New high-resolution pollen records from two lakes in Xizang (Tibet)

Studies on the pollen from cores of Lake Hidden (29°48.77' N, 92°22.37' E) and Ren Co (30°43.97' N, 96°40.97' E) in Southeast Xizang (Tibet) showed climatic changes and the evolution of vegetation since the Last Glacial Maximum (LGM). Before 16 ka BP pollen assemblages were dominated by Chenopodiaceae (20% ~ 50%) and Artemisia (10% ~ 30%) and pollen influx values were very low from Lake Ren Co in Basu county. The vegetation around the lake was probably a desert-steppe during the LGM. The data also suggest that the climate in the Basu area was cold and dry during the LGM and the last glacial time. The mean annual temperature was probably 4 ~ 6 °C colder than the present, and the mean annual precipitation was only 250 mm, about 40% of the present. The Southwest monsoon became stronger from 12 ka BP to 6 ka BP reaching its acme by 7 ka BP, but weakened gradually from 5 ka BP to the present. The paleovegetation was dominantly forest or forest-meadow around Lake Hidden and Ren Co during the 9.2 ~ 5 ka BP. The mean temperature in January was probably 2 ~ 3 °C higher than the present, and the mean annual precipitation was 100 mm more than the present. The timing of vegetational and climatic changes based on pollen records generally supports the results of global climatic-model experiments that predict a strengthened monsoon system during the early to middle Holocene followed by a weakened monsoon system.