HOLIVAR2006 Abstracts
Holocene climatic and limnologic changes at Sihailongwan maar lake (North-eastern China) as indicated by diatom remains: preliminary results.
Patrick Rioual1, Chu Guoqiang1, Han Jintai1, Jens Mingram2, Liu Qiang1, Georg Schettler2 and Liu Jiaqi1
1Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
2GeoForschungsZentrum Postdam, 14473 Potsdam, Germany
Contact: P. Rioual (prioual@mail.igcas.ac.cn)
North-eastern China, because of its location in the mid-latitude temperate area of East Asia under the influence of the Monsoon, has been identified by the International Geosphere-Biosphere Programme (IGBP) as a key region for investigating present and past climates.
The uppermost 4.8 m of a sedimentary sequence from Sihailongwan maar lake (Jilin Province) has been analysed for diatom assemblages with the aim to investigate the changes in limnology and climate that took place during the Holocene in that region. According to the chronology adopted, which is based on AMS 14C-dating of terrestrial plant remains, this sequence spans the period from c. 13,000 yr BP until the present. Diatom assemblages at the bottom of the sequence (before c. 12,200 yr BP) are dominated by Stephanodiscus parvus, indicating that the lake system was already productive during the Late-glacial. Between c. 12,200 and 11,300 yr BP, low microfossil concentration, diatom assemblages dominated by benthic species, and abundant Chysophyte cysts indicate low productivity and intensely cold conditions that are most likely equivalent to the Younger-Dryas stadial event. At the start of the Holocene the oligotrophic species Cyclotella comensis dominates before being replaced by S. parvus, suggesting a rapid increase in productivity of the lake system. A thick tephra layer (T2), at 10,500 yr BP, occurred during that interval and appears to interrupt this trend, as C. comensis again dominates just above the tephra layer. This is followed by a long period (c. 10,000 to 6000 yr BP) during which S. parvus dominates. This species indicates low Si:P ratio and therefore suggests vigorous spring mixing of the water column, which is generally associated with mild climatic conditions in late winter and spring. Around 8000 yr BP, the short-lasting development of Asterionella formosa and Cyclotella radiosa may be related to a cooling event. Between 6000 to c. 3400 yr BP, there is a sharp decrease in S. parvus while two species with a preference for high Si:P ratio, A. formosa and Fragilaria gracilis became abundant. This may reflect a reduced intensity and/or the delay of mixing of the water column in spring and may correspond to the early neoglacial cooling. The final period (3400 to present) is characterised by Cyclotella pseudostelligera and C. radiosa and lower microfossil concentration, therefore indicating lower productivity of the lake system. This period is also marked by the occurrence of the tephra layer T1 at 2150 yr BP.
A more detailed interpretation of these results requires a better knowledge of the present-day diatom flora of the lake in NE China. It is expected that quantitative reconstructions of limnological parameters will be performed once a surface-sediment calibration data-set for the lakes of this region (currently being developed) is available. The monitoring of the modern lake-system (using sediment traps and temperature recorders) should also provide more information on the seasonal preference of the main diatom species.
Patrick Rioual is visiting Professor at the Institute of Geology and Geophysics, Chinese Acdemy of Sciences in Beijing (China). He is also an Honorary Senior Research Fellow at the Environmental Change Research Center of University College London. His research interests include quaternary diatom analysis, palaeolimnology, and aquatic ecology.