Geochemical evidence for atmospheric pollution derived from prehistoric copper mining at Copa Hill, Cwmystwyth, mid-Wales, U.K.

Mighall, T.M.,¹ Abrahams, P.W.² Grattan, J.,² Hayes, D.² Timberlake, S.³. & Forsyth, S.¹

1. Centre for Quaternary Science, Geography, School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry, CV1 5FB, U.K. (corresponding author: t.mighall@cov.ac.uk)

2. Institute of Geography and Earth Science, University of Wales, Aberystwyth, Wales, SY23 3DB, U.K.

3. 98 Victoria Road, Cambridge, CB4, U.K.

ABSTRACT

This paper presents geochemical data from a blanket peat located close to a Bronze Age copper mine located on the northern slopes of the Ystwyth valley, Ceredigion, mid-Wales, U.K. The research objectives were twofold; firstly to test the comparability of different geochemical analytical techniques, and secondly to explore the possibility that the peats contained a geochemical record of the pollution generated by the mining activity. Four peat monoliths were extracted from the blanket peat to reconstruct the pollution history of the prehistoric mine. Three different geochemical measurement techniques were employed and 4 copper profiles have been reconstructed, one of which is radiocarbon-dated. Three of these profiles, located close to the mine, recorded copper enrichment of the peat during the known period of exploitation of the mine. In contrast copper did not enrich the fourth monolith, extracted approximately 1.5 km north of the mine. Whilst other possible explanations to explain the copper concentrations are discussed, it is argued that the high copper concentrations represent the first evidence of localised atmospheric pollution caused by Bronze Age copper mining in the British Isles.

INTRODUCTION

In northern and western Europe Roman and subsequent mining and smelting activity has been detected in the records of geochemical deposition in ombotrophic peat bogs. Less attention, however, has been given to European prehistoric mining and smelting despite the fact that ‘the detection of early mineral exploitation in sediments of this kind is an important step in understanding the extent and nature of mining and smelting in the past’ (West et al., 1997:343). In Britain, prehistoric metal mining sites have been difficult to identify and, until recently, there was a widespread belief that evidence of early mining was destroyed by subsequent activities. This view has been recently overturned by the discovery of several prehistoric copper mines in the British Isles including Copa Hill Cwmystwyth in west Wales (between 3500±50 and 3070±50 BP); Great Orme’s Head, Llandudno, north Wales (2940±80 BP); Mount Gabriel, Co. Cork, Ireland (3450±120 BP. and 3130±80 BP) (Timberlake, 1990a,b; Timberlake & Switsur 1988; Dutton and Fasham, 1994; O’Brien, 1994).

This paper aims to identify evidence for prehistoric copper mining pollution at Copa Hill, Cwmystwyth, mid-Wales, by (i) replicating copper profiles at four sites located within 600 metres of a prehistoric mine, using a variety of analytical methods and (ii) comparing these proximal profiles with a relatively distal profile located 1.5 km from the mine.

The prehistoric mining site of Copa Hill is situated 30 km ENE of Aberystwyth, mid-Wales (SN811751) on a copper/lead mineral vein on the northern slopes of the Ystwyth valley at approximately 420 m OD. Charcoal, recovered from the mineworking debris, has been dated to a period within the early to mid Bronze Age (Timberlake, 1990a,b, 1995). Metal mining for probably resumed in the Ystwyth valley, during Roman times when lead was the focus of activity and from the 16th century until the early 20th century for lead and zinc (Hughes, 1981). Thus, this location provides an ideal site to investigate pollution histories for mining activities spanning three millennia.

METHODS

An extensive area of blanket peat occupies the northern plateau of the Ystwyth valley. Four peat monoliths were extracted from freshly exposed sections. In order to detect any atmospheric pollution and to attempt to replicate the data four monoliths CH1, CH2 and CH4 were extracted 600 metres to the north of the mine, and are within 30 metres of each other. A fourth monolith CH3 was recovered from a freshly exposed section approximately 1.5 km NE from the prehistoric mine to act as a spatial control.

