Late Holocene Climate Change Revealed By Pollen Records From Lacustrine Sediments In The Qaidam Basin

Gahai Lake and Tonson Lake, two saline Lakes, are located on the northeastern Tibetan Plateau. We use Artemisia/Chenopodiaceae (A/C) ratios in sediment cores from these two lakes to reconstruct paleoclimate change over the past 1480 years in the Qaidam Basin.We present a 50-year pollen record at annual resolution from Gahai Lake in the Qaidam Basin. This record was used to evaluate the utility of the A/C ratio as a proxy for moisture change by comparing it with instrumental climate data and independent climate proxies. Chronology of the two 22-cm cores (GL05-1 and GL05-2) was established by ²¹⁰Pb and ¹³⁷Cs analysis. The pollen data from core GL05-2 at 0.5 cm intervals showed large changes in A/C ratios (<0.2 to 0.95) in the last 50 years. High (low) A/C ratios represent increased (decreased) steppe pollen and plants. These correspond to high (low) relative humidity observed at the nearby Delingha climate station. In addition, the A/C ratios curve is correlated to carbonate abundance, which can be a measure of lake-level change and regional effective moisture; low carbonate sediments are likely to be deposited in shallow water or wetland environments. On the basis of correlations with instrumental records and with independent climate proxy, we conclude that the A/C ratio is extremely sensitive to moisture change and can be a useful index in high-resolution paleoclimate reconstruction in arid and semi-arid regions.Building on this regional groundtruthing of A/C ratios, moisture variability over the past 1480 years was revealed from the Toson Lake sediment core (TL06-1). The result of pollen analyses shows that four zones can be partitioned according to the pollen assemblages, and each zone is characterized by high or low A/C ratios. The periods from 530 to 1060 AD and from 1460 to 1700 AD, it was relatively wet. Drier conditions prevailed from 1060 to 1460 AD, and from 1700 to the present, although this latter period shows a trend toward greater wetness. In addition, the A/C ratio curve for the past 1480 years correlates positively to organic matter and carbonate abundance based on LOI analyses. This may be interpreted as the result of enhanced deposition of carbonate in deep water relative to nearshore zones.Our 1480 year A/C ratio-based moisture record is consistent with nearby tree ring records of precipitation during the past 1437 years, particularly for century-scale variability. Our results are also coherent with records of pollen and particle contents in the Dunde ice core. This may reflect the complex relationship among the relative humidity, temperature and precipitation.