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Lecture: Weathering

Subaerial • Physical• Chemical• Weathering Rates• Products

SubmarineSoils

Atmospheric Carbon Dioxide

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Long-termCarbon Sinks & Sources

Feedbacks on carbon levels: Weathering & Burial1. Temperature - reaction rates2. Net precipitation - chemical reactions3. Corg production & burial

Long-term Carbon Sources and Sinks

long-term sources of CO2:• Volcanism (ridges, plumes) CO2

• carbonate metamorphism        CaCO3 + SiO2  →  CaSiO3 + CO2

• carbonate sedimentation     2HCO3- + Ca2+   →   CaCO3 + CO2 + H2O

• weathering of organic-rich seds. ("respiration"; CH2O + O2 →   CO2 + H2O)

long-term sinks of CO2:• carbonate rock weathering     CaCO3 + CO2 + H2O →  2HCO3- + Ca2+

• silicate rock weathering  CaSiO3 + 2CO2 + H2O  → 2HCO3-  + Ca2+ + SiO2

silicate weathering + carb sedimentation     CaSiO3 + CO2  →  CaCO3 + SiO2

• burial of organic matter   ("photosynthesis"; CO2 + H2O  → CH2O + O2)

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Rock Cycle &Sedimentary Rocks

are formed through a complex set ofprocesses that include:1. weathering of source rocks2. erosion3. sediment transport4. deposition5. Burial & diagenesis

Weatheringdecomposition and disintegration of rocks by

chemical, physical, and biological processesProducts:1. particles / grains2. dissolved ionsmay retain the same physical and chemical

characteristics of the parent rock or may be altered– chemical resistance - some minerals are completely

dissolved– authigenesis - new minerals are created

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Weathering Processes• Continental Crust

1. Physical (Mechanical): Large rocks broken intosmaller fragments with no change in composition• frost wedging, exfoliation, root wedging, running water and glacial

abrasion, solar heating2. Chemical: Rocks dissolved - chemical and

mineralogical composition can be altered• new minerals may form

3. Biological Weathering: plants & animals• Chemical (organic acids) and Physical (burrowing)

• Ocean Crust1. Chemical (i.e., hydrothermal systems)

1.Physical weathering1.Physical weathering

Exfoliation

Jointing

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Pressure Unloading and Exfoliation:• igneous rocks (granite) formed under pressure brought to

the surface and expand– Fractures tend to parallel to surface

Royal Arches and North Dome in Yosemite Natl. Park

Unloading/Exfoliation

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Unloading/Sheeting

Frost wedging, frost heavingFrost wedging, frost heaving

• freeze & thaw of waterin rock fractures– volume increases

~9%– most effective

process in coldenvironments

Ice-crystal

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Frost Wedging:

Antarctica

• Frost Wedging

Sierra Nevada

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Physical Weathering• Thermal expansion & contraction: caused by heating

and cooling, weakening bonds along grain boundaries– Solar heating

• particles carried by water, ice,wind, or gravity

Abrasion:

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2. Chemical weathering2. Chemical weathering• Dissolution of Rock• Dissolved ions transported to lakes/ocean

Chemical Weathering• What are the requirements?

– Water• pH = -log [H+]

H2O ⇔ H+ + OH-

Neutral solution H+ = OH- , [H+] = 1x10-7 M/liter– pH range of groundwaters: ~4.0 - 6.5

• Hydration– Dissolved gases

• O2• CO2

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Hydrolysis

orthoclase + H + H20 ⇒ kaolinite + silicic acid + potassium 2KAlSi3O8 + 2H+ + 9H20 ⇒ H4Al2Si2O9 + 4H4SiO4 + 2K+

Parent (i.e., granitic minerals) ⇒ product (clays)illite, albite (plagioclase feldspar) ⇒ kaolinite, smectite

• reaction - silicate minerals & acids (solutions containingH+ ions)1. dissolves silicate minerals2. releases cations & silica

• silicic acid (H4SiO4) &• as colloidal SiO2

Kaolin mine, central Georgia

hydration or dehydration• water molecules are added or removed to form a

new mineral.water + hematite = goethite

H2O + Fe2O3 ⇔ 2FeOOH (hydration)

water + anhydrite = gypsum2H2O + CaSO4 ⇔ CaSO4 • 2H2O

Spheroidal weathering injointed basalt, Culpeper Basin,Virginia

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Oxidation• loss of e- - oxidation Fe & Mn bearing minerals ⇒ oxide

or hydroxide (if H20 is present).Fe+2 ⇒ Fe+3 + e-

pyroxene + O2 +H2O ⇒ limonite + silicaFe2Si2O6 + O2 +4H2O ⇒ 2Fe(OH)3 + 2H4SiO4

pyrite + oxygen + water ⇒ hematite + sulfate + hydrogen 2FeS2 + 15/2O2 + 4H2O ⇒ Fe2O3 + 4SO4

-2+ 8H+

Nevada's Valley of Fire red

Solution• highly soluble minerals such as

– Calcite, dolomite, gypsum (highly soluble)Calcite + carbon dioxide + water ⇔ Calcium + bicarbonate

CaCO3 + CO2 + H2O ⇔ Ca + 2HCO3

Empire Cave, UCSC

Sinkhole, UCSC

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3. Biological Weathering• Root Wedging

• Animals: ground squirrels• Organic respiration/decay

– Organic acids, CO2

• Lichens:– Chelation - organic ligands or acids

(humic)

How important is precipitation/vegetation?

Precipitation (mm/yr)

Soil Organic Carbon & pH vs. rainfall

Kelly et al.,Biogeochemistry, 1998.

