The aim of this project is to study the effect of sea ice on the carbon dioxide (CO₂) system and dissolved oxygen in the surface waters of the Southern Ocean as well as the biogeochemical processes that drive this system during the productive summer season. The ocean fronts, the marginal ice zone, and the ice edge constitute interesting areas of study because of the relatively high level of biological production, resulting in high biological CO₂ uptake. As a result of variations in sea-ice cover, caused by the production and melting of sea-ice, a variable environment is created for biological production and CO₂ fluxes. The CO₂ gas exchange between sea and atmosphere (sea–air) is estimated and related to the physical and biological characteristics of the surface water. Furthermore, the importance of sea ice for CO₂ fluxes in the sea–ice–air interface will be investigated.

Background

Variation in extension of The Southern Ocean sea ice cover is largely attributed to melting in summer and freezing again in winter. The seasonality in sea ice conditions affects biological production and CO₂ fluxes. Due to the sea ice melt in spring and summer, strong surface water stratification is created, and in combination with nutrient availability and sufficient light, biological CO₂ drawdown will take place. Previously, sea ice has been considered to act as a protecting lid, preventing gas exchange between water and atmosphere. However, recent studies have indicated that sea ice is permeable to gases.

Ship-based work

Measurements commenced soon after departing the Strait of Magellan in the austral summer 2006 and proceeded successfully onboard icebreaker Oden both in open water and in sea ice until the McMurdo Sound. The project used on-line, high-frequency measurements of surface water partial pressure of CO₂ (pCO₂), dissolved oxygen, chlorophyll fluorescence (in collaboration with Dr. M. Chierici), salinity and temperature. Samples for total dissolved inorganic carbon (DIC), total alkalinity (AT) and nutrients (phosphate, nitrate, silicate) were collected and measured on shore (picture 1). This project is closely coordinated with Dr. Chierici’s project.

The automated pCO₂ instrument was successfully used for sea ice conditions for the first time in the Arctic expedition Beringia 2005, and was now used for measurements in the Southern Ocean. Seawater was pumped with a flow rate of approx. 15 l/min into a CO₂ equilibrator (tandem type combined with a static mixer type manufactured by Kimoto Electric Co., LTD (Kimoto and Harashima 1993, Harashima et al., 1997), passing through a thermosalinograph for continuous pCO₂ measurements. The mole fraction of CO₂ (xCO₂) in dry air, expressed by part per million (ppm), was continuously measured every minute by a non-dispersive infrared detector (NDIR, LiCOR^®, model 6262, Lincoln, USA). Four CO₂ standard gases and zero gas (0, 250, 350, 450, 550 μatm) were used for calibration and pCO₂ in air was alternately measured with the same instrument. Sea surface salinity and temperature were continuously measured and an oxygen sensor (Aanderaa) was used to measure dissolved oxygen every minute. The oxygen values were compared to the values obtained from the discrete samples analysed with the Winkler method.

Preliminary results

Along the sailing route, from open water to marginal sea ice, variability in surface water pCO₂ and oxygen was observed using 1-minute readings (figure 1). Preliminary analyses indicate that there was a substantial decline in pCO₂ and increase in oxygen (figure 1) and correlative peaks in chlorophyll a (Chl a) on the 17 and 18 December, suggesting that the ship passed through a phytoplankton bloom. The most striking feature was the high pCO₂ values, i.e. CO₂ oversaturation relative to the atmospheric level, observed for the first time since departing from Punta Arenas. It is probable that this was due to vertical mixing of CO₂-rich subsurface water with the surface, which is confirmed by the low oxygen values. The Chl a showed the lowest values during the time period, indicating low primary production. Coincident with this specific time point, the ship also passed through an area of pack ice and moderate densities of Crabeater seals and Adelie penguins which are feeding on zooplankton, which in turn feed on phytoplankton living under the sea ice. In general, pCO₂ was below atmospheric values, and the lowest pCO₂ values in combination with the highest oxygen and Chl a values were measured in the Ross Sea polynya, implying intense biological CO₂ drawdown.