Geological Oceanography Section II Lecture 1

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Geological Oceanography Section II Lecture 1

• Sources of Marine Sediments
• Weathering
• 7 February 2008

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Mountains to Muds
How? Where? Why?
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Clastic sedimentation
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How does a Geosystem work?

• Holistic approach to sedimentation
• Geology defines starting materials and context
• Physics and chemistry define processes
• Chemistry and biology define alterations
• Biology builds and destroys
• How do we pull all these parts together??

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A Geologic Perspective...

• What makes a sedimentary deposit?
• Sediment
• Deposition
• What leads to deposition?
• Decrease in competency of transport medium
• Where do sediments come from?
• Where do they go?

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Geology - Materials and context

• Source material
• sediment production and characteristics
• Tectonic setting
• grade and slope
• basins
• Climate
• Temperature
• Precipitation
• Interactions with physics, chemistry, biology

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Geological Physiography
From Open University Press
Sediments move downslope
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Physics - How sediments move

• Gravity, gravity, gravity and wind
• River flow (unidirectional)
• Winds (unidirectional)
• Waves and tides (bidirectional)
• Stratified flows
• estuarine and shelf circulation
• Density-driven flows

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The predominant mechanism of weathering and
transport
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Chemistry - Changing the rules

• Dissolution and weathering
• host rock, climate
• Interaction of grains
• flocculation of fine silts, clays
• water chemistry
• Precipitation and cementation
• fluid chemistry and flow
• authigenic sediments

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Biology - A dose of the living

• Influences physical and chemical weathering
• Alters erosion rates
• Influences transport
• bed roughness elements
• bioturbation
• Sequestering sediments
• wetlands, mats, lags, reefs
• Production of sediments
• shells, skeletal material (biogenic)
• bioerosion (clastic or biogenic)

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1. Pre-Phanerozoic before land vegetation
2. Paleozoic primitive land vegetation
3. Mesozoic flowering plants, pre-grasses
4. Cenozoic flowering plants and grasses
Relationship between precipitation and sediment
yield has changed through geologic time
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Power of Erosion

• At present rate of erosion, mass of materials
  carried to oceans by rivers would fill in ocean
  basins in 1 billion years (by)
• At present rates of erosion, continents would be
  reduced to sea level in 100 my.
• So, why do we still have continents if earth is
  4.8 by old?
• Sediment recycling
• Continental accretion--exotic terrains
• Isostatic readjustments

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Sedimentary cycle

• Mountains are eroded
• Weathering products are dispersed into oceans
• Sediments are lithified, deformed and uplifted
  into mountain ranges by plate tectonics
• New cycle of erosion begins

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Weathering

• Sedimentary deposits and rocks are the result of
  weathering of pre-existing rocks
• Including igneous, metamorphic and sedimentary
  rocks
• Mechanical weathering
• Disintegration of rocks into smaller particles of
  the same rock type
• Chemical weathering
• Changes chemical makeup of resulting sediments
• Alters minerals to other minerals
• Factors to consider
• Climate (temperature, moisture), pore fluids,
  surface area
• Composition of original rocks
• Relative resistance of minerals to chemical
  alteration
• Crystal sizes (e.g., intrusive vs. extrusive
  igneous rocks)
• Biota

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Mechanical Weathering

• Abrasion grain to grain, ice to rock, etc.
• Crystal formation, esp, ice and salts
• Volumetric change from water to ice 9, salts
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• Frost wedging increases with number of days
  freezing and thawing occur
• Thermal expansion/contraction with diurnal
  heating and cooling
• Expansion and contraction with wetting and drying
• Pressure release as overlying layers are removed
  (exfoliation)

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Mechanical Weathering
Ice gouged grooves in pillow lavas, Boulder Pass,
Glacier NP, MT
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Mechanical Weathering
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Mechanical Weathering - Exfoliation
Taken by Wing-Chi Poon on 2nd April 2005 in the
Enchanted Rock State Natural Area of Texas, USA.
The photo clearly shows the geological
exfoliation of granite dome rock.
http//en.wikipedia.org/wiki/Exfoliation
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Mechanical Weatheringglacial erosion, transport,
deposition
Lake Grinnell, Glacier NP, Montana
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Chemical weathering dissolution

• Chemical reactions between minerals and water or
  acids
• Example dissolution of limestone
• H2O CO2 ?? H2CO3 ?? H HCO3-
• CaCO3 H2CO3 ? Ca2 2HCO3-

