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Stable Isotope Principles


Isotope: an isotope is an atom whose nuclei contain the same number of protons but a different number of neutrons. Isotopes are broken into two specific types: stable and radioactive. There are over 300 known naturally occurring stable isotopes. The Stable Isotope Geosciences Facility focuses on the measurement of naturally occurring light element stable isotopes of nitrogen, carbon, hydrogen and oxygen.

Isotopic Abundances: light elements contain different proportions of at least two isotopes. Usually one isotope is the predominantly abundant isotope. For example, the average natural abundance of 14N is 99.64%, while the average abundance for 15N is 0.36%. Knowing these abundances helps the researcher determine if a sample is enriched or depleted in a specific isotope once the data is in hand. The table below outlines the average isotopic abundances of elements that are most commonly measured for stable isotope measurements in this facility.

Natural Isotopic Abundances of some light stable isotopes:

Heavy isotopes undergo all of the same chemical reactions as light isotopes, but, simply because they are heavier, they do it ever so slightly slower. These tiny differences in reaction rates cause the products of reactions to have different isotope ratios than the source materials. Knowing the precise isotope ratios in plant and animal tissues allows us to know about the processes by which the materials were formed. This can tell if a plant's roots are tapping recent rain or deep groundwater, the water-use efficiency of whole forests, what an animal has eaten throughout its life and where it sits on the food chain, and the global sources and sinks for carbon dioxide in the atmosphere. Historical materials, including those that may be many thousands of years old, can be analyzed in the same manner, allowing us to compare modern and ancient environments.

Isotopic Fractionation: Isotopic fractionation causes stable isotopic abundance variations. Fractionation is caused by the differences in the chemical and physical properties of a certain atomic mass and concerns the concepts of isotope exchange and kinetic processes in reaction rates. Changes in temperature are just an example of an isotope exchange process that can cause fractionation in an isotopic ratio. This is why temperature stability is a priority in many instrumentation facilities. Gas pressure can also have a significant role in determining the magnitude of fractionation effects. Some examples of a kinetic isotope processes are evaporation and condensation, diffusion, and dissociation reactions.

Delta value:Understanding the processes that may affect the isotopic relationship in a specific sample type is an important step toward understanding how isotopic delta values (d) are calculated. An average difference in isotopic composition between the sample and the reference gas is determined using this equation:

[(Rsample-Rstandard)/(Rstandard)] x 1000 = δsample-standard

Rsample is the ratio of the heavy isotope to the light isotope in the sample

Rstandard is the ratio of the heavy isotope to light isotope in the working reference gas which is calibrated against an internaionally known IAEA or NBS standard

δsample-standard is the difference in isotopic composition of the sample relative to that of the reference, expressed in per mil (‰)

Rstandard absolute ratio values for international standards.

Primary Reference Scales:

V-SMOW (Standard Mean Ocean Water) - used for δ2H and δ18O isotope measurement. The standard is an average of different ocean samples from around the world.

SLAP (Standard Light Antarctic Precipitation) - used for δ2H and δ18O isotope measurement. The standard is an average of different ocean samples from around the world.

V-PDB (Pee Dee Belemnite) - used for δ13C measurement. The standard is a CaCO3 from a belemnite from the Pee Dee formation in South Carolina.

Atmospheric Air - used for δ15N measurement. The air has a very homogeneous isotopic composition making this an ideal reference.

V-CDT (Canyon Diablo Troilite) - used for δ34S measurement.

Levels of Stable Isotope Ratio Specificity:

Now that you understand what stable isotopes are, how fractionation can affect isotope ratios, and how stable isotopes are measured, it is important to realize that there are several levels of specificity at which different materials can be measured. There are currently 3 levels of specificity for isotope analysis (bulk, compound, and position specific). The example shown below illustrates how a plant sample can be analyzed down to each of these levels for different purposes depending on the research being done.

International Reference Material Calibration

Typically, samples are measured along with several internal standards as a check on internal instrument precision. These internal standards are usually a matrix standard (which means that we analyze the sample along with a similar matrix standard: plant samples are analyzed with plant standards, animal tissues are measured with animal tissue standards, water samples are measured with water standards, carbonate samples are measured with carbonate standards, etc). Before analyzing samples along with internal standards they need to be calibrated for accuracy. For this calibration we use international reference materials. These reference materials are provided by the International Atomic Energy Agency (IAEA), National Institute of Standards and Technology (NIST) and the United States Geological Survey (USGS) and are typically in limited quantities. These reference materials are, in turn, calibrated against the above discussed primary reference scales. In some cases the samples are measured along with the international reference materials, rather than an internal standard. Samples measured in a dual inlet system are measured against an isotopically known reference gas. The MAT 253, for instance, has a known CO2 reference gas for carbonate analysis. Samples are measured against this gas and steps are taken to watch for any slight changes in calibration.

