Monsoonal forcing of paleoenvironmental change on the Central Tibetan Plateau inferred from a lacustrine record of Lake Nam Co (Xizang, China)

On the Tibetan Plateau (TP), lake sediments are valuable tools for reconstructing paleoenvironmental changes during the Late Quaternary. Concerning previous work on lacustrine records from the TP a high variability of both sampling resolution and age control is evident. Hence a detailed and comprehensive analysis of paleoenvironmental evolution at different sites is still outstanding, hampered by insufficient chronologies, varying reservoir effects and sparse proxy data. Despite these circumstances a monsoonal forcing of environmental change on the TP is obvious, highlighting the need for a long term, high resolution and multi-proxy based study on Late Quaternary paleo-monsoon dynamics. A sediment coring at Lake Nam Co (90°50'E, 30°40'N, 4718 m asl) addresses that issue, providing undisturbed and continuous lacustrine sediments reaching beyond the Last Glacial Maximum. Attributing to the complexity of the record only the Holocene era will be discussed within this study. Since most records of Nam Co cover solely the last ~8.9 ka cal BP (e.g. ZHU et al. 2008), a complete picture of Holocene paleoenvironmental conditions at Lake Nam Co is still lacking. Moreover, a linkage to different sedimentary records on a regional and supra regional scale was not possible for the entire Holocene up to now. Therefore this study aims to reconstruct the Holocene evolution of the Indian Ocean Summer Monsoon on the Central TP applying lacustrine sediments of Lake Nam Co on a decadal resolution. A multi-proxy approach is used to identify high and low amplitude changes of lake sedimentation which are supposed to reflect the interplay of monsoonal triggers and the sensitivity of the lake to record monsoonal pulses during time. Results obtained from geochemical and sedimentological analyses suggest high amplitude oscillations of lake productivity and allochthonous sediment influx during the Early Holocene culminating at around 9.5 ka cal BP. From 11.2 ka to 9.5 ka cal BP, all proxies point to an enhanced moisture supply at Lake Nam Co, probably related to strengthened summer monsoon intervals on the Central Tibetan Plateau, as reported from the Arabian Sea (OVERPECK et al. 1996). A higher lake level, possibly the Holocene maximum, is expected during this time. Thus, a sensitive response of the lake system to distinct, monsoonal forced paleoenvironmental changes can be assumed for the Early Holocene. For the Mid- to Late Holocene period only minor oscillations of low amplitude are visible, pointing to a weakened response of Lake Nam Co to short term fluctuations of monsoon strength.