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Techniques for validating the historic record of lake sediments. A demonstration of their use in the English Lake District
Vol. 1I, pp.21l-215, 1996 Copyright 0 1996Elsevier Science Ltd
Applied Geochemistry,
Pergamon
Printed in Great Britain. All rights reserved 0883-2927/96 $15.00 + 0.00
0883-2927(95)0004%
Techniques for validating the historic record of lake sediments. A demonstration of their use in the English Lake District Joy E. Rae and Andrew Parker Postgraduate Research Institute for Sedimentology, The University of Reading, P.O. Box 227, Whiteknights, Reading RG6 6AB, U.K.
Abstract-A lake sediment core from an upland catchment has been investigated in order to determine the nature of the anthropogenic record, and to demonstrate techniques by which historic records can be validated. The core was subjected to chemical analysis (major elements and selected trace metals), and sequential extractions. Two unrelated anthropogenic events were recorded in the sediments: the onset of woodland clearance ca. 1000 BP and of lead mining ca. 800 BP. Major element profiles indicating erosion rates can be validated using supporting organic carbon data, and the historic record of trace metal inputs can be validated by comparison of sequential extraction results expressed as percentages of total metal at each depth, with the results of similar non-pollutant species. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION Geochemical trends in lake sediment depth profiles have been extensively used as indicators of historical pollution (see for example review by Alderton, 1985). It has become increasingly apparent, however, that this approach requires an appreciation both of the sources and the nature of particulate material, and of the significance of early diagenetic processes (Engstrom and Wright, 1984; Farmer, 1991). The objectives of this study were two-fold, firstly to determine the nature of the Brotherswater lake sediment anthropogenic record, and secondly to demonstrate techniques by which such records can be validated.
today in the catchment are of local origin (Chambers, 1978). The history of mining in the area is documented by Adams (1988). An old lead mine (argentiferous galena) is situated in the woods above Hartsop Beck, one of the main streams. It is likely that small-scale mining started at a very early time; the earliest known lease for the mine is dated 1696. (Workings continued infrequently from 1696 until 1942 when the mine was abandoned.)
STUDY AREA The Brotherswater catchment in the English Lake District (Fig. 1) was selected as the study area, since it is a small catchment with well-defined sources of particulate material and with a documented historical input of pollutant Pb. In addition, a good deal of background information is available on processes of weathering and sediment transport (Chambers, 1978; Rae and Parker, 1993). The catchment bedrock is composed of Borrowdale Volcanic Series rocks (Ordovician); chloritized and sheared andesitic tuffs make up w 96% of the exposed rock surface and andesitic/rhyolitic lava flows the remaining 4% (Chambers, 1978). The surficial deposits of the study area probably relate to two periods of glaciation and retreat: that of maximum extent at 25,000 BP (Devonian), and the minor resurgence of glacial activity between 11,000 and 10,000 BP. It is thought likely that all the surficial deposits found 211
Fig. 1. Location map of the Brotherswater catchment in the English Lake District.
J. E. Rae and A. Parker
212
Table 1. Values of organic and inorganic carbon and total Fe and Mg, and Pb, Zn, Co and Cu for sediment samples from the Brotherswater lake core Sample no. (and depth/cm)
Organic C (%)
Inorganic C (%)
Total Fe (as % Fez%)
Total Mg (as %MgO)
Total Pb (ppm)
Total Zn (ppm)
Total Co (ppm)
2 (1418) 4 (22-25) 7 (32-35) 8 (35-38) 10 (42-45)
5.59 5.91 5.38 5.21 1.36
0.51 0.33 nd 0.26 0.43
8.2 8.1 8.0 8.2 8.2
2.3 2.4 2.2 2.4 2.8
332 211 451 412 587
239 231 205 202 181
21 23 20 23 25
25 24 22 23 23
13 13 11 12 13
8.39 5.85 nd
0.42 0.36 nd
9.4 8.0 8.0
2.2 2.5 2.3
155 64 68
306 223 221
22 22 25
31 22 21
13 12 11
8.60 7.21 8.95 9.20 6.52 9.01
0.33 0.40 0.30 0.40 0.73 0.38
6.7 6.4 6.9 6.1 6.2 6.2
1.8 1.8 1.9 1.8 1.9 1.6
60 59 63 61 57 59
195 195 208 202 175 245
18 16 21 20 17 18
18 18 20 19 16 22
10 10 10 10 10 10
Total Cu Total Cd (ppm) (ppm)
(1) 15 (62-68) 26 (102-105) 39 (152-155)
(2)
52 (202-205) 65 (252-255) 78 (302-308) 90 (352-355) 103 (402405) 113 (442-445)
nd = not determined; (1) = horizons above which Pb increases; (2) = horizons above which Fe, Mg increase and organic carbon decreases.
