Paleoprecipitation Reconstructions for the Brady Soil Based on Rock-Magnetic Analyses

Christoph E. GeissPooja ShakyaEmily Quinton

William C. JohnsonJoseph Mason

Ap

A

Bt

BC

C

250

200

150

100

50

0D

ep

th (

cm)

8E -7 (m 3 / m 3)

0 0.0002A R M (Am 2/ kg)

0.004 0.008IR M (Am 2/ kg)

0.02 0.04AR M / IR M

Carson Cem etery, IACAR 03-A (N 41.23984, W 095.40592)ppt: 820 m m /a

Pedogenic Magnetic Component

• fine grained (d < 0.1 μm) ferrimagnetic minerals (magnetite, maghemite)

• remarkably consistent in it’s magnetic properties throughout the Midwestern U.S.

• under certain conditions we can calculate its abundance in a sample

estimates of pedogenic magnetic minerals• ARM ratios (presented today)

– ARM is highly sensitive for fine magnetic particles

– easily measured

• magnetic coercivity distributions– more time consuming, but yields more

information about the samples

for moderns soils from the Midwestern U.S. both methods yield equivalent results

some magnetism basicssaturation magnetization (Ms)

magnetization of a sample when exposed to a (pretty strong) magnetic field

remanent magnetization (Mr)magnetization of a sample in zero fieldvarious flavors (ARM, IRM etc.)

simple linearized model:Mr = α × Ms

For many natural samples Mr is additiveMtot = M1 + M2

2-component mixing model

• pedogenic component (fine-grained maghemite)

• background component(here coarse-grained rest)

fped = relative volume (or mass) fraction of pedogenic component

r ped s background sM = α M + (1- ) α Mped pedf f

pedogenic contribution to magnetic remanence

background contribution to magnetic remanence

ARM Ratios (ARM / IRM)

• Sensitive for fine-grained (SD) ferrimagnets

• Can write simple model to predict ARM / IRM ratios

ped(ARM) s background(ARM) s

ped(IRM) s background(IRM) s

α M + (1- ) α MARM=

IRM α M + (1- ) α Mped ped

ped ped

f f

f f

Note: Js cancels out and if the ’s are known fped is the only variable lefttherefore, can calculate fped from ARM ratios

obse

rved

ran

ge

0.0 0.2 0.4 0.6 0.8 1.0f ped

0.02

0.04

0.06

0.08

0.10

0.12AR MIR M

AR M ratio vs. pedogen ic abundance

ARM-ratios of modern enhanced horizons

400 600 800 1000m ean annua l precip ita tion (m m / a)

0

5

10

15

20 A

RM

/ IR

M x

10

-4 (

m /

A)

r2 = 0 .70 (n = 76)

800

600

400

200

0

Pro

file

Dep

th (

cm)

2 3 4 5 6 7AR M / IR M (m /A

2 3 4 5 6 7AR M / IR M (m /A

2 3 4 5 6 7AR M / IR M (m /A

D evil's D en H arlan C ounty L .(N orth C ove)

W auneta

AR

M /

IRM

= 5

.7

0.3

1

0-4

m/A

AR

M /

IRM

= 5

.2

0.2

1

0-4

m/A

AR

M /

IRM

= 3

.6

0.2

1

0-4

m/A

precipitation reconstructions

300 400 500 600 700 800 900 1000m ean annua l precip ita tion (m m / a)

0

5

10

15

20 A

RM

/ IR

M x

10

-4 (

m /

A)

W auneta(400 50 m m /a)

D evil's D en(520 40 m m /a)

H arlan Lake(550 35 m m /a)

300 400 500 600

M ean Annual P recip ita tion (m m /a)

300

400

500

600R

econ

stru

cted

Mea

n A

nnua

l Pre

cipi

tatio

n (m

m/a

)

W auneta

D evil's D en

H arlan C ty. L .

drier than today

wetter than today

comparison with modern precipitation

Caveats !

• technique might underestimate ppt– no loss of pedogenic material– magnetic enhancement reaches equilibrium

state rapidly

• magnetic properties of parent material can be distinguished from pedogenic material– inherited pedogenic Fe-oxides / redeposited

soils