Holocene weak monsoon intervals indicated by low lake levels at Hulun Lake in the monsoonal margin region of northeastern Inner Mongolia, China

A 170 cm long sediment core spanning the last 10 000 years was recovered from Hulun Lake in the northeastern margin of the East Asian summer monsoon. The sediment core was analyzed at 1 cm intervals for grain-size distribution and sedimentary component partition. These data provide a proxy record of the monsoon variability on millennial to centennial scales during the Holocene. We used a lognormal distribution function fitting method to partition three to six components from fine to coarse modes within the individual polymodal distributions into overlapping lognormal distributions. Three coarse components representing nearshore suspension, saltation and traction, together with the sand-fraction percentage and the median grain size of bulk samples, indicate the lake levels that fluctuated in response to the intensity of the monsoonal precipitation. Higher percentages of the nearshore components accompanied by more sand-fraction proportions and coarser median grain sizes reflect lower lake stands resulting from weaker monsoon circulations. The results show low levels at Hulun Lake c. 8000-7850, 6400-6050, 5150-4900, 4500-3800, 3050-2800, 1650-1400, 1150-900, 700-600, and 400-350 cal. yr. BP, indicating the weakened East Asian summer monsoon during these intervals. We suggest that these weak monsoon events would not only result from the reduced ocean-atmosphere interacting processes in the western tropical Pacific, but could also be related to cold climatic conditions in the Northern Hemisphere high latitudes.