ausblenden:
Schlagwörter:
Nam Co
Tangra Yumco
Indian Monsoon
δ18Ocarb
Allochthonous minerogenic input
Zusammenfassung:
Two of the largest and deepest endorheic lakes on the Tibetan Plateau, Nam Co and Tangra Yumco, were targeted to infer Late Quaternary (24 ka) responses of their hydrological balance to environmental dynamics. For this purpose, high-resolution multi-proxy records were used to obtain information on lake level variations and how these are related to direct precipitation proxies. Allochthonous minerogenic input as reflected by potassium (K), is directly linked to precipitation driven by variations in Indian Summer Monsoon. Negatively correlated elements such as Ca, Sr and Mg and associated carbonate mineral phases represent autochthonous sediment production, which is coupled to evaporation in these closed systems. δ13C and δ18O of bulk carbonates represent lake water conditions, i.e., salinity and lake water volume, interpreted as precipitation-evaporation (P/E) balance. In combination with geomorphological features, i.e., lake level terraces and paleo-shorelines, the multy-proxy data set from Nam Co is used to reconstruct lake level dynamics. Low lake levels of >90 m below the recent one between 20 and 16 ka cal BP are associated with most enriched δ18Ocarb values and the precipitation of aragonite and high-Mg calcite in a shallow lake with highly dynamic allochthonous minerogenic input. Similar conditions are observed between 13.1 and 11.4 ka cal BP matching the Younger Dryas chronozone. Between 15.5 and 13.1 ka cal BP as well as during the Early Holocene (11.4 to 8.0 ka cal BP) distinctly depleted δ18Ocarb and δ13Ccarb coupled with high rates of allochthonous minerogenic input suggest substantial strengthening in precipitation. After 9.4 ka cal BP, the terrigenous input and thus precipitation is significantly reduced, whereas δ18Ocarb reveals only a slight enrichment trend and thus a slowly but steady decrease in lake level througout the Mid- to Late Holocene. The decoupling of this relation between precipitation and lake level suggest that lake level terraces of large endorheic lakes on the TP and also δ18Ocarb records are not a direct indicator of (monsoonal) precipitiation, whereas the allochtonous minerogenic input can be directly linked to precipitation dynamics. This is supported by the comparison to regional archives from the Arabian Sea and the northern Bay of Bengal, showing the very same pattern in precipitation variability. The presented monsoon precipitation record from Nam Co additionally reveals a link to supra-regional atmospheric circulation components as it is oscillating in- and anti-phase to El-Niño strength and frequency. A distinct precipitation change at 2 ka cal BP seems to have affected all archives, however leaving space for future studies to find the cause of this Late-Holocene climatic shift.
