Restricted utility of δ 13C 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 13C of bulk organic matter. The correlations suggest that the observed glacial–interglacial variations of δ 13C 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 13C analysis of bulk organic matter in some loess–paleosol sequences of the Chinese Loess Plateau.