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1.
New paleovegetation and paleoclimatic reconstructions from the Sierra Madre Occidental (SMO) in northwestern Mexico are presented. This work involves climate and biome reconstruction using Plant Functional Types (PFT) assigned to pollen taxa. We used fossil pollen data from four Holocene peat bogs located at different altitudes (1500‑2000 m) at the border region of Sonora and Chihuahua at around 28° N latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.; Ortega-Rosas, C.I., Peñalba, M.C., Guiot, J. Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico. Submitted for publication of Palaeobotany and Palynology). The closest modern pollen data come from pollen analysis across an altitudinal transect from the Sonoran Desert towards the highlands of the temperate SMO at the same latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.). An additional modern pollen dataset of 400 sites across NW Mexico and the SW United States was compiled from different sources (Davis, O.K., 1995. Climate and vegetation pattern in surface samples from arid western U.S.A.: application to Holocene climatic reconstruction. Palynology 19, 95–119, North American Pollen Database, Latin-American Pollen Database, personal data, and different scientific papers). For the biomization method (Prentice, I.C., Guiot, J., Huntley, B., Jolly, D., Cheddadi, R., 1996. Reconstructing biomes from paleoecological data: a general method and its application to European pollen data at 0 and 6 ka. Climate Dynamics 12, 185–194), we modified the pollen-PFT and PFT-biomes assignation of Thompson and Anderson (Thompson, R.S., Anderson, K.H., 2000. Biomes of western North America at 18,000; 6000 and 0 14C yr BP reconstructed from pollen and packrat midden data. Journal of Biogeography 27, 555–584) for a better representation of the modern vegetation of NW Mexico. The biome reconstruction method was validated with the modern pollen sites and applied to the fossil sites. Our results show that, during the early Holocene, a cool conifer forest extended at least down to 1700 m, while today this biome is present above 2000 m in the Chihuahua state. The Younger Dryas event was recorded in one site with cold and dry conditions. The reconstructed annual temperature for this period was 3°–6 °C colder than today, and annual precipitation was 250 mm lower than at present (900 mm/yr). The middle Holocene after 9200 cal yr BP was marked by a warming trend, reaching temperatures 2 °C warmer than today at 7000 cal yr BP, and by the installation of a warm mixed forest, the present day biome, at 1700 m elevation, while at higher elevations (1900 m) the cool conifer forest was still present. Summer precipitation was 200 mm/yr above the early Holocene values, suggesting that monsoon-like conditions strengthened since 9200 cal yr BP at this region. During the last 4000 yr, the same warm mixed forest was reconstructed below 1700 m and a conifer forest above 1700 m. A great variability of vegetation and climate patterns was recorded for the last 3000 yr particularly at high elevation sites, where warming and cooling trends would be coeval of the Medieval warm period and Little Ice Age, likely related to ENSO variability.  相似文献   

2.
Owing to the very gently sloping nature of the flood plain in the lower White Nile valley, which is underlain by a former lake-bed, the depositional record in that area is unusually well preserved. In Egypt and along the Blue Nile phases of erosion have destroyed segments of the sedimentary record, but the White Nile sequence is a good proxy for both the main Nile and the Blue Nile. During the last 15 ka, at least, times of high flow in the Blue Nile and main Nile were synchronous with those in the White Nile.Not all the White Nile flood deposits have been preserved but calibrated radiocarbon dates obtained on fossil freshwater and amphibious Pila shells and fish bones indicate that White Nile levels were high around 14.7–13.1 ka, 9.7–9.0 ka, 7.9–7.6 ka, 6.3 ka and 3.2–2.8 ka. The Blue Nile record is more fragmentary and that of the main Nile even more so except for the Holocene Nile delta. Calibrated radiocarbon ages for high Blue Nile flows indicate very high flood levels towards 13.9–13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka.Incision by the Blue Nile and main Nile has caused progressive incision in the White Nile amounting to at least 4 m since the terminal Pleistocene  15 ka ago and at least 2 m over the past 9 ka. The Blue Nile seems to have cut down at least 10 m since  15 ka and at least 4 m since 9 ka. The time-transgressive and relatively late inception of plant domestication in the Nile valley may partly reflect this history of incision. Nile incision would propagate upstream into the White Nile valley, draining previously swampy areas along the valley floor, which would then become accessible to cultivation.  相似文献   

