Identifying The Drivers Of Vegetation Change On The Chinese Loess Plateau Over The Last 200,000 Years

Abstract:

The Chinese Loess Plateau (CLP) is regarded as one of the best continental paleoclimate archives, as cycles of warm, wet interglacials and cool, dry glacials have been recorded in a sequence of alternating layers of loess and paleosol. Changes in the bulk organic carbon isotopic composition (δ¹³C_TOC) throughout these loess-paleosol sequences have revealed shifts in the relative occurrence of C₃ and C₄ plants in the past, with increased abundance of C₄ vegetation during interglacial periods. Although increased temperatures generally lead to an expansion of C₄ vegetation, increased humidity has an opposite effect, as does pCO₂. In spite of this contradiction, the absence of independent air temperature, precipitation, and vegetation records prevents identification of the drivers of past vegetation change in this region.

Here we present combined records of changes in continental air temperature, monsoon precipitation, and vegetation type during the past 200,000 years based on the distribution and isotopic value (δ¹³C and δ²H) of plant waxes (long chain n-alkanes) and temperature-sensitive soil microbial membrane lipids (brGDGTs) stored in the Lingtai and Yuanbao sections of the CLP. Glacial-interglacial temperature variability is faithfully recorded by the brGDGTs at these sites. However, fluctuations in plant wax-δ¹³C are only minor (<2‰), especially given that modern end-member plant wax-δ¹³C values for C₃ and C₄ vegetation on the CLP differ by over 10‰. We thus infer that temperature and pCO2 are not the primary drivers of vegetation change, although growing season temperatures may have been consistently too low for C₄ vegetation to thrive due to the relatively high elevation of our sites (>1300 m). Instead, variations in the average chain length (ACL, a plant functional type indicator) and plant wax-δ²H records mainly explain the small δ¹³C variations, indicating that the precipitation- evaporation ratio (driven by moisture stress, precipitation seasonality and temperature) thus leads differences in C₃ vegetation type (woody vegetation dominating during interglacials and shrub-dominated vegetation during interglacials), rather than large variations in C₃-C₄ vegetation.

Key words: Paleoclimate, Geochemistry, Monsoon, Vegetation