Lead-210 chronology in relation to levels of elements in dated sediment core profiles

Estuarine and Coastal Marine Science (1979) 8, zsg-269

Lead-210 Chronology in Relation to Levels of Elements in Dated Sediment Core Profiles

Robert J. Clifton and Eric I. Hamilton Natural Environment Research Council Institute for Marine Envir onmental Research, Prospect Place, The Hoe, Plymouth, Devon, U.K. Received 22 August 1977 and in revised form 13 February 1978

Keywords: geochronology;

radioactivity

measurements;

cores; sediments;

mines; zinc; lead; copper This paper describes the use of alaPb-*loPo chronology to determine sedimentation rates in the Sevem (Swansea Bay, Bridgwater Bay, Newport Deep) and Tamar estuaries of the United Kingdom. Samples taken from Swansea Bay and the Tamar show a significant decrease in the specific radioactivity of *lOPb with depth while samples from Newport Deep and Bridgwater Bay did not show this feature-instead the 210Pbactivity tended to be constant. Attempts are made to confirm the derived sedimentation rates using other data, such as lead isotopic compositions and the industrial history of the areas. These have been partially successful, but there is still a need for an unequivocal means of confirmation.

Introduction The effect of man’s technological

activities

on the natural environment

can be interpreted

only in the light of a knowledge of baseline or natural levels existing before his influence on the system became manifest. This is particularly relevant to estuaries which are often the receptacle of industrial

wastes but which also contain elements, often regarded as pollutants,

such as Fe, Zn, Cu and Pb, derived from natural geological processes not associated with man’s activities.

A continuously depositing sediment, for which each increment of deposited material remains in a closed system, will retain a historical record of industrial and naturally derived components. A knowledge of the sedimentation rate will allow different horizons, within the sediment column, to be dated and the relative concentrations of materials derived from natural and technical sources can be calculated. The 210Pb-210Po dating method described by Krishnaswami, Martin & Majbeck (rg71), Koide, Soutar & Goldberg

(rg72), and Robbins & Edgington

(1975) has been found particu-

larly suited for dating marine sediments over periods of approximately IOO years-i.e.

the

period in which man has had most impact on the environment.

However, this method has only been validated in relatively low energy systems such as marine varved clays and lakes. In this work we have attempted to assessthe 210Pb-s1aPo method as a means of dating estuarine sediments and to relate the variability of element concentrations within the sediment column to man’s industrial activity within that region.

OJOZ-3524/79/030259

+ I I 802*00/O

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Press Inc. (London)

Ltd.

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R.J. ClzJftw &SE. I. Hamilton

Two estuaries were selected for study; the energetic Bristol Channel-Sevem System and the less energetic ria estuary of the R. Tamar. The study reported in this paper considered the srOPb--2l’JPosystem; an extension of this work, described by Hamilton & Clifton (1979), investigates changes in the isotopic composition of lead in estuarine sediments for the purpose of identifying times in the past when lead of a particular composition can be used to identify an event recorded in deposited sediments. As part of this programme of research it is proposed to consider other radionuclides, for example those present in the effluent from the production of nuclear energy, such as the transuranides, together with 2s’JTh/sW, ss’JTh/ssrPaand W! dating techniques in order to identify particular aspects of sediment deposition in estuaries. Apart from establishing chronologies for discrete sediment cores, anomalies identified when using these techniques can also be used to describe and identify major estuarine processes, for example the stability

Figure I. Geographical estuary (B).

localities;

Bristol

Channel-Sevem

Estuary

(A), Tamar

Lead-2x0 chronology and sedimentation rates

261

of sediments with time, the dispersion of sediments and the extent to which sediments from defined areas act as sinks or sources for elements, radionuclides and compounds. Methods Sample collection and preparation

The geographical location of sampling areas is illustrated in Figure I. Gravity core samples, 7 cm in diameter, were taken in the St John’s Lake area of the Tamar estuary (Figure a) and

/‘\

0 Devon Gt. Consuls

c

Figure 2. Sites of old mines (O), smelting works (0) of the Tamar and St John’s Lake sediment core station ( n).

Valley,

Devon,

262

R.J. Clifton & E. I. Hamilton

from the Newport Deep, Bridgwater Bay and Swansea Bay areas of the Bristol Channel (Figure 3); samples were removed to the laboratory, extruded from the plastic liner, cut into I cm thick sections, weighed and freeze-dried. The dried samples were then lightly crushed in a plastic ball mill.

