The Lithosphere. Definition

Transcription

1 10/14/ The Lithosphere Ben Sullivan, Assistant Professor NRES 765, Biogeochemistry October 14th, 2014 Contact: Definition io9.com tedquarters.net Lithos = rocky; Rocky sphere Skin of an apple Two types: oceanic, continental Biogeochemical Focus is on continental, pedosphere (Schlesinger) Pedosphere = zone of soil formation interface of soil and rock 1

2 Multiple temporal and spatial scales: Geologic Landscape Soil processes blogs.ocweekly.com Rock weathering Two mechanisms: Chemical Physical (Mechanical) Biology can influence both Biology Soil formation Chemical weathering Mechanical weathering 2

3 Mechanical weathering Freeze/thaw cycles Wind Running water Root growth Seismic activity Glaciation discovermoab.com travellogs.us Chemical weathering Climate Atmospheric chemistry Rock type Rate of uplift Vegetation Microbial activity (Igneous rocks + acid volatiles = sedimentary rocks + salty oceans) 3

4 Chemical weathering Climate Atmospheric chemistry Rock type Rate of uplift Vegetation Microbial activity RELATIVE ROLES NOT WELL UNDERSTOOD likely a combination of all! (including mechanical) One area of disagreement Is weathering controlled by processes that reduce rock OR Uplift OR Both? 4

5 Soil forms from bottom up Regolith = unconsolidated rock material above bedrock (C horizon) The origin of primary minerals: Cooling magma (intrusive or extrusive) Silicates are the building blocks of all primary minerals 5

6 From the basic SiO 4 building blocks, different arrangements lead to different minerals (polymers) Examples: Chain silicates, double-chain silicates, sheet silicates Mafic (Have Mg) Silicate Minerals Felsic (Have Al) 6

7 Mafic (Have Mg) Silicate Minerals Felsic (Have Al) Olivine (Fe, Mg) 2 Si 4 O 16 7

8 Pyroxene (Ca, Mg, Fe)Si single chains of silica tetrahedra Amphibole (Horneblende) Dark minerals, composed of double chain SiO 4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures Ca 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2 8

9 Biotite mica Fe-rich version of mica (K, Fe, Mg)Si 4 O 10 -sheets -black glitter in granite Mafic (Have Mg) Silicate Minerals Felsic (Have Al) 9

10 Felsic Series Plagioclase (Ca-feldspar; CaAl 2 Si 2 O 8 ) -white or pink in color Albite (Na-feldspar; NaAlSi 3 O 8 ) Occurs in granitic masses 10

11 10/14/2014 Orthoclase (K-rich feldspar; KAlSi3O8) Common in granite and related rocks Muscovite mica (K, Al)Si4O10 -white mica -most common mica -cleaves into sheets; Sheet silicate 11

12 Quartz Composed of silicate tetrahedra Si0 4 (no substitutions) Biogeochemical Weathering Chemical Weathering is the chemical alteration of rocks and minerals There are 6 types of biogeochemical weathering 12

13 1. Hydration Water binds to minerals: (5Fe 2 O 3 ) + 9H 2 O Fe 10 O 15 9H 2 O Hematite Ferrihydrite Especially common with Fe and Al 2. Hydrolysis Water molecules SPLIT (H+ and OH-) and replace a cation KAlSi 3 O 8 + H 2 O HAlSi 3 O 8 + K + + OH - What is a cation? 13

14 3. Dissolution Water can dissolve ionic bonds and cause cations to dissociate (e.g., Gypsum) CaSO 4 2H 2 O + 2 H 2 O Ca 2+ + SO H 2 O H H - O + - O + H H Hydrogen bonds Strong electrostatic attraction between polar molecules creates the unique properties of water *Dissolution of salts 4. Carbonation & Acid Rxns Weathering is accelerated by acids: CO 2 + H 2 O H 2 CO 3 carbonic acid H 2 CO 3 + CaCO 3 Ca HCO 3 - Carbonic acid Calcite Solution Production of CO 2 during respiration in soils is a major contributor to weathering 14

