5 Stable and radioactive isotopes

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1 5 Stable and radioactive isotopes

2 Outline 1 Stable isotopes Measuring stable isotopic abundances Equilibrium isotope effects Kinetic isotope effects Rayleigh distillation

3 Isotopes: a mainstay of chemical oceanography Different ocean water masses Marine vs. terrestrially derived organic matter Temperature, geographic settings, transport mechanisms, ecology from fossils Passage of organic elements up trophic levels, climate change, marine productivity, formation and melting of continental glaciers.

Isotopes Bing Bang (15 Ga) H ( 99%) and He ( 1%) Subsequent nuclear reactions 92 naturally occurring elements on Earth number of protons (atomic number, Z) number of

4 Isotopes Bing Bang (15 Ga) H ( 99%) and He ( 1%) Subsequent nuclear reactions 92 naturally occurring elements on Earth number of protons (atomic number, Z) number of neutrons (N) atomic mass (A = Z + N) Atoms of the same element with different numbers of neutrons, and hence different atomic masses, are referred to as isotopes of that element.

5 Properties of isotopes Isotopes exhibit almost identical chemical properties. Heavier isotopes form slightly stronger bonds to other atoms. Molecules containing heavier isotopes move more slowly owing to their greater mass.

6 Chart of nuclides stair step pattern

7 Some of distinctive trends among isotopes 157 have even numbers of both protons and nuetrons (out of 264 stable isotopes). A third of 157 have odd numbers of either protons or neutrons Only 4 stable isotopes ( 2 H, 6 Li, 10 B, and 14 N) contain odd numbers of both protons and neutrons. * * * The number of neutrons in naturally occurring isotopes tends to be equal or greater than the number of protons. * * * Most nuclei beyond Hg, with Z in excess of 80 and A greater than 210, are unstable.

8 Outline 1 Stable isotopes Measuring stable isotopic abundances Equilibrium isotope effects Kinetic isotope effects Rayleigh distillation

9 Analyte gases and standard materials for light isotope analysis Air for N; Belemnite Vienna-PDB (VPDB)

10 (Isotope) Ratio mass spectrometer (IRMS) high vacuum; Fleming s left hand rule; m/z; sequential analysis of sample and standard

11 δ-notation H and L: heavy and light isotopes spl and std: sample and standard R: H/L δ is expressed in parts per thousand ( )

12 Equilibrium isotope effects They are most likely to be exhibited by inorganic chemical species that rapidly interconvert between forms containing the same elements. The extent of isotopic fractionation becomes less as the temperature of the system increases (K is at 0 C and at 30 C).

13 Fractionation factors fractionation factor (α) difference fractionation factor (ɛ) Refer to Appendix 5.1

14 δ 18 O paleotemperature method CaCO 3 (s) + H 2 18 O CaC 18 OO 2 + H 2 O isotopic equilibrium with dissolved CO 2 3? 18 O of the shell unchanged?

15 Laboratory-determined temperature dependence of the oxygen isotope difference H 3 PO 4 to CaCO 3 CO 2 H 2 O CO 2 equilibrated CO 2

16 One of the first applications (Urey et al., 1951) Four cool extremes separated by three warm periods ( diary of a belemnite)

17 The oxygen isotope record recorded in foraminifera in deep sea sediments planktonic vs. benthic; temperature changes + the waxing and waning of glacial ice

18 Kinetic isotope effects It takes place under non-equilibrium physical or chemical conditions. Physical: Consider 12 CO 2 and 13 CO 2 : 1 2 M 44 v 2 44 = 1 2 M 45 v 2 45 v 44 v 45 = = Thus 12 CO 2 travels 1.2% faster than 13 CO 2 (in pure CO 2 gas). Chemical: Heavier isotopes form stronger chemical bonds that break more slowly during chemical reactions.

19 Difference fractionation factors Isotope effects involved with the formation and destruction of organic matter are kinetic.

20 Difference fractionation factors (continued)

21 Exchanges and stable carbon isotope ratios among the reservoirs terrestrial vs. marine; you are what you eat

22 Surface enrichement of carbon isotope of DIC profiles in the Pacific

23 Isotope fractionation of molecular oxygen photosynthesis 0, respiration -20 ( carbon)

$Nitrogen isotope ratios in the sea During respiration, nitrogen isotopes are fractionated$

24 Nitrogen isotope ratios in the sea During respiration, nitrogen isotopes are fractionated by about +3 for each trophic level ( carbon) If animals feed at more than one trophic level?

25 Rayleigh distillation Equilibrium isotope effect for O: 9 at 20 C; 11 at 0 C 10 times larger for H than O. At 0 C the cloud would contain only a quarter of its initial (20 C) water. ( ) R (α 1) t = f (α 1) Ct = R 0 C 0 δ 18 O t = (δ 18 O ) f (α 1) 1000 Refer to Appendix 5.2

26 Oxygen isotopic fractionation among seawater, the atmosphere and rain

27 Annual mean δ 18 O in precipitation as a function of the annual mean air temperature δ 18 O = 0.695T 13.6 It became possible to estimate the temperature of regions where ice accumulates, as far back in time as ancient ice can be dated (Dansgaard, 1964).

28 Enrichment of oxygen isotopes in the surface seawater left behind

Stable Isotope Tracers

Stable Isotope Tracers OCN 623 Chemical Oceanography 5 March 2015 Reading: Emerson and Hedges, Chapter 5, p.134-153 (c) 2015 David Ho and Frank Sansone Outline Stable Isotopes - Introduction & Notation