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61.
中国西部地区全新世自然环境演变序列与特征 总被引:4,自引:0,他引:4
根据湖泊、山地冰川、黄土与古土壤序列、泥炭及沙漠演变等多种地质记录的综合对比分析,重建和恢复了我国酉部地区全新世以来的自然环境演变序列及特征。全新世期间我国西部地区环境演变存在着以水热配置关系不同为突出特征的两种不同的演变模式:即以新疆北部地区为代表的西风型和以青海湖及青藏高原为代表的季风型模式。其成因同区域性特征及大气环流形势密切相关。并划分出我国西部地区全新世自然环境演变过程中的几个重要时间界线,即距今12ka或10ka、9ka、3ka和0.5ka。众多地质记录间良好的对比关系对于深入理解我国西部全新世气候环境的演变过程,预测其变化趋向具有十分重要的意义。 相似文献
62.
The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan 总被引:1,自引:0,他引:1
This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition
and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black
pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits
of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence
of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical
spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the
pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with
very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in
thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin
section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting
of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred
from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high
water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and
low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption
probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.
Received: 3 February 1997 / Accepted: 28 July 1997 相似文献
63.
A multi‐proxy approach was used to examine the geomorphic dynamics and environmental history of an upper deltaic ?oodplain tract in the Sacramento–San Joaquin Delta, California. Three long cores were collected from the McCormack–Williamson Tract (MWT) and these cores were analyzed for bulk density, loss‐on‐ignition, ?ne (clay and silt) content, Al concentration, magnetic susceptibility, pollen, and charcoal. Radiocarbon dates obtained for the cores were converted into calendar years and an age–depth model was constructed. Long‐term vertical accretion and sedimentation rates were estimated from the age–depth model. Cross‐core relations show that coarse sediment generally accumulates more rapidly and has greater magnetic susceptibility compared to ?ne sediment. Percentage ?ne and LOI data show a strong linear relationship that indicates ?ooding is the primary mechanism for the deposition of particular organic matter on the ?oodplain and that landscape wash load has contributed a highly consistent fraction of persistent organic matter averaging 5·5 per cent to the site. Down‐core grain size pro?les show two hydrological domains in the cores, namely millennial ?ne–coarse ?uctuations superimposed on general up‐core ?ning. Coarse sediment is viewed as channel or near‐channel overbank deposits, whereas ?ne deposits are considered to be distal overbank ?ood deposits. The coarse–?ne ?uctuations are indicative of changing depositional settings as channels migrated laterally across MWT, whereas the upward ?ning trend re?ects a combination of self‐limiting overbank deposition as ?oodplain elevation increased and decreasing competence as sea‐level rise reduced ?ood‐pulse energy slopes. MWT has been cross‐cut and incised numerous times in the past, only to have the channels abandoned and subsequently ?lled by ?ne sediment. The channels around MWT attained their modern con?guration about 4000 years ago. MWT likely came under tidal in?uence at about 2500 cal BP. Wetlands have recently developed on MWT, but they are inorganic sediment dominated. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
64.
He Xiubin Tian Junliang Tang Keli Sun Jianzhong John A Matthews 《Journal of Asian Earth Sciences》2004,22(5):455
Micromorphology, heavy minerals, pollen and soil properties were examined in a typical Holocene loess profile in the north of the Loess Plateau, consisting of a palaeosol (locally known as the Ansai palaeosol) underlain by the Malan loess and overlain by modern loess. The palaeosol consists of an upper humus-rich (AB) horizon over a clay-rich (Bt) horizon. The humus-rich horizon is intensely weathered, contains precipitated calcitic material derived from the overlying modern loess, and has both high pollen content and diversity. Clay coatings in the clay-rich horizon indicate formation by eluviation–illuviation of clay. Pedogenetic characteristics and pollen analysis imply that the vegetation during the time of most intensive soil development is likely to have been a warm-temperate forest. A complex interpretation of the loess–palaeosol sequence recognizes several development stages. The phase of soil development, maximum and minimum limiting ages for which are ca. 8800 and 4400 14C years BP, involved continuing but reduced loess deposition, and successive Holocene bio-climatic environment overprinted new features on the palaeosol. 相似文献
65.
The global climate changings at the end of Pleistocene led to extinction of the typical representatives of Mammoth fauna–mammoth, woolly rhinoceros, wild horse, bison, muskox, cave lion, etc.–on the huge territories of Northern Eurasia. Undoubtedly the Mammoth fauna underwent pressure from the Upper Paleolithic Man, whose hunting activity also could play the role in decreasing the number of mammoths and other representatives of megafauna (large mammals). Archaeological data testify that the typical representatives of Mammoth fauna were the Man's hunting objects only till the end of the Pleistocene. Their bone remains are not usually found on the settlements of Mesolithic Man. Formerly it was supposed that the megafauna of ‘Mammoth complex’ was extinct by the beginning of Holocene. Nevertheless the latest data testify that the global extinction of the Mammoth fauna was sufficiently delayed in the north of Eastern Siberia. In the 1990s some radiocarbon data testified that the mammoths on the Wrangel Island existed for a long time during the Holocene from 8000 till 3700 y. BP. The present radiocarbon data show that wild horses inhabited the north of Eastern Siberia (the lower stream of the Enissey river, the Novosibirskie Islands, the East Siberian sea-shore) 3000–2000 y. BP. Musk-oxen lived on the Taimyr Peninsula and the Lena River delta about 3000 y. BP. Some bison remains from Eastern Siberia belong to the Holocene. The following circumstances could promote the process of preservation of the Mammoth fauna representatives. The cool and dry climate of this region promotes the maintenance of steppe associations – habitats of those mammals. The Late Paleolithic and Mesolithic settlements are not found in the Arctic zone of Eastern Siberia from the Taimyr Peninsula to a lower stream of the Yana River; they are very rare in the basins of the Indigirka and Kolyma Rivers. So, the small number of the Stone Age hunting tribes on the North of Eastern Siberia was another factor in the long-term preservation of some Mammoth fauna representatives. 相似文献
66.