METHODOLOGY A 4.5 m sediment core was obtained from the deepest part of Brotherswater (16m water depth), using a 6 m Mackereth pneumatic corer. The core was sliced into (normally 4 cm) intervals which were dried at 60°C for 24 h. The samples were ground using a tungsten carbide grinder; selected samples were also ground in an agate mortar. Comparison of trace metal values from samples prepared by both methods demonstrated no detectable contamination from the tungsten carbide system for the elements studied. Fifteen samples were selected for detailed chemical and mineralogical analysis from 14-445 cm depth (below the oxic-anoxic boundary), in order to include both the onset of Pb mining and any major change in land use in the area. Sequential extractions were performed on sediment samples according to the method of Tessier et al. (1979), in which five phases are defined according to the reagents used: these phases are exchangeable, carbonate, ion and manganese oxide, organic-sulphidic and residual. Subsequently Pb and a number of other trace metals (Zn, Cd, Co, Cu) not associated with the mining activity were determined by inductivelycoupled plasma spectrometry (ARL 35000) with standardisation using a range of international rock standards. Major elements were determined by X-ray fluorescence analysis of pressed powder pellets using a Philips PW 1480 spectrometer with dual-anode tube (SC/MO). Calibration was achieved using a wide range of international reference materials. Organic carbon was determined by the wet oxidation method of Gaudette et al. (1974) and inorganic carbon by calcimeter.
RESULTS Results of the analysis of organic and inorganic carbon, and total Fe, Mg, Pb, Zn, Co, Cu and Cd, are presented in Table 1, where trace metal totals represent the sum of the individual extracted phases.
(There was no significant variation with depth for the other major elements.) Total Fe and Mg are greatest above approximately 152 cm depth, with mean values of 8.2% and 6.5% FezOs, and 2.4% and 1.7% MgO, above and below this horizon respectively. Values of organic carbon appear to follow the opposite trend, with mean values of 6.1% above 152 cm and 8.2% below 152 cm. Inorganic carbon shows little variation in the profile. Total Pb displays a marked depth trend (Fig. 2) with a mean value of 61 ppm below about 100 cm, a “transitional” value of 155 ppm at -65 cm depth and higher values above (389 ppm mean value). In contrast Zn, Co and Cu display no significant depth trends. Results of the selective extractions are given in Table 2. The quantitative importance of each of the phases for the trace elements studied is: (1) Pb residual > Fe and exchangeable > carbonate (2) Zn residual > Fe and able > organic > carbonate (3) Co residual > organic carbonate/exchangeable (4) Cu organic > residual able > Fe and Mn oxide
Mn oxide > organic > Mn oxide > exchange> Fe and Mn oxide > > carbonate/exchange-
These results are broadly similar to those which have been obtained by other workers (e.g. KabataPendias, 1993), but detailed comparisons are difficult, since it is well known that phase association results depend on the laboratory methods used as well as sediment characteristics and the nature of trace metal inputs (Jones and Hao, 1993). Values of Pb within each of the five phases reveal a marked depth
The historic record of lake sediments
Fig. 2. Brotherswater lake sediment depth-profiles for total Pb and for Pb in each of 5 sequentially-extracted phases.
distribution (Fig. 2), whereas Zn, Co and Cu within each extracted phase are not significantly different with depth.
