Isotopic tracers of the marine nitrogen cycle: Present and past

Abstract

The oceanic nitrogen cycle consists of a web of microbially mediated transformations driven in part by the large range in possible nitrogen oxidation states. Many of these transformations have corresponding isotope fractionation effects, usually leaving the product depleted in 15N (δ15 N). Due to the complexity of the nitrogen cycle, observed patterns of isotopic ratio could be expected to defy explanation. However in reality, a few geographically separated processes dominate the larger spatial and temporal scales in the open ocean. These are (1) NO3 - assimilation by phytoplankton, (2) N2 fixation, and (3) denitrification. The latter two have particular importance as the principal source and sink, respectively, of combined nitrogen to the ocean. As such, they together control the oceanic inventory for combined nitrogen which in turn is a factor controlling marine plant production and organic matter flux from the surface to the ocean's interior. Taking into account the effective isotopic fractionation effects for N2 fixation and denitrification, the modern average δ15 N for the ocean is a potentially important constraint on the modern marine nitrogen budget. Past variation in these processes can be reconstructed on time scales from decades to millions of years from sediment cores with good preservation of organic matter. In particular, temporally well resolved δ15N records show large variations in the three major water column denitrification regions in response to climate variations. Collectively, these variations in denitrification likely produced significant changes in the oceanic combined nitrogen inventory which appears to be confirmed by global-scale changes in δ15N across the last deglaciation. © Springer-Verlag Berlin Heidelberg 2005.