Three methods were employed to reconstruct metal concentrations. These were, a) Inductively Coupled Plasma Mass Spectrometry using a partial acid digestion of the peat, b) Atomic Absorption Spectrophotometry using a Kjeldahl total digestion method to produce the aliquot c) Absorption Spectrophotometry using a microwave digestion method to produce the aliquot.

RESULTS and DISCUSSION

In all three proximal profiles copper distribution within the monolith followed a similar pattern. Copper is clearly concentrated towards the base of each core reaching 110 µg g-1 and are higher than a local mean background figure for local peats which may be extrapolated from CH3 of ~15 µg g-1. In complete contrast to the previous three sets of results copper concentrations at site CH3 show little stability in this monolith. Values range between 0 and 38 µg g-1. Copper in this monolith appears to be distributed entirely randomly. It is clear that each of the three proximal monoliths CH1, CH2 and CH4 record a similar pattern of copper deposition. Each of the extraction techniques and analytical methods used have produced broadly similar results. However, it is clear that the partial acid digest method used on CH1 dramatically reduced the concentrations detected, though not the pattern of distribution, which limits its comparative value as an extraction method.

Although the exact timespan of prehistoric mining cannot be determined, a basal radiocarbon date of the CH2 peat profile, from 134 to 133 cm depth, places peat initiation at 3470 + 35 years BP during the early Bronze Age. A second radiocarbon date of 2395 + 35 years BP was derived from peat between 105 and 106 cm. It is clear that the copper enrichment evident between 124–138 cm depth in monolith CH2 corresponds with the established dates of Bronze Age mining at Copa Hill 600 metres to the south. The different lengths of all three proximal monoliths suggest that since initiation peat accumulation has occurred at a slightly differing rate. However, the clear similarity of the geochemical profiles and their proximity to each other suggest that it is reasonable to propose that the copper enrichment indicated in all three monoliths is the result of the same depositional processes occurring at approximately the same time.

The replication of elevated copper concentrations three times using three different extraction and analytical methods suggests that the concentration of copper towards the base of each the proximal monoliths is unlikely to be an artefact of the chemical method. In fact, the pattern for copper throughout the peat profiles is fairly consistent although the values do vary slightly. The basal peak in copper concentrations described at sites CH1, CH2 and CH4 is not evident at site CH3. One explanation for the absence of high copper concentrations is that the site lies outside the atmospheric dispersion halo of prehistoric mining activity. It is likely that the pattern of copper deposition recorded in monoliths CH1, CH2 and CH4 reflects prehistoric copper mining activity ca. 3000 years ago in the remote uplands of central Wales. Having established the existence of this record it is probable that other geochemical records of ancient mining activity may be detected in upland blanket peats.

References

Dutton, A. and Fasham, P.J. 1994: Prehistoric copper mining on the Great Orme, Llandudno, Gwynedd. Proceedings of the Prehistoric Society 60: 245-286.

Hughes, S. 1981: The Cwmystwyth Mines. British Mining No17. Northern Mines Research Society.

O’Brien, W.F. 1994: Mount Gabriel: Bronze Age mining in Ireland. Galway: Galway University Press.

Timberlake, S. 1990a: Excavations at an early mining site on Copa Hill, Cwmystwyth, Dyfed 1989 and 1990. Archaeology in Wales 30: 7-13.

Timberlake, S. 1990b: Excavations and fieldwork on Copa Hill, Cwmystwyth, 1989. In Crew, P. and Crew, S., editors, Early mining in the British Isles. Plas Tan y Bwlch Occasional Paper No1, 22-29.

Timberlake, S. & Switsur, R. 1988: An archaeological investigation of early mine workings on Copa Hill: new evidence for prehistoric mining. Proceedings of the Prehistoric Society 54: 329-333.

West, S., Charman, D.J., Grattan, J.P. and Cherburkin, A.K. 1997: Heavy metals in Holocene peats from south west England: detecting mining impacts and atmospheric pollution. Water, Air, and Soil Pollution 100: 343-353.