J Paleolimnol (2009) 42:215–232 DOI 10.1007/s10933-008-9272-9
ORIGINAL PAPER
An integrated lake-catchment approach for determining sediment source changes at Aqualate Mere, Central England Nathan J. Pittam Æ Ian D. L. Foster Æ Tim M. Mighall
Received: 25 January 2008 / Accepted: 28 October 2008 / Published online: 15 November 2008 Springer Science+Business Media B.V. 2008
Abstract This paper raises fundamental questions about the sole use of paleolimnological techniques to identify sediment sources and develop catchment management plans. The concept of an integrated lake: catchment framework was established 30 years ago, yet paleolimnologists occasionally fail to appreciate the dynamics of the contributing catchment. This is especially critical when the predominant source of sediment accumulating in a lake is allochthonous. In this paper we argue that a detailed appraisal of catchment sources and investigation of historical
N. J. Pittam Department of Geography, Environment and Disaster Management, Coventry University, Priory St., Coventry CV1 5FB, UK I. D. L. Foster (&) Department of Molecular and Applied Biosciences, School of Biosciences, University of Westminster, Cavendish Campus, 115 New Cavendish Street, London W1W 6UW, UK e-mail:
[email protected] I. D. L. Foster Department of Geography, Rhodes University, P.O. Box 94, Grahamstown, 6140 Eastern Cape, South Africa T. M. Mighall Department of Geography and Environment, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, UK
documentary evidence is needed to identify and evaluate the relative significance of sediment sources. We used such an approach at Aqualate Mere, Shropshire, UK. Mineral magnetic and radionculide signatures of potential catchment sources and accumulating lake sediments were compared in an attempt to match the sources to sediments deposited in the Mere. Dated lake sediments indicate there has been an increase in sedimentation rate and the relative amount of minerogenic material delivered to the Mere over the last 200 years. In contrast to a previous study at the same site, there is no evidence to attribute this increase to an overspill from a nearby canal. Other catchment disturbances include landscaping in parkland surrounding the Mere in the early nineteenth century and drainage systems installed to improve catchment agriculture over the last ca. 150 years. Both activities may explain the change in sedimentation rates and types, independent of the hypothesized canal origin. Although our results exclude the canal as a major sediment source, identifying the contribution of other potential catchment sources remains problematic. 137Cs inventories for the lake are similar to those recorded at a local reference site, suggesting little influx of 137Cs-bearing topsoil, yet 137Cs activities remain high in the upper 20–30 cm of the lake sediment profile, indicating a topsoil origin. Combined radionuclide and mineral magnetic signatures proved to be relatively poor discriminators of potential sources, and the high atmospheric pollution load from the West Midland conurbation has probably altered
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recent lake sediment signatures. Although further research is required to identify the origins of recent (last ca. 200 years) minerogenic sediment inputs to the Mere, we suggest that the combined lake: catchment approach offers a more rigorous method for understanding the impact of catchment disturbance than analysis of the paleolimnological record alone. Keywords Palaeoenvironmental reconstruction Environmental magnetism Landscape disturbance Catchment source tracing Mere
Introduction Research over the last two decades demonstrates that it is possible to identify the sources of sediment deposited in a lake (Foster et al. 1990a, b, 2008; Foster and Walling 1994; Foster and Lees 1999a). To do so effectively requires an evaluation of catchment sources rather than use of the paleolimnological record alone (Foster 1995, 2006). In this paper we argue that detailed appraisals of catchment sources and historical documentary evidence are needed to support conclusions regarding catchment changes and delivery of sediment from potential sources. Inadequate source identification can lead to a management strategy for a lake or reservoir that erroneously targets potential sources that, in fact, contribute insignificantly to the problem. Although paleolimnology can provide an effective approach to the resolution of environmental problems, a method that utilizes the lake sediment record, catchment source information, and analysis of historical documentary evidence is necessary to understand catchment dynamics and develop an effective management strategy. Approaches to determine the origins of sediment deposited in lakes and reservoirs have been reviewed and synthesised by Foster (1995, 2006) and Foster and Lees (2000). Catchment sources can often be discriminated on the basis of their inherent properties, including mineral magnetic signatures, particle size, geochemistry and radionuclide signatures (e.g. Foster et al. 1998; Foster and Walling 1994; Kurashige and Fusejime 1997; Douglas et al. 2003) and then compared with paleolimnological records. Typically, several sources make up the sediment load of a river that contributes to accumulating lake
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sediments. These include arable and pasture topsoil, which are consistently found to be the two major contributors to contemporary suspended sediment transport (Walling and Woodward 1995; Walling 2005; Walling and Kane 1984), woodland soils, subsoils, channel bank sediments and road dust (Gruszowski et al. 2003). Recent studies have also highlighted the importance of alterations to sediment pathways, such as the installation of land drains (Foster et al. 2003; Pittam et al. 2006), to the delivery of eroded sediment to a lake. The sediment sourcing variables used must exhibit a high degree of independence for satisfactory discrimination to be achieved (Walling 2005), and the use of multivariable fingerprinting to trace the origins of lake sediment relies on assumptions that were considered in detail by Foster and Lees (2000). This paper aims to (1) demonstrate the importance of using an integrated lake-catchment approach to interpret the contribution of different sediment sources to a lake or reservoir and (2) evaluate the contribution of this knowledge to the development of an effective management strategy. These aims were addressed using Aqualate Mere as a case study. Three specific objectives were identified: 1.
2.
3.
Date and characterize the sediments accumulating in Aqualate Mere using radionuclide signatures, environmental magnetism and sediment geochemistry, Identify and characterize potential catchment sources contributing to Aqualate Mere using the same signatures and establish the relative importance of these sources. Analyse historical documentary evidence to identify changes in catchment management that may have led to a change in sediment sources and/or sediment delivery to the lake.