DISCUSSION
On the basis of the bulk chemistry and sequential extractions it appears that two (unrelated) anthropogenie events are clearly documented in the core. Firstly, sediments above approximately 152 cm (circa 1000 BP, Chambers, 1978) have lower values of organic carbon and higher values of total iron and magnesium than those below. These observations are consistent with the findings of Mackereth (1966). He reasoned that periods of active erosion should involve the transport of relatively unleached material, leading
213
to relatively high values of bedrock-derived elements such as Fe and Mg in lake sediments. In contrast, during episodes of relatively stable soils, deep weathering of mature soil profiles might diminish the content of bedrock-derived elements in the lake sediments since a high proportion of these elements would be lost from the catchment in solution, or associated with suspended material (Rae and Parker, 1993). Mackereth (1966) makes no mention of organic carbon values, but the finding of this study (the association of high organic carbon values in the lake sediments with low amounts of bedrock-derived elements) is consistent with periods of mature soil profile weathering. The onset of active erosion in the Brotherswater catchment at approximately 1000 BP is consistent with the suggestion that man became a major influence on soil erosion through woodland clearance in many parts of the English Lake District around 1000 BP (Pennington, 1964). The second example of a record of man’s influence in the catchment is related to the onset of mining. Values of total Pb (Table 1) demonstrate a clear depth distribution (Fig. 2) with the onset of higher levels at around 42 cm depth. This horizon (ca. 800 BP, Chambers, 1978) may well record the early onset of small-scale Pb mining in the catchment. [Note: a characteristic of the depth-time profiles of cores from lakes in the area is a steepening of the curve toward the top of the core, indicative of an increase in the rate of sediment accumulation.] Several authors have recently expressed concern over the use of sediment-depth profiles as historical records because of the potential for remobilisation of elements during early diagenesis (e.g. Allen and Rae, 1990; Farmer, 1991) In the Brotherswater Lake core, two major elements (Fe and Mg) are used to establish the horizon of errosion-rate change. Since major elements are less subject to remobilisation than trace elements (Engstrom and Wright, 1984) it is suggested that the record of woodland clearance has been preserved intact. Furthermore, additional supporting evidence is given by the levels of organic carbon, which are easily explained by the lake-core record of erosion. A clue to the significance of any early diagenetic remobilisation of Pb is provided by depth profiles of the other trace metals which are not associated with mining, but which behave in a similar manner to Pb during early diagenesis (Allen and Rae, 1990; Fergusson, 1990). The consistency of values of Zn, Co, Cu and Cd with depth suggests little early diagenetic remobilisation for these trace metals, and by implication for Pb. Furthermore, phase associations of the trace metals confirm this supposition (Table 2). Above 42-45 cm, the residual-phase Pb becomes less important relative to the other more weakly-bound phases, possibly owing to the exposure and weathering of Pb minerals on waste heaps. In contrast, the distribution of Zn, Co, Cu and Cd
between each phase is constant with depth. In particular, the sequential extraction values expressed
J. E. Rae and A. Parker
214
Table 2. Results of the sequential extractions for Pb, Zn, Co, Cu and Cd. All trace element values are given as ppm dry weight Phase Sample depth (cm) 14-18
22-25 25-28 32-35 35-38 42-45 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442-445
Exchange Pb
Carbonate
Fe/Mn oxide
Organic and sulphidic
Residual
Zn
Cd
Pb
Zn
Pb
Zn
Co
Pb
Zn
Co
Cu
Cd
Pb
Zn
Co
Cu
Cd
12 14 9 11 9 3 10 1 6 1 6 6 15122 2 161 I 4 1 15
1 1
17 12 15 22 20 40 4
1
3
53 41 49 100 72 129 22 6 8 9 6 3 7 7 6
25 24 30 26 21 22 28 21 31 26 22 22 26 18 31
