carbon isotope fractionation

Review of carbon isotopic fractionation inEmiliania huxleyi under high pCO2

Can alkenones be used as a paleobarometer?

Sollberger, S. (2009)

Chemical Oceanographic UnitUniversity of Liege

Background Experimental framework Calculation methods Results Discussion & Outlook References

Contents

1 BackgroundCarbon isotopic fractionationAlkenonesCoccolithophorid

2 Experimental frameworkBatch culturesMesocosms

3 Calculation methodsCarbon isotopic compositionIsotopic fractionation

4 ResultsRiebesell et al. (2000)Rost et al. (2002)Benthien et al. (2007)Environmental influences

5 Discussion & Outlook2 / 17

Carbon isotopic fractionation

Since 1968...

I Correlation between [CO2(aq)] and εp in marine phytoplankton

(Degens et al., 1968)

Use of carbon isotopic fractionation

I Reconstruction of paleo-CO2 (Rau et al., 1989, 1991;Freeman and Hayes, 1992; Jasper et al., 1994)

I BIAIS: species-specific CIF, irradiance, growth rate, physicalisolation (e.g. sea ice) etc.

Sediment tracer - alkenones

I Specific to Haptophyta E. huxleyi (Riebesell et al., 2000; Rostet al., 2002; Benthien et al., 2007)

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Since 1968...

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Since 1968...

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Since 1968...

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Since 1968...

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Alkenones

I Unsaturated ketones with unique structure

I Organic molecules uniquely traceable to specific organisms→ biomarkers

I 5-10 % of total cellular carbon

I Resistant to diagenesis (up to 100 Myrs.)

I Cellular energy storage (excess photosynthate)? Remainsunder debate

I Visualized by gas chromatography

I 37-39 carbon atoms with 2 or 3 double bonds (unsaturation)e.g. C37:2 or C37:3

I If T ↑ → then ↓ of multiple bonds

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Alkenones

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Alkenones

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Alkenones

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Alkenones

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Alkenones

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Alkenones

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Alkenones

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Alkenones

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Emiliania huxleyi

Main characteristics

I Large bloom former

I Major actor in the oceanic carbon export

I Calcifying organism

(a) serc.carleton.edu (b) www.co2.ulg.ac.be/peace

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Batch cultures

Table 1

Set of the different parameters tested on carbon isotopic fractionation andcomplementary information

Riebesell et al. (2000) Rost et al. (2002)Methods HCl, NaOH, NaHCO3 HCl, NaOH[CO2] (µmol l-1) 1.1 → 53.5 5 → 34PFD (µmol m-2 s-1) 150 15, 30, 80, 150Light:Dark cycle 16:8 16:8, 24:0Experimental run 2 2Analyzer GC-IRMS IRMS

Characteristics

I Addition of NaOH & HCl in order to modify [CO2]→ changes in TA while DIC remains constant (1st run)

I Addition of NaHCO3 in order to modify [CO2]→ changes in both TA and DIC (2nd run)

I Modern ocean concentration range: 8− 25µmol l-1

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A mesocosm bloom experiment

Nine 11 m 3 confined water environments (Bergen, 2001)

I Future conditions (meso. no 1,2,3)pCO2 fixed to ∼ 710 µatm

I Present conditions (meso. no 4,5,6)pCO2 fixed to ∼ 410 µatm

I Glacial conditions (meso. no 7,8,9)pCO2 fixed to ∼ 190 µatm

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Carbon isotopic composition

In a sample, relative to PeeDee belemnite standard (PDB)

I δ13CSample =

[(13C/12C)Sample

(13C/12C)PDB− 1

]× 1000

In CO2

I δ13CCO2 = δ13CDIC + 23.644−(

9701.5TK

)(Rau et al., 1996)

Alkenone unsaturation index

I UK ,37 = [C37:2]

[C37:2]+[C37:3](Prahl and Wakeham, 1987)

