Late Quaternary changes in the Westerly Winds over the Southern Ocean

Abstract

The latitudinal position and intensity of the Southern Hemisphere westerly winds (SHW) has important and far-reaching implications for global climate and the physical environment in the southern high latitudes. Despite this, our ability to project how they will change in the future is reduced by limited understanding of their behaviour over centennial to millennial timescales. Peatland archives on the sub-Antarctic Islands are uniquely located inside the core wind-belt (50-55˚S), and hence ideally situated to reconstruct changes in westerly wind behaviour. However, suitable proxies to develop reconstructions of wind-strength throughout the region are lacking. Westerly winds are shown to enrich the sub-Antarctic Islands with salt-spray in concentrations that are proportional to wind-strength. This research has tested the potential for peat-dwelling testate amoebae to act as a bioindicator for variations in salt-spray deposition through time. Measurements of communities in variably salt-enriched environments - spanning a gradient from predominantly freshwater to salt-marsh - revealed a strong relationship between the productivity of testate amoeba communities and salinity, which allows past salt-concentrations to be inferred from sub-fossil assemblages. Presented here are two reconstructions of SHW intensity over the South Atlantic, based primarily on changes in the productivity of testate amoeba communities. The first provides a high-resolution record of the changes in wind-intensity over recent decades, extending beyond the observational record to 1920 CE, and demonstrating that present-day wind conditions are unprecedented over the last century. The second record, collected from the same site, provides a c. 8000-year reconstruction of wind-intensity over the South Atlantic, based on changes in testate amoeba productivity as part of a multi-proxy analysis that includes three independent proxies to track the deposition of salt-spray aerosols and minerogenic particles into the peat record on Bird Island (sub-Antarctica). Three significant phases of intensified winds during this period (0.45-1.15, 2.8-3.65 and 4.45-8 k yr BP) indicate long-term correspondence between temperature and wind-strength at 54˚S. These observations suggest that with climatic warming in the 21st Century, the westerly wind belt will continue to intensify and displace southwards, leading to increased wind-stress over the Southern Ocean. Implications of this shift are expected to include; reduced precipitation supply to the Southern Hemisphere continents, reduced Antarctic ice-sheet stability and increased contributions to global sea-level, and weakening of the Southern Ocean carbon sink, allowing accumulation of more CO₂ in the atmosphere.