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  • 1
    Language: English
    In: Nature (London), 2007-01-04, Vol.445 (7123), p.74-77
    Description: The Asian-Australian monsoon is an important component of the Earth's climate system that influences the societal and economic activity of roughly half the world's population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling-Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4-6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.
    Subject(s): Tang dynasty ; Asia ; China ; Exact sciences and technology ; Art and archaeology ; Earth, ocean, space ; Paleontology: general ; Earth sciences ; External geophysics ; Climatology. Bioclimatology. Climate change ; Paleontology ; Meteorology
    ISSN: 0028-0836
    E-ISSN: 1476-4687
    Source: Academic Search Ultimate
    Source: Nature Journals Online
    Source: Alma/SFX Local Collection
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  • 2
    Language: English
    In: Nature geoscience, 2008-08, Vol.1 (8), p.520-523
    Description: The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679 yr BP, broadly coincident with other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate; we suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.
    ISSN: 1752-0894
    E-ISSN: 1752-0908
    Source: Nature Journals Online
    Source: ProQuest Central
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  • 3
    Language: English
    In: Journal of paleolimnology, 2007-05, Vol.37 (4), p.517-528
    Description: We present a unique, versatile piston corer for recovering continuous lake and bog sediment sequences with superior accuracy and quality. The main components of the system and their function are described, with special focus on measures for obtaining long, continuous lake sediment sections up to a current maximum length of 95 m. Examples of lake sediment profiles obtained with this system from different climatic zones are presented.
    Subject(s): Sedimentology ; Environment, general ; Quaternary lake sediments ; Geology ; Hydrobiology ; Climate Change ; Limnogeology ; Physical Geography ; Environment ; Coring equipment ; Piston corers ; Paleolimnology ; Sediments (Geology)
    ISSN: 0921-2728
    E-ISSN: 1573-0417
    Source: Alma/SFX Local Collection
    Source: ProQuest Central
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  • 4
    Language: English
    In: Nature (London), 1999-08-19, Vol.400 (6746), p.740-743
    Description: Oxygen-isotope records from Greenland ice cores, indicate numerous rapid climate fluctuations during the last glacial period. North Atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice-rafted debris. In contrast, very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents. Here we present high-resolution geochemical, physical and pollen data from lake sediments in Italy and from a Mediterranean sediment core, linked by a common tephrochronology. Our lacustrine sequence extends to the past 102,000 years. Many of its features correlate well with the Greenland ice-core records, demonstrating that the closely coupled ocean–atmosphere system of the Northern Hemisphere during the last glacial extended its influence at least as far as the central Mediterranean region. Numerous vegetation changes were rapid, frequently occurring in less than 200 years, showing that the terrestrial biosphere participated fully in last-glacial climate variability. Earlier than 65,000 years ago, our record shows more climate fluctuations than are apparent in the Greenland ice cores. Together, the multi-proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials, and provide a step towards determining the underlying mechanisms of the centennial–millennial-scale variability.
    Subject(s): Earth, ocean, space ; Earth sciences ; Marine and continental quaternary ; Surficial geology ; Exact sciences and technology ; Glacial epoch ; Research
    ISSN: 0028-0836
    E-ISSN: 1476-4687
    Source: Academic Search Ultimate
    Source: Nature Journals Online
    Source: Alma/SFX Local Collection
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  • 5
    Language: English
    In: Journal of Geophysical Research: Atmospheres, 2008-11-04, Vol.113 (D21), p.D21101-n/a
    Description: We present a continuous record of fossil diatoms from Huguang Maar Lake (HML) in southeastern China, spanning the time interval 17,500 to 6000 calendar years (cal years) B.P. The seasonal change in relative abundance of the dominant diatom taxa, Aulacoseira and Cyclotella species, can be used as a proxy of the strength of winter monsoon winds (WMW), which is supported by the results of a sediment trap experiment in HML and by an extensive review of the literature on the autoecologies of these species. In the sediment, high C. stelligera abundance and high‐diatom concentration, which indicate warm conditions and low wind‐driven turbulence of the water column, characterize an interval equivalent to the Greenland Interstadial 1. This is followed by an interval with low‐diatom concentration and with assemblages dominated by Aulacoseira species, which suggests high wind‐driven turbulence and therefore strong WMW. This interval corresponds with the Greenland Stadial 1. During the early and middle Holocene, another two episodes with strong WMW are evident from the data between 10,000 and 8500 and between 7000 and 6000 cal years B.P. The diatom record implies that strong winter monsoon episodes not only occurred during the last glacial‐Holocene transition but also during the Holocene “thermal maximum.”