IAEA Stable Isotope Reference Materials

2H and 18O in water samples

  • VSMOW2, Water
  • SLAP2, Water
  • GISP, Water
  • IAEA-304, Water

Materials with known 2H,13C, 15N and 18O isotopic composition

  • NBS 22, Oil
  • NBS 28, Quartz Sand
  • NBS 30, Biotite
  • USGS-24, Graphite
  • USGS-40, L-Glutamic Acid
  • USGS-41, L-Glutamic Acid
  • IAEA-600, Caffeine
  • IAEA-601, Benzoic Acid
  • IAEA-602, Benzoic Acid
  • IAEA-CH-3, Cellulose
  • IAEA-CH-6, Sucrose
  • IAEA-CH-7, Polyethylene
  • IAEA-303, Sodium-Bicarbonate

Materials with known 13C, 18O, and 7Li isotopic composition

  • NBS-18, Calcite
  • NBS-19, TS-Limestone
  • LSVEC, Lithium Carbonate
  • IAEA-CO-1, Marble
  • IAEA-CO-8, Calcite
  • IAEA-CO-9, Barium Carbonate
  • RM 8562, CO2 Gas
  • RM 8563, CO2 Gas

Current Research

Ethan Grossman

Dr. Grossman applies stable isotope geochemistry to understand environmental and paleoenvironmental change and its causes. Ongoing projects of current students include (1) the Neogene closing of the Central American Isthmus and its relation to climate change, circulation change, and the Caribbean extinction event (Kai Tao, Ph.D. candidate, and Aaron O'Dea, Smithsonian Tropical Research Institute); (2) circulation changes in the epicontinental seas of Carboniferous North America during formation of Pangea and its role in Late Paleozoic Ice Age (funded by NSF; Ryan Flake, M.S. candidate, and Debbie Thomas, Brent Miller, Tom Olszewski, Anne Raymond, Tom Yancey); (3) the causes of hypoxia off the Texas Coast through stable isotopic analyses of waters, mollusks, and fossil foraminifera (Josiah Strauss, Ph.D. candidate; funded by Texas' Hackerman Advanced Research Program).

Figure 1: Comparison of 13C/12C ratios (as δ13C) for (1) brachiopod shells from U.S. Craton and the Russian Platform, (2) micritic limestone from Arrow Canyon, Nevada (Saltzman, 2003), and (3) terrestrial organic matter (Peters-Kottig et al., 2006), and glacial events. Timing of Glacial events I, II, and III is from Isbell et al. (2003a) as modified by Montañez et al. (2007). The red curve represents the running average of all brachiopod shells (4 Ma window, 2 Ma steps) ±2 standard errors of the mean. Note the major increase in δ13C at the Mississippian boundary suggesting increased burial of organic carbon coincident with glaciation (from Grossman et al., 2008). 

Examples of Isotopic Research:

By studying the soil we can determine where plants get their water from through the stable isotopes 18O and 2H.

Where Do Plants Get Their Water From?

Figure 2: Mean isotopic composition of water extracted from woody, green stems (young), woody stems with well-developed bark (old), leaves and soil from five potted individuals of P. velutina. Two-tailed paired t-tests were used to determine the statistical significance of mean differences between the isotopic composition of water from stems and soil samples. 2H and 18O values of plant organs that were significantly different from that of the soil are marked with an asterisk (*P <0.01, **P <0.001, ***P <0.0001). Mean 18O values of water extracted from old stems were slightly more positive than that of soil water (P = 0.07).

By resolving nitrogen baselines from nitrogen fixation vs. NH4+ or NO3- uptake in forest ecosystems through the stable isotope 15N.

Nitrogen Fixation: refers to the natural process of nitrogen converting to ammonia (NH3) within the atmosphere. This process is vital to life as is a basic building block of life.

By establishing a base of a food chain through isotopes 13C and 34S and by creating the length of a food chain through isotope 15N.

