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Loch Ness Sediments: A Preliminary Report

Reproduced with the permission of the Scottish Naturalist
Copyright: May be used for private research. All other rights reserved 

 

By PETER H. JENKINS

School of Applied Sciences,

University of Wolverhampton

 

Introduction

The sediments of Loch Ness have received little attention in the past from palaeolimnologists.  Pennington, Haworth, Bonny and Lishman (1972) reported mainly grey microlaminated glacial clay in a core taken from a depth of 50 metres off Dores.  This may explain why further research on the sediments has been somewhat neglected until quite recently.

Loch Ness is some 230 metres deep, and it is therefore difficult to remove sediment cores from the bottom of the loch with the equipment readily available to palaeolimnologists.  The Loch Ness and Morar Project, however, has recently developed a technique capable of collecting cores from a depth of 220 metres.  Cores from this depth have been examined by members of the Loch Ness Sediment Group.  The sediments are rich in organic material and laminated to a depth of 2.0 metres.

This paper presents some of the preliminary bulk geochemical and mineral magnetic results from a 1.2 m core, collected in 1990 from a depth of 170 m in the North Basin (National Grid reference NH 572326).

The research being undertaken is set within the lake watershed ecosystem model, which has proved to be a useful tool in developing a spatially integrated understanding of lake and catchment processes.

Bormann and Likens (1969) identified the framework of this model by monitoring nutrient cycles within a catchment, and relating their results to climatic change and human activity in the catchment.  This concept was further developed by Oldfield (1977) to consider the sediment as a record of past environmental conditions within the lake and its catchment.  The sediments are analysed for variations in their physical, chemical and organic properties, which are then interpreted within the lake watershed ecosystem model framework.

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p67

The sediment can be considered as a diverse matrix of inorganic and organic materials, derived from the catchment (allogenic), from within the lake (authigenic), and within the sediment (diagenic).  To distinguish between these sources presents any one discipline with considerable problems, which can only be overcome by multi-disciplinary studies (Haworth and Lund, 1984).

Mineral magnetic studies examine the physico-chemical changes of the iron compounds brought about by changes in the environment.  Thompson and Oldfield (1986) present various examples of mineral magnetic studies, and Oldfield (1991) presents a recent review of the research being carried out in this discipline.  The advantage of this form of analysis is its speed and non-destructive method, which allows further methods of analysis to be carried out.  A brief interpretation of the magnetic parameters measured is presented in Table 1.

Geochemical studies elucidate the environmental processes which control the distribution of inorganic elements within the sediment.  Mackereth (1966) established methods of interpretation which have been further developed, and questioned, by other workers in this field, e.g. Engstrom (1983).

 Site Description

 Loch Ness is situated in the Highland Region of Scotland , and is one of several lochs, linked by the Caledonian Canal, which follow the Great Glen fault (Figure 1, 7K map).  Loch Ness is the largest body of natural freshwater, and has the greatest mean depth of any lake, in Great Britain.  The catchment of the loch spans almost the entire width of the Scottish mainland, and incorporates a variety of climatic domains.  The geology of the catchment exposure consists of a complex suite of metamorphic, intrusive igneous and old red sandstone rocks with post-Devensian deposits (Craig, 1991).  Land classification of the catchment is predominantly mountain moorland or rough pasture, with crop and grass land only a minor proportion in the glens. 

The main industries are agriculture and tourism.  Sheep farming and forestry dominate the upland areas, with minor livestock and arable farming in the crofting areas of the glens.  Hydro-electric schemes and various cottage industries are common in the region.  Aluminium smelting was carried out on the shore of Loch Ness at Foyers from 1895 to 1967.

 The morphometry of the loch and its catchment is presented in Table 2.  Hydrological and land use data are given in Table 3, which follows Maitland's (1981) method of catchment subdivision.

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p68

 

Table 1 

Mineral Magnetic parameters

 (Walden, 1990) 

Magnetic mass specific susceptibility: 
The ratio of magnetisation induced, to the intensity of a small applied magnetic field (no remanence is induced, so the sample retains its original magnetic properties).  It can be considered as being roughly proportional to the concentration of ferrimagnetic minerals within the sample. 