1 2 2 1 2 3 2 2 3 2 1 2 2 1 1
36 31 38 71 62 101 12 15 -
39 39 38 37 32 26 45 35 39 33 36 37 37 32 46
3 3 4 4 4 4 4 4 6 4 3 6 5 3 4
16 15 14 14 15 14 19 14 14 13 12 14 14 11 14
I 1
1 1
3 3 3 2 2 2 2 3 2 3
190 159 152 203 215 254 109 56 58 35 50 59 52 48 51
160 179 151 129 138 128 221 158 143 127 130 144 131 119 150
16 15 14 14 16 18 15 16 15 12 12 13 13 13 12
9 9 7 8 8 9 12 8 7 5 6 6 5 5 8
11 11 10 9 10 11 11 IO 9 7 8 8 8 8 8
36 28 29 55 43 63 8
1
1 1 1 1 1 1 1
1
1
12 1
1
1 1
2 3
1 1
1 1 1 1 1 2 1 1 1 1
1
Note: “ - ” represents < 0.5 ppm. There was consistently <0.5 ppm copper in the exchangeable, carbonate and ion oxide phases, < 0.5 ppm cobalt in the exchangeable and carbonate phases, and < 0.5 ppm cadmium in carbonate and iron oxide phases. Table 3. Phase associations of (a) Pb and(b) Zn in the Brotherswater lake sediment core (values given are percentages of totals at each depth)
(4 Sample depth (cm) 14-18 22-25 25-28 33-35 35-38 4245 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442445
%Pb exchangeable 11 10 IO 12 10 11 5 2 2 0 2 0 2 2 2
%Pb carbonate
%Pb Fe and Mn oxide
%Pb organic and sulphide
%Pb residual
5 4 5 5 5 7 3 2 2 2 3 2 2 2 2
16 15 17 22 18 22 14 9 11 15 10 5 11 12 10
11 11 13 16 15 17 8 0 0 25 0 0 0 0 0
57 60 55 45 52 43 70 87 85 58 85 93 85 84 86
%Zn carbonate
%Zn Fe and Mn oxide
%Zn organic and sulphide
%Zn residual
11 10 13 13 10 12 9 9 14 13 11 11 13 10 13
16 17 16 18 16 14 15 16 18 17 19 18 18 18 19
67 77 65 63 68 70 72 71 65 65 67 69 65 68 61
@I Sample depth (cm) 1418 22-25 25-28 32-35 35-38 4245 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442445
%Zn exchangeable 5 6 4 5 5 2 3 3 3 3 3 3 2 6
1 1 1
1 1 1 1
1 2
1 1 1
1 1
The historic record of lake sediments of the total metal (percentage phase associations) at each depth demonstrate this clearly (e.g. Table 3 for Pb and Zn). Details of mineralogy and grain size for the lakecore sediments are given by Chambers (1978): mean mineralogy is quartz 63.9%, feldspar 14.8%, chlorite 16% and illite 5%, and the silt-size fraction is predominant. Although there is some degree of variation with depth for both mineralogy and grain size, it is of a magnitude which is unimportant compared to the broad chemical trends.
REFERENCES
as percentages
CONCLUSIONS The Brotherswater lake sediment depth-profile records the onset of two anthropogenic catchment events. These are: (1) woodland clearance at ca. 1000 BP, and (2) Pb mining
at ca. 800 BP
Any element redistribution by early diagenetic processes appears to have been of minor significance compared to the preservation of the historical anthropogenic record. Lake sediment depth-profiles can be confidently used as historical records of anthropogenic activity, particularly where supporting evidence is available in addition to single-element profiles. In particular, it is suggested that the major-element profiles can be used confidently with supporting organic carbon data as indicators of erosion rates, and that trace-metal data can be used effectively with phase-association studies of a number of elements. The comparison of percentage phase associations of pollutants with those of similar non-pollutant species is suggested as a particularly useful tool. Editorial
handling:
Dr 0. Selinus.
215
Adams J. (1988)Mines of the Lake District Fells. Dalesman, Lancaster. Alderton D. H. M. (1985) Sediments. In Historical Monitoring, MARC Report No. 31, pp l-95. MARC, London. Allen J. R. L. and Rae J. E. (1990) Metal speciation (01, Zn, Pb) and organic matter in an oxic salt marsh, Severn Estuary, Southwest Britain. Mar. Pollut. Bull. 21, 574 580. Chambers K. C. (1978) Source-sediment relationships in the Cumbrian Lakes. PhD thesis, The University of Reading, Reading, U.K. Engstrom D. R. and Wright H. E. (1984) Chemical stratigraphy of lake sediments as a record of environmental change. In Lake Sediments and Environmental History (eds G. Y. Haworth and J. W. G. Lund), pp. 1l67. Leicester University Press. Farmer J. G. (1991) The perturbation of historical pollution records in aquatic sediments. Environ. Geochem. Health 13, 76-83.
