Definitions
Isotopes Atoms of the same element (i.e., same number of protons and electrons) but different numbers of neutrons.
Stable Isotope Do not undergo radioactive decay, but they may be radiogenic (i.e., produced by radioactive decay).
Usually the number of protons and neutrons is similar, and the less abundant isotopes are often “heavy”, i.e., they have an extra neutron or two.
Why are stable isotopes useful?
• Because of tiny differences in mass, different isotopes of a chemical element 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 atomic mass
2) Relatively large mass differences between stable isotopes
3) Element tends to form highly covalent bonds
4) Element has more than one oxidation state 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/light 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 lead 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/B = RA/ RB = (1000 + δ XA)/(1000 + δ XB)
Discipline Term Symbol Formula
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 carbon pump
depth
CH2O + O2 → CO2 + H2O
13 δ C CO2 or CaCO3 low high
25‰
-5‰
13 13 13 13 δ Ccarb = δ Cinput + forg(δ Ccarb- δ Corg) = -5 + 25forg
3 4 SNOWBALL Earth
It appears that several times (2x) between 800 and 600 Myr, and at 2.3 Gyr, the Earth ICED OVER COMPLETELY
5 Glaciers at sea level near equator
6 Thick layer of calcium carbonate (CaCO3) above glacial deposits
Geochemical evidence that Evidence? photosynthesis turned off
SNOWBALL Earth
How did it start? Lower solar intensity?
Removal of CO2 from air?
How did it start? Lower solar intensity?
Removal of CO2 from air?
SNOWBALL Earth
Run away Ice spreads from poles, Reflects sunlight, Planet cools
Albedo: reflectiveness of a surface (higher number - more reflective)
SNOWBALL Earth
How does it end? Volcanos keep erupting No photosythensis or rock weathering
CO2 levels rise
8 SNOWBALL Earth
Aftermath Greenhouse warming Rapid rock weathering leads to CaCO3 deposits
Climate and Isotopes Organisms sequester isotopes into their shells either • at the same ratios as in seawater • 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-water = (2.78x10 /T )-2.89 15 32.1 T in in Kelvin 10 33.3 5 34.6 18 18 Remember: αcc-water = (1000+δ Occ)/(1000+δ Occ)
T°C
18 18 δ Ocalcite-δ Owater
9 10 11