The Sarmiento and Bender groups use observations and modeling to study the role of the ocean in the carbon cycle and gain insight into history and future of the ocean carbon sink.


Understanding the Southern Ocean’s impact on carbon and climate

As part of their NSF proposal (see “New Initiatives” above), the Sarmiento group carried out a CMI analysis focusing on oceanic heat and carbon uptake in a new set of 19 IPCC-class climate models over the period 1861 to 2005. The model intercomparison study shows that 71 ± 24% of the excess heat and 43 ± 3% of anthropogenic carbon is entering the Southern Ocean south of 30°S (Figure 2), although the Southern Ocean only covers 30% of the global ocean surface area. Overall, multi-model variability in CO2 uptake remains largest over the Southern Ocean, but the multi-model spread is significantly reduced compared to earlier generation models.

Moving forward, the group plans to investigate the response of the Southern Ocean carbon cycle to changing Southern Ocean winds in response to changes in stratospheric ozone levels and greenhouse gas concentrations.

Figure 2: Oceanic uptake of anthropogenic CO2 and excess heat in CMIP5 models. (a) Cumulative oceanic CO2 uptake in year 1995 (represented by mean of period 1986 to 2005) and integrated from 90°S to 90°N such that the vertical scale from 0 at 90°S to the total uptake at 90°N. (b) Same as (a), but for excess heat.


Impacts of ocean acidification on phytoplankton

A third of the anthropogenic CO2 released to the atmosphere dissolves into the surface ocean, reducing its pH. What effects this ocean acidification will have on the ocean biota is a focus of research in the Morel group. Among the manifold potential effects of ocean acidification is a change in the cycling of nitrogen, the principal limiting nutrient for marine ecosystems.

The major input of “new” nitrogen in the open ocean is through nitrogen fixation, which is effected by a few species of cyanobacteria. In experiments with the dominant nitrogen-fixing species, Trichodesmium (Figure 3), Morel and colleagues demonstrated that lowering pH decreases both Fe uptake and, independently, the efficiency of N2 -fixation. Because the nitrogenase enzyme, which catalyzes the reduction of N2 to NH4 +, contains a very large number of iron atoms, and N2 fixing organisms thrive in regions where iron is scarce, these two effects may act synergistically to decrease the input of new nitrogen to marine ecosystems and impact global productivity.


Measuring dissolved CO2 in the ocean

To help quantify the fluxes of carbon into and out of the surface ocean, Michael Bender and colleagues have developed a new instrument for continuous, high precision measurements of the dissolved inorganic carbon concentration (DIC) of surface seawater. The DIC instrument achieves high precision using a method called isotope dilution, which allows concentrations to be determined by an isotope ratio measurement that is much easier and more accurate that the normal concentration measurements.

During 2012, the instrument was validated and successfully deployed on 3 cruises. It has since been modified, using a so-called “doublespike” technique, in a way that achieves higher precision while at the same time greatly simplifying the implementation. During the coming year, the group will use this instrument to characterize DIC on cruises in the Southern Ocean and the Arctic Ocean. Other groups have expressed interest in copying this instrument. The researchers expect that it will be widely used for measurements along cruise tracks of oceanographic ships, both to characterize the invasion of fossil fuel CO2 into ocean surface waters and to track the seasonal imprint on DIC of the annual cycle of biological activity.

Figure 3: Postdoc Sven Kranz performing an experiment with phytoplankton using a Membrane Inlet Mass Spectrometer.