δ15N in sedimenting particles as indicator of euphotic-zone processes

Stable nitrogen isotopes (δ15N) were analyzed in sedimenting particles in the northern North Atlantic at the Vöring Plateau at 500 m depth and in the Lofoten Basin at 500, 1000 and 3000 m depth over a period of up to 3 years. Suspended particles were sampled throughout the entire water column in early summer, autumn and winter in the Lofoten Basin only. From this data set a seasonal cycle of δ15N-PON (particulate organic nitrogen) was constructed and supplemented by calculations of δ15N-PON from nitrate data (after Rayleigh fractionation formulae). nt trap δ15N-PON values from 500 m showed a clear seasonal signal with a 7‰ drop from winter to spring and a similar increase again in winter. This seasonal pattern in δ15N of sedimenting particles also occurred in trap collections at 3000 m depth with a delay of approximately two months. It is assumed that fractionation during nitrate uptake in spring and early summer and increased sedimentation of isotopically light phytodetritus are the main reasons for this pronounced seasonal pattern. Surface water degradation processes, uptake of isotopically heavy nitrate, and heterotrophic activity cause the increase in δ15N-PON in sinking matter towards autumn. In winter the nitrogen isotope values remain isotopically heavy. The observed seasonal pattern and quantity of sinking fluxes compared well with the calculations. singly, the suspended particle pool below the winter mixed layer also had a seasonal signal, with a 5‰ increase in δ15N between June and September and a decrease towards November. In the absence of intense biological fractionation the sinking and suspended particle pools obviously undergo considerable exchange of material, which seems to be more intense during spring and less during autumn and winter. The nitrogen isotope signal in the particulate material in the deep northern North Atlantic is clearly dominated by the fractionation of the limiting nitrate pool, causing a much higher amplitude than at lower latitudes.