Lecture 5 Stable Isotopes

Isotopes of ElementsTypes of Isotopes

Chart of the NuclidesMeasurementsDelta NotationIsotope Fractionation Equilibrium Kinetic Raleigh

See E & H Chpt. 5

Key questions:

What are isotopes?What are the types of isotopes?How do we measure isotopes?How do we express measurements of isotopes?

What is isotope fractionation and how do we express it?What is equilibrium isotope fractionation?What is kinetic isotope fractionation?What is Raleigh distillation?What are some applications of stable isotopes?

Isotopes of Elements

Atomic Number = # Protons = defines which element and its chemistry

Atomic Weight = protons + neutrons = referred to as isotopes

Different elements have different numbers of neutrons and thus atomic weights.

Example: Carbon can exist as 12C, 13C, 14CHow many protons and neutrons in each of the C isotopes?

12C = 6P, 6N13C = 6P, 7N14C = 6P, 8N

1 chemical, many isotopes!

Where do Isotopes come from?

In the beginning (Big Bang), light elements of H and He were formed (and a little bit of Li)

Nuclear reactions (ie: fusion) in stars created the remaining elements (and are still creating), some of which have since decayed to more stable elements

There are 92 naturally occurring elements – some are stable, some are not

Types of Isotopes

Isotopes can be categorized into 2 categories:

Stable isotopes – Isotopes that do not decay over the timescale of earth history (4.5 billion years)

Radioactive isotopes – Isotopes that spontaneously convert into other nuclei at a discernable rate

The chart of the nuclides (protons versus neutrons) for elements 1 (Hydrogen) through 12 (Magnesium).

Valley of Stability

Xb decay

X

a decay

Most elements have more than one stable isotope.

Number of neutrons tendsto be greater than the number of protons

1:1 line

Full Chart of the Nuclides

1:1 line

Valley of Stability

Examples for H, C, N and O:

Atomic Protons Neutrons % AbundanceWeight (Atomic Number) (approximate)

Hydrogen H 1P 0N 99.99 D 1P 1N 0.01Carbon 12C 6P 6N 98.89

13C 6P 7N 1.1114C 6P 8N 10-10

Nitrogen 14N 7P 7N 99.615N 7P 8N 0.4

Oxygen 16O 8P 8N 99.7617O 8P 9N 0.02418O 8P 10N 0.20

% Abundance is for the average Earth’s crust, ocean and atmosphere

1/2 = 5730 yr

1. Input as gases2. Gases Ionized3. Gases/ions accelerated in vacuum4. Gases bent by magnetic field according to mass5. Gases detected

Isotope Ratio Mass Spectrometer (IRMS)

1.

2. 3.

4.

5.

How we measure stable isotopes – the IRMS

Isotopes are measured as ratios of two isotopes.Standards are run frequently to correct for instrument stability

Nomenclature – δ Notation

Report stable isotope abundance as ratio to most abundance isotope (13C/12C)- Why? The ratio can be measured very precisely.

BUT – any differences in the isotope ratio can be very very small so we use δ (“del”) notation

WhereH = moles of heavy isotope L = moles of light isotopeR = H/L

δ tells us how much the sample deviates from the standard

δ = “delta” or “del” (if you’re real savvy), units are per mil (‰)

Compared to the standard

The sign of δ

Standards Vary

Each standard has a well defined H/L ratio

Example 1:The IRMS standard for C is PDB (13C/12C = 0.011237)

Your sample has an 13C/12C = 0.010957.

What is δ13C in ‰ for the standard?For the sample?

Isotopic Fractionation

• All isotopes of a given element have the same chemical properties• Small differences in the distribution of the isotopes in materials because heavier

isotopes form stronger bonds and move slightly slower

A heavier mass = stronger bondYou would need a stronger string to hold two bowling balls together than you would need to hold two golf balls together

Isotope Fractionation = process that results is differences in delta values in products and reactants

Example: condensation of water vapor

H2O(g) <=> H2O(l)

In a closed water sample: δ 18O of H2O(g) = -1‰ (Atlantic)

δ 18O of H2O(l) = -10‰ (Atlantic)

Because of isotope fractionation!

and ε NomenclatureFractionation Factor =

If = 1, no fractionationIf >1, more heavy in productIf <1, more heavy in reactant

Difference fractionation Factor = ε

If ε = 0, no fractionationIf ε > 0 , more heavy in productIf ε < 0 , more heavy in reactant

ε is in permil (‰)

is unitless

Example #2: condensation of water vapor

H2O(g) <=> H2O(l)

In a closed water sample: δ 18O of H2O(g) = -1‰

δ 18O of H2O(l) = -10‰

What is ε and of this reaction?

