Geochemical cycles and models

•  Transfer of Carbon between chemical reservoirs
•  Reservoirs
•  Living things (biosphere)
•  Sediments (geosphere)
•  Organic matter/kerogen (20%)
•  Hydrocarbons
•  Oil
•  Gas
•  Clathrates
•  Methane ice
•  Carbonate/Limestone (80%)
•  CaCO₃
•  Oceans (hydrosphere)
•  HCO₃^-
•  Atmosphere (atmosphere!)
•  CO₂

 

---

Important reactions/processes in the carbon cycle

•  Weathering of silicate rocks
•  CaSiO₃ + 2CO₂ + H₂O -> Ca²⁺ + 2HCO₃^-+ SiO₂
•  Weathering of carbonate rocks
•  CaCO₃ + CO₂ + H₂O -> Ca²⁺ + 2HCO₃^-
•  Biogeochemical formation of carbonate
•  Ca²⁺ + 2HCO₃^- -> CaCO₃ + CO₂ + H₂O
•  Net weathering
•  CaSiO₃ + CO₂ -> CaCO₃ + SiO₂
•  Net metamorphism
•  CaCO₃ + SiO₂ -> CaSiO₃ + CO₂
•  Volcanic and hydrothermal venting of CO₂

 

---

BLAG Model (Berner, Lasaga, and Garrels)

•  Weathering rate based on total land area
•  Higher CO₂ -> higher temperature -> higher rates of weathering
•  What kind of feedback?
•  Rates of exchange
•  Ultimately, the geochemical cycle regulates the long term 
    atmospheric concentration of CO₂
•  Rate of plate tectonic cycling
• Seafloor spreading

---

Carbon isotopes as tracers

• Carbon
• ¹²C
• ¹³C
• ¹⁴C
• d¹³C, in ‰ PDB
• Carbon isotopic compositions
• Pure Rural Air, d¹³C = -7‰
• Land plants, d¹³C = -24 to -34‰
• Rivers, d¹³C = -3.8‰
• Marine carbonate (e.g., foraminifera), d¹³C = 0-1‰
• Close to oceanic value
• Very little fractionation
• Therefore act as a  value of the ocean water
• Marine organic matter, d13C = -24-25‰ less than carbonate,
• Generally between -20 and -27‰
• Petroleum, d¹³C = -18 to-34‰ , average -28‰
• Bacterial methane, d¹³C = -40 to -90‰

Mass balance relationships

• Changes in the balance of organic matter productivity, organic matter deposition, and carbonate deposition shift  thed¹³C value of ocean

 
• Changes in the whole ocean over time
• More (proportionally) organic carbon burial
• More (+) d¹³C in carbonate shells
• Less (proportionally) organic carbon burial
• More (-) d¹³C in carbonate shells
• Changes within the ocean at any one time
• Reflects deep water circulation patterns
• Older the bottom water
• More (-) d¹³C in water and in carbonate shells

---

Permian/Triassic carbon excursion

• Positively shifted d¹³C
• Development of vascular plants
• New way of fixing carbon
• More burial of organic matter -> increased free oxygen
• Worldwide Triassic red beds
• Why the Permian Extinction?
• Competing scenarios
• Sea level fall
• Bolide impact
• Oceanic anoxia
• Oceanic hypercapnia
• CO₂ build up in the depths of a stagnant sea
• Two cycles of buildup/release
• Acompanied by greenhouse cooling and warming

 
What would be the carbon isotopic signal?

Click your Mouse on "Back to the Beach!" below if you want to return to the California State University at Long Beach (CSULB) home page

Back to the Beach!

Or click  here to return to the Department of Geological Sciences home page

For any questions or problems with these pages contact
John Francis
Email: jfrancis@csulb.edu
Phone: 562-985-4928
written by R. Behl.
Last changes: 10 Oct. 1997