Temporal variations in ^1^3C and ^1^4C concentrations in particulate organic matter from the southern North Sea

As a new approach for the characterization and determination of the origin of particulate organic matter (POM) in coastal waters, we measured the 14C activity and 13C/ 12C isotope ratios and applied molecular analysis by means of AMS, IRMS and pyrolysis-GCMS for both bulk samples and isolated fractions of POM from the North Sea off the Dutch coast. The fractions were obtained by a sequence of chemical treatments separating polysaccharides, proteins, lipids and non-hydrolysable resistant compounds. The 13C/ 12C and 14C/ 12C isotope ratios, the 14C activities and the polysaccharide/protein contents were high in the spring and summer samples and significantly lower in the autumn and winter samples. This is explained by the high amount of marine phytoplankton in summer and spring, and the presence of detrital material in autumn and winter. It was noted that phytoplankton as found in the spring and summer samples was ca. 20% enriched in 14C with respect to natural values, very likely caused by 14C contamination of the water from the English Channel by the nuclear fuel reprocessing plant at La Hague, Normandy, France. The 13C/ 12C isotope ratios and 14C activity data show that the winter sample was not a simple mixture of relatively recent marine phytoplankton and POM from the rivers Rhine and Meuse. The main source of both terrestrial and marine organic matter in POM in winter is resuspended organic matter derived from eroded sea floor deposits with relatively low contributions of polysaccharides and proteins.