Two kinds of Isotope Fractionation Processes

1. Equilibrium Isotope effects

Occurs in equilibrium reactions (reactions can go both ways) if the system is in equilibrium

Chemical equilibriumPhase changes (closed system)

Distributes isotopes in a system so that the total energy of the system is minimized

Heavier isotope equilibrates into the compound or phase in which it is most stably bound

Within a molecule (CO2 vs HCO3-)

Between molecules (CO2(g) vs CO2(aq) )

Usually applies to inorganic species. Usually not in organic compoundsDue to slightly different free energies for atoms of different atomic weightUsually temperature dependent!

Differences in vibrational energy is the source of the fractionation.Heavier isotopes wind up in the compound where it is bound more strongly

Example #3: Condensation of Water Vapor in a closed container

H2O(g) <=> H2O(l)

H216O(l) + H2

18O(g) ↔ H218O(l) + H2

16O(g)

In a closed container: δ18O of H2O(g) = -1‰

δ18O of H2O(l) = -10‰

Is this reaction an example of an equilibrium isotope effect? How can you tell?

Does the 18O “prefer” to be in the gas or liquid phase? Why?

Example #4: Bicarbonate system

The carbonate buffer system involving gaseous CO2(g), aqueous CO2(aq), aqueous bicarbonate HCO3

- and carbonate CO32-.

One step of that reaction: CO2(aq) + H2O ↔ HCO3

- + H+

δ 13C of CO2(aq) = 1‰ = 1.0092 at 0ºC and 1.0068 at 30ºC (The IRMS standard for C is PDB (13C/12C = 0.011237))

Is this reaction an example of an equilibrium isotope effect? How can you tell?What is the final δ13C of HCO3

- at 0ºC at 30ºC?Is 13C more stable as CO2(aq) or HCO3

-?Is there more or less fractionation at higher temperatures?

2. Kinetic Fractionation

Occurs in unidirectional (irreversible) reactions reversible reactions that are not yet at equilibrium diffusion or differential bond breaking

Heavier isotopes move more slowly (KE = ½ mv2)Therefore react more slowly

Reaction products are depleted in the heavy isotope relative to the reactants All isotopes effects involving organic matter are kinetic

Why do heavier isotopes move more slowly?Same kinetic energy, despite isotopeE = ½ mv2

If E is the same and mass increases, the v must decrease

Examples of Kinetic FractionationThree types of kinetic fractionation:

1. Unidirectional reactions Example: Carbon fixation via photosynthesis:12CO2 + H2O -> 12CH2O + O2 faster13CO2 + H2O -> 13CH2O + O2 slowerOrganic matter gets depleted in 13C during photosynthesis (decreases in 13C)

2. Reversible reactions that are not yet at equilibriumExample: Evaporation of water vapor if not in equilibrium (net evaporation ie: N .Atlantic)H2

16O(l) -> H216O(g) faster

H218O(l) -> H2

18O(g) slowerWater vapor gets depleted in 18O during net evaporation (decreases in 18O)

3. Diffusion Example: Diffusion of H2Oacross a cell membraneH2

16O(l) outside cell -> H216O(l) inside cell faster

H218O(l) outside cell -> H2

18O(l) inside cell slowerWater vapor gets depleted in 18O during net evaporation (decreases in 18O)

Equilibrium Fractionation vs Kinetic Fractionation

The difference depends on the reason for the fractionation

Equilibrium fractionation occurs so that the total energy of the system is minimized via forming the most stable bonds possible

Equilibrium is related to bond stability of the isotope

Kinetic fractionation occurs because smaller molecules move faster than heavier molecules and therefore react more slowly

Kinetic is related to the speed of the isotope

E & H Fig. 5.6

d13C in carbon reservoirs

d13C of atmospheric CO2 versus time

See Quay, 1992, Science

Raleigh Fractionation

A combination of kinetic and equilibrium isotope effects

• Kinetic when water molecules evaporate from sea surface (net evaporation b/c system is not in equilibrium)

• Equilibrium effect when water molecules condense from vapor to liquid form

A isotope fractionation reaction where products are isolated immediately from the reactants will show a characteristic trend in isotopic composition.

