Download Rocks and Soil Outline: • Introduction • Rock Weathering o

Rocks and Soil
Outline:
• Introduction
• Rock Weathering
o Mechanical
o Chemical
• Secondary minerals
• Phosphorus minerals
• Soils chemical reactions
o Cation exchange capacity
o Soil buffering
o Anion exchange capacity
• Soil development
• Weathering rates
Please note that the majority of the text below is taken from Chapter 4 in:
Schlesinger, W.H. 1997. Biogeochemistry: an analysis of global change. 2 nd edition. Academic Press,
California.
Introduction
Since early geological time, the atmosphere has interacted with the exposed Earth’s crust
causing rock weathering
• Early on, volcanic gases dissolved in water to form acids that reacted with mineral
surfaces
• Later as O 2 accumulated in atmosphere, exposed rocks weathered due to oxidation
of reduced minerals (e.g., pyrite)
• Since arrival of land plants, rock mineral also exposed to high concentrations of CO 2
in soils due to respiration of soil microbes and plant roots
o Today carbonic acid (H2CO 3) derived from CO 2 reacting with soil water, is
major determinant of rock weathering in most ecosystems
• Most recently, acid rain (derived from human inputs of NO x and SO2 to the
atmosphere) has greatly accelerated rock weathering in some region.
Basic equation (proposed by Siever) summarizing close link between earth atmosphere and
crust:
Igneous rocks + acid volatiles = sedimentary rocks + salty oceans
Products of reaction are carried to oceans where they accumulate as dissolved salts or ocean
sediments
• Sedimentary rock has accumulated through geological time
• ~75% of exposed rocks at Earth’s surface are now sedimentary rocks uplifted by
tectonic action
•
•
These exposed sedimentary rocks are subject to further weathering reactions with
acid volatiles
Eventually geological processes carry sedimentary rock to deep earth, where CO 2 is
released and solid constituents are converted back to primary minerals under heat
and pressure
Rock weathering important to the bioavailability of elements with no gaseous forms
Table 4.1
Thus weathering basic to:
• soil fertility
• biological diversity
• agricultural productivity
Reactions between soil waters and minerals determines
• availability of essential elements to biota
• losses of elements in runoff
Rock Weathering
Weathering general term that encompasses variety of processes that decompose rock:
• Mechanical weathering
• Chemical weathering
Mechanical weathering
Definition: fragmentation or loss of materials without chemical reactions
Important in:
• extreme and highly seasonal climates
• areas with much exposed rock
Examples:
• Wind erosion: important in arid environments
• Expansion of frozen water in rock crevices (causing fracturing): important in cold climates
• Plant roots: also fragment rocks when growing in crevices
When products of mechanical weathering not removed rapidly by erosion, thick soils
develop, and landscape is said to be transport limited
• In other areas, finely divided rock removed from ecosystem by erosion
• Products of mechanical weathering may also be lost in catastrophic events like
landslides
•
Rivers draining mountain regions often carry huge sediment loads due to high rates
of mechanical weathering and erosion at elevation
Chemical weathering
Definition: loss of minerals from rocks and soils due to reactions with acidic and oxidizing
substances
• Usually involves water, followed by release of dissolved ions available for uptake by
biota or loss in runoff
• Mechanical weathering usually important in exposing rock minerals to chemical
attack
Rates of chemical weathering depend on mineral composition of rocks:
• Igneous and metamorphic rocks contain primary silicate minerals formed under
conditions of high temperature and pressure deep in earth
• Two classes, depending on crystal structure an presence of Mg versus Al in crystal
lattice:
