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Archaeological Center Research Facility for Stable Isotope Chemistry

What do we do? The Archaeological Center Research Facility (ACRF) is a stable isotope research laboratory located in

the Anthropology Department at the University of Utah. Our primary focus is stable isotope analysis and accelerator radiocarbon dating of skeletal hard and soft tissues for ecological, archaeological, forensic and paleontological applications.

We provide stable carbon, nitrogen, oxygen and sulphur isotope analysis as well as calibrated accelerator radiocarbon dating on a wide variety of organic materials for the University of Utah research community as well as researchers at other institutions, both national and international.

Under the direction of Dr. Joan Brenner Coltrain and colleagues, numerous projects have been facilitated by the molecular techniques offered at ACRF. These include:

  •  monitoring prey choice in endangered Florida panther populations
  •  reconstructing trophic level relationships among extinct Pleistocene megafauna
  •  determining reliance on maize agriculture among prehistoric horticulturalists in the American Southwest
  •  examining the importance of whaling among Thule foragers in the Eastern Arctic
  • dating population movement on the Aleutian Islands
  • sourcing the origins of unidentified individuals in Colonial American cemetaries
  • monitoring evidence for climate change using archaeofaunal isotope chemistry
  • identifying source water in archaeological maize.

For information on procedures, cost, sample submission and turn around time please contact Joan Coltrain. 

What is an isotope? Virtually all elements have a common and rare form or isotope. The rare form has at least one additional neutron in the nucleus of the atom. Carbon is a good example. It has one radioactive isotope, carbon-14, which decays at a constant rate. Thus the amount of 14C in a once living organism can be used to determine its age. Carbon also has two stable isotopes. Carbon-12 with 6 protons and 6 neutrons in the nucleus is the common isotope and makes up 98.8 % of all carbon atoms. Carbon-13 with 6 protons and 7 neutrons is the rare isotope represented by 1 % of all carbon atoms. With an extra neutron, this rare stable form or isotope possess greater mass and is discriminated against in physical or chemical reactions. In other words, the ratio of the common to rare stable isotope of carbon changes in a highly predictable manner as these reactions occur. This change is called fractionation and is an isotopic label allowing researchers to reconstruct biological processes and the implications that derive from them. A brief discussion of stable carbon and nitrogen isotope chemistry illustrates these principles.

Please feel free to contact us if you have questions regarding the applicability of stable isotope analysis to your research. 

Stable Carbon Isotope Analysis Stable carbon isotope values record the ratio of 13C/12C in the carbonates of tooth enamel or in the amino acid sequences that comprise bone collagen fibrils. These values reflect reliance on plants that use the C3 versus C4 and CAM photosynthetic pathways for the following reasons. When CO2 is taken up during photosynthesis, metabolic processes alter or fractionate the ratio of 13C/12C, depleting plant tissues in 13C relative to atmosphere (-7.7‰). This ratio is expressed in delta notation (δ13C) as parts per mil (‰) difference from an internationally recognized standard (PDB) assigned by definition a value of 0 ‰ and computed as follows:

Equation 1.
δ13C = Rsample - Rstandard x 1000 ‰
Rstandard where R = 13C/12C

Fractionation associated with photosynthesis co-varies with the kinetic properties of carbon uptake and enzymatic processes of carbon fixation (Farquhar et al. 1989). In terrestrial plants, carbon isotope fractionation is contingent upon which of three photosynthetic pathways (C3, C4, CAM) plants use to metabolize atmospheric CO2. Cool season grasses, trees, tubers and most bushy plants employ C3 photosynthetic mechanisms discriminating heavily against 13C, expressing a mean δ13C value of -26.7  2.7 ‰ (n=370) (Cerling et al. 1998:Figure 3). A small set of forbs and all warm-season grasses including maize (Zea mays), common to regions where daytime growing-season temperature exceeds 22oC and precipitation exceeds 25 mm (Ehleringer et al. 1997), use C4 photosynthesis resulting in less discrimination against 13C and an average δ13C value of -12.5 ± 1.1 ‰ (n=455) (Cerling et al. 1998:Figure 3). Cacti and some members of the Agavacaea (yucca and agaves) use the CAM pathway, which alternates between C3 and C4 photosynthetic mechanisms and can produce isotope signatures as positive as C4 photosynthesis. Plants grown before fossil fuel depletion of atmospheric CO2 are enriched 1-2 ‰ relative to the above averages (Tieszen & Fagre 1993a).

 δ13C values are passed from producer to consumer leaving a diagnostic signature in consumer tissues that does not co-vary with the skeletal element analyzed or sex of the sample independent of differences in feeding ecology (Hobson and Schwarcz 1986; Lovell et al. 1986).

