Recent Sediment Flux and Erosional Processes in a Welsh Upland Lake-Catchment Based on Magnetic Susceptibility Measurements

Abstract

Single sample magnetic susceptibility measurements are used to correlate synchronous levels in 16 cores of dated (210Pb, 137Cs) recent sediment taken from the deep and oligotrophic Llyn Peris, N. Wales, in order to provide a basis for calculating total sediment and chemical influx through time. Results show that sediment influx has steadily increased since c. 1750 A.D. to reach peak levels in the period 1966–1976 A.D., equivalent to a rise in erosion in the catchment from c. 5 t km−2 yr−1 to c. 42 t km−2 yr−1. Comparison of down-core susceptibility fluctuations with sediment pollen and organic pigment data indicates that maximum susceptibility values relate to periods of channel erosion and minimum susceptibility values relate to periods of slate debris inwash from spoil tips in the catchment. A study of historical records reveals that pre-twentieth-century peak levels of erosion were due to the effects of extractive industries, while twentieth-century erosion has been caused by overgrazing, increased trampling pressure, and heavy construction works in the catchment.

Magnetic studies of erosion in a Scottish lake catchment. 1. Core chronology and correlation1

“…4). In the adjacent Loch Meadhoin, forestry ploughing 6 months before coring resulted in the deposition of a distinctive orange sediment over much of the lakebed (Dearing 1979). This clear link between afforestation practices and the presence of orange sediment suggests that the stratigraphic change observed in these three cores is a datable horizon.…”

“…Although the complex morphometry has made total sediment influx calculations (cf. Bloemendal et al 1979;Dearing 1982Dearing , 1983Dearing et al 198 1) impossible, the value of information on sedimentation rates from different parts of the lake is nevertheless apparent. For example, the central cores provide no record of the rapid sedimentation recorded round the margins as a result of erosion caused by 20th century afforestation.…”

The Suitability of a South Pennine (UK) Reservoir as an Archive of Recent Environmental Change

“…The high proportion of pollen and spores transported from the eroding blanket bog and re-deposited in the reservoir sediments prevents the reservoir record from being a reliable archive of vegetation change in the surrounding area. The contamination of the lake sediment record in recent sediments by re-deposited pollen has also been noticed in natural lakes in the British Isles (Pennington et al 1972;Dearing et al 1981;Bradshaw and McGee 1988;Huang and O'Connell 2000).…”

Islands of Inquiry: Colonisation, seafaring and the archaeology of maritime landscapes (TA29) : Colonisation, seafaring and the archaeology of maritime landscapes

“…Each site will be subject to a range of sedimentological and biotic proxy analysis, with particular focus on the past 1000 years. These sedimentological methods include: (i) Grey-scale analysis (Petterson et al 1999) which records frequency of colour changes related to sediment type through the cores; (ii) Magnetic susceptibility which determines the concentration and composition of magnetic minerals in a sample (Nowaczyk 2001;Zolitschka et al 2001) and can be associated with climatic variability and/or anthropogenic disturbance, including fires and forest clearance (Dearing et al 1981;Dearing and Flower 1982;Nowaczyk 2001;Whitlock and Larsen 2001); (iii) Loss on ignition (LOI, Dean 1974) to estimate the organic and carbonate contents of sediment; (iv) X-ray fluorescence to quantify variations in 30 elements at 0.2 mm intervals (Croudace et al 2006).…”

“…Each site will be subject to a range of sedimentological and biotic proxy analysis, with particular focus on the past 1000 years. These sedimentological methods include: (i) Grey-scale analysis (Petterson et al 1999) which records frequency of colour changes related to sediment type through the cores; (ii) Magnetic susceptibility which determines the concentration and composition of magnetic minerals in a sample (Nowaczyk 2001;Zolitschka et al 2001) and can be associated with climatic variability and/or anthropogenic disturbance, including fires and forest clearance (Dearing et al 1981;Dearing and Flower 1982;Nowaczyk 2001;Whitlock and Larsen 2001); (iii) Loss on ignition (LOI, Dean 1974) to estimate the organic and carbonate contents of sediment; (iv) X-ray fluorescence to quantify variations in 30 elements at 0.2 mm intervals (Croudace et al 2006).A range of biotic proxies are also being used, including: (i) Palynology and micro-charcoal analysis to reconstruct vegetation succession over the late Holocene and yield specific information about human activities in relation to the flora; (ii) Testate amoebae preserved in Sphagnum bogs, which are sensitive to changes in hydrology, as palaeo-precipitation indicators; (iii) Diatom analysis to reconstruct water quality and depth changes through time.Of these three biotic indicators, palynology is most likely to yield significant information on human activity, due to the potential presence of cultivars associated with the Polynesian expansion into the east Pacific, such as sweet potato (Ipomea batatas), banana (Musa sp. ), coconut (Cocos nucifera), taro (Colocasia esculenta), flax (Phormium tenax), Pacific Island cabbage tree (Cordyline fruticosa) and Hibiscus .…”