o Ferromagnesian (or mafic) series
o Plagioclase (or felsic) series
• Rates of weathering tend to follow reverse order of sequence of mineral formation
during original cooling and crystallization of rock
o Minerals formed during rapid early crystallization of magma at high
temperatures contain few bonds that link crystalline structure
o Also have frequent substitutions of various cations (Ca, Na, K. Mg) and trace
metals (Fe, Mn) in crystal lattice, distorting the shape and increasing
susceptibility to weathering
Figure 4.1
Chemical weathering concentrated on relatively labile minerals
o Dissolution of some minerals may result in reduction of density of bedrock
o Sometimes little collapse or loss of initial rock volume (product of such
weathering called saprolite, and composes lower soil profile of many regions)
o Other times get collapse of soil profile, with apparent increase in
concentration of elements that remain (e.g., Zr, Ti, and Fe)
Quartz (Si 4O 8 in tetrahedral crystals linked in 3 dimensions):
o Very resistant to chemical weathering
o Sand fraction of soils largely quartz crystals that remain following chemical
weathering and loss of other constituents during soil development
Chemical weathering also depends on climate because it involves chemical reactions
o More rapid in tropical than temperate forests
o More rapid in forests than grasslands or deserts
Loss of Si due to chemical weathering directly related to precipitation globally
Figure 4.2
Dominant form of chemical weathering is the formation of carbonic acid in soil
solution:
H2O + CO 2 ? H+ + HCO 3- ? H2CO 3
Because plant roots and soil microbes release CO 2, concentration of H2CO 3 in soil waters
often higher than that in equilibrium with atmospheric CO 2
o Example: seasonal CO 2 concentrations of greater than 7% have been observed in
soils beneath Missouri wheat fields (CO 2 in atmosphere ~0.036%)
Soil CO 2 seems to vary as function of evapotranspiration of site
Figure 4.3
Plant growth greatest in warm, wet climates, maintaining highest levels of soil CO 2 and
carbonation weathering
THEREFORE, BY MAINTINING HIGH CONCENTRATIONS OF CO2 IN SOIL,
LIVING ORGANISMS EXERT BIOTIC CONTROL OVER THE GEOCHEMICAL
PROCESS OF ROCK WEATHERING ON LAND
One type of chemical weathering is incongruent dissolution:
Here, only some of the constituents of the primary mineral are released
o e.g., weathering of Na-feldspar:
2NaAlSi 3O 8 + 2H2CO 3 + 9H2O ? 2Na + + 2HCO 3- + 4H4SiO4 + Al 2Si2O 5(OH) 4
The opposite is congruent dissolution:
o e.g., in moist climates, dissolution of limestone during carbonate weathering:
CaCO 3 + 2H2CO 3 ? Ca 2+ + 2HCO 3o or oxidation of pyrite
4FeS2 + 2H2O +15O2 ? 2Fe2O 3 + 16H+ + 8SO 42The H+ produced accounts for acidity of runoff from many mining operations
Fe2O 3 usually precipitated out into soil profile or streambed
In addition to carbonic acid, living organisms release a variety of organic acids to soils:
e.g.,:
o Plant roots: acetic and citric acids
o Soil microbes: fulvic and humic acids from decomposition of plant litter
o Fungi: oxalic acids
In addition to increasing acidity, organic acids speed weathering by combining with some
weathering product in soil:
o Process called chelation:
o E.g. when Fe and Al combine with fulvic acid, they are mobile and move to lower
soil profile in percolating water
Summary for chemical weathering:
• Organic acids often dominate acidity of upper soil profile, while carbonic acid is
important below
• Organic acids dominate weathering processes in cool temperate forests where
decomposition processes are slow and incomplete, whereas carbonic acid drives
weathering in tropical forests where lower concentrations of fluvic acids remain after
rapid decomposition of litter
Secondary minerals
Formed as byproducts of weathering on Earth’s surface
e.g.,
• In temperate forest soils, often dominated by layered silicate or “clay” minerals.
