Late Quaternary changes in the Westerly Winds over the Southern Ocean

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Public summaryClimate changes have been attributed to the increasing concentrations of greenhouse gases (e.g. CO2) in our atmosphere. These measured increases in atmospheric CO2 are partly controlled by changes in the ability of the world's oceans to absorb CO2 at the surface (e.g. via diffusion and the biological pump) versus release of old carbon to the atmosphere from deep ocean reservoirs (e.g. via upwelling and out gassing).Of the world's oceans, the Southern Ocean has been identified (by models) as playing a major role in modulating global atmospheric CO2, particularly on glacial-interglacial timescales. This is because surface nutrients are high indicating their incomplete utilisation by the biological pump, and because wind driven changes in ocean circulation can bring old carbon stored in deep ocean reservoirs to the surface. Changes in the strength of the Southern Hemisphere Westerly Winds (SHW) influence Southern Ocean circulation and control how much of this carbon rich deep water reaches the ocean surface. Thus any change in the strength or position of the SHW such as the recently observed intensification of the winds, could influence whether the Southern Ocean acts as a net source or sink of atmospheric CO2.At present our understanding of past changes in the SHW is based mainly on geological proxy records from South America, one record from South Africa, and two from New Zealand. With the exception of Campbell Island there are no studies of changes in the SHW in the Southern Ocean where the core of the SHW wind belt is located. This lack of spatial resolution has been identified by Stager et al. (2012) as a major limitation in our understanding of past climate. As a result, although present General Circulation Models use a variety of processes (biology, ocean chemistry, and ocean physics), they either fail to produce the magnitude of past atmospheric CO2 variations or do not agree with geologic field data. Here we propose to substantially improve the spatial resolution of the geological data by generating proxy records in each of the three major sectors of the Southern Ocean, focusing on sub-Antarctic islands situated in the core belt of the SHW. We apply a novel diatom proxy for past wind strength independently controlled by a range of standard sedimentological and biogeochemical proxies. The new proxy is based on the direct transfer of sea spray across the islands by wind, and its effect on the salinity of west coast lakes and ponds. This works on sub-Antarctic islands where there is a marked west-east conductivity gradient in water bodies across the island. This conductivity gradient determines which diatom communities are present in the lakes. Once this diatom- conductivity relationship is established quantitatively, the subfossil diatom communities deposited in radiocarbon dated sediment cores can be used to reconstruct changes in conductivity through time, and hence past relative wind strength.We have demonstrated that this approach works at Macquarie Island and we have also tested its feasibility at Marion and Campbell Islands. This proposal is for funding to support further work on Campbell and Marion Islands, and in the Cape Horn archipelago. To interpret our data we will carry out a series of General Circulation Model runs to explore the long term changes in SHW strength and the processes driving them, by taking advantage of the new Paleoclimate Modelling Intercomparison Project 3 (PMIP3) AOGCM (climate model) simulations. These experiments will allow comprehensive model-observation evaluation of the new proxy wind strength reconstructions.Ultimately this work will help provide improved boundary conditions for models which simulate the impact of past changes in wind strength on the upwelling of deep ocean carbon reservoirs, and improve our understanding of the relationship between past changes in global atmospheric CO2 and temperature.