西藏塔若错末次冰盛期以来介形类定量重建的湖面变化及其气候意义

The Tibetan Plateau is located at a triple junction of influences from the East Asian monsoon, the Indian monsoon and the Westerlies, which makes it sensitive to global climatic changes. However, it remains unclear to what degree the Indian monsoon and the Westerlies influence the environmental changes of the Tibetan Plateau at different periods. Lake sediment is a good geological archive for climate changes, and lake level change is an important indicator of changes in the hydrological status of lakes. Taro Co, a lake located in the transitional zone of the Indian monsoon and the Westerlies, is an ideal site to investigate the interaction of these two circulation systems. The present proposal is to establish an ostracod-inferred water depth transfer function with improved reliability based on a larger database of modern ostracod assemblages from the surface sediments and relevant environment variables of the lake, then to quantitatively reconstruct the record of lake level fluctuations at Taro Co since the Last Glacial Maximum (LGM). Meanwhile, the real lake level changes indicated by the OSL dating of palaeoshorelines will be used to check the results. On this basis, combining with the multi-proxy data (stable isotope and trace element of ostracod shell), the controlling factor and climate significance of lake level change can be clarified, and consequently, the history of hydrological and environmental changes in the region since the LGM can be reconstructed. The results of this project is of great significance in assessing the relative roles of the Indian monsoon and the Westerlies in the region, thus will provide new evidence for the interplay of the two atmospheric circulation systems on glacial-interglacial timescales.

青藏高原位于东亚季风、印度季风和西风环流交互影响区，对全球气候变化异常敏感。然而这两大环流系统在不同时期对青藏高原气候变化的影响程度仍不清楚。湖泊沉积物是记录气候变化的良好地质载体，而湖面变化是湖泊水文状态改变的重要指示器。塔若错，位于季风与西风边缘过渡区，是研究两大环流系统相互作用的理想地点。因此，本项目通过完善塔若错表层沉积物介形类属种组合与相关环境因子数据库，建立可信度更高的介形类-水深转换函数，定量重建塔若错末次冰盛期以来的湖面变化，并利用古湖岸堤光释光测年指示的湖泊水位变化对定量重建结果进行验证；在此基础上，结合介壳稳定同位素和微量元素等多指标综合分析，明确湖面变化的控制因素及其气候意义，重建该地区末次冰盛期以来的湖泊水文变化过程和气候变化历史。研究结果对揭示季风和西风环流对该地区气候变化的影响程度具有重要意义，亦可为冰期-间冰期时间尺度上季风与西风的相互作用提供新的证据。

青藏高原既受到亚洲季风环流的影响，同时也受到西风环流的控制。这两大环流系统深刻地影响着青藏高原地区的环境变化，但目前仍不清楚他们对高原不同地区的影响程度及其发生时间。为此，本文基于介形类属种分布与水深的关系，湖岸堤年代学反映的湖面变化，探讨了季风与西风环流对青藏高原西南部地区的影响过程。采自塔若错（31.1°N, 84.1°E）钻孔TR14（水深130 m，长11 m）的介形类属种组合，TIC、Ca、粒度、孢粉浓度等其它指标，结合塔若错及扎布耶周边31个湖岸堤和湖相沉积物的光释光测年结果，重建了末次盛冰期以来塔若错和扎布耶的湖面变化过程：.（1）29 ka BP之前，该时期湖面较高，高于现湖面60 m。26.3-16.5 ka BP，气候干冷，湖面逐渐下降，其中26.3-23.5 ka BP，无介壳出现，粒度出现高值，不利于介形类生存；23.5-20.3 ka BP，出现?Leucuocythere dorsotuberosa，指示湖泊盐度升高，湖面下降；20.3-16.5 ka BP，出现Limnocythere inopinata，指示湖泊湖面较低，盐度较高，该种的出现说明LGM时塔若错湖泊水位降到最低值。16.5-12.3 ka BP，进入末次冰消期，气候转暖，湖面不断上升，在16 ka BP时，塔若错湖面至少高于现在湖面30 m；而在13-12.3 ka BP，湖面下降，喜浅种Fabaeformiscandona gyirongensis的出现，对应于YD事件，此时湖面仅高于现湖面8 m。12.3-3.8 ka BP，进入全新世，气候转暖，湖面升高且比较稳定，其中，9.9-3.8 ka BP，介壳被溶蚀而没有保存下来，此时塔若错可能处于持续外流的一个状态，湖岸堤结果显示在7 ka BP时，塔若错和扎布耶仍然是统一的大湖，到6 ka BP左右，两湖湖面出现高差，扎布耶和塔若错通过出湖河流连通。3.8-0 ka BP，湖面下降，介壳出现，孢粉浓度升高并达到最大值，说明此时塔若错可能已经封闭。（2）该地区在26.3-16.5 ka BP时期，气候寒冷干旱，湖面较低，湖水盐度较高，可能是该时期主要受西风环流控制的原因。而在16.5 ka BP之后，进入冰消期，湖面不断升高，该时期可能主要受到印度季风的影响。