EXAMINING ENVIRONMENTAL IMPACT FROM MINING USING MULTIPLE PROXIES ON A SEDIMENT CORE FROM CLEAR LAKE, COLORADO, USA

A sediment core from Clear Lake, an alpine lake in southwestern Colorado, records paleoenvironmental changes over the last 6000 cal. yr BP. Resource extraction from mining was historically important in the area, but the effect on lake productivity and chemistry has not been assessed. To examine if, when, and to what extent ore mining impacted Clear Lake, sediment was analyzed for magnetic susceptibility (MS), grain size, biogenic silica (Bsi), x-ray fluorescence (XRF), and total organic content (TOC). An age-depth model using radiocarbon and ²¹⁰Pb dating was constructed. Large and abrupt increases in metals such as iron, nickel, and silver are measured ca. 1870 and are interpreted to indicate the onset of mining in the catchment, which is consistent with historical records. The abundance of metals measured in the sediment decreases slightly to the present but remains elevated compared to the pre-mining background. Although mining activities have ceased, the elevated abundance of metals, despite a decreasing trend, suggests that runoff will continue to bring metals into the lake for the foreseeable future.