δ13C of sediment organic matter in coastal Gulf of Mexico

Peterson, 1999 (original data in Sackett & Thompson, 1963

Stable isotopes in Oceanography

Stable isotopes are used to:

• Trace the sources and sinks of material in the environment

• Determine the extent and type of biogeochmical processes which have acted on materials

• Provide information on paleooceanographic conditions

• Experimentally trace specific elements using stable isotope tracers i.e. 15N

Light vs. heavy isotopes

In all cases, the light isotope is the most abundant i.e.

12C >> 13C

16O >> 18O

1H >> 2H

32S >> 34S

All these elements are analyzed as gases. So material for analyses must be processed and converted to gas (usually by combustion)

by isotope ratio mass spectrometry

Stable isotopes are STABLE

Total Mass is Conserved

Different isotopes of the elements are moved around the Earth and partitioned slightly differently

in different compartments of the Earth system

Different isotopes of the same element have the same chemistry (same reactions, same bonds, etc.), but bond energies differ however - lighter isotopes have higher vibrational energy – therefore more likely to react. Slight differences in reaction rates of heavier vs. light isotopes ultimately results in fractionation.

H3C COOH

H313C COOH

Both are acetic acid

Fundamentally, each isotopic form of a compound has a slightly different free energy. Differences in free energy result in different rate constants for the different isotopic forms, and different equilibrium constants. This results in slightly different partitioning of the heavy and light isotopes in reactant and product pools. (see Emerson an Hedges Chap 5 for discussion)

Fig. 1.6. The extra neutron does make a very slight difference in some reactions; having an extra neutron usually results in slower reactions. This reaction difference is fractionation. From Fry, Stable Isotope Ecology, 2006

Isotopic composition of water - SMOW - Standard Mean Ocean Water

This water is the reference material for isotopic analyses of D (del-deuterium) and 18O.

Ocean water is made up of many different isotopic forms of water, the main ones of which are:

H216O H2

18O DH16O, D216O DH18O

Water containing 18O instead of 16O is two mass units heavier per molecule and thus is 12.5% more dense, and a tiny bit slower to evaporate or react in a chemical reaction, resulting in fractionation.

Mass 18 20 19 20 21

A positive value indicates the substance is enriched in the heavy isotope (relative to the

standard). Likewise, a negative value of indicates the substance is depleted in the heavy isotope (relative to the standard).

1000

12

13

12

13

12

13

13

std

stdsample

CC

CC

CC

C

10001

12

13

12

13

13

std

sample

CC

CC

C

1000113

std

sample

R

RC

Definition of the del notation using 13C as an example

R stands for the Ratio of the heavy to light isotope in sample or standard

del 13C ={ [Rsample/Rstd]-1} *1000

  # of atoms   [Rsample/Rstd]  

13C-sample 10800 Ratio 13C/12C sample = 0.0108 0.98182 del 13C = -18.18

12C- sample 1000000

13C-std 11000 Ratio 13C/12C std = 0.011

12C- std 1000000

Example calculation for δ 13C value

Differences in abundance of isotopes is very small !Differences in abundance of isotopes is very small !

Typical abundance of 13C relative to 12C is 1.1%

Negative value indicates that sample is depleted in 13C relative to standard

A difference of only A difference of only 200 atoms out of a 200 atoms out of a million gives a large million gives a large del del 1313C value!C value!

Isotope Discrimination - The instantaneous difference in isotopic composition, usually given in o/oo, between the parent substrate undergoing reaction and the product, at any given instant in time.

Discrimination factor (after Fry, 2006):D () = reactant - product = [(Rsample/Rstd)-1] x 103

D is positive when light isotope reacts faster. Expressed in per mille (mil) or o/oo.

Some definitions (note subtle differences in terminology:

Note: Emerson and Hedges use Difference Fractionation Factor:ε = product - reactant

ε is negative when light isotope reacts faster.

Example for typical δ13C o/oo values: -22 - (-20) = -2

This reaction results in a -2 per mil shift to lighter isotope. That is, the product is isotopically lighter by 2 o/oo

Hydrogen δ D

Carbon δ 13C

Oxygen δ 18O

Nitrogen δ 15N

Emerson & Hedges, Chap. 5

Fractionation factor () – (expressed in isotope ratios not del units) The realized isotopic composition difference between reactants and products.

= [13C/12C]products/ [13C/12C]reactants = Rproducts/Rreactants

For our example earlier the α was 0.98182

Value of will be close to 1 because isotopic differences are small!

The difference between Discrimination and Fractionation: A given chemical reaction/process, say photosynthesis, may have associated with it some isotope discrimination which would be constant if conditions were constant and the substrate was unchanging. In the real world scenario, conditions are variable and discrimination will change over time, ultimately producing some net isotope Fractionation.

