Sediment magnetic properties of a short core from Sharkey Lake, MN, record the effects of Euroamerican settlement and climate change over the last 150 yr. The onset of European-style farming led to increased erosion, reflected in high values of concentration-dependent parameters such as magnetic susceptibility (k), Isothermal Remanent Magnetization (IRM), and Anhysteretic Remanent Magnetization (ARM). These high values are only partially due to increased supply of terrigenous material to the lake, and recent sediment contains an additional component of authigenic fine (single-domain) magnetite, most likely magnetosomes from magnetotactic bacteria. High organic productivity in the lake during the 1920s to 1940s drought increased this authigenic component resulting in highly magnetic fine-grained sediment. A comparison with older Holocene sediment from the same lake shows that, over time, most of the fine magnetic signal is lost after deposition, leading to decreases in magnetization and a bimodal grain size distribution of ultrafine, superparamagnetic grains and coarser multidomain particles, evident from measurements of ARM/IRM ratios, hysteresis measurements, and low-temperature analyses. The effects of dissolution and the superposition of climate and land-use signals complicate the use of recent sediments as modern analogs for sediment magnetic analyses.
Bibliographical noteFunding Information:
We thank Margaret Sharkey for generous access to the site. The magnetic analyses were performed at the Institute for Rock Magnetism, which is funded by the W.M. Keck Foundation, the NSF Earth Sciences Division's Instrumentation and Facilities Program, and the University of Minnesota. Part of this study was funded by NSF/ATM grant 9909523 and grants by the Howard Hughes Medical Institute to Carleton and St. Olaf Colleges. L. Dvorak, J. Aldinger, and K. Walkup performed analyses of biogenic silica. The comments of S. Colman and an anonymous reviewer helped to greatly improve the manuscript. We thank Greg Spoden, MN State Climatology Office, for providing us with the unpublished climate data for Ft. Snelling, MN.
Copyright 2011 Elsevier B.V., All rights reserved.
- Magnetotactic bacteria
- Reductive dissolution
- Sediment magnetism