Site description Aqualate Mere is a shallow lake with a water surface area of ca. 72 ha that formed in a kettle hole left by retreating Devensian ice (Fig. 1; Table 1). It lies approximately 2 km west of Newport, Staffordshire, UK and is a Site of Special Scientific Interest (SSSI) and a National Nature Reserve (NNR). Additionally, Aqualate Mere was identified as internationally
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Fig. 1 Aqualate Mere and its catchment
Table 1 Lake and catchment characteristics of Aqualate Mere UK Ordnance survey grid reference
SJ771205
Lake area
0.716 km2
Catchment area
59.24 km2
Lake: catchment
Ratio 82.7
Maximum altitude
103 m
Minimum altitude
67 m
Maximum lake depth
1.4 m
Mean lake depth
1.2 m
Lake volume
107 9 104 m3
Date of formation
Late Devensian
important by Natural England in its Geological Conservation Review, with respect to its being a Devensian Glacial Site (Glasser 2003, 2004). The hydrological catchment of Aqualate Mere encompasses ca. 59 km2 of mainly agricultural land (Hutchinson 2003). Sediment is transported into the mere via a major inflow stream, the Coley Brook, and two other significant inflows, Humesford and Wood Brooks, in addition to numerous agricultural ditches and brooks that drain the surrounding agricultural land. A further potential sediment input is a canal overflow located at Norbury Junction on the
Shropshire Union Canal (Fig. 1) that was constructed in 1796 and feeds directly into the catchment through the Wood Brook. The major outflow is on the western side of the mere that feeds the River Meese. Well-established beds of Phragmites are currently expanding into the lake as a result of the shallowing and ‘‘terrestrialisation’’ at the margins of Aqualate Mere. South of the mere is an area of managed parkland dominated by grassland and isolated mature Quercus stands and early nineteenth century plantations (Yale 1994). Isolated coniferous plantations also grow northwest and southeast of the mere. The greater catchment area is predominantly pasture, with small areas of arable agriculture. The underlying geology is Permian-Triassic Old Red Sandstone and the Devensian glaciation left behind coarse-grained superficial deposits largely in the form of outwash gravels and eskers. Catchment soils are diverse, with nine soil series identified (Table 2). A study using solely lake sediment to evaluate the recent sedimentation history and identify potential sediment sources of Aqualate Mere, was undertaken by Hutchinson (2005). That detailed paleolimnological investigation included study of multiple cores, analysis of the physical (particle size and loss on
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Table 2 Soil types and potential sources identified within the Aqualate Mere catchment (numbers in each column refer to the number of samples collected from each potential source) Soil associationa
Code/source type
Arable Pasture Woodland
Adventurer 1
1024a
0
5
5
Clifton
711n
5
5
0
Bromsgrove
541b
5
5
0
Wick 1
541r
5
5
0
Newport 1
551d
5
5
0
Crewe Blackwood
712f 821b
5 5
5 5
0 0
Bridgnorth
551a
5
5
0
Wickham 1
572 m
5
5
0
Wickham 2
711f
5
5
0
Canal samples
5
Channel bank
15b
Road dusts
6
Subsoil
10
a
The names given relate to UK Soil Associations described by Ragg et al. 1984. These soil associations belong to two major soil series, the Salop and Newport series, which are Inceptisols and Entisols in the USDA classification respectively (Ragg and Clayden 1973) b
Five samples from each of three major tributaries
ignition), mineral magnetic and geochemical signatures and organochlorine pesticide residues of the sediment columns, and used 137Cs and 210Pb dating to provide a chronology. It was suggested by Hutchinson (2005) that a local canal had been responsible for the greatest contribution to increased sedimentation rates in Aqualate Mere over the last 200 years.
Field and laboratory methods Field sampling and sample pre-treatment An 11-m core was recovered using a 150-cmlong 9 5-cm-diameter Mackereth Corer and a 50-cmlong 9 5-cm-diameter Russian corer (Barber 1974; Aaby and Digerfeldt 1986). The Mackereth core was transported vertically back to the laboratory and extruded at 1-cm intervals using a slicing plate. The Russian cores were transported from the field in plastic tubing wrapped in polythene sleeves. The sediment was stored for a few days at 4C, and after
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sub-sampling, remaining sediment was placed in sealed petri dishes and stored at –16C. To ascertain sediment sources, 126 soil and sediment samples were collected from the Aqualate Mere catchment. This included topsoil and subsoil ([40 cm depth) from the major soil types and land uses (Table 2), using bulk density rings (5-cmdiameter 9 5-cm-long). Additional potential sources included canal sediment, river channel banks and road dust (Table 2). All potential source samples were screened to measure signatures on the \63 lm particle size fraction (see below). To calculate a 137Cs inventory and account for micro-scale variability of inventories within the soil due to soil heterogeneity and sampling variability, four replicate cores were obtained from an undisturbed (since the 1940s) flat area of open parkland adjacent to Aqualate Mere. Cores were retrieved by hammering a 7-cm-diameter 9 80-cm-long steel pipe vertically into the soil. Each core was retrieved using a chain hoist and tripod. A sub-sample of the deepest 5 cm of each core was collected separately for 137Cs analysis. If 137Cs were undetectable in this subsample, the complete fallout record was assumed to be contained in the upper section of the core, which allowed calculation of a reference inventory for comparison with the 137Cs inventory in the mere. The lower 5-cm sub-sample and the upper section of each core were dried, disaggregated and sieved to exclude the [2 mm particle size fraction (Zapata 2002). Subsamples of the \2 mm fraction were retained for 137 Cs analysis (see below). One-cm contiguous samples throughout the core from Aqualate Mere were taken for mineral magnetic measurements from the Mackereth and Russian cores. Potential catchment sources were dried, disaggregated using a pestle and mortar and dry sieved to recover the \63 lm fraction to provide a direct comparison with the lake sediment (see below). Laboratory analysis Loss on ignition (%LOI) was measured after combustion of samples at 550C for 3 h, as recommended by Bengtsson and Enell (1986). Particle size distributions of the lake sediments were measured using a Malvern Instruments Mastersizer 2000, following removal of organic matter using hydrogen peroxide, washing, and ultrasonic dispersal. Particle size
