Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties     

Diana Jordanova  & Nikolai Petersen 《Geophysical Journal International》1999,138(2):520-532

A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess ( Hus & Han 1992 ; Maher & Thompson 1992 ; Heller & Evans 1995 ; Hunt et al. 1995 ). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S₀ and first and second palaeosols S₁ and S₂ developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force H _c showing even higher values than those of the parent loess material. The older palaeosols (S₄ to S₆ ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200 °C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.  相似文献   

中国末次冰盛期以来湖泊水量变化及古气候变化机制解释   总被引：2，自引：0，他引：2  

薛滨  于革 《湖泊科学》2005,17(1):35-40

中国古湖泊数据库收录的42个湖泊,提供的湖泊水量每千年变化的空间信息,可以用来系统分析中国区域末次冰盛期以来大气环流变化的状况.研究结果表明:我国西部从末次冰盛期以来直至全新世中期均为较湿润的气候状况,推测冰期内的湿润条件主要与西风带的降水以及低温低蒸发密切相关,而全新世主要为夏季风降水增加所致;全新世晚期气候趋干明显.我国东部的大部分区域在冰盛期和晚冰期较为干旱;只是在全新世有效降水状况才有大幅度的改善,全新世中期夏季风降水的效应仍然相当显著,控制的范围可达整个中国西部,同时位于现代季风气候区的中国东部,有效降水的峰值区的变化似乎存在从北往南的穿时性,南方有效降水峰值出现在晚全新世.而西南季风区湿润状况的明显改善发生在晚冰期,比东南季风区发生的早,显然这与两个季风系统的相互消长有一定的关系.我国东北区的湿润状况改善的也较早,显示了独特的季风气候机制.  相似文献   

西藏扎布耶盐湖晚更新世沉积学及古气候意义   总被引：7，自引：1，他引：6  

李明慧  郑绵平 《湖泊科学》2005,17(1):24-27

从粘土矿物、碳酸盐矿物、孢粉、介形虫等方面,讨论了西藏扎布耶盐湖晚更新世的气候条件.603±2.7kaB.P.、142.2±7.4kaB.P.时期,扎布耶为间或有一定盐度的淡水湖,前期湖水含盐量比后期高.气候寒冷,风化作用不强,物理风化作用为主.高岭石(60.3±2.7kaB.P.)和厌热性介形虫Radix.Bowelli(142.2±7.4kaB.P.)的出现说明,142.2±7.4kaB.P.时期气候偏干冷,气温比现在低;60.3±2.7kaB.P.气候偏湿冷.比较方解石中Mg含量可知,60.3±2.7kaB.P.比142.2±7.4kaB.P.的湖水盐度高.  相似文献   

海南岛双池玛珥湖沉积中的碳、氮地球化学记录及其环境意义   总被引：5，自引：3，他引：5       下载免费PDF全文

白雁  刘春莲  郑卓  王建华  杨小强  王斌 《古地理学报》2003,5(1):87-93

根据岩芯的碳、氮含量及其比值分析并结合其他气候替代指标的分析结果，恢复了海南岛双池玛珥湖末次冰消期以来的古气候变化。全新世大暖期始于7,400a BP，结束于2,700a BP左右，期间热带地区气候湿热，植物茂盛，双池玛珥湖主要处于沼泽化阶段，蒸发量与降雨量持平。但大暖期中仍存在气候的快速波动事件，约7,200a BP和7,000a BP前后、6,000～5,900a BP、4,300～4,000a BP 期间气候转凉，有机质生产力降低。大暖期前后，分别有明显的凉温但季节性暴雨频繁的气候（8,000～7,400a BP、2,700～2,300a BP），表示气候模式发生变化，降雨量相对大于蒸发量。此时双池玛珥湖处于深水期，由于气温降低以及湖水上升引起的湖边陡壁植被覆盖面积减少，植物生产力降低，导致了岩芯中有机质的低含量。  相似文献   

Palaeoclimatic record in the loess–palaeosol sequence of the Strelitsa type section (Don glaciation area,Russia) deduced from rock magnetic and palynological data     

Elena I. Virina  Friedrich Heller  Stanislav S. Faustov  Nathalia S. Bolikhovskaya  Robert V. Krasnenkov  Tatyana Gendler  Ernie A. Hailwood  Jozeph Hus 《第四纪科学杂志》2000,15(5):487-499

Until now, palaeoclimatic reconstructions for the major stages in the development of the Quaternary loess–palaeosol sequence on the Russian Plain have been based on pedological, palynological and faunal (vertebrates and molluscs) analyses. In order to demonstrate the palaeoclimatic influence on the magnetic properties of this sequence, the magnetic susceptibility signature of the Strelitsa type section in the Upper Don basin is compared with a detailed landscape – climate reconstruction of loess and soil from palynological data. Large amplitude fluctuations of palaeoclimate and palaeoenvironment are reflected clearly in the lithology and in the rock magnetic properties, which usually are enhanced in wet and warm interglacial periods, but stay at low levels during cold glacial epochs. Palynological climate zonation, however, is sometimes in conflict with the pedologic–magnetic record. Strong climate fluctuations, as indicated by changing pollen assemblages, are not always paralleled by corresponding changes in lithology and/or rock magnetic properties. Alternatively, light coloured illuvial horizons with low magnetic signal sometimes appear to have formed during early stages of interglacials, and the top parts of some palaeosols apparently formed during glacial stages. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Stable climatic conditions in central Germany during the last interglacial     

