Preliminary results of a Tibetan stable C-isotope chronology dating from 1200 to 1994

Recently the set up of a stable carbon isotope chronology of juniper tree rings of the eastern Tibet timberline has been initiated. First results for the last 800 years will be presented. The dominant delta(13)C pattern shows higher values between 1200 and 1450. Thereafter, a decrease is observed until a minimum is reached at about 1650. Low values are observed until 1750. From then onwards the delta(13)C values increase again, followed by a sharp decline since around 1900. Local historical, glaciological, dendrochronological and icecore data suggest that the isotope values mirror a climatic signal between 1200 and 1900. The negative trend in the 20th century can probably be attributed to species specific physiological reactions induced by the anthropogeniocally elevated CO2 concentration. Since the tree rings of some time periods are extremely narrow, always 5 tree rings had to be combined. Because of this sampling procedure (pentad sequences) at present the 50 years of instrumentally measured meterological data could not be correlated to the corresponding delta(13)C record.