Applications of Aqueous Geochemistry to Weathering Studies

1
Applications of Aqueous Geochemistry to
Weathering Studies

• Michael Fortwengler
• Joniell Borges

2
What is weathering?

• Physical
• No chemical change involved
• Weathering vs Erosion
• Chemical
• Chemical change involved
• Biological
• Organic acids
• Chelation

3
Types of Chemical Weathering

• Hydrolysis
• 2NaAlSi3O8 2H 9H2O H4Al2Si2O9 4H4SiO4
  2Na
• Hydration/Dehydration
• CaSO4.2H2O CaSO4 2H2O
• Oxidation/Reduction
• FeSiO3 O2 2Fe2O3 4SiO2
• Solution
• H2O CO2 CaCO3 Ca2 2HCO3-
• Ion Exchange
• Na-clay H H-clay Na
• Chelation
• Metal ions (cations) chelating agent (from
  lichens) H chelateaq

4
Why is chemical weathering important?

• Climate change
• Drawdown of greenhouse gases (pCO2)
• Global Climate Modeling (Past, Present, Future)
• Anthropogenic vs Natural Weathering
• Acid Mine Drainage
• Groundwater Contamination
• Acid Rain
• Landscape Evolution (Geomorphology)

5
Climate Change Record
Petit et al., 1999
6
CO2 Sources Sinks

• Sources
• Volcanism
• Respiration
• Fossil Fuel Burning

• Sinks
• Silicate Weathering
• Photosynthesis
• Organic Burial

7
Weathering Reactions

• Silicate Weathering (net pCO2 drawdown)
• CO2(g) H2O H2CO3
• CaSiO3 2H2CO3 H2O Ca2 2HCO3- H4SiO4
• Ca2 2HCO3- CaCO3 CO2 H2O
• Net Reaction
• CaSiO3 CO2(g) 2H2O CaCO3 H4SiO4
• Carbonate Weathering (no net effect on pCO2)
• CaCO3 CO2 H2O Ca2 2HCO3-
• Ca2 2HCO3- CaCO3 CO2 H2O
• Evaporite Weathering
• NaCl Na Cl-
• CaSO4 Ca2 SO42-

Urey, 1953
8
Amazon River
Stallard and Edmond, 1993
9
Silicate vs Carbonate Terrains
Stallard and Edmond, 1983
10
Calcite Saturation State
Huh and Edmond, 1999
11
Conclusions

• Chemical weathering involves a number of aqueous
  geochemistry reactions
• Silicate weathering and climate change might be
  closely coupled
• Studying the geochemistry of current river
  systems may allow us to derive silicate
  weathering rates related to climate change
• Ancient climate systems can be studied by
  discovering sedimentary geochemical tracers