RECONSTRUCTION OF ARCHAIC COPPER MINING AND HOLOCENE ENVIRONMENTAL CONDITIONS ON ISLE ROYALE, MICHIGAN USING LAKE SEDIMENT BIOGEOCHEMISTRY

The Keweenaw Peninsula (KP) and Isle Royale (IR) (Michigan, USA) are host to North America’s largest native copper deposit. During the 19^th and 20^th centuries, thousands of copper mines were discovered throughout the region, and are now thought to be among the earliest examples of metallurgy in the world. Despite its potential significance, there is a dearth of studies investigating the temporal and spatial patterns of prehistoric copper mining in the region. In 2016 CE, sediment cores were recovered from a small pond (Pond 2) near prehistoric copper mines on the southwest side of IR and analyzed to investigate the timing of ancient mining emissions and to reconstruct past environmental conditions. Weakly sorbed trace metals including Pb, K, and Ti were analyzed in order to reconstruct past mining. Anomalously high Pb concentrations indicate that both historical and prehistoric mining emissions are detectable, and based on the field site’s location, emissions were likely delivered to the pond via atmospheric fallout. Our age model, constructed via Baysesian statistics (Bchron) with radiocarbon control points, indicates that the largest prehistoric increase in Pb concentrations occurs in sediment deposited approximately 6000 calendar years before present (cal BP). Accounting for age model uncertainties, peak Pb concentrations in Pond 2 correlate with lake records throughout the KP and IR. The use of biogeochemical proxies, including C:N ratios, δ¹³C of organic carbon, and lignin-phenol biomarkers provide information on paleoclimate conditions coincident with purported mining emissions during the middle Holocene. Between 7500 and 2500 cal BP C:N ratios decreased, potentially indicating a larger contribution of autochthonous organic matter to the sediment record. This same period shows an enrichment of δ¹³C suggesting a transition from depleted C3 terrigenous woody material to more enriched C3 algal sources. Both trends imply a transition from lower lake level (warmer, drier conditions) to higher lake level (cooler, wetter conditions). Lignin-phenol proxies further elucidates paleoenvironmental conditions by reconstructing trends in past catchment vegetation. This study represents one of the first contemporary biomarker-based records of paleoenvironmental change on Isle Royale.