Stable Isotopes

Stable isotopes have been used to determine many things about fossil organisms such as their diet, their trophic level, and the temperature of their environment. Several stable isotope analyses have been used in attempts to gain insights into the relationships among fossil vertebrates from Southeast Alaska (Champe and Heaton 1996). By far the most successful has been diet determination from carbon isotopes derived from bone collagen (Heaton 1995c), and this method will be described here.

IsotopeProtonsNeutronsStability
Carbon-12
6
6
Stable
Carbon-13
6
7
Stable
Carbon-14
6
8
Radioactive

Isotopes are varieties of an element with different numbers of neutrons. Carbon is found in three isotopes as shown in the table at the right. Carbon, by definition, has six protons in its nucleus, and the number of neutrons determines the atomic mass and the specific isotope. Carbon-14 is very rare and is radioactive (meaning it decays to nitrogen over time), so it is useful for dating fossils but not for diet determination. Carbon-12 and Carbon-13 are the two stable isotopes of carbon, and the ratio of the two is uniform in the atmosphere but differs in the tissues of plants and animals because of slight preferences in various biochemical pathways. In other words, when carbon is passed from one system to another (from the air to the tissue of a plant, from the food to the bones of an animal, etc.) there is a slight preference for either the heavier or the lighter isotope, so the ratio changes. The ratio is presented in parts per thousand (ppt) relative to a standard. This means that the values only have meaning in terms of comparing different materials. To make matters more confusing, all the values mentioned here are negative.

Tissue samples can be sent to various labs for stable isotope analysis. Luckily, stable carbon isotope values are also provided with radiocarbon dates as a correction factor. The reason for this is as follows: radiocarbon dates are only accurate if the original ratio in the bone or other tissue is known. But stable isotope studies demonstrate that the ratios are not identical between the atmosphere and various plant and animal tissues due to the slight isotopic preferences in biochemical pathways. Stable isotope analysis provides a way to correct for this. If a biochemical pathway has a preference for carbon-13 over carbon-12, for example, then it will have an even greater preference for carbon-14. So by comparing the ratio of the stable isotopes (which remain constant over time), the original amount of carbon-14 relative to the atmosphere can be accurately determined.

The plot below illustrates some of the pathways and typical values for stable carbon isotopes. Terrestrial plants in Southeast Alaska are all of the C3 photosynthetic type, which have a value around -27 ppt. The bones of terrestrial herbivores eating these plants have values increased about six ppt to around -21 ppt, and carnivores eating these herbivores have a slight increase to around -20 ppt. The pathways are more difficult to follow in the marine environment, but the bones of marine herbivores average around -14 ppt, while carnivores eating such herbivores have a slight increase to around -13 ppt. Expected ranges for each group are typically about 5 ppt wide, as shown in the plot below. Although herbivores and carnivores in the food chain are nearly impossible to distinguish from one another with these values, animals getting their food from terrestrial vs. marine sources can be distinguished very easily. The plot shows values obtained from fossils of various ages in Southeast Alaska as well as a few modern bears in the same region.

Expected and Actual delta-13C Collagen Values from Southeast Alaska

                                       delta-13C (ppt)
-27  -26  -25  -24  -23  -22  -21  -20  -19  -18  -17  -16  -15  -14  -13  -12  -11  -10  -9
 |----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|


Terrestrial                Terrestrial                          Marine
C3 plants                  Herbivores                           Herbivores
 * --------------------------> *                                    *

                              Terrestrial                          Marine
                              Carnivores                           Carnivores
                                ->*                                  ->*

                    TERRESTRIAL  HERBIVORES                MARINE   HERBIVORES
                   |-----------------------|            |-----------------------|

                       TERRESTRIAL  CARNIVORES   MIXED        MARINE   CARNIVORES
                      |-----------------------|  FEEDERS   |-----------------------|

