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