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Isotope Stratigraphy

Heather PattersonGeological 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 = RA/RB

• Delta value– δA = (RA/RSt -1) * 103 (‰)– δB = (RB/RSt -1) * 103 (‰)

• Evaporation/Condensation – lighter mol. prefer vapor phase

http://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 vibrationalfrequencies - 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

Standards

• Sr – SRM 987, EN-1, E & A (Depends on value of sample)

http://serc.carleton.edu/images/research_education/geochemsheets/techniques/table1.jpg

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Carbon

• 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

J. Hoefs, 1987

Oxygen

• 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

• 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://www.science.uottawa.ca/~eih/ch9/9stront.html

Mass Spectrometry

http://en.wikipedia.org/wiki/Mass_spectrometry

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

• Carbonate, chalk, barite• Belemnites, bivalves

• Need to make sure unaltered – 87Sr/86Sr to within 20 * 10-6 of original value

Scanning Electron microscope

A scanning electron microscope (SEM) image of the planktonic foraminifera Globogerinoidesfistulosus.

www.csiro.au/science/ps1qb.html

epswww.unm.edu/.../virtualtour/laboratories.htm

• Good ultrastructuremorphology = sample integrity

• not the best for aragonite

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

www.phy.cmich.edu/people/petkov/x-ray.html

www.esrf.eu/.../2003/Imaging/Imaging08

Amino Acid Analysis

• Organic matter probably more susceptible to alternation– Good preservation of OM = Good

preservation of carbonate

Cathodeluminescene

Cathodoluminescent image showing non-luminescent calcite

• If carbonate luminesces that it is regarded as recrystalized

Chemical Analysis

• Major elements– Al conc. warning of silicates– HCl acid leaches contaminates &

changes Sr value– Contamination ok if doesn’t

change Sr value

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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

• Internal consistency = original• Model alteration• Selective dissolution – minimize contamination

Sr Dating

• Depends on– Slope of 87Sr/86Sr

• Depends upon gradient– Accuracy of age model used

• 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)

Applications in Stratigraphy

• 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

Wierzbowski, 2002

Purpose: Provide 13C & 18O data from brachiopods & belemnites from the Oxfordian (161.2 ± 4.0 MA) and discuss

www.plingfactory.dehttp://www.tonmo.com/science/public/belemnites.php

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)

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Results & Discussion• 2.5-3‰ difference in δ13C between belemnites &

brachiopods – habitat? Vital effect??

•Max δ13C middle Oxfordian –related to sea level change

• δ18O values similar• Uniform values around 0‰

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