利用活塞采样器在位于青藏高原西南部塔若错60m水深处获得3．1m沉积岩芯．通过AMS14^C测年、花粉、粒度和无机碳分析，恢复了青藏高原西南部全新世以来的植被和气候变化．结果显示，全新世初期（10200～8900 cal a BP），植被由高寒草原向高寒草甸转换，气候由冷干向暖湿发展：全新世早期（8900～7400 cal a BP），植被由高寒草甸到高寒草原的转换，气候相对干旱，但偏暖：全新世中期（7400～3300cal a BP），植被从高寒草原向高寒草甸或草甸化草原转换，气候以冷湿为主：全新世晚期（3300c ala BP至今），植被逐渐被高寒草原取代，气候变得寒冷干旱．塔若错全新世早期和中晚期的特征气候事件表明，该地区明显受到西风区环境变化的影响，而在全新世中期更多受印度季风环流控制．尤其是中晚全新世塔若错的湖泊沉积环境记录对于进一步阐述西风／季风在该地区的影响程度具有非常重要的意义．
In this study,67 surface sediment samples collected from Nam Co in central Tibet were analyzed for total carbon,total organic carbon and total nitrogen,and 51 of these samples were also analyzed for n-alkanes.The origin and spatial distribution of organic matter were then investigated using these proxies,and the control factors responsible for the spatial distribution patterns and paleolimnological significance were discussed.The results indicated that the origin of organic matter in surface sediment of Nam Co is consistent with the sources of n-alkanes,which were primarily submerged plants,followed by terrestrial plants,and then aquatic algae and bacteria.The organic matter in surface sediments of the lake showed typical spatial variability.Because of the great influence of underwater topography,river inputs and water quality,the spatial distribution of organic matter is enriched from the source to the deposit center.This spatial variability of organic matter in the lake indicates that the sediments in different areas have different sensitivities to environmental changes,which is important to reconstruction of paleoenvironments and paleoclimate using lake sediment cores.
The content and hydrogen isotope ratios of n-alkanes extracted from recent lake surface sediments sampled from nine lakes in three different climate zones on the Southwestern Tibetan Plateau were analyzed.Values were compared with n-alkanes in plants from lake drainages,and dD values of meteoric water,lake water,and mean annual precipitation.The results showed that n-C23was predominantly derived from aquatic plants,and n-C27–n-C33from terrestrial higher plants.The average carbon chain length of n-C27–n-C33(ACL27–33)was positively correlated with the mean annual precipitation.dD values of the long-chain n-alkanes n-C29and n-C31of terrestrial origin(varying between-214%and-169%,and-226%and-185%,respectively)were inversely correlated with mean annual precipitation;but in accordance with the average annual variations in dD(OIPC),dD values of n-C31were strongly related to the dD values of growing season meteoric water(R2=0.74).The large difference between dD values of n-C23of aquatic origin and n-C31(an average of about 27%)demonstrates the enrichment of the lake water dD compared to precipitation,caused by strong evaporation in the semiarid–arid areas of the southwestern Tibetan Plateau.Average value of en C25 31=p(-95%)is evidently higher than the value observed in European wet regions(-128%);besides,en C31(about-116%)is constant along the study transect(SD=9),which indicates that n-C31is a useful proxy for the environment.
Xing HuLiping ZhuYong WangJunbo WangPing PengQingfeng MaJianfang HuXiao Lin
A 310-cm-long sediment core,covering the last10,200 years,was collected from Taro Co on the southwestern Tibetan Plateau and analyzed for pollen,grain size and total inorganic carbon content.The pollen data showed that vegetation changed from alpine steppe to alpine meadow during 10,200–8,900 cal a BP,to alpine steppe dominated by Artemisia during 8,900–7,400 cal a BP,to alpine meadow during 7,400–3,300 cal a BP and to alpine steppe after 3,300 cal a BP.Correspondingly,the pollen,grain size and total inorganic carbon content results revealed climatic change in this area over four stages.The initial stage was from 10,200 to 8,900 cal a BP,during which the climate changed from cold-dry to warm-humid.The second stage(8,900–7,400 cal a BP)was characterized by a warm and dry climate.However,at approximately7,400 cal a BP,the climate began to become cold and humid,which continued until 3,300 cal a BP.The last stage,from 3,300 cal a BP to present,was characterized as cold and increasingly arid.Climatic events of the early and mid-late Holocene showed that the area was significantly affected by the westerlies.However,the mid-Holocene climate in Taro Co was controlled by the Indian monsoon.The mid-late Holocene depositional environment record of Taro Co was very important to further elaborate the degree of influence by the westerlies or Indian monsoon.
