Mid-Holocene drought and lake-level change at Elk Lake, Clearwater County, Minnesota: Evidence from CHIRP seismic-reflection data

摘要

Despite many studies of Holocene paleoclimate records from small lakes in the mid-continent of North America, direct estimates of lake-level changes associated with mid-Holocene aridity are rare. Varved sediments from Elk Lake, Clearwater County, Minnesota, are among the best studied in terms of paleoenvironmental proxies, yet the sedimentary architecture of those sediments has not been previously studied and the hydrological responses of the lake - changes in level and volume - are poorly known. High-resolution seismic-reflection (CHIRP) profiles of Elk Lake reveal complex sedimentary basins in the lake, a pattern of nearshore onlap of sediments onto older substrates, and the focusing of sediments into several deep basins. Biogenic gas obscures sediments in the deepest parts of the basins, but beneath the rest of the lake, a three-part Holocene sequence is clear. The transitions between these parts are correlated with lithological changes defined in earlier core studies. Sediments of the modern stage are less focused than those of the prairie lake stage. A prominent erosional unconformity occurs within the Holocene sequence, separating sediments from the prairie and modern stages of the lake. Erosion associated with this unconformity extends to a depth of 18.2 m below the modern water surface, at which point the unconformity grades into a conformable horizon within the sequence in the deep basins of the lake. This transition is an analog of the onlap of modern sediments onto older substrates, which occurs at a depth of about 8.2 m. The configuration of the erosional unconformity and its modern analog indicates a lake-level fall in the mid Holocene of 10.0 m. At its mid-Holocene minimum, the lake was 39% of its present size and 30% of its present volume, providing quantitative evidence to aid in constraining and evaluating reconstructions of mid-Holocene aridity in the mid-continent of North America.