Stable Isotopes & Paleoclimates · PDF fileOxygen Isotopes : Seasonal Variations in Modern...

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1 0 1 2 3 4 Water depth (Kilometers) Reconstructing Earth History with Stable Isotopes & trace elements of Marine Microfossils Foraminifera tests ~ calcite (CaCO CO 3 ) Mass Spectrometers CO2 Isotopes: 13 C/ 12 C 18 O/ 16 O Notation: δ 13 C (‰) δ 18 O (‰) Δ Δ Ocean Temperature Ocean Temperature As T increases, δ 18 O decreases 1‰ ~ 4°C Basic Temperature Equation T=16.9-4.0 (δ 18 O c - δ 18 O sw ) (Shackleton, 1974) δ 18 O (‰) -3 -2 -1 0 1 2 3 0 1 2 3 4 Water depth (Kilometers) Mixed layer planktonic foraminifera Benthic foraminifera 25 20 15 10 5 0 Temperature (°C) Stable Isotopes & Paleoclimates O & C Basics – Notation – Fractionation Greenhouse Case Study Icehouse Case Study Who is this person? Dr. Harold Urey

Transcript of Stable Isotopes & Paleoclimates · PDF fileOxygen Isotopes : Seasonal Variations in Modern...

Page 1: Stable Isotopes & Paleoclimates · PDF fileOxygen Isotopes : Seasonal Variations in Modern Foraminifera • Plankton tows, Bermuda (Williams et al. , 1981, Palaeo3, v.33, p. 71-102)

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ater

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Kilo

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Reconstructing Earth Historywith Stable Isotopes & trace

elements of Marine MicrofossilsForaminifera tests~ calcite (CaCOCO3)

MassSpectrometers

CO2

Isotopes:13C/12C 18O/16ONotation: δ13C (‰) δ18O (‰)

∆∆ Ocean Temperature Ocean TemperatureAs T increases, δ18Odecreases 1‰ ~ 4°C

Basic Temperature EquationT=16.9-4.0 (δ18Oc- δ18Osw)(Shackleton, 1974)

δ18O (‰)-3 -2 -1 0 1 2 3

0

1

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ater

dep

th (

Kilo

met

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Mixed layer planktonicforaminifera

Benthic foraminifera

25 20 15 10 5 0

Temperature (°C)

Stable Isotopes & Paleoclimates

• O & C Basics– Notation– Fractionation

• Greenhouse Case Study• Icehouse Case Study

Who is this person?Dr. Harold Urey

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What’s an Isotope?• Isotopes - of an element - same number of protons (P),

different number of neutrons (N)– (from the Greek isos, meaning “same,” and topos, signifying

“place”)

nmEm = mass number (A)n = atomic number (protons)A = P + N

16O, 17O, 18OP/N : 8/8, 8/9, 8/10

12C, 13C P/N : 6/6, 6/7

•Extranuclear structure remains the same!

What is Isotopic fractionation?

Knowledge of isotopic fractionation is derivedmainly from?

• laboratory calibration studies:– equilibrium precipitation experiments– exchange or kinetic effects between different

phases or speciessupplemented with:

• semi-empirical calculations - data and statisticalmechanics

• measurement of natural samples whoseformation conditions are well known

partial separation of isotopes between two substancesduring physical and chemical processes

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Notation for Expressing Isotopic Compositions(abundances)

Delta Value (δ) - differences in ratios• Geochemical applications - difference in

absolute isotopic ratios betweensubstances is adequate

– more accurately measured thanabsolute ratios

• Ratios reported relative to a standard (ref.gas) in the delta notation expressed asparts per thousand or permil (‰)

10.0‰, = 1.0%R = absolute isotope ratios (heavy/light)13C/12C18O/16O

δA (‰)= RAsamp − RSTD

RSTD

x 103

δA(‰)= ( RAsamp

RSTD

-1) x 103

δ13C(‰)= 13C/12C(samp)!-!13C/12C(std)!

13C/12C(std) x 103

δ13C(‰)= ( 13C/12C(samp)!

