中文摘要：

对青藏高原北部地区新生代陆相地层中湖相碳酸盐岩进行了系统采样分析，以碳氧同位素作为古环境和古气候变化的替代指标，试图从古湖泊演化的角度，阐明高原新生代早期环境演变历史和过程。研究结果表明，该区所有分析样品中生物碎屑灰岩类样品的δ^18O和δ^13C显示最低值，反映它们沉积在一个水体滞留时间短的开放性淡水湖泊系统中。泥晶灰岩类样品的δ^18O和δ^13C之间具有正相关关系，表明它们发育在蒸发作用明显的封闭性成水湖泊体系中，而叠层石灰岩类明显富集^13C可能与微生物活动有关。从始新世到渐新世至中新世，湖相碳酸盐岩δ^18O的值具有逐渐增大的趋势，δ^13C则由古近纪的负值变为新近纪的正值，反映该区古湖泊系统经历了一个由开放到封闭的过程，预示古近纪到新近纪之交高原古气候格局发生了重大变革。

英文摘要：

Through systematic sampling and analyses of the lacustrine carbonate rocks in the Cenozoic continental strata in the northern Qinghai-Tibetan Plateau, by using the carbon and oxygen isotopes as proxies, the environmental evolution of the Early Cenozoic is revealed according to the palaeo-lacustrine evolution. The results indicate that the δ^18O and δ^13C in the bioclastic limestone samples have the lowest values in all the samples in this area, reflecting that they were deposited in an open freshwater lacustrine environment with a short period of water residence. There is a positive relation between the δ^18O and δ^13C in the micritic limestone samples, reflecting that they were deposited in an obvious closed saline water laustrine environment. The obviously enriching of δ^13C in stromatolitic limestones may be related to the microbial activities. The δ^18O value of the lacustrine carbonate rocks gradually increased from Eocene to Oligocene and then to Miocene. The δ^13C changed from a negative value in the Paleogene to a positive value in the Neogene, which reflects that the lacustrine system in this area evolved from an open system to a closed one. Thus the prediction can be made that the palaeoclimate changed greatly throughout the Paleogene-Neogene period.