Postglacial floral and climate changes in southeastern China recorded by distributions of n-alkan-2-ones in the Dahu sediment-peat sequence

Abstract

Long-chain n-alkane-2-ones occur widely in lacustrine and peat depositional settings; however, their paleobotanical and paleoclimate implications are less understood, compared with their n-alkyl counterparts (n-alkanes, n-fatty acids). In this study, we have performed a molecular stratigraphic investigation of changes in the plant contributions and depositional conditions using n-alkan-2-ones in the Dahu lacustrine-peat sequence spanning the last 17 kyr. A series of n-alkan-2-ones ranging from C23 to C33 with a maximum at C29 appears to be contributed by microbial oxidation of both n-alkanes and n-alkanoic acids. The values of the average chain length (ACL) and the ratios of mid-to long-chained n-alkan-2-ones display a trend comparable with the abundance of trees relative to herbs and ferns. In addition, the carbon predominance index (CPI) of n-alkan-2-ones varied in pace with lithology/paleoenvironment, with higher values seen in peat deposits. The alternating layers represent a succession of shallow lakes and peatlands that filled the depositional basin in response to paleoclimate changes between wetter and less wet conditions. The succession of paleohydrologic conditions is responsible for the variation of ketone CPI values in this peat-sediment sequence.