云南中甸纳帕海湖泊沉积物的磁化率及环境意义
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摘要
研究区母岩特性、搬运距离和搬运营力决定了磁性矿物主要赋存在砂砾级颗粒中。纳帕海湖泊沉积物的磁化率与颗粒粒径、沉积相和有机质含量存在密切的相关关系。滨浅湖环境中砂砾级颗粒含量增加以及波浪淘洗对磁性矿物的富集作用 ,磁化率较高 ;浅湖环境中粉砂增多 ,砂砾减少 ,磁化率较之滨浅湖有所降低 ;在低能的半深湖 /深湖环境中 ,由于缺乏磁性矿物的输入 ,磁化率再次降低 ;有机质含量很高的湖岸湖沼环境由于有机质的稀释作用磁化率值最低 ,磁化率等环境替代指标以及和北半球气候变化阶段、深海氧同位素阶段的对比 ,将纳帕海近 6ka年来划分成 6个主要的环境演化阶段 ,32kaB .P .(14 .99m)和 15kaB .P .(6 .0 6m)分别是环境和气候变化的重要转折点。 32~ 15kaB .P .期间出现的高湖面表明 ,虽然大气温度下降但有效湿度增加 ,全新世湖泊的急剧萎缩预示在大气温度增加的同时有效湿度降低 ,暖偏干构成了研究区全新世气候演化的特色。
Magnetic susceptibility measurements have been made on a long sediment core from Napahai, northwestern Yunnan, China. These results are set alongside those derived from total organic carbon and granulometric analysis to reconstruct the environmental and climatic processes recorded in the radiocarbon dated sediment column. It is shown that the variations of magnetic susceptibility in this sediment core are mainly the result of changes in size constitution, hydrological and sedimentological regimes and organic content, implying shifts in the sedimentary environment and climatic condition. Decided by the characterization of sediment source and proximal transport, the magnetic minerals, in this case, are mainly concentrated in coarser sand size fractions. The values of magnetic susceptibility in an organic free shallow lake correlate positively with coarser sediment. The proportions of magnetic minerals are strongly linked to the increase of runoff around the catchment and high energy conditions in the environment. The magnetic minerals are mainly concentrated in silty sand in low energy conditions below wavebase and the values of magnetic susceptibility decrease due to the lack of coarser sediment, the major carrier of magnetic minerals. The magnetic susceptibility has no definitely link to a fixed grain size fraction in lakeshore environment and extremely low values of magnetic susceptibility are encountered due to the dilution of organic matter. Six major environmental sages during the last 60,000 years from the core record have been identified on the basis of magnetic susceptibility. Two major environment shifts occurred in approximately 32 ka B. P. (14.99?m) and 15 ka B.P. (6.06?m) respectively, in response to climate change. Cold/moist and warm/dry assemblages dominated the area and a typical cold/moist climate occurred from 32 ka B. P. to 15 ka B. P. The incline of precipitation during this stage is due to an increased mass exchange between the cold/dry air of northern continent and the water vapor from the northern Indian ocean.
Magnetic susceptibility measurements have been made on a long sediment core from Napahai, northwestern Yunnan, China. These results are set alongside those derived from total organic carbon and granulometric analysis to reconstruct the environmental and climatic processes recorded in the radiocarbon dated sediment column. It is shown that the variations of magnetic susceptibility in this sediment core are mainly the result of changes in size constitution, hydrological and sedimentological regimes and organic content, implying shifts in the sedimentary environment and climatic condition. Decided by the characterization of sediment source and proximal transport, the magnetic minerals, in this case, are mainly concentrated in coarser sand size fractions. The values of magnetic susceptibility in an organic free shallow lake correlate positively with coarser sediment. The proportions of magnetic minerals are strongly linked to the increase of runoff around the catchment and high energy conditions in the environment. The magnetic minerals are mainly concentrated in silty sand in low energy conditions below wavebase and the values of magnetic susceptibility decrease due to the lack of coarser sediment, the major carrier of magnetic minerals. The magnetic susceptibility has no definitely link to a fixed grain size fraction in lakeshore environment and extremely low values of magnetic susceptibility are encountered due to the dilution of organic matter. Six major environmental sages during the last 60,000 years from the core record have been identified on the basis of magnetic susceptibility. Two major environment shifts occurred in approximately 32 ka B. P. (14.99?m) and 15 ka B.P. (6.06?m) respectively, in response to climate change. Cold/moist and warm/dry assemblages dominated the area and a typical cold/moist climate occurred from 32 ka B. P. to 15 ka B. P. The incline of precipitation during this stage is due to an increased mass exchange between the cold/dry air of northern continent and the water vapor from the northern Indian ocean.
引文
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[26] SirockoF ,GarbeSchonbergD ,MclntyreAetal.Teleconnectionsbetweenthesubtropicalmonsoonsandhigh latitudeclimatesduringthelastdeglaciation[J].Science,1996,272:526-529.
[27] DansgaardW ,JohnsenSJ ,ClausenHBetal.Evidenceforgener alinstabilityofpastclimatefromaGRIPicecore[J].Nature,1993,364:203-207.
[28] 羊向东,王苏民,童国榜,等.云南鹤庆古湖晚更新世的孢粉记录及其古气候学意义[J].第四纪研究,1998,(4):335~343.
