OC583 Isotope Biogeochemistry
• Paul Quay, Rm 417, 5-8061, pdquay@u.
• Meets Tues and Thurs at 12:00 – 1:20 in Rm 425 OSB
• Lectures, problem sets, paper discussions
• Bring lecture notes and figures to class
• Grade: midterm, problem sets and project (either a
proposal or oral presentation)
• Web page: http://courses.washington.edu/oc583
1
OC583: Course Objectives
• Learn how to use isotopic measurements to improve
our understanding of major biogeochemical cycles.
• Learn to use isotopic measurements in a quantitative
manner whenever possible.
• Use lectures, problem sets and paper discussions to
demonstrate concepts, variety of applications and
calculations involving isotopic measurements.
2
Isotope Biogeochemistry
• Tremendous variety of applications across the
earth and biological sciences.
-e.g., oceanography, geology, atmospheric sciences,
plant physiology, hydrology, paleoclimatology
• Even >50 years after its introduction, innovative
applications of isotopic measurements to
biogeochemical problems continue to occur.
-e.g., mass independent fractionations,
clumped isotopes, in-situ measurements
3
Isotope Purists
• Richet et al (1977) “It is our experience that
geochemists, even those who specialize in stable
isotope geochemistry, are badly informed on the
theory of stable isotope fractionation.”
• Criss (1998) “Several academic posts in this field
are held by individuals who have little competence
in physical chemistry or differential equations, and
even some who have never had a course in either
subject.”
• Does it matter?
4
Isotope Biogeochemistry
• The field is dominated by empirical, rather
than theoretical, results.
• One doesn’t need to understand the quantum
mechanical reason for isotope effects in order to
successfully utilize isotope measurements.
• One does need to accurately estimate the
uncertainty or errors associated with isotopic
applications.
• Often one uses isotopic measurements to
compliment other measurements.
5
Isotopic Abundances of ‘Light’ Elements
6
Isotopic Notation and Standards
• del = δ (‰) = [Rsample/Rstandard – 1]*1000,
– where R = rare isotope / abundant isotope
• Standards:
• Oxygen (δ18O) = Standard Mean Ocean Water (SMOW)
= Pee Dee Belemnite (PDB)
= Air (AIR)
•
•
•
•
Carbon (δ13C) = Pee Dee Belemnite (PDB)
Nitrogen (δ15N) = AIR
Hydrogen (δD) = SMOW
Sulfur (δ34S)
= Canyon Diablo (Meteorite)
7
CO2 in air
Natural
Range of
δ13C, δ18O
and δ15N
Isotopic
Values
(vs PDB)
O2 in air
(vs SMOW)
(vs AIR)
8
H2 in air
Natural
Range of δD
and δ34S
Isotopic
Values
(vs SMOW)
(vs Canyon Diablo)
9
Molecular Properties of Isotopologues
10
Energy Levels of Isotopologues of Molecular Hydrogen
11
Equilibrium Isotope Effects
• Reaction: H2S + HDO ↔ HDS + H2O
• Keq = [HDS][H2O] / [H2S][HDO]
• Keq = α = (D/H)H2S / (D/H)H2O = 0.45 (measured)
• Implies that D preferentially substitutes in water
relative to hydrogen sulfide under equilibrium
conditions. Why?
12
Energy Increasing
Energy States of the Isotopologues of H2O
and H2S
___________ HHS
___________ HDS
_______________HHO
_______________HDO
The slightly greater energy decrease that occurs when
substituting a D for H in a water molecule relative to
hydrogen sulfide molecule causes a slight preference for
D to accumulate in water relative to hydrogen sulfide.
13
Equilibrium Isotope Effects between Species
H2S + HDO ↔ HDS + H2O
• ln(Keq) = -? E/RT, thus when –? E<0, then α <1.
• Energy change caused by the isotopic substitution is:
? E = ZPEHDS + ZPEHHO – ZPEHHS – ZPEHDO
-? E = [ZPEHHS – ZPEHDS] - [ZPEHHO – ZPEHDO]
• Energy changes for isotopic effects are small
(calories) compared to reaction energies (Kcals)
14
Equilibrium
Fractionations
(empirically
determined)
15
Equilibrium Isotope Effects between Phases
Reaction: HHO(l) + HDO(g) ↔ HDO(l) + HHO(g)
Energy Increasing
Keq = α = (D/H)H2O(l) / (D/H)H2O(g) = 1.08 to 1.11
___________ HHO(g)
___________ HDO(g)
_______________HHO(l)
_______________HDO(l)
The slightly greater energy decrease that occurs when
substituting a D for H in a liquid water molecule relative to
gaseous water molecule causes a slight prefer for D to
accumulate in liquid water relative to water vapor.
16
Equilibrium
Fractionation
Effects between
Phases of Water
17
Theoretical vs Experimental Isotopic
Fractionation Effects
• ½C16O2(g) + H218O(l) ↔ ½C18O2(g) + H216O(l)
• Experimental α = 1.0412±0.001
• Theoretical α = 1.0408
• Theoretical α based on quantum mechanical derived
estimates of free energy changes resulting from
isotopic substitutions (e.g., Richet et al, 1977)
• Can sometimes work well (simple gas molecules).
18
Temperature
Dependence
of
Fractionation
Effects
19
Mass
Dependent and
Independent
Fractionation
Effects for
Oxygen Isotopes
in Silicates in
Meteorites
20
Mass Independent Fractionation of Oxygen
Isotopes in O3 and CO2
Lab Experiments: O3 Production
Observed Stratospheric CO2
Mass Independent
Mass Dependent
21
Mass Spectrometer Components
22
Continuous Flow Inlet to Mass Spectrometer
Carrier Gas (He)
- small sample sizes (nano- and pico-molar)
- in-line separation capability
- slightly less precise (±0.1 ‰)
23
Laser-based Isotope Ratio Measurements
- in situ continuous measurement capability
- high precision (better than ±0.1 ‰)
24
- cheap and portable instruments