soil development in absence of water examples so far illustrate general trend of chemical weathering...
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Soil Development in Absence of Water
•Examples so far illustrate general trend of chemical weathering over time
–Chemical weathering
–Element loss
• Rates of this trend will of course vary with:
–1. Bedrock mineralogy
–2. Climate
•What happens as climate (rain) diminishes to zero?
•Atacama Desert in Chile is an ideal area to test this question.
•Purpose of this section will be to:
–1. Examine soil physical and chemical processes as MAP approaches 0
–2. Examine the role of atmospheric inputs to soil chemistry in absence of weathering
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Why is Atacama Desert Dry?
Location
From Valero-Garcés et al. 1999
22°S
24°S
26°S
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Present Climate
0
1
2
3
4
5
6
Feb Apr Jun Aug Oct Dec
antofagasta
Montly Precipitation (mm)
Month
0
1
2
3
4
5
6
Feb Apr Jun Aug Oct Dec
copiapo
Monthly Precipitation (mm)
Month
Dry Site “Wet” Site
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Atacama Desert Well Known for Nitrate: why is it there?
•We will examine cycling of atmospherically derived solutes and their concentration in soils
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Soil Morphology: South to North (dry to drier)
•Parent material = grantic alluvium
•Age = ??? (hundreds of thousands to > 4 million years)
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Soil Profile Excavation Techniques
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Soil Morphology: Wettest Site
A
AB
Bw
Bk
Bkm
Bykm
By
Byk
Etc.
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Soil Morphology: middle site
A
Byk
Bykm1
Bykm2
Bykm3
Bykm4
Bykm 5
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Soil Morphology at Surface of Middle and Dry Sites
C
By
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Formation of Desert Pavement and Gravel Lag
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Soil Morphology: Dry Site
C
By
Byk1
Byk2
Bykm1
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Surface gypsum layers/prisms
gypsum (CaSO4• 2H2O) anhydrite (CaSO4)
Polygonal cracking at surface
Gypsum prisms determine polygons
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Soil Morphology: lower horizons
Bykm2
Bykm3
Bykm4
Bzm
BCzyk1
BCzyk2
BCzyk3
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Where do salts come from and why do they have the pattern observed?
Source: coastal fog (marine salts,aerosols) and playa deflation
Distribution in soils: related to solubility
NaCl =35.7 g/100cc = 6.2 M
NaNO3=81.5= 9.6M
CaSO4 (gypsum)= 0.241= 0.015M
CaCO3 =0.00153=0.000153M
Soil salts with increasing depth (and increasing rainfall with latitude) should be (deepest, most soluble, first):
NaNO3>NaCl>gypsum>carbonate
This is what we observe…………
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Mass Balance View of Soil Chemistry and Physical Changes
•Volumetric expansion increases with increasing aridity
0
20
40
60
80
100
120
140
0 200 400
yungay
altimira
copiapo
Soil Depth (cm)
ε (%)
0
20
40
60
80
100
120
140
0 2000 4000
yungay
altimira
copiapo
Soil Depth (cm)
τ ( )Cl
0
20
40
60
80
100
120
140
0 400 800 1200
yungay
altimira
copiapo
τ ( )S
0
20
40
60
80
100
120
140
0 400 800 1200 1600
yungay
altimira
copiapo
τ (CaCO3
)
•Salt content with depth and latitude consistent with hydrology (but unique for most of Earth)
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Measuring Rate and Composition of Atmospheric Inputs
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Air Chemistry: 2002
Yields ~ 5 g-h-1 (1.5 g-m-3)
Little variation among sites
Salt chemistry strongly suggests marine aerosols
ratio obs SW sea salt aerosol*
Br/Cl 2x10-3 3.5x10-3
Cl/Na 0.7 1.8 0.2-1.8
SO4/Na 0.2 0.2 0.2-4.4
NO3/Na 0.05-0.1 <1x10-5 0.04-0.1
NH4/Na 0.2-0.5 <1x10-5 0.2-1.2
K/Na 0.04 0.02-0.06 0.04
Ca/Na 0.14 0.04 0.01-0.16
*Parungo et al. 1987
Organic carbon = 8-10% (9x105 mol/g)
Nitrate = 150 mol-g-1 (450 mol-g-1 at Yungay)
Ammonium = 1400 mol-g-1 (790 mol-g-1 at Yungay)
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Air Chemistry Summary
•Salt chemistry ratios similar to sea water and marine aerosols
•Air solids are 8-10% organic C !
•Air N is mainly NH4 rather than nitrate
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Nitrate in Soils
Atmospheric NDeposition(Iex; IexRex)
N2 Fixation
(Ifix; IfixRfix)
N Losses toenvironment(Nskex;
15Nskexαex)
Plan t Nuptakefro m soil (Nskp; 15Nskpαp) Plan t Nreturn to
soil(Npks; 15Npks)
SOIL PLANTSSS
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Nitrate in Soils
Yungay
0
50
100
1500 5000 10000 15000 20000
ug NO 3-N/g soil
depth (cm)
Yungay
0
50
100
1500 2 4 6 8
ug NH 4-N/g soil
depth (cm)
Copiapo
0
50
100
1500 50 100
ug NO 3-N/g soil
depth (cm)
Copiapo
0
50
100
1500 2 4 6 8
ug NH 4-N/g soil
depth (cm)
NITRATE AMMONIUMRain
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Soil Biology vs. Rainfall
Life (?) in the Atacama Desert
Courtesy of F. Rainey, M. Vinson, B. Gatz, J. Battista, and C. McKay.
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0
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7
Log
10 cfu/g
S97-3 AT97-3 AT01-03
AT01-16
AT01-19
AT01-23
AT01-22
Sample Sites
1/10 PCA
RM
Figure 2. Cfus/g of soil from 6 Atacama Desert sites and a Sonoran Desert site. Soils were dilution plated on both low nutrient (1/10 PCA) and high nutrient (RM) agar. Counts were determined after 20 days incubation.
Table 1: Total viable counts for heterotrophic bacteria determined on 1/10 PCA and RM agarTable 1: Total viable counts for heterotrophic bacteria determined on 1/10 PCA and RM agar
_____________________Sample Sites______________________________________________Sample Sites_________________________
Media S97-3 AT97-3 AT01-03 AT01-16 AT01-19 AT01-23 AT01-22
_____________________________________________________________________
1/10 PCA 9.6x106 1.3x104 NG* 7.2x103 7.8x103 1.9x105 2.2x106
_____________________________________________________________________
RM 5.5x106 4.5x103 NG 1.8x103 8.3x103 1.2x105 1.6x106
_____________________________________________________________________*NG = no growth on agar plates
Culturable Bacteria Experiments
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Summary of Soil Formation with Decreasing Precipitation
•Chemical weathering of silicate rocks appears to decline to ~ 0
•Soils gradually accumulate exceedingly soluble salts that in most environments are readily lost
•Salt distribution with latitude, and within a given soil, relate to salt solubility
•Driest site has:
–No plants
–Almost no microbiology (virtually sterile soil)
–Almost no loss of incoming atmospheric inputs
–NaCl cemented soil horizons
–Commerical grade nitrate accumulations