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Many lakes undergo anthropogenically driven eutrophication and pollution leading to decreased water and sediment quality. These effects can enhance seasonally changing lake redox conditions that may concentrate potentially toxic elements. Here we report the results of a multi-method geochemical and sediment microfabric analysis applied to reconstruct the history of cultural eutrophication and pollution of the North and South Basins of Windermere, UK. Eutrophication developed from the mid-19th to the earliest 20th centuries. Enhanced lake productivity is indicated by increased sedimentary δ13C, and increased pollution by a higher concentration of metals (Pb, Hg, and As) in the sediment, likely enhanced by incorporation and adsorption to settling diatom aggregates, preserved as sedimentary laminae. In the South Basin, increasing sediment δ15N values occur in step with Zn, Hg, and Cu, linking metal enrichment to isotopically heavy nitrate (N) from anthropogenic sources. From around 1930, decreases in Mn and Fe-rich laminae indicate reduced deep-water ventilation, whereas periods of sediment anoxia increased, being most severe in the deeper North Basin. Strongly reducing sediment conditions promoted Fe and Mn reduction and Pb-bearing barite formation, hitherto only described from toxic mine wastes and contaminated soils. From 1980 there was an increase in indicators of bottom water oxygenation, although not to before 1930. But in the South Basin, the continued impacts of sewage are indicated by elevated sediment δ15N. Imaging and X-ray microanalysis using scanning electron microscopy has shown seasonal-scale redox mineralisation of Mn, Fe, and Ba related to intermittent sediment anoxia. Elevated concentrations of these metals and As also occur in the surficial sediment and provide evidence for dynamic redox mobilisation of potentially toxic elements to the lake water. Concentrations of As (up to 80 ppm), exceed international Sediment Quality Standards. This process may become more prevalent in the future with climate change driving lengthened summer stratification.

Many lakes in industrialised areas have undergone anthropogenically driven eutrophication and increases in pollution leading to decreased water and sediment quality. In some cases, these effects are enhanced by seasonally changing lake redox conditions that may act to concentrate potentially toxic elements sufficiently to exceed internationally recognised Sediment Quality Standards, impacting key species and jeopardizing water supply. A combined, geochemical and sediment microfabric analysis is applied to reconstruct the history of cultural eutrophication and pollution in the North and South Basins of Windermere, England’s largest natural lake. We also document a record of seismicity and link increased sedimentation rates and sediment instability. The onset and development of eutrophication in Windermere occurred from the mid-19th to the early 20th centuries. Raised lake productivity is indicated by an increase in sedimentary δ13C, and increased pollution by elevated sedimentary trace metals (Pb, Zn, Cu, Hg, and As), likely enhanced by incorporation and adsorption to settling diatom aggregates, preserved as sedimentary laminae. In the South Basin of the lake, contemporaneous increasing sediment δ15N values also occur in step with increasing Zn, Hg, Cu from this time, linking metal enrichment to the input of isotopically heavy nitrate (N) from anthropogenic sources including sewage. From around 1930, a decrease in Mn and Fe-rich laminae indicate reduced deep water ventilation, and increased incidence of sediment anoxia, being most intense in the deeper North Basin where benthic activity intermittently ceased. Strongly reducing conditions in the sediment promoted Fe and Mn reduction and the formation of unusual Pb-bearing barite, hitherto only described from toxic mine wastes and contaminated soils. In the North Basin cores a clay rich laminae dated 1979-1980 is shown to be a mass transport deposit linked to large scale slope failure likely caused by the 4.7 ML 1979 Carlisle earthquake. Slope failure was exacerbated by preconditioning principally by increased sedimentation as a result of anthropogenic activities. From 1980 there was a partial recovery in oxygenation with Mn and Fe rich laminae returning in some parts. But in the South Basin, the continued impacts of sewage discharge is indicated by elevated δ15N of organic matter. Imaging and X-ray microanalysis using scanning electron microscopy has enabled the identification of seasonal-scale redox mineralisation of Mn, Fe and Ba related to intermittent sediment anoxia. Elevated concentrations of Mn, Fe, Ba, and As also occur in the surficial sediment and provide evidence for dynamic redox mobilisation of potentially toxic elements that may be released to the lake waters. Concentrations of As, in particular, exceed international Sediment Quality Standards. These surface enrichments in As and other toxic elements may become more prevalent in the future with climate change driving lengthened summer stratification in the lake.

