Presentation on theme: "Isotopic Fossils Acknowledgements:"— Presentation transcript:
1 Isotopic Fossils Acknowledgements: Hedges/Keil/Wakeham U. Washington: Aquatic Org.GeochemistryML Fogel and LA Cifuentes, Isotope fractionationduringprimary productionJM Hayes: Fractionation of the isotopes of carbonand hydrogen in biosynthetic processesAP Murray et al. PhD Thesis, Curtin University
2 Light Isotope Abundances & Effects Reactant ProductAn Isotope Effect Fractionationis a phenomenon an observable quantityarising from themass differencebetween two isotopesQuestion: what’s incongruent about this?
3 Origins of Mass Dependent Isotopic Fractionation Equilibrium in a reversible reaction, where the heavier isotopeconcentrated in the more strongly bonded form:13CO2(g) + H12CO3-(aq) = 12CO2(g) + H13CO3-(aq)Different rates of diffusive transport where:12CO2 diffuses ~1% faster than 13CO2Different rates of reaction in kinetically controlledconversions - the light isotope tends to react faster:most biochemistry
4 Principles of Isotopic Measurement Either a pure gas via a duel inlet system for sample and standardOr a stream of gas containing sample ‘slugs’ interspersed withstandard ‘slugs’
5 NB Standard for δ18O/16O in carbonates is PDB TerminologyNB Standard for δ18O/16O in carbonates is PDB
6 Terminology ε is also called the isotope effect or epsilon !! α = is the kinetic fractionation factor =Where p is product, s is substrate, h is heavy and l is light.Written precisely this isA general approximation isε is also called the isotope effect or epsilon !!Equilibrium isotope effects are simply related to kinetic effects bya = k2/k1
8 Isotopic compositions of materials in a reaction network are complex to computebut can be very informative about processesin food chains and also biosyntheticpathways. The reactions which have mosteffect are not necessarily the ones withbigger e.
9 Fractionation of C-Isotopes during Autotrophy Theoretical max. depletion from RUBISCO from CO2 atm (-7‰) & e = 29.4therefore δ13C biomass -36.4‰Usually, a much lower fractionation is observed eg δ13C biomass of plants isroughly -27‰ ie ε = 20‰Theoretical models and practical experiments show that [CO2] inside andoutside are different and controlled by diffusion through the stomata. ε notmaximal as a result of water relations and minimization of transpirationε = α + (Ci/Ca) (β-α)where α fractionation factor for diffusionand 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
10 Fractionation of C-Isotopes during Autotrophy Formation of oxaloacetate from PEP (Phosphoenolpyruvate) catalysed by PEPcarboxylaseC4 pathwayMesophyll cell Bundle-Sheath cellQuestion: What are the differences between C4 and CAM plants??
12 Questions:What is the distinction between C4 pathway and C4 plantsWhat does RUBISCO stand for?What does the RUBISCO do when CO2 is low and light is highIs this good or bad for the plant?What advantages does C4 pathway confer on plants that useit?Observations:C4 evolution seen in the Cenozoic C-isotopic record of terrestrialorganic matter and also in teeth of grazing animals
13 Cerling, T. E. , Ehleringer, J. R. , and Harris, J. M Cerling, T. E., Ehleringer, J. R., and Harris, J. M., 1998, Carbon dioxide starvation, thedevelopment of C4 ecosystems, and mammalian evolution. Philosophical Transactions ofthe Royal Society B. 353:Cerling, T. E., Harris, J. M., 1999, Carbon isotope fractionation between diet andbioapatite in ungulate mammals and implications for ecological and paleoecologicalstudies. Oecologia 120:Cerling, T. E., Harris, J. M., and Leakey, M. G., 1999, Browsing and grazing in modernand fossil proboscideans. Oecologia 120:Ekart, D. D. and T. E. Cerling, 1999, A 400 million year carbon isotope record ofpedogenic carbonate: Implications for atmospheric carbon dioxide. American Journal ofScience. 299:Cerling, T. E., J. M. Harris, B. J. MacFadden, 1998, Carbon isotopes, diets of NorthAmerican equids, and the evolution of North American C4 grasslands. In (Griffiths, H.,Ed.)Stable Isotopes and the Integration of Biological, Ecological, and GeochemicialProcesses.. Bios Scientific Publishers, Oxford, p
15 Intramolecular C-isotopic Differences (DeNiro and Epstein, 1977; Monson and Hayes, 1980,1982; reviewed Hayes, 2001)Reactions occur between molecules but isotope selectivityis expressed as chemical bonds that are made or broken atparticular carbon positions.Isotope effects pertain to those specific positions andcontrol fractionations only at that reaction site, notthroughout the whole molecule.To calculate changes in the isotopic compositions of whole moleculeswe must first calculate the change at the site and then allow for the restof the molecule because the isotopic shift is diluted by mixing withcarbon that is just along for the ride………………..Hayes, 2001
19 The major nonbiological fractionation process affecting stable hydrogen (andoxygen) isotopes is the hydrologic cycle in whichwater molecules containing lighter isotopes (1H &16O) are preferentially evaporated and retainedin a cloud (vs. 2H & 18O).The net result of this fractionation process is thatprecipitation at increasingly inland, higher altitude(cooler) sites is depleted in both 18O and 2H (D).
20 In general, tree sap isotopically resembles local meteoric water, whereas leaf water is isotopically depleted. Organic matter is depleted in D versus the leafwater 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 sameplant, 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 aproxy for the hydrogen isotope composition of past environments in which thewood was made