Contrasting sources of plant wax n-alkanes and n-alkanoic acids in gulf of mexico sediments (ODP 625B)

Abstract

Long chain plant waxes (n-alkanes, n-alkanoic acids, and n-alcohols) and their carbon isotopic compositions (δ13C) in geologic archives are valuable tools for paleovegetation reconstruction. However, the sensitivity of different plant wax constituents to vegetation shift is not well understood. This study explores controls on the variation in d13C values of long-chain n-alkanes (C27 to C33) and n-alkanoic acids (C26-C30) in the Gulf of Mexico core sediments (ODP 625B) near the Mississippi River delta. n-Alkanoic acids’ δ13C values were higher than those of n-alkanes by 1-2‰ on average and such a pattern is the opposite from their isotope fractionation observed in living plants: 1-2‰ smaller in n-alkanes than nalkanoic acids. We attribute this offset to contributions from aquatic plants or microbes that produce high concentrations of 13C-enriched long-chain n-alkanoic acids. The sensitivity of n-alkanes and n-alkanoic acids to vegetation and climate varied among chain lengths. The n-C33 alkanes were most sensitive to C4 grassland expansion among n-alkane homologues, while no specific trend was observed in n-alkanoic acids. This is due to the similarity in n-alkanoic acid concentrations between C3 and C4 plants by homologues and low terrestrial plant-derived n-alkanoic acid contributions to the sediments. The results of this study suggest that long chain n-alkanoic acids’ δ13C values in sediments may be influenced by contributions from different sources such as aquatic plants or microbial inputs and therefore interpretations regarding this matter should be cautiously formulated. We suggest that there is a need for further studies on characterizing long-chain n-alkanoic acids (C26-C34) in aquatic plants and microbes from various climates and environments in order to investigate their production and integration into sedimentary archives. © 2019, Korea Ocean Research and Development Institute. All rights reserved.