&

Sediment

BRISTOL

core stations (Area A)

CHANNEL

Km.

20

40

60

80

Figure 3. Sediment core stations in the Bristol Channel.

Radiochemical methods As sr0Pb is a very weak beta emitter (0.017 MeV) it is relatively difficult to determine, especially at the natural level (a few pCi), therefore 210Po,an alpha emitter, in assumed secular equilibrium with siaPb is more readily determined and was used as an indicator of the aloPb levels in sediments. In order to determine chemical yields, approximately 2 g of the dried sediment was spiked with 2oePotracer and taken to dryness in an air oven. The sample was then treated with 5 ml of concentrated nitric acid to extract the radionuclides and heated to dryness on a water bath. The dried residue was then treated with 5 ml of concentrated nitric acid, warmed, followed by the addition of concentrated hydrochloric acid to expel the nitric acid; the sample was then leached with 6 N-HCl for one hour at 90 “C and the volume adjusted to 20 ml, centrifuged, followed by the separation of srOPofrom the leachate according to the method of Flynn (1968). The s*sPoactivity was then determined by alpha spectrometry using a 200 mm2 silicon surface barrier detector and corrected for yields using -PO activity; samples were counted for 10~ min. In the sediment horizons attempts were made to measure la7Cs (derived both from weapons testing and from the nuclear power industry) in order to confirm the 210Pbsedimentation rates; 50 g sediment samples were gamma counted using a 3 inch x 3 inch NaI(T1) crystal connected to a multi-channel analyser. This system had a detection limit for l*rCs of 0.3 pCi/g dry sediment. Element analysis Sediment samples were analysed for major and minor elements on a Philips

1220

X-ray

Lead-210 chronology and sedimentation rates

263

fluorescence spectrometer. The determinations were performed upon aliquots of dried sediment ground to pass a 400 #sieve and then pressed into approximately 2 cm diameter tablets. A gold target X-ray tube and a LiF(2oo) analysing crystal were used for the measurement of Pb, Zn, Fe and Ca; copper was determined using a tungsten target X-ray tube and lighter elements, such as Al, Si and Ti, by using a Cr target X-ray tube and a PE analyser crystal.

Results and Discussion The conventional assumptions made in this method of dating and which were accepted for present purposes, are : (a)

srsPb, 210Bi and 2roPo are in secular equilibrium and there is no post-depositional migration of any of these radionuclides; (b) the flux of sroPbto the system, and the sedimentation rate are constant; (c) the 2roPb background is constant throughout the sediment column. If these assumptions are valid, a plot of log, excess slsPb versus corrected depth should be linear thus illustrating the radioactive decay of unsupported 21sPbpresent in the sediments. The corrected depth (T,), allowing for compaction, was calculated by normalizing the porosity of all sediment horizons to the porosity at the sediment water interface (0,). Then

TX =

(1)

i; t,

where x is the number of sediment sections taken and t1 is the thickness of an individual horizon expressed by :

tl=+(l+-&-)

(2)

where ws and p are the weight and density of the dry sediment in that horizon and Y is the internal radius of the core liner. The specific activity (A’) of the excess210Pbin a particular sediment section is expressed by: log, A’=M(D)+log,

A,

(3)

where M is the slope of the line, D the corrected depth of the section and A, the specific activity of the excess sr0Pb at the water sediment interface. The background was taken as the mean of the 210Pbspecific activities of the horizons whose 210Pbcontent showed no linear relationship with depth. The sedimentation rate may be expressed in terms of either : (a) Centimetres per year of ‘wet’ material (R) R=

0.693 Mx 22.26

(4)

or: (b) Grams of dry material per square centimetre per year (W) W=R(I -ao)p

(5)

where the half-life of sr0Pb is 22.26 years and the density (p) of St John’s Lake and Swansea Bay sediment material was found to be 2.61f0.16 and 2*54&0*21 g/cm8 respectively.

R. ‘J. Clifton & E. I. Hamilton

264

The flux (P) of zloPb to the water sediment interface (pCi cm -a year-l) is given by: P=WxA,

(6)

The values for R, Oo, W and P are presented for two areas in Table

I.