Fergusson J. E. (1990) The Heavy Elements: Chemistry, Environmental Impact and Health Effects. Pergamon Press, Oxford. Gaudette H. F., Flight W. R., Toner L. and Folger D. W. (1974) An inexpensive titration method for the determination of organic carbon in recent sediments. J. Sed. Petrol. 44, 24%253.
Jones J. M. and Hao J. (1993) Sequential extraction method: a review and evaluation, Abstr. Environ. Geochem. Health 15, 185.
Kabata-Pendias A. (1993) Behavioural properties of trace metals in soils. Appl. Geochem. Suppl. 1, Iss. 2, 3-9. Mackereth F. J. H. (1966) Some chemical observations on post-glacial lake sediments. Phil. Trans. R. Sot. B 250, 165-213.
Pennington W. (1964) Pollen analyses from the deposits of six upland farms in the Lake District. Phil. Trans. R. Sot.
B 248, 205-244. Rae J. E. and Parker A. (1993) Sources, solid-phase transport and geochemical associations of Co and Cu in a small upland catchment, English Lake District. Appl. Geochem. Suppl. 1, Iss. 2, 263-268. Tessier A., Campbell P. G. C. and Bisson M. (1979) Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844-851.
Applied Geochemistry,
Pergamon
Printed in Great Britain. All rights reserved 0883-2927/96 $15.00 + 0.00
0883-2927(95)0004%
Techniques for validating the historic record of lake sediments. A demonstration of their use in the English Lake District Joy E. Rae and Andrew Parker Postgraduate Research Institute for Sedimentology, The University of Reading, P.O. Box 227, Whiteknights, Reading RG6 6AB, U.K.
Abstract-A lake sediment core from an upland catchment has been investigated in order to determine the nature of the anthropogenic record, and to demonstrate techniques by which historic records can be validated. The core was subjected to chemical analysis (major elements and selected trace metals), and sequential extractions. Two unrelated anthropogenic events were recorded in the sediments: the onset of woodland clearance ca. 1000 BP and of lead mining ca. 800 BP. Major element profiles indicating erosion rates can be validated using supporting organic carbon data, and the historic record of trace metal inputs can be validated by comparison of sequential extraction results expressed as percentages of total metal at each depth, with the results of similar non-pollutant species. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION Geochemical trends in lake sediment depth profiles have been extensively used as indicators of historical pollution (see for example review by Alderton, 1985). It has become increasingly apparent, however, that this approach requires an appreciation both of the sources and the nature of particulate material, and of the significance of early diagenetic processes (Engstrom and Wright, 1984; Farmer, 1991). The objectives of this study were two-fold, firstly to determine the nature of the Brotherswater lake sediment anthropogenic record, and secondly to demonstrate techniques by which such records can be validated.
today in the catchment are of local origin (Chambers, 1978). The history of mining in the area is documented by Adams (1988). An old lead mine (argentiferous galena) is situated in the woods above Hartsop Beck, one of the main streams. It is likely that small-scale mining started at a very early time; the earliest known lease for the mine is dated 1696. (Workings continued infrequently from 1696 until 1942 when the mine was abandoned.)
STUDY AREA The Brotherswater catchment in the English Lake District (Fig. 1) was selected as the study area, since it is a small catchment with well-defined sources of particulate material and with a documented historical input of pollutant Pb. In addition, a good deal of background information is available on processes of weathering and sediment transport (Chambers, 1978; Rae and Parker, 1993). The catchment bedrock is composed of Borrowdale Volcanic Series rocks (Ordovician); chloritized and sheared andesitic tuffs make up w 96% of the exposed rock surface and andesitic/rhyolitic lava flows the remaining 4% (Chambers, 1978). The surficial deposits of the study area probably relate to two periods of glaciation and retreat: that of maximum extent at 25,000 BP (Devonian), and the minor resurgence of glacial activity between 11,000 and 10,000 BP. It is thought likely that all the surficial deposits found 211
Fig. 1. Location map of the Brotherswater catchment in the English Lake District.
