to the intensified and/or more frequent ENSO activity in the Galapagos Islands. Our contribution describes, for the first time, a ria lake sedimentary succession from the northern Andes and demonstrates the high potential of these hitherto undervalued deposits for the reconstruction of the paleohydrological history of the northern Andes.  相似文献   

3.
Paleohydrological changes in an Amazonian floodplain lake: Santa Ninha Lake   总被引：1，自引：0，他引：1  
Holocene environments have been reconstructed by sedimentological, mineralogical and organic geochemical analysis of a 270-cm core from Santa Ninha Lake, a floodplain lake in lower Amazonia. Dated by fourteen AMS-radiocarbon dates, the sediment core has a basal age of 5,600?cal?years BP and different sedimentary units were identified. These units document various hydrologic phases in the evolution of this lake. Reduced Amazon River influence, with reduced high-water levels of the river, characterized the period between 5,600 and 5,100?cal?years BP. Comparison with other Amazonian and Andean paleoclimate studies point to a dryer climate during this phase. After 5,100?cal?years BP coarse sediments and quartz increase which suggest a higher inflow of the Amazon River. Between 5,000 and 2,300?cal?years BP the coarse sediments and quartz remain high but the organic carbon showed the lowest values. The riverine inflow caused dilution of the organic material produced in the lake and consequently low rates of carbon flux in these phases were recorded. These results show that the hydrodynamics of the Amazon River strongly influence the behavior, productivity and consequently the sedimentation process in the floodplain lakes.  相似文献   

4.
The floodplain of the Upper Paraná River, Brazil, is strongly influenced by hydrology, which in turn affects geomorphological and environmental conditions, and controls the form of islands in the river. Such islands develop by deposition of river-borne sediment that creates small lateral sediment bars. Geomorphological processes can produce a variety of aquatic environments on such islands, e.g. channels, backwaters, lakes, transitional areas, and swamps. Our objective was to test whether subfossil diatoms preserved in the sediment on an island in the Upper Paraná River floodplain responded to changes in limnological conditions brought about by such geomorphological modifications. We hypothesized that the composition of diatom assemblages in the sediment shifted in response to past geomorphic, and hence limnological conditions. We analyzed diatom subfossils in a 2-m-long sediment core with a calibrated date near the base of 1047–1224 cal yr AD. Absence of diatoms at the bottom of the sequence was associated with the channel phase, followed by appearance of diatoms 1229–1381 cal yr AD that were adapted to flow, in the backwater phase. After another 100–200 years, presence of Eunotia species in the lake phase suggests a decrease in pH, phosphorus and nitrogen. Replacement of Eunotia spp. by Diadesmis species, following a transition phase, suggests different environmental conditions, with reduced water depth. Diatoms in surface deposits are distinct from assemblages in the other phases in the core and contain taxa that suggest a disturbed environment, with variations in water depth and flow. The data illustrate the importance of physical and hydrological factors in shaping diatom communities and show the utility of diatoms as bioindicators in this floodplain environment.  相似文献   

5.
A 95-cm-thick peat sequence obtained from Daping Swamp in the western Nanling Mountains provides evidence for climate variability in the past ~3,000 year. Multi-proxy records (including organic carbon isotopes, humification degree, organic matter content, and dry bulk density) revealed three intensified Asian summer monsoon (ASM) intervals (i.e.~2900–2700, 2500–1700 and 1000–600 cal. yr BP) and three weakened ASM intervals (i.e.~2800–2500, 1700–1000 and 600–200 cal. yr BP). Our δ13C record shows a possible correlation with the sunspot number and residual atmospheric 14C records on multi-centennial scale, especially for the period between 2960 and 2200 cal. yr BP. A spectral analysis of δ13C record reveals three significant cycles (i.e., 396, 110 and 102 yr) and all these cycles could be related to solar activity, suggesting that solar output may have influenced the late Holocene climate variability in the study region.  相似文献   

