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

摘要

Compound-specific C-13/C-12 ratios of the C-23, C-25, C-27, C-29 and C-31 n-alkanes in the Hongyuan peat sequence from southwest China were measured to decipher paleoenvironmental information recorded in the delta C-13 variations over the last 13 ka. The delta C-13 values of the n-alkanes range between -35.4 parts per thousand and -30.5 parts per thousand, falling within the range of those from modern C-3 peat-forming vegetation. However, the vertical trends do not match with those of the delta C-13 value for the C-3 peat-forming plant cellulose. Such a discrepancy between the delta C-13 profiles implies that the n-alkane delta C-13 values are unlikely to reflect signals from emerged aquatic plants in the bog. Because submerged/floating aquatic plants are major contributors of mid-chain (C-23 and C-25) n-alkanes in the sequence, the decoupling between the C-23 and C-25 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 delta C-13 values of submerged/floating aquatic plant n-alkanes (C-23 and C-25) reveals two prominent positive excursions (0.8-2.4 parts per thousand) during the early to middle Holocene. The excursions coincide with peat accumulation maxima and stronger Indian monsoon activity in southwest China, indicating that the delta C-13 variations in submerged/floating aquatic plants are closely related to changes in bog primary productivity controlled by the Asian monsoon activity. (C) 2010 Elsevier Ltd. All rights reserved.