| The Asian monsoon is a key component of the earth’s climate system that directly affects the livelihood of50million people on the loess plateau of central China. At the far edge of monsoonal influence, this region is especially vulnerable to future changes in temperature and evaporation/precipitation. Therefore, paleoclimatic information on the natural sensitivity of the region to changes in monsoon driven aridity is crucial. Despite the need for multiproxy records of Holocene climate from this region, reconstructions are rare, because of the low resolution of loess deposits and the scarcity of other paleoclimate archives (e.g. natural lakes, speleothems). Here we present multiple proxy records from Tianchi Lake, one of the few nature lakes on the western loess plateau. The chronology is well constrained by a high-resolution (28AMS14C dates) radiocarbon age-model, spanning the past6000years. Here we present magnetic susceptibility, grain size, total organic carbon, carbon and nitrogen ratio, plant macrofossils, Glycerol dibiphytanyl glycerol tetraethers (GDGTs), lake macrophytes (paq) and plant-wax hydrogen isotope to reconstruct regional climate change during the middle to late Holocene.Evidence from n-alkane Paq, C/N and δ13Corg data suggest increasing relative abundance of macrophytes over past6000years, which we interpret (based on lake morphology) as decreasing lake-level. Plant macrofossils and other environmental parameters suggest that the vegetation in the catchment was decreased from6000cal yr BP and then more rapidly from2200cal yr BP. This is coeval with documentary records of increasing local population density and infers historical human impact on the catchment.Using Sun et al (2011) regional calibration we derive mean annual GDGT based temperatures (MBT/CBT-MATs) with reasonable ranges. Our temperature reconstruction closely correlates on millennial to centennial timescales with the independent D/H measurements on C28fatty acid (δDC28), whose signal is assumed to derive primarily from terrestrial plant waxes and the δDC28values to reflect local changes in precipitation. Synchronous evolution of reconstructed temperature (MBT/CBT-MAT) and precipitation (δDC28) from Tianchi Lake suggest that the climate on western loess plateau shifted between "warm/wet" and "cold/dry" during the middle to late Holocene. Five evolutionary stages could be divided as follow:(a)6000～5200Cal yr BP, regional climate seems still in the end of Holocene Megathermal, a warm and wet climate can be inferred.(b)5200～4800Cal yr BP, abrupt cold and dry climate event was existed during this period.(c)4800～3800Cal yr BP, abrupt warm and wet climate event was happened. The temperature increased rapidly and was especially high.(d)3800～1200Cal yr BP, the reconstructed temperature and precipitation in this stage was low and relatively constant. A cold and dry climate can be inferred.(e)1200Cal yr BP～present, asunchronous characteristic of increased reconstructed temperature and decreased reconstructed precipitation was showed. The climate seemed to be a warm and dry condition.Comparisons of our independent GDGT temperatures and plant-wax δDC2s records with stalagmite δ18O records from the monsoon region and NH summer insolation suggest strongly that our record reflects regional changes in monsoon strength forced by NH summer insolation. Superimposed on the longer-term insolation driven changes are centennial scale variations, recorded by both the independent reconstructions of precipitation (δDC28) and temperature (MBT/CBT-MAT).Trade-wind driven of the frequency/intensity of ENSO cycle on the longer-term scale and the SST changes of the western tropical pacific might be the key point of the monsoon strength variations in study area. |
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