Environmental influences over the last 16 ka on compound-specific δ 13C variations of leaf wax n-alkanes in the Hani peat deposit from northeast China

Abstract

Compound-specific carbon isotope ratios (δ 13C) of leaf wax n-alkanes (C 21–C 33 odd carbon numbered n-alkanes) were measured in the Hani peat sequence from northeast China. These data were compared with lipid biomarker compositions to assess changes in local vegetation and paleoclimate for the last 16 ka. The δ 13C values of n-alkanes range between −36.6 and −30.7‰, showing that the compounds originate from C 3 plants. Much larger variations (~ 5.4‰) in the n-alkane δ 13C values than those of atmospheric CO 2 during the last 16 ka (< 0.5‰) indicate that the isotopic values were affected by environmental factors in addition to the postglacial δ 13C variations in the atmospheric reservoir. The stratigraphic records of δ 13C reveal decoupled fluctuations among the individual n-alkanes, particularly between 15.5 and 11.4 ka. Synchronous excursions in the δ 13C offsets among individual n-alkanes (Δδ 13C) and lipid biomarker paleoplant proxies (P aq, and C 23/C 31 and C 27/C 31) from 14.9 to 13.2 ka and 12.7 to 11.6 ka suggest that vegetational changes are the most likely causes for the decoupled δ 13C variations. Parallel fluctuations of the δ 13C values of terrestrial higher plant-derived C 29 and C 31 n-alkanes and the n-alkane average chain-length (ACL) from 11 to 6 ka indicate that the δ 13C variations responded to net evaporation changes. Negative shifts in the n-alkane δ 13C values coinciding with the ACL decreases at 10.5–9.3 ka and 8.1 ka indicate the short-term onset and fluctuations of the summer monsoon strength in eastern China during the early Holocene.