C, N STABLE ISOTOPE RECORDS OF ENVIRONMENTAL CHANGES IN BOSTEN LAKE DURING THE PAST 200 YEARS: C, N STABLE ISOTOPE RECORDS OF ENVIRONMENTAL CHANGES IN BOSTEN LAKE DURING THE PAST 200 YEARS

Two sediment cores were recovered from the Bosten Lake in order to investigate the changes in both the lake system and source of organic materials during the last 200 years.Based on 210Pb/137Cs chronology,stable isotopes of organic carbon and nitrogen as well as Loss on Ignition(LOI),grain size and magnetic susceptibility(MS) were analyzed.The results indicate that the agricultural activities in the catchment of the Bosten Lake have much earlier influence in the lake center rather than in the river mouth.Due to the general increase in regional humidity during 1810—1883 AD,δ13Corg declined rapidly in the core BST16,while abrupt increases in δ15N were found as more terrestrial material could reach the lake center.After the establishment of Xinjiang Province in 1884 AD,intensified agricultural activities have led to marked decrease in the medium grain size,and relatively higher δ13Corg and lower δ15N in the core BST16.Since 1950 AD,large scale agricultural activities have led to the rapid decrease in δ13Corg and increase in δ15N,indicating a general increase in the trophic level.Further decrease in δ15N together with the increasing δ13Corg after the 1990's indicates that nitrogen fixation by algae began to dominate the lake primary productivities.