1. Oxygen isotopes in marine carbonate
SOES Revision Lecture

2. Papers Zachos, J., et al. (2001). Lear C. H., et al. (2000).
Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present. Science 292(5517):
Lear C. H., et al. (2000).
Cenozoic Deep-Sea Temperatures and Global Ice Volumes from Mg/Ca in Benthic Foraminiferal Calcite. Science 287(5451): 4.

3. Outline What is a proxy? Foraminifera
Controls on oxygen isotopes in calcite
Composition of ambient seawater
Temp dependent equilibrium fractionation
Disequilibrium or vital effects
Summary

4. What is a Proxy?
A measurable parameter that stands in for a desired but unobservable target parameter
e.g. Mg/Ca in biogenic calcite → Temp proxy
Problems?
Proxies are not perfect as they are often dependent on more than one variable
Mg/Ca
Temp [CO32-]

5. Oxygen isotopes: Delta notation
Values given in per mil ‰
Allows comparison between measurements
Positive δ18O
Enrichment of heavier isotope (18O) in the sample relative to a standard
Negative δ18O
Depletion of heavier isotope (18O) in the sample relative to a standard

6. What can we measure δ18Oc in?
Courtesy of Jan Derk
Courtesy of Dave Bunnell
Under Earth Images
Courtesy of
Earth Science
Australia (link)
Molluscs
Coral
Speleothems
Bulk sed
(nanno fossils)
Courtesy of Wikipedia
Foraminifera

7. Foraminifera Protists (single-celled) Microscopic (50 -1000μm)
University of Southampton
Protists (single-celled)
Microscopic ( μm)
Planktonic & benthic forms
Most forams precipitate a calcite shell (CaCO3) or ‘test’
Good environmental recorders because:
Good fossilisation potential
Long record
Planktonic ~ 200 Ma
Benthic ~ 550 Ma
Wide distribution
Abundant!
Courtesy G Schmedl
University of Southampton

8. δ18Oc in forams Factors controlling δ18Oc Key assumption:
δ18O of ambient seawater
Temperature at which shell precipitates
Key assumption:
Forams precipitate their test in isotopic equilibrium with seawater
Complicated by
Disequilibrium processes or “vital effects”
Diagenetic alteration

9. δ18O of ambient sea water (δ18Osw)
Variation in δ18Osw on long timescales
Changes in continental ice volume
Rayleigh fractionation
Residual vapour
Ice = 18Osw
Ice = 18Osw
Net transfer of heat and moisture to higher latitudes
Evaporation in tropics
16O pref into vapour phase
University of Southampton

10. δ18O of ambient sea water (δ18Osw)
Glacial & Interglacial cycles
Ice = 18Osw
Data: EPICA Dome C Ice Core Data
PEAKS → INTERGLACIALS (Less ice)
TROUGHS → GLACIALS (More ice)
University of Southampton

11. Temp dependent equilibrium fractionation
Equilibrium fractionation factor (δc-w) between calcite and water changes with temperature (O’Neil 1969)
1 ‰ change in δ18Oc ≈ 4°C
T = δ18Oc
Effectively the diff between
δ18Oc and δ18Ow decreases
University of Southampton

12. 18Oc and water depth T Depth + δ18Oc - Depth Depth T
Depth δ18Oc (more +ve)
University of Southampton
Application: Determine depth habitat of ancient foraminifera
Sexton et al., 2006

13. δ18O of ambient sea water (δ18Osw)
Glacial & Interglacial cycles
PEAKS → INTERGLACIALS (Less ice )
TROUGHS → GLACIALS (More ice )
and/or warming)
and/or cooling)

14. 18Oc as a palaeothermometer
δc measured δ18Oc of calcite
δw = δ18O seawater
Present day δw = 0 (SMOW)
δw has not remained constant through time!
From: Zachos, J.C., Pagani, M., Sloan, L., Thomas, E.,Billups, K.,(2001) ‘Trends, rhythms, and aberrations in global
climate 65 Ma to present’, Science, v. 292, p. 686–693. Reprinted with permission from AAAS. This figure may be used
for non-commercial, classroom purposes only. Any other uses requires the prior written permission from AAAS.

