The hydrogen isotopic compositions of sedimentary mid-chain n-alkanes record ecological change at a Portuguese paleowetland

Abstract

The origin of mid-chain (C23 and C25) and C27n-alkanes in sedimentary archives can be ambiguous. In coastal Portugal, a peat deposit representing a mid-Holocene paleowetland (PRC-South) that was initially dominated by Sphagnum presents an ideal setting for assessing methods of mid-chain n-alkane source elucidation. In particular, we examine a comprehensive record of the difference in δD between mid-chain (δDmid) and C29n-alkanes (ΔDmid-C29) in relation to n-alkane molecular distributions and compound-specific δ13C values in order to improve interpretations of paleoenvironmental change and evaluate the reliability of δDmid values for hydrologic reconstructions. Mid-chain n-alkane production remains significant at PRC-South after a substantial Sphagnum decline, yet an increase in ΔDmid-C29 values indicates a primarily terrestrial origin of mid-chain n-alkanes in the upper 50 cm of the deposit. This progression towards more positive ΔDmid-C29 values coincides with a relative increase in n-C27 abundance and potentially represents the replacement of Sphagnum moss by tree species that produce abundant mid-chain n-alkanes, such as Betula, Quercus, or Fagus spp. According to our analysis, ΔDmid-C29 displays promise as a tool for mid-chain n-alkane source attribution in Sphagnum paleoenvironments, but interspecies variability of hydrogen isotope fractionation in Sphagnum mosses and terrestrial vegetation could critically hinder its application. Our use of ΔDmid-C29 at PRC-South ultimately exemplifies the importance of accounting for vegetation composition when qualitatively or quantitatively interpreting sedimentary δDn-alkane values, particularly in Sphagnum wetlands.