Palaeoseismology from microfabric and geochemical analysis of lacustrine sediments, Windermere, UK

Abstract

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 lake-basin scale processes. X-radiography and micro-XRF of cores are complemented by backscattered electron imagery and energy dispersive X-ray microanalysis of resinembedded 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 likely represent the distal turbidite generated by a major mass flow deposit identified from multibeam swath bathymetry and sediment grab sampling. A likely trigger for the mass flow and associated turbidity current is the 4.7 ML 1979 Carlisle earthquake. The lake basin slope was likely 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 characterisation of event layers in sediments where bioturbative disruption has occurred.

Convergent human and climate forcing of late-Holocene flooding in northwest England

“…This depositional mechanism contrasts with lakes that experience powerful turbidity currents capable of transporting coarse material extended distances across the lake bed (Girardclos et al, 2007;Osleger et al, 2009 and Matter, 1978). Basin slope angle is susceptible to seismic destabilisation (Wilhelm et al, 2016) but we do not observe sedimentological features associated with 20 th -century regional seismicity (Fielding et al, 2018) nor can any abnormal deposit be linked to the largest documented historical earthquake (CE 1247, Irish Sea, Mw = 5.47 ± 0.47;Stucchi et al, 2013).…”

Convergent human and climate forcing of late-Holocene flooding in Northwest England

“…This depositional mechanism contrasts with lakes that experience powerful turbidity currents capable of transporting coarse material extended distances across the lake bed (Girardclos et al, 2007;Osleger et al, 2009;Sturm and Matter, 1978). Basin slope angle is susceptible to seismic destabilisation (Wilhelm et al, 2016) but we do not observe sedimentological features associated with 20 th -century regional seismicity (Fielding et al, 2018) nor can any abnormal deposit be linked to the largest documented historical earthquake (CE 1247, Irish Sea, Mw = 5.47 ± 0.47; Stucchi et al, 2013).…”

Tracing lake pollution, eutrophication and partial recovery from the sediments of Windermere, UK, using geochemistry and sediment microfabrics

“…The age-depth model and LSR generated from 210 Pb and 137 Cs data for the upper part of the cores (SC68 = 27cm, SC64 = 20.5cm, SC67 = 18.5cm, SC57 = 22.5cm) is described in detail in Fielding et al (2018). In all cores linear interpolation between the deepest 210 Pb activity and the upper most radiocarbon date in the corresponding piston cores (Table 2) yielded LSR an order of magnitude lower than those generated by the CF:CS model in the upper part of the core.…”

“…The main difference is the thicker upper pale brown mud, our Unit I, reflecting an additional 3-4 decades of recent sediment accumulation, the transition to Unit I having been around 1970-1975. The thin clay-rich interval in the North Basin (Unit III) is interpreted as an event bed generated by a slope failure restricted to the North Basin caused by the 1979 Carlisle earthquake (Fielding et al, 2018;Musson and Henni, 2002). The transition from Unit I to Unit II occurs earlier in the deep South Basin cores (1870 in SC67; 1883 in SC57) than in those from the North Basin (1916 in SC64;1920 in SC68).…”