More definitions:

Fig. 3.1. 13C distribution in ecosystems. Single arrows indicate CO2 fluxes. The double arrow signifies an equilibrium isotope fractionation. Numbers for pools indicate 13C values (o/oo) and numbers of arrows indicated the fractionation (, o/oo) occurring during transfers. Negative 13C values indicate that less heavy isotope is present than in the standard (which has a 1.1% 13C content; Table 1.2a), not that isotope concentrations are less than zero. From Peterson and Fry (1987). Reprinted, with permission, from the Annual Review of Ecology and Systematics, Volume 18, copyright 1987 by Annual Reviews www.annualreviews.org.

Fractionation factor for carbon in photosynthesis is the same for marine and terrestrial plants – but they draw on isotopically different CO2 pools!

From Fry, 2007

Factors affecting isotope fractionation

• Temperature – Affects kinetic (reaction rate-dependent) isotope fractionation - Fractionation decreases with increasing temperature. As total energy of the system increases (i.e. thermal energy), the fractional difference between the bond energies of the heavy and light isotopes becomes less significant.

• Kinetics - heavier isotopes less likely to react – therefore react slower. (affected by temperature)

• Equilibrium processes – phase changes reactions (gas/liquid or solute/mineral)

• Diffusion – light isotopes diffuse slightly faster.

Kinetic Isotope Fractionation (depends on differential rate of reaction for light vs. heavy isotopes)

For reaction sequence of 4 different compounds containing carbon:

A --> B --> C --> D

If all A is converted to D, then no fractionation will take place (this is a simple mass balance - if you start with a certain amount of 13C, you will finish with the same amount)

If however, only a portion of A is converted to B, and then A is replenished, then fractionation is likely.

Thus, even if all B is converted to C and all C is converted to D, fractionation will be evident in D and the fractionation factor, A -> B will be the same as A-> D

δ18O

0 7 14

δ18O of O2

O2 minimum zone

Emerson & Hedges, Chap. 5

Equilibrium isotope effects

Equilibrium effects are caused by a preferential enrichment of one isotope in a crystal lattice site (or mineral phase) relative to another, based on thermodynamic stability.

Molecules containing the heavy isotope are more stable and have higher bond dissociation energies. Thus, the heavy isotope will tend to partition into the solid phases or larger complexes. This type of equilibrium fractionation is strongly affected by temperature. This is the basis of stable isotope thermometry!

Likewise, light isotopes will preferentially partition into the aqueous phase over crystals, or gaseous phase over aqueous phase.

When seawater evaporates, the heavy water (H2

18O) is preferentially left behind yielding isotopically heavier (more positive 18O) water and isotopically lighter H2O vapor.

As water vapor moves through the atmosphere, precipitation removes the heavier isotope preferentially (same principle as in the evaporation) and the vapor becomes lighter still.

Example of equilibrium isotope effect – 18O buildup in ocean during glaciations

+9o/oo enrichment of precip

+9o/oo enrichment of precip

Since water vapor transport is generally from tropics to high latitude, snow deposited at high latitudes has a lighter 18O isotopic composition than precipitation at lower latitudes. This shifting of the isotope signatures of natural waters can be used to trace processes such as ice sheet buildup during glacial periods, paleotemperatures and ocean temperatures.

http://earthobservatory.nasa.gov/Study/Paleoclimatology_OxygenBalance/oxygen_balance.html

Different ocean water masses have different isotope signatures that behave as conservative tracers, aiding in distinction of mixing patterns in the ocean.

For a closed or semi-closed system, the isotopic composition of the products and reactants will depend on the extent of the reaction. This is an example of a “Rayleigh Distillation”.

From Peterson and Fry, 1986

“Rayleigh Distillation”

Initial δ value of substrate

Little substrate left – what is left is enriched in heavy isotope

δ value of first products formed

Heavie

r

An example An example of a semi-of a semi-closed closed system – system – sulfate sulfate depletion in depletion in a marine a marine sedimentsediment

Foraminifera (CaCO3, calcite-depositing) preserved in sediments have proved invaluable in determining paleo conditions in the ocean, especially temperature and ocean water volume.

Relationship between temperature and the 18O content

of carbonates and water is (Emerson & Hedges Chap 5):

Tcalcite = 17.04 - 4.34(Calcite - water) + 0.16(Calcite - water)2Forams deposit CaCO3 that is in isotopic equilibrium with the seawater. Because each species has slightly different fractions factors, it is necessary to use a single species of foram in the analysis.

Benthic and pelagic species are known

Elphidium excavatum clavatum

Buccella frigida

Benthic forams

Globigerina sp.

The δ18O in CaCO3 shells reflects the temperature at which the organisms grew.

