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Adjusted Magnetic
susceptibility
(emu Oe x 10 )
Magnetic susceptibility
(emu Oe x 10 )
Quartz Peak Ht.
(Relative units)
1
5
1
5
0
50
100
8 10
12
14 16
0
10
20
Lake Ann
Elk Lake
Elk Lake
Figure 2.3. Quartz peak height and magnetic susceptibility from sediments in Elk
Lake and Lake Ann, Minnesota, USA (from Dean et al ., 1996).
associated with several elements including aluminium, potassium, sodium, tita-
nium and several trace elements (Fig. 2.2). The sodium (Na) can be used as a proxy
foravailable moisture at the time of deposition because the Na is removed from
therecord through the decomposition of plagioclase feldspar during moister
periods. The Na levels remain relatively high during the intervening dry episodes.
Drier episodes, when aeolian clastic material entered the lake, are also marked
by varves with higher concentrations of silt-sized quartz and increased magnetic
susceptibility (Fig. 2.3).
Dean (1997)examined the 10 000 year long varve record of Elk Lake and noted
that the highest concentrations of aluminium (Al) and Na occurred in the mid-
Holocene (Fig. 2.2). Drought episodes then were clearly more severe than any that
have occurred over the last 4000 years. Dean (1997)was also able to examine the
record over the last few centuries at a finer scale because the varve sediments
provide a record of annual resolution. Interestingly, the largest peaks in Na and
Al concentrations during the more recent time period correspond to the Little
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