Stable Isotopes

Isotopes are atoms of the same atomic weight but different atomic mass.

Most elements of biological importance (C, H, O, N, and S) have two or more stable isotopes - with the lightest in much greater abundance. Most of the heavy isotopes are in abundance of 1% or less.

Stable isotope geochemistry focuses on the variations of the isotopic compositions of light elements arising from chemical fractionations rather than nuclear processes. The most commonly studied stable isotopes are H, Li, B, C, N, O, Si, S, and Cl.

12C - 12 amu - 98.89%

13C - 13 amu - 1.11%

These elements have several common characteristics:1)They have low atomic mass;2)The relative mass difference between the isotopes is large;3)They form bonds with a high degree of covalent character;4)They exist in more than one oxidation state, form a wide variety of compounds;5)The abundance of the rare isotope is sufficiently high (at least tenths of a percent) to facilitate analysis.

Terrestrial Abundance of Stable Isotopes:Element Isotope Abundance %Hydrogen 1H 99.985

2H 0.015Carbon 12C 98.89

13C 1.11Nitrogen 14N 99.63

15N 0.37Oxygen 16O 99.759

17O 0.03718O 0.204

Sulfur 32S 95.0033S 0.7634S 4.2236S 0.014

Measurement Notation:Results from environmental and agricultural studies using isotopically enriched tracers report in units of atom percent (At %). This value gives the absolute number of atoms of a given isotope in 100 atoms of total element:

At%15N = 15N/14N + 15N (100 At%)

At%13C = (13C/12C + 13C + 14C) (100 At%)

Note: For the At% 13C calculation of the amount of naturally present 14C is usually treated as negligible and the sum of 12C and 13C is taken to be total C.

The Notation: Studies examining stable isotopes at or near natural abundance levels are usually reported as delta, a value in parts per thousand or per mil (%o) to make into larger numbers.

Delta values are not absolute isotope abundance but differences between sample readings and one of another of the widely used natural abundance standards (air for Nitrogen , Pee Dee Belemnite for C).

Pee Dee Belemnite (Cretaceous marine fossil, Belemnitella americana from South Carolina) has a higher 13C/12C ratio than nearly all other carbon based substances for convenience it is assigned a delta 13C value of zero giving almost all other naturally occurring samples negative delta values.

PDB now used up so NBS-21 graphite carbon has replaced it. Absolute ratios (R) are measured for sample and standard, and the relative measure delta is calculated:

For example, if a leaf sample is found to have a 15N/14N ratio R greater than the standard’s by 5 parts per thousand, this value is reported as delta 15N = 5 delta %o

Isotopic Fractionation:

Isotopic fractionation can originate from both kinetic (evaporation, diffusion, and dissociation reactions) and equilibrium (energy of a molecule such a vibration motion) effects. (Temperature-dependent equilibrium isotope fractionations arise from quantum mechanical effect in vibrational motions).

Kinetic effects might intuitively be expected since lighter isotopes will diffuse faster than heavier ones.16O will react about 15% faster than 18O in oxygen molecules.

Oxygen Isotopes and Paleoclimate Change:

16O will evaporate more quickly than 18O: water vapor above ocean is typically 18O of around -13 per mil. Temp. effects on fractionation of isotopes (Urey, 1941). Heavier isotope is enriched in the evaporation process. Thus, atmospheric water vapor is depleted in heavy isotopes relative to the sea water from which is evaporates

Interglacial Scenario

High sea-level, little ice at the poles, and relatively little storage of light oxygen isotope in the ice caps leads to relatively depleted heavy oxygen containing seawater. Forams living in the ocean will fractionate this high 18O proportion in their carbonate shells. Clouds contain high proportion of light isotope because of its higher vapor pressure

Glacial Scenario

Sea-level lowered (ca. 120 m), more ice at poles. Polar ice stores more of the light isotope and sea water will contain higher proportions of the heavy isotope. This proportion will be mirrored by forams that live here and fractionate oxygen into their carbonate shells. Clouds contain high proportion of the light because of it’s higher vapor pressure

Fractionation in the Biosphere:

Autotrophs and heterotrophs can fractionate isotopes.

The most important process producing isotopic fractionation of carbon is photosynthesis. Early work by Park and Epstein (1960) - several steps.

Stomatal conductance correlated with 13C of plant species - indicating that diffusion is an important process.

Aquatic :

4.4 %o difference between diffusion coefficients (12C is faster) - so a -4.4 %o fractionation is expected. Marine algae and aquatic plants can utilize either dissolved CO2 of HCO3 for photosynthesis. Atmospheric CO2 at about -7 per mil

At equilibrium fractionation of +9 per mil is associated with dissolution (+7 to +12 depending on temp.) occurs during hydration and dissolution of CO2.