Figure 3(at the left): The ratio of 15N/14N presents a characteristic distinction between herbivores and carnivores, as the movement up along the food chain tends to concentrate the15N isotope, by 3-4‰ with each step of the food chain (terrestrial plants have an isotopic ratio of 2-6‰ of N).

The tissues and hairs of vegans contain a significantly lower percentage of 15N than those who eat mostly meat.

By using Glacial Ice Cores, we can determine past climate changes from 13C,2H, and 18O isotopes.

Figure 4 (at the left): Through stable isotopes found within ice cores like this one from Vostok, Antarctica, scientists can plot the carbon dioxide and methane levels in reference to a specific age in time.

Recent Publications

Niall Slowey

  • Tripsanas, E., W. Bryant, N. Slowey, A. Bouma and D. Berti, submitted. Oxygen Isotope Stage 6 canyon and spillover deposits of Bryant and Eastern Canyon Systems: a sedimentological approach into their flow behaviour, Sedimentology.

  • Vance, D., A. Scrivner, P. Beney, M. Staubwasser, G. M. Henderson, and N. C. Slowey, in press. The use of foraminifera as a record of the past neodymium isotope composition of seawater,Paleoceanography, v. 19, p. PA2009 (1-17).

  • Tripsanas, E., W. R. Bryant, N. C. Slowey, and D. Bean, in press. "Layered/Laminated (Rhythmites) Mud Deposits of the northwest Gulf of Mexico: processes leading to their deposition, and their relation to sediment failures", SEPM Special Publication, Siltstones, Mudstones and Shales.

  • Slowey, N. C., W. R. Bryant, D. A. Bean, A. G. Young, and S. Gartner, 2003. Sedimentation in the vicinity of the Sigsbee Escarpment during the last 25,000 yrs, Proceedings of OTRC 2003 International Conference, 15 pp.

  • Young, A. G., W. R. Bryant, N. C. Slowey, J. Brand, and S. Gartner, 2003. Age dating of past slope failures of the Sigsbee Escarpment within Atlantis and Mad Dog developments, Proceedings of OTRC 2001 International Conference, 24 pp.

Brendan Roark

  • Costa, E., Dunbar, R.B., Kryc, K.A., Mucciarone, D.A., Brachfeld, S., Roark, E.B., Manley, P.L., Murray R.W., and Leventer. A. Solar forcing and El Niño-Southern Oscillation (ENSO) influences on productivity cycles interpreted from a late-Holocene highresolution marine sediment record, Adélie Drift, East Antarctic Margin. 10th International Symposium on Antarctic Earth Sciences Proceedings. In review.

  • Roark, E.B., Guilderson, T.P., Dunbar, R.B., and Ingram, B.L., 2006. Radiocarbon based ages and growth rates: Hawaiian deep-sea corals. Marine Ecology Progress Series. 327:1-14. (Feature Article)

  • Guilderson, T.P., Roark, E.B., Quay, P.D., Flood-Page, S., and Moy, C., 2006. Seawater evolution in the Gulf of Alaska: 2002 observations. Radiocarbon, 48:1-15.

  • Schimmelmann, A., Lange, C.B., Roark, E.B., and Ingram, B.L., 2006. A 6,700-year stratigraphy and R record based on varve-counting and 14C-AMS dating for Santa Barbara Basin. Journal of Sedimentary Research, 76:74-80.

  • Roark, E.B., Guilderson, T.P., Flood-Page, S., Dunbar, R.B., Ingram, B.L., Fallon, S.J., and McCulloch, M., 2005. Radiocarbon-based ages and growth rates of bamboo corals from the Gulf of Alaska. Geophysical Research Letters, 32, L04606, doi:10.1029/2004GL021919.

Ethan Grossman

  • DiMarco, S.F., Strauss, J., May, N., Smith, R., Mullins, R.L., Shormann, D., Bianchi, T.S.,Grossman, E.L., Quigg, A.S., and Walker, N., 2010. Texas Coastal Hypoxia: Linkages to the Brazos River. Aquatic Geochemistry (in review).

  • Tao, K., and Grossman, E.L., 2010. Origin of high productivity in the Pliocene of the Florida Platform: Evidence from stable isotopes and trace elements. Palaios (in review).

  • Grossman, E.L., 2010. Oxygen isotope stratigraphy. In Gradstein, F.M., Ogg, J.G., Smith, A., A New Geologic Time Scale, Cambridge Univeristy Press (invited, in press).

