For the better part of the past three decades, stable isotope geochemistry has become an increasingly common tool vertebrate paleontologists use to find out more about the biology and ecology of extinct organisms. The diet and ecology of an animal can often remain elusive when fossils are merely examined morphologically. Luckily, stable isotopes can tell us specific characteristics about what an animal is eating and the environments it lives in during its lifetime. Despite the fact this method is commonly used these days, there is a lot of background and jargon associated with it that many paleontologists have not been exposed to. In my two part series I am going to describe the foundations of these methods as they apply to paleontology, and then in part two I will highlight some recent work that illustrates how geochemical methods should be seen as an indispensible tool for paleontologists. Hopefully this brief description will help the method of stable isotope ecology become more accessible to all types of paleontologists!
What is an isotope?
Some elements have multiple stable forms with a different number of neutrons in the nucleus, known as isotopes. For example, a normal carbon atom has 6 protons and 6 neutrons in its nucleus, for an atomic weight of 12, but there is also a less abundant form of carbon that exists with an atomic weight of 13, meaning there are 6 protons and 7 neutrons in its nucleus. Heavier stable isotopes tend to be more rare on earth. The differing physical properties of isotopes of the same element lead to a predictable and often times systematic variation in the ratio of heavy to light isotopes in organic materials.
How are they measured?
Stable isotope ratios are measured using a isotope ratio mass spectrometer (IRMS). The basic mass spectrometer design has not been greatly modified since its invention by Alfred Nier in the 1940s while working on the Manhattan Project, hence the design is still called “Nier-type” today. A Nier-type IRMS is constructed of a source, flight tube, magnet, and collector cups. At the source, the sample gas is ionized and then accelerated into the flight tube. A giant magnet deflects the sample based on mass into collection cups, known as Faraday collectors. This collection creates an electrical current that is measured by a connected computer and converted into an isotope ratio. Samples are measured relative to a standard in “per mil” notation. Unknown samples are measured against known standards, which in the case of carbonates is Vienna Pee Dee Belemnite or V-PBD.
Isotopes in the environment
As bones, teeth, and eggshells mineralize, stable isotopic signatures from the water and food an organism is consuming will be locked in and recorded. Once mineralized, this signature will not change over the life of an animal. When we find a fossil, it is possible to measure these ratios using an IRMS, as previously mentioned, but what do they tell us about the environment specifically?
Perhaps on of the most useful stable isotope systems to understand for paleoecologists is the pattern of carbon isotopic fractionation in plants. Due to a physical isotope fractionation, C3 type plants preferentially fix 12C bearing Co2 during photosynthesis, resulting in δ13C values that are strongly discriminated from the δ13C of the atmosphere (currently ~-7‰), ranging between -35 and -25‰ depending on climate and ecology. In contrast, C4 plants have δ13C values worldwide between -15 and -11‰. C3 and C4 plants tend to grow under different climate regimes, and this clear delineation in δ13C values of these plants provides a way to track changes in ecosystems and understand dynamics of environments in deep time.
Similar predictable fractionations occur with water in the environment and water in the body of an organism, which allows oxygen isotope ratios, δ18O, to be useful for reconstructing the type of water the organism was drinking in their environment, for example, if it was highly evaporated or not. Oxygen isotope ratios recorded in tooth enamel are related to the drinking water of the organism, which is obtained from the environment. The δ18O of environmental water is determined by the δ18O of precipitation, which in turn is determined by temperature, evaporation, and the source of the precipitation air mass (Dansgaard 1964). Lighter isotopes of oxygen evaporate while heavier ones condense, meaning the greater the distance between the ocean and the air mass, rain will be lighter because heavy isotopes rainout preferentially earlier in travel of the air mass. Enrichment of δ18O is greatest under arid, hot conditions. These systematic variations in the enrichment and fractionation of oxygen isotopes allows us to glean environmental information from the δ18O contained in fossil tooth enamel.
As you can see, there is a lot that can be described surrounding stable isotope ratios in fossils. I have only touched on carbon and oxygen, which are the two most commonly used in vertebrate paleontology isotopic studies, but I will be sure to highlight others in part two of this series where I will dive more into the applications of this method in recent case studies.
Dansgaard, Willi. 1964. Stable isotopes in precipitation. Tellus 16 (4): 436-468.
Koch, Paul. 2007. Isotopic study of the biology of modern and fossil vertebrates. In Stable Isotopes in Ecology and Environmental Science. 2nd ed. Boston: Blackwell Publishing.
Sulzman, E W. 2007. Stable isotope chemistry and measurement: A primer. Stable Isotopes in Ecology and Environmental Science 1-21.