Evolution and climatic change of Nam Co of Tibet and an ancient large lake in the northern Tibetan Plateau since the late Pleistocene

Abstract：The paper discusses the Evolution and climatic change of Nam Co of Tibet and an ancient large lake in the northern Tibetan Plateau since the late Pleistocene based on the field leveling survey and laboratory analysis. There are 6 lake terraces along the lake shore of Nam Co less than 48 m above the lake level and highstand lacustrine sediments 48 m to 139.2 m above the lake level. Research shows that the development of Nam Co and evolution of the ancient large lake in the southeast of the northern Tibetan Plateau underwent three stages: ① The ancient large lake stage at 116—37 ka BP, ② the outflow lake stage at 37—30 ka BP, and ③ the Nam Co stage since 30 ka BP. In the ancient large lake stage, a large number of large, medium-sized and small, modern lakes in the southeast of the northern Tibetan Plateau such as Nam Co, Siling Co, Zhari Namco Co and Tangra Yumco were joined together to form a single great lake, whose areal extent might overstep the watershed between the northern Tibet endorheic drainage system and exorheic drainage (Nujiang River) system. It might also be joined with other ancient lakes in the south and west of the north Tibetan Plateau, forming a unifying “ancient large lake”in the northern Tibetan Plateau. Through the comparison between the ages of the lacustrine sediments of Nam Co and the deep-sea oxygen isotope records and an integrated study of more soluble salts, pH values, geochemistry and ostracods and sporopollen, the authors have found that the lacustrine sediments record the information of lake evolution and climatic change since the late Pleistocene. Data show that: during the ancient large lake stage the lake level was the highest and the climate was mild and slightly dry; during the outflow lake stage the lake level lowered sharply and the air temperature and humidity were slightly higher than the present; and the climate got arid increasingly, temperatures fluctuated and the lake level dropped continuously after the warm and humid climate in the Holocene Climatic Optimum since the Nam Co stage. Based on the general trend of getting arid gradually, the climate in the region experienced several marked cold-warm and dry-humid fluctuations since Late Pleistocene.