 Enamel δ13C values record the intake of C3 versus C4 carbohydrates during the juvenile period of a human's lifetime since the enamel on permanent teeth, with the exception of third molars, forms early in life, derives primarily from the carbonates in blood and does not turn over once laid down. Fractionation between plant tissues and consumer tooth enamel is 12-13 ‰. Conversely, collagen ?13C values represent a weighted average of long term dietary intake since carbon in the amino acid sequences that make up adult bone collagen turns over, albeit slowly, requiring ca. 30 years to replace existing carbon with an equivalent amount of carbon (Stenhouse and Baxter 1977, 1979:333; see also Harkness and Walton 1972; Libby et al. 1964). Fractionation between plant tissues and bone collagen is 5 ‰ and approximates 1‰ at higher trophic levels. Accordingly the bone collagen of individuals with diets comprised primarily of wild C3 plant foods will exhibit mean collagen ?13C values in the -22 ‰ to -19 ‰ range, while individuals heavily reliant on a C4 domesticate such as maize will express δ13C values in the -10 ‰ to -6 ‰ range (e.g., Coltrain and Leavitt 2002; Decker and Tieszen 1989; Ezzo 1993; S. Martin 1999; Matson and Chisholm 1991; Spielmann et al. 1990).

Work with rodents on experimental diets (Ambrose and Norr 1993; Tieszen and Fagre 1993b) has led to the assumption that bone collagen δ13C values are heavily biased by the δ13C value of ingested animal protein. Although protein biasing does occur, it has become increasingly clear that the degree of biasing is highly correlated with source carbon used for synthesis of non-essential amino acids (neAAs) (Schwarcz 2001). Non-essential amino acids occupy two of three positions in bone collagen's cross-linked amino acids chains. If protein intake is adequate to supply both essential and neAAs, the stable carbon isotope signature of bone collagen will reflect that of dietary protein. This is a condition seldom obtained even by carnivores since they experience periodic resource stress and at least occasionally rely on stored macronutrients for the manufacture of neAAs (Schwarcz 2001). In omnivores, the carbohydrate fraction of the diet may frequently supply carbon for neAA synthesis. In these cases, bone collagen δ13C values more closely reflect the isotope signature of total diet. A recent study with burials from the Great Salt Lake wetlands (Coltrain and Leavitt 2002) provides inferential support for this understanding. Although dietary protein carried a C3 or isotopically depleted signature (Coltrain and Leavitt 2002:Table 5), some individuals expressed C4, or isotopically enriched stable carbon isotope ratios. Clearly, enriched values reflected the carbohydrate, (i.e., maize) component of sampled diets. Thus in settings where protein intake is moderate to low, collagen δ13C values provide a conservative but useful measure of C4 consumption, facilitating diagnostic estimates of sampled diets for comparative purposes. 

Stable Nitrogen Isotope Analysis Stable nitrogen isotope analysis follows from the understanding that 15N/14N increases by approximately 2-4 ‰ with each increase in trophic level due primarily to fractionation during urea production, enriching the isotope signature of nitrogen available for protein synthesis (Schoeller 1999). The ratio of 15N/14N is also expressed in delta notation and calculated by substituting R = 15N/14N into Equation 1. Organic δ15N is commonly a positive value since atmospheric nitrogen is the standard at 0 ‰. The trophic level effects of nitrogen metabolism are clearly illustrated in nursing infants whose δ15N values are typically 3 ‰ above adult diets (Coltrain 1996; Coltrain and Leavitt 2002; Coltrain et al. 2006 b, c; Katzenberg 1993).

Most terrestrial plant taxa obtain nitrogen from soil ammonium (NH4+) or nitrate (NO3-) and have mean δ15N values of 3-6 ‰ with a 0-9 ‰ range contingent upon temperature and aridity (Pate 1994). Studies indicate a general relationship between aridity and δ15N values such that vegetation δ15N declines 1.0-1.3 ‰ per 100 mm of rainfall (Ambrose 1991; Gröcke et al. 1997; Heaton et al. 1986; Pate et al. 1998; Robinson 2001; Schwarcz et al. 1999). Independent of effective moisture, plants that fix atmospheric nitrogen such as legumes have a mean δ15N value of 1 ‰, with a −2 to 2 ‰ range (Evans and Ehleringer 1994; Pate 1994). Low nitrogen values are also characteristic of taxa growing in association with nitrogen-fixing mycorrhizae in biological soil crusts found in arid settings throughout the American Southwest. Herbivores in temperate climates typically exhibit δ15N values of 6-9 ‰ (Coltrain and Leavitt 2002:Table 5), while arid-land species and non-obligate drinkers, those that recycle urea, reflect their water-conservation strategies in more positive δ15N values (Ambrose 1991).   

Last Updated: 3/24/21