Shell

Small excursions of δ18O, but measurements are very precise

Planktonic foram

Benthic foram

Otoliths from Bluefin Tuna show depletion of 13C in response to changes in Earth’s atmospheric δ 13C

1947

2006

Atmospheric δ13CO2 is going down due to Suess Effect – input of fossil carbon with light isotopic signature

Several isotopes of N have been used with utility in the study of nitrogen cycling

14N is the most abundant stable form of N

15N is stable and has a natural abundance of 0.365 atom%

13N is radioactive with a half life of 10 minutes - not very useful, but it has been used in some studies

Atmospheric N2 is the reference for 15N (i.e. 15Natmos = 0)

Fractionation of N occurs through each level of the food chain, with each trophic level becoming isotopically heavier (higher 15N).

Phytoplankton fractionate N (take lighter isotope preferentially) when N is available. When N is severely limiting, fractionation decreases. Thus, 15N values can tell us something about nutrient status. Useful for paleo-reconstructions.

Fig. 3.2. Representative 15N values in natural systems. See Fig. 1.3a for explanation of symbols. From Peterson and Fry (1987). Reprinted, with permission, from the Annual Review of Ecology and Systematics, Volume 18, copyright 1987 by Annual Reviews www.annualreviews.org.

Wastewater NO3

- ~ +10 to +20

Sigman et al. 2009

Typical del 15N values for marine N poolsDeep ocean nitrate +5 (up to +12 o/oo in denitrification zones)

Atmospheric N 0 o/oo

Phytoplankton -4 to +8 o/oo

N-fixer biomass 0 o/oo (they draw on atmospheric N2)

Consumers Variable –trophic enrichment of 15N along food chain – about 3 o/oo per trophic level

See Karl et al. for data on changes in del 15 N with changes in Trichodesmium abundance.

Primary producers

15N enrichment at higher trophic levels on Georges Bank

Sulfur isotopesSeawater sulfate +21 o/oo

Sedimentary sulfides (FeS2) -10 to -40 o/oo

Marine Plankton +19 o/oo

Spartina alterniflora -8 to +2 o/oo

Upland plants +4 to + 6 o/oo

The dissimilatory sulfate reduction process fractionates sulfur (taking the lighter isotope preferentially) and other sedimentary sulfur cycle processes further fractionate the reduced sulfur such that sulfides preserved in sediments are isotopically light. The large global burial of this “light” sulfur, explains why the remaining seawater sulfate pool is heavy (+20 o/oo) compared to the primordial CDT standard.

The dissimilatory sulfate reduction process fractionates sulfur (taking the lighter isotope preferentially) and other sedimentary sulfur cycle processes further fractionate the reduced sulfur such that sulfides preserved in sediments are isotopically light. The large global burial of this “light” sulfur, explains why the remaining seawater sulfate pool is heavy (+20 o/oo) compared to the primordial CDT standard.

Fig. 3.3. Representative 34S values in natural systems. See Fig. 1.3a for explanation of symbols. From Peterson and Fry (1987). Reprinted, with permission, from the Annual Review of Ecology and Systematics, Volume 18, copyright 1987 by Annual Reviews www.annualreviews.org.

Availability of substrate affects fractionation“Beggars can’t be choosers”

If substrate is non-limiting (and constantly renewed) maximum fractionation will take place.

If substrate is limiting (and virtually all is used, with slow replacement), fractionation will be low

Examples

CO2 limitation of phytoplankton affects 13C

Nitrate availability affects phytoplankton 15N

NO3- concentration

(μM)

δ 15

N o

f pl

ankt

on b

iom

ass

(o/o

o)

0

+5

+10

+15

5 10

High N availability – significant fractionation

Low N availability – Little fractionation

N-fixing organisms

Seawater NO3- δ15N

value

δ 15N values of plankton depend on the source of N (e.g NO3

- vs. N2) and availability of the nutrient.

Isotopes in food web studies

You are what you eat!

• Consumer isotopic composition reflects the isotope composition of the food source.

• Little fractionation along trophic levels for Carbon or Sulfur

• Some trophic enrichment of 15N with higher trophic levels (+3 to +4 o/oo per trophic level) due to preferential excretion of light isotope.

Typical values for del 13C

Sea water DIC +2 o/oo

Atmospheric CO2 -7 o/oo

Marine POC -20 to -22 o/oo

Terrestrial plants -27 o/oo

Marsh grasses (C4) -14 o/oo

Benthic algae -17 o/oo

Values for biogenic materials are approximate, and subject to variation depending on factors such as temperature and availability of substrates (e.g. CO2)

New data are emerging all the time!