3.
We obtained the high-resolution record of terrestrial biomarkers (C29 and C31 n-alkanes) for the last 26,000 years from Oki Ridge in the south Japan Sea that enabled us to discuss millennial scale climate changes. Our sampling resolution for the biomarker during the major deglaciation period (10–19.5 cal ka BP) is 300 years and for the elemental analyses (total organic carbon and total nitrogen) is as good as ca 200 years. The estimated mass accumulation rate of these molecules during the last glacial period is substantially higher than during the Holocene. They also exhibited two distinct peaks at 17.6 cal ka BP and 11.4 cal ka BP, which are coincident with Heinrich Event 1 and the latest stage of the Younger Dryas, respectively. The unique oceanographic setting of the Japan Sea tends to preferentially preserve organic material of aeolian origin. The nature of our biomarker record in fact suggests a strong aeolian signal, and hence their flux to the Japan Sea potentially reflects the climate conditions of the dust source regions and transport intensity. Our results are consistent with previously reported monsoon variations based on other proxies that is indicative of a strong linkage between North Atlantic climate and Asian monsoon intensity.  相似文献   

4.
In the present paper, we report on micropaleontological (dinocysts) and isotopic (18O and 13C in foraminifers) analyses performed in Holocene sediments from fifteen cores raised from the central and northwest North Atlantic. Sea-surface temperature (SST), sea-surface salinity (SSS), thus potential density, and sea-ice cover are reconstructed based on dinocyst assemblages. After proper calibration, oxygen isotope data on the mesopelagic foraminifer Neogloboquadrina pachyderma left coiled (Npl) are converted into potential density values deeper in the water column, thus allowing documentation of vertical density gradients and identification of intervals favourable for winter convection to occur with formation of intermediate Labrador Sea Water (LSW). The most important findings from this study include: (1) the existence of an early-mid Holocene thermal optimum with positive anomalies up to 6 °C above present along the main SW–NE axis of the North Atlantic Current, but no significant SST maximum at most sites along eastern Canadian margins; (2) the evidence for larger than present amplitude of annual SSTs during the early Holocene, thus for a stronger seasonality; (3) minimum sea-ice cover from 11 500 to 6000 cal years BP, and a slight increase of sea-ice variability, and average seasonal duration of 0.5 to 1 month per year afterwards; (4) variable SSS during the entire Holocene, suggesting changes in the routing and rates of freshwater–meltwater discharges from the Arctic and eastern Canada; (5) the setting of conditions compatible with LSW production after 8 ka only, and likely a more steady production during the late Holocene; (6) an overall trend for a potential density increase of the Labrador Sea, throughout the Holocene, matching a decreasing trend eastward, thus suggesting a progressive enhancement of the western branch of the Atlantic Meridional Overturning with respect to its northeastern route; and (7) indication of maximum production and fast dispersal of LSW in the entire North Atlantic during recent times only, as suggested by linearly-converging δ18O-values of Npl from all sites, towards its modern relatively homogeneous composition ( 2.5/2.6‰). The overall picture of the Holocene North Atlantic arising from this study is that of a basin marked by a strong regionalism with large discrepancies in hydrographical trends and high frequency oscillations, at least partly controlled by freshwater–meltwater routes and rates of export from the Arctic.  相似文献   

