1. Isotope · • Eby (2004) Principles of Environmental Geochemistry, Thomson & Brooks/Cole,...

Isotopengeochemie

1. Isotope

Lehrmaterial

CAU - Organische Geochemie – Lehrmaterialien – Bachelor – Sommersemester

• Allègre (2008) Isotope Geology, Cambridge Press, pp. 512

• Eby (2004) Principles of Environmental Geochemistry, Thomson & Brooks/Cole, pp. 514

• Engel & Macko (1993) Organic Geochemistry, Plenum Press, pp. 861

• Faure (1998) Principles and Applications of Geochemistry, Prentice Hall, pp. 600

• Hoefs (2009) Stable Isotope Geochemistry. Springer Press, pp. 285

• Rundel et al. (1988) Stable Isotopes in Ecological Research, Springer-Verlag, pp. 525

• Tyson (1995) Sedimentary Organic Matter, Chapman & Hall, pp. 615

Literatur

C

12.011

Kohlenstoff

Element

Atommasse

Ordnungszahl (Z)

Elementsymbol

6

12

Massenzahl (A)

Elemente des Periodensystems

C

12.011

Kohlenstoff

6

12

C

12.011

Kohlenstoff

6

13

C

12.011

Kohlenstoff

6

14

Elemente des Periodensystems

C

12.011

Kohlenstoff

6

12 C

12.011

Kohlenstoff

6

13 C

12.011

Kohlenstoff

6

14

Isotope Menge (%) Masse (u)

98.9 12.000000

1.1 13.003354

< 10-9 14.003241

C 12

6

13

6

14

6

C

C

Atommasse von C = 0.989*12.000 + 0.011*13.003 + 10-11*14.003 = 12.011

Elemente des Periodensystems

Isotope sind unterschiedliche Arten von Atomkernen eines chemischen

Elements, die dieselbe Anzahl an Protonen (d.h. gleiche Ordnungszahl)

aber eine unterschiedliche Anzahl an Neutronen besitzen.

A

Z Elementsymbol

A = Massenzahl

(Anzahl an Protonen

und Neutronen im

Atomkern)

Z = Ordnungszahl

Isotope

1. Stabile Isotope sind Nuklide

eines Elements die (scheinbar)

nicht weiter in ein anderes Nuklid

weiter zerfallen (z.B. 1H, 2H, 12C, 13C, 14N, 15N)

2. Radioaktive Isotope sind Atome mit

einem nicht stabilem Nuklid, die entlang

einer radioaktiven Zerfallsreihe in ein stabiles

Isotope eines anderen Elemente übergehen.

Während dieses Prozesses sendet das

Radionuklid subatomare Partikel und/oder

Röntgenstrahlung aus

Isotope

2. Typen von Isotopen

Paläoumweltstudien

Paläoökologie

Altersbestimmung

• Die meisten Elemente bestehen aus einer Mixtur aus

stabilen und radioaktiven (d.h. unstabilen) Isotopen

• Zurzeit sind ~250 stabile und über ~3050 radioaktive Isotope

bekannt

• 26 Elemente kommen in monoisotopischer Form vor, d.h.

sie besitzen nur ein stabiles Isotop (dies sind z.B. Beryllium,

Fluor, Natrium, Aluminium, Phosphor, Gold oder Plutonium)

Isotope

The Symmetric Rule states that in a stable nuclide with low atomic number the number of protons is approximately equal to the number of neutrons

The electrostatic Coulomb

Repulsion of the positively

charged protons grows rapidly

with increasing Z. Therefore,

more neutrons than protons are

incorporated into the nucleus with

increasing atomic weight

Neu

tro

ns

Protons

1.2:1

1.4:1

1.5:1

Hoefs (2009)

