Paleoenvironmental significance of compound-specific δ13C variations in n-alkanes in the Hongyuan peat sequence from southwest China over the last 13 ka

Compound-specific 13C/12C ratios of the C23, C25, C27, C29 and C31 n-alkanes in the Hongyuan peat sequence from southwest China were measured to decipher paleoenvironmental information recorded in the δ13C variations over the last 13 ka. The δ13C values of the n-alkanes range between −35.4‰ and −30.5‰, falling within the range of those from modern C3 peat-forming vegetation. However, the vertical trends do not match with those of the δ13C value for the C3 peat-forming plant cellulose. Such a discrepancy between the δ13C profiles implies that the n-alkane δ13C values are unlikely to reflect signals from emerged aquatic plants in the bog. Because submerged/floating aquatic plants are major contributors of mid-chain (C23 and C25) n-alkanes in the sequence, the decoupling between the C23 and C25 n-alkanes and the peat cellulose likely reflects the situation that these mid-chain homologues primarily record the isotopic signals of submerged/floating aquatic plants. The stratigraphic profile of δ13C values of submerged/floating aquatic plant n-alkanes (C23 and C25) reveals two prominent positive excursions (0.8–2.4‰) during the early to middle Holocene. The excursions coincide with peat accumulation maxima and stronger Indian monsoon activity in southwest China, indicating that the δ13C variations in submerged/floating aquatic plants are closely related to changes in bog primary productivity controlled by the Asian monsoon activity.