6.
Diatom assemblages recovered from a Colorado River delta core in northern Patagonia, Argentina were analyzed in order to assess past environmental conditions. A total of 35 samples were selected from a 172-cm core extracted 37 km from the mouth (39°36′19.6″S; 62°29′26.1″W). One-hundred and thirty-eight taxa were recognized and grouped according to life forms. Two diatom zones were identified by cluster analysis. At the base of the sequence, the Diatom Zone I (DZI; 4132 ± 35 ??2919?±?27 14C yr BP), consisted of clays, and was characterized by assemblages dominated by aerophilous and benthic taxa and chrysophyte stomatocysts, which led to inferences of a sedimentary environment corresponding to a pond experiencing dry periods. The upper section (DZII) was dominated by fine sands and silts encompassing the last ~?150 yr with abundant planktonic and tychoplanktonic diatoms. Benthic diatoms were abruptly replaced by planktonic forms in this zone indicating a shift to deeper waters. These results characterize the meandering dynamics of a deltaic system. During the Mid-Holocene, more arid periods would have favored the deactivation of meanders and the formation of riverine and oxbow wetlands. In Late-Holocene and historical times, more humid conditions and the hydrological system across the floodplain reactivated the paleochannel. Today, the dominant diatom species are brackish/freshwater fragilaroids. A non-multidimensional scaling analysis showed a lack of analogy between fossil and modern samples. The change in diatom floras in recent historical times was attributed to anthropogenic disturbances, a consequence of the regulation of the river flow. This regulation is evidenced by less discharge, morphological modifications in the floodplain and increased salinity in the last decade.  相似文献   

7.
An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.  相似文献   

8.
The evolution and current state of landscapes around Lake Teletskoye have not previously been studied in detail. In the valley of the Malye Chily River, which flows into Lake Teletskoye, the timing of dam failure and draining of two moraine-dammed lakes has been identified. Botanical analysis, ash content determination, and radiocarbon dating of two peat profiles provide insight into postglacial evolution of wetlands related to this landscape. We found clear evidence of the disappearance from the peat of higher vascular species that, today, grows mostly in the plains of Siberia. Correlation of the data obtained with the accepted chronology of the Holocene events in the Russian Altai suggests the following stages of postglacial environmental change in the Malye Chily River valley: (1) the continuation of the Late Würm glaciation degradation (before 7000?cal. yr BP); (2) Holocene Climate Optimum (7000–5000?cal. yr BP); (3) Akkem cooling (5000–4200?cal. yr BP); (4) warm period (4200–3700?cal. yr BP); and (5) Historical cooling (3700–1600?cal. yr BP).  相似文献   

9.
A 4000-yr history at Clark Lake reveals natural variations in sediment geochemistry and nutrient levels and anthropogenic influence on twentieth century sedimentation rates. Sediment texture and ridge and swale topography indicate that the channel system creating Clark Lake is now occupied by the Yazoo River. Between 1.24 and 0.60 m (2522–865 cal yr BP), total carbon percentages and the C/N ratio average 42% and 17, respectively. The base of the 1650-yr interval and onset of high organic activity in the lake coincides with the abandonment of the Yazoo Meander Belt. The top of the interval is marked by a drop in C/N ratio to 11 and a geochemical transition zone. No change in the rate of sediment accumulation or clay mineralogy was observed despite increased cumulative percentages of Al, Fe, K, Mg, Mn, Ca, and Na. The decrease in organic activity at 865 cal yr BP is attributed to unfavorable growth conditions related to the entrenchment of the Yazoo River and changes in the hydroperiod of the area. Significant floodplain alteration was completed in 1978, but the only major effect was increased sedimentation in the late twentieth century due to the completion of the Whittington Auxiliary Channel.  相似文献   