Isotones

Iso

top

es

Stabilität von Isotope

The Oddo-Harkins rule argues

that elements with odd atomic

numbers have one unpaired proton

and are more likely to capture

another, thus increasing their

atomic number. In elements with

even atomic numbers, protons are

paired, with each member of the

pair offsetting the spin of the other,

enhancing stability

Z-N combination Number of stable nuclides

Even-even 160

Even-odd 56

Odd-even 50

Odd-odd 5

Stabilität von Isotope

Isotope

Mass differences of the

light elements are fairly

large enabling a rapid

measurement of isotopic

differences

Advances in analytical

precision allow the

analysis of isotopic

differences of heavier

elements (Fe, Zn) with

less pronounced mass

differences

100

80

60

40

20

0 O s Fe Zn N C H

Elements

Mass d

iffe

ren

ce

s (

%)

Mass Differences of Isotopes

Element Stable Isotope Mass Abundance Mass

difference (%)

Hydrogen 1H 2H

1.007825

2.014000

99.985

0.015 99.8

Carbon 12C 13C

12.000000

13.003355

98.90

1.10 8.36

Nitrogen 14N 15N

14.003074

15.000108

99.63

0.37 7.12

Oxygen

16O 17O 18O

15.994915

16.999131

17.999160

99.762

0.038

0.200

12.5

Sulfur

32S 33S 34S 35S

31.972070

32.971456

33.967866

35.967080

95.02

0.75

4.21

0.02

6.24

Mass Differences of Isotopes

modified after Bigeleisen (1965)

Schematic potential energy

curve for the interaction of two

atoms in a stable molecule

repulsive

forces

Attractive

forces

r

E = ½h x v

h = Planck’s constant

v = frequency of vibration

Isotope Effects

E = (n + ½)hv

Interatomic distance

Pote

ntial en

erg

y (

kca

l/m

ole

)

ZPE

109.4

Dissociated atoms

103.2

104

105.3

H-H

H-D

D-D

where n = vibrational energy level (n = 0,1,2, etc)

h = Planck’s constant (6.624 10-34 Jsec-1)

v = frequency

v = 1

2

ks

μ 1

2 ks ( 1

ma

1

mb + ) =

Zero-Point Energy of Molecules with Different Isotopes

Property H216O D2

16O H218O

Density 0.997 1.1051 1.1106

Temperature of greatest density (°C) 3.98 11.24 4.30

Melting point (°C) 0.00 3.81 0.28

Boiling Point (°C) 100.00 101.42 100.14

Vapor Pressure 760.00 721.60

Viscosity 1.002 1.247 1.056

Physical Properties associated with Isotopes

Mass Spectrometer Elemental Analyzer

Isotope Ratio Mass Spectrometry

Isotope Ratio Mass Spectrometry

Nitrogen (N2) Carbon (CO2)

12C16O16O (44 Da) 12C16O16O (44 Da)

15N14N (29 Da) 14N14N (28 Da)

Isotope Ratio Mass Spectrometry

The collectors measure the number of ionized molecules hitting for the

given masses of carbon and oxygen

The IRMS reports ratios - not the abundance - of individual isotopes

For CO2, three masses are reported 44, 45, 46

Composition Mass Abundance 12C 16O 16O 44 >99 % 13C 16O 16O 45 <1 % 12C 16O 18O 46 <1 %

Composition Mass Abundance 12C 16O 16O 44 >99 % 13C 16O 16O 45 <1 % 12C 16O 18O 46 <1 %

Mass 45/44 for δ13C Composition Mass Abundance 12C 16O 16O 44 >99 % 13C 16O 16O 45 <1 % 12C 16O 18O 46 <1 %

Mass 46/44 for δ18O

Measurement of Stable Carbon and Oxygen Isotopes

The isotope ratio reflects the proportion of heavy versus light isotopes in a given sample or standard and is expressed as:

R = Xh

Xl

Rcarbon = 13C 12C

Where Xh and Xl refer to the heavier (e.g., 2H, 13C, 15N, 18O, 34S) and the lighter isotope (e.g., 1H, 12C, 14N, 16O, 32S), respectively.

