CO2 maximum in the oxygen minimum zone (OMZ)

A. Paulmier, D. Ruiz-Pino, V. Garçon
2010 Biogeosciences Discussions  
Oxygen minimum zones (OMZs), known as suboxic layers which are mainly localized in the Eastern Boundary Upwelling Systems, have been expanding since the 20th "high CO 2 " century, probably due to global warming. OMZs are also known to significantly contribute to the oceanic production of N 2 O, a greenhouse gas (GHG) more efficient than CO 2 . However, the contribution of the OMZs on the oceanic sources and sinks budget of CO 2 , the main GHG, still remains to be established. We present here
more » ... We present here the dissolved inorganic carbon (DIC) structure, associated locally with the Chilean OMZ and globally with the main most intense OMZs (O 2 <20 µmol kg −1 ) in the open ocean. To achieve this, we examine simultaneous DIC and O 2 data collected off Chile during 4 cruises (2000)(2001)(2002) and a monthly monitoring (2000)(2001) in one of the shallowest OMZs, along with international DIC and O 2 databases and climatology for other OMZs. High DIC concentrations (>2225 µmol kg −1 , up to 2350 µmol kg −1 ) have been reported over the whole OMZ thickness, allowing the definition for all studied OMZs a Carbon Maximum Zone (CMZ). Locally off Chile, the shallow cores of the OMZ and CMZ are spatially and temporally collocated at 21 • S, 30 • S and 36 • S despite different cross-shore, long-shore and seasonal configurations. Globally, the mean state of the main OMZs also corresponds to the largest carbon reserves of the ocean in subsurface waters. The CMZs-OMZs could then induce a positive feedback for the atmosphere during upwelling activity, as potential direct local sources of CO 2 . The CMZ paradoxically presents a slight "carbon deficit" in its core (∼10%), meaning a DIC increase from the oxygenated ocean to the OMZ lower than the corresponding O 2 decrease (assuming classical C/O molar ratios). This "carbon deficit" would be related to regional Correspondence to: A. Paulmier (aurelien.paulmier@legos.obs-mip.fr) thermal mechanisms affecting faster O 2 than DIC (due to the carbonate buffer effect) and occurring upstream in warm waters (e.g., in the Equatorial Divergence), where the CMZ-OMZ core originates. The "carbon deficit" in the CMZ core would be mainly compensated locally at the oxycline, by a "carbon excess" induced by a specific remineralization. Indeed, a possible co-existence of bacterial heterotrophic and autotrophic processes usually occurring at different depths could stimulate an intense aerobic-anaerobic remineralization, inducing the deviation of C/O molar ratios from the canonical Redfield ratios. Further studies to confirm these results for all OMZs are required to understand the OMZ effects on both climatic feedback mechanisms and marine ecosystem perturbations.
doi:10.5194/bgd-7-6353-2010 fatcat:bc2u64f6tjck3oc4lf2dbdq3nm