Use of lipid biomarker patterns as a proxy of environmental variability in the coastal sedimentary record from the Gulf of Cádiz, SW Spain

Abstract

Surface sediments from the Gulf of Cádiz were analyzed using gas chromatography-mass spectrometry in order to estimate the origin, spatial distribution and degradation extent of the sedimentary organic matter (OM) from the inner continental shelf of the southwest Iberian Peninsula. A wide variety of lipid assemblages (n-alkanes, n-alkan-2-ones, n-aldehydes, n-fatty acids, α,ω-alkanedioic acids, diterpene resin acids and isoprenoids) were extracted with organic solvent and exhibited local variations depending on OM sources. Whereas the sediments located under the Guadiana river's influence showed dominant continental derived, long chain (>C20) distributions, typically marine-derived short chain (<C20) alkyl compounds were detected in the offshore sediments. Miscellaneous OM source signatures, derived from algae, sea grasses, microbial biomass and higher plants, were displayed by those sediments of intermediate location, indicating a NW-to-SE transition from terrigenous to marine dominant composition. The relative abundance of unsaturated C16:1 and C18:1 fatty acids, the presence of the strong reductive nature of the n-aldehydes, the occurrence of molecular sulfur in most of the extracts and the low pristane/phytane ratio values detected appear to suggest little drastic alteration in the sedimentary OM in the area. The study illustrates the relevance of using molecular biomarkers combined with bulk geochemical proxies to evaluate OM sources and continental influence in river dominated ocean margins.