Effects of redox conditions and temperature on the degradation of Sphagnumn-alkanes

Abstract

Leaf wax n-alkanes in peat deposits are important proxies for paleoenvironmental reconstructions. The n-alkane compositions of Sphagnum species, which are important peat forming plants, are commonly dominated by the C23 and C25 homologs. However, questions remain about the stability of Sphagnum-derived n-alkanes to early diagenetic alterations. In this study, a 13-month litterbag decomposition study using Sphagnum palustre collected from the Dajiuhu peatland, central China, was conducted under controlled laboratory conditions to assess the influences of redox (aerobic vs anaerobic) conditions and environmental temperatures (15 °C vs 25 °C) on the chain-length distributions and carbon isotopic compositions of Sphagnum n-alkanes. Results clearly reveal the preferential degradation under aerobic conditions of mid-chain n-alkanes relative to their long-chain homologs (C29 and C31), resulting in the increase of the relative proportions of long-chain n-alkanes. The change in n-alkane distribution is accompanied by an increase in the δ13C values of the mid-chain n-alkanes, whereas those of the long-chain n-alkanes remain nearly constant. In contrast, neither chain-length distributions nor n-alkane δ13C values were much affected under anaerobic degradation conditions. Statistical analysis reveals that redox conditions play significant roles in the degradation of Sphagnum n-alkanes, whereas the influence of air temperature is not significant. These findings will improve applications of n-alkane paleoenvironmental proxies to reconstruct the types of plants and the depositional conditions that contributed to peat sequences, particularly under settings with fluctuating water levels that affect redox conditions.