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[30] 于革,薛滨,王苏民,等.末次盛冰期中国湖泊记录及其气候意义[J].科学通报,2000,45:250~255
[2] OldfieldF ,BarnoskyC ,LeopoldEBetal.Mineralmagneticstudiesoflakesediments[J].Hydrobiologia,1983,103:37-44.
[3] ThompsonR ,OldfieldF .EnvironmentalMagnetism[M ].London:GeorgeAllen&Unwin.1986:10-20.
[4] HironsKR ,ThompsonR .Palaeoenvironmentalapplicationofmag neticmeasurementsfrominter drumlinhollowlakesedimentsnearDungannon,Co.Tyrone,NorthernIreland[J].BOREAS ,1986,15:117-135.
[5] YuLZ ,OldfieldF ,WuYS ,etal.PaleoenvironmentalimplicationsofmagneticmeasurementsonsedimentcorefromKunmingBasin,SouthwestChina[J].JournalofPaleolimnology,1990,3:95-111.
[6] OldfieldF .Environmentalmagnetism-Apersonalperspective[J].QuaternaryScienceReviews,1991,10:73-85.
[7] 吴瑞金.湖泊沉积物磁化率、频率磁化率的环境意义—以青海湖、岱海近代沉积为例[J].湖泊科学,1993,5(2):128~135.
[8] DearingJA ,ElnerJKandHappey-woodCM .RecentsedimentfluxanderosionalprocessesinaWelshuplandlakecatchmentbasedonmagneticsusceptibilitymeasurements[J].QuaternaryResearch,1981,16:356-372.
[9] DearingJAandFlowerRJ .Themagneticsusceptibilityofsediment ingmaterialtrappedinLoughNeagh,NorthernIreland,andit’sero sionalsignificance[J].Limnol.Oceanogr,1982,27:969-975.
[10] 胡守云,王苏民,AppelE ,等.呼伦湖湖泊沉积物磁化率变化的环境磁学机制[J].中国科学(D辑),1998,28:334~339.
[11] AnZS ,KuklaG ,Porteretal.MagneticsusceptibilityevidenceofmonsoonvariationontheLoessPlateauofcentralChinaduringthelast130,000Years[J].QuaternaryResearch,1991,36:29-36.
[12] 王苏民,窦鸿身.中国湖泊志[M].北京:科学出版社,1998.
[13] 张谊光.横断山区气候区划[J].山地研究,1989,7:28~30.
[14] 王苏民,王云飞,吉磊,等.若尔盖盆地湖泊深钻岩芯记录[A].施雅风,李吉均,李炳元(主编).青藏高原晚新生代隆升与环境变化.广东:广东科技出版社,1998.161~213.
[15] MaherBA .Magneticpropertiesofsomesyntheticsub-micromag netites[J].GeophysicalJournal,1988,94:83-96.
[16] Bj rckS ,DearingJAandJonssonA .MagneticsusceptibilityoflateWeichseliandepositsinSoutheasternSweden[J].Boreas,1982,111:99-111.
[17] 王建,刘泽纯,姜文英,等.磁化率与粒度、矿物的关系及其古环境意义[J].地理学报,1996,51(2):155~163.
[18] HiltonJ .QuaternaryResearch,1987,27:160-166.
[19] 张树夫,肖家仪,俞立中,等.沉积物矿物磁性测量在古环境研究中的应用[J].地理科学,1991,11(2):182~193.
[20] 刘泽纯,黄巧华.昆明盆地两孔岩芯的沉积特征及环境磁学分析[J].沉积学报,1992,10:54~61.
[21] 黄春长.环境变迁[M].北京:科学出版社,1998.
[22] ColinC .Reconstructiondur啨gimedel’啨rosiondescha?neshi malayenneetbirmanesaucoursdeuxdernierscyclesclimatiques(300dernierska)[D].Memoiredel’UniversiteenGeologieetGeochimieSedimentaires.1997,272p.
[23] 姚檀栋,ThompsonLG ,施雅风,等.古里雅冰芯中末次间冰期以来气候变化记录研究[J].中国科学,D辑,1997,27:447~452.
[24] 姚檀栋.末次冰期青藏高原的气候突变—古里雅冰芯和格陵兰GRIP冰芯对比研究[J].中国科学,D辑,1999,29:175~184.
[25] 吴敬禄,李世杰,王苏民等.若尔盖盆地兴错湖沉积记录揭示的近代气候与环境[J].湖泊科学,2000,12(4):291~296.
[26] SirockoF ,GarbeSchonbergD ,MclntyreAetal.Teleconnectionsbetweenthesubtropicalmonsoonsandhigh latitudeclimatesduringthelastdeglaciation[J].Science,1996,272:526-529.
[27] DansgaardW ,JohnsenSJ ,ClausenHBetal.Evidenceforgener alinstabilityofpastclimatefromaGRIPicecore[J].Nature,1993,364:203-207.
[28] 羊向东,王苏民,童国榜,等.云南鹤庆古湖晚更新世的孢粉记录及其古气候学意义[J].第四纪研究,1998,(4):335~343.
[29] 蒋雪中,王苏民,羊向东.云南鹤庆盆地30ka以来的古气候与环境变迁[J].湖泊科学,1998,10(2):10~16.
[30] 于革,薛滨,王苏民,等.末次盛冰期中国湖泊记录及其气候意义[J].科学通报,2000,45:250~255
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