10.
Sub-bottom profiling was conducted at eight sub-basins within the lower French River area, Ontario, to investigate deposits preserved within the ancient North Bay outlet. Ten cores were collected that targeted the four depositional acoustic facies identified in the sub-bottom profiling records. The rhythmically laminated/bedded glaciolacustrine deposits of facies I are interpreted to have aggraded within glacial Lake Algonquin and its associated recessional lakes that persisted between 13,000 and 11,300 cal BP (~11,100 and 9,900 BP). The majority of the facies II, III and IV lacustrine deposits accumulated between about 9,500 cal BP (~8,500 BP) and the mid-Holocene, based on radiocarbon-dated organic materials. These deposits represent sedimentation within a ‘large’ lake during the late portion of the Mattawa-Stanley phase, and the Nipissing transgression, Nipissing Great Lakes and post-Nipissing recession phases of lake levels. Two sets of organic-rich sand beds are preserved within facies II deposits and reveal that the large lake lacustrine depositional environment was interrupted during the late Mattawa-Stanley phase between 9,500–9,300 and 9,000–8,400 cal BP (~8,500–8,300 and ~8,000–7,600 BP), when the water surface of Lake Hough fell below the outlet threshold and the lake basin became hydrologically closed. Pre-9,500 cal BP (~8,500 BP), the early and middle portions of the Mattawa-Stanley phase were dominated by erosion, as reflected by an unconformity at the base of facies II that occurs widely in the sub-basins and the general lack of preserved deposits for these intervals in the cores. This erosion is attributed to wave action and fluvial scouring within the outlet mouth during the early and mid-Stanley-Hough low stages and relates specifically to the period when the flowing portion of the North Bay outlet was situated over the lower French River area. This study reveals that the majority of the post-glacial deposits accumulated after the outlet threshold had shifted permanently eastwards and the lower French River area was inundated under the multiple phases of the large lake occupying the Nipissing Lowlands and Georgian-Huron basins, extending well into the mid-Holocene. The occurrence of deposits marking two closed-basin intervals during the late Stanley-Hough stage are well preserved locally within the lacustrine depositional sequence, but identifying earlier closed-basin intervals from the French River stratigraphy is hindered by the lack of preserved pre-9,500 cal BP (~8,500 BP) post-glacial deposits.  相似文献   

11.
Three lake sediment sequences (lakes Nero, Chashnitsy, Zaozer’e) from the Rostov-Jaroslavl’ region north of Moscow were studied to provide information on palaeoclimatic and palaeoenvironmental changes during the past 15,000 cal yr. The multi-proxy study (i.e., pollen, macrofossils, mineral magnetic measurements, total carbon, nitrogen and sulphur) is chronologically constrained by AMS 14C measurements. Lake Nero provided the longest sedimentary record back to ca. 15,000 cal yr BP, while sediment accumulation began around ca. 11,000 cal yr BP in the two other lakes, possibly due to melting of permafrost. Limnic plant macrofossil remains suggest increased lake productivity and higher mean summer temperatures after 14,500 cal yr BP. While the late glacial vegetation was dominated by Betula and Salix shrubs and various herbs, it appears that Betula sect. Albae became established as early as 14,000 cal yr BP. Major hydrological changes in the region led to distinctly lower lake levels, starting 13,000 cal yr BP in Lake Nero and ca. 9000 cal yr BP in lakes Chashnitsy and Zaozer’e, which are situated at higher elevations. These changes resulted in sedimentary hiatuses in all three lakes that lasted 3500–4500 cal yr. Mixed broad-leaved – coniferous forests were widespread in the area between 8200 and 6100 cal yr BP and developed into dense, species-rich forests between 6100 and 2500 cal yr BP, during what was likely the warmest interval of the studied sequences. Agricultural activity is documented since 500 cal yr BP, but probably began earlier, since Rostov was a major capital by 862 A.D. This apparent gap may be caused by additional sedimentary hiatuses around 2500 and 500 cal yr BP.  相似文献   

12.
Sponge spicules are siliceous microfossils that are especially useful for analysis of sandy fluvio-lacustrine sediments. Sponge spicules in a long sediment core (~550 cm below surface), consisting of fine sand, sandy silt, and organic-rich mud, recovered from the floodplain of the Nabileque River, southern Pantanal, Brazil (S20°16′38.3″/W57°33′00.0″), form the basis of a novel paleoenvironmental interpretation for this region. Optically stimulated luminescence dates constrain the timing of deposition to the middle-late Holocene and all spicules identified are typical of the Brazilian cerrado biome. The base of the section is dominated by Oncosclera navicella Carter 1881, Metania spinata Carter 1881, and Corvospongilla seckti Bonetto and Ezcurra de Drago 1966, which indicate a lotic to semi-lotic environment strongly influenced by an actively meandering river channel at ~6.7–5.7 ka BP. The appearance of Heterorotula fistula Volkmer-Ribeiro and Motta 1995, Dosilia pydanieli Volkmer-Ribeiro 1992 and Radiospongilla amazonensis Volkmer-Ribeiro and Maciel 1983 at ~340 cm downcore suggests a reduction in flowing water and a more stable lentic environment, consistent with deposition in an oxbow lake. This oxbow lake environment existed during an interval of regional aridity between ~4.5 and 3.9 ka BP. Spicules, as well as phytoliths and diatoms, are highly variable moving up-section, with species from both lotic and lentic ecosystems present. Above ~193 cm, the total abundance of spicules declines, consistent with wetter climate conditions and development of an underfit river similar to the modern floodplain. Results support hypotheses related to migration of the Paraguay River inferred from geomorphological studies and add a key southern-region dataset to the emerging Holocene database of paleoenvironmental records from the Pantanal wetlands.  相似文献   

