A Phosphate Minimum in the Oxygen Minimum Zone (OMZ) off Peru

The Oxygen Minimum Zone (OMZ) off Peru is known to be associated with the advection of Equatorial SubSurface Waters (ESSW), rich in nutrients and poor in oxygen, through the Peru-Chile UnderCurrent (PCUC), but this circulation remains to be refined within the OMZ. During the Pelágico cruise in November-December 2010, measurements of phosphate revealed the presence of a phosphate minimum (Pmin) in various hydrographic stations, which could not be explained so far and could be associated with a specific water mass. This Pmin, localized at a relatively constant layer (~20<220 m) and with a patchy distribution mainly between 10 and 16°S, is confirmed and characterized in details from the complementary hydrological data acquired during the German Meteor cruise M77 (Legs 3 and 4, January-February 2009). The significant Pmin present an intense minimum with a mean vertical phosphate decrease of 0.6 µM but highly variable between 0.1 and 2.2 µM. In average, these Pmin are associated with a predominant mixing of SubTropical Under- and Surface Waters (STUW and STSW: ~20 and ~40%, respectively) within ESSW (~25%), complemented evenly by overlying (ESW, TSW: ~8%) and underlying waters (AAIW, SPDW: ~7%). The hypotheses and mechanisms leading to the Pmin formation in the OMZ are further explored and discussed, considering the physical regional contribution associated with various circulation pathways ventilating the OMZ and the local biogeochemical contribution including the potential diazotrophic activity.