C3 and C4 pathways:

Most plants use enzyme called ribulose bisphosphate carboxylase (RuBP) to catalyze reactions whereby RuBP reacts with one molecule of CO2 and water to produce 2 molecules of 3-phosphoglycerate (carboxylation).

Such plants are called C3 plants this process is called the Calvin-Benson Cycle. C3 plants constitute about 90% of all plants including alage, autotrophic bacteria, cultivated plants (wheat, rice etc.).

Kinetic fractionation associated with carboxylation is about ---29.4 %o in higher terrestrial plants. Bacterial carboxylation is about -20 %o.

C4 or Hatch-Slack Cycle :

Plants use phosphoenol pyruvate carboxylase (PEP) to initially fix CO2 and form oxalacetate - compound with 4 carbons . A fractionation of about -2 to -2.5 %o occurs here. C4 plants have an average 13C of -13 %o . Occur in dry climates (grasses, marshgrass, corn, sugarcane) -efficient use of water.

C4 photosynthesis: high WUE, low photorespiration, high ATP cost, low leaf N, hot climate, high light conditions.

A third group of plants, the CAM plants, have a unique metabolism called the Crassulacean acid metabolism. They generally use the C4 pathway but can use the C3 under certain conditions. Arid plants (pineapple and many cacti).

CAM: day- stomates close, C4 acids decarboxylated, fixed by normal C3 pathway (RuBp); night stomates open, CO2 fixed by PEP carboxylase, C4 acids stored in vacuoles

C3 plants: Higher plants 13C of avg. -27 %o ; algae and lichens 13C -12 to 23 %o.

C4 plants have an average 13C of -13 %o

CAM: 13C of -14 %o

Fractionation in surface and deep oceanic waters

Short-term Carbon Cycle and Anthropogenic Inputs

13C of fossil fuel has varied from -24 %o in 1850 to -27.3 %o in 1980 as coal has been replaced by oil and gas. Thus, we might expect to see a decrease in the 13C of atmopheric CO2. Based on measurements in tree rings and ice cores, the 13C of atmospheric CO2 has declined by about 1.5 since 1800.

13C in Antarctic Ice Core

1700-1850 range from -6.2 to -6.6 %o

1900-1950 range from -6.7 to -6.9 %o

Recent air samples from South pole :

1980-2000 range from -7.4 to -7.8 %o

There is a -19 equilibrium fractionation in the conversion of NH3 to NH4. NH3 is generally the form used, most natural waters have NH4 as the dominant form.

Ammonium assimilation and fractionation by two principle reactions (Fogel and Cifuentes, 1993) :

1) Formation of glutamate from alpha-ketoglutarate via glutamate dehydrogenase and; 2) formation of glutamine from glutamate via the enzyme glutamine synthetase

A positive fractionation (enrichment 15N) occurs for both reactions (about 2-4 %o ) - this is common in reversible reactions because the heavy N is concentrated in the molecule with the strongest bond (glutamate).

Passive Diffusion Model when enzyme or diffusion limited (Fogel and Cifuentes, 1993):

= Eq + D + (Ci/Co)(Eenz + D)

where, Eq is the equilibrium fractionation between NH3 and NH4; D is the fractionation between associated with diffusion in and out of the cell, Ci/Co is the ratio of concentration of ammoinia inside to outside the cell, and Eenz is the fractionation associated with enzymatic fixation of by either glutamine synthetase or glutamate dehydrogenase.

Nitrogen Isotopes:

Forms of inorganic nitrogen: N2, NO3, NO2, NH3 and NH4. Equilibrium and kinetic occur between these five forms of N. Ammonia is dominant form utilized by plants. N2 can be taken up through N fixation (reduction) in some plants (legumes) and cyanobacteria.

NO3 and NO2 can be utilized as well, in these cases N must be reduced by the action of reductase enzymes.

15N fractionations of 0 to -24 %o have been measured for assimilation of NO3. Fractionation of 0 to -20 %o has been measured for assimilation of NH4.

Fractionations of -3 to +1 %o have been measured for fixation of N2.

Nitrogen Isotopes in Organic Matter

Atmospheric = 15N 0 %o

Non-fixing plankton or macroalgae: 15N -3 to + 18 %o

Terrestrial sources: 15N -6 to + 18 %o

Cyanobacteria: -2 to +4 %o and N-fixing terrestrial plants -6 to +6 %o