Air-sea exchange of CO2 over the Antarctic seasonal ice zone

The Antarctic Seasonal Ice Zone (ASIZ) is potentially a large contemporary sink for anthropogenic CO2 due to the formation of bottom water along the Antarctic coast. However, south of 55°S, the lack of measurements of the fugacity of CO2 in surface seawater (fCO2), or the concentration and ratio of stable carbon isotopes of atmospheric CO2, has meant that it has been difficult to determine whether the ASIZ acts as a net source or sink for atmospheric CO2. This study contributes to, and is largely based on, new measurement programmes of oceanic fCO 2 and the concentration and 13C/12C ratio of atmospheric CO2 over the region of the Southern Ocean between Australia and the Antarctic continent, with particular emphasis on data from regions of pack ice. Using fCO2 data from six voyages of the RSV Aurora Australis, it was estimated that between 1 October 1992 and 31 March 1993 the ocean south of 55°S, between 60°E and 150°E, sequestered 0.025 ± 0.013 Gt C over an area of ocean equivalent to 19% of the maximum area of open water south of 55°S. The CO2 sink was most pronounced west of 105°E (0.026 ± 0.013 Gt C), where it was associated with intense summer phytoplankton blooms following the melting of sea-ice. In conjunction with the sampling of oceanic fCO 2, flasks were regularly filled on the ship with dry air and later analysed for levels of CO2 and its 13C/12C ratio. This provided the opportunity to observe atmospheric variations directly forced by fluctuations in fCO2, temperature, and the 13C112C ratio of dissolved inorganic carbon (DIC) in the surface ocean. Sea surface temperature and 13C/12C-DIC effects are transmitted to the atmosphere by gross air-sea fluxes of CO2 in the absence of net exchange. Over the ice-free region of the Southern Ocean between 44°S and 60°S, from 85°E to 160°E, atmospheric 13CO2/12CO2 values were dominated by a linear dependence on sea surface temperature (0.0041 ± 0.0003 °/00 °C-1 ), due to the \equilibrium\" isotopic fractionation of CO2 during air-sea exchange. During late spring and summer over the region of the ASIZ south of 60°S between 60°E and 105°E the effect of sea surface temperature on atmospheric 13CO2/12CO2 values was overwhelmed by the effect of high marine productivity on 13C/12C -DIC. It is demonstrated that the impact of net air-sea flux of 13CO2 on atmospheric ratios of 13CO2/12CO2 can be measured more easily than the impact of net CO2 flux on atmospheric mixing ratios of CO2 . Long-term changes in sea surface temperature and productivity over the ASIZ and therefore net ocean uptake can be more accurately determined from isotopic ratios of 13CO2/12CO2 in baseline air samples from a coastal Antarctic station than from mixing ratios of CO2 in the same samples."