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analysis was used to determine the required particle size fraction in potential contributing sources to reduce the influence of particle size on fingerprint signatures (see Foster and Lees 2000), and to determine downcore changes in particle diameter, sorting, and specific surface area. The 10, 50 and 90th percentile particle sizes were determined (D10, D50, D90) and a sorting index (Span) and specific surface areas (SSA) were calculated (see Foster et al. 1991). From the lake sediment analysis, (see Fig. 5) the D90 particle size varied ca. a diameter of 63 lm, which was subsequently used as the upper cutoff for potential source contributions. This limit is the same as that recommended by Foster and Lees (1999a, b) and Foster et al. (2003) in their analysis of a number of UK lake and reservoir sediment sequences. LOI was used to characterize the organic matter content of the mere sediments and to correct the mineral magnetic measurements for the diluting effect of organic material (see below). A number of samples (upper 60 cm of the lake sediment column and a composite canal sediment sample) were analysed for geochemical signatures using methods described by Foster and Lees (1999b) and Foster et al. (2007). Approximately 0.5 g (weighed to 0.0001 g) of the disaggregated sample was digested in a CEM Mars 5 microwave in a 4:1 solution of concentrated nitric acid and hydrogen peroxide. The digestion programme ran for 17 min during which energy levels were increased from 250 to 580 W. Once cooled, samples were filtered, 3 ml LaCl were added, and the sample volume was made up to 50 ml with deionised water. Analysis was undertaken using either a Unicam 939 Flame Atomic Absorption Spectrometer (Pb, Cu and Zn) or an Inductively Coupled Plasma-Atomic Emission Spectrometer using the Perkin Elmer Plasma 400 (other elements). Digestion of certified reference materials using this procedure showed extraction efficiencies between 93% and 98% and results reported here were not corrected for recovery efficiency. Seven elements were analyzed: Pb, Cu, Zn, Ni, Fe, Al and Mg. Quality control included the use of replicates and spiked standards. Analytical accuracy was between ±3%. Results were expressed on a mass specific basis (as either mg g-1 or lg g-1). Mass specific magnetic susceptibility and remanence characteristics (Table 3) were determined on ca. 5–10 ml sub-samples of oven-dried (40C)
219 Table 3 Mineral magnetic properties used in the analysis of sediment sources and reservoir sediments Propertya
Measured (M)/ Units Derived (D)
J
M
Volume susceptibility
vlfmin
M
10-6 m3 kg-1
vhfmin
M
10-6 m3 kg-1
vfdmin
D
10-9 m3 kg-1
vfd%
D
%
ARM
M
10-3 Am2 kg-1
M
10-3 Am2 kg-1
IRM0.88T
min
(SIRM)
b
IRMloss (24 h)
M
%
IRM-0.1T
M
10-3 Am2 kg-1
varm
D
10-6 m3 kg-1
D D
Dimensionless 10-3 Am2 kg-1
c
Sratio HIRMd a
The subscript min is used in the text in association with the magnetic symbol to indicate where the measured property is expressed on a minerogenic basis b It is assumed that most magnetic minerals will be saturated at a field of 0.88 T and the abbreviation SIRM (Saturated Isothermal Remanent Magnetisation) has been used throughout the text) c
SRatio: -1 9 (IRM-0.1T/IRM0.88T)
d
HIRM: (IRM0.88T/(1-Sratio))/2
sediment sieved to \63 lm and, where necessary, were corrected for loss on ignition at 550C (Foster et al. 1998). Low- and high-frequency magnetic susceptibility were measured on a Bartington MS2 susceptibility meter with an MS2B dual frequency sensor (Dearing 1999). Anhysteretic (ARM) and Isothermal (IRM) remanences were measured in a Molspin rotating magnetometer. ARM was grown with a DC bias field of 40 lT and a peak alternating field of 100 mT in a Molspin a.f. demagnetiser. The ARM values are given as susceptibility of ARM (varm) and were normalised to the strength of the DC field. IRMs were grown in a Molspin pulse magnetiser to a maximum field strength of 880 mT (see Walden 1999). Measurement of loss of IRM after 24 h (IRMloss) was recorded after samples had been magnetised at the maximum field strength (880 mT). Loss of remanence over time, like frequency dependent susceptibility, is often a function of fine ‘viscous’ grains on the transition from stable single domain to superparamagnetic (Higgitt et al. 1991). Units of measurement and ratio definitions are given
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in Table 3. A detailed explanation of magnetic measurements and their interpretation is given by Foster et al. (2008). A combination of dating methods was employed, which allows for a mutually supportive and reliable chronological framework. These included analysis of radiocarbon (14C), Spheroidal Carbonaceous Particles (SCPs), lead-210 (210Pb) and caesium-137 (137Cs). Two samples from the upper 4.9 m of the sediment column (centred on 4.75 m and 2.45 m ±2 cm depth) were extracted and submitted to Beta Analytic, Miami, for conventional radiocarbon dating. Dates were calibrated using the CALIB 5.0.2 radiocarbon calibration program and IntCal04 (Reimer et al. 2004). Spherical Carbonaceous Particles (SCPs) are produced from the high-temperature combustion of fossil fuels as a by-product of industrial activity. SCPs are widely dispersed in the atmosphere and large numbers are deposited onto lakes and become incorporated into lake sediments (Rose et al. 1995). The quantity of SCPs released into the atmosphere has varied throughout the industrial and post-industrial period as a result of industrial growth and decline. The peaks and troughs in SCP concentrations can often be used to identify key dates, especially the initial rise in concentration in the 1830s and a major peak in the early 1950s. SCPs have long been used for dating lake sediments in the United Kingdom (e.g. Rose et al. 1995; Shotbolt et al. 2001; Vukic and Appleby 2005). SCPs cover a period, the industrial and post-industrial period, not datable by other techniques, and bridge the gap between 210Pb and
C dating. The extraction of Spheroidal Carbonaceous Particles (SCP) followed the method of Rose et al. (1995), and 5% of the final residue was mounted onto fixed microscope slides using Naphrax adhesive. The total number of SCPs counted on each slide was converted to units of particles per cm3 as recommended by Rose et al. (1995). The disaggregated potential source samples were sieved to recover the \63 lm (\2.0 mm for 137Cs Reference Inventory Samples) fraction and packed into pre-weighed PTFE sample holders and lightly compressed to provide a sample depth of 40 mm (to match the geometry of the active Ge volume in the well detectors used for analysis; see Appleby et al. 1986). After reweighing, the sample holders were sealed using Subaseals and paraffin wax to prevent 222 Rn gas escape. All sealed sample holders were then stored for a minimum of 21 days to allow for equilibration between 226Ra and daughter radionuclides used to estimate 226Ra. Radionuclide activities were measured using calibrated Hyper-Pure Germanium Well Detectors with count times typically between 36 and 96 h. The seven radionuclides measured are given in Table 4. Detailed descriptions of the methods and calibration techniques are given by Foster et al. (2006).
