Late-Holocene palaeoecological reconstruction of Southern Hemisphere Westerlies variability on Subantarctic Macquarie Island

Abstract

As the world’s strongest zonal surface winds, changes in the Southern Hemisphere Westerlies (SHW) can profoundly affect atmospheric carbon dioxide (CO₂) concentrations, ocean-cryosphere domains and precipitation patterns in the Southern Hemisphere mid- to high-latitudes. The strengthening and poleward migration of the SHW in the last few decades points to an increase in Southern Ocean ventilation and CO₂ outgassing with significant implications for future global temperatures and Southern Hemisphere mid-latitude precipitation variability. A more in-depth and longer-term understanding of past SHW behaviour is required to improve projections of future SHW variability. Macquarie Island (54°S), located within the modern core belt of the SHW, is an ideal location because the influx of westerly wind-blown sea spray creates a strong conductivity gradient in lakes from west to east across the island. We used the known positive correlation between increasing wind-blown sea spray driven water conductivity and halophilic and halophobic diatoms preserved in lake sediments to reconstruct past changes in SHW influence on Macquarie Island over the last 2500 years. Low values in the accumulation rate of the dominant low-conductivity Aulacoseira principissa and Psammothidum taxa at ~2500–2300 cal. yr BP and after ~800 cal. yr BP suggest relatively stronger SHW over Macquarie Island at those times. Contrastingly, an increase in Psammothidum taxa at ~2300–800 cal. yr BP signals weaker SHW as a consequence of lower sea spray input. This is supported by the coeval, anti-phased changes observed in the relative abundance of high-conductivity species such as Platessa oblongella, Planothidium renei, Pinnularia sp. and Fragilaria capucina. This study provides a detailed and valuable record of SHW variability for the last few millennia in the Pacific sector of the Southern Ocean.