Geology: A 120 yr record of widespread contamination from mining of the Iberian pyrite belt

A metal-enriched seawater plume entering the western Mediterranean Sea through the Strait of Gibraltar originates 300 km to the west in the Rio Tinto estuary of southwestern Spain. Mining of Rio Tinto ore, one of the largest metal-rich sulfide deposits in the world, started well before Roman times. Contemporary Rio Tinto waters draining the region are highly acidic (pH 2.5) with dissolved cadmium, zinc, and copper concentrations 105–106 times higher than in uncontaminated surface water of the Gulf of Cadiz. Two dated sediment cores from the Spanish continental shelf show that metal inputs to the region increased with the onset of intensive mining activities during the second half of the 19th century. Although the impact of mining may have decreased over the past few decades, the Tinto river and estuary remain highly contaminated. Figure 1.Map of Gulf of Cadiz, Strait of Gibraltar,and Alboran Sea, showing distribution of dissolved Zn in surface waters. Symbols correspond to Zn concentration ranges shown in inset. Most samples from Gulf of Cadiz east of mouth of Guadiana River and all samples from Strait of Gibraltar and Alboran Sea were collected on board USNS Lynch in March–April 1986 (van Geen and Boyle, 1990; van Geen et al., 1991). Samples off coast of Portugal were collected on board RV Noruega in October 1988 (van Geen et al., 1990). Samples from Rio Tinto and its estuary were collected from a small boat and from shore in December 1992. Spanish shelf waters are entrained into Mediterranean inflow and also into northward-flowing coastal current (Frouin et al., 1990). Elevated concentrations of Zn 20–100km west of Atlantic Moroccan coast are advective feature of fresher water from metal-enriched Spanish shelf water (van Geen et al.,1991).Crosses on Gulf of Cadiz shelf show locations of sediment cores TG25b and TG22. Isobaths are plotted at 100 and 500 m depth.Three historically most important mining sites within Iberian pyrite belt:RT—Rio Tinto,Th—Tharsis,LZ—La Zarza. on 17 July 2009 geology.gsapubs.org Downloaded from