CO2 maximum in the oxygen minimum zone (OMZ)

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.

East Siberian Sea, an arctic region of very high biogeochemical activity

Shelf seas are among the most active biogeochemical marine environments and the East Siberian Sea is a prime example. This sea is supplied by seawater from both the Atlantic and Pacific Oceans and has a substantial input of river runoff. All of these waters contribute chemical constituents, dissolved and particulate, but of different signatures. Sea ice formation during the winter season and melting in the summer has a major impact on physical as well as biochemical conditions. The internal circulation and water mass distribution is significantly influenced by the atmospheric pressure field. The western region is dominated by input of river runoff from the Laptev Sea and an extensive input of terrestrial organic matter. The microbial decay of this organic matter produces carbon dioxide (CO2) over-saturating all waters from the surface to the bottom relative to atmospheric values, even if the nutrient concentrations of the surface waters showed recent primary production. The eastern surface waters were under-saturated with respect to CO2 illustrating the dominance of marine primary production. The drawdown of dissolved inorganic carbon equals a primary production of ∼1 mol C m−2, which when multiplied by half the area of the East Siberian Sea, 500 000 km2, results in an annual primary production of 0.5×1012 mol C or 6×1012 gC. Even though microbial decay occurs through much of the water column it dominates at the sediment surface where the majority of organic matter ends up, and most of the decay products are added to the bottom water. High nutrient concentrations and fugacity of CO2 and low oxygen and pH were observed in the bottom waters. Another signature of organic matter decomposition, methane (CH4), was observed in very high but variable concentrations. This is due to its seabed sources of glacial origin or modern production from ancient organic matter, becoming available due to sub-sea permafrost thaw and formation of so-called taliks (layers of thawed sediments within the permafrost body). Riverine transport as well as leakage of groundwater rich in methane from decay in fresh water systems could add to the CH4 shelf water inventory as minor sources. The decay of organic matter to CO2 as well as oxidation of CH4 to CO2 contribute to a natural ocean acidification making the saturation state of calcium carbonate low, resulting in under-saturation of all the bottom waters with respect to aragonite and large areas of under-saturation down to 50% with respect to calcite. Hence, conditions for calcifying organisms are very unfavorable.

NSF grants will fund studies of ocean acidification

SAN DIEGO — With increasing levels of carbon dioxide accumulating in the atmosphere and moving into marine systems, the world’s oceans are becoming more acidic.

To address the growing concern of acidifying marine ecosystems, the National Science Foundation has awarded 21 grants, including awards to scientists at Scripps Institution of Oceanography at UC San Diego, under the Ocean Acidification theme of NSF’s Climate Research Investment. The projects will foster research on the nature, extent and effects of ocean acidification on marine environments and organisms in the past, present and future — from tropical systems to icy seas.

Maine Oceanographers study climate change in the South Atlantic

Oceanographers from Maine are among a team of 23 scientists sailing across the South Atlantic studying the effects of climate change on marine life and future ability of the ocean to sustain life.

Leading the 5-week expedition is Dr. Barney Balch, from the Bigelow Ocean Sciences Lab in West Boothbay, Maine. In the challenging, remote and often turbulent southern oceans, the team is collecting samples and conducting experiments aboard the research vessel Melville, as it sails the 7,000 miles from Chile to Cape Town, South Africa, where it’s due to arrive in 2 weeks time.

New ocean acidification web site at PMEL

The Carbon group of the NOAA Pacific Marine Environmental Laboratory has just launched a new web site which comprises a section on ocean acidification.