TABLE I. Data for sedimentation rate, surface porosity, aloPb flux to sea surface and sediment background for sediment-core samples from Swansea Bay and the Tamar estuary

Station Swansea Bay ra Swansea Bay rb Tamar

eloPb Flux Porosity at at surface (P) (pCi cmsa year-r) m

Sedimentation rate (cm year-r)(R) (g cm-* year-‘) m 0~19*0~02

0.15

0.14fo.02

0'12

1.14&0.08

0.41

0.69 0.66 0.86

0.24

0.19 0.28

*r”Pb Mean background Wig) 0.4 0.3 0'2

The anJPbspecific activity of the cores from St John’s Lake and Swansea Bay show a marked correlation with depth, as illustrated in Figure 4, namely: St John’s Lake, 0.94; Swansea Bay ra, (o-94), and Swansea Bay rb, (0.97). N o such correlation was found for samples from Bridgwater Bay (0.72) and Newport Deep (o’II), indicating that these sediments are either being accreted at a relatively fast rate (> IO cm year-l) or were unstable; this has been l

Newport Deep

0 Swonsea Bay la

l

St. John’s Lake

n

l

Bridgewater

I 20

I

I 60

Swanseo Bay lb

Bay

I

I ID0

I

I

I

I

I

I

20 60 100 Corrected depth (cm)

I

I

I 160

profiles for sediment columns from Figure 4. a1oPo activity (log scale)/depth 2a Newport Deep (A), Bridgwater Bay (+) and St John’s Lake (m): 2b Swansea Bay-ra(U)and rb (H).

Lead-210 chronology and sedimentation rates

265

confirmed by a study of compaction profiles of these stations. The samples from Swansea Bay and St John’s Lake also showed a marked decrease in porosity with depth while the Newport Deep and Bridgwater Bay samples showed little or no evidence of compaction for the top 100-200 cm. The calculated value of P (Table I) for the Swansea Bay and St John’s Lake sites agree with a value of 0.23 pCi 210Pocm -2 year-l obtained by Peirson, Cambray & Spicer (1966) for the total zloPb flux for precipitation and dry fallout at Milford Haven situated at the extreme western end of the Bristol Channel. The geochemistry of atmospheric radon and radon products relevant to dating sediments and similar materials has been discussed by Jawrowski (1966) and Turekian, Nozaki & Benninger (1977). Two sediment core samples were taken at the Swansea Bay station within IOO m of each other. The sedimentation rates derived for these two samples are in reasonable agreement, especially when considering the effect of spatial variation of different types of mineral debris within the area. From the sedimentation rates derived for the Swansea Bay and St John’s Lake stations it was possible to calculate an age for discrete sediment sections. From these data it was also possible to undertake a time-related study of certain elemental concentrations throughout the sediment column. Identification of nuclear fallout horizons, such as 13’Cs and 23gPu,has been used by some workers, for example Wahlgren & Nelson (1973), as a confirmation of the validity of dating obtained by this method. In this study several attempts were made to determine levels of r3’Cs in samples of dry sediment, but no activity was detected above background levels of the counting system. Industrial history and sediment composition Tumar Valley. The Tamar Valley, draining the Dartmoor granite and its mineralized aureole, is rich in copper, lead, arsenic, tin, manganese and zinc minerals. The area has been mined since at least the thirteenth century but in about 1840 there was rapid increase in mining with the opening of the Devon Great Consuls group of copper mines, especially between 1850 and 1880. As the lodes became exhausted or difficult to work because of flooding by water, the mines fell into rapid decline and little or no significant mining activity has occurred in this area since the turn of the century. With the exception of the Royal Naval Dockyard at Plymouth, and some reprocessing of spoil tips in the upper reaches of the Tamar, present-day activity in the Tamar Valley is mainly confined to agriculture. Most of the spoil tips and many of the mine adits remain to this day and runoffs from these workings enter the Tamar at a variety of points along its length, especially in the region of the Devon Great Consuls (Figure 2). At some localities bright-blue coloured waters issue from old adits, illustrating the leaching of copper ores in the flooded mine workings. We have attempted to quantify the mining activity within the Tamar Valley region in terms of tonnage of copper ore mined per year (see Booker, 1971), and this picture superimposed upon the element/age profile of the St John’s Lake sediment core is illustrated in Figure 5. The silicon levels have remained essentially constant except during the period of peak mining activity when they reflect ore production, possibly as a result of ground disturbance in removing overburdens and the initial working of the near surface quartz ore lodes. However, the levels of zinc, lead and copper in the sediments rise sharply approximately 25 years after the onset of peak mining and these levels remain to this day. Zinc, lead and copper enter the Tamar as a result of weathering processes from the spoil tips and seepageof underground water, hence the maintenance of present-day levels of these elements can be accounted for by leaching of mineral debris by acid solutions produced from oxidation of sulphide ores.

