AN 11,000 YEAR FIRE HISTORY OF THE KEWEENAW PENINSULA FROM COPPER FALLS LAKE

An 11,000 year record of forest fires is reconstructed at Copper Falls Lake using macroscopic fossil charcoal (>125 μm) preserved in lake sediment. Copper Falls Lake (47.418° N, 88.192° W; 392 m above sea level) is situated along the elevated copper-bearing ridge in the interior of Keweenaw Peninsula, on the south shore of Lake Superior. Charcoal in lake sediment records have been shown both empirically and through models to reflect the local (0–1.0 km) occurrence of individual or multiple high-severity forest fires (“fire events”). Charcoal was counted from continuous 1 cm³ samples, and dated using ²¹⁰Pb, ¹³⁷Cs, and nine AMS ¹⁴C ages to document the temporal patterns of fire occurrence over the interglacial (~1,000-12,000 BP). A charcoal accumulation rate (CHAR, counts cm^-2 year^-1) was calculated and linearly interpolated to 10-year time steps. CHAR_Back levels were estimated using a 500-year moving average to quantify background “red-noise” in the series to estimate residual CHAR “peaks.” Current research seeks to develop a statistical threshold to identify significant fire-related CHAR_Peaks from CHAR_Back to distinguish local fire events. CHAR_Back could be influenced by a wide range of factors including: vegetation shifts, summer temperature, and effective moisture.

Following deglaciation ~12,100 (cal ¹⁴C year) BP and transition to interglacial conditions, Copper Falls Lake records an abrupt rise in CHAR_Back. Distinct CHAR_Peaks occur at ~11,600, ~9,900, and ~9,600 BP as a result of local fire events during the early interglacial. A pollen reconstruction from nearby Mud Lake (47.128° N, 88.320° W; 189 m asl) find a ~300 year rise in CHAR_Back at ~8,600 BP coincides with the rapid migration of white pine (Pinus Strobus) into the region (Booth et al. 2002). By ~7,000 BP, CHAR_Back levels decline to levels typical of the late Holocene, possibly influenced by lake-effect snow development in the region. At ~5,600 BP, a ~200 year rise in CHAR_Back and distinct CHAR_Peaks correspond to a rise in birch (Betula) and hemlock (Tsuga) across the region. The Copper Falls charcoal record highlights the role of vegetation change on the development of fires.

Booth RK, Jackson ST, Thompson TA (2002). Paleoecology of a Northern Michigan lake and relationship among climate, vegetation, and Great Lakes water levels. Quaternary Research. 57:120-130.