DISTRIBUTION OF RECENT OSTRACODA AND THEIR USE IN PALAEOENVIRONMENTAL RECONSTRUCTION OF TANGRA YUMCO, CENTRAL TIBETAN PLATEAU
Thirty five samples from surface sediments were examined quantitatively for ostracods (dead + living) from >200 µm size fraction. Aquatic habitats and water parameters for ostracod samples were monitored simultaneously. All the samples contained fossil ostracods, of which twenty eight yielded living ostracods. There are at least nine living and fossil ostracod species in Tangra Yumco and its catchment: Within the lake, the typical species are Leucocythere dorsotuberosa, Leucocytherella sinensis, Limnocythere inopinata, Fabaeformiscandonda gyirongensis, and Candona xizangensis. Eucypris spp. and Ilyocypris sp. are characteristic for small streams and temporary waters also dwelled by Heterocypris incongruens. Candona candida occurs both in the lake and in flowing waters. A very rare genus is Potamocypris found in a modern stream with a single valve. Water depth indicator species identified include Leucocythere dorsotuberosa and Fabaeformiscandona gyirongensis in medium and deeper water; shallow water species are Leucocytherella sinensis and Eucypris gyirongensis.
The applicability of Recent ostracod analysis to fossil associations was tested using a short core drilled from 223 m water depth in Tangra Yumco (central part of the northern basin). The core covers the last approximately 3500 cal yrs BP and yields all typical Recent ostracod species confirming sedimentation in a deep water environment. The upper part of the core represents a lowered lake level. It is dominated by L. inopinata, tolerating high salinity. This points to a high evaporation/precipitation ratio during the last approximately 1000 cal yrs BP. The last centuries are an exception when the proportion of L. inopinata decreases probably due to a rising lake level. The lower part of the core is dominated by L. sinensis and partly L. dorsotuberosa. There is a trend change to a drier climate starting earlier, around 2000 cal yrs BP, when L. dorsotuberosa increased in proportion.