中国近千年来的水文气候变化大致以暖期时季风区相对湿润而西北西风区相对干旱为特征,处在季风边缘区的青海湖对验证此特征因此显得特别重要.然而相互不一致的青海湖研究结果还时常出现,多数时候与沉积物年代及所采用指标的解释的不确定性有关.本文重新分析了青海湖东部靠近沙岛水深5m处的一根2m的沉积柱里的有机指标,长链烯酮和正构烷烃的分布特征.长链烯酮U37K+古温度指标、％C37∶4古盐度指标和直链烷烃的水生植物比(Paq)指标显示当代暖期(1850～ 2000A.D.)和中世纪暖期(800 ～ 1400A.D.)时湖水相对温暖,盐度较低和湖水深度增加,而小冰期时(1400～ 1850A.D.)相反;并且中世纪暖期相对当代暖期要温暖湿润,证实了以前从湖心深水沉积柱获得的烯酮结果.本文结果也显示至少在盐度变化明显的青海湖长链烯酮古温度指标不包括C37∶4可能更合理一些;另外,气候从冷到暖的转变过程中,湖水温度和盐度变化似乎不同步,可能与径流输入的突然性增加有关.%Hydroclimatic changes in China over the past millennium are characterized by the association of warm periods with wetter conditions in monsoonal regions but with drier conditions in westerly regions.Lake Qinghai,located at the margin of monsoonal regions,is thus critical to evaluating this proposal by studying its sediment archive.However,paleoclimatic results from this lake are still controversial,mostly due to uncertainties in sediment chronology and proxy interpretations.We here again analyze biomarkers,long-chain alkenones and n-alkanes,from a 2m sediment core,newly retrieved from the eastern lake (37°02′17.80″N,100°27′14.07″E),at a relatively shallow (5m) water depth.The core chronology was established with 14C dates,indicating the core spanning the last ca.1600 years.We have analyzed sediment materials at every one centimeter,with 204 samples in total,and based on our chronology,achieved a sampling resolution of ca.8 years.Over the past 1600 years,alkenone UK+37 values varied between 0.15 and 0.50,and ％C37:4 varied between 15％ and 80％,much larger than the range documented previously from a core retrieved from the lake center of the same lake (0.1 unit for U37K+ and 15％ ～45％ for ％C37:4).Paq (proportion of aquatic plants) values range from 0.2 to 0.7.High U37K+,％C37:4 and Paq values occurred during the current warm period (since 1850A.D.)and the Medieval Warm Period (800～1400 A.D.),while during the Little Ice Age (1400～1850A.D.),those values showed little variability and remained low.During the transition from colder toward warmer conditions (ca.1850A.D.and 800A.D.),it appears that U37K+ and Paq values increased first while ％C37∶4 changes lagged by ca.50～100 years (ca.900A.D.and 1900A.D.).Thus the biomarker records of temperature (U37K+),salinity (％C37:4) and water depth (Paq) suggest that during the current warm period and the Medieval Warm Period,lake water was warmer and fresher and water level was higher,while being the opposite during the Little Ice Age.Despite different magnitude of changes in the alkenone indices documented from the shallow and deep water cores,the pattern of their changes is largely consistent.It confirms that in Lake Qinghai,warmer conditions were generally associated with wetter/fresher conditions and vice versa,the typical pattern in monsoonal regions and opposite to the association in westerly regions.It also appears that the extent of warmth and wetness during the current warm period has not exceeded that during the Medieval Warm Period.％C37:4 value,up to 80％,during the Medieval Warm Period suggest extremely fresh conditions,at least in nearshore shallow waters in Lake Qinghai at the time.Our results further suggest that in Lake Qinghai with substantial salinity changes in the past,alkenone C37:4 should not be included for temperature calculation.Lastly,during climate transition from cold toward warm stages,salinity changes appear to lag temperature changes,possibly due to abrupt riverine input.If correct,then our current warm period is still at a transitional stage and has not entered into a stable warm stage yet.
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