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The high values in the samples from ==2500 years ago are not due to uncontrolled contamination of the corresponding core sections, because the radial concentration profiles for copper (and aluminum) across all but one (6) section show excellent plateaus of concentration, which confirms that transfer of outside contamination to the inner parts of the cores cannot be responsible for the observed high values. The high concentrations cannot be explained by changes in contributions from natural sources. Ancillary measurements of aluminum, sodium, sulfate, and ammonium (10) show (11) that copper from natural sources such as rock and soil dust, sea salt spray, volcanoes, and biomass burning is not responsible for the observed high values. We interpret these concentrations to be the result of early large-scale copper pollution of the atmosphere of the Northern Hemisphere long before the Industrial Revolution.
During ancient and recent times, copper emissions mainly originated from copper mining and smelting activities, with only rather minor contributions from other human activities
such as production of iron and other nonferrous metals, wood combustion, and so on (12)
. As a first step toward obtaining a time series for copper emissions to the atmosphere that can be used to interpret our Greenland ice data, we reconstructed past changes in copper production (Fig. 2) from a comprehensive review (13) of available literature.
Copper was first produced from native copper ==7000 years ago (14). Its production became substantial ==5000 years ago, after the development of techniques for smelting carbonate and oxide ores (15) and the introduction of tin-bronze, which heralded the real Bronze Age. Production then continuously increased after the development of new techniques for smelting sulfide ores ==4500 years ago (14-16). Cumulative production from ==4000 to 2700 years ago was ==500,000 metric tons (17).
The Roman period marked the beginning of a sharp rise in the production of copper in response to the rapidly increasing use of copper alloys for military and civilian purposes, especially in coinage (17, 18). Total production peaked at over ==15,000 metric tons per year ==2000 years ago (Fig. 2). The main production districts were in Spain (which accounted for more than half of worldwide production during Roman times), Cyprus, and central Europe (15, 18). Cumulative production from ==2250 to 1650 years ago was ==5 million metric tons.