The Power of Varved Lake Sediments for High-Resolution Studies of Lead Pollution History

 

M -L. Brännvall, R. Bindler, I. Renberg (Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden), O. Emteryd (Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden)

Corresponding author: Maja-Lena.Brannvall@eg.umu.se

 

Abstract

We have used varved lake sediments from four lakes in northern Sweden for a high-resolution study of atmospheric lead pollution during the last 2000 years. The pollution lead in the sediments is to a large extent transported from continental Europe and the British Isles. After a pollution peak in Greek-Roman times (0 AD) and a cleaner period 400-900 AD, lead deposition increased in the late 10th century and peaked around 1200. Thereafter, lead pollution declined and reached a minimum about 1350, at the time when the plague reduced the European population by 25%. Lead pollution increased again during the mid-15th century and reached a marked peak about 1530. The peaks 1200 and 1530 coincide with two periods of economic development and extensive metal production. The decline in lead pollution after 1530 followed the conquest of the Inca Empire and the increasing import of noble metals from the Americas to Europe, and the subsequent crises in European mining and metal industry. Lead pollution increased substantially after World War II, peaked about 1970, and has since declined and is nearly back to Medieval levels.

 

Introduction

Varved (annually-laminated) lake sediments, which often have records extending back several thousand years from the present, are found in several parts of the world. In a varved lake sediment each individual year can be distinguished as a separate feature, often consisting of seasonal layers (Figure 1). Varved lake sediments have a great potential for high-resolution studies of past pollution. At least theoretically, they could be used to study pollution history year by year from ancient time to the present day. It is important to notice, however, that due to material influx from the catchment and sedimentation processes in the lakes (e.g. sediment focusing), a single core from a varved lake sediment cannot be used to calculate atmospheric flux rates of pollutants in absolute terms. This is only possible in glacier ice and ombrotrophic peat bogs, which receive all their pollutants from the atmosphere (Hong et al., 1994; Shotyk et al., 1998).

This paper presents a two thousand year long high-resolution history of atmospheric lead pollution in northern Sweden, Europe, based on analyses of lead concentrations and stable lead isotopes (206Pb and 207Pb) in varved sediments from four lakes (Brännvall et al. 1999). Temporal changes in the lead pollution records of the sediments are discussed in relationship to the history of mining and metallurgy in Europe.

 

Material and Methods

Sediment cores from the four lakes Kassjön (63°42¢ N, 20°00¢ E), Koltjärn (62°57¢ N, 20°59¢ E), Grånästjärn (64°53¢ N, 20°38¢ E), Norrtjärnssjön (63°55¢ N, 18°31¢ E) were sampled with a Russian peat corer and a freeze corer. ICP-MS was used to analyse lead concentrations and 206Pb and 207Pb isotopes. Uncertainty of the analyses are <10% for concentrations and <0.5% for isotopes. Pollution lead concentrations in the sediments (total lead minus natural lead) were calculated using isotopes and a mixing model. The chronology is based on varve counting. The sediment cores were sub-sampled into contiguous ten and twenty year samples (Kassjön, Koltjärn), and in irregular discontinuous samples (Grånästjärn, Norrtjärnssjön). For details see Brännvall et al. (1999).

 

 

Figure 1 The climate in northern Sweden, with marked seasonal changes, favours the formation of varves, since alternating layers of different material are deposited over the course of a year. The spring layer (May) is characterised by mineral grains (light colour), the summer layer (grey colour) is formed by the biological production (June-September), and the winter layer (thin black) consists of fine-grained organic material deposited while the lake is covered with ice (November-May) (Renberg 1981). Each varve is approximately 0.5 mm thick.

 

Results and Discussion

There is a high degree of similarity between the lead pollution records of the four lakes in northern Sweden (Figure 2), and between these and the history of lead production and use in Europe.

1.    A small peak in lead pollution concentration, mirrored by a trough in the 206Pb/207Pb ratio, appeared about 2000 years ago. This peak coincides with a period of extensive production of lead and silver during the Greek-Roman times (Settle & Patterson, 1980; Nriagu, 1983).

2.    >From about 400 to 900 AD the pollution lead concentration declined in the sediments and the 206Pb/207Pb ratio increased. After the fall of the Roman Empire a period of reduced metal production followed in Europe, although production never fully stopped (Nef, 1987).

3.    The 10th century was the starting-point of a significant and permanent increase in pollution lead concentration that peaked about 1200 AD in the four lakes. In Kassjön the 206Pb/207Pb ratio declined from 1.46 at 900 AD to 1.32 at 1200 AD. In Europe 900 to 1200 AD was a flourishing time with extensive metal production, economic growth, and increasing population (Molenda, 1976; Nef, 1987).

4.    During the 14th century the pollution lead concentration declined to about 50% in the sediments. This decline coincides with an economic depression and the Black Death that killed about 25% of the European population in the 1350s (Gottfried, 1983; Nef, 1987).

5.    A new conspicuous peak appeared around 1530 AD in the sediments of the four lakes, twice as high compared to the pollution peak 1200 AD. New techniques were developed after the mid-15th century, which made it possible to extract silver from silver-rich copper ores, and large amounts of lead were needed in this process (Molenda, 1976). About 60-85% of all lead produced in Europe at this time was used for this purpose (Blanchard, 1976). After 1530 AD the production of silver and lead declined in Europe and especially after 1550 when a large import of precious metals from South America occurred following the conquer of the Inca empire. The metal production continued to be low during the 17th century, partly because of the Thirty Years War (Nef, 1987; Burt, 1995)

6.    A new peak appeared in the 18th century in the sediments. Thereafter, there is a tendency towards greater inconsistency in specific events, probably as a consequence of the influence from more local lead emission sources and increased local disturbance.

7.    Contrary to what is commonly assumed, Koltjärn and Kassjön show no remarkable concentration increase in connection with the Industrial revolution (1800 AD). The real increase did not occur until the mid 20th-century, with increased fossil fuel combustion and extensive use of leaded petrol. The concentration peaked in the 1970s, and the pollution lead concentration has now declined below the 1530s pollution level.

 

 

Figure 2 206Pb/207Pb isotope ratios and pollution lead concentrations from four lakes with varved sediments. In Norrtjärnsjön the time-scale must be considered a floating chronology because of a disturbance in the varve record near the sediment surface, and, therefore here, the results are matched to the other lakes using the lead isotope curves. Redrawn from Brännvall et al., 1999.

 

 

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