Lead pollution history from 256 BC to AD 2005 inferred from the Pb isotope ratio (206Pb/207Pb) in a varve record of Lake Korttaj?rvi in Finland

摘要

The lead pollution history, based on the accumulation rate of total Pb and ratio of stable isotopes (~(206)Pb/~(207)Pb), was studied in the annually laminated sediment of a small lake in Finland (62°20′ N; 25°41′ E). The sediment chronology based on varve counting provided a unique opportunity to explore and date signals of Pb emissions, including the ancient metallurgical activities of the Roman Empire at the beginning of the Current Era. Changes in the ratio of stable isotopes gave a pronounced signal of the atmospheric Pb fallout in AD 32-392, although this was not distinguishable in the accumulation rate of total Pb, as it was observed in previous work. Calculated accumulations of the ancient pollution Pb were low, the highest values being 0.2-0.3 mg m2 a-1 in AD 144-392, corresponding 14-21% of the accumulation of total Pb. The accumulation of pollution Pb collapsed in the fifth century and remained at or close to the background level up to the eleventh century. After this, the accumulation rate of pollution Pb began to increase and reached 1.2 mg m~2 a~(-1) in AD 1420-1439, corresponding to 44% of the total Pb accumulation. During five centuries, from AD 1420-1895, the average accumulation of pollution Pb was 2.6 mg m2 a-1, the variation being from 0.8 to 4.8 mg m2 a-1. The accumulation of Pb started to increase exponentially in the early twentieth century, and the highest accumulations of pollution Pb (11-2~2 mg m~2 a~(-1), corresponding 50-76% of the annual accumulation of total Pb) were dated to AD 1926-1985. The banning of the use of leaded fuel has led to a pronounced decrease in the accumulation of pollution Pb since the 1980s, and the present accumulation rate represents the level that prevailed 80-120 years ago.