J. E. Rae and A. Parker
212
Table 1. Values of organic and inorganic carbon and total Fe and Mg, and Pb, Zn, Co and Cu for sediment samples from the Brotherswater lake core Sample no. (and depth/cm)
Organic C (%)
Inorganic C (%)
Total Fe (as % Fez%)
Total Mg (as %MgO)
Total Pb (ppm)
Total Zn (ppm)
Total Co (ppm)
2 (1418) 4 (22-25) 7 (32-35) 8 (35-38) 10 (42-45)
5.59 5.91 5.38 5.21 1.36
0.51 0.33 nd 0.26 0.43
8.2 8.1 8.0 8.2 8.2
2.3 2.4 2.2 2.4 2.8
332 211 451 412 587
239 231 205 202 181
21 23 20 23 25
25 24 22 23 23
13 13 11 12 13
8.39 5.85 nd
0.42 0.36 nd
9.4 8.0 8.0
2.2 2.5 2.3
155 64 68
306 223 221
22 22 25
31 22 21
13 12 11
8.60 7.21 8.95 9.20 6.52 9.01
0.33 0.40 0.30 0.40 0.73 0.38
6.7 6.4 6.9 6.1 6.2 6.2
1.8 1.8 1.9 1.8 1.9 1.6
60 59 63 61 57 59
195 195 208 202 175 245
18 16 21 20 17 18
18 18 20 19 16 22
10 10 10 10 10 10
Total Cu Total Cd (ppm) (ppm)
(1) 15 (62-68) 26 (102-105) 39 (152-155)
(2)
52 (202-205) 65 (252-255) 78 (302-308) 90 (352-355) 103 (402405) 113 (442-445)
nd = not determined; (1) = horizons above which Pb increases; (2) = horizons above which Fe, Mg increase and organic carbon decreases.
METHODOLOGY A 4.5 m sediment core was obtained from the deepest part of Brotherswater (16m water depth), using a 6 m Mackereth pneumatic corer. The core was sliced into (normally 4 cm) intervals which were dried at 60°C for 24 h. The samples were ground using a tungsten carbide grinder; selected samples were also ground in an agate mortar. Comparison of trace metal values from samples prepared by both methods demonstrated no detectable contamination from the tungsten carbide system for the elements studied. Fifteen samples were selected for detailed chemical and mineralogical analysis from 14-445 cm depth (below the oxic-anoxic boundary), in order to include both the onset of Pb mining and any major change in land use in the area. Sequential extractions were performed on sediment samples according to the method of Tessier et al. (1979), in which five phases are defined according to the reagents used: these phases are exchangeable, carbonate, ion and manganese oxide, organic-sulphidic and residual. Subsequently Pb and a number of other trace metals (Zn, Cd, Co, Cu) not associated with the mining activity were determined by inductivelycoupled plasma spectrometry (ARL 35000) with standardisation using a range of international rock standards. Major elements were determined by X-ray fluorescence analysis of pressed powder pellets using a Philips PW 1480 spectrometer with dual-anode tube (SC/MO). Calibration was achieved using a wide range of international reference materials. Organic carbon was determined by the wet oxidation method of Gaudette et al. (1974) and inorganic carbon by calcimeter.
RESULTS Results of the analysis of organic and inorganic carbon, and total Fe, Mg, Pb, Zn, Co, Cu and Cd, are presented in Table 1, where trace metal totals represent the sum of the individual extracted phases.
(There was no significant variation with depth for the other major elements.) Total Fe and Mg are greatest above approximately 152 cm depth, with mean values of 8.2% and 6.5% FezOs, and 2.4% and 1.7% MgO, above and below this horizon respectively. Values of organic carbon appear to follow the opposite trend, with mean values of 6.1% above 152 cm and 8.2% below 152 cm. Inorganic carbon shows little variation in the profile. Total Pb displays a marked depth trend (Fig. 2) with a mean value of 61 ppm below about 100 cm, a “transitional” value of 155 ppm at -65 cm depth and higher values above (389 ppm mean value). In contrast Zn, Co and Cu display no significant depth trends. Results of the selective extractions are given in Table 2. The quantitative importance of each of the phases for the trace elements studied is: (1) Pb residual > Fe and exchangeable > carbonate (2) Zn residual > Fe and able > organic > carbonate (3) Co residual > organic carbonate/exchangeable (4) Cu organic > residual able > Fe and Mn oxide
Mn oxide > organic > Mn oxide > exchange> Fe and Mn oxide > > carbonate/exchange-
These results are broadly similar to those which have been obtained by other workers (e.g. KabataPendias, 1993), but detailed comparisons are difficult, since it is well known that phase association results depend on the laboratory methods used as well as sediment characteristics and the nature of trace metal inputs (Jones and Hao, 1993). Values of Pb within each of the five phases reveal a marked depth
The historic record of lake sediments
Fig. 2. Brotherswater lake sediment depth-profiles for total Pb and for Pb in each of 5 sequentially-extracted phases.
distribution (Fig. 2), whereas Zn, Co and Cu within each extracted phase are not significantly different with depth.
