Seasonal and depth-related changes in the source of sinking particles in the North Atlantic

Abstract

LARGE, fast-sinking particles are important in the downward transport and redistribution of biogeochemical species in the deep ocean. Using nitrogen isotope ratio, ¹⁵N/¹⁴N, as an in situ tracer, we investigate the source and transformation of these particles in the North Atlantic ocean. We observe seasonal variations in δ¹⁵N associated with seasonal changes in near-surface nitrate concentration and particle flux; the nitrogen isotope variations are consistent with, but much larger than, previously observed variability^1,2. Our results show that the signal from these near-surface changes propagates rapidly into the deep ocean, but is modified depending on the phase of the seasonal production cycle. Surprisingly, we find that δ¹⁵N values for sinking particles decrease with depth during low-flux periods—behaviour that may occur generally in the open ocean. The sinking particles must therefore be either gaining light nitrogen or losing heavy nitrogen, an effect that we believe requires there to be another source of sinking particles, apart from recent surface production.