266

R. J. Clifton & E. I. Hamilton

The Bristol Channel. The mid-18th century saw the beginning of change away from agriculture in favour of heavy industry at Cardiff, Avonmouth, Bristol and Swansea. In the Swansea region this change accelerated especially rapidly during the 1850s and has been described by Balchin (1971). The manufacture of non-ferrous metals such as lead, zinc, copper and tinplate together with iron and steel, was until recently the major industry of the area, and dependent on locally mined coal.

40300

1950 -7..

1925

t

4

1900 t

1875 -t

1850 JI

F p F 8“35-

t x

E

j

.5 E 8

g

z w30-

200

t E E 8

IOC

2525

20

15 Sample no.

IO

5

Figure 5. Element (Log Scale)/age pro&s for the St John’s Lake sediment core. The production of Cu ore has been superimposed.

A further change in the industrial bias of the area started in 1948 when the output of coal began to decline. Copper has not been smelted in this region since 1929 and the period 1945-1975 was one of rundown of old industries and change to new. The region now produces steel, tinplate, aluminium, nickel and titanium which, together with other industries such as chemicals, oil refining and light engineering, gives the Swansea region a very different industrial balance from that of 30 years ago. The many variables associated with the industrial and urbanization history of the Swansea region make it impossible to describe the area as precisely as the Tamar Valley. It is, however, possible to make certain generalized comparisons of the industrial history of the Swansea region with the time-related record of element concentrations within the sediment column, for example there has been a gradual change in aluminium and silicon levels in deposited sediments during the past N IOOyears ; the silt component of the deposited material has increased in relation to the sand. A plot of log, (element concentration) versus Si/Al ratio down the sediment column revealed the following : (a) A correlation which can be expressed by two lines, one of which incorporates the post-1876 horizons and the other the pre-1858 horizons such as those found for zinc, as illustrated in Figure 6. (b) A correlation which can be expressed by a single line, incorporating all horizons, such as that found for Ca and Ti.

Lead-210 chronology and sedimentation rates

267

It may be deduced that elements falling into category (a) have a component which is significant in relation to the natural levels expected from the Si/Al ratio and which may be technologically derived; those represented by category (b) are present at natural levels and any technologically derived component is considered to be negligible. Zinc, iron, lead, nickel and L

9, $ "0 iIn t x -loo.-6 E f

-

_ -

I 5 Si/Al

I 7 ratio

I 9

Figure 6. Relationships between Zn (log scale) and the Si/Al ratio, for pre-1858 (A) and post-1876 (B) levels.

copper all showed a two-component relationship; to allow for variations in the silt and sand ratios for excess levels of these elements in the sediment column, the equation of the element concentration versus silicon/aluminium relationship (SA) was determined for pre-1858 sediment horizons, namely : Cu=SA(-0*22)+4’52 Fe=SA(-o*2o)+I*86 Zn=SA(-o*z5)+5*6r Ni=SA( -o*o7)+3*54 Pb=SA(o.ss)+o.77 It should be noted that unlike Cu, Fe, Zn and Ni, the correlation between Pb and SA shows a positive slope, the significance of which is being considered further. From these equations the natural, and hence excess level of an element in a particular horizon could be determined. Figure 7 shows the time-related profiles for excess Fe, Zn, Pb and Cu calculated by this method. All of these element profiles show an increase in excess levels starting between 1850 and 1900. The concentration of these elements would appear to peak ~1950 and show some evidence of a gradual decreasein latter years. Conclusions The correlation between mining activity in the Tamar Valley and the concentration of elements in dated horizons indicates that the sediment column of the St John’s Lake station

