Reconstructing the 800,000-year history of C4 herbs on the Chinese loess plateau using n-alkane carbon isotopes

Abstract

C₄ plants constitute a critical component in the modern ecosystem on the semi-arid Chinese Loess Plateau (CLP), and the changes in their abundances and distributions on geological time scales in response to climate factors such as temperature, precipitation, and atmospheric CO₂ are a subject of intense investigation and debate. Reliable reconstruction of the percentage of C₄ plants and a better understanding of its long-term variations in association with environmental and climatic changes are essential for interpretation of terrestrial ecosystem evolution. In this paper, we determine the carbon isotope and biomass characteristics of leaf wax on the CLP. We identify Bothriochloa ischaemum (L.) Keng as the dominant C₄ grass, which accounts for ∼90% of the total C₄ grass biomass and an average of 70% of the herbaceous community in the area. Using CO₂ and precipitation corrected carbon isotope compositions from leaf wax n-alkanes as a proxy, we reconstruct a continuous history of C₄ vegetation changes on the CLP during the Pleistocene. Our data indicate that a persistent grassland ecosystem existed on the tableland of the CLP, with C₄ plants accounting for up to 40–53% during interglacial phases and 24–40% during glacial phases. Our findings suggest that temperature and precipitation, influenced by the East Asian monsoon, are key factors affecting the abundance of C₄ vegetation in the region. The interactions with the monsoonal climate with warm-season precipitation in this grassland ecosystem throughout the past eight glacial cycles have important implications for the inhabitation of ancient humans in this region and underscore the challenges of vegetation restoration on the CLP under the projected warmer climate in the near future.