A preliminary reconstruction of the paleoecological and paleoclimatic history of the Chinese Loess Plateau from the application of biomarkers

摘要

This study provides a preliminary reconstruction of paleoecological and paleoclimatic history over the central Chinese Loess Plateau (CLP) during the last 8.1 Ma based on biomarker records from the earliest of the Chaona stratigraphic section. Throughout the section, we found variations in n-alkane and n-alkan-2-one distributions and dramatic changes in six other biomarker proxies: 1) n-alkanes (C-27 + C-29)/(C-31 + C-33) ratios, 2) n-alkanes C-27/C-31 ratios, 3) CPI (carbon preference index) values for CPI((H))ALK, 4) values for CPI(H)KET, 5) n-alkane mean chain lengths ACL-ALK, and 6) n-alkan-2-ones C-29/C-31 ratios. The C-29 n-alkanes dominate the red clay sediments with little variability, indicating that trees dominated the CLP and that the climate was relatively stable and less variable during the 8.1-2.6 Ma period. In contrast, the C-31 n-alkanes dominate the loess-paleosol sediments, and biomarkers vary with relatively greater amplitude and higher frequency, indicating that grasses dominated the CLP and the climate was more and and variable. These biomarker records chronicle a drying and cooling trend on the CLP since 4 Ma. These records can be further divided into four stages with boundaries around 5.6, 3.8 and 2.6 Ma, indicating that the CLP vegetation and climate experienced four evolutionary phases, broadly consistent with those inferred from other available proxy data.