Determining the temporal relationships of large-scale atmospheric and oceanic fluctuations is crucial for advancing understanding of the mechanisms controlling heat transfer between the Northern and Southern Hemispheres. The thermal bipolar see-saw caused asynchronous interhemispheric climatic changes during the last glacial period and Southern Ocean marine records and the Antarctic ice-cores are valuable archives recording this past climatic variability. Ascertaining the precise phasing of the climatic variability between these records provides crucial boundary conditions for testing models simulating the future behaviour of the bipolar see-saw and assessing potential large-scale oceanic and atmospheric reorganisations under anthropogenic forcing. In addition, establishing tighter constraints on phase relationships between sedimentary evidence for deep-water ventilation of CO2, and ice-core evidence for past atmospheric CO2 variations is key to determining the future response of the Earth system to rising CO2 levels.
This project aimed to address these challenges by ascertaining the rate, timing and phasing of Southern Hemisphere climatic changes between 40-10 kyr BP. Using tephrochronology to independently synchronise the palaeoclimatic sequences using common horizons of volcanic ash as time-synchronous tie-lines. Recognition of ash horizons not visible upon core inspection (cryptotephras) within sequences increasingly distal from volcanic regions has increased the scope of this technique. As such, a prime objective of this project was applying recently developed techniques and protocols for cryptotephra identification and assessment to several marine sequences from the South Atlantic sector of the Southern Ocean to build a framework of isochronous volcanic events present in these records. To achieve this high-resolution profiles of tephra shard concentration were gained and any identified deposits were robustly geochemically characterised. These physical characteristics were then be used to assess whether the deposits were deposited via primary airfall, and can be utilised as isochronous markers, or if secondary processes, which would affect their temporal integrity, were responsible for tephra delivery.
A combined assessment of the tephra shard concentration profiles and the geochemical analyses from deposits identified in two marine records (MD07-3076CQ and TN057-21) shows that the dominant mechanisms for tephra delivery to the region are secondary processes. This is shown by the wide spread of tephra in the sequences and the heterogeneous geochemical signature of deposits, indicative of the mixing of tephra shards from several eruptions. Secondary processes that could have been depositing tephra shards at the sites include iceberg or sea-ice rafting and bottom current reworking through the migration of currents or variations in their velocity. These processes cause ‘non-isochronous’, i.e. delayed, deposition of material and as such these deposits can not be used as time-synchronous markers to synchronise the marine records to the Antarctic ice-cores. Therefore, it has not been possible to address the research questions regarding providing better constraints on the rate, timing and phasing of past climatic and CO2 changes in the region. Work is ongoing to identify the primary drivers of the tephra deposition and assess the relationship between deposits in the two cores and assessing whether the processes driving tephra deposition were local-scale, i.e. site-specific, or regional-scale, i.e. oceanwide.
