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

600

500

400

300

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( )zz

surfacez 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 profilemodeled profile after 250 yearsmodeled profile after 500 yearsmodeled profile after 750 yearsmodeled 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 profilemodeled profile after 1000 yearsmodeled profile after 2000 yearsmodeled profile after 3000 yearsmodeled 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

11

10

9

8

7

6

5

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 !