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I δ13CSample =

[(13C/12C)Sample

(13C/12C)PDB− 1

]× 1000

In CO2

I δ13CCO2 = δ13CDIC + 23.644−(

9701.5TK

)(Rau et al., 1996)

I UK ,37 = [C37:2]

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I δ13CSample =

[(13C/12C)Sample

(13C/12C)PDB− 1

]× 1000

In CO2

I δ13CCO2 = δ13CDIC + 23.644−(

9701.5TK

)(Rau et al., 1996)

I UK ,37 = [C37:2]

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I δ13CSample =

[(13C/12C)Sample

(13C/12C)PDB− 1

]× 1000

In CO2

I δ13CCO2 = δ13CDIC + 23.644−(

9701.5TK

)(Rau et al., 1996)

I UK ,37 = [C37:2]

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Isotopic fractionation

In POC

I εPOC =

(δ13CCO2aq

δ13CPOC+1000− 1

)× 1000

(Freeman and Hayes, 1992)

In alkenones

I εAlk =

(δ13CCO2aq

δ13C37:X +1000− 1

)× 1000

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In POC

I εPOC =

(δ13CCO2aq

δ13CPOC+1000− 1

)× 1000

In alkenones

I εAlk =

(δ13CCO2aq

δ13C37:X +1000− 1

)× 1000

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In POC

I εPOC =

(δ13CCO2aq

δ13CPOC+1000− 1

)× 1000

In alkenones

I εAlk =

(δ13CCO2aq

δ13C37:X +1000− 1

)× 1000

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Batch cultures (Riebesell et al., 2000)

Table 2

A few results from Riebesell et al. (2000): response of the growth rate, the C:Nratio and the alkenones content per cell under increasing [CO2]

Run Growth rate C:N εAlk

1st ⇑ ⇑ ⇑2nd ⇓ ⇓ ⇑

Discussion

I Differences between the runs are not clear → experimental context??

I Increase in alkenones is less significant when normalized to POC

I Small effect of ∆ [CO2] on alkenones and Uk′37

I ε is affected rather by POC than by ∆ [CO2]

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Batch cultures (Rost et al., 2002)

Table 3

A few results from Rost et al. (2002): response of the growth rate, POC andPIC under increasing [CO2]

Run Growth rate POC PIC

1st ⇑ ⇑ ⇓2nd ⇑ ⇑ ⇓

Discussion

I Stronger effect of light on growth rate→ independence of µ to [CO2]

I PIC is enhanced under increasing light

I εP is largely independent of the ambient [CO2] (∆ less than 2 ‰)

I Discrimination of 13C is higher under high irradiance

I Values of ε at 16:8 cycle are lower than at continuous light

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Mesocosms (Benthien et al., 2007)

Results

I No notable effect of CO2

I Differences in δ13CPOC → differences in δ13CCO2 at d0

I ↓ δ13CPOC in present & future conditions

I ↓ εPOC during POC ↑ in present & past conditions

I Cellular alkenones concentration remains cst duringexponential growth but ↑ when nutrient are exhausted

I Isotopic offset between treatments due to differences inδ13CCO2aq or δ13CDIC

Discussion

I Variability between treatments must be < variability betweenone triplicate

I PEP and TEP are enriched in 13C12 / 17

Mesocosms (Benthien et al., 2007)

Results

I No notable effect of CO2

I Differences in δ13CPOC → differences in δ13CCO2 at d0

I ↓ δ13CPOC in present & future conditions

I ↓ εPOC during POC ↑ in present & past conditions

I Cellular alkenones concentration remains cst duringexponential growth but ↑ when nutrient are exhausted

I Isotopic offset between treatments due to differences inδ13CCO2aq or δ13CDIC

Discussion

I Variability between treatments must be < variability betweenone triplicate

I PEP and TEP are enriched in 13C12 / 17

Environmental influences

Table 3

Review of the major factors influencing alkenones unsaturation andεPOC within the actors

Authors [CO2] Growth rate PFD OtherDeuser et al. (1968)

