Restricted utility of δ¹³C of bulk organic matter as a record of paleovegetation in some loess–Paleosol sequences in the Chinese Loess Plateau

Abstract

Molecular stratigraphic analyses using gas chromatograph-mass spectrometry have been performed in the upper section (S 0 , L 1 , S 1 ) of the Yuanbo loess–paleosol sequences in northwest China, with a record extending from the last interglaciation through the present interglaciation. The CPI (Carbon Preference Index) values of both n -alkanols and n -alkan-2-ones display variations between loess deposits and paleosols, showing a correlation with the magnetic susceptibility record, an indicator of the East Asian summer monsoon. The observed variations in the indexes in relation to changes in lithology/paleoclimate are proposed to result from microbial degradation of higher plant lipids in the paleosols. The CPI values of n -alkanes, n -alkanols, and n -alkan-2-ones are negatively correlated with δ 13 C of bulk organic matter. The correlations suggest that the observed glacial–interglacial variations of δ 13 C data in the loess stratigraphy reflect the relative importance of the contribution of paleovegetation compared with microorganisms (including both the degradation and the addition of organic matter) and allochthonous loess/soil parent materials. It is thus necessary to evaluate the contributions of the latter two before the paleovegetation can be reconstructed based on the δ 13 C analysis of bulk organic matter in some loess–paleosol sequences of the Chinese Loess Plateau.