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Rates of WeatheringFactors that control rates of chemical weathering:1. Climate (Temperature/Moisture)

• more rapid in hot, wet climates than cold, dryRates of Weathering of Clean Rock Surfaces (micro-meters/1000years)Rock Type Cold Climate Warm, Humid ClimateBasalt 10 100Granite 1 10Marble 20 200

2. Renewal of fresh weathering surfaces• Slopes• Glaciers

3. Rock resistance• Surface area exposure - porosity• Mineral composition

Importance of Surface AreaHow much does the surface area of 1 m3 block change if it

crushed into 1000 blocks?

= 6 m2 (0.1m/1.0m)2*1000 = 60 m2

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Weaker mineralsWeaker mineralsweather firstweather first

Mica -> clay

Feldspar -> clay

Quartz is more robust…

Less mature sediment-->Less mature sediment-->

More mature sediment-->More mature sediment-->

Mineral CompositionStability ~Reverse of Bowens Reaction Series:• Least stable - Hi T minerals (e. g. olivine) - disequilibrium w/

surface conditions (small crystals)• Most stable - Lo T minerals (e.g. quartz) - larger crystals

Most stable ⇔ Least stable(mafic minerals)

Biotite Amphibole Pyroxene Olivine(felsic minerals)

Qtz, muscovite, K-feldspar, Na- Ca, Ca-Na, Ca plagioclase

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• http://imnh.isu.edu/digitalatlas/geo/basics/diagrams.htm

How important is mineral composition?

Granite - Silicate rocks (continental shields) have high87Sr/86Sr(>0.7200),

Limestones have low 87Sr/86Sr (<0.7080)

River Water Sr concentration vs Sr ratios

Edmond, Science, 1992

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Products of Subaerial Weathering

3 types of weathering products:1. source-rock residues - chemically resistant

minerals & rock fragments2. secondary minerals - formed by in-situ

chemical recombination3. soluble constituents - released by chemical

weathering and transported by fluids

Products of Subaerial Weathering1. source-rock residues - chemically resistant

minerals & rock fragments• igneous/metamorphic parent rocks -

– immature soils - chemically unstable minerals:• biotite, pyroxenes, horneblende, Ca-Plag.

– mature soils - quartz, muscovite, maybe K-feldspars• siliciclastic sedimentary parent rocks -

– chemically stable minerals (been through theweathering cycle once before)

• limestone parent rocks -– residues, insoluble silicates, fe-oxide residues

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Products of Subaerial Weathering2. secondary minerals - formed by in-situ chemical

recombination• clay mineral groups (phyllosilicates)

– immature - illite (Fe, Mg, Al)6(Si, Al)4O10(OH)8),chorite, smectite

– mature - kaolinite (Al2Si2O5(OH)4)• iron oxides & hydroxides (Fe+3 highly insoluble)

– Hematite, goethite (limonite)• aluminum oxides hydroxides (Al+3 highly

insoluble)– reflect very intense weathering - gibbsite

(Al(OH)3)

Products of Subaerial WeatheringLaterite - humid climate, leached, rich in Al+3 and Fe+3

(e.g., kaolinite, gibbsite, limonite)

laterite soil cross section in Brazil. The deep red color is due to Fe3+ hydroxide minerals in the soil

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Products of Subaerial Weathering3. soluble constituents

– released by chemical weathering– transported by rivers & groundwater– raw materials for biogenic & chemical precipitates -

e.g., cherts, limestones, evaporites• primary soluble products:

HCO3- (bicarbonate)

Ca+2

H4SiO4 (Silicic acid)SO4

-2

Cl-

Na+

Mg+2

K+

(most abundant dissolved ions in decreasing abundance)

Long-Term Rates of WeatheringMost important influences on

long term erosion rates ofcontinents:

1. Mountain Building– increased relief - gravity/fluvial

(rivers, streams)2. Precipitation

– amount– seasonality

3. Glacial erosion– Gouge, pluck– grind (stones imbedded at the

base of a glacier)

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Submarine weathering

350°C<10°C

• Hydrothermal fluid circulation– Hi-T (mid-ocean ridges) & Lo-T (old ocean crust)– mid ocean ridges– Dissolved constituents

• Hi-T: Mg,Sr• Zn, Mn, Fe, & Cu

Chemical Erosion of Deep SeaSediments

40 80 120 160 200 240 CO3 (µmol/kg)

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2

3

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Depth (km)

calcite saturation

aragonite saturation

CaCO3preservation

dissolution

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Erosion and Transport• water and ice erode continental rock & soil

1. Rivers - transport particulates and dissolved ions to the coast• large particles settle out near the coast

2. Gravity - Slumps, Slides

3. Winds - transport fine grain, silts and clays, in suspension(loess)• Abyssal plain

4. Glaciers & Icebergs - deposit assorted debris• high latitude oceans - coast to 1000 km from source.

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Sediment CharacteristicsSize Sorting/Settling

1. energy2. density

Classification by size(grain diameter)

1. CLAY - < 4 µm2. SILT - 4 to 62 µm3. SAND - 62 to 2000

µm4. Granules, pebbles,

cobbles, boulders ->2000 µm

Products - SoilsHORIZONSO, (humus), loose organic matterA,coarse grain minerals, zone of

leaching, removal of soluble andfine grain material (clay, Fe, Aloxides)

B, zone of accumulation (clay, Fe, Aloxides), evaporation leavesminerals,

C, transition zone, partially weatheredparent material (may or may not bebedrock)

bedrock

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Caldeira & Wickett, 2003

Buffering: a negativefeedback

• Increased dissolution ofcarbonate in the watercolumn & on theseafloor

Rise in Anthropogenic CO2, GlobalWarming, and Ocean Acidification

4500 Gt

Rock Cycle: Source of Sediments