5.
We utilize a regional climate model with detailed land surface processes (RegCM2) to simulate East Asian monsoon climates at 0 ka, 6 ka and 21 ka BP, and evaluate the changes in hydrology process, including vapor transportation, precipitation, evapotranspiration and runoff in the eastern and western China during these periods. Results indicate that the Tibetan Plateau climate presents a wet–cold status during the LGM while it exhibits a wet–warm climate at 6 ka BP. The LGM wetter climate over the Tibetan Plateau mainly results from the increased vapor inflow through its south boundary, while the increase in the vapor import over the Tibetan Plateau at 6 ka BP mostly sources from its west boundary. The increase in the LGM runoff over the Tibetan Plateau is mainly caused by the decrease in evapotranspiration, while the increase in runoff at the 6 ka BP mainly by the enhanced precipitation. Eastern China (including southern China) presents a dry status during the LGM, which precipitation and runoff decreases significantly due largely to weakened Asian summer monsoon that results in the decreased vapor inflow through the south boundary of eastern China. The variation pattern in the hydrological cycle in eastern China is contrary to that in western China during the LGM. The increase in precipitation and runoff at 6 ka BP in eastern China is tightly related to the strong Asian summer monsoon that leads to increased vapor import through the south boundary. Long term decrease trend in precipitation and runoff in northern China since the last 20 000 years may be attributed to the steady increase in vapor export through the east boundary as a result of the changes of East Asian monsoon and the adjustments of local atmospheric circulations in this area.  相似文献   

6.
The history (45–0 ka BP) of the aquatic vegetation composition of the shallow alpine Lake Luanhaizi from the NE Tibetan Plateau is inferred from aquatic plant macrofossil frequencies and aquatic pollen and algae concentrations in the sediments. C/N (range: 0.3–100), δ13C (range: −28 to −15‰), and n-alkane measurements yielded further information on the quantitative composition of sedimentary organic matter. The inferred primary production of the former lake ecosystem has been examined in respect of the alternative stable state theory of shallow lakes [Scheffer, M., 1989. Alternative stable states in eutrophic, shallow freshwater systems: a minimal model. Hydrobiological Bulletin 23, 73–83]. Switches between clear and turbid water conditions are explained by a colder climate and forest decline in the catchment area of Lake Luanhaizi. The macrofossil-based reconstruction of past water depth and salinity ranges, as well as other organic matter (OM) proxies allowed climatic inferences of the summer monsoon intensity during the late Quaternary. Around 45 ka BP, conditions similar to or even moister than present-day climate occurred. The Lake Luanhaizi record is further evidence against an extensive glaciation of the Tibetan Plateau and its bordering mountain ranges during the Last Glacial Maximum. Highest lake levels and consequently a strong summer monsoon are recorded for the early Holocene period, while gradually decreasing lake levels are reconstructed for the middle and late Holocene.  相似文献   

7.
Uranium series dating has been carried out on secondary uranyl silicate minerals formed during sub-glacial and post-glacial weathering of Proterozoic uraninite ores in south west Finland. The samples were obtained from two sites adjacent to the Salpauselkä III ice marginal formation and cover a range of depths, from the surface to more than 60 m. Measured ages fall into three distinct groups, 70–100 ka, 28–36 ka and < 2500 yr. The youngest set is associated with surface exposures and the crystals display clear evidence of re-working. The most likely trigger for uranium release at depths below the surface weathering zone is intrusion of oxidising glacial melt water. The latter is often characterised by very high discharge rates along channels, which close once the overpressure generated at the ice margin is released. There is excellent correspondence between the two Finnish sites and published data for similar deposits over a large area of southern and central Sweden. None of the seventy samples analysed gave a U–Th age between 40 and 70 ka; a second hiatus is apparent at 20 ka, coinciding with the Last Glacial Maximum. Thus, the process responsible for uranyl silicate formation was halted for significant periods, owing to a change in geochemical conditions or the hydrogeological regime. These data support the presence of interstadial conditions during the Early and Middle Weichselian since in the absence of major climatic perturbations the uranium phases at depth are stable. When viewed in conjunction with proxy data from mammoth remains it would appear that the region was ice-free prior to the Last Glacial Maximum.  相似文献   