15. Separating temp & ice vol signals
δ18Oc = temp effect + ice volume effect
Multi proxy approach
Mg/Ca in foram calcite - independent temp proxy
TEX86 - membrane lipids of crenarchaeota (varies with SSTs).
UK37 - C37 alkenones from organic matter (varies with SSTs).
Sr/Ca in corals (varies with SSTs).
Paired planktonic/benthic foram δ18O ratios.

16. Disequilibrium fractionation (vital effects)
Deviation of foraminiferal isotopic composition from thermodynamic equilibrium due to physiological factors causing fractionation
Ontogeny
Gametogenic calcite
Photosymbionts
Carbonate ion effect

17. 1. Ontogeny δ18Oc of test with growth
Juvenile chambers depleted (~ -1.15‰)
Final chamber less depleted (~ -0.3 ‰)
Relative to equilibrium
Depth migration through water column from shallow to deep through life cycle?
More
+ve
δ18O
More
-ve
δ18O
Courtesy of NOAA

18. 2. Gametogenic calcite
Several planktonic species secrete veneer of calcite on test at end of life cycle
Up to 30% mass of test
“Thick-walled specimens” e.g. G.sacculifer & O.universa
Gametogenic calcite heavier (more +ve δ18O) relative to earlier stages of shell growth.
Cause unknown
calcified in deeper, colder waters?
Courtesy: Earth Sciences Australia
University of Southampton

19. 3. Photosymbionts effect
Algae (diatoms or dinoflagellates) that live with planktonic forams
Between pseudopodia or in chambers
Mutual benefits for both
Irradiance, Growth rate, δ18O
Kinetic discrimination against heavier isotopes

20. 4. Carbonate ion effect [CO32-]
[CO32-], δ18O
Species specific
A-biogenic effect (also seen in inorganic calcite)
Kinetic fractionation
Again related to increase
in growth rates
University of
Southampton
O.universa
Reprinted by permission from Macmillan Publishers Ltd: Spero, H., Bijma, J., Lea, D.W.,
Bemis, E.B., (2000) Seawater carbonate chemistry and carbon and oxygen isotopes
during experiments with planktonic foraminifera Orbulina universa and Globigerina
bulloides, Nature, v. 390, p (copyright 1997), Not under CC licence.

21. Summary δ18Oc    Temp effect + Ambient seawater + Vital effects
  
Temp effect + Ambient seawater + Vital effects
T = 18O
Multi-proxy approach
eg. Mg/Ca
Ice = 18O
Volume of continental ice
Preferential removal of lighter isotope into ice through Rayleigh fractionation
Ontogeny
Gametogenic calcite
Photosymbionts
Carbonate ion effect

22. Summary δ18Oc    Temp effect + Ambient seawater + Vital effects
  
Temp effect + Ambient seawater + Vital effects
Multi-proxy temperature estimation
Ice free conditions
(Cenozoic before 34Ma)
Core top analysis
Culture experiments
T = 18O
Multi-proxy approach
eg. Mg/Ca
Ice = 18O
Volume of continental ice
Preferential removal of lighter isotope into ice through Rayleigh fractionation
Ontogeny
Gametogenic calcite
Photosymbionts
Carbonate ion effect

23. Summary Questions? δ18Oc   
  
Temp effect + Ambient seawater + Vital effects
T = δ18O
Multi-proxy approach
eg. Mg/Ca
Ice = δ18O
Volume of continental ice
Preferential removal of lighter isotope into ice through Rayleigh fractionation
Ontogeny
Gametogenic calcite
Photosymbionts
Carbonate ion effect
Questions?

24. Rayleigh fractionation
Initial δ18O of liquid
Initial δ18O of water vapour
First condensate enriched in 18O making residual vapour isotopically lighter
δ18Ovapour
Final δ18O of water vapour
Much more -ve
Fraction of vapour
removed by condensation 1

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