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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. [ABSTRACT FROM AUTHOR]