Environmental influences over the last 16 ka on compound-specific δ13C variations of leaf wax n-alkanes in the Hani peat deposit from northeast China

Compound-specific carbon isotope ratios (δ13C) of leaf wax n-alkanes (C21–C33 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 δ13C values of n-alkanes range between −36.6 and −30.7‰, showing that the compounds originate from C3 plants. Much larger variations (~ 5.4‰) in the n-alkane δ13C values than those of atmospheric CO2 during the last 16 ka (< 0.5‰) indicate that the isotopic values were affected by environmental factors in addition to the postglacial δ13C variations in the atmospheric reservoir. The stratigraphic records of δ13C reveal decoupled fluctuations among the individual n-alkanes, particularly between 15.5 and 11.4 ka. Synchronous excursions in the δ13C offsets among individual n-alkanes (Δδ13C) and lipid biomarker paleoplant proxies (Paq, and C23/C31 and C27/C31) 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 δ13C variations. Parallel fluctuations of the δ13C values of terrestrial higher plant-derived C29 and C31n-alkanes and the n-alkane average chain-length (ACL) from 11 to 6 ka indicate that the δ13C variations responded to net evaporation changes. Negative shifts in the n-alkane δ13C 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.

Research Highlights
► Stable carbon isotope ratios of n-alkanes were measured in the Hani peat deposit.
► δ13C profiles reveal decoupled fluctuations among the individual n-alkanes.
► A comparison between δ13C records and n-alkane ratios reveals vegetational changes.
► Negative shifts in δ13C variations indicate enhancement of summer monsoon rainfall.