COMPARING MAGNETIC SUSCEPTIBILITY MEASUREMENT TECHNIQUES FROM RECENT LAKE SEDIMENTS

Magnetic susceptibility (MS) is commonly measured on lake sediments and is used as a proxy for clastic input and soil erosion, for correlating among cores in the same lake basin, and for aligning successive overlapping drives of the same core. There are several common techniques for measuring MS, each with its own advantages. Here we review three such techniques measured on a sediment core from Lake Pepin, a natural impoundment of the upper Mississippi River: 1) loop-sensor MS logging on wet sediment of the intact core; 2) point-sensor MS logging on wet sediment of a split (lengthwise) core; and 3) discrete MS measurements of dried sub-samples using a susceptibility bridge. To obtain further information about the origin of down-core MS variability, additional magnetic measurements were performed on the discrete samples to model ferrimagnetic sedimentary components. Overall trends and individual features in the MS curves agree reasonably well between MS techniques; however, the amplitude of local minima and maxima varies according to the technique used. All three MS techniques captured distinct events ca. 1900 and 1940, attributed to increases in allochthonous ferrimagnetic components. A notable increase in interacting single domain particles in the late 1970s was captured by the point sensor log and susceptibility bridge, but was not distinct in the loop sensor MS curve. This highlights possible discrepancies between MS techniques on lake sediment cores. The ferromagnetic particle flux has declined over the past half century despite increasing sediment accumulation in Lake Pepin. This may suggest a possible shift in sediment sources from agricultural fields to stream banks, or a depletion of the primary supply of ferrimagnetic particles.