Reconstruction of C₄ plant content in Cenozoic lacustrine sediments: An example from the Dahonggou section, Qaidam Basin

Global climate change has posed serious effect on the vegetation of our ecosystem. In order to understand the response of plant ecosystems to environmental and climate changes, we need to learn well from the earth’s past. C₄ plants play a pivotal role in the modern ecosystems. Previous studies have shown that C₄ plants originated prior to the Eocene-Oligocene transition, but the expansion of C₄ grasses had not happened until the Middle to Late Miocene. What caused the origin and expansion of C₄ plants is important in understanding the ecosystem evolution. The current method for estimating C₄ contribution is mainly based on the differences in C₃/C₄ plant carbon isotopes and their structures. The commonly used samples include (paleo-) soil organic matters, pedogenic carbonates, terrestrial herbivore tissues, biomarkers, pollen and phytolith. The application of isotopic difference in these different samples has played an important role in the reconstruction of the relative biomass of C₄ plants in modern ecosystems and geological records. This article summarizes the mainstream views related to the driving forces that triggered the origin and expansion of C₄ plants, and briefly introduces the basic principles of various materials archiving the biomass of C₄ plants evolution history. Finally, taking the Dahonggou section in the Cenozoic Qaidam Basin as an example, the methods and shortcomings of the reconstruction of C₄ plant content in lacustrine sediments are discussed in detail based on the long-chain n-alkanes and carbon isotope analysis of Cenozoic terrestrial higher plants. The δ¹³C_alk values of long-chain alkanes vary from~-30‰ to~-26‰, and the δ¹³C_alk values are positive. We believe that the double superposition effect of drought and C₄ plant expansion leads to the positive excursions of δ¹³C_alk values. The expansion of C₄ vegetation in the Cenozoic Qaidam Basin may be controlled by dry and wet conditions and climate change. But this hypothesis needs to be tested by carbon isotope analysis from single pollen grains.