Rock-magnetic Analyses of two Lake Sediment Records Yield a Late-Holocene Climate Record for Eastern South Dakota, U.S.A.

We obtained rock magnetic and geochemical records from Pickerel Lake (45.5114 ° N, 97.2796 ° W) and Enemy Swim Lake (45.4409 ° N, 97.2650 ° W), two lakes in eastern South Dakota. Susceptibility data from Pickerel Lake show cyclical fluctuation which are thought to reflect short term climate change. Similar susceptibility variations, however, are absent in nearby Enemy Swim Lake, which casts doubt on the value of magnetic susceptibility as a climate proxy. At Pickerel Lake magnetic susceptibility and remanence parameters (ARM, IRM) covary and are caused by a dilution of the ferrimagnetic component by diamagnetic material, with reductive dissolution playing a secondary role. At Enemy Swim Lake the susceptibility record is consistently high throughout the top 5 m of sediment and drops off below that depth. Remanence parameters, however, do show cyclical variations similar to the ones observed at Pickerel Lake, and the two lakes can be correlated using variations in ARM. At Enemy Swim Lake variations in magnetic remanence are not due to the dilution of a ferrimagnetic component as they are poorly correlated with the abundance of dia- or paramagnetic magnetic material. Variations in magnetic grain size as expressed by ARM/IRM ratios are more pronounced and regular at Enemy Swim Lake than at Pickerel Lake, where ARM/IRM ratios and hysteresis data show little systematic change throughout the record. The different magnetic responses to a regional climate signal are likely due to the different hydrologic settings of the two lakes. Enemy Swim Lake is a closed basin lake, while Pickerel Lake is a throughflow lake with presumably more stable hydrologic conditions. Under these circumstances dryer climatic conditions are likely reflected in the increased deposition of (windblown?) detrital material, resulting in higher values for concentration dependent magnetic parameters. Variable hydrologic conditions at Enemy Swim may have caused changes in sediment redox conditions leading to varying degrees of reductive dissolution of ferrimagnetic grains and an abundance of ultrafine superparamagnetic (SP) minerals. These changes lead to a susceptibility record controlled by the SP component.