ESM 203: Formation of Soil Resources Biogeochemical Role of the Lithosphere

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ESM 203 Formation of Soil ResourcesBiogeochemica
l Role of the Lithosphere

• Jeff Dozier Tom Dunne
• Fall 2007

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Summary of previous two lectures

• Global tectonic processes generate global
  patterns of rock types and landscape types
  (defined by their form and functioning)
• Lithosphere reacts with atmosphere, hydrosphere,
  and biosphere
• Incorporation of water into subducted sediments,
  tectonic and volcanic breakage and erosion of
  rocks, chemical reactions
• Intensity of interaction varies with the type of
  geologic environment
• Interactions most intense at plate margins

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These interactions

• Sustain the nutrition of the biosphere, mainly
  through release of lithologic elements into
  solution in hydrosphere
• Generate a water-holding soil that sustains
  primary production
• Keep some ecosystems impoverished
• Create some toxic hydrochemical and soil
  environments
• Cause high rates of erosion and sedimentation
• Margins of mountain belts and sedimentary basins
  (downwarps in Earths crust)
• Major floodplains and deltas
• Sedimentation stores rock minerals, carbon, and
  incorporated solutions
• Modulates CO2 content of the atmosphere and ocean
  over the long term (104 yr) by incorporating CO2
  into weathered minerals

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The effects result from a set of biogeochemical
processes called weathering, defined as

• Mechanical disintegration and chemical
  decomposition of rock minerals in situ near
  Earths surface (within 1 km), and
• Short-distance translocation of the dissolved
  substances within the near-surface environment

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General weatheringreaction
Solutes (Na, Ca2 , K, Fe2, Mg2, Zn2, etc.)
Solid residuum Secondary minerals separated
into fragments (clay to gravel), coated with
precipitates of amorphous oxides, storing cations
and trace metals
Primary rock minerals H2O, CO2, HNO3, O2,
organic acids, ? changes in temperature and
pressure
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General weatheringreaction
Solutes (Na, Ca2, K, Fe2, Mg 2, Zn2,
etc. temporary storage in soils
Solid residuum Secondary minerals separated
into fragments (clay to gravel), coated with
precipitates of amorphous oxides, storing cations
and trace metals
Primary rock minerals H2O, CO2, HNO3, O2,
organic acids, ? changes in temperature and
pressure
To groundwater and streams (water quality)
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Mechanical weathering

• Unloading or pressure release
• Crystal growth (salt or ice)
• Create microfractures and joints in rocks that
  allow penetration of water

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Chemical weathering process1Needs water, acids
(CO2, SO2, HNO3, organic acids from plants)

• Solution of carbonates, e.g.
• CaCO3 CO2 H2O Ca(HCO3)2
• Insoluble Soluble
• hard water

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Chemical weathering process 2Needs water,
acids, oxygen

• Oxidation, e.g.
• Especially Fe- and Mn-bearing minerals (turns
  dark minerals reddish or yellowish)
• Catalyzed 1000 times by bacteria

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Chemical weathering process 3Most common
weathering process

• Silicate hydrolysis

Other rock minerals also dissolve in the presence
of acid H to yield a variety of solutes
(nutrients), clay minerals, and relatively
insoluble minerals such as quartz
(SiO2) Incorporation of H ions from soil-water
solution increases pH
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Primary silicate mineral structurePotassium
feldspar (KAlSi3O8)
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Silicate hydrolysis
Other rock minerals also dissolve in the presence
of acid H to yield a variety of solutes
(nutrients), clay minerals, and relatively
insoluble minerals such as quartz (SiO2)
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Chemical weathering 4

• Chelation
• Large, soluble organic molecules such as
  peptides and sugars, produced during plant
  decomposition, form complexes with metals (Cu,
  Zn, Fe, Hg)
• The metals may be very insoluble in water
• The complexes are soluble
• Virtually all biochemicals (including
  manufactured products) exhibit the ability to
  dissolve metal cations
• Involved in bioremediation to remove metals

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Chemical weathering process 5

• Cation exchange (1)
• Clay minerals produced by silicate hydrolysis
  have sheet structures lt0.002 mm across
• The sheets consist of layers of Si, Al, and O
  atoms stacked in various configurations forming
  different clay minerals

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Cation exchange (2)

• The sheets of atoms have negative charges on
  their edges
• These attract and hold the positively charged
  ions released into solution by weathering,
  liming, fertilization, etc.
• The resulting electrical bonds are weak, and the
  cations can be leached and replaced by other
  ions, especially hydrogen (H), and thus become
  available for incorporation into plants via roots
  that exude H

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Cation exchange (3)

• Clay minerals differ in their capacity to store
  plant nutrients in this way, i.e. their cation
  exchange capacity (CEC) - Smectite 100meq/100g
  Illite 30meq /100g
• - Kaolinite 8meq/100g Organic matter in soils
  200meq/100kg
• Cations stored on clays can be displaced by high
  concentrations of other cations.
• - Na if inundated by sea water or evaporated
  irrigation water
• - H from large amounts of recharge H2O ? H
  OH-
• - H from acid rain.