13C/12C(std) -1 ) x 103

If Ra > Rb, heavier, higher, more positiveδ18O, δ2H or δD, δ15N, δ34S,

Why not absolute values?• convenience: working w/ small differences

(i.e. 13C = 1.1230% 1.1210%)• ensures greater consistency over time and between laboratories

-differences can be measured with greater precision

Equilibrium FractionationDefined as a redistribution of isotopes of an element among

various species or compounds– aA1 + bB2 ⇔ aA2 + bB1– where A & B represent two species, subscripts 1 &2 isotopes

that are substituted, a & b # of molesFor example; 1/3 CaC16O3 + H2

18O ⇔ 1/3CaC18O3 + H216O

Requirements for Equilibrium?• closed system and/or well mixed• Stable (on some time-scale that is rate dependent)• complete equilibrium is achieved when forward / backward reactions

are equal

• Highly temperature dependent !

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General observations of Equilibrium Fractionation:

• Oxidation State:– heavier isotope accumulates in species/compound with higher

oxidation statei.e., SO4

-2 (S+6)is enriched in 34S relative to SO3 -2 (S+4) and FeS (S-2)

• Multiple phases:– higher density accumulates heavier isotope– δ18OS > δ18Ol > δ18Og

Condensation - heavier isotope is concentrated in the liquidphase, lighter isotope in the vapor phase

– For both δ13C and δ18O ~ CO2 < HCO3 < CaCO3• Temperature:

– Increase T difference in equilibrium isotope compositionsdecreases.

• Fractionation factor (α ) - defines the fractionation associated w/equilibrium exchange reaction between two substances.R = N*/N, N*=heavy isotope, 18O/16O

αA-B = RA

RB

1/3 CaC16O3 + H218O ⇔ 1/3CaC18O3 + H2

16Oα CaCO3 -H2O = 1.031 at 25°C

General observations concerning α• can be determined experimentally and theoretically• values tend to be close to unity (~1.00xx)• sign and magnitude dependent on many factors

• temperature (most important)• chemical composition• chemical structure• pressure

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• Conversion of α to delta:δA= RA

RSTD

− 1

x 103 (‰)

and

δB= RB

RSTD

− 1

x 103 (‰)

αA-B = Ra

Rb

αA-B = δ A + 1000δ B + 1000

Express Ra & Rb in terms of δ and substitute into;

If the δ18Ol of the ocean is 1.0‰, and the water temperature is 25°C,assuming equilibrium, what is the δ18Oc of calcite that precipitates fromthis water? α c-l = 1.028 at 25°C

δ18Oc = α(1.0‰+103) - 103

δ18Oc = 29.03‰

Oxygen Isotopes-Temperature and Ice-volume

• most abundant element on earth(gas, liquid, solids)

• three stable isotopes16O - 99.763%17O - 0.037518O - 0.1995

• omnipresent, involved in mostnaturally occurring geologicprocesses

- sea water- hydrologic cycle- biosphere

• geothermometer– Partitioning of isotopes is

temperature sensitive

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Isotope Ratio Measurements1. Gas Source Mass Spectrometers2. Convert Sample to CO2

– Carbonates -• reaction with 100% phosphoric acid (H3PO4) at

temperature to create CO2

CaCO3 + H3PO4 ⇒ CO2 +H2O + CaHPO4

• reactions temperatures - 25-90°C• advantages of higher temperature:

- increase reaction rates- decreases fractionation difference between carbonates

• O-isotope fractionation during reactionα CO2-CaCO3 = 1.01025 at 25°C

2. Waterequilibration w/ CO2 at a fixed temperature (25°C)

– Must know the fractionation factor:αCO2 -H2O = 1.041 at 25°C

• H2O >> CO2

Standards (18O/16O):

Standard abs. ratio(x106)

δ18O

v- SMOW Standard Mean Ocean

Water

2005 (±0.43)373

0‰

v SLAP Stand. Light Ant. Precip -55.5‰

v PDB Pee Dee Belemnite 2067 (±2.1) 28.64 (-2.20PDB)

Conversion - SMOW and PDB scalesδSMOW = 1.03086 δPDB + 30.86δPDB = 0.97002 δSMOW - 29.98

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O-Fractionation Mechanisms• phase transitions of water - most

effective means of fract.– vapor - liquid - ice

• evaporation• condensation

– Vapor Pressure and, to a lesserextent, Freezing PointDifferences: Mass dependant

