High-resolution records of deglaciation and palaeomagnetism in the Late-Quaternary sediments of Windermere, UK

Abstract

Windermere, in the Lake District, UK, is a glacial ribbon lake that has accumulated sediment since its exposure after the retreat of the British and Irish Ice Sheet (BIIS). A coring campaign by the British Geological Survey in partnership with the University of Southampton has recovered a suite of sediment cores from Windermere, which document lacustrine sedimentation from ~15 ka to the present day. 
The varved nature of some of the sediments has offered the opportunity to investigate the nature and timing of events in the northern UK over the Lateglacial (~14.7 – 11.7 ka BP). In particular, the placement of Windermere between continental Europe and Greenland provided the impetus to investigate the laminations further, with the possibility that climatic leads and lags across the region may be identified should the chronology prove robust enough. Also, the seismic work already carried out in Windermere implied that the initial retreat of ice from the lake during the last deglaciation was fairly rapid, and the recovery of the cores was intended in part to test this claim using suspected varves recovered. A third possibility, once the cores had been recovered and other investigations were underway (prior to this study), was that the palaeomagnetic nature of the sediment may be ascertained and the UK palaeomagnetic master curve of Turner & Thompson (1981; obtained in part from Windermere sediments) may be updated and extended using newer measurement and dating techniques. 
A suite of techniques were used to analyse the cores, including (but not limited to) scanning electron microscope imagery of resin-embedded thin sections, Itrax XRF core scanning, palaeomagnetic measurements, and grain size analysis. The upper part of the varve sequence is more organic-rich and was radiocarbon-dated to provide a Holocene (~11.7 ka – present) age-depth model. Palaeomagnetic measurements from the Holocene were placed on this age-depth model and stacked to form a new Holocene geomagnetic palaeosecular variation curve for the UK, WINPSV-12K. The pre-Holocene (~15 – 11.7 ka BP) sediments were not suitable for palaeomagnetic stack creation, thus the original UK master curve of Turner & Thompson (1981) was not able to be extended. 

The pre-Holocene (~15 – 11.7 ka BP) sediments of Windermere yield several sequences of annually laminated sediments, or varves. The lowermost varves represent a period of ~250 yr prior to the onset of the Lateglacial Interstadial (~14.7 – 12.9 ka BP), and document ice retreat of the Lake District ice cap (which had separated from the receding British and Irish Ice Sheet) up the North Basin of the lake. The final collapse of the Trout Beck valley glacier occurred in 36-40 vyr. Seismic reflection imaging indicates the possibility of a further >40 m of varves in the South Basin. The Younger Dryas-age (~12.9 – 11.7 ka BP) sediment sequences of the South Basin are interrupted by large mass transport deposits. 
The Younger Dryas-age varves in the North Basin appear to predominantly nival (i.e. snowmelt-induced) in style. The early Younger Dryas-age varves exhibit evidence of stormy conditions (e.g. bioturbation from wind mixing), as reported from other North Atlantic- influenced sites. The later varves show higher instances of less-bioturbated varves and more precipitation (i.e. thicker varves), as the sea ice extent decreased and the polar front-related storm tracks moved north of the UK. Our results show the potential for construction of precise annually resolved record throughout the Younger Dryas interval. 
Spectral analysis of selected varve sequences from the pre-Interstadial period (pre- ~14.7 ka BP) and the Younger Dryas age (~12.9 – 11.7 ka BP) sediments shows that interannual variability between 2 – 4 yr, often associated with the Quasi-Biennial Oscillation and teleconnections of El Niño-La Niña events, was ubiquitously present. Interannual variability in the 4 – 7 yr band was however absent in the pre-Interstadial sequence but present in the Younger Dryas sequences, in keeping with current thinking that variability in El Niño strength was damped prior to ~14 ka BP.
 This study has shown that Windermere is a site sensitive to regional climatic and geomagnetic variability, and the annual resolution of the pre-Holocene sediments has afforded many insights into the deglacial history of the Lake District.

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ORCID for Rachael Avery: orcid.org/0000-0002-4439-4174

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