A high-resolution environmental and climate record of change in the Holocene sediments of Windermere, UK

Abstract

The Holocene (11,750 Yrs. B.P. – present day) provides valuable examples of climate change in response to natural and anthropogenic forcing, by which future forecasting models can be validated. However, reliable climate and environmental observations rarely extend beyond the past 200 years. In this case proxy-based reconstructions can extend the record further.
The sediments and water of Windermere, NW England, have been studied since the 1930s. These studies show the potential of the sediments to create a record of environment and climate change which extends from the Pleistocene to the present day. It’s location in the NE Atlantic region means it is ideally suited to record changes in climate and environment which are affected by globally important systems such as the North Atlantic Oscillation, Atlantic sea surface temperatures and the North Atlantic Currents.
This thesis aims to firstly provide preliminary results of a multiproxy study of the whole Holocene sediment sequence from Windermere’s North Basin. A combination of organic, geochemical, and sediment microfabric analysis complemented by a chironomid inferred mean July temperature and pollen community reconstruction show the potential for the Holocene sediments of Windermere to record major climate events such as the 4.2 k. Yrs. B.P. cooling event. More detailed analysis has identified mass transport deposits (MTDs) in the early Holocene, likely caused by seismic instability induced by isostatic readjustment following deglaciation.
The sediments of Windermere have also been impacted by anthropogenic activities since at least the beginning of the industrial revolution. However, the full impact of this activity is as yet unknown. With this in mind this thesis aims to provide a detailed history of anthropogenic impacts on the water column and sediments. Using gravity cores collected from Windermere in 2014 this thesis presents a novel combination of techniques to relate microscopic sediment fabric features to lake-basin scale processes. Together microfabric and geochemical methods enabled the identification of MTDs which, despite bioturbational mixing, can be dated to 1979 and 1979-1980 respectively. The timing of these features make a likely trigger the 4.7 ML 1979 Carlisle earthquake. Slope failure was likely to be the result of preconditioning principally by increased sediment in-wash as a result of anthropogenic activities. This study constitutes the first evidence of seismic activity-induced MTDs preserved in lake sediments in the UK and is published in the Journal of the Geological Society of London.
Further to this, this study presents the results of a multi-method organic- and geochemical, and sediment fabric analysis, applied to reconstruct the history of eutrophication and pollution in Windermere. Eutrophication developed in the late 19th and earliest 20th centuries and is marked by changes in the sediment microfabric, organic chemistry and geochemistry. Δ13C values show increased lake productivity is coeval with Pb, Zn, Cu, Hg, and As enrichment. Δ15N values in the South Basin sediment correlate with Zn, Hg and Cu, suggesting a major source of pollution to be from human sewage or farm runoff marked by isotopically heavy nitrate. In peak eutrophic conditions, a strongly reducing environment promoted Fe dissolution and the formation of anglesite-barite mineralisation, hitherto undescribed in lake sediments. Partial recovery is shown to occur after 1980 across both basins, but elevated δ15N in the South Basin shows the continued impacts of sewage discharge. Results also show elevated concentrations of Mn, Fe, Ba, and As in the surficial sediment.
Scanning Electron Microscope (SEM)-led methods additionally identified preserved diatom algae seasonal blooms, some of which may be matched with bloom occurrence record from Windermere. Millimetre-scale laminations of Fe and Mn minerals are also further analysed from the surface and pre-eutrophication intervals, and are shown to record seasonal cycles of lake ventilation. Results also show tight coupling of Fe and P, which indicates the potential redox-driven release of P to the water column with implications for lake eutrophication.
This study highlights the power of microstratigraphic techniques in the recognition and characterisation of event layers in sediments where bioturbative disruption has occurred

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