Isotopes: Climate, Sea Level,

Ecology

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

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

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

i.e., 10‰ = 1% + value = relatively more H than standard - value = relatively less H than standard

δ18O is spoken aloud as “delta O 18” Don’t ever say “del”. Don’t ever say “parts per mil”. These make you sound like a knuckle-head.

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.

Fractionation mechanisms

Equilibrium Isotope Fractionation A quantum-mechanical phenomenon, driven mainly by differences in the vibrational energies of molecules and crystals containing atoms of differing masses.

Kinetic Isotope Fractionation Occur in unidirectional, incomplete, or branching reactions due to differences in reaction rate of molecules or atoms containing different masses.

Fractionation mechanisms

Equilibrium Isotope Fractionation A quantum-mechanical phenomenon, driven mainly by differences in the vibrational energies of molecules and crystals containing atoms of differing masses.

Kinetic Isotope Fractionation Occur in unidirectional, incomplete, or branching reactions due to differences in reaction rate of molecules or atoms containing different masses.

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

Discipline Term Symbol Formula Geochemistry Often equilibrium fractionations, put heavy

isotope enriched substance in numerator Separation ΔA-B δA - δB

Enrichment εA-B 1000(αA/B -1)

Biology Often kinetic fractionations, put light isotope enriched substance in numerator

Discrimination ΔA-B 1000(αA/B -1)

Enrichment εA-B 1000 lnαA/B

Multiple Approximations 1000 lnαA-B ≈ δA - δB ≈ ΔA-B ≈ εA-B

Climate and Isotopes Organisms sequester isotopes into their shells but

fractionate them in constant or predictable manner

CaCO3

13C/12C 18O/16O

Seawater

Oxygen Isotopes & Water

16O: 99.763% 17O: 0.0375% 18O: 0.1995%

δ18O/Temperature Calibration Experiment

T°C

δ18Ocalcite-δ18Owater

Temp d18Oc-d18Ow30 28.825 29.820 30.915 32.110 33.35 34.6

H218O + CaC16O3 ⇔ H2

16O + CaC18O16O2

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

0

5

10

15

20

25

30

35

28 29 30 31 32 33 34 35

Deep-sea Oxygen Isotope Record

Benthic Foraminifera Minimal variations in temperature & salinity

A record of global temperature and ice volume

During H2O evaporation, 16O concentrated in vapor Vapor pressure: H2

16O > H218O

at 25°C, αl-v = 1.0092 if δ18Ol = 0.0‰, then δ18Ov = -9.2‰

18O/16OV/18O/16OVo = fα-1

where f is fraction of vapor remaining, and Vo is initial vapor

For vapor: δ18OV = (δ18OVo + 1000)f (α-1) -1000 For rain: δ18OR = α(δ18OVo + 1000) -1000

Bowen & Wilkinson (2002) Geology

Fricke & O’Neil (1999) EPSL

Sea-level Change 150 ka to present

120 meters

Carbon isotope correlation

How to measure the “integrated” isotope composition of vegetation?

Bump et al. (2007) PRSB

Koch et al. (2003) GSA Spec. Pub.

-8-10-12-14-16

δ13C(PDB)

25r

25n

24r

24n.

3nTi

-5Ti

ffani

anC

f-2C

f-1C

f-3C

lark

fork

ian

Was

atch

ian

Wa-

4W

a-0

to W

a-3

Wa-

6W

a-7

Pale

ocen

eEo

cene

NAL

MA

Sub-

Zone

s

Wa-5

24n.2n

Age(Ma) Po

larit

y

57.5

57.0

56.5

56.0

55.5

55.0

54.5

54.0

53.5

53.0

52.5

43210-1

Clarks Fork BasinCentral Bighorn BasinMcCullough Peak

Bighorn Basinsoil carbonates

Benthic foraminifera

Benthic MarineForaminifera

δ13C(PDB)

δ18O(‰, PDB)

δ13C(‰, PDB)

0 1 2 3

C26

C25

C24

C23

-1-0.500.51

50

52

54

56

58

60Colder Warmer

Early EoceneWarm Interval

Ma

-1

P-EThermal

Maximum

More 12C More 13C

δ18O(‰, PDB)

δ13C(‰, PDB)

0 1 2 3

C26

C25

C24

C23

-1-0.500.51

50

52

54

56

58

60Colder Warmer

Early EoceneWarm Interval

Ma

-1

P-EThermal

Maximum

More 12C More 13C

-8-10-12-14-16

δ13C(PDB)

25r

25n

24r

24n.

3nTi

-5Ti

ffani

anC

f-2C

f-1C

f-3C

lark

fork

ian

Was

atch

ian

Wa-

4W

a-0

to W

a-3

Wa-

6W

a-7

Pale

ocen

eEo

cene

NAL

MA

Sub-

Zone

s

Wa-5

24n.2n

Age(Ma) Po

larit

y

57.5

57.0

56.5

56.0

55.5

55.0

54.5

54.0

53.5

53.0

52.5

43210-1

Clarks Fork BasinCentral Bighorn BasinMcCullough Peak

Bighorn Basinsoil carbonates

Benthic foraminifera

Benthic MarineForaminifera

δ13C(PDB)

-8-10-12-14-16

δ13C(PDB)

25r

25n

24r

24n.

3nTi

-5Ti

ffani

anC

f-2C

f-1C

f-3C

lark

fork

ian

Was

atch

ian

Wa-

4W

a-0

to W

a-3

Wa-

6W

a-7

Pale

ocen

eEo

cene

NAL

MA

Sub-

Zone

s

Wa-5

24n.2n

Age(Ma) Po

larit

y

57.5

57.0

56.5

56.0

55.5

55.0

54.5

54.0

53.5

53.0

52.5

43210-1

Clarks Fork BasinCentral Bighorn BasinMcCullough Peak

Bighorn Basinsoil carbonates

Benthic foraminifera

Benthic MarineForaminifera

δ13C(PDB)

T. brandti T. belgica T. asiatica

20,000 km in ~20,000 years (living mammals 1 to 10 km/yr)

Rapid speciation along dispersal

Crossed Turgai and Bering Straits in all directions

Smith et al. (2006) PNAS