Leaf wax lipids as paleovegetational and paleoenvironmental proxies for the Chinese Loess Plateau over the last 170 kyr

Abstract

High resolution records for long chain n-alkane and n-alkanol biomarkers have been obtained by extraction of sediments from the profiles of Luochan (0–36 ka) and Xunyi (0–170 ka) on the Chinese Loess Plateau (CLP). The two records are similar where they overlap over the last 36 ka. At Xunyi, distributions of the n-alkane and n-alkanol homologues remain relatively constant over the full 170 kyr. The n-alkanes are characterized by high odd over even carbon number preference and predominance of the C 31 n-alkane, whereas the n-alkanols are dominated by even over odd carbon numbers and by the C 28 n-alkanol. An Alkane Index [A.I., C 3l/(C 29+C 31)] and an Alkanol Index [Alc.I., C 30/(C 28+C 30)] are both positively related with magnetic susceptibility of the sediment, with higher values during interglacials. A Lipid Index, L.I., averaging the normalized Alkane and Alkanol indices, gives a measure of changing vegetation for palaeo-environmental construction. A further useful index is provided by the δ 13 C value of the most abundant n-alkane, C 31, after adjustment for the calculated effect of the estimated atmospheric pCO 2 and precipitation. The record of this index over the last 170 kyr, which varies from −36 to −32‰, indicates that the vegetation on the CLP has been mainly C 3 plants and became even more so in the glacial periods (especially MIS 6 and 2) as the minor population of warm season grasses (C 4) favored in interglacial times declined when temperatures fell. The lipid distributions and carbon isotope values are in general accord with the pollen and phytolith evidence recorded for the Yuan areas of the CLP in indicating steppe or meadow-steppe vegetation, with abundant cold season grasses (C 3) and some herbs, bushes and trees, especially in glacial times.