    Subject(s): Global Change ; Paleoclimatology ; Regional climate change ; Biosphere/atmosphere interactions ; Atmospheric Composition and Structure ; Atmospheric Processes ; Abrupt/rapid climate change ; diatom ; Asian winter monsoon ; the last glacial Holocene ; Earth, ocean, space ; Earth sciences ; Exact sciences and technology
    ISSN: 0148-0227
    E-ISSN: 2156-2202
    Source: Wiley Online Library All Backfiles
    Source: Alma/SFX Local Collection
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  • 6
    Language: English
    In: Journal of paleolimnology, 2009-12, Vol.42 (4), p.587-609
    Description: We present varve chronologies for sediments from two maar lakes in the Valle de Santiago region (Central Mexico): Hoya La Alberca (AD 1852--1973) and Hoya Rincon de Parangueo (AD 1839--1943). These are the first varve chronologies for Mexican lakes. The varved sections were anchored with tephras from Colima (1913) and Paricutin (1943/1944) and 210.sup.Pb ages. We compare the sequences using the thickness of seasonal laminae and element counts (Al, Si, S, Cl, K, Ti, Mn, Fe, and Sr) determined by micro X-ray fluorescence spectrometry. The formation of the varve sublaminae is attributed to the strongly seasonal climate regime. Limited rainfall and high evaporation rates in winter and spring induce precipitation of carbonates (high Ca, Sr) enriched in 13.sup.C and 18.sup.O, whereas rainfall in summer increases organic and clastic input (plagioclase, quartz) with high counts of lithogenic elements (K, Al, Ti, and Si). Eolian input of Ti occurs also in the dry season. Moving correlations (5-yr windows) of the Ca and Ti counts show similar development in both sequences until the 1930s. Positive correlations indicate mixing of allochthonous Ti and autochthonous Ca, while negative correlations indicate their separation in sublaminae. Negative excursions in the correlations correspond with historic and reconstructed droughts, El Nino events, and positive SST anomalies. Based on our data, droughts (3--7 year duration) were severe and centred around the following years: the early 1850s, 1865, 1880, 1895, 1905, 1915 and the late 1920s with continuation into the 1930s. The latter dry period brought both lake systems into a critical state making them susceptible to further drying. Groundwater overexploitation due to the expansion of irrigation agriculture in the region after 1940 induced the transition from calcite to aragonite precipitation in Alberca and halite infiltration in Rincon. The proxy data indicate a faster response to increased evaporation for Rincon, the lake with the larger maar dimensions, solar radiation receipt and higher conductivity, whereas the smaller, steeper Alberca maar responded rapidly to increased precipitation.
