The role of Southern Ocean processes in orbital and millennial CO 2 variations – A synthesis

Abstract

Recent progress in the reconstruction of atmospheric CO 2 records from Antarctic ice cores has allowed for the documentation of natural CO 2 variations on orbital time scales over the last up to 800,000 years and for the resolution of millennial CO 2 variations during the last glacial cycle in unprecedented detail. This has shown that atmospheric CO 2 varied within natural bounds of approximately 170–300 ppmv but never reached recent CO 2 concentrations caused by anthropogenic CO 2 emissions. In addition, the natural atmospheric CO 2 concentrations show an extraordinary correlation with Southern Ocean climate changes, pointing to a significant (direct or indirect) influence of climatic and environmental changes in the Southern Ocean region on atmospheric CO 2 concentrations. Here, we compile recent ice core and marine sediment records of atmospheric CO 2, temperature and environmental changes in the Southern Ocean region, as well as carbon cycle model experiments, in order to quantify the effect of potential Southern Ocean processes on atmospheric CO 2 related to these orbital and millennial changes. This shows that physical and biological changes in the SO are able to explain substantial parts of the glacial/interglacial CO 2 change, but that none of the single processes is able to explain this change by itself. In particular, changes in the Southern Ocean related to changes in the surface buoyancy flux, which in return is controlled by the waxing and waning of sea ice may favorably explain the high correlation of CO 2 and Antarctic temperature on orbital and millennial time scales. In contrast, the changes of the position and strength of the westerly wind field were most likely too small to explain the observed changes in atmospheric CO 2 or may even have increased atmospheric CO 2 in the glacial. Also iron fertilization of the marine biota in the Southern Ocean contributes to a glacial drawdown of CO 2 but turns out to be limited by other factors than the total dust input such as bioavailability of iron or macronutrient supply.