Isotope Stratigraphy
Isotope Stratigraphy
• Isotope - Same chemical element, differing
by # neutrons; atomic weight
Heather Patterson
Geological Oceanography
Spring 2008
• Stratigraphy – Branch of geology studying
rock layers
Heather’s Guide to Fractionation as
related to Geology
Isotopic Fractionation
• Isotope exchange - the chemical process
doesn’t change, but the distribution of the
isotopes change
• Mass differences lead to different
physiochemical properties
• Fraction factor α = RA/RB
• Delta value
– δA = (RA/RSt -1) * 103 (‰)
– δB = (RB/RSt -1) * 103 (‰)
• Evaporation/Condensation – lighter mol. prefer
vapor phase
J. Hoefs, 1987
http://serc.carleton.edu/images/microbelife/research_methods/environ_sampling/isotope_fractionation.jpg
Isotopic Fractionation
• Kinetic Processes – rate of rxn sensitive to atomic
mass
Standards
– Diffusion
• Chemical Composition - Ions w/high ionic
potential, low atomic mass, have high vibrational
frequencies - therefore preferentially incorporate
the heavier isotope
–
18O
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
Cole & Chakraborty 2001
• Sr – SRM 987, EN-1, E & A (Depends on value of sample)
http://serc.carleton.edu/images/research_education/geochemsheets/te
chniques/table1.jpg
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Carbon
Oxygen
• Fractionation
– Photosynthesis – concentration light (12C) in
organic material
– Chemical exchange – enrichment (13C) in
bicarbonate
– Not the same in all organic matter (lipids,
carbohydrates, proteins)
– Temperature in calcium carbonate – effects
the precipitation point
• Diffusion is different between wet and dry
conditions
• Water-Rock interactions – if rock greater,
fluid modified, if water greater than rock
modified
J. Hoefs, 1987
Strontium
J. Hoefs, 1987
Mass Spectrometry
• Used as indicator of water-rock interaction,
a tracer for groundwater movement and
the origin of salinity
• Depends on contributions of weathering
from land and hydrothermal activity
• Is geochemically substituted for Ca2+
http://en.wikipedia.org/wiki/Ma
ss_spectrometry
http://www.science.uottawa.ca/~eih/ch9/9stront.html
McArthur, 1994
Purpose: To review uses and misuses of
strontium isotope stratigraphy
Background info.
• Can date back the Cenozoic, but not easily
further
– Can’t find unaltered fossils/rocks
• Variation of 87Sr/86Sr
– related to loss from hydrothermal circulation
– leaching from basalts
– weathering from land
• Sr residence time 106 (longer than mixing 103)
– Sr well mixed
– Flux from rivers (except 4 not enough to make a
difference)
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Samples used
Scanning Electron microscope
• Carbonate, chalk, barite
• Belemnites, bivalves
• Good
ultrastructure
morphology =
sample integrity
• Need to make sure unaltered
– 87Sr/86Sr to within 20 * 10-6 of original value
• not the best for
aragonite
epswww.unm.edu/.../virtualtou
r/laboratories.htm
A scanning electron microscope
(SEM) image of the planktonic
foraminifera Globogerinoides
fistulosus.
www.csiro.au/science/ps1qb.html
X-ray Diffraction
• Can detect to 0.5%
of crystal
contamination
• But calcite &
aragonite can
recrystalize without
altering their
mineralogy
• But since its rare for
2° aragonite, may be
best method for
aragonite
Amino Acid Analysis
www.phy.cmich.
edu/people/petko
v/x-ray.html
• Organic matter probably more susceptible
to alternation
– Good preservation of OM = Good
preservation of carbonate
www.esrf.e
u/.../2003/Im
aging/Imagi
ng08
Cathodeluminescene
Chemical Analysis
• Major elements
– Al conc. warning of silicates
– HCl acid leaches contaminates &
changes Sr value
– Contamination ok if doesn’t
change Sr value
• If carbonate luminesces that
it is regarded as recrystalized
Cathodoluminescent image
showing non-luminescent
calcite
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• Trace elements
–
–
–
–
Mg, Sr, Fe, Mn to track alterations
Not useful for small degree alterations
Aragonite – Mg >100 µg/g = contamination
Biogenic carbonate – Fe/Mg >100 µg/g =
contamination (anoxic/suboxic conditions)
– Sr/Ca ratios lower than modern = recrystalization
Sr Dating
• Depends on
– Slope of 87Sr/86Sr
• Depends upon gradient
– Accuracy of age model used
• Internal consistency = original
• Model alteration
• Selective dissolution – minimize contamination
• Need correct boundaries
– Analytical quality of data – lab bias
• Appropriate standard
• Differs between labs = 61 * 10-6
• Appropriate understanding of Rb-decay
Statistics
• Linear
• Polynomial
• Non-parametric regression (LOWESS)
Wierzbowski, 2002
Purpose: Provide 13C & 18O data from
brachiopods & belemnites from the
Oxfordian (161.2 ± 4.0 MA) and discuss
Applications in Stratigraphy
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Date sediments
Determine subsidence at atolls
Define sea level change
Aid in oil exploration
Distinguish between marine & non-marine fish
Dating phosphogenesis
Defining inflexions
ID disaster, K/T boundary
Methods
• Thin slabs of organisms
– Polarizing microscope
– Cathodoluminscence
(no/slight luminescence)
– SEM – preserved
microstructure
(Cross-polarizing microscope photographs of typical
structures of volcanic rock (left) and plutonic rock (right) http://www.sand4students.net/en/images/text04_p5.jpg)
www.plingfactory.de
http://www.tonmo.com/science/public/belemnites.php
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Results & Discussion
• 2.5-3‰ difference in δ13C between belemnites &
brachiopods – habitat? Vital effect??
• δ18O values similar
• Uniform values around 0‰
•Max δ13C
middle
Oxfordian –
related to
sea level
change
Discussion Points
• Do you “buy” the use of isotopes?
• Are there any biases/misuses that you can
think of?
- “Vital effects”
• How might you use them in your own
research?
• The paleotemperature equation for calcite developed by
Epstein et al. (1953) and modified by O’Neil et al. (1969)
is:
– T (◦C) = 16.9–4.38(δ18Oca − δ18Ow) +0.01(δ18Oca − δ18Ow)2
• where δ18Oca is the oxygen isotope ratio for a calcite
• sample and δ18Ow the oxygen isotope ratio for seawater.
• The equation developed by Grossman and Ku (1986) for
aragonite in mollusc shells is:
T (◦C) = 21.8–4.69(δ18Oar − δ18Ow)
– where δ18Oar is the oxygen isotope ratio for an aragonite
– sample and δ18Ow the oxygen isotope ratio for seawater
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