603-pp25 - Isotope Stratigraphy Heather Patterson...

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1 Isotope Stratigraphy Heather Patterson Geological Oceanography Spring 2008 Isotope Stratigraphy • Isotope - Same chemical element, differing by # neutrons; atomic weight • Stratigraphy – Branch of geology studying rock layers Heather’s Guide to Fractionation as related to Geology • Mass differences lead to different physiochemical properties J. Hoefs, 1987 Isotopic Fractionation • Isotope exchange - the chemical process doesn’t change, but the distribution of the isotopes change • Fraction factor α = R A /R B •De l ta va lue δ A= (R A /R St -1) * 10 3 (‰) δ B= (R B /R St -1) * 10 3 (‰) • Evaporation/Condensation – lighter mol. prefer vapor phase ht p:/ serc.carleton.edu/images/microbelife/research_methods/environ_sampling/isotope_fractionation.jpg Isotopic Fractionation • Kinetic Processes – rate of rxn sensitive to atomic mass – Diffusion • Chemical Composition - Ions w/high ionic potential, low atomic mass, have high vibrational frequencies - therefore preferentially incorporate the heavier isotope 18 O is bound in quartz, and magnetite is deficient • Crystal Structure - Heavier isotope packed in well ordered structure – Graphite and diamond, fractionation is 11.5‰ • Pressure – Can be important in rocks, change volatility Standards
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2 Carbon • Fractionation – Photosynthesis – concentration light ( 12 C) in organic material – Chemical exchange – enrichment ( 13 C) in
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603-pp25 - Isotope Stratigraphy Heather Patterson...

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