Westerly winds and the Southern Ocean CO2 sink

The capacity of the Southern Ocean to absorb anthropogenic CO2 has recently been limited (according to some studies) by an observed increase in the strength of the Southern Hemisphere Westerly Winds (SHW). These are causing turbulent mixing which is drawing CO2 saturated waters from the deep ocean back to the surface, causing a net outgassing. This proposed positive climate feedback between winds and CO2 means that the ocean may no longer function as a net sink of CO2, driving up atmospheric greenhouse gases and accelerating rates of global warming. Thus, reconstructing past changes (and the range of natural variability), in the strength and position of the SHW, and evaluating whether the SHW have modulated the CO2 sink in the past, is now a major priority for palaeoclimate science.

Recognising the urgency of this issue, De Vleeschouwer and ECOLAB have instigated a series of projects studying the history of the SHW around the Southern Ocean from changes in atmospheric dust deposition recorded in peat and lake sediments. So far, these records are from the northern-margin of the SHW at Amsterdam Island and Tierra del Fuego (IPEV-funded programmes: PARAD – PI F. De Vleeschouwer; and PALATIO – PI E. Michel/N. van der Putten, ANR JCJC PI FDV). However, there remains a major gap in our understanding of SHW behaviour in their core belt in the higher latitudes of the Southern Indian Ocean sector. This can be addressed by wind reconstructions from subantarctic islands which lie in the core belt of the SHW. Here we propose a sampling programme on the west coast of Ile de la Possession (Crozet Archipelago) which provides a representative site for the Southern Indian Ocean Sector. This will contribute to international initiatives to reconstruct the SHW in the other sectors (Atlantic, Pacific) to gain a better understanding of the zonal behaviour of the winds in their core-belt.

Our research involves analysing radiocarbon-dated peat and lake sediments for past changes in mineral aerosols/dust, and sea salt aerosol flux. We do this using geochemical methods (ITRAX core scanning and ICP-MS) together with novel biological proxies (diatom and testate amoebae), which record changes in salinity from wind-driven sea spray. A recent paper by our collaborators in Nature Geoscience has shown that, combined, these proxies provide reliable, independent, reconstructions of changing wind strength. We are focusing on reconstructing changes through the major transition into the current interglacial (last 15ka) and changes in the last 1000 years; periods associated with major shifts in atmospheric CO2 concentrations.

By using agreed international analytical protocols, our wind reconstructions will provide a reliable indicator of past SHW around the Southern Ocean that can be compared with records of CO2 and temperature (in ice cores), and past ocean upwelling (from marine sediment cores). Ultimately, the combined datasets will be compared with Global Climate Model simulations that will help us understand the drivers of past changes in the SHW and atmospheric CO2.