Frequency dependent susceptibility: 
The variation of susceptibility with respect to the frequency of the applied field.  Viscous magnetic grains within the stable single domain/superparamagnetic boundary show a delayed response to the applied field. 

Anhysteretic remanent magnetisation:
The remanence induced in the sample by the applied field tends to be retained by ferrimagnetic stable single domain size grains.

Saturation isothermal remanent magnetisation:
The maximum amount of magnetic remanence which can be imparted to a sample by a large applied field.  (With the fields used, not all magnetic minerals will have been saturated).  This parameter relates to mineral type and concentration.

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p69

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p70

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p 75

It cannot always be assumed that the waters and transported eroded materials flow directly from the catchment to the loch, since they may pass through other lochs before they enter Loch Ness.

Study Methods

The core was sealed immediately after collection, for subsequent transportation to the School of Applied Sciences at the University of Wolverhampton.  The core was extruded in the laboratory and samples were taken at 3.0 cm intervals.  These samples were oven dried at a temperature not greater than forty degrees centigrade.  The samples were homogenised and prepared for mineral magnetic analysis using standard methods. A standard suite of mineral magnetic measurements was carried out. On completion of these non-destructive magnetic measurements, the samples were prepared for bulk geochemical analysis by X-ray fluorescence.

The results are presented using a statistical method which normalises the data in such a way that all measurements can be presented on the same scale. This method allows the magnitude of change in the properties measured to be compared.  A brief description of this method is given below.


Data Normalisation Method

The down core results of each variable are considered as a column.  The Mean and Standard Deviation (standard deviation of the sample) of the column are calculated.  Each element of the column has the column Mean subtracted from it.  It is then divided by the Standard Deviation and the square root of the number of samples in the column.  Results from this method should then fall between +1.0 and -1.0.  This method is used by Walden (1990) as part of the procedure of simultaneous R-mode and Q-mode factor analysis.

The normalised data then represents either positive or negative variations from the mean of the data set.


Results and Interpretation

The four magnetic parameters (Figures 2, 3, 4, 5 5K graphs) show three distinct zones, from the top of the core to 21 cm, from 21 cm to 84 cm, and from 84 cm to 120 cm.  There are various anomalies within each zone, e.g. frequency dependent

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p76

susceptibility at 75 cm is positive, while all other parameters are negative or just positive.  This can be considered as indicating the presence of viscous grains.  The possible source for this is soil erosion within the catchment (Mullins and Tite, 1973).

The positive zones can be considered as having enhanced input of magnetic grains from various environmental sources, such as reduction within the loch or catchment soils, atmospheric inputs, or biological sources such as bacteria.  The negative zones can be considered as the reverse, or dilution of the magnetic grains by the increased input of diamagnetic or paramagnetic sediments from the catchment (Thompson and Oldfield, 1986).

Figures 6, 7, 8, 9, (5Kgraphs) comprising the alkali and alkaline earth elements, calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg), also show three distinct zones, which almost coincide with the magnetic zones but have the opposite sign.  The positive zone implies a period of enhanced erosion in the catchment of diamagnetic or paramagnetic sediment which is diluting the magnetic grains.  Na and Mg mirror the magnetic properties, whereas Ca tends to respond before the magnetic properties at a depth of 24 cm, and K responds after the magnetic properties at a depth of 15 cm.

Iron (Fe) and manganese (Mn) (Figures 10 and 11, 5K graphs ) follow the trend of three zones except for the top 21 cm of the core.  Mn is increasing while Fe is close to the mean; this implies variations in redox of the sediment (Mackereth, 1966).

The heavy metals, lead (Pb) and zinc (Zn) (Figures 12 and 13, 5K graphs) do not appear to correspond to the three zones discussed above.  Closer inspection, however, shows three zones which could be considered to follow the alkali and alkaline earth elements, except for the top 33 cm.

With the advent of the Industrial Revolution (1760), certain elements were introduced to the atmosphere by the increased burning of fossil fuels; these were then deposited on the land during rainfall (Hunt, Jones and Oldfield, 1984).  The enhancement of the heavy metals is considered to be the start of the Industrial Revolution, and the peak of Pb at 15 cm could be associated with the decline in the 1970s of industrial production throughout Europe.