  • Ruebush, L.E., Grossman, E.L., Miller, S.A., North, S.W., Schielack, J.F., and Simanek, E.E, 2009. Scientists' perspective on introducing authentic inquiry to high school teachers during an intensive three-week summer professional development exercise. School Science and Mathematics, v. 109 (3), p. 162-174.

  • Grossman, E.L., Yancey, T.E., Jones, T.E., Chuvashov, B., Mazzullo, S.J., and Mii, H.S., 2008. Glaciation, aridification, and carbon sequestration in the Permo-Carboniferous: The isotopic record for low latitudes. Palaeogeography, Palaeoclimatography, Palaeoecology, v. 268, p. 222-233.

Art Kasson

  • Taghipour, B., Mackizadeh, M.A., Pourmoghan, M., Kasson, A., Taghipour, S., 2008. Geology and mineralogy of advance argillic alteration in the keche area (Mt. Karas), Central Iran. Central European Geology, 51 (1): 85-98.

  • Batoul, T., Mackizadeh, M.A., Kasson, A., Huertas, A.D., 2010. Mineralogical and geochemical studies of acid-sulfate alteration in the Shahrzad area, east of Esfahan, Central Iran. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen, 256(2): 129-139.

Josiah Strauss

  • Strauss, J., Grossman, E.L., DiMarco, S.F., 2010. Oxygen and carbon isotopes of hypoxic waters from the 2009 Louisiana hypoxic zone: Indicators of fresh water sources and benthic respiration.AGU Ocean Sciences Abstract.

  • Strauss, J., Oleinik, A.E., and P. Swart, 2009. Oxygen and carbon stable isotopes of marine gastropod shells: growth rates, environmental interpretation, and latitudinal effects. Submitted toPalaeogeography, Palaeoclimatology, Palaeoecology.

Kai Tao

  • Tao, K., and Grossman, E.L.,2009. Late Neogene marine temperatures reconstruction of the Florida Platform from molluscan stable isotopic and Sr/Ca records. American Geophysical Union Fall Meeting.

  • Tao, K., and Grossman, E.L., 2008. Pliocene marine temperatures and nutrient sources on the Florida Platform: Evidence from molluskan stable isotopes and trace element signatures. Geol. Soc. America Abstracts withPrograms, 40.

XinXin Li

  • X.X. Li, G.P. Yang, X.Y. Cao. 2008. Sorption behaviors of sodium dodecylbenzene sulfonate (SDBS) on marine sediments. Water, Air & Soil Pollution, 194, 23-30.

  • G.P. Yang, X.X. Li, X.Y. Cao. 2008. Sorption behaviors of Tween20 on marine sediments.Periodical of Ocean University of China, 38(2), 309-314. (In Chinese). 


The Stable Isotope Geosciences Facility offers courses related to stable isotopes and stable isotope methods.

See below for a list of existing and future courses.


GEOL 681, GEOG 681, OCNG 681: Seminar in Stable Isotope Geoscience (1 credit). Current and classic literature in light stable isotopes (H, C, N, O, S) applied to the study of Earth system processes and history; stable isotope technique and conventions, stable isotope modeling; climate change; paleobiology; biogeochemistry; ocean and atmospheric, circulation and chemistry; hydrogeology; petrology; organic and petroleum geochemistry. This course provides an overview of the principles and current applications of stable isotopes.

GEOL 648: Stable Isotope Geology (3 credits). Stable isotopes of oxygen, carbon, sulfur, nitrogen, and hydrogen applied to problems in climate change, biogeochemistry, oceanography, biogeology, carbonate diagenesis, petroleum exploration, and igneous and metamorphic petrology; analytical methods; theory of isotopic fractionation. This course provides in-depth knowledge of the principles and applications of stable isotopes.

Planned Courses

GEOG 689: Stable Isotope Methods (3 credits). Principles of stable isotope ratio analysis by isotope ratio mass spectrometry (IRMS) and cavity ring-down spectroscopy (CRDS); principles of gas chromatography, vacuum technique, and isotope fractionation; methods for measuring H, C, N, O, and S isotopes in a variety of geoscience and bioscience samples including air, water, animal and plant tissue, sedimentary organic matter, gaseous and liquid hydrocarbons, and carbonate minerals, including data reduction protocols. This course is designed to give students hands-on experience in performing stable isotope measurements of samples to be analyzed for their thesis and dissertation research.