Rnitrogen = 15N 14N

The Isotope Ratio

The isotope ratio is given in δ notation (the calculated isotope ratio

multiplied by 1000) to make the resulting ratio more meaningful

Rsample = sample isotope ratio (e.g. 13C:12Csample)

Rstandard = standard isotope ratio (e.g. 13C:12Cstandard)

The δ value is given as per mil (‰) difference compared to a standard

Rsample – Rstandard

Rstandard δ (‰) = x 103

The Delta Value (d)

The delta (δ) notation is most commonly used in reporting isotopic compositions of biological and geological materials and relates the isotopic ratio of a sample to that of a standard:

Rsample = isotope ratio of the sample (e.g. 13C:12Csample) Rstandard = isotope ratio of the standard (e.g. 13C:12Cstandard)

Rsample – Rstandard

Rstandard δX (‰) = x 103

13C/12Csample – 13C/12Cstandard

13C/12Cstandard δ13C (‰) = x 103

The Delta Value (d)

heavier lighter

enriched depleted

more positive more negative

δ + -

If a sample is said to have a δ13C value of -27‰ then it is:

27 parts in 1000 depleted in 13C compared to the standard

If a sample is said to have a δ13C value of +5‰ then it is:

5 parts in 1000 enriched in 13C compared to the standard

12C

13C

The Delta Value (d)

SMOW (standard mean oceanic water)

V-SMOW (Vienna standard mean ocean water)

SLAP (standard light Antarctic precipitation)

Oxygen and hydrogen

isotope ratios in water

PDB (PeeDee belemnite) = oxygen and carbon isotope ratios in organic

matter and carbonates

V-PDB (Vienna PeeDee belemnite) = δ13CNBS-19/V-PDB = 1.95

δ18ONBS-19/V-PDB = -2.20

Air = nitrogen isotopes

CDT (Canyon Diablo troilite) = sulfur isotopes

© M. Kampf

Isotope Standards

The standard employed for 13C analysis was originally the Pee Dee

Belemnite (PDB)

It is the rostrum of a Cretaceous belemnite (belemnitella americana) found

in the Pee Dee Formation in South Carolina

The 13C:12C ratio of its rostrum is anomalously high and this 13C values was

established as zero

Pee Dee Belemnite

Element Standard Ratio

Hydrogen V-SMOW 2H/1H = 155.76 x 10-6

Carbon PDB* 13C/12C = 1123.75 x 10-5

Oxygen V-SMOW 18O/16O = 2005.2 x 10-6

PDB* 18O/16O = 2067.2 x 10-6

Nitrogen NBS-14 15N/14N = 367.6 x 10-5

Sulfur CDT 34S/32S = 449.94 x 10-4

from Kyser (1987) * PDB is now exhausted and replaced by NBS-19

Stable Isotope Ratios of Standards

The partitioning of isotopes between two substances or two phases of the same substance with different isotope ratios is called isotopic fractionation. The main phenomena producing isotopic fractionation are:

1. Isotope exchange reactions (equilibrium isotope distribution)

2. Kinetic processes that depend primarily on differences in reaction rates of

isotopic molecules

Isotopic Fractionation (a)

For isotope exchange reactions in geochemistry, the equilibrium constant K is often replaced by the fractionation factor α. The fractionation factor is defined as the ratio of a number of any two isotopes in one chemical compound A divided by the corresponding ratio for another chemical compound B:

αP-S = Xh,p/Xh,s

Xl,p/Xl,s

RP

RS

=

where Xh is the heavy isotope, Xl is the light isotope, s is the substrate, and p is the product.