Results Results from the field survey, dating, geochemical, physical and mineral magnetic analyses from the
Table 4 Gamma-emitting radionuclides measured (After Foster et al. 2002) Isotope
Energy(keV)
Main origin
Secondary origin
Notes
210
46.5
Atmospheric fallout (unsupported)
226
Atmospheric from 222Rn (radon gas) 226 Ra from 238U decay series
234
63.3
Natural
238
Pb Th
Ra decay (supported) U decay series
235
143 & 186
Natural
235
214
295 & 351
Natural
238
137
662
Fission
Fission
Weapons fallouta
Nuclear accidentsb 232
U Pb(226Ra) Cs
228
338 & 911
Natural
40
1,461
Natural
Ac
K
U decay series U/226Ra decay First occurrence 1954a, peaks in 1963a & 1986b
Th decay series
a
Produced global fallout with first occurrence in N Hemisphere environmental samples in 1954
b
The Chernobyl nuclear power station accident produced regional fallout
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Aqualate Mere core are presented. Interpretation focuses on the uppermost 1.5 m to evaluate the contribution of different sediment sources to the lake during the past 200 years. Chronology for the Aqualate Mere sedimentary sequence was determined and the source of sediment from the upper 1.5 m of the core was established using multiple variables. The Mackereth and Russian corers retrieved ca. 11 m of sediment, of which the upper 4.90 m was used for this study. The upper 4.90 m of Aqualate Mere sediment (Table 5) is a broadly homogenous, organic-rich gyttja, but a change occurs at ca. 1.1 m depth where organic-rich gyttja is overlain by minerogenic clay. The basal section of the 11-m core was also organic-rich gyttja. Radiocarbon results suggest that the sequence to a depth of 4.75 m covers the last ca. 3,000 years, while the sample at 2.45 m depth yielded a calibrated age of 560AD (1390 BP) (Table 6). The initial increase in SCP concentration occurs at a depth of 1.12 m (Fig. 2a). Initial presence of SCPs occurred in the 1830s and has been used as a stratigraphic marker in many paleoenvironmental studies (e.g. Rosen and Dumayne-Peaty 2001; Vukic
Table 5 Description of the sedimentary units identified from the field survey using the classification proposed by TroelsSmith (1955) Depth (cm)
Description
0–100
Fine inorganic clay Ag 4.0, Nig. 3.0, Strf. 1.0, Sicc. 1.5
100–126
Light grey organic gyttja, slight banding. Dh3.5. 4.0, Nig. 2.0, Strf. 1.5, Sicc. 2.0
126–130
Dark grey organic gyttja Dh3.5 4.0, Nig. 2.0, Strf. 1.5, Humo, 3.5, Sicc. 2.0
130–220
Dark grey Organic Gyttja Dh3 4.0, Nig. 3.0, Strf. 0.5, Sicc. 2.5
220–490
Light grey banding (reed fragment @ 265 cm) Dh3.5 4.0, Nig. 3.0, Strf. 2.0, Sicc. 2.5
and Appleby 2005). Peaks in SCP concentration also occur at 42 and 22 cm depth, but their use as chronological tools is equivocal. A major increase in SCP concentrations has been dated to the early 1950s. If the increased concentration at 42 cm correlates to the early 1950s (e.g. Rose et al. 1995), then the peak at 22 cm is unlikely to date to the 1970s. Similarly, if the peak at 22 cm depth is from the 1970s, then sedimentation rates in the lake would have slowed significantly in recent decades, a claim that is not supported by other lines of evidence. The double peak in the SCP profile was resolved by supporting evidence from the 137Cs profile, which dates the 42 cm depth in the early 1950s. The double peak in SCP concentration was also detected by Rose (pers. comm.) in Aqualate Mere sediments, but no explanation has yet been proposed. The 137Cs profile (Fig. 2b) demonstrates an initial increase in 137Cs activity up-core from a depth of ca. 60 cm. The long tail of low 137Cs activities below 50 cm depth may be due to post-depositional diffusion of 137Cs through the profile (Foster et al. 2006). The increase at 50 cm depth probably corresponds to the initiation of atmospheric weapons fallout in 1954. The peak in concentration at ca. 37 cm is attributed to the 1963 peak in fallout from atmospheric weapons testing. A second peak, at ca. 20 cm depth, was probably caused by fallout from the Chernobyl nuclear reactor accident in 1986 (Bonnett and Cambray 1991). A 210Pb chronology was derived from the application of the ‘Constant Rate of Supply’ (crs) model described by Appleby (2001), using the excess or unsupported 210Pbun activities in the sediment column. The analysis (Fig. 2b) dates the sediment at 58 cm depth to the beginning of the twentieth century, providing a chronological marker that lies between the initial increase in SCP concentrations and the 137 Cs profile. The margins of error with the 210Pb dates at Aqualate Mere are high (±ca. 40 years in the basal date) because of the low activities of 210Pbun throughout the profile (also reported by Hutchinson 2005). The independent chronology provided by the
Table 6 Conventional and calibrated radiocarbon dates Sample code
Beta analytic code
Conventional
Calibrated (2r)
Intercept on calibration curve
AM243-247
Beta-189864
1530 ± 50BP
Cal AD 432-638
560 AD
AM473-477
Beta-184151
2900 ± 70BP
Cal BC 1307-908
1060 BC
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Fig. 2 Spheroidal carbonaceous particle (SCP) concentration profile in the Aqualate Mere sediments (a) and 137Cs (b) and depth-age curve derived from 210Pbun dating using the ‘crs’ model of Appleby (2001) (c) 137
