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Definitions

IsotopesAtoms of the same element (i.e., same number ofprotons and electrons) but different numbers ofneutrons.

Stable IsotopeDo not undergo radioactive decay, but they may beradiogenic (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, differentisotopes of a chemical element are be sorted bybiological, chemical or physical processes.

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

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

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

What makes for a stable isotopesystem that shows large variation?

1) Low atomic mass

2) Relatively large mass differences between stableisotopes

3) Element tends to form highly covalent bonds

4) Element has more than one oxidation state orforms bonds with a variety of different elements

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

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Since natural variations in isotoperatios are small, we use δ notation

δHX = ((Rsample/Rstandard) -1) x 1000

where R = heavy/light isotope ratio forelement X and units are parts per thousand(or per mil, ‰)

δ13C = (13C/12Csample/13C/12Cstandard) -1) x 1000δ18O = (18O/16Osample/18O/16Ostandard) -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 ofprotons and roughly the same number ofelectrons, hence they undergo the samechemical (and physical) reactions.

2) Differences in mass can, however, influencethe rate or extent of chemical or physicalreactions, or lead to partitioning of isotopesdifferentially among phases.

3) Isotopic sorting during chemical, physical, orbiological processes is called Fractionation.

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Fractionation terminology

Fractionation factor: αA/B = HRA/HRB = (1000 + δHXA)/(1000 + δHXB)

1000(αA/B -1)εA/BEnrichment

δA - δBΔA/BSeparation

FormulaSymbolTermDiscipline

Multiple Approximations1000 lnαA/B ≈ δA - δB = ΔA/B ≈ εA/B

Isotopic consequence of biological carbon pump

CH2O + O2 →CO2 + H2O

depth

δ13C CO2 or CaCO3low high

-5‰

25‰

δ13Ccarb = δ13Cinput + forg(δ13Ccarb- δ13Corg) = -5 + 25forg

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SNOWBALL Earth

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

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Glaciers at sea level near equatorGlaciers at sea level near equator

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Thick layer of calciumThick layer of calciumcarbonate (CaCOcarbonate (CaCO33))above glacial depositsabove glacial deposits

Geochemical evidence thatGeochemical evidence thatphotosynthesis turned offphotosynthesis turned offEvidence?Evidence?

SNOWBALL EarthSNOWBALL Earth

How did it start? Lower solar intensity? Removal of CO2 from air?

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SNOWBALL EarthSNOWBALL Earth

How did it start? Lower solar intensity? Removal of CO2 from air?

SNOWBALL EarthSNOWBALL Earth

Run away Ice spreads from poles, Reflects sunlight, Planet cools

Albedo: reflectivenessof a surface (highernumber - more reflective)

SNOWBALL EarthSNOWBALL EarthHow does it end? Volcanos keep erupting No photosythensis or rock

weathering CO2 levels rise

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SNOWBALL EarthSNOWBALL Earth

Aftermath Greenhouse warming Rapid rock weathering leads to

CaCO3 deposits

Climate and IsotopesOrganisms sequester isotopes into their shells either

• at the same ratios as in seawater• fractionate them in constant or predictable

manner

CaCO3

13C/12C18O/16O

Seawater

δ18O/Temperature Calibration Experiment

T°C

δ18Ocalcite-δ18Owater

Temp d18Oc-d18Ow

30 28.8

25 29.8

20 30.9

15 32.1

10 33.3

5 34.6

H218O + CaC16O3 ⇔ H2

16O + CaC18O16O2

1000lnαcc-water = (2.78x106/T2)-2.89T in in Kelvin

Remember: αcc-water = (1000+δ18Occ)/(1000+δ18Occ)

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