California State University Long Beach 

Stable isotopes as geochemical tracers

• Carbon isotopes; d¹³C
• ¹²C (98.9%)
• ¹³C (1.1%)
• ¹⁴C (variable trace)
• Carbon mainly fractionated by life processes
• Photosynthesis
• Respiration
• Bacterial decay
• Oxygen isotopes; d^18O
• ¹⁶O (99.6%)
• ¹⁷O (0.04%)
• ¹⁸O (0.2%)
• Oxygen mainly fractionated by physical processes
• Evaporation
• Precipitation
• Temperature
• Nitrogen; d¹⁵N
• ¹⁴N (99.6%)
• ¹⁵N (0.4%)
• Nitrogen mainly fractionated by life processes
• Most useful for studies of present time

Oxygen Isotopic fractionation

• All oxygen isotopes have the same chemistry
• Separation of different isotopes (~distillation) by some process due to mass differences & differences ground state vibrational energy

• Physical properties
• E.g., different vapor pressures
• Kinetics
• Lighter isotopes react faster
• Bond strengths
• Heavier isotopes have stronger bonds
• Tend to remain in more "immobile" phases

Oxygen Isotopes

• Fractionated in water by physical processes
• Evaporation
• Precipitation
• Temperature
• Fractionated during mineral precipitation
• Bones
• Shells -calcite (CaCO3)
• Bivalves
• Foraminifera
• ¹⁸O preferentially remains in the more "solid" phase
• How much depends on temperature of formation

Delta notation

• Parts per thousand, i.e., "per mil (‰)"
• Compared to standards
• SMOW = Standard Mean Ocean Water
• PDB = Pee Dee Belemnite (calcite)
• Same for carbon isotopes (¹³C /¹²C)
• Higher ¹⁸O /¹⁶O ratio in sample
• Enriched in heavy (rarer) isotope
• More (+) positive d¹⁸O
• Lower ¹⁸O /¹⁶O ratio in sample
• Depleted in heavy (rarer) isotope
• More ( ­ ) negative d¹⁸O

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Calcite paleothermometer

• At isotopic equilibrium
• ~4°C change in temperature per 1‰ change in d¹⁸O
• Warmer temperatures =
• More ( ? ) d¹⁸O in calcite
• Cooler temperatures =
• More ( + ) d¹⁸O in calcite
• Important complication and benefit
• Ice sheets/glaciers isotopically depleted
• Due to repeated evaporation/precipitation cycles as water vapor transported from equator
• Average composition = - 30‰ d¹⁸O_SMOW
• Greater global ice volume
• remaining seawater has more ( + ) d¹⁸O
• Smaller global ice volume
• remaining seawater has more ( - ) d¹⁸O
• Clearly shows glacial/interglacial cycles
• d¹⁸O of calcite microfossils records a combination of
• Local temperature (where they lived)
• Global ice volume (glacial/interglacial state)

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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: 24 Oct. 1997