A multi-proxy approach to reconstruct hydrological changes and Holocene climate development of Nam Co, Central Tibet

Abstract

Holocene lake level fluctuations were reconstructed from a 2.7-m sediment core from Nam Co, Central Tibet, China dating to [7.2 cal ka BP. Results were compared to existing lake records from the Tibetan Plateau to infer variations in the strength of the Asian Monsoon. Geomorphological features in the Nam Co catchment, such as beach ridges and lake terraces, indicate high lake stands during the late Glacial. A major low stand is suggested for the Last Glacial Maximum (LGM). Sands and sandy silts at the base of the core are transgressive facies, with material transported by melt water and deposited under rising lake level conditions that followed the LGM low stand. Variations in grain size, major elements, biomarker stable isotopes and minerals in the core suggest a climate evolution reflected in at least five depositional units and subunits. Sediments in Unit I (*7.2 to *5.4 cal ka BP) were deposited at highest lake levels. Large amounts of allogenic minerals and allochthonous organic matter suggest high precipitation and melt water input, implying positive water balance. Increasing aquatic productivity points to favourable environmental conditions. Unit II (*5.4 to *4.0 cal ka BP) marks a transition between favourable, stable hydrological conditions and lake level decrease. Lower lake levels were a consequence of drier climate with less monsoonal precipitation, higher evaporation rates, and increased moisture recycling in the catchment. Unit III (*4.0 to *1.4 cal ka BP) reflects the driest periods recorded, at *3.7 cal ka BP and 1.6 cal ka BP. Lake shrinkage and salinization was interrupted as suggested by the deposition of Unit IV (*1.4 to *0.8 cal ka BP), when increased precipitation and runoff that might be related to the Medieval Warm Period, led to a stable, but still low lake level. Unit V (800 cal years BPpresent) is characterized by progressive lake shrinkage due to intense evaporation. Paleolimnol (2010) 43:625-648 DOI 10.1007 data indicate the lake level is rising. Comparison of the Nam Co record with other lake records from the Tibetan Plateau suggests general agreement with the broader picture of Holocene environmental evolution. The timing of dry and wet climate conditions at lake sites across Tibet indicates a gradually decreasing influence of the southern monsoon during the Holocene, from NW to SE. Nevertheless, further research is needed to improve our understanding of Holocene spatio-temporal hydrological variations across the Asian continent.

Landscape evolution of a bedrock peneplain on the southern Tibetan Plateau revealed by in situ-produced cosmogenic 10Be and 21Ne

“…2). The climate in this region is cold and arid with a mean annual temperature of 0°C and an annual precipitation of ~300 mm Mügler et al, 2010).…”

“…On the Tibetan Plateau, cold and dry conditions during marine isotope stage 2 were followed by a warm and humid period with high lake levels in the early-mid Holocene (e.g. Wang et al, 2002;Mügler et al, 2010). A subsequent phase of increased aridity, which started 5-6 ka ago, is documented by falling lake levels (e.g.…”

“…A subsequent phase of increased aridity, which started 5-6 ka ago, is documented by falling lake levels (e.g. Lister et al, 1991;Lee et al, 2009;Mügler et al, 2010). These past changes in temperature and moisture conditions result from a variable strength of the monsoon system (e.g.…”

Biogeochemical evidence for freshwater periods during the Last Glacial Maximum recorded in lake sediments from Nam Co, south-central Tibetan Plateau

“…humidity and temperature) at Nam Co (Xia et al 2008;Mügler et al 2010), such analyses have not yet been applied to LGM sediment records. Sediment n-alkane distribution reflects sources of organic matter input, and the nalkane isotopic composition reflects the signal of source water, which can thus be used to distinguish between meltwater and precipitation water (Aichner et al 2010b;Günther et al 2013;Hu et al 2014).…”

Hydrodynamic process of Tibetan Plateau lake revealed by grain size: Case study of Pumayum Co

“…In other Tibetan lakes such as Pumayum Co, three periods of larger grain size may correspond to a warm and wet TP environment [19]. In Nam Co, coarser sediments from 7 cal ka BP may indicate inflow increase [20]. In Lake Cuoe, coarser sediments may indicate inflow increase caused by precipitation increase [21].…”