Are sediment-derived long-chain n-alkanes a reliable paleoenvironmental indicator?

Abstract:

In soil, the distribution and carbon isotopic composition of n-alkane (δ¹³C_n-alk) is extremely source-specific. Generally, higher plants are primary producers of long-chain n-alkanes (C₂₇-C₃₃) while short-chain n-alkanes (C₁₄-C₂₀) are mainly contributed by microorganisms with distinct δ¹³C_n-alk values. Hence, the distribution and δ¹³C_n-alk values in soil is often used to characterize the source(s) of organic matter and to gain insight into paleoenvironmental conditions. However, information related to the post-depositional modification of plant-derived n-alkanes in soil is largely enigmatic. In order to comprehend the factors affecting the distribution and δ¹³C values of long-chain n-alkanes in soil, we have analyzed biomarkers from three soil profiles developed within an oxbow lake setting from the lower Gangetic plain, India. The n-alkanes exhibit a bimodal distribution suggesting contributions from both terrestrial higher plants and soil microbes. The concentration of C₂₇-C₃₃ homologues within the soil profiles decreases with depth, indicating a significant loss of n-alkanes during soil-forming processes. In the soil profiles, the δ¹³C_n-alk values exhibit a high degree of variability across C₂₇-C₃₃ homologues. On the contrary, the δ¹³C_n-alk values of the modern C₃ and C₄ plants show minimal variation (~2‰) across C₂₇-C₃₃ homologues. This suggests that the observed variation in the δ¹³C_n-alk values is a result of microbial degradation of n-alkanes during its transfer from leaves to soil. Additionally, the inter-homologue variation in the δ¹³C_n-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 δ¹³C values of C₂₇-C₃₃ homologues have been primarily used to estimate the relative abundance of C₃ and C₄ plants in the past, significant inter-homologue variations in the δ¹³C_n-alk values would lead to 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 δ¹³C_n-alk values is a cumulative product of temporal changes in the vegetation composition and post-depositional modifications experienced by the biomarker.