- Doug Wallace, Dalhousie University
- Kumiko Azetsu-Scott, Dalhousie University
- Roxanne Maranger, University of Montreal
- Denis Gilbert, Institute Maurice Lamontagne
- To determine how competing processes control the carbon and oxygen budgets of newly formed deep-water.
- To characterize the concentrations and air-sea fluxes of key greenhouse gases.
Gas measurements are essential in the VITALS plan to investigate Labrador Sea breathing and lateral exchange. This team integrates gas measurement activities across the other teams. As one of the very few regions where the deep-sea communicates directly with the surface (Lab Sea Group 1998), the Labrador Sea is one of the most important and accessible places where we can study what controls oceanic carbon and oxygen. The deep-sea will become the worldâ€™s largest reservoir of anthropogenic carbon, reducing atmospheric carbon dioxide concentrations and increasing ocean acidity. Oxygen concentrations control deep-sea habitat for a variety of organisms. VITALS will focus its efforts on developing a mechanistic understanding of the forcing factors that control the oxygen and carbon concentrations of newly formed deep-water exported from the Labrador Sea.
Surface pCO2 concentrations in the Labrador Sea undergo a large seasonal cycle (KÃ¶rtzinger et al. 2008; DeGrandpre et al. 2006). In late spring, photosynthesis draws down surface carbon while warming partially offsets this by raising pCO2. The organic carbon produced by photosynthesis is respired in deeper waters, leading to increased subsurface pCO2. In autumn and early winter, air-sea gas exchange increases pCO2 while falling temperatures again oppose this effect. Finally, during the convection period, high pCO2 waters are mixed into the surface leading to nearly atmospheric levels. At the end of the convection period, the pCO2 level of surface waters is close to atmospheric but not because the water has equilibrated with the atmosphere through gas exchange, which is much too slow. The near-equilibrium effect is caused by a delicate balance of the annual biological, temperature, gas exchange, and mixing cycles whose interactions are poorly known at present. In addition, water column CO2 profiles from repeat hydrography have indicated that deep convection acts a sink for atmospheric CO2 with significantly larger sinks during extremely deep convection in severe winters. These competing processes and their overall role in the Labrador Sea as a sink for anthropogenic carbon and conduit into the deep sea need to be reconciled to understand the role of deep convection in the global carbon cycle.
Dissolved oxygen cycling is important in its own right and plays a complementary role in teasing apart processes controlling carbon. Oxygen is produced by photosynthesis and consumed by respiration in a specific ratio to carbon, leading to a large surface oxygen excess during the spring bloom and a deficit during deep convection. However, oxygen exchanges with the atmosphere about ten times faster than carbon, is much more affected by air injection from bubbles, and has a fundamentally different relationship with temperature than pCO2. Oxygen is not a simple mirror image of carbon; it will provide a different constraint over the competing processes at work (Keeling et al. 1993). Lightweight, low-power, fast response oxygen sensors mean that we can measure oxygen across all platforms.
Carbon and oxygen cycling have previously been studied in the Labrador Sea (e.g. KÃ¶rtzinger et al. 2004; 2008), but VITALS will approach this problem in a more comprehensive way than has been possible. The SeaCycler can measure in both time and space where previous observations have been made with sensors at fixed depths on moorings or at one time in snapshot hydrographic cruises. Here we will investigate the biological cycle, air-sea gas exchange at high wind speeds, and the interaction of the natural carbon cycle and anthropogenic overprinting with attendant acidification by deploying multiple sensors on both profiling and fixed moorings, on mobile sampling platforms, and on traditional research cruises. We will also characterize some other important greenhouse gases. With a purely anthropogenic origin, CFCs and SF6 provide complementary information on ventilation and mixing processes during convection that will also help disentangle the influences on carbon and oxygen (Azetsu-Scott et al. 2003; Wallace and Lazier 1988). This work is part of the on-going AR7W program. Nitrous oxide (N2O) is a potent greenhouse gas likely produced by the activity of nitrifying microbes in the well-oxygenated intermediate waters of the Labrador Sea. In winter, the exposure of N2O-rich subsurface waters to the atmosphere during convection must allow outgassing to the atmosphere.
