[MIS25-P03] Are sediment-derived long-chain n-alkanes a reliable paleoenvironmental indicator?
キーワード：Biomarker, Carbon Isotope, Microbial degradation, Paleoenvironment
In soil, the distribution and carbon isotopic composition of n-alkane (d13Cn-alk) is extremely source-specific. In general, terrestrial higher plants are primary producers of long-chain n-alkanes (C27-C33) while short-chain n-alkanes (C14-C20) are mainly contributed from microorganisms with distinct d13Cn-alk values. Hence, the distribution and d13C values of n-alkane in sediments have been primarily used to characterize the source(s) of organic matter in a given depositional environment as well as to gain insight into paleoenvironmental conditions. However, very little is known about post-depositional modification of plant-derived long-chain n-alkanes within the soil. Therefore, the present study aims to understand the factors affecting the distribution and d13C values of long-chain n-alkanes in the soil as well as evaluate their potential as paleoenvironmental indicators. In this context, three soil profiles developed within an oxbow lake setting from the lower Gangetic plain, West Bengal, India has been used. The n-alkanes exhibit a bimodal distribution suggesting contributions from both terrestrial higher plants and soil microbes. The concentration of long-chain n-alkanes (C27-C33) within the soil profiles decreases with depth, indicating a significant loss of biomarkers during soil-forming processes. In the soil profiles, the d13Cn-alk values exhibit a high degree of variability across the higher molecular weight homologues (C27-C33). On the contrary, the d13Cn-alk values of the modern C3 and C4 plants from the study area show minimal variation (~2‰) in the isotopic composition across its homologues (C27-C33). This suggests that the observed variation in the d13Cn-alk values in the soil is a result of microbial degradation of n-alkanes during its transfer from leaves to soil. Additionally, the inter-homologue variation in the d13Cn-alk values shows an increasing trend with depth (from 6.0‰ to 9.6‰) suggesting that older soil samples have undergone more microbial alteration compared to the younger ones. As the d13C values of higher molecular weight homologues (C27-C33) have been primarily used to estimate the relative abundance of C3 and C4 plants in the past, significant inter-homologue variations in the d13Cn-alk values can result in erroneous paleovegetational interpretations. Therefore, the present study suggests that caution must be exercised while using n-alkanes as a paleoenvironmental indicator as the variations in d13Cn-alk values in sediments is a cumulative product of temporal changes in the vegetation composition and post-depositional modifications experienced by the biomarker.