Quantitative reconstruction of precipitation changes on the NE Tibetan Plateau since the Last Glacial Maximum – extending the concept of pollen source area to pollen-based climate reconstructions from large lakes

摘要

Pollen records from large lakes have been used for quantitativepalaeoclimate reconstruction, but the influences that lake size (as a resultof species-specific variations in pollen dispersal patterns that smallerpollen grains are more easily transported to lake centre) and taphonomy haveon these climatic signals have not previously been systematicallyinvestigated. We introduce the concept of pollen source area to pollen-basedclimate calibration using the north-eastern TibetanPlateau as our study area. We present a pollen data set collected from largelakes in the arid to semi-arid region of central Asia. The influences thatlake size and the inferred pollen source areas have on pollen compositionshave been investigated through comparisons with pollen assemblages inneighbouring lakes of various sizes. Modern pollen samples collected fromdifferent parts of Lake Donggi Cona (in the north-eastern part of theTibetan Plateau) reveal variations in pollen assemblages within this largelake, which are interpreted in terms of the species-specific dispersal anddepositional patterns for different types of pollen, and in terms of fluvialinput components. We have estimated the pollen source area for each lakeindividually and used this information to infer modern climate data withwhich to then develop a modern calibration data set, using both themultivariate regression tree (MRT) and weighted-averaging partial leastsquares (WA-PLS) approaches. Fossil pollen data from Lake Donggi Cona havebeen used to reconstruct the climate history of the north-eastern part ofthe Tibetan Plateau since the Last Glacial Maximum (LGM). The mean annualprecipitation was quantitatively reconstructed using WA-PLS: extremely dryconditions are found to have dominated the LGM, with annual precipitation ofaround 100 mm, which is only 32% of present-day precipitation. Agradually increasing trend in moisture conditions during the Late Glacial isterminated by an abrupt reversion to a dry phase that lasts for about 1000 yrand coincides with "Heinrich event 1" in the North Atlanticregion. Subsequent periods corresponding to the B?lling/Aller?dinterstadial, with annual precipitation (P_ann) of about 350 mm, and theYounger Dryas event (about 270 mm P_ann) are followed by moistconditions in the early Holocene, with annual precipitation of up to 400 mm.A drier trend after 9 cal. ka BP is followed by a second wet phase in themiddle Holocene, lasting until 4.5 cal. ka BP. Relatively steady conditionswith only slight fluctuations then dominate the late Holocene, resulting inthe present climatic conditions. The climate changes since the LGM have beenprimarily driven by deglaciation and fluctuations in the intensity of theAsian summer monsoon that resulted from changes in the Northern Hemispheresummer solar insolation, as well as from changes in the North Atlanticclimate through variations in the circulation patterns and intensity of thewesterlies.