1. isotope · • eby (2004) principles of environmental geochemistry, thomson & brooks/cole,...
TRANSCRIPT
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Isotopengeochemie
1. Isotope
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Lehrmaterial
CAU - Organische Geochemie – Lehrmaterialien – Bachelor – Sommersemester
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• 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
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C
12.011
Kohlenstoff
Element
Atommasse
Ordnungszahl (Z)
Elementsymbol
6
12
Massenzahl (A)
Elemente des Periodensystems
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C
12.011
Kohlenstoff
6
12
C
12.011
Kohlenstoff
6
13
C
12.011
Kohlenstoff
6
14
Elemente des Periodensystems
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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
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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
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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
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• 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
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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
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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
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Isotope
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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
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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
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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
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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
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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
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Mass Spectrometer Elemental Analyzer
Isotope Ratio Mass Spectrometry
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Isotope Ratio Mass Spectrometry
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Nitrogen (N2) Carbon (CO2)
12C16O16O (44 Da) 12C16O16O (44 Da)
15N14N (29 Da) 14N14N (28 Da)
Isotope Ratio Mass Spectrometry
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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
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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
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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)
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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)
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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)
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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
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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
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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
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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)
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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)
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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
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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
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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
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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
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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
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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
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What is the δ13C of the organic matter?
Stable Isotopes of Organic Matter
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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
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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
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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
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inorganic carbon source
CO2
H2CO3
HCO3-
Organic Matter
Photosynthesis
Solution ɛ = -8‰
ɛ = -10 to -25‰
Carbon Isotopes
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Meyers et al. (1993)
δ13C = -28.3‰
d13Corg versus TOC/TN
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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
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• 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
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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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