      Beaver  |-----Marmot-----|   |--Saiga---|        Fish   Whale Fish Human  Fish
        *     *   *   * *     *2   * *        *         *       *     *   * *    *

Fossils:                                  |----------Arctic/Red Fox----------|
                                          2   *   *     *  *    *     2  *   *

      |---Deer----|                  |Caribou|          |------Ringed Seal-----|
      *  **  3    *                   * 2*   *          2*  * ** 2* *23*4*223*22

                                       |----Brown Bear---|               |-River Otter|
                                       *    *223    * 2***               *  *         *

                 |--------Black Bear--------|             |---Sea Birds----|
                 *  *   * 22*2***3* *2 3*  22             **    *          2

Bone Artifacts:     *          * Black Bear ?                        * Steller's Sea Lion ?

 |----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
-27  -26  -25  -24  -23  -22  -21  -20  -19  -18  -17  -16  -15  -14  -13  -12  -11  -10  -9
                                       delta-13C (ppt)

Animals of Southeast Alaska with known diets fit the expectations fairly well. Obligate plant eaters, such as ungulates (deer, caribou, saigas) and rodents (marmots, beavers), have very low values while fish and marine birds and mammals (whales, seals, otters) have much higher values. This model allows us to determine the diets of potential omnivores or multi-source feeders such as bears, foxes, and humans. The values illustrated in the plot above indicate that black bears get virtually all of their food from land-based sources, that brown bears eat an even mix of terrestrial and marine food, and that foxes have a highly variable diet that is dominated by marine foods (probably a mix of fish and sea birds plus rodents and carrion). Two values were obtained from the human bones recovered in On Your Knees Cave, and they indicate that this person had a virtually pure marine diet (Dixon et al. 1997).

It is often difficult or impossible to determine the animal origin of bone artifacts because they have been so extensively modified. Stable isotopes provide one of the most useful clues. The spear tip from Kushtaka Cave, with a value of -23.3 ppt, and the tool found with the human remains in On Your Knees Cave, with a value of -21.1 ppt, are probably made from black bear since it is the only thick-boned mammal with corresponding isotope values. Another artifact from On Your Knees Cave appears to be made from a segment of a thick spongy rib that matches Steller's sea lion, and the isotope value of -13.4 ppt is consistent with a marine mammal (two teeth from On Your Knees Cave that probably belong to Steller's sea lion have values of-13.1 and -14.2 ppt).

Even within the terrestrial and marine dietary groups, different species appear (based on the limited sample) to have somewhat different isotopic signatures. River otters, which live on land but feed almost exclusively in the ocean in Southeast Alaska, have the highest carbon isotope values. This could result from a peculiar diet of tidepool fishes or invertebrates, but not enough is known about the isotopic character of these foods to know for sure. Seals and sea birds tend to feed farther from shore and to eat fish more exclusively, and their isotopic signatures are similar even though they are very different kinds of animals.

The isotopic differences are even more extreme among the terrestrial herbivores. Some values on deer and beaver are far lower than expected, whereas caribou and saiga fall on the high end of the expected range for terrestrial plant eaters. Marmots fall between these two extremes. Peculiar diets may in part account for these differences. Caribou eat primarily lichens while deer browse on leaves and young shoots of trees and bushes. Another factor may be the forest canopy. Caribou, saiga, and marmot date to before the Last Glacial Maximum (and also immediately after LGM for caribou) when tundra or alpine conditions prevailed. The deer and beaver shown in the plot above date to less than 8,200 years ago, after a dense forest canopy had developed. In a dense forest carbon recycling can occur. At night the plants release carbon dioxide that underwent isotopic selection when it was absorbed during the daytime, and it can be held under the forest canopy without being fully mixed with the atmosphere. Understory plants (the kind herbivores tend to eat) can pick up this already-altered carbon and subject it to a second round of isotopic selection, thus leading to carbon isotope ratios of less than -24 ppt for herbivores that eat these plants.

   
© 2002 by Timothy H. Heaton