    Subject(s): Clay ; Rock-salt ; Radiation ; Geophysics ; Fluorescence ; Calcite crystals ; X-ray spectroscopy ; Limnology ; Droughts ; Hypersonic planes ; Water, Underground ; Silt ; Universities and colleges ; Carbonates ; Rain and rainfall
    ISSN: 0921-2728
    E-ISSN: 1573-0417
    Source: Alma/SFX Local Collection
    Source: ProQuest Central
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  • 7
    Language: English
    In: Journal of paleolimnology, 2006-02, Vol.35 (2), p.239-270
    Description: The varved sediments of the dimictic Lake Sihailongwan (Long Gang mountain area, Jilin Province, Northeast China) represent a palaeoclimatic archive which documents the local precipitation frequency during the summer monsoon, and variations in the aeolian flux of dust with their remote sources in the arid and semi-arid regions of inner Asia. Based on a detailed discussion of sediment genesis in Lake SHL, dust flux rates and palaeohydrological conditions were reconstructed on a decadal scale over the past 220 years. The aeolian influx by dry and wet deposition was quantified and characterised in its chemical composition. Photosynthetic production in the lake is positively correlated with the inflow of nutrient-rich groundwater. The groundwater discharge largely reflects the strength of the summer monsoon. Net accumulation rates for biogenic silica were determined for annually laminated sediments from the centre of the U-shaped lake basin based on sediment data. In a Si-balance model of the modern lake, the depositional flux of biogenic silica could be independently quantified on the base of hydrochemical monitoring data. Comparison of the both estimates allowed to asses the focussing of the particle flux in the lake. Though water retention in Lake SHL is rather high (ca. 30 years), changes in the hydrological conditions are sensitively recorded in the sediments because (i) nutrient-rich groundwater discharges into the productive zone of the lake, (ii) a substantial proportion of the total dissolved Si-inventory of the mixed lake (ca. 30%) is annually consumed by diatom growth, and (3) sediment accumulation is substantially focussed towards the flat bottom of the lake basin. The bulk siliciclastic sediment fraction (ca. 75 wt.%) largely originates from influx of dust of remote provenance. In sediment thin-sections, the dry-deposited dust fraction is microscopically identifiable as seasonal silt layer. Aeolian input by wet-deposition shows a distinctly higher variability than the influx of dust by dry-deposition. As diatom production, wet-deposition of dust is positively correlated with the rainfall during the summer monsoon. The inferred positive correlation between rainfall and dust flux during the summer monsoon implies that dust deposition is determined by the out-wash efficiency of mineral particles for a permanent high atmospheric dust concentration over Northeast China in the last 220 years.
    Subject(s): Sedimentology ; Environment, general ; Hydrology ; Hydrobiology ; Climate Change ; Physical Geography ; Palaeoclimate ; East-Asian monsoon ; Geochemistry ; Environment ; Sediments ; Dust flux
    ISSN: 0921-2728
    E-ISSN: 1573-0417
    Source: Alma/SFX Local Collection
    Source: ProQuest Central
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  • 8
    Language: English
    In: Geophysical journal international, 2008-11, Vol.175 (2), p.462-476
    Description: Three up to 520-cm-long sediment cores from Hoya San Nicolas in Guanajuato, Mexico, were analysed for various magnetic properties in order to better define a palaeomagnetic secular curve for Central Mexico. The results—magnetic susceptibility, hysteresis cycles, anhysteretic and isothermal remanent magnetization—suggest that the remanent magnetization of the sediments is controlled by ferrimagnetic minerals (pseudo-single domain magnetite), which are suitable recorders of the geomagnetic field. The age–depth model indicates average deposition rates of 0.32 (interval 146–198 cm) and 0.45 mm yr−1 (interval 200–520 cm) and a basal age of about 11 600 calibrated years BP (cal. yr BP). We used magnetic susceptibility and natural remanent magnetization to correlate the three cores. A composite palaeomagnetic secular variation (SV) record was obtained from the cores with a stretching and stacking process, and a chronology established with accelerator mass spectrometer dates on microscopic charcoal and stratigraphic correlations with other well dated Holocene records from Mexico and Guatemala. The declination and inclination results show oscillatory behaviour varying in a narrow range, although a distinctive directional swing is evident between 9060 and 9810 cal. yr BP. The San Nicolas palaeomagnetic SV curve is similar to palaeomagnetic master curves from Europe and North America, in shape, occurrence and synchronicity of directional features, especially with respect to inclination.
