CONSTRAINING THE AMOUNT OF SEA-LEVEL RISE THAT TRIGGERED THE ABRUPT CLIMATE EXCURSION 8200 YEARS AGO

• Main Author: Lawrence, Thomas
• Other Authors: Faculty of Science
• Format: Thesis
• Language: English
• Published: University of Plymouth 2011
• Subjects: Ice Sheet; Labrador Sea; North Atlantic Deep Water; North Atlantic
• Online Access: http://hdl.handle.net/10026.2/1459

The abrupt global cooling centred on 8200 cal yr BP was the most significant and abrupt climatic deterioration of the Holocene. Regional temperatures in the circum- North Atlantic dropped by up to 5°C within a 150 year time window and was likely caused by a sudden freshwater perturbation into the Labrador Sea following the rapid drainage of proglacial lakes Agassiz and Ojibway. This significantly destabilised the salt-water balance of the thermohaline circulation and prevented North Atlantic Deep Water formation, one of the key drivers of heat transport within the global ocean system. Quantifying the amount of meltwater that triggered such widespread cooling is therefore crucial information for modelling studies that seek to elucidate the response of the thermohaline circulation to freshwater perturbations. Current estimates of meltwater release occupy an exceptionally large range of 0.4-3.0 m. Sea-level fingerprint theory predicts that the UK constitutes a suitable testing ground for constraining the magnitude of an outburst with a Laurentide source given its reasonable distance from the margin of the former Laurentide Ice Sheet. Saltmarsh sequences represent natural archives of relative sea-level change, and if rapid sea-level rise did occur, then evidence for synchronous and widespread bio- and litho-stratigraphic change should be registered within saltmarshes fronting the British coast. Microfossil techniques such as foraminifera offer the potential for high resolution reconstructions and this thesis utilises foraminifera-based transfer functions to predict the indicative meanings of fossil assemblages from five sites in the southwest, northeast and northwestern corners of the mainland UK. Two independent sea-level records from South Devon each contain two metre-scale fluctuations that are similar in structure and magnitude. The radiocarbon dates of 8424-8349 and 8333-8014 cal yr BP suggest that they could be associated with the 8.2 climate event. However, further age control is required before these fluctuations can be confidently labelled the British sea-level fingerprint of the 8.2 ka cooling causal mechanism. In addition, sea-level records from Northumberland and Morecambe Bay both contain evidence of a decimetre-scale fluctuation in local groundwater, but subsequent dating of this deposit at Alnmouth suggests that the section is too young to be attributed with to the 8.2 ka event. However, the record from Roudsea wood has the potential to be reconciled with the record at Alnmouth. Finally, lack of sufficient data from Woodlane highlights the potential inferiority of foraminifera in comparison to diatoms, as the latter are possibly more-often preserved in species-rich fossil assemblages that allow for the quantification of robust sea-level reconstructions. Faculty of Science