Isotopes of the same element (i.e., same number of and ) but different numbers of .

Stable Do not undergo , but they may be radiogenic (i.e., produced by radioactive decay).

Usually the number of protons and neutrons is similar, and the less abundant are often “heavy”, i.e., they have an extra or two.

Why are stable isotopes useful?

• Because of tiny differences in , different isotopes of a are be sorted by biological, chemical or physical processes.

• These naturally produced variations in isotope ratios are small (part per thousand), but easily measured.

• These differences in isotope ratio can be used as natural “labels” or tags.

• These differences can be used to monitor the rate or magnitude of processes.

What makes for a stable isotope system that shows large variation?

1) Low

2) Relatively large mass differences between stable isotopes

3) Element tends to form highly covalent bonds

4) Element has more than one or forms bonds with a variety of different elements

5) Rare isotopes aren’t in too low abundance to be measured accurately

1 Since natural variations in isotope ratios are small, we use δ notation

H δ X = ((Rsample/Rstandard) -1) x 1000 where R = heavy/ isotope ratio for element X and units are parts per thousand (or per mil, ‰)

13 13 12 13 12 δ C = ( C/ Csample/ C/ Cstandard) -1) x 1000 18 18 16 18 16 δ O = ( O/ Osample/ O/ Ostandard) -1) x 1000 i.e., 10‰ = 1% + value = relatively more heavy isotope than standard - value = relatively less heavy than standard

δ18O is spoken aloud as “delta O 18”

Isotope Fractionation

1) Isotopes of an element have same number of protons and roughly the same number of electrons, hence they undergo the same chemical (and physical) reactions. 2) Differences in mass can, however, influence the rate or extent of chemical or physical reactions, or to partitioning of isotopes differentially among phases. 3) Isotopic sorting during chemical, physical, or biological processes is called Fractionation.

2 Fractionation terminology

Fractionation factor: H H H H αA/ = RA/ RB = (1000 + δ XA)/(1000 + δ XB)


Separation ΔA/B δA - δB

Enrichment εA/B 1000(αA/B -1)

Multiple Approximations

1000 lnαA/B ≈ δA - δB = ΔA/B ≈ εA/B

Isotopic consequence of biological pump


CH2O + O2 → CO2 + H2O

13 δ C CO2 or CaCO3 low high



13 13 13 13 δ Ccarb = δ Cinput + forg(δ Ccarb- δ Corg) = -5 + 25forg


It appears that several (2x) between 800 and 600 Myr, and at 2.3 Gyr, the Earth ICED OVER COMPLETELY

5 Glaciers at level near equator

6 Thick layer of carbonate (CaCO3) above glacial deposits

Geochemical evidence that Evidence? photosynthesis turned off


How did it start? Lower solar intensity?

Removal of CO2 from ?


How did it start? Lower solar intensity?

Removal of CO2 from air?


Run away Ice spreads from poles, Reflects , cools

Albedo: reflectiveness of a surface (higher number - more reflective)


How does it end? Volcanos keep erupting No photosythensis or weathering

CO2 levels rise


Aftermath Greenhouse warming Rapid rock weathering to CaCO3 deposits

Climate and Isotopes sequester isotopes into their shells either • at the same ratios as in • fractionate them in constant or predictable manner

CaCO3 Seawater 13C/12C 18O/16O

δ18O/Temperature Calibration Experiment Temp d18Oc-d18Ow 18 16 16 18 16 H2 O + CaC O3 ⇔ H2 O + CaC O O2 30 28.8 25 29.8 6 2 20 30.9 1000lnαcc- = (2.78x10 /T )-2.89 15 32.1 T in in 10 33.3 5 34.6 18 18 Remember: αcc-water = (1000+δ Occ)/(1000+δ Occ)


18 18 δ Ocalcite-δ Owater

9 10 11