Annually laminated sediments (varves) provide excellent temporal resolution to study rapid environmental change, but are rare in the early part of the Last Termination (~19-~11.7 ka BP). We present a new >400 varve year (vyr) varve sequence in two floating parts from Windermere, a lake at the southern margin of the mountains of northwest England. This sequence records the final retreat of the Windermere glacier at the southern edge of the Lake District Ice Cap during the transition from Heinrich Stadial 1 (~18 – ~14.7 ka BP) into the Lateglacial Interstadial (~14.7- ~12.9 ka BP). Laminated sediments from four lake cores from Windermere’s northern and southern basins were investigated and shown to be varved. These sequences are integrated with seismic reflection evidence to reconstruct south-to-north deglaciation. Seismic and sedimentological evidence is consistent with gradual stepped ice retreat along the entire southern basin and into the northern basin between 255 and 700 vyr prior to the appearance of significant biota in the sediment that heralded the Lateglacial Interstadial, and had retreated past a recessional moraine (RM8) in the northern basin by 121 vyr prior to the interstadial. The Lateglacial Interstadial age of this biota-bearing unit was confirmed by 14C-dating, including one date from the northernmost core of ~13.5 cal ka BP. A change in mineralogy in all four cores as the glacier retreated north of the Dent Group (the northernmost source of calcareous bedrock) and a decrease in coarse grains in the varves shows that the ice had retreated along the entire North Basin at ~70 vyr prior to the Lateglacial Interstadial. The estimated retreat rate is 70-114 m yr-1 although buried De Geer moraines, if annual, may indicate retreat of 120 m yr-1 with a >3 year stillstand at a recessional moraine halfway along the basin. The glacier then retreated north of the lake basin, becoming land-terminating and retreating at 92.5 - 49 m yr-1. The northernmost core has a varve sequence ending at least 111 vyr after the other core chronologies, due to the increased proximity to remnant ice in the catchment uplands into the early Lateglacial Interstadial. We show that almost all of the glacier retreat in the Windermere catchment occurred before the abrupt warming at the onset of the Lateglacial Interstadial, in keeping with similar findings from around the Irish Sea Basin, and suggesting a similar retreat timescale for other radial valley glaciers of the Lake District Ice Cap. The seismic and core evidence also show the potential for a much longer varve chronology extending at least 400 and potentially over 1000 vyr further back into Heinrich Stadial 1 (18-14.7 ka BP), suggesting that glacier retreat in the Windermere valley initiated at least before 15.5 ka BP and perhaps 16 ka BP.

The global climate transition from the Lateglacial to the Early Holocene is dominated by a rapid warming trend driven by an increase in orbital summer insolation over high northern latitudes and related feedbacks. The warming trend was interrupted by several abrupt shifts between colder (stadial) and warmer (interstadial) climate states following instabilities of the Atlantic Meridional Overturning Circulation (AMOC) in response to rapidly melting ice sheets. The sequence of abrupt shifts between extreme climate states had profound impacts on ecosystems which make it challenging to reliably quantify state variables like July temperatures within a non-analogue climate envelope. For Europe, there is increasing albeit inconclusive evidence for higher stadial summer temperatures than initially thought. Here we present a comprehensive floral compilation of plant macrofossils from lake sediment cores of 15 sites from S-Scandinavia covering the period similar to 15 to 11 ka BP. We find evidence for a continued presence of plant species indicating high July temperatures throughout the last deglaciation. The presence of hemiboreal plants in close vicinity to the southern margin of the Fennoscandian Ice Sheet implies a strong thermal summer forcing for the rapid ice sheet melt. Consistent with some recent studies, we do not find evidence for a general stadial summer cooling, which indicates that other reasons than summer temperatures caused drastic setbacks in proxy signals possibly driven by extreme winter cooling and/or shorter warm seasons. (C) 2020 The Authors. Published by Elsevier Ltd.

Lake sediments commonly contain detrital layers that record events such as floods or earthquakes, but these may be disturbed or partially destroyed by bioturbation. Here we use a novel combination of techniques to relate microscopic sediment fabric features to processes at the lake basin scale. X-radiography and micro-X-ray fluorescence analysis of cores are complemented by back-scattered electron imagery and energy-dispersive X-ray microanalysis of resin-embedded sediment. Together, the microfabric and geochemical methods enable the identification of clay layer mass transport deposits despite bioturbational mixing of the original end-members. Two cores with robust radionuclide chronologies contain correlative clay layers dated to 1979 (1974–1982) and 1979–1980 (1973–1986), respectively. These clay layers probably represent the distal turbidite generated by a major mass flow deposit identified from multi-beam swath bathymetry and sediment grab sampling. A probable trigger for the mass flow and associated turbidity current is the 4.7 ML 1979 Carlisle earthquake. The lake basin slope was probably preconditioned for failure by increased sedimentary biogenic gas production and sediment in-wash as a result of anthropogenic activities, coupled with sediment disruption and dredging. This study highlights the effectiveness of microstratigraphic techniques in the recognition and characterization of event layers in sediments where bioturbative disruption has occurred.