CLIMATE-DRIVEN GLACIAL/INTERGLACIAL VEGETATION CHANGES ON THE CHINESE LOESS PLATEAU

We employ isotope compositions of organic matter and pedogenic carbonate nodules preserved in several loess/palaeosol couplets (S0, S1, S5, S15-S19) from the Jiaodao section, Shaanxi Province, to reconstruct past variations in floral composition at the site. Carbon isotope values of soil organic matter (SOM) consistently increase from minimum values in loess (-24 to -22 ‰ VPDB) to maximum values in paleosols (-19 to -21 ‰ VPDB). These data are interpreted to record repeated shifts in the ratio of C3 to C4 vegetation at the site during glacial/interglacial cycles. The proportion of C4 vegetation was consistently higher (up to 60%) during warm and wet interglacial periods than during dry and cold glacial periods (30% or less). Carbon isotope values of SOM correlate well with magnetic susceptibility values suggesting that the two proxies responded to the same climatic forcing. Pedogenic carbonate nodules at the bases of the studied paleosols were both microsampled and analyzed in bulk. Carbon isotope values of pedogenic carbonate nodules vary between -4 and -7 ‰ VPDB and suggest that the nodules formed in the presence of an elevated percentage of C4 vegetation. Oxygen isotope values of microsamples vary between -8.5 and -10.5 ‰ VPDB and correlate negatively to the corresponding carbon isotope values suggesting that peak C4 vegetation occurred during periods of intensified precipitation (peak interglacials). Bulk nodule isotope compositions (d¹³C=-5.1 to -6.5 , d¹⁸O=-8.9 to -10.0 ‰ VPDB) do record long-term conditions at the site, but they mask smaller-scale temporal environmental variations.