H H C C 1.11 N N 0.37 O O O S 0.014

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Brandon Harrington
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1 Stable isotopes

2 Element Isotope Abundance (%) Hydrogen [1] 1 H H Carbon [6] 12 C C 1.11 Nitrogen [7] 14 N N 0.37 Oxygen [8] 16 O O O Sulfur [16] 32 S S S S 0.014

3 delta notation % per cent (parts per hundred) per mil (parts per thousand) R = ratio of two isotopes of the same element (e.g., 13 C/ 12 C, 34 S/ 32 S) δ = 1000 (R sample R standard ) / R standard The heavy isotope is the numerator, so positive δ values mean the sample is heavier than the standard, and negative values mean it is lighter e.g., 13 δ C = C C sample C C C C standard standard

4 Stable isotope standards Element d value Ratio (R) Standard R of Standard Hydrogen δ D 2 H/ 1 H Standard Light Antarctic Precipitation (SLAP) Vienna Standard Mean Ocean Water (VSMOW) Carbon δ 13 C 13 C/ 12 C Vienna Pee Dee Belemnite (VPDB) Nitrogen δ 15 N 15 N/ 14 N Air Standard Light Antarctic Precipitation (SLAP) Oxygen δ 18 O 18 O/ 16 O Vienna Standard Mean Ocean Water (VSMOW) Vienna Pee Dee Belemnite (VPDB) Sulfur δ 34 S 34 S/ 32 S Canyon Diablo Troilite (CDT)

5 Stable isotope standards H, O Standard Mean Ocean Water (SMOW) (actually VSMOW is a mixture of distilled water samples from the world s oceans) C, O Pee Dee Belemnite (PDB) N S (VPDB is finely-ground and homogenized belemnite fossils from the Cretaceous Pee Dee Formation of South Carolina) atmosphere Canyon Diablo meteorite (CD) (troilite, an iron sulfide, from the Canyon Diablo iron meteorite that formed Meteor Crater in northern Arizona)

6 Stable isotope standards H, O Standard Mean Ocean Water (SMOW) (actually VSMOW is a mixture of distilled water samples from the world s oceans) C, O Pee Dee Belemnite (PDB) N S (VPDB is finely-ground and homogenized belemnite fossils from the Cretaceous Pee Dee Formation of South Carolina) atmosphere Canyon Diablo meteorite (CD) (troilite, an iron sulfide, from the Canyon Diablo iron meteorite that formed Meteor Crater in northern Arizona)

7 Stable isotope standards H, O Standard Mean Ocean Water (SMOW) (actually VSMOW is a mixture of distilled water samples from the world s oceans) C, O Pee Dee Belemnite (PDB) N S (VPDB is finely-ground and homogenized belemnite fossils from the Cretaceous Pee Dee Formation of South Carolina) atmosphere Canyon Diablo meteorite (CD) (troilite, an iron sulfide, from the Canyon Diablo iron meteorite that formed Meteor Crater in northern Arizona)

O Pee Dee Belemnite (PDB) N S (VPDB is finely-ground and homogenized

Carolina) atmosphere Canyon Diablo meteorite (CD) (troilite, an iron

8 Stable isotope standards H, O Standard Mean Ocean Water (SMOW) (actually VSMOW is a mixture of distilled water samples from the world s oceans) C, O Pee Dee Belemnite (PDB) N S (VPDB is finely-ground and homogenized belemnite fossils from the Cretaceous Pee Dee Formation of South Carolina) atmosphere Canyon Diablo meteorite (CD) (troilite, an iron sulfide, from the Canyon Diablo iron meteorite that formed Meteor Crater in northern Arizona)

9 Why do stable isotopes vary? Isotopes of a given element vary in their nuclear properties, but since these are stable isotopes, their nuclear properties don t affect their behaviour in ordinary life Their mass is the remaining difference, and it can result in fractionation of isotopes At a given temperature, all isotopes of an element will have (on average) the same energy E Since E = ½ m v 2, heavier isotopes will have smaller v, and will move more slowly than light isotopes Mass-dependent fractionation can therefore result from several processes

10 Diffusion Light Heavy With equal energy, light atoms are faster than heavy ones. They travel further in the same time. Collisions with other atoms don t alter their average equality of energy. [ 235 UF 6 diffuses through a membrane more rapidly than 238 UF 6 ]

11 Evaporation Light Heavy When a liquid is evaporating, the faster light atoms (or molecules) have more tries at breaking loose from the liquid bonds, and so the vapour is enriched in light atoms relative to heavy. [ 2 H 1 H 16 O will evaporate faster than 1 H 18 2 O, but slower than 1 H 16 2 O] The fractionation effects are normally much smaller than shown here a few percent at most.

the liquid is enhanced in the heavy atoms relative to the light.