Cs data, however, appears to match the 210Pb dating very closely (Fig. 2c). A depth-age profile (Fig. 3), which combines 14C, SCP, 210Pbun and 137Cs dates, shows that sediment accumulation rates from 2.45 to 4.9 m are ca. 0.14 cm year–1 (labeled Phase 1). The second phase (2) shows a decrease in accumulation rates between 1.2 and 2.45 m depth to ca. 0.06 cm year-1. The profile demonstrates an increasing rate of sediment accumulation in the upper 1.2 m (3). This stratigraphic change is dated to the late eighteenth or early nineteenth century AD. The sediment accumulation rate for the upper, clay-rich sediment (3) is ca. 0.7 cm year-1, and is much higher than the average of 0.08 cm year-1 for the previous ca. 1,300 years. This refined chronology supports the assertion made by Hutchinson (2005) that there has been an increase in sediment accumulation rates at Aqualate Mere over the past two centuries. The 137Cs inventory in the lake sediments of Aqualate Mere (176.2 ± 24.4 mBq cm-2) is remarkably similar to the average reference inventory (185.9 ± 10.6 mBq cm-2) obtained from an area of flat, undisturbed ground SW of the mere (Table 7).
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This could be interpreted to suggest that little or no eroded topsoil, with its associated 137Cs, has been delivered from the catchment to the lake since the 1960s. Many lowland UK lakes have much higher inventories of 137Cs than the reference inventory for their contributing catchments, suggesting significant amounts of 137Cs are delivered with eroded topsoil (e.g. Foster and Walling 1994; Zhang and Walling 2005; Foster 2006). Although the reference and lake sediment inventories are similar to each other, high 137 Cs activities, up-core of the peak from Chernobyl fallout, are problematic. This atmospheric input should have created a large, but isolated peak in the profile in the absence of 137Cs-rich sediment delivery to the lake. However, 137Cs activities up-core of the Chernobyl peak follow a pattern that would normally be interpreted as resulting from the delivery of 137Cs-rich topsoil from the catchment (Zhang and Walling 2005). One possible explanation is that Aqualate Mere has a low sediment trap efficiency and that a significant proportion of sediment delivered to the lake is lost via the outflow. This hypothesis requires testing to account for the uncertainty in what controls the 137Cs inventory and maintains high activities near the sediment surface.
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Fig. 3 The combined (14C, SCP, 137Cs and 210Pbun) chronology for the upper ca. 5 m of the Aqualate Mere sediment core. Planting records from the early nineteenth century document extensive Pinus plantations. It was reported by Pittam (2006) and is shown here as a biostratigraphic marker. Further explanation is provided in a later section of the paper
Table 7 137Cs reference inventories for the four soil cores, the Aqualate Mere 137Cs inventory and 137Cs activities in contemporary canal sediments Site reference
137
AMINV1
125.6
20.2
AMINV2
172.1
27.2
AMINV3 AMINV4
231.0 126.6
25.7 20.7
Average
176.2
24.4
Lake inventory 185.9
10.6
Canal sediment
Cs Inventory 137Cs Inventory error (±1r) (mBq cm-2) (mBq cm-2)
137
Cs activities 5.21 ± 0.6 mBq g-1a
a Based on a single analysis of five homogenised samples from the canal taken in 2004
The chronology suggests that the upper 30 cm of the sediment record of Aqualate Mere post-dates 1963. Average geochemical signatures for the last ca. 45 years were calculated and are presented in Table 8 along with five homogenized canal sediment samples that were subjected to the same chemical analysis. A key problem in the historical reconstruction of sediment sources is that signatures in the source may change through time (Foster and Lees 2000). Canal boats used a sequence of horse, steam and, eventually, diesel power, and it is possible that the signatures retained in the canal sediments changed as a result of variable contaminant loadings through time. It is unlikely, however, that these signatures changed over the last 45 years, and comparison between the lake and canal sediments probably remains valid over this timescale. Although there are similarities between the canal and lake sediment Zn concentrations, absolute concentrations and ratios of the other chemical elements in the canal and the lake sediments vary significantly. This supports the conclusion drawn from the 137Cs activities, that for the last ca. 45 years, the canal has not made a significant contribution to the sediments accumulating in Aqualate Mere. LOI in the upper *1 m of the lake sediments is much lower than in the deeper sediments, averaging only ca. 15%. LOI values increase downcore to a depth of 268 cm where concentrations stabilize around 50 % (Fig. 4d). Lake sediments dominated by catchment inputs often have LOI values \10% (Foster and Lees 1999b). In most of this sequence, organic matter levels are considerably higher, suggesting that internal lake processes dominate the sediment record of Aqualate Mere. High concentrations of organic matter are unusual in lowland UK lakes that receive significant inputs from their catchments. Foster and Lees (1999b) report maximum LOIs of ca. 35% in a survey of 9 UK lakes with mean LOIs ranging from 12.9% to 25.3%. While the upper ca. 1 m of the sediment in Aqualate Mere lies within this range, the remainder of the sequence has LOI values of [50 %. These data suggest that sediments accumulating in the mere below ca. 1 m depth are dominated by autochthonous and atmospheric inputs rather than catchment inputs. There are, however, two depths at which LOI levels decline dramatically and remain low. The first occurs at ca. 2.3–2.5 m depth, where LOI drops from [50% to ca.
