Stable Isotopes in Ostracod Shells: Their Uses in Palaeoenvironmental Studies

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Stable Isotopes in Ostracod ShellsTheir Uses
in Palaeoenvironmental Studies

Dr. Kevin Keatings Environmental Change Research
Centre University College London London, UK
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Outline of Presentation

• Stable isotopes
• Stable isotopes in palaeoenvironmental studies
• Ostracods
• Initial results from Lake Qarun

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Stable Isotopes

• Isotopes
• Atoms consist of nucleus surrounded by electron
  cloud
• Nucleus contains protons and neutrons
• Number of protons Number of electrons
• Elements comprise atoms with the same number of
  protons (atomic number)
• Each element exists as a number of isotopes
• Isotopes distinguished by number of neutrons
• Some isotopes decay
• Radioactive
• Some isotopes do not decay
• Stable Isotopes

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Stable Isotopes of Hydrogen, Carbon and Oxygen
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• An Example

• The nuclei of 98.89 of carbon atoms have six
  protons and six neutrons
• These atoms form the most common carbon
  isotope, 12C

• The nuclei of 1.11 of carbon atoms have six
  protons and seven neutrons
• These atoms form the less common carbon
  isotope, 13C

12C
13C
Extra Neutron
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Stable Isotopes in Palaeoenvironmental Studies
Mass differences among isotopes cause natural
variation and fractionation of isotopes in
nature.

• Molecules that comprise only light isotopes have
  more energy than molecules that contain heavy
  isotopes
• Light isotopes form weaker bonds than heavy
  isotopes
• Molecules with more energy, and weaker bonds, are
  more likely to participate in chemical reactions
  and physical processes
• Fractionation occurs
• Molecules with the lighter isotope are more
  easily transformed
• Molecules with the heavier isotope are
  concentrated in the original substance

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Isotopic Fractionation During Evaporation
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• Relative proportions of stable isotopes are
  measured by mass spectrometer

• Mass 44 12C, 16O, 16O
• Mass 45 13C, 16O, 16O
• Mass 46 12C, 18O, 16O

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• How isotope ratios are indicated
• Ratios 2H/1H, 13C/12C, 18O/16O, expressed as
  deviation from a standard.
• Units are (per mil)
• Expressed relative to the heavy isotope

• Standards
• VSMOW Oxygen and hydrogen isotopes in water
• VPDB Oxygen and carbon isotopes in carbonate
• An increase in the delta value (d2H (dD), d18O,
  and d13C) indicates an increase in the amount of
  the heavy isotope present

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• The factors controlling d18O and d13C values
  differ.
• Controls on d18O in lake water
• The d18O of precipitation
• Ratio of precipitation to evaporation
• Controls on d13C in lake water
• Equilibration with atmospheric CO2 produces an
  increase
• Primary productivity within the lake produces an
  increase
• Methanogenesis produces an increase
• Input to the lake of soil organic matter produces
  a decrease

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• The isotopic composition of lake water
  influences the isotopic composition of minerals
  that precipitate within the lake.
• Calcium carbonate (CaCO3)
• Common constituent of lake sediment
• Biogenic and inorganic
• Source of C and O for stable isotope work

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Ostracods

• Microscopic, bivalved crustaceans
• Secrete low-Mg calcite shells
• Ostracod shell carbonate is ideal for isotopic
  analysis because
• Known to have formed within the lake
• Knowledge of habitat can provide information for
  different parts of lake
• Knowledge of time of year of growth can provide
  seasonal data
• Need to be aware of vital effects, which are
  species specific
• Need good foundation of ecology and taxonomy

Cypris sp. On algal filament
Cypris sp. Exoskeleton following death or ecdysis
Images supplied by www.micrographia.com
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• Ostracod Carbonate
• Factors controlling d18O value
• Lake water d18O
• Temperature
• 1C rise in water temperature decreases d18O by
  0.23
• Vital effects
• Factors controlling d13C value
• d13C of the total dissolved inorganic carbon
  (TDIC)
• Habitat

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Initial Results for Lake Qarun
April 2003 Autumn 2003
Craig, H. (1961) Isotopic variations in meteoric
waters, Science, 133, 1702-1703
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• 1997 Sediment core taken from lake bed with
  Livingstone corer
• Recovered 1.3m core
• Initial microfossil analysis by Ian Hawkes, Earth
  Sciences, UCL
• Other analyses done at
• ECRC, University College London and
• NIGL, British Geological Survey, Keyworth, UK

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Dates determined from 137Cs peaks
Rasmy, M. Estefan, S.F. (1983) Geochemistry of
saline minerals separated from Lake Qarun brine.
Chemical Geology, 40, No. 3-4. p.
269-277 Meininger, P.L. Gamil, A.M. Atta (1990)
Ornithological studies in Egyptian Wetlands
1989/1990. FORE Report Number 94-01, WIWO
Report Number 40, Vlissingen/Zeist
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• Acknowledgements
• Thanks to Melanie Leng and Hilary Sloane at the
  NERC Isotope Geosciences Laboratory, Keyworth, UK
• Thanks to The Leverhulme Trust for their
  contribution in funding a portion of this
  research
• References
• Craig, H. Isotopic variations in meteoric waters,
  Science, 133, 1702-1703, 1961b
• Rasmy, M. Estefan, S.F. (1983) Geochemistry of
  saline minerals separated from Lake Qarun brine.
  Chemical Geology, 40, No. 3-4. p. 269-277
• Meininger, P.L. Gamil, A.M. Atta (1990)
  Ornithological studies in Egyptian Wetlands
  1989/1990. FORE Report Number 94-01, WIWO Report
  Number 40, Vlissingen/Zeist