• In tropical soils, often dominated by crystalline oxides and hydrous oxides of Fe and
Al
Phosphorus minerals
P often in limited supply for plant growth
Only primary mineral with significant P content is apatite
Can undergo carbonation weathering in congruent reaction to release P:
Ca 5(PO 4)3OH + 4H2CO 3 ? 5Ca 2+ + 3HPO 42- + 4HCO 3- + H2O
Large portion not taken up by plants, but involved in reactions with other soil minerals
leading to its precipitation in unavailable forms
• Occluded P is held in the interior of crystalline Fe and Al oxides and essentially
unavailable to biota
• Nonoccluded P includes forms held on surface of soils minerals by variety of reactions
including anion adsorption
Figure 4.4
P most available to biota at pH 7
• In acid soils, P availability controlled by precipitation with Fe and Al
• In alkaline soils, P is often precipitated with Ca minerals
Figure 4.5
Apatite weathers rapidly
• Initially held in nonoccluded forms or taken up by biota
• Over time, oxide minerals accumulate, and P precipitated in occluded forms
• At even later stages of weathering and soil development, occluded and organic P
dominate
o Here, almost all available P found in organic forms in upper soil profile,
while P found at depth mostly bound to secondary minerals
• Plant growth may depend almost entirely on release of P from dead organic matter
(defining biogeochemical cycle of P in upper soil horizons)
NOTE: Organic acids can also influence weathering of P from rocks, and P availability in
soils
• e.g., Organic acids can inhibit crystallization of Al and Fe oxides, reducing rates of P
occlusions
Soil Chemical Reactions
Following release by weathering, availability of essential elements to biota controlled by
number of reactions that determine equilibrium between concentrations in soil solution and
contents in soil organics and minerals
“soil exchange” reactions happen more rapidly than weathering
• specific reactions differ depending on how soil development progresses under
influence of climate, time, biota, topography, parent material, etc.
Cation exchange capacity
Layered silicate clays that dominate temperate soils have net negative charge that attracts and
hold dissolved cations in soil solutions
Negative because:
• Ionic substitutions within silicate clays (e.g., Al3+ replaced by Mg2+)
• -OH groups exposed on edges of clay pa rticles (strength of H+ binding dependant
on pH; pH dependant charge)
o H+ usually dissociated, leaving -O - to bind with cations
• Soil organic matter also contributes to cation exchange, with pH dependant charges
originating from phenolic (-OH) and organic acids (-COOH)
Cation exchange function of mass balance with soil solution
• In general cations are held and displaced one another in sequence (assuming equal
molar concentrations in initial soil solution):
Al3+ > H+ > Ca 2+ > Mg2+ > K+ > NH4+ > Na +
Cations other than Al and H are called “base cations” because they tend to form bases when
released to soil solution [e.g., Ca(OH)2]
Soil buffering
Cation exchange can buffer soils, by exchanging H+ in solution for cations (especially Ca) on
clay or organic matter
• As long as there are sufficient base cations, soils can be buffered from acidification
In strongly acid soils (e.g., humid tropics) there is little base cation buffering
• Instead, soils buffered by geochemical reactions involving Al, which leads to
formation of H+ as Al3+ is precipitated as Al hydroxide:
Al3+ + H2O ? Al(OH) 2+ + H+
Al(OH)2++ H2O ? Al(OH) 2+ + H+
Al(OH)2+ + H2O ? Al(OH) 3 + H+
Reactions are reason soils acid, but since reactions are reversible, so buffered against
additions of H+ by dissolution of Al hydroxide
• Acid rain in NE U.S. dissolves gibbsite [Al(OH)3] from forest soils, leaching Al3+
which may be toxic to fish
Anion exchange capacity
Tropical soils dominated by oxides of Al and Fe show variable charge depending on soil pH
• When acidic, soils possess positive charge (from association of H+ with surface OH
group)
Figure 4.7
Anion absorption then follows the sequence:
PO 4-3 > SO42- > Cl- > NO 3This accounts for low P availability in tropical soils
Anion adsorption capacity inhibited by soil organic matter (especially organic anions) that
tends to bind to reactive surfaces of Fe and Al.