– H218O (20) < H2O (18)

• For liquid/vapor: O isotopefractionation decreases withincreasing temperatureFor 0-350°C (Horita & Wesolowski,

1994)1000 ln α l-v = -7.685+6.7123(103/T) -

1.6664 (106/T2) + 0.35041(109/T3)

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0 20 40 60 80 100 120

18O/16O Fract. (liquid-vapor)

1000ln a (F&O,77)1000ln a (H&W,95)

1000

ln α

l-v

°C

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* O always enriched in the C bearing species.CO2 dissolution (hydrolysis):

CO2 (g) ⇔ CO2 (d) + H2O ⇔ H2CO3 ⇔ H+ + HCO3 ⇔ H+ + CO3

•Assuming each species is in isotopic equilibrium with water a species (19°C) 103ln αa-H2OH2CO3 38.7HCO3 34.5CO3 18.2CO2 (g) 41.6

Partitioning of O between dissolved C & H2O

Paleothermometry in Marine Carbonates

• Isotope Exchange reaction between H2O andHCO3, CaCO3 enriches calcite in 18O.

Ca + HCO3 ⇒ CaCO3 + H2O + CO2

CaCO163 + H2O18 ⇒ CaCO18

3 + H2O16

αCO3−H2O = Rc/Rw = 1.028 at 25°C– Foraminifers are roughly 28‰ enriched relative

to seawater-– Fractionation can deviate for several reasons!

• Vital effects - inclusion of metabolic CO2

• Precipitation rates - move away fromequilibrium

– First temperature equation (Epstein et al. 1953)• T°C=16.5-4.3(δcc- δw) + 0.14(δcc- δw)2

δcc== CO2 at 25°C

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Temperature dependence - fractionation increases with coolertemperatures

1000 ln αc-w = 2.78!x!106!!T2 - 2.89 (3)

(Erez and Luz, 1983).

The fractionation factor α c-w(Freidman & O’Neil, 1977)

Oxygen Isotopes : Seasonal Variations in Modern Foraminifera

• Plankton tows, Bermuda (Williams et al. , 1981, Palaeo3, v.33, p. 71-102)

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Oxygen Isotopes: Lab and Field studies of Modern Foraminifera

• Lab culturing experiments ∆Orb-H2O = 3.19-0.208T

•Plankton Tows & Box Cores, Indian Ocean (20°N-30°S)∆Orb-H2O = 3.50-0.214T

(Bouvier-Soumagnac et al., 1985, JFR, v.15, p. 302-320)

Oxygen Isotopes: Pelagic Benthic Foraminifera

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Oxygen Isotopes : Modern Benthic Foraminifera

•Box Cores•Rio Grande Rise, Norwegian Greenland Sea, E. PacificRise(Belanger et al.,1981, Palaeo3, v.33, p.205-220)

C. wullerstorfi Oridorsalis Pyrgo

Oxygen Isotopes: Modern Benthics

• Box Cores• California margin(Grossman, 1984, Palaeo3, v. 47, p.301-327)

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Heterococcoliths

Calcidiscus leptoporus Emiliania Huxleyi

Coccoliths: Oxygen Isotopes• Culturing studies

– multiple species– 6‰ range

• Calciteδ18O variations as afunction of:– Growth rate– Temperature

• Species dependant

Ziveri, Stoll et al., 2003, EPSL, v. 210, p.137

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Coccoliths: temperature

Ziveri, Stoll et al., 2003, EPSL, v. 210, p.137

Natural Abundances of Oxygen Isotopes: Water & Carbonates

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O-isotope of precipitation• For liquid/vapor: O isotope fractionation increases with decreasing

temperature– for 0-100°C (Friedman & Oneil, 1997):

• Condensationδ18Ol > δ18Ov

Clouds become progressivelydepleted in 18O

δ18Ov decreases toward highlatitudes

Ice-sheet δ18O = -30 to -50

O-isotopes Natural Abundances: General Observations• Ocean Water

– Open Ocean - Relatively homogeneousδ18O = 1.5 to - 0.5‰δD =12 to -8‰

evaporation/precipitation/runoffδD = M δ18O

where M is the slope of linear trajectories (decreases w/increasing evaporation/precipitation)