12 Condensation Light Heavy The slower heavy atoms are more easily captured by the bonding that holds the liquid together, and so the liquid is enhanced in the heavy atoms relative to the light. [water in rain is heavier than the water in the cloud it formed from]

13 Molecular bonds Light Heavy When bound, together in a solid, or with a smaller group of atoms in a molecule, atoms are constrained by the chemical bonding to vibrate in place. Light atoms vibrate faster than heavy ones. This difference can either strengthen or weaken the bond, depending on the details of the bonding. In this example, the light atom broke away, but there is no simple way of predicting how mass differences will affect the chemical bond.

$greater the potential mass fractionation.$ $they have considerable potential to fractionate isotopes.$

14 The more chemical processes an atom passes through, the greater the potential mass fractionation. Living systems are webs of ceaseless chemical reactions, so they have considerable potential to fractionate isotopes. Water cycle Carbon cycle

16 Isotopes and diet The most important application in archaeology is inferring diet (and related factors) from stable isotopes in bones and teeth Related factors include Hunting, gathering and agricultural practices Economic and social structures Climate patterns Patterns of migration and commerce Sometimes changes of diet over an individual s lifetime can be determined (especially from teeth)

17 Isotopes and diet Most food plants follow a chemical pathway for capturing CO 2 called C 3, because it forms a molecule with three carbon atoms This mechanism is followed by most plants, and leads to δ 13 C values around 27 ±7 These plants include rice, wheat, rye, barley, cassava, potatoes, algae and spinach Many plants in semitropical regions, with high temperatures and abundant sunlight, follow a pathway called C 4, forming four-carbon molecules This mechanism is more efficient in using water and CO 2, and leads to δ 13 C values around 13 ±4 These plants include maize, sorghum, sugarcane and millet There is also a third mechanism, called CAM, but it operates in only a limited number of desert plants

18 Isotopes and diet Most plants derive their nitrogen from the soil. Soil δ 15 N values are variable, depending on local conditions. δ 15 N values are plus a few per mil (~3-5 ) Some plants (legumes) have symbiotic bacteria that can allow the plant to use atmospheric nitrogen. δ 15 N values are near zero per mil (~1-2 ) At each step up the food chain, about +5 is added, so δ 15 N values are an index of level in the food chain, and can distinguish plant and meat diets, for instance. Note however that agricultural practices, such as the use of animal manure as fertilizer, may also affect nitrogen isotopes.

19 Isotopes and diet For archaeologists, the most useful material for analyzing dietary isotope ratios is bone or teeth. Bone contains a protein called collagen, which can be separated by dissolving the hard bone away. Bone collagen has a δ 13 C about 5 greater than the δ 13 C of the food source. The mineral part of bone and teeth is largely the mineral apatite, which also contains carbon. The δ 13 C of apatite is about 10 greater than the δ 13 C of the food source. Collagen is a protein, so it can be analyzed for both C and N isotopes. Apatite can only be analyzed for C. In both cases, careful sample preparation is required to ensure that only the proper component is being sampled. Can also directly analyze food residue found in storage and cooking utensils.

21 Carbon vs. nitrogen isotope ratios in common New World food groups.

22 Human collagen from Patagonia

23 Dietary change in Belize

24 Coastal Ecuador and Peru

25 Coastal Ecuador and Peru

26 Hydrogen and oxygen isotopes H and O isotopes in organisms are mostly derived from H 2 O. Isotope ratios vary geographically due to regional variations in the water cycle (precipitation, runoff, ground water) and therefore local water supply. There is no guarantee that these differences persist over periods of significant climate change! Further research and calibration of isotope variations with time is needed to make geographic inferences reliable.

Stable Isotope Tracers

Stable Isotope Tracers OCN 623 Chemical Oceanography 5 March 2015 Reading: Emerson and Hedges, Chapter 5, p.134-153 (c) 2015 David Ho and Frank Sansone Outline Stable Isotopes - Introduction & Notation