DISCUSSION
On the basis of the bulk chemistry and sequential extractions it appears that two (unrelated) anthropogenie events are clearly documented in the core. Firstly, sediments above approximately 152 cm (circa 1000 BP, Chambers, 1978) have lower values of organic carbon and higher values of total iron and magnesium than those below. These observations are consistent with the findings of Mackereth (1966). He reasoned that periods of active erosion should involve the transport of relatively unleached material, leading
213
to relatively high values of bedrock-derived elements such as Fe and Mg in lake sediments. In contrast, during episodes of relatively stable soils, deep weathering of mature soil profiles might diminish the content of bedrock-derived elements in the lake sediments since a high proportion of these elements would be lost from the catchment in solution, or associated with suspended material (Rae and Parker, 1993). Mackereth (1966) makes no mention of organic carbon values, but the finding of this study (the association of high organic carbon values in the lake sediments with low amounts of bedrock-derived elements) is consistent with periods of mature soil profile weathering. The onset of active erosion in the Brotherswater catchment at approximately 1000 BP is consistent with the suggestion that man became a major influence on soil erosion through woodland clearance in many parts of the English Lake District around 1000 BP (Pennington, 1964). The second example of a record of man’s influence in the catchment is related to the onset of mining. Values of total Pb (Table 1) demonstrate a clear depth distribution (Fig. 2) with the onset of higher levels at around 42 cm depth. This horizon (ca. 800 BP, Chambers, 1978) may well record the early onset of small-scale Pb mining in the catchment. [Note: a characteristic of the depth-time profiles of cores from lakes in the area is a steepening of the curve toward the top of the core, indicative of an increase in the rate of sediment accumulation.] Several authors have recently expressed concern over the use of sediment-depth profiles as historical records because of the potential for remobilisation of elements during early diagenesis (e.g. Allen and Rae, 1990; Farmer, 1991) In the Brotherswater Lake core, two major elements (Fe and Mg) are used to establish the horizon of errosion-rate change. Since major elements are less subject to remobilisation than trace elements (Engstrom and Wright, 1984) it is suggested that the record of woodland clearance has been preserved intact. Furthermore, additional supporting evidence is given by the levels of organic carbon, which are easily explained by the lake-core record of erosion. A clue to the significance of any early diagenetic remobilisation of Pb is provided by depth profiles of the other trace metals which are not associated with mining, but which behave in a similar manner to Pb during early diagenesis (Allen and Rae, 1990; Fergusson, 1990). The consistency of values of Zn, Co, Cu and Cd with depth suggests little early diagenetic remobilisation for these trace metals, and by implication for Pb. Furthermore, phase associations of the trace metals confirm this supposition (Table 2). Above 42-45 cm, the residual-phase Pb becomes less important relative to the other more weakly-bound phases, possibly owing to the exposure and weathering of Pb minerals on waste heaps. In contrast, the distribution of Zn, Co, Cu and Cd
between each phase is constant with depth. In particular, the sequential extraction values expressed
J. E. Rae and A. Parker
214
Table 2. Results of the sequential extractions for Pb, Zn, Co, Cu and Cd. All trace element values are given as ppm dry weight Phase Sample depth (cm) 14-18