• M ~ 6 (low latitudes) to 8 (high latitudes)– equatorial - P > E– subtropical - E > P– high lat - P > E

– N. Atlantic - end member -21‰• Deep Ocean - δ18O - mixing line between 0 and -0.5‰• Marginal Seas − δ18O - -3.0‰

high freshwater input

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The “Salinity” Effect and δw

∆ Evaporation/Precipitation Rates

δ18O of precipitation

Condensation/Precipitationδ18Ol > δ18Ov

Clouds become progressivelydepleted in 18O

δ18Ov decreases toward highlatitudes

Ice-sheet δ18O = -30 to -50

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Glacial Ice-volume and Ocean δ18O

Ice-sheets (18 kya, present day)

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Sea-level Change, 150 kya to present

120meters

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Carbon IsotopesC12 - 98.89%C13 - 1.11%

δ13C(‰)=13 C 12 C samp

13 C 12 C sta

− 1

x 103

Atlantic δ13CGlacial to interglacial cycles

Fractionation Mechanisms

1. Partitioning of C between CO2- and other hydrated speciesCO2 (g) ⇔ CO2 (d) + H2O ⇔ H2CO3 ⇔ H+ + HCO3 ⇔ H+ + CO3

– temperature dependent (~small):1000 ln a = A + B(106/T2)– When in isotopic equilibrium with CO2 (g)

Species A B 103ln α= ∆H2CO3- CO2 (g) -0.91 0.0063 -0.8HCO3- CO2 (g) -4.54 1.099 7.7CO3- CO2 (g) -3.4 0.87 6.4CaCO3- CO2 (g) -3.63 1.194 9.8

Deines et al. (1974)

CO3 can combine with divalent cations to form various carbonate minerals 2 sources for disequilibrium:

• precipitation rate - as rates increase the degree of fract. decreases• vital effects - metabolic C from respired CO2 may be incorporated

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C fractionation w/ respect to HCO3

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Carbon Isotopes: Natural Abundances

depletion of 13C in lipids relative to biomass as a function

Kinetic Effects: Photosynthesis∆CH2O-CO2 ~ -10-25‰ (all values are in terms of discrimination against 13C)• Why is plant δ13C lower than that of CO2 in the atmosphere?• Two key steps where fractionation can occur:

CO2(a) ⇔ CO2(i) ⇒ R-COOH1. diffusion - 12CO2 diffuses more rapidly2. bonding strengths - 12CO2 weaker (lower activation energy)

Uptake and conversion of CO2 involves three steps1. Diffusion - CO2 is transported through a boundary layer and the stomata into

a the internal gas space• ∆a-I = δa − δi = 4.4‰

2. Dissolution - CO2 dissolves into cell sap and diffuses to chloroplast (as HCO3in C4)• isotope fractionation ~-0.7-1.1‰ (i.e, 13CO2 is less soluble)

3. Carboxylation - carbon fixation• chloroplast - enzymes - catalysts for fixing CO2• isotope fractionation variable:

C3 vs. C4

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Planktonic Foraminifera Isotopes as a function of size

symbiotic and non-symbiotic plankton foraminifera (Orbulina universa and Globigerina bulloides(Spero et al., 1997, Nature)

Modern Benthic Foraminifera δ13C

Grossman, 1984, GCA, v.48, p. 1505-1512

DIC δ13C

Porewater δ13C

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Modern Benthic Foraminifera δ13C

Belanger et al. 1981, PPP, v. 33, p.205-220.

Heterococcoliths

Calcidiscus leptoporus Emiliania Huxleyi

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Coccoliths: Carbon Isotopes• Culturing studies,

– Constant T (17°) & light– multiple species– 5‰ range

• Calcite δ13C variations as afunction of:– Growth rate– Cell diameter– Surface area/volume

Ziveri, Stoll et al., 2003, EPSL, v. 210, p.137

Coccoliths: Sources of IsotopeDisequilibrium

Three potential effects:(1) kinetic fractionation during calcite precipitation or

cellular uptake(2) fractionation of the intracellular C pool during

photosynthesis(3) pH-dependent fractionation