Sediment-magnetic signature of land-use and drought as recorded in lake sediment from south-central Minnesota, U.S.A.

              Sediment-magnetic signature of land-use and drought as recorded
in lake sediment from south-central Minnesota, USA
Christoph E. Geissa,*, Subir K. Banerjeeb, Phil Camillc, Charles E. Umbanhowar Jr.d
aDepartment of Physics, Trinity College, Hartford, CT 06106, United States
bDepartment of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
cDepartment of Biology, Carleton College, Northfield, MN 55057, United States
dDepartment of Biology, St. Olaf College, Northfield, MN 55057, United States
Received 20 August 2003
Abstract
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 (j), 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.
D 2004 University of Washington. All rights reserved.
Keywords: Sediment magnetism; Holocene; Limnology; Drought; Reductive dissolution; Magnetotactic bacteria
Introduction
In recent years, sediment-magnetic investigations have
played an increasingly important role in reconstructing
paleoenvironmental and paleoclimatic change from con-tinental
and marine records. Multi-proxy studies that include
sediment-magnetic analyses have successfully reconstructed
late Pleistocene and Holocene climate variations (e.g.,
Oldfield et al., 2003; Rosenbaum et al., 1996), have clarified
sediment provenance (Reynolds et al., 1997), or established
the fire history of a watershed (Gedye et al., 2000).
Sediment-magnetic parameters are easily measured and
allow for the rapid construction of high-resolution records of
paleoclimate independent from other paleoclimate proxies,
such as pollen or geochemistry. They are, however, no direct
recorder of climate or environmental change. Interpretation
of magnetic parameters in such terms requires identification
of the underlying processes that influence the input,
distribution, preservation, or neoformation of the minerals
that make up the magnetic component of the sediment.
Research described in this paper helps to improve the
interpretation of sediment-magnetic parameters by compar-ing
them to known climatic or environmental changes.
Geiss et al. (2003) studied several lakes along the prairie-forest
ecotone in Minnesota to determine the processes that
0033-5894/$ - see front matter D 2004 University of Washington. All rights reserved.
doi:10.1016/j.yqres.2004.06.009
* Corresponding author. Department of Physics, Trinity College, 300
Summit Street, Hartford, CT 06106.
E-mail addresses: christoph.geiss@trincoll.edu (C.E. Geiss)8
banerjee@umn.edu (S.K. Banerjee)8 pcamill@carleton.edu (P. Camill)8
ceumb@stolaf.edu (C.E. Umbanhowar).
Quaternary Research 62 (2004) 117– 125
www.elsevier.com/locate/yqres