CO<sub>2</sub> maximum in the oxygen minimum zone (OMZ)

Abstract

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 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 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 regionalCorrespondence 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.

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…This behavior is known as diel vertical migration (DVM) and also contributes to the activity of the biological pump as it enhances the export of organic matter from the photic zone by continued respiration, excretion and egestion in midwater layers (Burd et al, 2010;Hannides et al, 2009;Robinson et al, 2010;Steinberg et al, 2000). In addition to changes in temperature with depth, DVM organisms experience low O 2 concentrations during the daytime in OMZ regions (Brewer and Peltzer, 2009;Paulmier et al, 2011), and O 2 concentrations below a certain threshold level can restrict DVM of most zooplankton and nekton (e.g., Hauss et al, 2016). On a regional scale, the upper boundary of the oxycline is the single most critical factor structuring the habitat of most zooplankton organisms in the Peruvian upwelling system (Escribano et al, 2009).…”

The metabolic response of pteropods to acidification reflects natural CO&lt;sub&gt;2&lt;/sub&gt;-exposure in oxygen minimum zones

“…OMZs can therefore also be thought of as persistent carbon maximum zones (Paulmier et al, 2011). Specifically, in the ETP and Gulf of California around depths of 200 m, CO 2 levels reach higher than 1000 ppm (Fabry et al, 2008;Paulmier et al, 2011). This high CO 2 condition creates a zone of low pH, and correspondingly a decreased saturation state of calcium carbonate.…”

“…In some pelagic ecosystems, such as the Eastern Tropical Pacific (ETP) and the Gulf of California, a pronounced oxygen minimum zone (OMZ) exists in which respiration below the photic layer outpaces mixing to create a region of low oxygen and elevated carbon dioxide. OMZs can therefore also be thought of as persistent carbon maximum zones (Paulmier et al, 2011). Specifically, in the ETP and Gulf of California around depths of 200 m, CO 2 levels reach higher than 1000 ppm (Fabry et al, 2008;Paulmier et al, 2011).…”

Hypoxia and acidification in ocean ecosystems: coupled dynamics and effects on marine life

“…In general, low tides are likely to maximize the influence of local metabolic rates while high tides may bring less intense metabolism if the water mass is originating from a region further from land with lowered amounts of organic matter, respiration and benthic influence [11,16]. Regardless, within deeper, stratified ecosystems including OMZs, hypoxic and acidified water can persist for weeks, months or longer, as benthic and/or deep water respiration continually consumes oxygen and produces CO 2 faster than it is replenished via diffusion and mixing from surface waters [10,17].…”