Soil liquefaction and associated ground failures have been a major source of damage during the past earthquakes. The risk of liquefaction and associated ground deformation can be reduced by various ground-improvement methods including the stone column (gravel drain) technique. This paper presents the current state of the stone column technologies as a liquefaction countermeasure. A comprehensive review is provided aiming to: (a) identify key considerations for the general use of stone columns as a liquefaction countermeasure, (b) provide insights for design and construction, (c) compile the latest research developments, and (d) identify sources of useful information. Case histories of field applications and observed field performance are cited to portray different stone column applications and observed effectiveness. The paper identifies areas where more research is needed and includes recommendations for future research and development. 相似文献
67.
Although recent studies have recognized peatlands as a sink for atmospheric CO2, little is known about the role of Siberian peatlands in the global carbon cycle. We have estimated the Holocene peat and carbon accumulation rate in the peatlands of the southern taiga and subtaiga zones of western Siberia. We explain the accumulation rates by calculating the average peat accumulation rate and the long-term apparent rate of carbon accumulation (LORCA) and by using the model of Clymo (1984, Philosophical Transactions of the Royal Society of London Series B 303, 605-654). At three key areas in the southern taiga and subtaiga zones we studied eight sites, at which the dry bulk density, ash content, and carbon content were measured every 10 cm. Age was established by radiocarbon dating. The average peat accumulation rate at the eight sites varied from 0.35 ± 0.03 to 1.13 ± 0.02 mm yr−1 and the LORCA values of bogs and fens varied from 19.0 ± 1.1 to 69.0 ± 4.4 g C m−2 yr−1. The accumulation rates had different trends especially during the early Holocene, caused by variations in vegetation succession resulting in differences in peat and carbon accumulation rates. The indirect effects of climate change through local hydrology appeared to be more important than direct influences of changes in precipitation and temperature. River valley fens were more drained during wetter periods as a result of deeper river incision, while bogs became wetter. From our dry bulk density results and our age-depth profiles we conclude that compaction is negligible and decay was not a relevant factor for undrained peatlands. These results contribute to our understanding of the influence of peatlands on the global carbon cycle and their potential impact on global change. 相似文献
68.
Oxygen isotope profiles along the growth axis of fossil bivalve shells of Macoma calcarea were established to reconstruct hydrographical changes in the eastern Laptev Sea since 8400 cal yr B.P.. The variability of the oxygen isotopes (δ18O) in the individual records is mainly attributed to variations in the salinity of bottom waters in the Laptev Sea with a modern ratio of 0.50‰/salinity. The high-resolution δ18O profiles exhibit distinct and annual cycles from which the seasonal and annual salinity variations at the investigated site can be reconstructed. Based on the modern analogue approach oxygen isotope profiles of radiocarbon-dated bivalve shells from a sediment core located northeast of the Lena Delta provide seasonal and subdecadal insights into past hydrological conditions and their relation to the Holocene transgressional history of the Laptev Sea shelf. Under the assumption that the modern relationship between δ18Ow and salinity has been constant throughout the time, the δ18O of an 8400-cal-yr-old bivalves would suggest that bottom-water salinity was reduced and the temperature was slightly warmer, both suggesting a stronger mixture of riverine water to the bottom water. Reconstruction of the inundation history of the Laptev Sea shelf indicates local sea level ∼27 m below present at this time and a closer proximity of the site to the coastline and the Lena River mouth. Due to continuing sea level rise and a southward retreat of the river mouth, bottom-water salinity increased at 7200 cal yr B.P. along with an increase in seasonal variability. Conditions comparable to the modern hydrography were achieved by 3800 cal yr B.P. 相似文献
69.
Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry semideciduous forests. Lowered atmospheric CO2 concentrations, in combination with a longer dry season, caused expansion of dry forests and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene, although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 14C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 2000 14C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2-3 km beyond the lake shore, whereas the grass- and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the Milankovitch Astronomical Theory. 相似文献
70.
Records of stable carbon isotopes (δ13C) are presented from cores collected from four San Francisco Bay marshes and used as a proxy for changes in estuary salinity. The δ13C value of organic marsh sediments are a reflection of the relative proportion of C3 vs. C4 plants occupying the surface, and can thus be used as a proxy for vegetation change on the marsh surface. The four marshes included in this study are located along a natural salinity gradient that exists in the San Francisco Bay, and records of vegetation change at all four sites can be used to infer changes in overall estuary paleosalinity. The δ13C values complement pollen data from the same marsh sites producing a paleoclimate record for the late Holocene period in the San Francisco Bay estuary. The data indicate that there have been periods of higher-than-average salinity in the Bay estuary (reduced fresh water inflow), including 1600-1300 cal yr B.P., 1000-800 cal yr B.P., 300-200 cal yr B.P., and ca. A.D. 1950 to the present. Periods of lower-than-average salinity (increased fresh water inflow) occurred before 2000 cal yr B.P., from 1300 to 1200 cal yr B.P. and ca. 150 cal yr B.P. to A.D. 1950. A comparison of the timing of these events with records from the California coast, watershed, and beyond the larger drainage of the Bay reveals that the paleosalinity variations reflected regional precipitation. 相似文献