Isotopic Fossils
Acknowledgements:
Hedges/Keil/Wakeham U. Washington: Aquatic Org.
Geochemistry
ML Fogel and LA Cifuentes, Isotope fractionation
duringprimary production
JM Hayes: Fractionation of the isotopes of carbon
and hydrogen in biosynthetic processes
AP Murray et al. PhD Thesis, Curtin University
Light Isotope Abundances & Effects
Reactant
An Isotope Effect
is a phenomenon
arising from the
mass difference
between two isotopes
Product
Fractionation
an observable quantity
Question: what’s incongruent about this?
Origins of Mass Dependent Isotopic
Fractionation
Equilibrium in a reversible reaction, where the heavier isotope
concentrated in the more strongly bonded form:
13CO
2(g)
+ H12CO3-(aq) = 12CO2(g) + H13CO3-(aq)
Different rates of diffusive transport where:
12CO
2
diffuses ~1% faster than 13CO2
Different rates of reaction in kinetically controlled
conversions - the light isotope tends to react faster:
most biochemistry
Principles of Isotopic Measurement
Either a pure gas via a duel inlet system for sample and standard
Or a stream of gas containing sample ‘slugs’ interspersed with
standard ‘slugs’
Terminology
NB Standard for δ18O/16O in carbonates is PDB
Terminology
α = is the kinetic fractionation factor =
Where p is product, s is substrate, h is heavy and l is light.
Written precisely this is
A general approximation is
ε is also called the isotope effect or epsilon !!
Equilibrium isotope effects are simply related to kinetic effects by
a = k2/k1
Some Equilibrium Isotope Effects
Isotopic compositions of materials in a
reaction network are complex to compute
but can be very informative about processes
in food chains and also biosynthetic
pathways. The reactions which have most
effect are not necessarily the ones with
bigger e.
Fractionation of C-Isotopes during
Autotrophy
Theoretical max. depletion from RUBISCO from CO2 atm (-7‰) & e = 29.4
therefore δ13C biomass -36.4‰
Usually, a much lower fractionation is observed eg δ13C biomass of plants is
roughly -27‰ ie ε = 20‰
Theoretical models and practical experiments show that [CO2] inside and
outside are different and controlled by diffusion through the stomata. ε not
maximal as a result of water relations and minimization of transpiration
ε = α + (Ci/Ca) (β-α)
where α fractionation factor for diffusion
and b is the fractionation factor for C-fixation.
If Ci/Ca is 1, ie unlimited CO2, e controlled by β.
If Ci/Ca <<1 ε controlled by α
Fractionation of C-Isotopes during
Autotrophy
Formation of oxaloacetate from PEP (Phosphoenolpyruvate) catalysed by PEP
carboxylase
C4 pathway
Mesophyll cell
Bundle-Sheath cell
Question: What are the differences between C4 and CAM plants??
Fractionation of C-Isotopes during
Autotrophy
Questions:
What is the distinction between C4 pathway and C4 plants
What does RUBISCO stand for?
What does the RUBISCO do when CO2 is low and light is high
Is this good or bad for the plant?
What advantages does C4 pathway confer on plants that use
it?
Observations:
C4 evolution seen in the Cenozoic C-isotopic record of terrestrial
organic matter and also in teeth of grazing animals
Cerling, T. E., Ehleringer, J. R., and Harris, J. M., 1998, Carbon dioxide starvation, the
development of C4 ecosystems, and mammalian evolution. Philosophical Transactions of
the Royal Society B. 353: 159-171.
Cerling, T. E., Harris, J. M., 1999, Carbon isotope fractionation between diet and
bioapatite in ungulate mammals and implications for ecological and paleoecological
studies. Oecologia 120: 347-363.
Cerling, T. E., Harris, J. M., and Leakey, M. G., 1999, Browsing and grazing in modern
and fossil proboscideans. Oecologia 120: 364-374.
Ekart, D. D. and T. E. Cerling, 1999, A 400 million year carbon isotope record of
pedogenic carbonate: Implications for atmospheric carbon dioxide. American Journal of
Science. 299: 805-817.
Cerling, T. E., J. M. Harris, B. J. MacFadden, 1998, Carbon isotopes, diets of North
American equids, and the evolution of North American C4 grasslands. In (Griffiths, H.,
Ed.)Stable Isotopes and the Integration of Biological, Ecological, and Geochemicial
Processes.. Bios Scientific Publishers, Oxford, p. 363-379.
Natural Variability in Bulk C-Isotopes
Intramolecular C-isotopic Differences
(DeNiro and Epstein, 1977; Monson and Hayes,
1980,1982; reviewed Hayes, 2001)
Reactions occur between molecules but isotope selectivity
is expressed as chemical bonds that are made or broken at
particular carbon positions.
Isotope effects pertain to those specific positions and
control fractionations only at that reaction site, not
throughout the whole molecule.
To calculate changes in the isotopic compositions of whole molecules
we must first calculate the change at the site and then allow for the rest
of the molecule because the isotopic shift is diluted by mixing with
carbon that is just along for the ride………………..Hayes, 2001
Two Origins for Isoprenoids
The major nonbiological fractionation
process affecting stable hydrogen (and
oxygen) isotopes is the hydrologic cycle in which
water molecules containing lighter isotopes (1H &
16O) are preferentially evaporated and retained
in a cloud (vs. 2H & 18O).
The net result of this fractionation process is that
precipitation at increasingly inland, higher altitude
(cooler) sites is depleted in both 18O and 2H (D).
In general, tree sap isotopically resembles local meteoric water, whereas leaf
water is isotopically depleted. Organic matter is depleted in D versus the leaf
water from which it is biosynthesized (values given above are typical for N.
American plants).
In general, lipids are depleted in D versus lignin and cellulose from the same
plant, whereas cellulose is slightly more depleted than coexisting lignin
(Rundle et al., 1989).
Cellulose is usually analyzed in nitrated form (NO3 replacing OH on each C),
so that only the nonexchangeable H directly bound to C is analyzed.
Because the D in cellulose nitrate reflects local water, woods can be used as a
proxy for the hydrogen isotope composition of past environments in which the
wood was made