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 C31n-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 C25n-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.