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Summary of weathering of a polymineralic rock
(e.g. granite)

• Granite usually consists of quartz, feldspar,
  mica, and Fe-Mg-rich minerals
• Quartz survives as quartz (sand)
• K feldspar ? clay mineral, dissolved Si, and K
• Na feldspar ? clay mineral, dissolved Si, and K
• K mica ? clay mineral, dissolved Si, and K
• Fe-Mg-Mn minerals ? clay minerals, dissolved Si,
  and Fe2 ? Fe3 (rust-colored precipitate)
• The result is a weathered layer, which if invaded
  and churned by the biosphere becomes a soil ,
  consisting of sand, clay minerals, and solutes,
  some of which are leached out and some are held
  on clay minerals.

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A number of redistribution processes
differentiate the soil into horizons
Ap Decaying plant matter
Ao Mineral horizon with some organic matter
A1 Leached most organic and clay and dissolved
material removed
B Accumulation of clays, oxides, and solutes
leached from upper horizons
C Unconsolidated, earthy, disturbed but little
or no bioturbation
D Parent material with little or no weathering
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Soil characteristics depend on

• Rock mineralogy (minerals weather at various
  rates to various soil minerals)
• Climate (T, P ? weathering rate, leaching
  intensity)
• Vegetation (source of CO2 and other acids)
• Topography (affects drainage and erosion)
• Time (age of soil)

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Global patterns of soil characteristics

• Global tectonics and global climate interact to
  generate regional patterns of these soil-forming
  factors
• There is much local variation superimposed on the
  regional patterns by topography and local
  variations of rock-type, but broad
  generalizations can be made

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Boreal forest/ tundra landscape, N. Canada
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Cool, humid climate with coniferous forest in
Pacific Northwest
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Cool, wet regions

• Cool, wet climate with copious primary
  production, CO2 and organic acids promotes all
  forms of weathering, ? soils with clays and
  oxides.
• Slow decomposition of organic material in cool
  climate allows survival of organic acids
  (chelating agents).
• Intense leaching high (P-E) of dissolved
  products from topsoil, so few nutrients stored.
  Low fertility, acid-rich soils
• Chelating agents leach even the Fe oxides,
  leaving bleached upper horizon.)
• Clays, Fe oxides deposited in subsoil as a dense
  horizon (sometimes impedes drainage)
• Podzol or spodosol

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Podzol or spodosol Organic-rich topsoil, leached
shallow horizon accumulationof clays and iron
in subsoil on well-drained sites
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(Former!) Tropical rainforest, Kenya
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Wet tropicsHigh T, P, primary production with
rapid decomposition to CO2.Intense weathering
and leaching to deep soils with clays and
oxides. Organic matter and even dissolved
organic acids quickly decomposed to CO2, so there
little or no chelation, and iron oxide remains
immobile, coating soil particles red. Few
nutrients stored on clays because of leaching by
high soil water recharge nutrients mainly in the
small amount of organic matter near surface.Low
fertility once the efficient recycling mechanisms
in the roots of primary forest are removed
(source of organic matter)Latosol or oxisol
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Mid-latitude grassland
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Temperate continental grasslandsModerate
rainfall and temperature regime.Organic-rich
surface horizon significant weathering to clays
but not heavily leached.Fertile with good
water-holding characteristicsChernozem or
mollisolGoldilocks!
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Desert landscape in Basin and Range Province
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DesertsWeathering slow (low P and primary
production)Thin soils, usually with low clay
content Significant fertility when watered
because solutes are leached from profile only
slowly. Solutes released by weathering may be
re-deposited during evaporation within the soil
profile as a layer of CaCO3 (caliche), Fe2O3
(iron pan), etc.If irrigation water is
evaporated from the soil without drainage,
concentration of solutes causes salinization of
the soil.Aridisols
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Tundra landscape, Alaska
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Cold regions with impeded drainageWeathering
slow due to low temperatures and water-logging.
Thin profile because of slow weathering and
short life of soil. Organic-rich topsoil due to
slow decomposition.Reducing conditions keeps
iron in ferrous (2) state, coloring soil
blue-to-olive. Acidic and nutrient- poor.
Gley soil or inceptisol
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Global patterns of soil characteristics
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Soil profile depth

• In addition to regional differences in soil
  profile and chemistry, depth is an important
  characteristic
• Soil depth (z) is a major resource within a soil
  region affects crop yields and forest site
  quality
• Depth available for rooting
• Water-holding capacity (?fc - ?wp)z (mm)
• Nutrient holding capacity CEC?bz

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Soil depth is the product of the mass balance of
soil formation and removal over a time period (T)
z
Soil
Weathering (W)
Erosion (E)
Primary rock
W and E are expressed as rates per unit area (kg
m-2yr-1) ?b is soil bulk density (kg m-3)
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Mass balance of soil formation over time
A wave of erosion chases the wave of
decomposition into the bedrock Reflect on a two
extreme cases (1) Amazon basin craton, low
gradients, thick forest vegetation, erosion rate
(E) low high T, P, primary production, rate of
weathering (W). Long time. Result is old, deep
soil, leached of nutrients by high rainfall and
acid production. (2) Utah canyonlands or Sierra
Nevada active tectonics, steep landscape (/-)
thin vegetation, high erosion rate (E)
weathering (W) slow (dry or cool) time short
because soil material doesnt stay on slope very
long, or because recently glaciated. Result is
thin soil not very weathered many minerals in
primary unweathered condition gravelly/sandy
with little clay for CEC).