• Vapor depleted in 18O compared to ocean water

• Air masses transported to higher latitudes where it is cooler.

• Rain enriched in 18O, removed from system (cloud)

• Cloud gets lighter

• Rain enriched in 18O, removed from system (cloud), but less enriched

Raleigh Fractionation - Concept

Example: Evaporation – Condensation Processesd18O in cloud vapor H2O(g) and condensate (H2O(l) rain)plotted versus the fraction of remaining vapor for a Raleigh process. Idealized:• 20ºC – All vapor -9‰• Just colder than 20ºC – Condensate starts to form,

more enriched in 18O, but is removed from the system (rained out)

• The vapor continues to condense as the temperature decreases – becoming more and more depleted in 18O

• Fractionation increases with decreasing temperature

• Same pattern for D/H isotopes - different scale because more fractionation during the condensation (ε = +78‰ rather than +9‰)

• This trend is used to reconstruct local paleotemperature from in Antartica and Greenland from ice cores

Raleigh Fractionation – Characteristic trend

d18O variation with time in Camp Centuryice core.

d18O was lower in Greenland snow during last ice age

15,000 years ago d18O = -40‰10,000 to present d18O = -29‰

Reflects 1. d18O of precipitation2. History of airmass – cumulative depletion of d18O

http://www.youtube.com/watch?v=nZC5EMPZDFA

Applications of Stable Isotopes

There are many applications of stable isotopes – especially in the study of past conditions on earth

Three case studies in oceanography:

1. Paleothermometer from foraminifera shells2. Origin of organic matter 3. Estimate primary production in marine systems

Case study: 18O of forams in sediment to reconstruct paleotemperature

HCO3- + Ca2+ ↔ CaCO3(s) + H+

Fractionation of 18O is temperature dependent and well quantified in labs

The 18O of CaCO3 precipitated in forams reflects the temperature

Preserved in marine sediments

Complicated because although thisrelationship is well defined, depends on a known 18O of water . That may changedue to ice volume.

d18O of planktonic and benthic foraminiferafrom piston core V28-238 (160ºE 1ºN)

Planktonic and Benthic differ due to differencesin water temperature where they grow.

Planktonic forams measure sea surface TBenthic forams measure benthic T

Case Study: Estimation of temperature in ancient ocean environments

CaC16O3(s) + H218O CaC18O16O2 + H2

16O

The exchange of 18O between CaCO3 and H2OThe distribution is Temperature dependent

Assumptions:1. Organism ppted CaCO3 in isotopic equilibrium with dissolved CO3

2-

2. The δ18O of the original water is known3. The δ18O of the shell has remained unchanged

last glacialHolocene

lastinterglacial

Does the 18O of water in the ocean change over time?

Large scale Raleigh distillation Net transfer of water from ocean to continental ice sheets make ice very depleted in 18O and the oceans enriched in 18O, increasing the d18O of water about 1‰

Case study: 18O of forams in sediment to reconstruct paleotemperature

In some cores, pore water can be measured directly, which gets around this issue.

Case study: 13C of bulk organic matter to determine source

Many people are interested in the preservation in organic matter in marine sediments

By looking at the d13C of an organic material, it can say something to how it was produced (marine or terrestrial) because the starting material is so different in d13C

Complicated by C4 and CAM plants.

C4 = grassesCrassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions

Case study: Profiles of DI13C and 18O to estimate primary productivity

The profiles of DI13C and 18O can be used to estimate primary productivity

More photosynthesis in surface results in a heavier DI13C, resulting in a more positive d13C in surface DIC

During respiration, 16O is preferentially taken up, resulting in a more positive d18O “left over” in the water (obvious at O2 minimum)

Why does the d13C decrease slightly at the O2 minimum?

North Atlantic data