Abstract

Measurements of dissolved gases (O2, N2O), nutrients (NO3 -, NO2 -, PO4 3-), and oceanographic variables were performed off northern Chile (∼21°S) between March 2000 and July 2004, in order to characterize the existing oxygen minimum zone (OMZ) and identify processes involved in N2O cycling. Both N2O and NO3 - displayed sharp, shallow peaks with concentrations of up to 124 nM (1370% saturation) and 26 μM, respectively, in association with a strong oxycline that impinges on the euphotic zone. NO2 - accumulation below the oxycline's base reached up to 9 μM. The vertical distribution of physical and chemical parameters and the existing relationships between apparent oxygen utilization (AOU), apparent N2O production (ΔN2O), and NO3 - revealed three main layers within the upper OMZ. The first layer, or the upper part of the oxycline, is located between the base of the mixed layer and the mid-point of the oxycline (around σt=25.5 kg m-3). There the O2 declines from ∼250 to ∼50 μM, and strong (but opposing) O2 and NO3 - gradients and their associated AOU-ΔN2O and AOU-NO3 - relationships indicate that nitrification produces N2O and NO3 - in the presence of light. The second layer, or lower part of the oxycline, represents the upper OMZ boundary and is located between the middle and the base of the oxycline (25.9<σt<26.1 kg m-3). In this layer NO3 - reduction begins at O2 levels ranging from ∼50 to ∼11 μM and accumulation of 41-68% of the ΔN2O pool occurs. The accumulation of N2O (but not of NO2 - or NH4 +) and the observed AOU-ΔN2O and AOU-NO3 - relationships (which are opposite to those of the overlying first layer) suggest that a coupling between nitrification and NO3 - reduction is involved in N2O cycling in this second layer. The third layer is the OMZ core, where the O2 concentration remains constant (O2<11 μM). It coincides with σt>26.2 kg m-3, which is typical of Equatorial Subsurface Water (ESSW). In this layer, N2O and NO3 - continue to decrease, but a large NO2 - accumulation is observed. Considering all the data, a biogeochemical model for the upper OMZ off northern of Chile is proposed, in which nitrification and denitrification differentially mediate N2O cycling in each layer. © 2006 Elsevier Ltd. All rights reserved.