268

R. J. Clifton &f E. I. Hamilton

holds a historic record of element deposition which can be related to the mining history of the region. The assumptions made for the ar’JPbdating technique must be considered an oversimplification as such phenomena as surficial mixing, redeposition and erosion are all important factors in determining the average sedimentation rates. These factors are especially pertinent in high energy areas where the remobilization of sediments may be effective to a depth of several metres and may result in zloPb profiles similar to those obtained for Newport Deep and Bridgwater Bay (see Figure 4). It is probable, however, that below the zone of resuspension there is an accreting sediment column that may be described by the zloPb or some other dating technique covering a longer time span. The Swansea Bay cores indicate at this site that the resuspension zone is of the order of 2-3 cm and that the sedimentation rate of N o-17 g cm --2 year-r derived from the 210Pbprofile is a reasonable average for the past IOO years. However, it should be noted that the data presented for Swansea Bay, and Newport Deep together with Bridgwater Bay, represent extremes and only relate to the core profiles which have been examined. In all of these areas different parts of the bottom experience sedimentation and resuspension at various times in the past and although areas of continuous sedimentation are rare they can be identified. While the 210Pomethod has been used with considerable success in lake and near shore sediments, data are not available for highly energetic estuaries such as the Bristol Channel/ Severn Estuary system. In such areas a considerable effort is required to identify regions of stable sedimentation and some areas are restricted over very short distances; nevertheless

0 Fe l

Zn

A Pb ‘3 cu

Figure

7. Element

(log x&)/age

profiles

for Swansea

Bay sediment

cores.

Lead-210 chronology and sedimentation rates

269

once they are identified they serve as a reference sequence against which other areas which have experienced complete or partial resuspension can be examined, with the emphasis being placed upon understanding the types of processes which have taken place, when they occurred and the duration over which they took place. By utilizing features of disequilibrium for natural and man produced radionuclides events may be dated over the time span of < I day to greater than IO ooo years. In future work we hope to describe more precisely the processes of resuspension taking place in such areas as Bridgwater Bay by taking longer cores and measuring nuclear fallout horizons such as 23sPuand 13’Cs in conjunction with the 210Pbdating technique; also the use of other natural radionuclide parent-daughter systems. A combination of all these methods has become essential when determining age sequences in the top 10-30 cm of estuarine and near-shore sediments which are poorly understood and which contain sites of intense interactions between physical, chemical and biological processes taking place in such environments.

References Balchin, of Booker, Flynn,

W. J. V. Ed. (1971) British Association Meeting Publ. British Association for the Advancement Science. F. (1971)The Irzdustriul Archaeology of the Tamar Pulley. Publ. David & Charles, Exeter. W. W. (1968) The determination of low levels of polonium-2ro in environmental materials.

Analytica Chimica Actu 43,221. Hamilton, E. I. & Clifton, R. J. (1979) Isotopic abundances of lead in estuarine sediments, Swansea Bay, Bristol Channel. Estuarine and Coastal Marine Science 8,271. Jawrowski, 2. (1966) Temporal and geographical distribution of radium D (lead-210). Nature 212, 886. Koide, M., Soutar, A. & Goldberg, E. D. (1972) Marine geochronology with Pb-210. Earth and

planets. Science Letters 14,442. Krishnaswami,

Lal. D., Martin,

J. M. & Majbeck,

M.

(1971)

Geochronology

of lake sediments.

Earth and Planet. ScienceLetters II, 407. Peirson, D. H., Cambray,

R. S. & Spicer, G. S. (1966) Lead-210 and polonium-2ro

in the atmosphere.

Tellus 18,427. Robbins, J. A. & Edgington, R. R. (1975)Determination of recent sedimentation rates in Lake Michigan using Pb-2ro and Cs-137. Geochimicaet CosmochimicaActn 39,285. Turekian, K. Y., Nozaki, Y. & Benninger, L. K. Geochemistry of atmospheric radon and radon products. In Annual Review of Earth and Planetary Sciences. (Donath, F. A., Stehli, F. G. & Wetherill, G. W. eds.) Vol. 5. Annual Rev. Inc. Palo Alto, Calif. USA. (1977) p. 227. Wahlgren, M. A. & Nelson, D. M. (1973)Residence Time of asDPuand l”Cs in Lake Michigan Water. Radiological and Environmental Res. Div. Annual Report, Ecology, Jan.-Dec. 1973. Argonne National Lab. Argonne III. ANL-8060, Part III, 85.