10/12/2015 www.komar.de The Lithosphere Ben Sullivan, Assistant Professor NRES 765, Biogeochemistry October 14th, 2015 Contact: bsullivan@cabnr.unr.edu Definition io9.com tedquarters.net Lithos = rocky; Rocky sphere Skin of an apple Two types: oceanic, continental Biogeochemical Focus is on continental crust, soil formation (Schlesinger) 1
How do landforms develop? Is topography controlled by rock weathering OR geologic uplift OR both? Lithosphere is foundation of life s building blocks "201 Elements of the Human Body 01" by OpenStax College Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Earth Crust Scanned by CamScanner Schlesinger & Bernhardt p. 21 2
In terrestrial ecosystems, it all starts with the geology (with some atmosphere mixed in for good measure). Rocks > soil > veg > animals Soil forms from the bottom up Soil formation starts with rock weathering discovermoab.com travellogs.us 3
Soil formation and ecosystem devlelopment: Primary succession Lava: Soil before it was soil. 4
Weathering of rock = soil, plus salty oceans Kilauea (active shield) Maui (400K y) Kohala (150K y) Kohala Kilauea Kauai (4 million y) The origin of primary minerals: Cooling magma (intrusive or extrusive) Silicates are the building blocks of all primary minerals 5
Silicate Minerals Aka: Mafic (Have Mg) Have Al Resistance to weathering Weathering involves disintegration & synthesis 6
Chemical Weathering of rocks and minerals There are 6 types of chemical weathering 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 7
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? 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 4 2- + 4H 2 O - O H + - O H + H H Hydrogen bonds Strong electrostatic attraction between polar molecules creates the unique properties of water *Dissolution of salts 8
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 2+ + 2HCO 3 - Carbonic acid Calcite Solution Production of CO 2 during respiration in soils is a major contributor to weathering The oxidation of organic matter drives weathering CH 2 O + O 2 ----> 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 9
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! An example of chemical weathering 10
Carbon dioxide is a major weathering agent More CO 2 at depth in soil. Why? See Figure 4.5 in Schlesinger. Climate influences weathering 11
Where are rates of weathering highest? Boreal vs. tropical soils 12
Mineralogy of sand, silt & clay < 0.002 mm 0.002 0.05 mm > 0.02 mm The building blocks of clay minerals Tetrahedron Tetrahedra: You know what that is! Octahedra: Al or Mg surrounded by OH- Octahedron 13
Structure of layer silicates Silicon tetrahedron Tetrahedral sheet O 2- Si 4+ O 2- O 2- Octahedron O 2- Octahedral sheet 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 14
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) Stages of Clay Formation 15
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 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! 16
Buffering capacity Bowman et al., 2008 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 17
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) Weathering releases elements, weathered soil has CEC and keeps elements This balance depends on the degree of weathering, the elements involved, climate, topography, plant productivity, and the geologic substrate. 18
The weathering of rock releases nutrients N, P, K, Ca, Mg, Na Morford et al., 2011; Johnson and Turner, 2014 Atmospherically derived Rockderived (And about 23 others!) 19
Weathering of rock derived nutrients Available P Nitrogen Unavailable P Walker and Syers, Geoderma, 1976 20
Soils are chromatographic columns: continuously changing, point by point Horizons 21
Factors of Soil Formation Dukochaev 1883, Jenny 1941: Simple but mathematical Soil = f(time, parent material, climate, biota, topography.) Fisher and Binkley (present): Complex, but likely more realistic Driving Variables: State Variables: Climate Parent Material Topography Vegetation, animals, microbes Soil structure, chemistry, fertility Dimension through which soil devleopment proceeds: Time Land use A tale of two chronosequences Hawaii (7 million years) and Arizona (3 million years) 22
Weathering of rock = soil Kilauea (active shield) Maui (400K y) Kohala (150K y) Kohala Kilauea Kauai (4 million y) Kilauea Iki (56 y): Rock Kauai (4 million y): Oxisol Thurston (300 y): Andisol Laupahoehoe (20 Ky): Andisol Maui (400 Ky): Andisol 23
Slower process in AZ 750 K y: Mollisol 3 million y: Alfisol 1 K y: Entisol 55 K y: Andisol 1 K y: Rock & Entisols 750 K y: Mollisol 55 K y: Andisol 3 million y: Alfisol 24