Autonomous gas sensors will be deployed on all the platforms (moorings, gliders, floats, and shipboard) with intensive discrete sampling from the ship to calibrate the sensors and provide analyses for gases not yet available autonomously.
The planned gas measurements are:
|Platform||Planned gas measurements|
|Central moorings||pCO2/pH (carbon system parameters), oxygen, gas tension (N2)|
|Gliders / floats||oxygen|
|Shipboard underway||pCO2, oxygen, O2/Ar ratio|
|Shipboard discrete||DIC/Alkalinity/pH (carbon system parameters), oxygen, DI13C, CFCs/SF6, O2/N2/Ar ratios and Ar, nitrous oxide (N2O)|
Carbon on the moorings will be measured with pCO2 sensors (ProOceanus CO2-Pro) supplemented by pH sensors (Satlantic SeaFET). We will take advantage of the capabilities of the different moorings to deploy technologies with different response times. On the SeaCycler, we will deploy a pair of oxygen sensors, one with a faster response time but some calibration drift (SeaBird Clark Electrode 43) and one with a slower response time but little drift (SeaBird optode 63). Optode sensors will additionally be deployed near-surface (~100 m) on the fixed moorings. Similarly, we will employ a fast response gas tension device (ProOceanus HGTD) on the SeaCycler and a more robust gas tension device with a slower response (ProOceanus GTD-Pro) near-surface on one of the fixed moorings. Gas tension measurements provide the total dissolved gas content, which combined with oxygen give N2 estimates.
The three gliders mapping the wider region will each carry an optode oxygen sensor (Aanderaa 3830), which will provide key information on spatial oxygen variability in the central basin. The three EM-APEX floats will carry fast-response optode sensors (Aanderaa 4330F). These will provide further information on spatial variability and allow investigation of near-surface bubble plumes (D'Asaro and McNeil 2007). Four profiling Argo floats will be released in the Labrador Sea in years 1-4 as part of DFO's contribution to the international Argo program. VITALS will provide for each float to be equipped with an oxygen optode (SeaBird optode 63). Further spatial variability will be investigated using underway measurements of pCO2 (General Oceanics pCO2 system) and oxygen (optode) during all research cruises associated with VITALS. Spring / summer cruises will additionally feature dissolved O2/Ar ratio measurements for estimating net community production (see Biological Processes team).
Finally, shipboard discrete sampling will provide important calibration data for the autonomous sensors as well as data that can be obtained only through direct chemical analysis. Carbon system parameters measured by the mooring sensors will be calibrated against DIC/Alkalinity/pH data collected on every cruise, measured following international protocol (Dickson et al., 2007). In addition, these observations will extend to deeper waters to constrain the whole water column and the contribution of deep convection to surface values. Isotopic composition of dissolved inorganic carbon (DI13C) will be analyzed using a Picarro CRDS. Full profiles of dissolved oxygen, measured by standard Winkler titration, will be collected near each mooring and any accessible gliders/floats on every cruise to provide calibration points. To calibrate the GTD measurements, samples for the dissolved N2/Ar ratio and absolute argon concentration will be collected near the mooring and analyzed using cryogenic gas processing and mass spectrometry techniques (Emerson et al. 1999, Hamme and Severinghaus 2007). Additional surface samples will be collected on spring/summer cruises for calibration of the underway O2/Ar measurements and analyzed by similar techniques. We will target additional greenhouse gases for shipboard sampling. CFCs and SF6 will be measured at sea using established gas chromatography techniques. Water for N2O will be collected in sealed glass serum bottles, preserved with HgCl2. Triplicate subsamples will be equilibrated and analyzed using a Varian CP-3800 gas chromatograph.
In collaboration with the other teams, we will
- make dissolved gas meaurements across all platforms including providing calibration data for autonomous sensors,
- analyze the gas and ancilliary data to disentangle competing influences on surface ocean oxygen and carbon,
- realistically incorporate the key processes into numerical models to allow projections for other years, and
- provide new measurements of other key greenhouse gases.