From Libes, 1992, Chapter 29. Original data from Peterson & Howarth, 1987

consumers

End

Fig. 1.3. You are what you eat - stable isotopes in a 50 kg human who is composed of mostly of light isotopes with a small amount of heavy isotopes.

People are mostly water, so hydrogen and oxygen isotopes dominate at >35kg along with carbon isotopes at >11 kg. Then comes N isotopes. S isotopes are missing – they should be here at about 220g for the light isotope 32S and 10g for the heavy isotope 34S.

Have you had your isotopes today? (from Wada and Hattori, 1990; reproduced with permission of CRC Press LLC). From Fry, Stable Isotope Ecology, 2006

If I ate only

marine fish I would

have 138.1 g of 13C

O2 consumption results in an O2 pool which is isotopically heavier because “light” O2 is used preferentially

In H2O

Bacterial enzymes will show a slight preference for the light isotope of reactants, yielding isotopically light products.

Consider a steady state pool of glucose in seawater (production = consumption). Given the unchanging reservoir of glucose carbon, fractionation can take place. If the 13C of carbon in glucose is -18 o/oo, then the 13C of CO2 produced from respiration of this glucose carbon may be -30 o/oo (preference for light

isotope). By mass balance, the residual steady-state glucose pool will have a slightly heavier 13C than the source glucose entering the pool (say from phyto exudate)

Oxygen consumed in this respiration reaction may also be fractionated. The enzymes will slightly prefer the 16O, leaving the residual O2 pool isotopically heavier.

Biogenic CH4 can be distinguished isotopically from thermogenic CH4. Why?

Bad example because carbon is not subtantially fractionated in respiration

Kinetic vs. equilibrium isotope effects

Kinetic isotope effects are common both in nature and in the laboratory and their magnitudes are generally much larger than those of equilibrium isotope effects.

Kinetic isotope effects are normally associated with fast, incomplete, or unidirectional processes like evaporation, diffusion, dissociation reactions and biological “enzymatic” reactions. The examples of diffusion and evaporation are explained by the different rates of reaction (i.e. phase transfer) by the different isotopic forms of molecules as they move through a phase or across a phase boundary.

Examples of kinetic effects include evaporation, leaf respiration, bacterial respiration.This is Redundant

Example of equilibrium isotope effect Isotopic exchange

H216O + C18O16O(aq) <=> H2

18O + C16O16O(aq)

HC16O16O16O- + H218O <=> HC16O16O18O- + H2

16O (bicarbonate)

In seawater the total DIC pool is in isotopic equilibrium with water

At low temperature, more 18O partitions into HCO3-

therefore at low temperature CaCO3 is enriched in 18O (i.e. It becomes isotopically heavier)

These are only fractional enrichments! Not all 18O will partition into CO3

2-.

This is too long – cut out some of the Kinetic fractionation stuff

• Organize better. Organize better.

• Focus more on applicationsFocus more on applications

• Focus on the food web stuff. Find a new Focus on the food web stuff. Find a new paper that uses multiple stable isotopes in a paper that uses multiple stable isotopes in a food web study.food web study.

• Develop some problems to work on for HWDevelop some problems to work on for HW

Isotope composition of natural material differs

0-10-30 -20

0-10-30 -20

0-10-30 -20

SW DIC

CO2 in air

Why do these isotopic variations exist?

Closed system vs. open system

Much water in clouds originates from evaporation of ocean water in tropics. Cloud condensation reactions – tend to enrich cloud water in lighter isotopes (i.e. 16O), leaving heavy isotopes (i.e. 18O) behind in the ocean. Poleward transport of cloud water, and eventual deposition of this on polar regions where it forms ice caps, leads to build up of isotopically light water in the ice, and isotopically heavy water in the oceans. During glacial ice build-up, the oceans become “heavier”. This record is preserved in marine carbonates and other paleoindicators.

Element Standard CommentsCarbon

Oxygen

Pee Dee Belemnite (PDB)

PDB fossil carbonate has long since been used up. New standards such as NB-20 are used, and directly related to PDB

Oxygen

Hydrogen

Standard Mean Ocean Water (SMOW)

Ocean water is largest reservoir of water on the planet. Good reference.

Nitrogen Atmospheric Nitrogen

Isotopic composition of the atmosphere is very constant, therefore a good reference

Sulfur Canyon Diablo Troilite (CDT)

The CDT is a meteor, therefore it represents “primordial” sulfur

Isotopic differences in materials are measured relative to a standard material.

Standards used for different stable isotopes

From Libes, 1992, Chapter 29

From Broecker, 1997

Glacial period (cold)

Interglacial period (warm)

YD- 1000y cool period within warm

Fractionation factor for carbon in photosynthesis is the same for marine and terrestrial plants – but they draw on isotopically different CO2 pools!