Late Holocene lake ecosystem change and the Southern Hemisphere Westerlies on sub-Antarctic Macquarie Island

The Southern Hemisphere Westerlies (SHW) are the worlds strongest zonal surface winds, and they profoundly influence ocean-atmospheric CO2 exchange, southern mid-latitude precipitation patterns and ocean-cryosphere dynamics. Stronger and more poleward-shifted SHW over the last few decades have been linked to anthropogenic warming and increased Southern Ocean ventilation and CO2 outgassing. A more in-depth understanding of past natural SHW variability is required to investigate the SHW response to future, anthropogenically impacted climate change scenarios (e.g., IPCC2023). Macquarie Island (5430S, 15857E) is located in the Southern Ocean within the SHW core belt, providing an ideal location for reconstructing past changes in the SHW. A strong and decreasing west-east conductivity gradient exists in lakes across the island due to the input of westerly wind-blown sea spray. Moreover, since diatom species present in surface sediments from these lakes are strongly determined by conductivity, we reconstructed variations in the SHW over the last 3000 years using a sediment record from Lake Tiobunga on the west coast. Decreases in the sediment accumulation rate and the dominant, low-conductivity Psammothidium taxa imply that the SHW over Macquarie Island were relatively stronger between ~30002300 cal BP and in the last thousand years. Conversely, an increase in Psammothidium taxa implies weaker SHW ~23001000 cal BP. Superimposed on these longer-term trends are centennial-scale fluctuations and a dramatic increase in diatom production after 1900 CE, which we associate with the well-documented invasive rabbit infestation. Our results provide a record and improve the understanding of the complex SHW atmospheric system for the last few millennia in the Pacific sector of the Southern Ocean.