    Subject(s): Environmental magnetism ; Rock and mineral magnetism ; Palaeomagnetic secular variation ; Geography ; Magnetite ; Geoarchaeology ; Magnetization ; Analysis ; Geographers ; Magnetism ; Geographical research ; Sediments (Geology) ; Magnetic properties
    ISSN: 0956-540X
    E-ISSN: 1365-246X
    Source: Alma/SFX Local Collection
    Source: Oxford Journals 2016 Current and Archive A-Z Collection
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  • 9
    Language: English
    In: Journal of paleolimnology, 2006-02, Vol.35 (2), p.271-288
    Description: Palaeovariations of the atmospheric 210Pb flux in Northeast China (Long Gang area, Jilin province), quantified by high resolution 210Pb measurements on seasonally laminated sediments of Lake Sihailongwan are presented on decadal scale. The mean flux of unsupported 210Pb between 1790 and 1970 for the centre of the lake basin is 517 Bq  m−2 yr−1 with maximum deviations between −23% and +27% of this value. Flux rates above this average were found between 1783 and 1813, around 1836, and between 1860 and 1901. The mean 210Pbexc flux rates derived were clearly lower around 1821, 1908, 1930, and 1953. The 210Pbexc flux reached its minimum of 344 Bq  m−2 yr−1 during the period 1977–1982. The atmospheric flux of unsupported 210Pb is correlated with the precipitation frequency during the summer monsoon and shows coinciding variability with geochemical proxies that document the groundwater inflow into the lake. Al2O3-rich dust of remote provenance scavenged by wet-deposition in the rainy season is the major carrier of the atmospheric 210Pbexc flux.
    Subject(s): Sedimentology ; Environment, general ; Hydrology ; Hydrobiology ; Climate Change ; Physical Geography ; Palaeoclimate ; East-Asian monsoon ; Environment ; Lead-210 ; Sediments ; Dust flux
    ISSN: 0921-2728
    E-ISSN: 1573-0417
    Source: Alma/SFX Local Collection
    Source: ProQuest Central
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  • 10
    Language: English
    In: Journal of geophysical research. Solid earth, 2013-06, Vol.118 (6), p.2648-2669
    Description: This study presents paleomagnetic data from 59 independent lava flows from the trans‐Mexican volcanic belt (TMVB) with ages from 6.4 Ma to recent, 52 being younger than 1 Ma, and 11 new 40Ar/39Ar age determinations. Most remanence carriers are Ti‐poor titanomagnetite of pseudosingle‐domain magnetic structure, nine lavas contain small amounts of titanomaghemite, and four lavas additional (titano‐) hematite. Paleosecular variation of lava flows younger than 1.7 Ma is consistent with latitude‐dependent Model G and also in agreement with other Pleistocene paleomagnetic data from the TMVB. The directional record of Brunhes and Matuyama Chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions. One lava flow dated at 592 ± 20 ka has a fully reversed paleodirection and most likely erupted during the Big Lost excursion. Another fully reversed flow, dated at 671 ± 12 ka, gives new volcanic evidence for the Delta/Stage 17 excursion. This excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 ± 10 ka. From the Matuyama age lavas, one flow with normal polarity magnetization, dated at 949 ± 37 ka, could either be related to the Kamikatsura or the Santa Rosa excursion and a normal polarity flow, dated at 1628 ± 56 ka, could have been emplaced during the Gilsa excursion. The results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions. Key Points Evidence for at least 4 geomagnetic excursions in 59 Brunhes‐Matuyama lava flows 11 new Ar‐Ar age determinations Results confirm in 1 but disagree in 4 cases with previously published data
    Subject(s): excursions ; geochronology ; Mexico ; paleomagnetism
    ISSN: 2169-9313
    E-ISSN: 2169-9356
    Source: Alma/SFX Local Collection
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