Weathered Proterozoic stromatolitic
limestone, Glacier National Park, Montana
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Chemical weathering hydrolysis

• Replacement of cations in mineral structure by
  hydrogen ions
• Releases cations to solution
• Chemically changes original mineral to different
  mineral or dissolves completely
• 2NaAlSi3O8 11H2O 2CO2 ? Al2Si2 O5(OH)4 2Na
  2HCO3- H4SiO4

kaolinite (clay mineral)
sodium feldspar
dissolved silica
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Chemical weathering oxidation/reduction reactions

• Oxidation atoms or ions lose electrons
• Rusting Fe2 ? Fe3
• Fe4SiO6 O2 8H2O ? 4Fe(OH)3 H4SiO4
• Pyroxene to limonite clay and dissolved silica
• CaCO3 H2CO3 ? Ca 2 2HCO3-
• Reduction atoms or ions gain electrons
• Fe3 ? Fe2
• 2Fe(OH)3 3H2S ? 2FeS S 6H2O
• Limonite plus hydrogen sulfide can yield pyrite
  and free sulfur
• Can occur in anoxic environments

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Chemical weathering hydration

• Mineral reacts with water to create a hydrated
  mineral
• CaSO4 2H2O ? CaSO4-2H2O
• Anhydrite hydrates to gypsum
• Fe2O3 3H2O ? 2Fe(OH)3
• Hematite hydrates to limonite

H2O
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Quartz crystals

• Chemical weathering of an igneous rock
• grain boundaries weaken and
• the rock disintegrate into fragments

Granite
Quartz crystals become sand grains
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Mineralogy and weathering

• Mineral susceptibility to weathering
• Function of availability of cations for
  hydrolysis or solution
• Crystal size
• Extrusive gtgt intrusive (of same mineralogy)
• E.g., rhyolite will weather more quickly than
  granite
• Crystal structure of silicates
• Single tetrahedrongtchaingtsheetgtframework
• Weathering susceptibility mirrors Bowens
  reaction series

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Bowens reaction series
Most susceptible to chemical weathering
Most resistant to chemical weathering
http//hyperphysics.phy-astr.gsu.edu/hbase/geophys
/bowen.html
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Weathering and climate chemical weathering and
frost action require moisture
Revised from Peltier 1950, Annals, Association of
American Geographers. v. 40
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Influence of vegetation

• Roots, infaunal organisms, microbes
• Mechanically erode
• Chemically change interstitial environments
• Production of reducing agents (organic matter)
• Production of weak acids (e.g., by respiration)
• CH2O O2 ? H2O CO2 ? H2CO3 ? H HCO3-
• Hold sediments in place allowing prolonged
  chemical weathering
• Mix sediments

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Relative depth of weathering with climate and
vegetation assumes minimal relief
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Granitic terrain in Brazil deeply weathered in
tropics, now denuded in arid climate
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Persistence of important minerals with weathering
depth (after Wahlstrom 1948)
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Sediments classified by size
www-odp.tamu.edu/publications
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Sedimentary Products of Weathering
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Weathering and sand constituents
100 quartz chert
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100 rock fragments
100 feldspar
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Weathering and clays
100 kaolinite Al and Fe(Fe3) oxides
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100 rock flour (chlorite)
100 illite
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Solution Load
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Sediments reflect climatic conditions
(Schopf 1980)
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What you now know about weathering

• What is weathering?
• What processes influence weathering?
• What is the Sedimentary Cycle?
• What are the major mechanical weathering
  processes?
• What are the major chemical weathering processes?
• What are the sedimentary products of weathering?

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Web links to information on weathering processes

• http//grunwald.ifas.ufl.edu/Nat_resources/weather
  ing/weathering.htm
• http//www.physicalgeography.net/fundamentals/10r.
  html
• http//soil.gsfc.nasa.gov/soilform/weather.htm
• http//www.tulane.edu/sanelson/geol111/weathering
  .htm
• http//www.geo.ua.edu/intro03/Eros.html
• http//www.ndsu.nodak.edu/instruct/schwert/geosci/
  g120/wxing.htm
• http//www.globalchange.umich.edu/globalchange1/cu
  rrent/lectures/soils/soils.html
• http//www.stmarys.ca/conted/webcourses/GEO/GEO99/
  pubweather/overview.html

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Web links to practice tests on weathering

• http//geoweb.tamu.edu/courses/geol101/grossman/We
  athering.pp.html
• http//www.ndsu.nodak.edu/instruct/schwert/geosci/
  g120/wxing.htm