13.
A late Quaternary diatom stratigraphy of Lago Puyehue (40°40′ S, 72°28′ W) was examined in order to infer past limnological and climatic changes in the South-Chilean Lake District. The diatom assemblages were well preserved in a 1,122 cm long, 14C-dated sediment core spanning the last 17,900 years, and were in support of an early deglaciation of Lago Puyehue. The presence of a short cold spell in South Chile, equivalent to the Younger Dryas event in the Northern Hemisphere, the Antarctic Cold Reversal in Antarctica, or the Huelmo-Mascardi event in southern South America, was not clearly evidenced in the diatom data, although some climate instability may have occurred between 13,400 and 11,700 cal. yr. BP, and a relatively long period (between 16,850 and 12,810 cal. yr. BP) with low absolute abundances and biovolumes could be tentatively interpreted as a period of low rainfall and/or temperatures. An increase in the moisture supply to the lake was tentatively inferred at 12,810 cal. yr. BP. After 9,550 cal. yr. BP, inferred stronger and longer persisting summer stratification, may have been the result of the higher temperatures associated with an early-Holocene thermal optimum. The mid-Holocene appeared to be characterized by a decrease in precipitation, culminating around 5,000 cal. yr. BP, and rising again after 3,000 cal. yr. BP, likely associated with a previously documented lowered frequency and amplitude of El Niño events. An increase in precipitation during the late Holocene (3,000 cal. yr. BP–present) might have marked subsequent increased frequency of El Niño occurrences, leading to drier summers and slightly moister winters in the area.  相似文献   

14.
The ostracod record from Kajemarum Oasis in the Sahel zone of Northeastern Nigeria covers the last c. 4000 cal. years of a 5500 cal. year lake-sediment sequence. The first appearance of ostracods, around 4000 cal. yr BP, reflects the switch from a very dilute lake during the mid-Holocene, to slightly oligosaline conditions that favoured the occurrence and preservation of ostracods. Between 3800 and 3100 cal. yr BP, the lake remained permanent and fresh or slightly oligosaline, with a Ca-Mg-HCO3 composition. A rise in salinity c. 3100 cal. yr BP, accompanied by a change to more variable conditions on a seasonal to interannual timescale, led to the influx of more-euryhaline taxa. Oligosaline conditions continued between 3100 and 1500 cal. yr BP. Around 1500 cal. yr BP, there was a sharp rise in salinity, probably accompanied by a shift to Na-CO3-type water, with marked seasonal and interannual variability. Salinity decreased after 900 cal. yr BP, although short-term variations were marked between 900 cal. yr BP and the top of the sequence, 95 cal. yr BP. Changes in the species assemblages and ostracod abundance were a response to climate-driven variations in the seasonal and interannual stability of the lake, together with changes in its salinity and solute composition, but there is no simple relationship between ostracod faunas and salinity. Within Kajemarum, there is no evidence of ostracod assemblages typical of deep, fresh water, nor of hypersaline Na-Cl waters. The sediments associated with the freshest waters at Kajemarum did not favour ostracod preservation, and the driest climatic conditions were associated with oligosaline to mesosaline water of Na-CO3-type. The species-poor assemblages reflect the short-term instability of the lake, coupled with the limited opportunities for the colonisation of this isolated basin.  相似文献   

