CO₂ maximum in the oxygen minimum zone (OMZ)

Abstract. 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 CO₂" century, probably due to global warming. OMZs are also known to significantly contribute to the oceanic production of N₂O, a greenhouse gas (GHG) more efficient than CO₂. However, the contribution of the OMZs on the oceanic sources and sinks budget of CO₂, the main GHG, still remains to be established.

We present here the dissolved inorganic carbon (DIC) structure, associated locally with the Chilean OMZ and globally with the main most intense OMZs (O₂<20 μmol kg⁻¹) in the open ocean. To achieve this, we examine simultaneous DIC and O₂ data collected off Chile during 4 cruises (2000–2002) and a monthly monitoring (2000–2001) in one of the shallowest OMZs, along with international DIC and O₂ 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 CO₂. 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 O₂ decrease (assuming classical C/O molar ratios). This "carbon deficit" would be related to regional thermal mechanisms affecting faster O₂ 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.