Compound-Specific δ13C of n-Alkanes: Clean-Up Methods Appraisal and Application to Recent Sediments of a Highly Contaminated Bay

Abstract

Different clean-up methods to determine isotopic composition of n-alkanes were tested. Sources of organic matter in Guanabara Bay were re-examined through the δ 13 C of individual n-alkanes in surface sediment samples. The n-alkanes were efficiently isolated without significant losses of the compounds of interest. The δ 13 C of n-alkanes (n-C 17 to n-C 35 ) ranged between -34.0 and -26.4‰. More depleted δ 13 C values associated to higher carbon preference index (CPI > 4) suggest prevalence of biogenic n-alkanes from terrestrial origin in the inner stations influenced by inputs from mangrove areas. Small isotopic differences between odd and even-numbered chains and 13 C-enrichment in long-chain homologues indicate a petrogenic hydrocarbons contribution. Higher concentrations of short-chain compounds associated to a more 13 C-enriched total organic carbon (TOC) were found near the most intense sewage discharges. Results show that δ 13 C of n-alkanes can be applied to a highly degraded system as the Guanabara Bay and help improve the characterization of the organic matter. Keywords: GC-C-IRMS, UCM, urea adduction, organic matter, Guanabara Bay IntroductionThe composition and distribution of hydrocarbons in sediment samples provide information about the organic matter sources and transformation processes occurring in the environment. 1 Studies on n-alkanes have often been conducted to assess the origin of hydrocarbons, such as biogenic (marine or terrestrial) or petrogenic, found in the marine environment. The assessment makes use of the difference in the n-alkane chain length to draw conclusions on the sources of organic matter. In higher plants, for example, n-alkanes from epicuticular waxes show long chains and an odd over even-numbered carbon chain predominance. 2 Algae are characterized by shorter chain lengths with no expressive odd-over-even preference. 3 In contrast, n-alkanes in fossil fuels have a wide composition range, without predominance of odd or even-numbered chains. 4 However, despite these typical fingerprints, there are cases in which n-alkane profile of different classes of organisms overlap, resulting in ambiguous interpretations of the organic material sources. 5 Given this multiplicity of origins, an interpretation of the different sources exclusively by the concentrations of n-alkanes is very limited in more complex environments. 6 During the last decades, the carbon isotopic analysis of some biomarkers have been widely used as a valuable tool to better distinguish the sources of organic matter in marine environments. 5,[7][8][9][10][11][12][13][14] The carbon isotope composition in the biosynthesized organic material depends on both the isotopic composition of the carbon source used by organisms and the isotope discrimination that occurs during carbon fixation, which includes the steps of absorption and intracellular diffusion, and also the biosynthesis of the cell components. 15 Thus, when two classes of organisms present an overlap in the n-alkane distribution, additional information can b...