D. Ruiz Pino, A. Paulmier,
Y. Du Penhoat, O. Ulloa

Laboratoire de Biogéochimie et Chimie Marines
(LBCM), Case Courrier 134, Tour 24, 5ème étage,
Université Pierre et Marie CURIE (PARIS VI) 4, pl.
Jussieu, 75 252 PARIS Cédex 05, France,
E-mail: ruiz@ccr.jussieu.fr, paulmier@ccr.jussieu.fr

The increasing concentrations of greenhouse gasses (CO₂ and N₂O) in the atmosphere are the most striking of perturbations measured at global scale. In this context, it becomes of primary concern to quantify the ability of the ocean to absorb or outgas carbon dioxide. The role of the northern ocean has been thoroughly investigated in the framework of the JGOFS (Joint Global Ocean Flux Study) and WOCE (World Ocean Circulation Experiment). However, the variability of the greenhouse gases, regarding their sources and sinks has been poorly documented in the southern ocean. Then, in the Southern hemisphere the strongly increasing population of developing countries represents a potential anthropic source of CO₂ and N₂O for the atmosphere.

Among the regions of intense gaseous exchanges between the ocean and the atmosphere and the more pronounced OMZ are the upwelling regions located along the western boundaries of the continents. There, due to equatorward coastal winds and the Coriolis force, coastal surface waters are advected off shore and replaced by cold nutrient rich deep waters which generate a high productivity when reaching the illuminated layers. This productivity results into storage of carbon by sequestration of organic matter into the deep ocean. However, this carbon sink may be compensated by the outgassing of the deep waters brought to the surface, which have been enriched in CO₂ along their path in the deep ocean. As a consequence it remains unclear whether upwelling areas are sources or sinks of carbon for the atmosphere (Liu et al. 2001).

The Humbolt current system, located along the western coasts of South America, is one of the most productive regions of the global ocean. The productivity in this upwelling region is strongly modulated by the El Niño phenomenon, which generates interannual fluctuations of the equatorward winds and of the nutricline depth near the coast. Moreover, subsurface coastal waters contain the smallest quantities of dissolved oxygen encountered in the ocean. This anoxic water mass, named the Oxygen Minimum Layer (OML), is fed by the coastal Peru-Chile undercurrent that transports poleward nutrient rich anoxic waters of equatorial origin. The spatial and temporal variability of the OML constrains the productivity by consuming nutrients during denitrification processes, produces nitrous oxide, a major greenhouse gas, and perturbs higher trophic levels (Ulloa et al., 2001). Both the origin and variability of the OML and its feedback on productivity and climate effects are among the less known mechanism in the ocean. Moreover, the future and past climatic changes associated with the atmosphere greenhouse gases increase drive to an Oxygen Minimum Layer (OML) extension or contraction. What are the physical and biogeochemical processes responsible for the OML formation? What are the OML feedbacks on both marine productivity and CO₂ and N₂O fluxes?

The main objectives of the CHOC (Chile-Peru Oxygen and Circulation) experiment is to study the dynamical and biogeochemical processes that modulate the OML variability, and to analyze the OML extension impact on productivity and CO₂/N₂0 fluxes from days to the Holocene time scales. The CHOC goals are related to the international SOLAS program (Surface Ocean Low Atmosphere Study) and associated to the International IGBP initiative PJTT (Paleo Joint Global Ocean Flux Study Task Team).

This project is organized through collaborations between French and Chilean research groups. The CHOC program is based in an international cooperation between French, Danish, North and South American oceanographic laboratories. It is financially supported by the French INSU (Institut National de Sciences de l'Univers), the Chilean COPAS-FONDAP and Inter-American Fundacion Andes.

The experimental design is based on a multidisciplinary cruise between Callao (10°S) and Puerto Montt (45°S). Hydrological, biological, biogeochemical and sediment parameters will be measured during two legs in December 2003-January 2004. The cruise transects have been designed to study the vertical and cross shore and meridional structure of the Peru-Chile undercurrent, of the OML and the spatial variability of the ocean atmosphere fluxes of CO₂ and N₂O. Five fixed stations will be located in upwelling cells of different intensity and oxygen concentration, in the north along the Peruvian coast where the most intense upwelling is quasi permanent during non El Niño years, and further south where the upwelling is weaker and seasonal. During this cruise, the variability of biogeochemical tracers and the high frequency variability of the undercurrent linked to internal and coastal-trapped waves will be measured for three days. Data from current meters and sediment traps deployed before and during the cruise will be used to replace the quasi synoptic observations in the context of intra seasonal, seasonal and interannual variability of the current system and carbon /nitrogen fluxes. Sediment cores will be used to reconstruct the spatial variability of the palaeoproductivity and minimum O₂ concentration in the various upwelling provinces. The acquisition of simultaneous sediment cores and sediment trap data in an area with a very high productivity could be used to infer paleoproxies. The CHOC experiment will provide a data set that will be analysed in the years 2003-2005 in the light of a hierarchy of models, ranging from relatively simple inverse models to more complex biogeochemical prognostic models.

On-going work for the development of a biogeochemical model called MINOC (Min…), adapted to the Peru-Chile area and taking into account processes associated with the oxygen minimum and CO₂ and N₂O fluxes, will be presented.