15.
Lacustrine records from the northern margin of the East Asian monsoon generate a conflicting picture of Holocene monsoonal precipitation change. To seek an integrated view of East Asian monsoon variability during the Holocene, an 8.5-m-long sediment core recovered in the depocenter of Dali Lake in central-eastern Inner Mongolia was analyzed at 1-cm intervals for total organic and inorganic carbon concentrations. The data indicate that Dali Lake reached its highest level during the early Holocene (11,500–7,600 cal yr BP). The middle Holocene (7,600–3,450 cal yr BP) was characterized by dramatic fluctuations in the lake level with three intervals of lower lake stands occurring 6,600–5,850, 5,100–4,850 and 4,450–3,750 cal yr BP, respectively. During the late Holocene (3,450 cal yr BP to present), the lake displayed a general shrinking trend with the lowest levels at three episodes of 3,150–2,650, 1,650–1,150 and 550–200 cal yr BP. We infer that the expansion of the lake during the early Holocene would have resulted from the input of the snow/ice melt, rather than the monsoonal precipitation, in response to the increase in summer solar radiation in the Northern Hemisphere. We also interpret the rise in the lake level since ca. 7,600 cal yr BP as closely related to increased monsoonal precipitation over the lake region resulting from increased temperature and size of the Western Pacific Warm Pool and a westward shifted and strengthened Kuroshio Current in the western Pacific. Moreover, high variability of the East Asian monsoon climate since 7,600 cal yr BP, marked by large fluctuations in the lake level, might have been directly associated with variations in the intensity and frequency of the El Niño-Southern Oscillation (ENSO) events.  相似文献   

16.
Small lakes and wetlands from high elevation within the Sierra Nevada Range (southern Spain) preserve a complete post-glacial Holocene record. Isotopic, TOC and C/N analyses, carried out on a sediment core, show various stages in the evolution of the Borreguiles de la Virgen, which today constitute a small bog at about 2,950?m above sea level. Glacial erosion generated a cirque depression, which became a small lake during the first phase of infilling (from?8,200 to 5,100?cal?yr BP), as suggested by sedimentary evidence, including an atomic C/N ratio generally below 20, low TOC values and the highest ??13C and ??15N values of the record. These results imply significant algal productivity, which is confirmed by the microscopic algal remains. Drier conditions became established progressively in this area from?5,100 to 3,700?cal?yr BP. Subsequently, the lake evolved into a bog as shown by geochemical evidence (C/N ratios above 20, high TOC content and low ??13C values). Unstable conditions prevailed from?3,600 to 700?cal?yr BP; an extremely low sedimentation rate and scarcity of data from this period do not allow us to make a coherent interpretation. Fluctuating conditions were recorded during the last?~700?cal?yr BP, with wetter conditions prevailing during the first part of the interval (with C/N rate below 20) up to 350?years ago. In general, a gradual trend toward more arid conditions occurred since?~6,900?cal?yr BP, with a further increase in aridity since?~5,100?cal?yr BP. This evidence is consistent with other contemporaneous peri-Mediterranean records.  相似文献   

17.
Despite their sensitivity to climate variability, few of the abundant sinkhole lakes of Florida have been the subject of paleolimnological studies to discern patterns of change in aquatic communities and link them to climate drivers. However, deep sinkhole lakes can contain highly resolved paleolimnological records that can be used to track long-term climate variability and its interaction with effects of land-use change. In order to understand how limnological changes were regulated by regional climate variability and further modified by local land-use change in south Florida, we explored diatom assemblage variability over centennial and semi-decadal time scales in an ~11,000-yr and a ~150-yr sediment core extracted from a 21-m deep sinkhole lake, Lake Annie, on the protected property of Archbold Biological Station. We linked variance in diatom assemblage structure to changes in water total phosphorus, color, and pH using diatom-based transfer functions. Reconstructions suggest the sinkhole depression contained a small, acidic, oligotrophic pond ~11000–7000 cal yr BP that gradually deepened to form a humic lake by ~4000 cal yr BP, coinciding with the onset of modern precipitation regimes and the stabilization of sea-level indicated by corresponding palynological records. The lake then contained stable, acidophilous planktonic and benthic algal communities for several thousand years. In the early AD 1900s, that community shifted to one diagnostic of an even lower pH (~5.6), likely resulting from acid precipitation. Further transitions over the past 25 yr reflect recovery from acidification and intensified sensitivity to climate variability caused by enhanced watershed runoff from small drainage ditches dug during the mid-twentieth Century on the surrounding property.  相似文献   

