Transcript Paleoprecipitation Reconstructions for the Brady Soil

Paleoprecipitation
Reconstructions for the Brady
Soil Based on Rock-Magnetic
Analyses
Christoph E. Geiss
Pooja Shakya
Emily Quinton
William C. Johnson
Joseph Mason
0
Ap
A
Carson Cemetery, IA
CAR 03-A
(N41.23984, W095.40592)
ppt: 820 mm/a
50
Bt
Depth (cm)
100
BC
150
200
C
250
8E-7
 (m3 / m3)
0
0.0002
ARM (Am2/ kg)
0.004 0.008
IRM (Am2/ kg)
0.02
0.04
ARM / IRM
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 basics
saturation magnetization (Ms)
magnetization of a sample when exposed to a
(pretty strong) magnetic field
remanent magnetization (Mr)
magnetization of a sample in zero field
various flavors (ARM, IRM etc.)
simple linearized model:
M r = α × Ms
For many natural samples Mr is additive
Mtot = M1 + M2
2-component mixing model
• pedogenic component
(fine-grained maghemite)
• background component
(here coarse-grained rest)
Mr = fped αped Ms + (1- fped ) αbackground Ms
pedogenic contribution to
magnetic remanence
background contribution to
magnetic remanence
fped = relative volume (or mass) fraction of pedogenic
component
ARM Ratios (ARM / IRM)
• Sensitive for fine-grained (SD)
ferrimagnets
• Can write simple model to predict
ARM / IRM ratios
fped αped(ARM) Ms + (1- fped ) αbackground(ARM) Ms
ARM
=
IRM
fped αped(IRM) Ms + (1- fped ) αbackground(IRM) Ms
Note:
Js cancels out and if the a’s are known fped is the only variable left
therefore, can calculate fped from ARM ratios
ARM ratio vs. pedogenic abundance
ARM IRM
0.12
0.10
0.08
observed range
0.06
0.04
0.02
f ped
0.0
0.2
0.4
0.6
0.8
1.0
ARM-ratios of modern enhanced
horizons
20
ARM / IRM x 10-4 (m /A)
15
10
5
r2 = 0.70 (n = 76)
0
400
600
800
mean annual precipitation (mm / a)
1000
0
Wauneta
Devil's Den
Harlan County L.
(North Cove)
ARM / IRM = 5.7 0.3  10-4 m/A
600
ARM / IRM = 5.2 0.2  10-4 m/A
400
ARM / IRM = 3.6 0.2  10-4 m/A
Profile Depth (cm)
200
800
2
3
4
5
6
ARM / IRM (m/A 
7
2
3
4
5
6
ARM / IRM (m/A 
7
2
3
4
5
6
ARM / IRM (m/A 
7
precipitation reconstructions
20
ARM / IRM x 10-4 (m /A)
15
10
Harlan Lake
(550  35 mm/a)
5
Wauneta
(400 50 mm/a)
Devil's Den
(520 40 mm/a)
0
300
400
500
600
700
800
mean annual precipitation (mm / a)
900
1000
Reconstructed Mean Annual Precipitation (mm/a)
comparison with modern precipitation
wetter than
today
600
Harlan Cty. L.
Devil's Den
500
400
Wauneta
drier than
today
300
300
400
500
Mean Annual Precipitation (mm/a)
600
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