Weathering Processes and Soil Formation: A Geochemical Perspective - Prof. T. Dunne, Study notes of Environmental Science

Download Weathering Processes and Soil Formation: A Geochemical Perspective - Prof. T. Dunne and more Study notes Environmental Science in PDF only on Docsity! 11/21/2002 ESM 203 - Weathering 1 1 ESM 203: Formation of Soil Resources Biogeochemical Role of the Lithosphere Jeff Dozier & Tom Dunne Fall 2007 2 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 3 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 Earth’s 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 4 The effects result from a set of biogeochemical processes called weathering, defined as
Mechanical disintegration and chemical decomposition of rock minerals in situ near Earth’s surface (within ~1 km), and …
Short-distance translocation of the dissolved substances within the near-surface environment 11/21/2002 ESM 203 - Weathering 2 5 General weathering“reaction” ‘Primary’ rock minerals + H2O, CO2, HNO3, O2, organic acids, ± changes in temperature and pressure 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 6 General weathering“reaction” ‘Primary’ rock minerals + H2O, CO2, HNO3, O2, organic acids, ± changes in temperature and pressure 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 To groundwater and streams (water quality) 7 Mechanical weathering
Unloading or pressure release
Crystal growth (salt or ice)
Create microfractures and joints in rocks that allow penetration of water 8
Solution of carbonates, e.g. CaCO3 + CO2 + H2O Ca(HCO3)2 Insoluble Soluble “hard water” Chemical weathering process1 Needs water, acids (CO2, SO2, HNO3, organic acids from plants) 11/21/2002 ESM 203 - Weathering 5 17 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. 18 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 19 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) 20 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 11/21/2002 ESM 203 - Weathering 6 21 Boreal forest/ tundra landscape, N. Canada 22 Cool, humid climate with coniferous forest in Pacific Northwest 23 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 24Podzol or spodosol: Organic-rich topsoil, leached shallow horizon; accumulation of clays and iron in subsoil on well-drained sites 11/21/2002 ESM 203 - Weathering 7 25(Former!) Tropical rainforest, Kenya 26 Wet tropics High 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 27Mid-latitude grassland 28 Temperate continental grasslands Moderate rainfall and temperature regime. Organic-rich surface horizon; significant weathering to clays but not heavily leached. Fertile with good water-holding characteristics Chernozem or mollisol Goldilocks!