The enhancement of the heavy metals in the sediment record can be used to estimate the deposition rate, and the date of the sediment with respect to depth. Pb and Zn results from Loch Ness show an enrichment of Pb and Zn at sediment depth of 33 cm. Therefore:

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p77

Mean depth of enhanced Pb and Zn = 33 cm

Years since Industrial Revolution = 230 years

Annual deposition rate = 33/230 = 0.14 cm per year

Deposition, however, cannot be considered as occurring at a constant rate, because of the nature of environmental processes.

 

Conclusions

It is considered that the results presented here show the importance of Loch Ness to the study of palaeolimnology. The 1.2 metre core is believed to encompass some 850 years of environmental history.

The heavy metals indicate the start of the Industrial Revolution in 1760 (33 cm depth).  The positive zone of the alkali and alkaline earth elements may indicate the start and finish of the Little Ice Age, a cold wet period between 1500 (78 cm) and 1850 (24 cm).

Future work

The method of normalising the data presented here appears to give a qualitative representation of environmental change, with a relative representation of the magnitude of change within the sediment record.  This method may be used to compare changes between different lakes.  At present a sediment trap is in position in Loch Ness to collect one year's deposition.  The sediment collected will then be analysed and the results will be used as the mean for the method of normalisation.

Loch Ness offers considerable scope for palaeolimnology, with respect to environmental change during the late Devensian and Holocene times.

 Acknowledgements

The author would like to thank Mr. Adrian J. Shine and all colleaques at the Loch Ness and Morar Project for their assistance; also Dr. J.P. Smith, Dr. K.M. Farr and Mr. Brian Bucknall at the University of Wolverhampton for their help and advice.

Vol 105, The Scottish Naturalist: Loch Ness Sediments: A Preliminary Report p86

 

References 

Bormann, F.H. and Likens, G.E.  (1969).  The watershed/ecosystem concept and studies of nutrient cycles.  In: The Ecosystem Concept in Natural Resource Management.  (Ed. G.M. Van Dyne).  Pages 49-76.  New York: Academic Press.

Craig, G.Y. (Ed.) (1991).  Geology of Scotland.  Third edition.  London:  Geological Society.

Engstrom, D.R.  (1983).  Chemical Stratigraphy of Lake Sediments as a Record of Environmental Change.  Ph.D. Thesis, University of Minnesota.

Haworth, E.Y. and Lund, J.G.W. (Eds.) (1984).  Lake Sediments and Environmental History.  Leicester University Press.

Hunt A., Jones, J. and Oldfield, F.  (1984).  Magnetic measurements and heavy metals in atmospheric particulates of anthropogenic origin.  Science of the Total Environment, 33: 129-139.

Mackereth, F.J.H.  (1966).  Some chemical observations on post-glacial lake sediments.  Philosophical Transactions of the Royal Society of London, Series B, 250: 165-213.

Maitland, P.S.  (1981).  Introduction and catchment analysis.  In: The Ecology of Scotland's Largest Lochs: Lomond, Awe, Ness, Morar and Shiel. (Ed. P.S. Maitland).  Monographiae Biologicae, 44: 1-27. The Hague: Junk.

Mullins, C.E. and Tite, M.  (1973).  Magnetic viscosity, quadrature susceptibility and frequency dependence of susceptibility in single domain assemblages of magnetite and maghaemite.  Journal of Geophysical Research, 78: 804-809.

Oldfield F.  (1977).  Lakes and their drainage basins as units of ecological study.  Progress in Physical Geography, 1: 460-504.

Oldfield, F.  (1991).  Environmental magnetism - a personal perspective.  Quarternary Science Review, 10: 73-85.

Pennington, W., Haworth, E.Y., Bonny, A.P. and Lishman, F.P.  (1972).  Lake sediments in Northern Scotland.  Philosophical Transactions of the Royal Society of London, Series B, 264: 191-294.

Thompson, R. and Oldfield, F. (1986).  Environmental Magnetism.  London: Allen & Unwin.

Walden, J.  (1990).  The Use of Mineral Magnetic Analysis in the Study of Glacial Diamicts.  Ph.D. Thesis, University of Wolverhampton.

 

Received May 1993 

Mr. Peter H. Jenkins, School of Applied Sciences,

University of Wolverhampton, Wulfruna Street,

WOLVERHAMPTON WV1 1SB.

 

 

 

 

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