22-25 25-28 32-35 35-38 42-45 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442-445
Exchange Pb
Carbonate
Fe/Mn oxide
Organic and sulphidic
Residual
Zn
Cd
Pb
Zn
Pb
Zn
Co
Pb
Zn
Co
Cu
Cd
Pb
Zn
Co
Cu
Cd
12 14 9 11 9 3 10 1 6 1 6 6 15122 2 161 I 4 1 15
1 1
17 12 15 22 20 40 4
1
3
53 41 49 100 72 129 22 6 8 9 6 3 7 7 6
25 24 30 26 21 22 28 21 31 26 22 22 26 18 31
1 2 2 1 2 3 2 2 3 2 1 2 2 1 1
36 31 38 71 62 101 12 15 -
39 39 38 37 32 26 45 35 39 33 36 37 37 32 46
3 3 4 4 4 4 4 4 6 4 3 6 5 3 4
16 15 14 14 15 14 19 14 14 13 12 14 14 11 14
I 1
1 1
3 3 3 2 2 2 2 3 2 3
190 159 152 203 215 254 109 56 58 35 50 59 52 48 51
160 179 151 129 138 128 221 158 143 127 130 144 131 119 150
16 15 14 14 16 18 15 16 15 12 12 13 13 13 12
9 9 7 8 8 9 12 8 7 5 6 6 5 5 8
11 11 10 9 10 11 11 IO 9 7 8 8 8 8 8
36 28 29 55 43 63 8
1
1 1 1 1 1 1 1
1
1
12 1
1
1 1
2 3
1 1
1 1 1 1 1 2 1 1 1 1
1
Note: “ - ” represents < 0.5 ppm. There was consistently <0.5 ppm copper in the exchangeable, carbonate and ion oxide phases, < 0.5 ppm cobalt in the exchangeable and carbonate phases, and < 0.5 ppm cadmium in carbonate and iron oxide phases. Table 3. Phase associations of (a) Pb and(b) Zn in the Brotherswater lake sediment core (values given are percentages of totals at each depth)
(4 Sample depth (cm) 14-18 22-25 25-28 33-35 35-38 4245 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442445
%Pb exchangeable 11 10 IO 12 10 11 5 2 2 0 2 0 2 2 2
%Pb carbonate
%Pb Fe and Mn oxide
%Pb organic and sulphide
%Pb residual
5 4 5 5 5 7 3 2 2 2 3 2 2 2 2
16 15 17 22 18 22 14 9 11 15 10 5 11 12 10
11 11 13 16 15 17 8 0 0 25 0 0 0 0 0
57 60 55 45 52 43 70 87 85 58 85 93 85 84 86
%Zn carbonate
%Zn Fe and Mn oxide
%Zn organic and sulphide
%Zn residual
11 10 13 13 10 12 9 9 14 13 11 11 13 10 13
16 17 16 18 16 14 15 16 18 17 19 18 18 18 19
67 77 65 63 68 70 72 71 65 65 67 69 65 68 61
@I Sample depth (cm) 1418 22-25 25-28 32-35 35-38 4245 62-68 102-105 152-155 202-205 252-255 302-308 352-355 402-405 442445
%Zn exchangeable 5 6 4 5 5 2 3 3 3 3 3 3 2 6
1 1 1
1 1 1 1
1 2
1 1 1
1 1
The historic record of lake sediments of the total metal (percentage phase associations) at each depth demonstrate this clearly (e.g. Table 3 for Pb and Zn). Details of mineralogy and grain size for the lakecore sediments are given by Chambers (1978): mean mineralogy is quartz 63.9%, feldspar 14.8%, chlorite 16% and illite 5%, and the silt-size fraction is predominant. Although there is some degree of variation with depth for both mineralogy and grain size, it is of a magnitude which is unimportant compared to the broad chemical trends.
REFERENCES
as percentages
CONCLUSIONS The Brotherswater lake sediment depth-profile records the onset of two anthropogenic catchment events. These are: (1) woodland clearance at ca. 1000 BP, and (2) Pb mining
at ca. 800 BP
Any element redistribution by early diagenetic processes appears to have been of minor significance compared to the preservation of the historical anthropogenic record. Lake sediment depth-profiles can be confidently used as historical records of anthropogenic activity, particularly where supporting evidence is available in addition to single-element profiles. In particular, it is suggested that the major-element profiles can be used confidently with supporting organic carbon data as indicators of erosion rates, and that trace-metal data can be used effectively with phase-association studies of a number of elements. The comparison of percentage phase associations of pollutants with those of similar non-pollutant species is suggested as a particularly useful tool. Editorial
handling:
Dr 0. Selinus.
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