8.
Recent observations showing substantial diurnal changes in velocities of glaciers flowing into the ocean, measured at locations far inland of glacier grounding lines, add fuel to the ongoing debate concerning the ability of glaciers to transmit longitudinal-stress perturbations over large distances. Resolution of this debate has major implications for the prediction of glacier mass balance, because it determines how rapidly a glacier can respond dynamically to changes such as weakening or removal of an ice shelf. Current IPCC assessment of sea-level rise takes little account of such changes, on the assumption that dynamic responses would be too slow to have any appreciable effect on ice discharge fluxes. However, this assumption must be questioned in view of observations showing massive increases in glacier velocities following removal of parts of the Larsen Ice Shelf, Antarctica, and of others showing diurnal velocity changes apparently linked to the tides.Here, I use a simple force-perturbation model to calculate the response of glacier strain rates to tidal rise and fall, assuming associated longitudinal-force perturbations are transmitted swiftly far inland of the glacier grounding line. Results show reasonable agreement with observations from an Alaskan glacier, where the velocity changes extended only a short distance up-glacier. However, for larger Antarctic glaciers, big velocity changes extending far upstream cannot be explained by this mechanism, unless ice-shelf “back forces” change substantially with the tides.Additional insight will require continuous measurement of velocity and strain-rate profiles along flow lines of glaciers and ice shelves. An example is suggested, involving continuous GPS measurements at a series of locations along the centre line of Glaciar San Rafael, Chile, extending from near the calving front to perhaps 20 km inland. Tidal range here is about ± 0.8 m, which should be sufficient to cause a variation in ice-front velocity of ± 2 cm h− 1 about its average value of 75 cm h− 1, assuming local seawater depth of 150 m and glacier thickness of 200–400 m.  相似文献   

9.
A digital 3D-reconstruction of the Baltic Ice Lake's (BIL) configuration during the termination of the Younger Dryas cold phase (ca. 11 700 cal. yr BP) was compiled using a combined bathymetric–topographic Digital Terrain Model (DTM), Scandinavian ice sheet limits, Baltic Sea Holocene bottom sediment thickness information, and a paleoshoreline database maintained at the Lund University. The bathymetric–topographic DTM, assembled from publicly available data sets, has a resolution of 500 × 500 m on Lambert Azimuthal Equal Area projection allowing area and volume calculations of the BIL to be made with an unprecedented accuracy. When the damming Scandinavian ice sheet margin eventually retreated north of Mount Billingen, the high point in terrain of Southern central Sweden bordering to lower terrain further to the north, the BIL was catastrophically drained resulting in a 25 m drop of the lake level. With our digital reconstruction, we estimate that approximately 7800 km3 of water drained during this event and that the ice dammed lake area was reduced by ca. 18%. Building on previous results suggesting drainage over 1 to 2 years, our lake volume calculations imply that the freshwater flux to the contemporaneous sea in the west was between about 0.12 and 0.25 Sv. The BIL reconstruction provides new detailed information on the paleogeography in the area of southern Scandinavia, both before and after the drainage event, with implications for interpretations of geological records concerning the post-glacial environmental development.  相似文献   

10.
The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global carbon cycle. Ice core studies however, show little variability in atmospheric CO2. Resolving any possible centennial to decadal CO2 changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship between stomatal index (measured on sub-fossil leaves) and atmospheric CO2 to complement ice core records between 11,230 and 10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct CO2 decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to expanding vegetation in the Northern Hemisphere. The increasing instability in CO2 after 10,600 cal. yr BP occurs during a period of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the region.The reconstructed CO2 changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO2 concentrations fluctuated via rapid responses in climate.  相似文献   

11.
We estimate the intensity of Late-glacial and Holocene methane emissions from peatlands based on their paleo net primary production (PNPP). The PNPP is derived from the carbon accumulation rates of the studied bog profile (Etang de la Gruère, Switzerland), which are corrected for the degree of peat degradation. The obtained PNPP curve is taken as a proxy for methane emissions. It shows relatively high values (90 g C m− 2 yr− 1) early in the Bolling/Allerod and drops to low values (40 g C m− 2 yr− 1) during the Younger Dryas cold period. With the onset of the Holocene the PNPP increases strongly up to 150 g C m− 2 yr− 1 around ca. 10,000 Cal. yr bp. This is followed by a decline to minimum values (30 to 40 g C m− 2 yr− 1) between 6500 and 4000 Cal. yr bp. Thereafter, the PNPP starts to increase again to reach its highest value (175 g C m− 2 yr− 1) around 1000 Cal. yr bp.The PNPP curve correlates well with the evolution of the atmospheric methane concentrations as derived from Greenland ice-cores. For example, minima in atmospheric methane reported during the Younger Dryas and around 5200 Cal. yr bp are coinciding with the lowest values of PNPP and the negative atmospheric methane peak at 8200 Cal. yr bp corresponds to a marked decrease in PNPP.Our PNPP curve suggests that the methane emissions from northern peatlands evolved similar to those of low latitude wetlands and together they largely determined the evolution of atmospheric methane throughout the Late-glacial and the Holocene. The abruptness of the rise of atmospheric methane at the end of the Younger Dryas probably points to an additional source (e.g. marine gas hydrates), but very early in the Holocene the peatlands have likely become the dominant source of atmospheric methane.  相似文献   