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Table 8 Element concentrations in the upper 30 cm (post-1963) of the Aqualate Mere sediment core compared with those of contemporary canal sediments
Mean SD Canala a
Zn (lg g-1)
Pb (lg g-1)
Ni (lg g-1)
Cu (lg g-1)
Fe (mg g-1)
Al (mg g-1)
Mg (mg g-1)
295.1
143.7
106.8
177.7
20.3
10.4
3.0
66.7
20.2
41.6
30.5
2.2
1.9
0.7
297.0
10.3
46.9
109.2
12.4
5.0
2.0
Based on a single analysis of five homogenised samples from the canal taken in 2004
Fig. 4 Downcore trends in low frequency magnetic susceptibility (vlf) (a), varm (b), SIRM (c) and loss on ignition (LOI) (d)
40%, but this predates the period of interest covered here. The second occurs between ca. 1.5 and 1.0 m depth, where LOI declines from ca. 40% to \20%, and spans the period dated by the SCPs to the 1830s (1.12 m). Downcore trends in particle size analysis are plotted in Fig. 5. It is evident that major changes occurred in particle size in the upper ca. 1 m of the sediment column. The diameters of the 10, 50 and 90th percentile particle sizes (D10, D50 and D90) decrease towards the sediment surface, while specific surface area of the particles increases. Span increases in the upper ca. 1 m, suggesting that the sediments are more poorly sorted in this section of the core. Although small changes are recorded in the deeper
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sediments, the changes that are dated to the end of the eighteenth and beginning of the nineteenth centuries appear to be most dramatic and indicate an increase in the delivery of finer, but less well sorted minerogenic sediments to the mere. Downcore trends in selected mineral magnetic parameters are plotted in Fig. 4a–c. Several factors can influence the magnetic properties of lake sediment, such as the influx of eroded soil or sediment of varying magnetic mineralogy, and the addition of atmospheric pollutants. Within the lake, dissolution of magnetic minerals and/or the formation of authigenic greigite and bacterial magnetite can influence the preservation of the mineral magnetic record (Foster et al. 2008). Internal lake processes alter the detrital magnetic
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Fig. 5 Downcore trends in particle size characteristics D10, D50 and D90, the sorting index (span) and specific surface area (SSA, see text for explanation)
properties of the lake sediment so that they reflect internal processes rather than external catchment sources and change the mineral magnetic signatures of the lake sediments in such a way that they are unlikely to reflect the signatures of source sediments. The presence of authigenic greigite can be detected using the ratio of SIRM against vlf, with values exceeding 30 9 103 A m-1 being indicative of sediments with magnetic signatures altered by greigite formation (Oldfield et al. 1999; Foster et al. 2008). Values for the SIRM/vlf ratio at Aqualate Mere fall well below this threshold value and the magnetic signatures can be regarded as largely unaltered by the presence of greigite. The dashed line on Fig. 6a represents the 30 9 103 A m-1 threshold, suggesting that authigenic greigite does not make a significant contribution to the mineral magnetic signatures in this sequence. Oldfield et al. (1999) also used the ratio of varm against SIRM to detect the presence of bacterial magnetosomes in sediment records. It was concluded that ratios exceeding 2 9 103 A m-1 (dashed line on Fig. 6b) were indicative of their presence. Values of
varm against SIRM at Aqualate Mere only exceed this threshold at depths between 300 and 370 cm (Fig. 6b) and are therefore not considered to be a major influence on magnetic signatures of the majority of the lake sediment record, especially in the upper ca. 1.3 m, which represents the last ca. 200 years of sedimentation history. Evidence for dissolution diagenesis as described, for example, by Foster et al. (1998, 2008), does not exist in the preserved records of these lake sediments. Although organic matter levels are high, there is no evidence from the downcore trends that fine-grained magnetites are preferentially removed by dissolution, as such a process usually causes a downcore decrease in most mineral magnetic parameters, which is not evident in any of the plots (Fig. 4). The mineral magnetic record reflects sources with little influence from internal lake processes. All mineral magnetic signatures in Fig. 4, however, increase in the upper ca. 50 cm of the sediment sequence, a pattern often seen in UK lakes that display relatively slow accumulation rates and are located in areas of high industrial pollution fallout
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Fig. 6 SIRM: vlf ratio (a) and the varm/SIRM ratio (b) in the Aqualate Mere sediments. (The dashed lines are critical thresholds indicating the likely presence of authigenic greigite and bacterial magnetite)
(Foster et al. 1990a, b; Foster and Charlesworth 1996). A strong mineral magnetic overprint derived from atmospheric pollutant fallout may have implications for interpreting the mineral magnetic signatures in the last ca. 50 years of the record. Catchment sources We used environmental magnetism and gammaemitting radionuclides to identify the likely source of sediment accumulating in Aqualate Mere, along with a considerable body of documentary evidence concerning land use change and land management in the catchment over the last 200 years. Detrended correspondence analysis was carried out using the statistical package SPSS on the
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radionuclide and mineral magnetic signatures measured on all potential sources. Neither 210Pbun nor 137 Cs was used in this analysis because their activities decline through time with half-lives of ca. 22 and 30 years, respectively, and the pattern of 137Cs fallout has varied significantly through time leading to temporal changes in the initial activities of topsoils. Five radionuclide signatures (228Ac, 40K, 226Ra, 234Th and 235U; Table 4) and all mineral magnetic concentration variables (Table 3) were included in the analysis. Similar approaches for other UK lakes have shown that combined radionuclide and mineral magnetic signatures provide good source discrimination (e.g. Foster 2006; Pittam et al. 2006), but in this analysis, two mineral magnetic signatures alone, SIRM and ARM, accounted for over 95% of the
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Fig. 7 SIRM versus ARM for potential catchment sources (a) with the range of values for the same parameters in the lake sediments (b)