Tatjana Boettger  Frank W. Junge  Thomas Litt 《第四纪科学杂志》2000,15(5):469-473

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Late Quaternary vegetational and climatic change in the Popayán region,southern Colombian Andes     

Hermann Behling  Alvaro Jos Negret  Henry Hooghiemstra 《第四纪科学杂志》1998,13(1):43-53

Late Pleistocene and Holocene vegetational and climatic change have been studied palynologically at a site at 1750 m elevation in the subandean vegetation belt near Popayán, in the southern Colombian Andes. Time control on the pollen record is based on six AMS ¹⁴C ages, ranging from possibly Middle Pleniglacial time (around 50000 yr BP) to 1092 ± 44 yr BP. Because of the presence of two hiatuses only the Middle Pleniglacial and Late Holocene periods (the last 2300 yr BP) are represented. Pollen data indicate the presence of closed subandean forest during glacial time. Changes in the contribution of pollen originating from the uppermost and lowermost subandean forest belts, changes in the contribution of a number of other subandean forest taxa, and changes in species composition between the three pollen zones, suggest that the climate during the Middle Pleniglacial was markedly colder, and perhaps also wetter, than during the Late Holocene. Pollen assemblages from the Late Holocene indicate that the landscape has been affected by deforestation and agriculture since at least 2300 yr BP, but that human impact decreased in the last 780 yr BP. © 1998 John Wiley & Sons, Ltd.  相似文献   

A mid to late Holocene record of palaeoclimatic changes from Nal Sarovar: a palaeodesert margin lake in western India     

SUSHMA PRASAD  SHEELA KUSUMGAR  S. K. GUPTA 《第四纪科学杂志》1997,12(2):153-159

With a view to obtain palaeoclimatic data from a climatically sensitive region we have studied core samples from Nal Sarovar, a large shallow lake lying within the palaeodesert margin of Thar in western India. A combination of C/N ratios and δ¹³C on a radiocarbon-dated core section have been used as climatic proxies. A high-resolution record extending back to ca. 6.6 ka BP has been reconstructed. The data indicate that, in the past, climate has varied from drier to wetter than present on time-scales of few hundreds to few thousands of years. There are, however, significant differences on the timing of these wet and dry periods, when compared with the available data from lakes farther north, in Rajasthan. Further, it looks unlikely that during the 6.6 ka the catchment areas of Nal Sarovar experienced such a significant increase in rainfall as has been suggested for the Rajasthan lakes. Interestingly, drier periods in Nal Sarovar data appear to correlate well with periods of glacier expansion in Eurasia indicating that the palaeoclimatic variations recorded in Nal Sarovar are a regional feature. © 1997 John Wiley & Sons, Ltd.  相似文献   

Chironomid-based Lateglacial climatic reconstruction for southeast Scotland     

STEPHEN J. BROOKS  FRANCIS E. MAYLE  JOHN J. LOWE 《第四纪科学杂志》1997,12(2):161-167

This paper presents the first chironomid-based climatic reconstruction for the UK Lateglacial, obtained from a lake basin in southeast Scotland, Whitrig Bog. Comparison of chironomid, lithological, geochemical, and pollen data reveals a marked vegetation lag behind the warming following deglaciation; warm-water chironomid taxa (e.g. chironomus) had replaced cold-water chironomid taxa (e.g. Paracladius) prior to the arrival of shrubs and trees and when local soils were still poorly developed. In addition to clear evidence for the Younger Dryas Stadial, chironomid data also reveal two short-term cold episodes that punctuate the Lateglacial Interstadial, both of which are also reflected in pollen and lithological data as reversals from open birch scrub/woodland to open herbaceous tundra coinciding with inwash of minerogenic matter into the basin. © 1997 John Wiley & Sons, Ltd.  相似文献   

Multiproxy surface wetness records from replicate cores on an ombrotrophic mire: implications for Holocene palaeoclimate records     

Dan J. Charman  Dawn Hendon  Susan Packman 《第四纪科学杂志》1999,14(5):451-463

Ombrotrophic mires can provide records of palaeoclimate over the mid- to late- Holocene in several areas of the world. Their potential is currently partly limited by difficulties with scaling indices based on plant macrofossils and humification, and the need to account for the internal variability of the mire system. This paper explores the use of testate amoebae analysis as a third technique and assesses the minimum within-site variability by comparing results from two closely spaced cores. Reconstruction of surface wetness changes was carried out on cores from the centre of an intermediate raised-blanket mire, Coom Rigg Moss, Northumberland, by analysis of testate amoebae, plant macrofossils and humification. Surface wetness changes were expressed as mean annual water table changes inferred from testate amoebae assemblages, two separate indices based on plant macrofossils and percentage transmission of humification extracts. Comparisons between the proxies suggest good agreement of general trends in Sphagnum peats but some differences in monocot and ericaceous peats. The magnitude of surface wetness changes also differs within Sphagnum peats. The records from the separate cores converge over time and replicability between cores is best in the last 1000 yr. Changes over this period are similar to those shown by estimates based on documentary sources. Good agreement is obtained between a normalised plant macrofossil index and normalised reconstructed water-table values and it is suggested that this approach could form the basis for improved composite, multiproxy records from peatlands. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献