Tracing sinking organic matter sources in the NW Iberian upwelling system (NE Atlantic Ocean): comparison between elemental, isotopic and molecular indicators

Abstract

We studied the seasonal fluctuations in the composition of sinking particulate organic matter (POM) in the only upwelling-affected continental margin of Europe, to improve our understanding of the carbon (C) cycle of such systems. The methods employed targeted the elemental, stable isotope (δ13Coc and δ15N) and molecular (Py-GC-MS) composition. The results showed that the sinking POM in this margin is predominantly of marine origin, with δ15N (5.2 ± 0.3 ‰) and C/N (9.7 ± 1.0) values similar to those of marine plankton, and high abundances of pyrolysis products with isoprenoid structures, N-containing products from proteins (indoles, cyanobenzenes, etc.), and N-acetylated polysaccharides (acetamide, acetamidofurans) from chitin and peptidoglycan. Furthermore, the dataset demonstrates seasonal differences in POM composition under different oceanographic scenarios: i) dominance of most N-compounds, fatty acids and isoprenoid products during highly productive upwelling seasons, and ii) an increase in the relative abundance of linear alkanes/alkenes, phenols, lignin and PAH reflecting terrestrial influence during downwelling periods. In addition, the detection of isoprenoid alkylthiophenes traces (compounds formed under reducing conditions in the sediment) point to resuspended sediment in the trap material due to hydrodynamic effects of currents and waves on the seafloor. The results show that analytical pyrolysis is a useful tool for identifying different types of autochthonous and terrestrial POM, and therefore identification of the sources of sinking POM.