Lead pollution can be detected in North America for thousands of years

Humans have contaminated environments across the world with lead (Pb). Clair Patterson was the first to realize that Pb in the 20^th century environment was elevated above natural levels (Patterson, 1965). Lee and Tallis (1973) showed that this modern increase in Pb could be detected in lake sediments. Soon after, researchers compiled historical and archaeological evidence to estimate Pb production rates over the past 5000 years (Settle and Patterson, 1980). These records revealed a distinct rise in Pb production during both the modern and Roman periods, suggesting that Pb pollution could extend into antiquity. Ice cores from Greenland later confirmed that the rise in Pb production during the Greek and Roman periods could be detected (Hong et al., 1994). Since the 1990s, the use of sediment and ice cores to document pollution chronologies has revealed complex regional histories of heavy metal pollution across Europe (Renberg et al., 1994; Cooke and Bindler, 2015; Longman et al., 2018), Asia (Lee et al., 2008; Hillman et al., 2014, 2015), and South America (Abbott and Wolfe, 2003; Cooke and Bindler, 2015; Uglietti et al., 2015). Despite the exceptionally long tradition of metal use in North America, few sediment-based reconstructions of metal pollution extend back more than 500 years. Until this past decade, nothing was known about the sources and geographical patterns of pre-Columbian (i.e., before 1492 CE) Pb pollution in North America.

Pre-Columbian metalworking in the United States never reached the metallurgical complexity of the Old World or South America, yet metal artifacts date back to at least 8500 yr B.P. (Reardon, 2014). Metals used by Native Americans included copper (Martin, 1999), meteoric iron (McCoy et al., 2017), and galena (Walthall, 1981). There is no evidence of pre-Columbian smelting in the United States. Rather, metallic ores were ground or shaped using rock tools and fire. Metalworking in the United States consisted primarily of annealing, the process of repeatedly heating metal in fire, cooling it in water, and working it with rock hammers. Pompeani et al. (2013) were the first to use lake sediments to reconstruct prehistoric copper mining and annealing emissions near prehistoric copper mines on Michigan’s Keweenaw Peninsula. Copper annealing emitted heavy metal impurities, found in the native copper ore at parts-per-million concentration (Kerfoot et al., 2018), the surrounding bedrock (Woodruff et al., 2003; Pompeani et al., 2015), and in wood smoke (Larson and Koenig, 1994). Using sediments from three lakes, Pompeani et al. (2013) suggested that distinct decadal- to century-scale spikes in Pb concentrations found in sediments deposited between 8000–5000 yr B.P. were the result of copper mining and annealing activity. They proposed that during annealing, Pb associated with the native copper was volatized and carried in plumes of heated air and wood smoke particulates that were dispersed into the local airshed by wind (Pompeani et al., 2013). Pompeani et al. (2015) expanded upon earlier work to include an additional sediment core reconstruction and proxies (i.e., copper) adjacent to the largest prehistoric copper mining area on Isle Royale, Lake Superior, Michigan (i.e., Minong Ridge). A sharp rise in the concentration of Pb suggests that mining activity peaked on the island between 6500 and 5400 yr B.P. (Pompeani et al., 2015). Together, lake sediments recovered near prehistoric copper mines in Michigan suggest that Pb pollution can be detected in North America since at least the middle Holocene (Pompeani, 2015).

Lake sediment cores recovered near two pre-Columbian cities in the U.S. Midwest have yielded new evidence for Pb pollution during the past 2500 years (Bird et al., 2019a; Pompeani et al., 2019). Lake sediments recovered from Horseshoe and Avery Lakes, near Cahokia (near East St. Louis, Illinois) and Kincaid (near Brookport, Illinois), respectively, record a distinct increase in sedimentary Pb concentrations when the lake watersheds were populated during the Mississippian Period (1000–1450 CE). Although the exact sources of Pb were poorly understood, the increases in Pb during the Mississippian Period were distinct, and corroborated by archaeological evidence, suggesting an increase in land-use intensity (Fig. 1).

In this issue of Geology, a groundbreaking paper by Bird et al. (2019b) uses Pb stable isotopes and a mixing model at Avery Lake to determine the specific mineral source of the Pb pollution. Increased ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios indicate that galena was likely the primary source of Pb during the Mississippian Period at Kincaid. Although Mississippians did not smelt galena, archaeological evidence suggests that galena ore was crushed into powder and used ritually (Bird et al., 2019b). Therefore, Pb pollution at Kincaid was likely galena powder that was washed or blown into Avery Lake (Bird et al., 2019b). The Pb isotope mixing model approach taken by Bird et al. (2019b) demonstrates that it is possible to estimate both the source and amount of Pb pollution derived from the long, mostly unknown, tradition of metal use in North America. Future research using Pb isotopes and sediments recovered near archaeological sites will likely reveal more information regarding the sources, spatial pattern, and duration of pre-Columbian Pb pollution in North America.