青藏高原中部湖泊沉积正构烷烃氢同位素特征及其古环境意义研究

The Tibetan Plateau is the focus of global geo-science research. The lakes on the Tibetan Plateau are quite sensitive to climate changes due to its unique natural environment. Biomarkers and their individual isotopes in lake sediments are well applied to reconstruct paleotemperature, paleovegetation and paleoprecipitation, which becomes one of important approaches for the research of global changes in the past. Zigetang Co and Lake Cuona are situated in the central Tibetan Plateau, southern Tanggula Mountains, and belong to Amdo County, Xizang Autonomous Region, China. The water supply in both lake systems mainly depend on precipitation and seasonal inflows and directly reflect the changes of southwest monsoon due to lacking glacial meltwater inputs in the catchment. This provides excellent sediments for palaeoclimatic and palaeoenvironmental reconstruction. Based on the advancements in the field of gas chromatography (GC)/thermal conversion (TC)/isotope ratio mass spectrometry (IRMS), n-alkanes from lake water, surface soils, plants and lake sediments and compound-specific hydrogen isotopes at Lake Cuona will be analyzed to explore the process of the source, transport and storage of the biomarkers in the lake system and its climatic and environmental significance. In addition, the reconstruction of effective moisture availability will be performed in the central Tibetan Plateau according to the variation in the amount of n-alkanes, n-alkane indicator ratios (e.g. carbon preference index (CPI), average chain length (ACL), aquatic/terrigenous ratio (ATR)), compound specific hydrogen isotope values and other environmental proxies. In comparison of other records, the project plans to further discuss different records between glacial lakes and non-glacial meltwater-fed lakes and its driving mechanism. Our records improve the understanding of environment evolution in non-glacial meltwater-fed lakes and complicated regional heterogeneity on the reconstruction of the Holocene monsoon dynamics on the Tibetan Plateau.

青藏高原是全球地球科学界瞩目的焦点，其独特的自然环境条件使得高原湖泊对气候变化尤为敏感。湖泊沉积物生物标志化合物及其单体同位素在恢复古温度、古植被、古降水等方面具有较好的应用前景，逐步成为过去全球变化研究的重要手段之一。青藏高原中部安多地区的兹格塘错及错那流域内无明显的冰川分布，湖泊补给主要依靠降水，直接反映了西南季风的强弱，是古气候/古环境重建的理想场所。本项目主要基于湖泊系统的现代过程研究，借助于气相色谱—质谱联用技术，阐明错那湖泊水体、湖区表土、流域植被及湖相沉积物正构烷烃及其氢同位素特征，探讨其气候、环境指示意义，并结合其他环境指标，恢复该流域晚冰期以来有效湿度变化。对比分析前人研究成果，进一步讨论冰川融水湖泊与非冰川融水补给湖泊所记录的环境信号的差异及其驱动机制，这对理解非冰雪融水补给湖泊的环境演化过程和高原气候系统的变化机制具有十分重要的学术价值。

本项目选取了青藏高原中部另一个未受冰川融水补给的湖泊——错那作为研究对象，对其湖泊水体、湖区表层土壤、流域植被及湖相沉积物正构烷烃及其氢同位素的组成进行了系统的研究和总结，更好地验证了兹格塘错沉积记录（已有研究成果）的可靠性与真实性，为青藏高原地区不同补给类型的湖泊气候环境重建提供了新的证据。得出如下研究成果：. （1）错那湖泊沉积岩芯中的正构烷烃分子分布特征与兹格塘错极为相似，以n-C15/16/17为主峰碳的短链正构烷烃占据主导地位，其丰度占正构烷烃总量的比例远远高于以n-C21/23/25 为主的中等链长及n-C27/29/31占优势的长链正构烷烃所占其比重，指示了湖泊自生浮游藻类与菌类等低等生物对湖泊沉积岩芯中的有机质贡献高于大型水生植物和陆生高等植物叶蜡所产生的有机质。. （2）错那湖泊沉积岩芯中的正构烷烃参数（如：碳优势指数CPI值和平均碳链长度ACL值），以及分别反映源区降水、湖区生态环境特征的正构烷烃n-C29、n-C17的δD值在时间序列上的变化趋势与兹格塘错基本趋于一致，进一步验证这一代用指标所记录的区域环境信号的准确性与真实性。. （3）与其邻近的冰川融水补给湖泊（如：纳木错、色林错及帕鲁错）记录相比较，全新世早期，纳木错、色林错及帕鲁错记录均指示气候较为暖湿，湖泊水位较高，兹格塘错及错那的沉积记录却揭示相对较为干旱的气候特征。这可能由于纳木错、色林错及帕鲁错在全新世早期气候变暖的条件下大量冰川融水注入湖泊，而兹格塘错及错那流域因缺乏冰川融水补给，太阳辐射引起的强烈蒸发超过西南季风所带来的降水，使得该区的有效湿度减少，湖水氢同位素更加富集。全新世中期，兹格塘错及错那记录显示气候较为湿润，有效湿度明显增加，与西风控制区（如：博斯腾湖及巴里坤湖）的气候特征极为相似。其水汽来源可能受到太阳辐射、西风环流、大陆冰盖的消融、季风叠加效应、局地环流等多种因素的影响。