12.
The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka = 1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (< 300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position.  相似文献   

13.
Two Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr− 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr− 1) and the smaller trends in the Aegean Sea (0.5 mm yr− 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region.  相似文献   

14.
The growth of two high-elevation inland lakes (at 4600 m) was analyzed using satellite imagery (2000–2005) and data were collected over the last decade (1997–2006) at a plateau meteorological station (at 4820 m) and stream gauging data from a station (at 4250 m) in central Tibet. We examined the lake water balance responses to meteorological and hydrological variables. The results show that the lake areas greatly expanded by a maximum of 27.1% (or 43.7 km2) between 1998 and 2005. This expansion appears to be associated with an increase in annual precipitation of 51.0 mm (12.6%), mean annual and winter mean temperature increases of 0.41 °C and 0.71 °C, and an annual runoff increase of 20% during the last decade. The changes point to an abrupt increase in the annual precipitation, mean temperature and runoff occurring in 1996, 1998 and 1997, respectively, and a decrease in the annual pan evaporation that happened in 1996. The timing of lake growth corresponds closely with abrupt increases in the annual precipitation and runoff and with the decrease in the annual evaporation since the mid-1990s. This study indicates a strong positive water balance in these permafrost highland lakes, and provides further evidence of lake growth as a proxy indicator of climate variability and change.  相似文献   

15.
In this study, more than 13 yr of merged altimetry sea level anomalies (SLA) data were used to analyze the trends of sea level variations in the South China Sea (SCS). The result shows that the mean sea level over the SCS has a rise rate of 11.3 mm/yr during 1993–2000 and a fall rate of 11.8 mm/yr during 2001–2005. The geographical distribution of the sea level variations over the SCS is asymmetric with a pronounced variation existing in the deep water. The trends of thermosteric sea level variations were also examined using Ishii data and MITgcm assimilation data. The result indicates that the thermal change of the upper layer of the SCS has a significant contribution to the sea level variations. Heat budget analysis suggests that heat advection may be a key factor influencing the thermal change. Apart from thermal contribution, the effect of water exchange on the sea level variations was also studied.  相似文献   

16.
The Chinese Loess Plateau (CLP) comprises an extensive record of eolian deposition that contains important information about climate change. The objective of this study is to investigate if mineralogy can provide more insight into the long-term evolution of the East Asian monsoon. Comparisons between mineralogy and other paleoclimatic records (grain size and magnetic susceptibility) from the CLP have been made to evaluate the efficacy of mineralogy as a paleoclimatic tool.Here we present data from a mineralogical study of a red clay–loess sequence at Lingtai, central Chinese Loess Plateau. Changes related to source area(s), transport processes and weathering regime over time are recorded in mineral variation.Higher average concentrations of kaolinite, chlorite and quartz in the red clay, together with abrupt changes in relative mineral abundances across the red clay–loess boundary suggest a change of source area at 2.6 Ma. From 2.6 Ma to about 1.7 Ma the summer monsoon influence increases, destroying chlorite and contributing fine illite particles to the sediment. At around 1.7 Ma the mineralogy becomes relatively constant, suggesting that the monsoon was fairly stable during this period. At 0.7–0.5 Ma an increase of both summer monsoon and winter monsoon activity is inferred from illite, kaolinite, chlorite and plagioclase concentrations. Over the last 0.5 Ma mineralogy suggests an aridification of source area(s) as chlorite and plagioclase concentrations increase where illite concentration decreases. The last major change occurred around 0.07 Ma and indicates reducing summer monsoon influence as chlorite and quartz concentrations increase and illite concentration, as well as the < 2 μm size fraction, decreases. The mineralogical trends and differences between loess and paleosols units suggest different source areas in the last 0.5 Ma.  相似文献   

