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
Transcript
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
Why is Atacama Desert Dry?
Location
From Valero-Garcés et al. 1999
22°S
24°S
26°S
Present Climate
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Feb Apr Jun Aug Oct Dec
antofagasta
Montly Precipitation (mm)
Month
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copiapo
Monthly Precipitation (mm)
Month
Dry Site “Wet” Site
Atacama Desert Well Known for Nitrate: why is it there?
•We will examine cycling of atmospherically derived solutes and their concentration in soils
Soil Morphology: South to North (dry to drier)
•Parent material = grantic alluvium
•Age = ??? (hundreds of thousands to > 4 million years)
Soil Profile Excavation Techniques
Soil Morphology: Wettest Site
A
AB
Bw
Bk
Bkm
Bykm
By
Byk
Etc.
Soil Morphology: middle site
A
Byk
Bykm1
Bykm2
Bykm3
Bykm4
Bykm 5
Soil Morphology at Surface of Middle and Dry Sites
C
By
Formation of Desert Pavement and Gravel Lag
Soil Morphology: Dry Site
C
By
Byk1
Byk2
Bykm1
Surface gypsum layers/prisms
gypsum (CaSO4• 2H2O) anhydrite (CaSO4)
Polygonal cracking at surface
Gypsum prisms determine polygons
Soil Morphology: lower horizons
Bykm2
Bykm3
Bykm4
Bzm
BCzyk1
BCzyk2
BCzyk3
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…………
Mass Balance View of Soil Chemistry and Physical Changes
•Volumetric expansion increases with increasing aridity
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yungay
altimira
copiapo
Soil Depth (cm)
ε (%)
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yungay
altimira
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Soil Depth (cm)
τ ( )Cl
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yungay
altimira
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τ ( )S
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altimira
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τ (CaCO3
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•Salt content with depth and latitude consistent with hydrology (but unique for most of Earth)
Measuring Rate and Composition of Atmospheric Inputs
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)
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
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
Nitrate in Soils
Yungay
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1500 5000 10000 15000 20000
ug NO 3-N/g soil
depth (cm)
Yungay
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ug NH 4-N/g soil
depth (cm)
Copiapo
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1500 50 100
ug NO 3-N/g soil
depth (cm)
Copiapo
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ug NH 4-N/g soil
depth (cm)
NITRATE AMMONIUMRain
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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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
