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...
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 â€˜non-isochronousâ€™, 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.
During this reporting period there has been a strong focus on defining the marine tephra framework. Tephrochronological investigations of two South Atlantic marine cores, MD07-3076CQ and TN057-21, have been completed over the 40-10 kyr BP period. These are the highest resolution tephra records now available from any South Atlantic cores and tephra content has been investigated down to finer grain-sizes than previous investigations of cores from the region. Tephra deposits were identified in both cores, however in general, the shard concentration profiles do not display distinct peaks in concentration and are atypical for isochronous deposits deposited via primary airfall. The delivery of tephra to the site could have been impacted by secondary processes such as iceberg or sea-ice rafting of tephra, reworking by bottom currents, changes in oceanographic conditions such as migration of currents and/or variations in their velocity and capacity to transport fine-grained material and aerial erosion from terrestrial settings. These processes affect the temporal integrity of tephra deposits and may make them inappropriate as isochronous tie-lines to the Antarctic ice-cores. This assertion is supported by the robust geochemical analysis of deposits conducted within the reporting period. The data shows that the geochemical signatures of deposits are highly heterogenous indicative of the mixing of tephra shards from multiple volcanic events, from multiple sources that occurred at different times. This prevents their use as isochronous tie-lines to correlate the marine records to ice-core sequences. Therefore, work is ongoing to compare the new tephra records to existing proxy records from the cores to provide further insights into which processes may have been impacting tephra delivery to the sites.
One potential tie-line was further investigated as in one core a more discrete peak in tephra shard concentration, more characteristic of primary deposition, was identified. Geochemical analysis of that tephra peak did display some homogeneity in composition, again potentially indicative of primary deposition. Ice-core samples with a similar timing were investigated to see if a correlation could be defined, however, these samples did not contain volcanic glass shards to allow an isochronous tie-line to be defined.
Results are currently being prepared for publication and dissemination.
The methods being applied within this work for the identification of cryptotephras are state of the art as they have only been developed in recent years and this in their first application to marine records from the Southern Ocean. While it has not been possible to provide a greater understanding of the timing of past climatic changes and variations in CO2, which will aid the testing of models for future climatic change the data should provide information on palaeoceanographic conditions and changes during the study period, providing a greater understanding of the past operation of the Earth System.
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