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

Christoph E. GeissPooja ShakyaEmily Quinton

William C. JohnsonJoseph Mason

ApA

Bt

BC

C

250

200

150

100

50

0D

epth

(cm

)

8E -7 (m 3 / m 3)

0 0 .0002AR 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 fpedogenic contribution to

magnetic remanencebackground 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 M

ped ped

ped ped

f ff 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

rang

e

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 MIRM

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 (c

m)

2 3 4 5 6 7A R M / IRM (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 600M ean Annua l 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