Paleoprecipitation Reconstructions for the Brady Soil Based on Rock-Magnetic Analyses Christoph E....
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Transcript of Paleoprecipitation Reconstructions for the Brady Soil Based on Rock-Magnetic Analyses Christoph E....
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