Molecular characterization of the sedimentary organic matter deposited off central Peru (12 – 14ºS): first insights into preservation processes

Phytoplankton production represents the ultimate source of organic matter in the ocean; thus, the study of organic compounds can give us information related to organic matter (OM) origin and transformations. Usually less than 1% of the OM produced in the ocean surface reaches the seafloor, although in highly productive regions nearly 10% can be buried and subjected to further degradation. The Peruvian upwelling system is among the most productive marine ecosystems in the world ocean, with high primary production sustained mainly in a year-round upwelling. Along the Peruvian continental margin, variations in primary production, bottom dissolved oxygen, and depth influence OM accumulation and preservation, and thus determine the existence of different depositional environments. Previous geochemical and palaeoceanographic studies have shown that the best records of well-preserved OM are found towards the central area of the Peruvian continental margin, between 12°S and 14°S. Therefore, the study of organic compounds, particularly lipids, deposited in surface sediments could give us information regarding early diagenetic processes related to OM degradation/preservation. The objective of this study was to characterize both the solvent extractable OM fraction (i.e. free lipids) and the insoluble OM fraction (i.e. protokerogen) in order to elucidate possible preservation mechanisms involved in OM accumulation. A total of 14 surface sediment samples from different locations between 12°S and 14°S were analyzed by means of gas chromatography mass spectrometry. Organic compounds such as short-chain and long-chain alkanes and fatty acids were quantified in the solvent-extractable OM fraction, which allowed the calculation of a pristane/phytane index and a carbon preference index. In the insoluble OM fraction, alkanes and fatty acids were also quantified together with dithiophene and benzothiophene compounds and organic sulfur heterocompounds. Overall, our results allowed a detailed geochemical molecular characterization of the OM deposited in surface sediments beneath one of the most productive areas of the Peruvian coast. The differences observed in both the n-alkane and fatty acids distribution between the solvent-extractable OM fraction and the insoluble OM fraction, together with the quantification of sulfur compounds in the insoluble fraction, suggests that complex diagenetic processes occur in surface sediments. An important part of the freshly-produced OM in the highly productive surface waters off central Peru reaches the seafloor and undergoes preservation mechanisms mainly related to natural sulfurization and selective preservation, tightly coupled to the reduced conditions that characterize surface sediments in the area.