18.
A multi-proxy analysis of two sediment cores from Rantin Lake are used to reconstruct past lake-level changes and to make inferences about millennial-scale variations in precipitation/evaporation (P/E) balance in the southern Yukon, Canada between 10,900 and 3,100?cal?yr BP. Analyses of calcium carbonate and organic matter concentration, magnetic susceptibility, titanium content, dry bulk density, and macrofossils are used to reconstruct water-level changes. The development of sand layers and deformed sediments at the deep-water core site (i.e. Core A-06) prior to ~10,900?cal?yr BP suggest that lake level was lower at this time. Fine-grained organic sediment deposited from 10,600 to 9,500?cal?yr BP indicates a rise in lake level. The formation of an unconformity at the shallow cores site (Core C-06) and the deposition of shallow-water calcium carbonate-rich facies at the Core A-06 site between ~9,500 and ~8,500?cal?yr BP suggest lower lake levels at this time. Shallow-water facies gradually transition into a sand layer that likely represents shoreline reworking during an extreme lowstand that occurred at ~8,400?cal?yr BP. Following this low water level, fine-grained organic-rich sediment formed by ~8,200?cal?yr BP, suggesting deeper water conditions at core site A-06. Calcium carbonate concentrations are relatively low in sediment deposited from ~6,300 to 3,100?cal?yr BP in Core A-06, indicating that lake level was comparatively higher during the middle and late Holocene. In general, results from this study suggest that the early Holocene was characterized by high P/E from ~10,500 to 9,500?cal?yr BP, low P/E from ~9,500 to 8,400?cal?yr BP, and return to higher P/E from ~8,200 to 3,100?cal?yr BP.  相似文献   

19.
High-resolution quantitative analysis of ostracod assemblages from 4.3-m-thick freshwater tufa-rich sediments, deposited during the last 12.8 ka in Lake Sinijärv, northern Estonia, yielded information on water level, trophic state conditions, and temperature changes since the late glacial. AMS 14C dates from aquatic mosses provided time constraints on the palaeoenvironmental development of the region. In the ostracod assemblage structure, four faunal zones (OFZ) were determined. The most significant change in the ostracod fauna occurred at 10,590 cal. y BP, when a typical littoral, polythermophilic fauna was replaced by a mostly sublittoral, species-rich meso- to stenothermophilic fauna. The ostracod data indicate two major low-water-level periods in the lake at 12,800–10,590 and 7,600–3,700 cal. y BP. Sediment analysis indicates the most intensive tufa precipitation occurred during these low stand periods, rather than during the warmest climate in Estonia between 8,000 and 4,500 cal. y BP. The late glacial low water level in the groundwater-fed Lake Sinijärv at 12,800–10,590 cal. y BP coincides partly with the regression in the Lake Peipsi basin (14,000–12,100 cal. y BP) and with the last drainage event of the Baltic Ice Lake at 11,600 cal. y BP. The low-water-level period in Lake Sinijärv occurred earlier than in lakes in the SE sector of Scandinavian glaciation. The change from low to high water level in Lake Sinijärv at 10,590 cal. y BP preceded the first post-glacial transgression events in the small lowland lakes of Estonia, southern Sweden, Poles`ye in Belarus, and Valday in NW Russia. In general, the mid-Holocene low-water-level period in Lake Sinijärv between 7,600 and 3,700 cal. y BP is concurrent with the regressions in the lakes of the SE sector of Scandinavian glaciation.  相似文献   

20.
We determined the sequence of formation of deposits on the first terrace above the Tarbagataika river floodplain, and a change of the sedimentation and soil-formation stages for the last 7 kyr. We reconstructed the accumulation dynamics of the sediment layer and calculated the mean sediment accumulation rates for different time intervals. The phases of intensification of erosion-accumulation and aeolian processes were recorded. The study revealed the main stratigraphic boundaries reflecting the fundamental changes of the settings of sedimentation: 7 cal. kyr BP, a change of riverbed sedimentation for floodplain sedimentation; 5.4 cal. kyr BP, intensification of erosion-accumulation processes due to climate aridization on the Atlantic-Subboreal boundary, and active input of erosion products to the floodplain; 4 cal. kyr BP, active filling of the river valley with horizontally layered polygenetic fine-grained sands and sandy loams containing intercalations of humic warps in conditions of the overlapped (built-up) floodplain with an increase in humidification in the middle of the Subboreal period and, as a consequence, an increase in the height of floods; 3.4 cal. kyr, the layer under investigation leaves the floodplain regime of sedimentation and accumulation of aeolian-deluvial deposits, and 1 cal. kyr, intensification of erosion-accumulation processes due to climatic and anthropogenic changes. We reconstructed the chronology and specific features of the pedogenesis stages which record the phases of temporal cessation of floodplain and slope sedimentation on the study territory. The results obtained are in good agreement with data on the hydraulicity of rivers, the stages of soil formation and on the manifestation of exogenous processes on the territories of Russia and Mongolia.  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