Climate-driven hydrologic transients in lake sediment records: multiproxy record of mid-Holocene drought

Abstract

Oxygen and carbon isotopes, trace element concentrations, and ostracode paleoecology are used with ground water flow modeling to assess the magnitude of climate-driven hydrologic changes during mid-Holocene time for Elk Lake, Grant County, Minnesota. During mid-Holocene time, two extended droughts were recorded in the sediment record from Elk Lake at 5385 and 5221 calendar yr BP. A drop in lake level to 14.9 m below the 1994 level and 10.0 m below the maximum 1930s decline at about 5385 calendar yr BP is interpreted for Elk Lake and its water table, based on Heterocypris fretensis occurrence and higher δ 18O, δ 13C, and Mg/Ca values. Numerical modeling suggests this “low-water” event could have resulted from conditions drier than those of the 1930s drought, sustained over a time frame of approximately a century. The later drought event at approximately 5221 calendar yr BP is marked by a shift in the dominant source of water to Elk Lake from surface run-off to ground water from the till aquifer. The shift in source water is identified by lower δ 18O values and the presence of Limnocythere staplini. Estimates of climatic sensitivity for Holocene lakes should not be directly based on modern conditions, but must take into account the changing shape of the lake basin through time and its effect on the paleohydrology.