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Rates of Magnetic Enhancement in Loessic Soils Estimated From Profiles of Rapidly Eroding Soils
C. E. GeissL. D. UrbanoC. Munroe
0 0.02 0.04ARM / IRM
0 0.1 0.2 0.3Mped / Mtotal
(from coercivity analyses)
0 0.2 0.4ARM(A/m)
0 0.002Volume Susceptibility
(m3/m3)
1600
1500
1400
1300
1200
1100
1000
900
800
700
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400
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200
100
0Depth(cm)
Fox Hill loess core, Illinoissamples courtesy of H.Wang, ISGS(Wang et al., Geology 31, 179 - 183)
incipient Pleistocenepaleosols
Rates of Magnetic Enhancement
Two “end members”• B. Maher – magnetic properties
develop over short time periods (especially in loess)
• M. Singer – California chronosequences show that magnetic enhancement might progress over very long time periods
And many others in between or even farther out…
2009
Simple 1D Magnetic Enhancement ModelRate of Magnetic
Susceptibility Enhancement
SoilProfileDepth
z0
soil surface
Rate of magnetic susceptibilityenhancement decays exponentiallywith depth:
(Anderson and Hallet, Quat.Res. 41, 1996)
Model allows for lowering of soil surfaceBy specifying an erosion rate
0( )z
zsurfacez e
Is exponential decay appropriate ?
0 2E-007 4E-007 6E-007
(m3/kg)
Mount Calvary Cemetery, IA(and many others in the region)
Deep , magnetically enhanced A-horizon
Exponential decay model:accumulation of dust leads to thick A-horizons(e.g., Mason and Jacobs, Geology, 26, 1998)
Rate of MagneticSusceptibility Enhancement
SoilProfileDepth
zone ofbiologicalactivity
Map courtesy ofIsabel Iwachiw
T7
C (m3/kg)
4E-007 6E-007 8E-007magnetic susceptibility
(SI)
120
80
40
0Depth(m)Badger Ridge profile T7
measuredsusceptibility profile
modeled profile after 250 years
modeled profile after 500 years
modeled profile after 750 years
modeled profile after 1000 years
model parametersdt : 5 time step
Pdot_0 : 1.50E-09 mag. Enhancement rate at surfacez_star : 0.2 characteristic pedogenic depth
ε : 0.001 erosion (negative) / deposition (positive)rate chi_o : 5.00E-07 m3/kg initial susceptibility of loess
tmax = 1000
yearSI/yearmm/year
4E-007 8E-007 1.2E-006magnetic susceptibility
(m3/kg)
200
150
100
50
0Depth(m)
Mt. Calvary Cemeterymeasuredsusceptibility profile
modeled profile after 1000 years
modeled profile after 2000 years
modeled profile after 3000 years
modeled profile after 4000 years
model parametersdt : 20 time step
Pdot_0 : 7.00E-10 mag. Enhancement rate at surfacez_star : 0.1 characteristic pedogenic depth
ε : 0.00012 erosion (negative) / deposition (positive)rate chi_o : 4.00E-07 m3/kg initial susceptibility of loess
tmax = 4000
yearSI/yearmm/year
PK0
PK1
PK2
PK2
PK3
PK3
0 2E-007 4E-007 6E-007Mag. Susceptibility(m3/kg)
13
12
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10
9
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4
3
2
1
0
Depth(m)
Dolni Vestonice Brickyard(Oches and Banerjee, Stud. Geophys. Geod., 40, 1996)
Eemian (stage 5e)parabraunerde
Additional Limitationsmineral transformations
mt only intermediate stage?(e.g., Oches, 1996, Barron and Torrent, 2003 etc.)
equilibrium with climatein stable soils enhancement never ceases, does not reach stable equilibrium
Thank you !
