Abrupt changes in climatic conditions across the late-glacial/Holocene transition on the N. E. Tibet-Qinghai Plateau: evidence from Lake Qinghai, China

A multi-proxy investigation of two sediment cores from the large closed-basin Lake Qinghai provides evidence of abrupt changes in paleolimnological conditions across the late-glacial/Holocene transition. The chronology of the lacustrine sediment sequence is framed by four AMS ¹⁴C ages for aquatic-plant macrofossil seeds. Four distinct stratigraphic units are identified on the basis of abrupt shifts in lithology, carbonate composition, ¹⁸O of authigenic carbonates, magnetic susceptibility characteristics, and total nitrogen content. These units represent four environmental stages that were each initiated by three abrupt changes in hydro-climatic regime at 11,600, 10,700, and 10,000 ¹⁴C yrs B.P. Each of the four environmental stages thus represents a characteristic precipitation-to-evaporation balance for the lake catchment. The paleoenvironmental evidence indicates that the lake before 11,600 ¹⁴C yrs B.P. was very shallow with carbonate production and organic productivity much lower than in the Holocene, suggesting a much colder and drier climate than in the Holocene. From 11,600 to 10,700 ¹⁴C yrs B.P., the presence of clastic laminations and Ruppia fossil seeds suggests an increased inflow of sediment-laden waters into the lake. Between 10,700 and 10,000 ¹⁴C yrs B.P., the development of a carbonate playa lake indicates that a negative water balance persisted. From 10,000 ¹⁴C yrs B.P. an abrupt increase in rainfall is suggested by a sudden termination of the playa lake environment and the diluted lake waters, as evidenced by negative shift in both total carbonate content and ¹⁸O values of mineral carbonate. However, the lake level during the early Holocene was about 20 m shallower than today, indicating that the effective moisture then was much lower than it is today. The multi-proxy record suggests a step-wise pattern of climatic change across the late-glacial/Holocene transition along with abrupt shifts in P-E balance on the N. E. Tibet-Qinghai Plateau. This pattern is characterized by reorganization of Asian monsoon circulation, which probably was determined by increasing summer insolation and changes in surface boundary conditions accompanying regional deglaciation. The arid event at 10,700–10,000 ¹⁴C yrs B.P. is interpreted as a Younger Dryas equivalent, although climatic cooling is not indicated by the evidence at hand.