Soil Development
Soils consist of number of layers (horizons) that form complete soil profile (pedon)
• Soil development influenced by:
o Rock weathering
o Water movement
o Organic decomposition
o Climatic conditions
Soil Horizons
Horizon
Organic horizons
L
F
H
Mineral horizons
A
B
C
Description
Litter: intact, easily recognizable plant structures
Fragmented, partially decomposed
Well decomposed, plant structures indiscernible
• enriched with organic matter (h); or
• leached of OM, clay, or iron/aluminum (e)
• enriched with OM (h), clay (t), or iron/aluminum (f); or
• structure developed (m); or
• color changed (m)
parent geologic material
Not all forest soils show layer differentiation
• e.g., in tropical forests decomposition of fresh litter so rapid there is little L layer
• boreal region has thick L layer (mor) due to slow decomposition of conifer needles
• northern peatlands characterized by waterlogged, anaerobic, acidic soils
Downward transport of Fe and Al in conjunction with organic matter is known as
podzolization
• Podzolization particularly intense in boreal and temperate forests
o Coniferous forests produce litterfall rich in phenolic compounds and organic
acids (pH of soil solutions often as low as 4)
• Decomposition is slow and incomplete, and large quantities of fulvic acid percolate
from litter into underlying A-horizon
A comparative index of soil formation and the degree of podzolization is seen in the ratio of
Si to sesquioxides (Fe and Al) in soil profile
• In boreal soils, Si is relatively immobile and Fe and Al are removed, which results in
high values for this ratio in the A-horizon
Region
Boreal
Cool-temperate
Warm-temperate
Tropical
Mean Si/sesquioxides ratio
A-horizon
B-horizon
10.65
4.07
3.77
1.47
7.40
2.28
3.15
1.61
Note: removal of Fe and Al results in high values in boreal and cool temperate soils,
especially in A-horizon. In tropical soils there is little differentiation between horizons due
to removal of Si from entire profile in long periods of weathering.
PLEASE SEE SCHLESINGER FOR SOIL FORMATION IN GRASSLANDS AND
DESERTS
Weathering Rates
Rock weathering and soil formation rates difficult to quantify because:
• Processes slow
• Nearly impossible to NOT disturb chemical reactions of interests when sampling soil
profile
However, information important to understanding biogeochemistry in watersheds where
essential elements for biota derived from underlying rock
Chemical weathering rates
Best known attempt at measuring chemical weathering rates was by Likens et al. at Hubbard
Brook forest in New Hampshire
• Area underlain by impermeable bedrock with no groundwater flow
They reasoned that if atmospheric inputs of elements were subtracted from streamwater
losses, the difference would reflect releases from rock weathering (MASS-BALANCE,
INPUT-OUTPUT STUDY):
Weathering =
(Ca lost in streamwater) – (Ca received in precipitation)
(Ca in parent material) – (Ca in residual material in soil)
Solution of such equations show different weathering rates for different elements
Table 4.4
Losses of Ca and Na imply higher weathering rates than calculated using K and Mg.
• Possibly K and Mg are accumulating in secondary minerals in soils
• As well, trees may temporarily store essential elements in long-lived tissues
Rock weathering is dominant source of many elements at Hubbard Brook:
Table 4.5
In most areas underlain by silicate rock, loss of elements in streamwater relative to their
concentration in bedrock follows order:
Ca > Na > Mg > K > Si > Fe > Al
Order reflects tendency for:
• Ca- and Na- silicates to weather easily
• Limited incorporation of Ca and Na in secondary minerals
• Fe and Al retained in lower soil profiles as oxides and hydroxides
Table 4.6
Temperate forest soils dominated by clay minerals with permanent negative charges
• Therefore loss of cations determined by availability of anions (usually bicarbonate
HCO 3-) that carry cations through soil to streamwaters
Adsorption of SO42- in tropical soils reduces its importance as a mobile anion
Losses of base cations has increased in certain areas due to large amounts of H+ and SO42- in
acid rain
• Some loss due to cation exchange
• Others lost to increased rock weathering due to acidity
• Cations move through soil to streams, with SO42- acting a balancing anion
Chemical weathering of primary minerals in igneous rock accounts for ~27% of dissolved
constituents delivered to oceans
• Chemical weathering of sedimentary rocks accounts for remainder
Total denudation rates
In addition to chemical weathering, mechanical weathering results in large particulate or
suspended loads
• Total denudation of land dominated by mechanical weathering
o Exceeds chemical weathering by 3-4 times worldwide
o Importance increases with elevation
Figure 4.14