Copper mining on Isle Royale (Michigan, U.S.A.) 6500–5400 years ago

In this post we present selected parts of the very interesting paper titled “Copper mining on Isle Royale 6500–5400 years ago identified using sediment geochemistry from McCargoe Cove, Lake Superior“, by David P Pompeani et al. (2015).

NovoScriptorium: Please see here for Isle Royale

Copper use in the regions surrounding Lake Superior represents one of the oldest examples of metalworking, yet the timing and pattern of past copper mining activity is poorly constrained (Beukens et al., 1992; Martin, 1993; Pompeani et al., 2013). Indigenous peoples mined copper from open bedrock features such as trenches and pits and heated or annealed the copper with wood fires to aid in extraction and processing (Laronge, 2001; Martin, 1999; Schroeder and Ruhl, 1968). High densities of pre-contact (or prehistoric) mines are found along Minong Ridge on Isle Royale (Gillman, 1873; Lane, 1900; Winchell, 1881). Excavations of these pits revealed depths up to 20 m and included tools such as rock hammers (i.e. hammerstones) and deposits of charcoal. Field surveys show that the mine pits are concentrated along a 3-km section of Minong Ridge starting from the western shoreline of McCargoe Cove (Gillman, 1873).”

The Great Lakes

“Previous lead (Pb) pollution reconstructions from lake sediments on the Keweenaw Peninsula document several periods of heightened sediment Pb loadings associated with copper mining from 8000 to 5000 yr BP (Pompeani et al., 2013). The restricted geographic distribution of the Keweenaw Peninsula study area relative to the larger Michigan Copper District region limited the scope of the investigation and raised new questions regarding the timing, intensity, and spatial pattern of the ancient copper mining industry. For example, on the Keweenaw Peninsula, Pb emissions were not detected after 5000 yr BP, even though copper artifacts are dated to this period (Crane and Griffin, 1965; Martin, 1993). A reduction in Pb emissions suggests that an intensive copper industry was absent or had shifted to other regions after 5000 yr BP. To explore the spatial pattern of ancient metalworking, we recovered lake sediments from McCargoe Cove; a long, narrow inlet of Lake Superior located on Isle Royale.

The Portage Lake Volcanic bedrock was targeted for mining because it contains large deposits of native copper ‘fissure veins’ and vesicles containing secondary native copper (Pompeani et al., 2013; Rosemeyer, 2009). Copper mines and related piles of poor rock, tailings, and soil (i.e. overburden) are frequently found around Lake Superior where the Portage Lake Volcanic bedrock is exposed or near the surface. Poor rock contains low copper concentrations and is commonly piled near the mine entrance (or adit). Tailing refers to the remaining source rock and residues associated with the copper ore that remains after processing (i.e. stamp mill, smelting, annealing, etc.). Runoff from indigenous overburden piles and mine pits on Minong Ridge flows directly into McCargoe Cove, providing a unique location in which to measure past sedimentological and geochemical disturbances associated with pre-contact mining. Hereafter, this is referred to as the Old Copper mining period.”

Radiocarbon dating the prehistoric industry

The development of radiocarbon dating established the temporal context of copper use around the Upper Great Lakes. Willard Libby was the first to radiocarbon date charcoal preserved with copper artifacts from Oconto archaeological site. These analyses returned dates of 5600 ± 600 and 7510 ± 600 14C yr BP (or ~6400 and 8400 yr BP) (Libby, 1954; Stuiver et al., 2010). Subsequent radiocarbon measurements of organic matter associated with copper artifacts confirmed the dates obtained by Libby (Binford, 1962; Mason and Mason, 1961). Additional work expanded the number of radiocarbon measurements by dating charcoal and wood preserved in copper pit mines on Isle Royale, extending the potential age range for copper mining to younger periods (Crane and Griffin, 1965).

The development of accelerator mass spectrometry (AMS) radiocarbon dating made it possible to measure smaller masses of organic macrofossils associated with copper artifacts. Copper is a known bactericide and fungicide that helps to preserve organic matter in the case of the Old Copper societies. Beukens et al. (1992) were among the first to pioneer techniques to date organic matter preserved with copper artifacts. The median calibrated AMS radiocarbon age of ~6800 yr BP (5940 ± 90 14C yr BP) from South Fowl Lake is still considered one of the oldest reliable dates for worked copper in North America (Beukens et al., 1992; Martin, 1999; Stuiver et al., 2010). More recently, carbon preserved at the Oconto site, including a piece of string, produced five AMS radiocarbon ages ranging between 8400 and 5600 yr BP (Pleger, 2001). At another prominent Old Copper site in Osceola, Wisconsin, an estimated 500 individuals were buried with copper objects. Two pieces of charcoal preserved in the grave pit at this site had ages of ~4600 and 3700 yr BP (4080 ± 70 and 3450 ± 250 14C yr BP) (Kuehn, 2002; Stoltman, 1997). In general, these and other radiocarbon measurements (Martin, 1993) indicate that copper use in the Upper Great Lakes region probably occurred over a period of several thousand years, beginning by the Early Archaic (~11,500–6700 yr BP) (Pleger and Stoltman, 2009).

Copper artifacts left by people of the Old Copper Complex

“Metal pollution detected at multiple locations on both Isle Royale and the Keweenaw Peninsula demonstrates mining activity occurred across the greater Lake Superior region from 6500 to 5400 yr BP . After 5000 yr BP, lake sediment records from McCargoe Cove on Isle Royale do not contain any significant Pb increases until European settlement, consistent with earlier findings from lakes on the Keweenaw Peninsula (Pompeani et al., 2013). Changing spatial and temporal patterns of pollution could be related to shifts in the location of copper mining activity during the prehistoric mining period. Therefore, identifying and separating human from natural changes will improve interpretations of lake sediment proxies from the region. Notably, the final cessation of Pb emissions at multiple lakes coincide with the onset of sustained dry conditions detected at several sites around the Great Lakes (Kirby et al., 2002; Smith et al., 1997, 2002; Yu et al., 1997), raising the possibility that metalworking activity associated with hunter-gatherer economies in North America was susceptible to influences of climate change.”

Research-Selection for NovoScriptorium: Philaretus Homerides

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