17.
An integrated dust storm modeling system is developed for the prediction of dust storms. The system couples a wind erosion scheme, a dust transportation model and the Penn State/UCAR modeling system (MM5) with a geographic information database. The system can be used for the prediction of dust emission rate and dust concentration associated with individual dust storm events. Two severe dust storm events occurred in spring 2002, one on the 19th–22nd of March and the other on the 6th–9th of April. The integrated modeling system is used to simulate the two events. The numerical results are compared with surface weather records and satellite images and good agreement is found between the model results and observation in dust concentration distribution and evolutions. The Gobi Desert in southern Mongolia and the Badain Jaran Desert, Tengger Desert and Hunshandake sandy land in Inner Mongolia (China) are identified to be the dust sources for the two events. The dominant modes of dust particles over western Inner Mongolia and Mongolia are from 2 to 11 μm in size, and 2 to 22 μm over Beijing and its surrounding area. The emission of particles in the 2–11 μm size range is found to be most important for Northeast Asian dust storms.  相似文献   

18.
A previous study of Fox [Fox, A.N. 1993. Snowline altitude and climate at present and during the Last Pleistocene Glacial Maximum in the Central Andes (5°–28°S). Ph.D. Thesis. Cornell University.] showed that for a fixed 0 °C isotherm altitude, the equilibrium-line altitude (ELA) of the Peruvian and Bolivian glaciers from 5 to 20°S can be expressed based on a log–normal expression of local mid-annual rainfall amount (P). In order to extrapolate the function to the whole Andes (10°N to 55°S) a local 0 °C isotherm altitude is introduced. Two applications of this generalised function are presented. One concerns the space evolution of mean inter-annual ELA for three decades (1961–1990) over the whole South American continent. A high-resolution data set (grid data: 10′ for latitude/longitude) of mean monthly air surface temperature and precipitation is used. Mean annual values over the 1961–1990 period were calculated. On each grid element, the mean annual 0 °C isotherm altitude is determined from an altitudinal temperature gradient and mean annual temperature (T) at ground level. The 0 °C isotherm altitude is then associated with the annual precipitation amount to compute the ELA. Using computed ELA and the digital terrain elevation model GTOPO30, we determine the extent of the glacierised area in Andean regions under modern climatic conditions. The other application concerns the ELA time evolution on Zongo Glacier (Bolivia), where inter-annual ELA variations are computed from 1995 to 1999. For both applications, the computed values of ELA are in good agreement with those derived from glacier mass balance measurements.  相似文献   

19.
The microcharcoal content (particles < 180 µm) of overlapping sedimentary sequences from two crater lake basins in central Turkey are used to reconstruct the regional fire history of the East Mediterranean oak–grass parkland zone from the Last Glacial Maximum to the present-day. These results are correlated with stable isotope and pollen data from the same cores in order to assess the changing role of climate, vegetation and human activity in landscape burning. This indicates that climatically-induced variation in biomass availability was the main factor controlling the timing of regional fire activity during the Last Glacial–Interglacial climatic transition, and again during Mid-Holocene times, with fire frequency and magnitude increasing during wetter climatic phases. Spectral analysis of the Holocene part of the record from Eski Acıgöl indicates significant cyclicity with a periodicity of ~ 1500 years that may be linked with large-scale climate forcing. Although proto-agricultural societies were established in this region as early as 10,000 years ago, it is only during the last two to three millennia that the pacing of wildfire cycles appears to have become decoupled from climate and linked instead to human-induced changes in land cover and fuel load availability.  相似文献   

20.
Airborne laser altimetry survey of Glaciar Tyndall, Patagonia   总被引：1，自引：1，他引：0  
The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of − 3.1 ± 1.0 m a− 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of − 7.7 ± 1.0 m a− 1 at the calving front at 50 m a.s.l. and minimum values of between − 1.0 and − 2.0 ± 1.0 m a− 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at  600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of − 3.2 m a− 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions.  相似文献   

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