Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems

Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C-17, n-C-19, n-C-21, n-C-23 and n-C-25 alkanes, algae (Rhodophyta, Chlorophyta) produced n-C-15 and n-C-17 and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C-25, n-C-27, n-C-29, n-C-31 and n-C-33. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53 +/- 19%) and terrestrial (ca. 41 +/- 17%) material, followed by algae (ca. 6 +/- 9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones - the TAR (C-27 + C-29 + C-31/C-15 + C-17 + C-19; terrestrial to aquatic ratio) and the P-aq (C-23 + C-25/C-23 + C-25 + C-29 + C-31; aquatic plant %) - with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems. (C) 2015 Elsevier Ltd. All rights reserved.