青海湖古气候与古环境未解重要科学问题的研究

The significance of Lake Qinghai as a key-site for the global paleoclimatic and paleoenvironmental reconstruction has been recognized since 1985 when a Sino-Swiss expedition successfully recovered high-resolution seismic profiles of 300 km and a number of sediment cores for the study. Among a number of field campaigns thereafter, the field investigation of 2005 is of prominence, which has recovered a total length of 548-m sediment cores with a GLAD800 drilling rig. Climate history over the past 32 ka is well reconstructed based on high-quality proxy records. However, uncertainty and controversy remained regarding MIS 3 paleoclimate condition, and it is yet unclear about what is the cause of the difference in sediment character between the eastern and southern sub-basins for the sediments deposited during the LGM. In addition, the δ13C-δ18O covariance has proven to be a useful tool for paleohydrological analysis of ancient lacustrine basin, yet the isotopic ratios from such a large, closed-basin lake appear invariant when using all the ratios of primary carbonates. The proposed project sets to investigate these key research issues, aiming (1) to figure out the paleo-lake size during MIS 3 for indicating whether the climate then was warmer and wetter than today, (2) to examine whether the deposition of the loess-like sediments is commonplace in the eastern sub-basin, representing the down-wind deposition in consequence of severely cold climate during the LGM, (3) to establish a δ18O record of authigenic carbonates as a proxy of the epilimnion δ18O for detecting its response to changes in water balance or moisture source, and (4) to unravel the mechanism of ?δ13C-δ18O covariance and to set up a method for identifying the original covariant trend of closed lakes. The ultimate goal of the isotope study is to establish a series of isotopic covariant trends being a function of the basin morphology, climate and hydrology.

青海湖古气候与古环境研究起步较早、起点较高、研究程度较深，是倍受国内外关注的关键研究点。然而，几个重要科学问题仍须深入探究：MIS 3古气候特征，LGM在东湖盆的沉积特征，以及δ13C-δ18O协变机理。依据高分辨率地震地层结构，选点钻探取岩芯，开展多学科综合研究，旨在1）验证MIS 3时段古湖区范围，2）探明东湖盆LGM沉积特征，了解极端干冷气候下各湖盆沉积亚相异同，3）用自生碳酸盐δ18O记录揭示冰后期上层湖水氧同位素值变化记录，与已有底栖介壳δ18O记录对比，求解它们对水化学、水位变化的综合反应，与水量平衡、西风/季风水汽来源变化的关联，4）系统测定碳酸盐碳氧稳定同位素，摸清δ13C-δ18O协同变化的机理，建立代表性协变趋势线的正确方法，为深入探究同位素协变趋势线与湖盆形态、气候、水文之间的关系提供创新成果。

青海湖是我国最大的内陆水体位于东亚夏季风外部边缘区，封闭半咸水大湖的水位和水化学变化历史敏感反映了区域气候变化，是西风和季风环流共同作用的结果。青海湖因此成为国内外关注度最高、最近十年中重大重点项目的关键研究点。本项目选择了几个未解重要科学问题开展研究，取得了以下重要结果。界定了LGM，MIS 3，MIS 4期间青海湖古湖岸线位置，发现MIS 4期间松散风砂沉积覆盖面积最大；MIS 3期间古青海湖扩展，但远没有达到现在的湖岸带，清楚指示MIS 3期间青藏东北部的气温、降水量都比全新世低；LGM期间的沉积物以碎屑为主，受极端干冷气候的影响夏季短暂，冬季冰封时间长，水位比MIS 3期间低，湖泊自生碳酸盐和有机产率皆很低；本项研究发现，MIS 4期间青海湖的干冷程度比LGM期间严酷很多，湖区被风砂大面积覆盖。需要古气候、古冰川、古环境研究学者高度关注。 获得的AMS 14C年代学数据准确界定了青海湖Younger Dryas（YD）干化气候时段（~12.9 to 11.7 ka），没有变冷的证据。越来越多的证据表明，YD发生在全球气候变化期，但绝非是退回到冰期的全球性事件。利用青海湖冰后期碳酸盐沉积序列摸清了d13C-d18O协变机理，提议采用正确的方法建立封闭湖泊可靠的同位素协变线，为准确分析和重建古湖盆水文特征服务。对比青海湖、大柴旦湖、黄旗海三个湖泊沉积记录的结果显示，LGM时段都反映极端干冷气候；末次冰消期各湖沉积物都以多变为基本特征；全新世开始时皆从碎屑向碳酸盐或盐湖相沉积快速转变。尽管沉积序列差异较大，千年尺度上对区域气候变化的反应很是相近。与国内外合作研究云南湖泊沉积的研究成果深化了西南季风演变和驱动机理、湖泊受自然与人类活动双重影响程度的了解。青海湖湖相白云石成因及其环境指示、硅藻的保存及其环境指示、叠层石水下平台的环境指示等项研究成果，对于全面深入系统了解青海湖环境演变及其与青藏东北部气候演变关联有着不可或缺的科学价值。