variance in the data, suggesting that best discrimination would be achieved historically through the use of these two variables alone. A bivariate scatterplot, showing the relationship between all sources for SIRM and ARM is given in Fig. 7a. This analysis provided clear discrimination between three major potential source groups: canal sediment, road dust, and catchment soils. The latter group was a composite group of arable and pasture topsoil, as well as channel bank and subsoil, which could not be discriminated adequately using the parameters measured. The overlap between arable and pasture topsoil may arise because, historically, both land use practices may have been utilized in the same field, even if at different times. While the UK soil classification system used to subdivide sample subsets often distinguishes soil types on the basis of small differences in particle size, depth, and other variables, it was not possible to distinguish among soil types using the fingerprint variables applied in this study. Some lake sediment samples plot outside the major source envelopes by having high ARM and relatively low SIRM (Fig. 7b). All samples that plot outside the potential source range are from the upper ca. 30 cm of the sediment column (Fig. 4) probably because of significant atmospheric contaminant loading. Samples from below 30 cm depth, however,
largely plot within the source envelope curve and appear most similar to channel side and/or subsoil samples. The lake lies on the fringe of a major industrial conurbation with a well-documented industrial and pollution history (Crompton 1991; Stobart and Raven 2005). Industrial processes such as the production of steel and cement, generate airborne magnetic material, but coal-burning power stations are usually the most significant sources, as the high-temperature combustion of coal produces large quantities of strongly ferromagnetic iron oxides (Evans and Heller 2003). The potential contribution of contaminants, especially the metallurgy industry in Wolverhampton (ca. 15 km to the southwest) and a coal-fired power station at Coalbrookdale (ca. 10 km to the southeast), which became operational in 1969, could have altered the magnetic signatures of the upper lake sediments. The large increases in vlf, varm, and SIRM (Fig. 4), dated using 137Cs and 210Pbun, correlate temporally with the opening of this power station. Lake sediment samples below 30 cm depth generally fall within the catchment source envelope, but show no similarity with road dusts and contemporary canal sediments. Although it is tempting to discount both of these sources, the analysis assumes that the characteristics of source materials are time-invariant (Foster and Lees 2000). This assumption, with
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respect to the characteristics of the canal sediments, is un-testable. It might be argued that these characteristics would change as the power sources used for canal barges shifted from horse to steam to diesel, thereby changing the pollution loading and type to the canal, and changing the signatures of the potential sources over time. Despite this problem, characteristics of sediments deposited over the last ca. 50 years show no similarities with those of the canal sediments, but there is evidence to suggest that these sediments closely mirror the characteristics of the catchment samples and appear to cluster more closely with samples of sub-soil and channel bank material in having both low SIRM and varm. Historical evidence for catchment disturbance Evidence provided by Hutchinson (2005) suggested that construction of the canal in the 1790s was the only significant disturbance within the catchment at that time. Evidence provided by Yale (1994), however, suggests other major disturbances to the catchment in the immediate vicinity of the mere. Aqualate Park has extensively landscaped gardens and orchards and significant numbers of trees ca. 200 years old. There is documentary evidence of this landscaping in the early nineteenth century, which took 5 years to complete. Planting of trees is exhaustively documented, with figures for the number of individual trees planted each year between 1805 and 1810, with the exception of pine, as both semi-mature trees (10–12 ft high) and whips (2–3 ft high). In total, 121,304 whips and 1,393 semi-mature, mainly indigenous British trees were planted, (Yale 1994). In addition to the large number of mature forest trees and whips planted during landscaping, it is also noted in parkland records that a significant number of pine whips were also planted and at least 10,000 ornamental and fruit trees, although the exact number of pine whips is not recorded in the Accounts Books. The extent of landscaping and tree planting is confirmed in an early nineteenth century watercolour of Aqualate Park, by S. Shaw, which is housed in the William Salt Library, Stafford. The reported landscaping at Aqualate Mere would have had a significant impact on soil stability within the immediate vicinity of the mere. Reports from the Deer Park in 1810 suggest that there was ‘‘considerable activity…with teams moving soil into the
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garden’’ (Yale 1994, p. 36). Consequent instability within the catchment soils and subsequent erosion is detectable within the mineral magnetic signatures of the lake sediment presented in Fig. 4, which all increase at around this time. Delivery of large quantities of eroded minerogenic material is supported by the LOI data of Fig. 4d, which shows a time-synchronous decline. The organic matter content of the lake sediment continues to decrease upcore from this point in the sediment record. Grassland and arable land use were dominant in the catchment from the early nineteenth century onwards, and from historical maps it would appear that there was little alteration to the extent of woodland within the immediate vicinity of the mere. The only major alteration in the past century has been the planting of Castle Wood, a small area of woodland some 200 m south of the mere, which was not present on the Ordnance Survey map of 1892. Although extensive planting of trees and landscaping may have triggered significant increases in erosion, the influx of minerogenic sediment would be expected to decline over time as the disturbed parkland soils became re-vegetated: an observation that is not supported by the general decline in organic matter content of the mere sediments in the upper ca. 1 m of the sequence. In evaluating the possible impact of the canal, Hutchinson (2005) estimated that as much as 69,000 tonnes of sediment had accumulated in the upper minerogenic layer. If this had been transported from the canal and through the Wood Brook to Aqualate Mere since canal construction in 1793, it would equate to over 1 tonne of sediment per day since that time. If no sediment was delivered from the canal to the lake in the last 50 years, it would equate to an annual sediment yield over the shorter time period of ca. 34 t km-2 year-1 in addition to the natural sediment load transported by the river. The Wood Brook has a relatively small catchment (ca. 13 km2) and enters the lake through a thick fringing reed bed (Phragmites) at least 100 m wide and with no distinct river channel. Wood Brook also has a sedimentation pond (now unused) that would have trapped some of the sediment delivered to the mere. The S. Shaw watercolour of Aqualate Park shows that a reed fringe already existed in the early nineteenth century, although it does not record the presence or absence of a distinct channel connecting the Wood Brook
