HOLOCENE ENVIRONMENTAL-CHANGES IN BANGONG CO BASIN (WESTERN TIBET) .4. DISCUSSION AND CONCLUSIONS

A 12.4 m core taken in Lake Bangong provides a continuous Holocene cli matic record. We summarize information on changes in stable isotope an d radiocarbon balances in the lake, hydrobiology and vegetal cover in the catchment, deduced from detailed analytical results given in the t hree preceding papers. The Bangong record is then compared with the en vironmental history of the neighbouring Lake Sumxi also constructed fr om multidisciplinary analyses. The two records show a major environmen tal change at approximate to 10-9.5 ka B.P., attributed to a rapid str engthening of the summer monsoonal circulation which led to wet-warm c onditions. This event was followed by a long-term trend toward aridity which culminated around 4-3 ka B.P.. In Western Tibet, maximum monsoo n rainfall seems to have occurred from approximate to 9.5 to 8.7 ka B. P, and from approximate to 7.2 to 6.3 ka B.P., as two wet pulses separ ated by a reversal event centered on 8.0-7.7 ka B.P. Our results broad ly agree with the records from Lake Seling in Central Tibet, and Lake Qinghai at the plateau's northeastern margin, and with palaeoclimatic studies in western China which document conditions wetter and warmer t han those of today during the early-middle Holocene. The environmental fluctuations recorded in western Tibet appear in phase with climatic changes recognized in tropical North Africa, suggesting that the 8.0-7 .7 ka B.P. and the 4.0-3.0 ka B.P. dry events were caused by abrupt di sequilibrium in the climatic system.