Popp et al. (1998)√

Burkhardt et al. (1999)√

Riebesell et al. (2000)√ √

Rost et al. (2002)√ √

Benthien et al. (2007)√

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Discussion & Outlook

Experiments

I Use of alkenones as a paleobarometer can only be applied as afirst approximation since the response is not proved in modernseawater [CO2]

I Effect of light intensity and irradiance cycle as strong orstronger than the effect of [CO2]. Need extensive knowledgeof environmental conditions determining phytoplanktongrowth

I Varying nutrient conditions influence the CO2 signal

Improvements

I Different experimental approaches lead to variations of themagnitude of the results

I No adaptation of E. huxleyi to increasing pCO2 during theexperiments

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Literature cited I

Benthien, A., Zondervan, I., Engel, A., Hefter, J., Terbr’uggen, A., andRiebesell, U. (2007). Carbon isotopic fractionation during a mesocosmbloom experiment dominated by Emiliania huxleyi : Effects of CO2

concentration and primary production. Geochim. et Cosmochim. Acta,71:1528–1541.

Burkhardt, S., Riebesell, U., and Zondervan, I. (1999). Effects of growth rate,CO2 concentration and cell size on the stable isotope fractionation in marinephytoplankton. Geochim. et Cosmochim. Acta, 63:3729–3741.

Degens, E. T., Guillard, R. R. L., Sackett, W. M., and Hellebust, J. A. (1968).Metabolic fractionation of carbon isotopes in marine plankton. i.temperature and respiration experiments. Deep-Sea Res., 15:1–9.

Deuser, W. G., Degens, E. T., and Guillard, R. R. L. (1968). Carbon isotoperelationships between plankton and sea water. Geochim. Cosmochim. Acta,32:657–660.

Freeman, K. H. and Hayes, J. M. (1992). Fractionation of carbon isotopes byphytoplankton and estimates of ancient CO2 levels. Global Biogeochem.Cycles, 6:185–198.

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Literature cited II

Jasper, J. P., Hayes, J. M., Mix, A. C., and Prahl, F. G. (1994).Photosynthetic fractionation of 13c and concentrations of dissolved CO2 inthe central equatorial pacific during the last 255,000 years.Paleoceanography, 9:781–798.

Popp, B. N., Laws, E. A., Bidigare, R. R., Dore, J. E., Hanson, K. L., andWakeham, S. G. (1998). Effect of phytoplankton cell geometry on carbonisotopic fractionation. Geochim. Cosmochim. Acta, 62:69–77.

Prahl, F. G. and Wakeham, S. G. (1987). Calibration of unsaturation patternsin long-chain ketone compositions for paleotemperature assessment. Nature,330:367–369.

Rau, G. H., Froelich, P. N., Takahashi, T., and Des Marais, D. J. (1991). Doessedimentary organic δ13c record variations in quarternary ocean [CO2(aq)]?Paleoceanography, 6:335–347.

Rau, G. H., Riebesell, U., and Wolf-Gladrow, D. (1996). A model ofphotosynthetic 13c fractionation by marine phytoplankton based on diffusivemolecular CO2 uptake. Mar. Ecol. Prog. Ser., 133:275–285.

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Literature cited III

Rau, G. H., Takahashi, T., and Des Marais, D. J. (1989). Latitudinal variationsin plankton δ13c: Implications for CO2 and productivity in past oceans.Nature, 341:516–518.

Riebesell, U., Revill, A. T., Holdsworth, D. G., and Volkman, J. K. (2000). Theeffects of varying CO2 concentration on lipid composition and carbonisotope fractionation in Emiliania huxleyi. Geochim. et Cosmochim. Acta,64:4179–4192.

Rost, B., Zondervan, I., and Riebesell, U. (2002). Light-dependent carbonisotope fractionation in the coccolithophorid Emiliania huxleyi. Limnol.Oceanogr., 47(1):120–128.

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carbon isotope fractionation

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