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directly to the mere. The filtering effect of the reed beds would undoubtedly have reduced the delivery of sediment from the canal to the mere. This suggests that the amount of material released from the canal must have been significantly higher than the 69,000 tonnes deposited in the lake, as a significant amount would have been stored within the catchment and reed fringe—an area not explored in detail either by Hutchinson (2005) or in our study. What is apparent in the watercolour, and in the parkland today, is that the southern shore of the mere is largely unprotected by vegetation over a distance of ca. 0.75 km where a deer park extends down to the mere shoreline. Large sections of this shoreline have vertical banks up to ca. 75 cm high that are exposed to wave action. A shallow, possibly wave-cut beach, extends more than 50 m out from the southern shore into the mere, and it is not unreasonable to postulate that shoreline erosion may have made a substantial contribution to the sediment accumulating in the mere over the last 200 years. It is not known whether the early nineteenth century landscaping removed pre-existing shoreline vegetation to open up views of the lake from the Manor, but such a change could have sustained the delivery of large quantities of fine sediment to the mere. Shoreline erosion has been invoked in several recent studies as a potential source of lake sediments (e.g. Bruk 1985; Bloesch 1995) and measurement of current rates of shoreline erosion would indicate whether this process could produce sufficient material to account for accelerated rates of sedimentation. Aqualate Mere is not the only mere in the region reported to show increases in minerogenic sediment supply and eutrophication in the recent past (e.g. Reynolds 1979; Twigger and Haslam 1991), yet it is the only site where a unique source, a canal overflow, has been invoked as the dominant source of sediment. The poorly drained soils of the larger southern catchment (the Back Brook) have led to extensive alterations in the drainage pattern, evident on current and historical Ordnance Survey maps of the area. Although the dates of initial cutting of these ditches and channels are not recorded, their existence, and periodic dredging and cleaning, could also be a source of sediment accumulating in the mere over the last 200 years. Twigger and Haslam’s (1991) study on environmental change in Shropshire notes that the last 200 years have witnessed significant land use change and intensification during the historical period,
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coupled with increases in the rates of soil erosion. The poorly drained soils of the catchment resulted in widespread installation of subsurface land drains before 1939 (Robinson 1986). Replacement and improvement of many of these drainage schemes is also recorded in the 1970s (Robinson and Armstrong 1988). Recent paleoenvironmental reconstructions of sediment yield and sediment delivery to Kyre Pool, Worcestershire (Foster et al. 2003; Pittam et al. 2006) have shown that sediment yields in this region increased by a factor of ca. 4 as a direct result of land drainage. This study was not able to meet one of its key objectives, i.e. identifying and quantifying the dominant source(s) of sediment accumulating in Aqualate Mere. Many of the historical catchment disturbances, however, would have caused an increase in the delivery of topsoil to the lake. Although the 137Cs activities in the upper section of the sediment column support this interpretation, comparison of the lake sediment 137Cs inventory with the reference soil inventory does not support it, unless sediment was lost through the outflow due to low trapping efficiency in the lake. There is no evidence, however, to support the contention that the canal now, or in the past, has contributed significantly to increased sedimentation rates in the Mere.
Discussion and conclusions This case study demonstrates that a solely paleolimnological approach to reconstruct land use changes and sediment sources in a lake catchment is limited and can produce inadequate and/or erroneous interpretation of data. These limitations can be overcome through adoption of a more holistic lake-catchment approach, which also identifies major sources that contribute to the accumulation of lake sediment, and includes an examination of documentary sources. The lake-catchment strategy used in this study at Aqualate Mere does not support Hutchinson’s (2005) contention that overflows from the canal were the major source of increasingly minerogenic sediment being deposited over the last 200 years. Results presented here suggest that without a detailed assessment of catchment sources, future management and rehabilitation strategies of catchments and lakes could be ineffective. In this example, a management strategy focused on the canal would be unwarranted as well as
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time consuming and financially ineffective, despite the fact that the research reported here has yet to assess quantitatively the exact source of the sediments now entering Aqualate Mere. In order for management and restoration to be effective, a much more detailed analysis of contemporary catchment sediment dynamics is required. This study highlighted the dangers of speculating about catchment dynamics based on the palaeoenvironmental record alone. It is 30 years since Oldfield (1977) published his seminal paper on the lake: catchment ecosystem framework, an idea adopted and developed in many subsequent publications concerned with catchment erosion and sediment delivery to lakes and reservoirs (e.g. Foster et al. 1998, 1990b). Failure to understand the dynamics of the contributing catchment leaves a substantial area of research methodology under-utilised in the context of catchment management and rehabilitation. This study failed in one of its major objectives, i.e. to quantify the contributions from all catchment sources. Additional measurements of potential source sediment geochemistry may have resolved this problem and remain a high priority for future research in the catchment of Aqualate Mere. Further work is also required to understand the controls on the 137Cs inventory and concentration profile before an acceptable management strategy can be developed for the catchment. Aqualate Mere contains a high-resolution, 11-m paleoenvironmental record and may be unique in the English Midlands. Partial destruction of such a highresolution and important record by dredging, suggested by Hutchinson (2005), without collecting and preserving complete sediment sequences for future paleolimnological research, would be ill-considered and unjustified. Acknowledgements We thank Coventry University for funding a Ph.D. project (Pittam), Bob Hollyoak and Liz Turner for laboratory assistance, and Alison Sandison and Jenny Johnson at the University of Aberdeen for drawing the diagrams. We also thank Neil Rose for help with SCP analysis.
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