15 The oxidation of organic matter drives weathering CH 2 O + O > CO 2 + H 2 O...combined with Acid hydrolysis CO 2 + H 2 O ----> H+ + HCO - 3 or CO 2 + H 2 O ----> 2H+ CO 2-3 Remember too that soils also contain stronger acids that are more effective weathering agents 6. Complexation Organic acids associate (chelate) with Al +++ ions and remove them from minerals, subjecting them to further disintegration The production of biological acids drives such rapid rates of weathering we see! 15

16 Chemical Weathering Mechanisms Albite Weathering 2 NaAlSi 3 O 3 + 2H 2 CO 3 + 9H 2 O 2Na + + 2HCO H 4 SiO 4 + Al 2 Si 2 O 3 (OH) 4 (Albite + Acid = Sodium (Cations!) Bicarbonate + dissolved Silica + Kaolinite [a secondary mineral]) 16

17 Weathering involves disintegration & synthesis Mineralogy of sand, silt & clay < mm mm > 0.02 mm 17

18 Mineralogy of sand, silt & clay < mm mm > 0.02 mm Stages of Clay Formation 18

19 The building blocks of clay minerals Tetrahedron Tetrahedra: You know what that is! Octahedra: Al or Mg surrounded by OH- Octahedron Structure of layer silicates Silicon tetrahedron Tetrahedral sheet O 2- Si 4+ O 2- O 2- Octahedron O 2- Octahedral sheet 19

20 Continued Weathering Leads to the Formation of 1:1 Clays, Like Kaolinite 2:1 layer silicates Tetrahedral sheet Octahedral sheet Tetrahedral sheet 1:1 layer silicates Octahedral sheet Tetrahedral sheet Silicate Clay Crystal (Mycelle) Adsorption of Cations & Anions Mycelle = colloid particle in sheetlike layers - Usually negatively charged Cation adsorption (cations are exchangeable) Very small (>0.002 m) 20

21 Clay formation (and type) drives CATION EXCHANGE CAPACITY Clay particles have negative charge 2:1 clays have greatest surface area, hence greatest CEC Strongest electrostatic charge Greatest water holding capacity (OH ) charge (remember, water has slight charge) ph dependent Most soils slightly acidic (except those derived from limestone and deserts The weathering of rock releases nutrients N, P, K, Ca, Mg, Na Morford et al., 2011; Johnson and Turner,

22 Cation musical chairs Al 3+ > > Sr 2+ > Ca 2+ > Mg 2+ > Rb + > K + > NH 4+ > Na + > Li + Changing acidity? Change the cations. ph doesn t change. BUFFERING CAPACITY!! Soils with high CEC have high buffering capacity! Buffering capacity Bowman et al.,

23 Anion exchange? Not so much! Called anion adsorption ph dependent in acidic (tropical) soils Anion adsorption is extremely important for nutrient retention PO 4 3 > SO 4 2 > Cl > NO 3 P loss, last plant available N form ph dependent: zero point of charge LOW ph ZPC Higher ph Positive charge (anion adsorption) H+ No net charge Net negative charge (cation exchange) Vegetation can influence ph (litter inputs, fungi, root exudates) 23

24 Summary of soil forming factors ClORPT! Climate Organisms Relief Parent material Time 1941 Dukochaev (original idea) Jenny (mathematical formulation) soils = f(cl,o,r,p,t) Summary of soil forming factors ClORPT! Climate Organisms Relief Parent material Time 1941 Dukochaev (original idea) Jenny (mathematical formulation) soils = f(cl,o,r,p,t) 24

25 Climate influences weathering Where are rates of weathering highest? 25

26 Time influences soil development Walker and Syers, 1976 Boreal vs. tropical soils 26