Isotopic Fractionation (a)

In equilibrium reactions forward and backward reaction rates are

equal for each isotope

Only occurs in closed systems at chemical equilibrium

equilibrium non-equilibrium

Light Isotope

Heavy Isotope

Equilibrium Reactions

For example 18O has a equilibrium fractionation factor of (αl-v) of 1.0098 at 20°C for the liquid-vapor phase transition This means that the δ18O of the water phase is 9.8‰ higher than the one of vapor at equilibrium

Equilibrium Reactions

Kinetic fractionation reactions are unidirectional reactions in which

reaction rates are dependent on the masses of the isotopes and their

vibrational energies

Due to the continues removal of reactive products the isotopic fractionation associated with the kinetic fractionation reactions is much larger compared to equilibrium fractionation

Kinetic Fractionation Reactions

The Rayleigh fractionation is an exponential relation that describes the partitioning of isotopes between reservoirs as one reservoirs decreases in size

1. Material is continuously removed

2. The fractionation is always described by the fractionation factor

3. α does not change during the process

(R/R°) = (Xl/ Xl°)α-1

Where R is the ratio of isotopes (e.g. 18O/16O) in the reactant, R° is the initial ratio, Xl is amount of the more abundant lighter isotope (e.g. 16O), and Xl° is the initial concentrations

Rayleigh Equation

Rayleigh Fractionation

A

B

D

E

C

+50

+40

+30

+20

+10

0

-10

δ1

8O

(‰

)

0.75 0.5 0.25 1 0

Open System A = remaining water B = Vapour C = accumulated vapour

Closed System D = remaining water E = Vapor

Residual water fraction

Rayleigh Fractionation

δ13C of alga depends on (1) the inorganic carbon source and (2) the isotopic

fractionation associated with its uptake

photic zone given δ13C value of alga

δ13CCO2 ~ -8‰

Stable Isotopes of Organic Matter

What is the δ13C of the organic matter?

Stable Isotopes of Organic Matter

38

1. Sampling (0.5 gr)

2. Grinding (Swing mill, mortar)

3. Decalcification (10% HCl + Neutralization)

4. Sample preparation (tin cups)

Carbonates OM (C3)

100% 100%

-28‰ +1‰ -15‰

50%

Mixture

Sample Preparation for Isotope Measurements

Requirements

(1) 13C:12C of sample and standard & (2) δ equation

13C:12Csample is measured

The measured 13C:12C value is 0.0010921

0.0010921 – 0.0011237

0.0011237 x 103 δ13Corg (‰) =

δ13Corg (‰) = -28.1

Rsample-Rstandard

Rstandard δ13Corg (‰) = x 103

Isotope Measurement

What is the δ13C of the organic matter? δ13Corg = -28.1‰

The 13C:12C ratios is 28 parts-per thousand or ca. 3% lower than the 13C:12C of the VPDB standard

Stable Isotopes of Organic Matter

inorganic carbon source

CO2

H2CO3

HCO3-

Organic Matter

Photosynthesis

Solution ɛ = -8‰

ɛ = -10 to -25‰

Carbon Isotopes

Meyers et al. (1993)

δ13C = -28.3‰

d13Corg versus TOC/TN

Terrestrial runoff

Algae

Bacteria

Algae: Diatoms, dinoflagellates, green algae (Botryococcus) Bacteria: green sulfur bacteria, purple non-sulfur bacteria

Zooplankton

Bacterial reworking

Oxidation

Heterotrophic bacteria

Inorganic source

Atmospheric deposition

Aquatic macrophytes

Chemocline

Organic matter Sources

• Isotopic signal of the source

• Availability of the inorganic nutrient source

• Growth rate

• Cell Size

• Uptake and diffusion through the cell membrane

• Enzymatic pathways involved in organic matter production

• Decay of organic matter

• Temperature

• Salinity

Factors affecting the isotope signal

Datum Thema

10.04.19 Isotope – Definition, Messung und Formeln

17.04.19 Kohlenstoffisotope (Organik) und deren Applikation

24.04.19 Kohlenstoffisotope (Anorganik) und deren Applikation

08.05.19 Stickstoffisotope und deren Applikation

15.05.19 Wasserstoffisotope und deren Applikation

22.05.19 Organische Geochemie (Schwark)

29.05.19 Organische Geochemie (Schwark)

05.06.19

Organische Geochemie (Schwark)

19.06.19 Organische Geochemie (Schwark)

26.06.19 Organische Geochemie (Schwark)

03.07.19 Organische Geochemie (Schwark)

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