An improved methodology of the modern analogues technique for palaeoclimate reconstruction in arid and semi-arid regions
Résumé
This study presents an improved method of the plant functional type modern analogues technique (PFT-MAT) in which environmental proxies and a moisture index (alpha, i.e. ratio of actual evapotranspiration to potential evapotranspiration) are used to constrain the selection of modern analogues. The method is tested using high-resolution, precisely dated palaeorecords (pollen, Pediastrum and delta 18O of authigenic carbonate) from Lake Bayanchagan, northern China. The unconstrained and constrained PFT-MAT produces general agreement for Holocene climate changes, with a wet period between 11 000 and 5500 cal. yr BP and a warm interval between 11 000 and 8000 cal. yr BP. However, there are significant differences in the details of their reconstruction. The constrained PFT-MAT generally yields smaller error bars for the reconstructed climate parameters than the unconstrained PFT-MAT. In addition, three prominent climatic events are identified from the constrained reconstructions; namely, a cold event around 8400 cal. yr BP and two warm events around 6000 and 2000 cal. yr BP, which is consistent with other regional palaeoclimatic records. Our data show that changes in tree components correlate well with alpha variations during the entire Holocene, with the highest tree components and highest alpha values between 8000 and 5500 cal. yr BP, indicating the dominant role of alpha in the growth of trees in northern China rather than single temperature or precipitation. The improved PFT-MAT is therefore an efficient method for quantitative reconstructions of palaeoclimate in arid and semi-arid regions.