DELTA-N-15 IN SEDIMENTING PARTICLES AS INDICATOR OF EUPHOTIC-ZONE PROCESSES

Stable nitrogen isotopes (delta(15)N) were analyzed in sedimenting par ticles in the northern North Atlantic at the Voring Plateau at 500 m d epth and in the Lofoten Basin at 500, 1000 and 3000 m depth over a per iod 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 delta(15)N-PON (par ticulate organic nitrogen) was constructed and supplemented by calcula tions of delta(15)N-PON from nitrate data (after Rayleigh fractionatio n formulae). Sediment trap delta(15)N-PON values from 500 m showed a c lear seasonal signal with a 7 parts per thousand drop from winter to s pring and a similar increase again in winter. This seasonal pattern in delta(15)N of sedimenting particles also occurred in trap collections at 3000 m depth with a delay of approximately two months. It is assum ed that fractionation during nitrate uptake in spring and early summer and increased sedimentation of isotopically light phytodetritus are t he main reasons for this pronounced seasonal pattern. Surface water de gradation processes, uptake of isotopically heavy nitrate, and heterot rophic activity cause the increase in delta(15)N-PON in sinking matter towards autumn. In winter the nitrogen isotope values remain isotopic ally heavy. The observed seasonal pattern and quantity of sinking flux es compared well with the calculations. Surprisingly, the suspended pa rticle pool below the winter mixed layer also had a seasonal signal, w ith a 5 parts per thousand increase in delta(15)N between June and Sep tember and a decrease towards November. In the absence of intense biol ogical fractionation the sinking and suspended particle pools obviousl y undergo considerable exchange of material, which seems to be more in tense during spring and less during autumn and winter. The nitrogen is otope signal in the particulate material in the deep northern North At lantic is clearly dominated by the fractionation of the limiting nitra te pool, causing a much higher amplitude than at lower latidudes.