Research Part 4: Stable Isotope Analysis

We’re on the fifth blog post in our series on making dioramas, and we’re still doing research! Don’t worry, we’re almost on to the design phase, but first, let’s talk a little bit about one especially important field of study, stable isotope analysis.

Stable Isotope Analysis

stable isotopes aren't radioactive

Stable isotopes analysis: no radioactivity involved!

This one’s a little more complicated but it’s invaluable to paleoecologists, so bear with me. Isotopes are atoms of the same element (like Potassium or Oxygen) that have the same number of protons, but a different number of neutrons in their nuclei, giving them different atomic weights. Some isotopes are radioactive, spontaneously emitting high-energy alpha and beta particles and gamma rays while they decay. Radioactive isotopes are useful in other areas of science, but what we’re interested in are the stable isotopes — that is, the ones that aren’t radioactive and don’t decay into other elements.

Because stable isotopes of a given element have the same number of protons and electrons as each other, they usually behave similarly in chemical reactions, but certain reactions might ‘prefer’ one isotope over another because of the differences in their mass. For example, a heavier isotope might have a harder time letting go of its electrons, so the lighter isotope will be used more often in reactions. In certain systems, these “preferences” in chemical reactions eventually lead to differences from the natural ratio of isotope A to isotope B. And this is important, because stable isotopes accumulate differently in various plants, animals and trophic levels (a.k.a. levels of the food chain). Knowing this, scientists use mass spectrometry to figure out the ratio of one isotope to another in a given sample and make inferences about the larger system.

Let’s look at two different elements, oxygen and carbon. 99.76% of oxygen has an atomic weight of 16 amu (8 protons + 8 neutrons), but 0.2% of Oxygen isotopes weigh 18 amu (8 protons + 10 neutrons). Because Oxygen 18 is heavier than Oxygen 16, it comes out of clouds as rain or snow more readily, but it’s slower to evaporate out of sea water. Thanks to these evaporation/precipitation patterns, the ratio of the two isotopes varies from location to location. Knowing the isotopic ratios of oxygen can tell you a lot about global weather patterns and local topography, thereby giving you an idea of how hot and wet an area was.

a diagram of evaporation, condensation, and oxygen isotopes under different climates

A nifty little diagram of how oxygen isotope ratios are affected by climate. (Image from http://www.noc.soton.ac.uk/gg/IPY/background.html#climate)

  • Example #1: when global climate is very cold, like during an ice age, more Oxygen 16 evaporates out of the ocean than usual (remember, it’s lighter), and because the clouds contain more Oxygen 16, there ends up being more Oxygen 16 locked into the ice and snow on land. The ice caps are enriched with Oxygen 16, while seawater is enriched with Oxygen 18. When the world gets warmer, this pattern flips.
  • Example #2: because Oxygen 18 is heavier, there’s preferential condensation of Oxygen 18 when clouds pass over land. This means Oxygen 18 rains out first (i.e., closest to coasts), so as you go inland and up in altitude, there’s less Oxygen 18 remaining to precipitate in rain or snow — making inland lakes and glaciers richer in Oxygen 16.

When animals drink water, oxygen atoms accumulate in their bodies, so the ratio of oxygen isotopes found in the bones and teeth of ancient animals reflect what the water was like while they were alive, which in turn tells us what the climate was like in that time and place. You are what you eat, even if, in the case of fossils, your last meal was millions of years ago.

grassland biome with scattered trees

Oh hey, a grassland scene! All we need are some giant herbivores…

As for Carbon, the various isotopes are useful for determining what kinds of plants were prevalent at a given time in history, but for our purposes we mostly focused on grass. The story of Cenozoic Wyoming (65 mya till right now) is really the story of the spread of grasses. There are three photosynthetic pathways that plants can use to fix carbon when making sugars — C3, C4, and CAM, which fits in between, is a little weird, and is the least common of the three so we won’t deal with it here — and these pathways differ in their carbon isotope composition. Normally, 98.89% of Carbon is Carbon 12, while 1.11% is Carbon 13. However, C4 chemical reactions tend to fix slightly more Carbon 13 than C3 reactions do, so C4 plant tissues have higher isotopic values (meaning, the ratio is skewed toward the heavier isotope). This difference in Carbon 13 ratios, which gets preserved in pollen and in the teeth and bones of herbivores, is big enough that it’s actually pretty easy to distinguish between a C4 grassland, a C3 grassland, and a forest. Think of the ratios of carbon isotopes as markers on a sliding scale, with C4 plants grouped on one end of the scale and C3 plants on the other. Even within one of these groups, there will be a range of slightly different ratios, determined by the plant’s own physiology and environmental factors like dryness. By looking at variations within the expected range of isotopic ratios for, say, C4 plants, you can determine whether individual plants and animals were living in open/dry habitats or under a forest canopy.

photo of human-sized switchgrass

Switchgrass (Panicum virgatum) is a great big example of a C4 grass native to North America

Even within the grass family Poaceae, some are C3 and others are C4 photosynthesizers. C3 grasses dominate at high latitudes and altitudes, under high levels of atmospheric carbon dioxide, and in climates with cool-season rains and snows. C4 grasses, on the other hand, are often found in subtropical lowlands or places with warm-season rains. They fix carbon efficiently under high temperatures, high aridity (i.e., dryness), low atmospheric CO2 levels, high salinity (saltiness), and waterlogging. C4 grasses didn’t make an appearance until the Late Miocene — what guesses can you make about Late Miocene climate?

Funny enough, the same traits that helped C4 grasses adapt to swampy habitats may have also helped them develop drought resistance, which came in handy as Wyoming cooled and dried. For example, if you’re a grass, having meristems (the region of cells capable of growth and division) located at your stem and leaf nodes is advantageous under both extreme aridity and in water-saturated, nutrient-deficient conditions. Essentially, these intercalary meristems are an adaptation to stress, even though the stress factors of swampiness and aridity seem totally opposite.

Long story short, by doing stable isotope analysis on animal and plant fossils and paleosols (fossilized soil), the oxygen tells you about the climate and the carbon tells you about the environment, which definitely helps you visualize the landscape.

*   *   *

Paleobotany, Palynology, Leaf Margin Analysis and Stable Isotope Analysis are by no means the only ways of determining what ancient ecosystems look like, but they certainly helped us figure out which plants to include in the various dioramas. Now that we’ve completed this initial research (and you’ve been patient/curious enough to follow along), it’s finally time to actually design the display — which you can read all about in the next post!

For more on stable isotopes and grasses:

NASA Earth Observatory website: “Paleoclimatology: The Oxygen Balance” http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/

Nelson, D., Sheng Hu, F., and Michener, R. (2006). Stable-carbon isotope composition of Poaceae pollen: an assessment for reconstructing C3 and C4 grass abundance. The Holocene, 16(6): 819-825 http://www.life.illinois.edu/hu/publications/Nelson%20et%20al-The%20Holocene%202006.pdf

Nelson, D. et al. (2007). Carbon-isotopic analysis of individual pollen grains from C3 and C4 grasses using a spooling-wire microcombustion interface. Geochimica et Cosmochimica Acta, 71: 4005–4014. http://isites.harvard.edu/fs/docs/icb.topic881205.files/Carbon-isotopic%20analysis%20of%20individual%20pollen%20grains%20from%20C3%20and%20C4%20grasses%20using%20a%20spooling-wire%20microcombustion%20interface

Strömberg, C. (2011). Evolution of Grasses and Grassland Ecosystems. Annual Review of Earth and Planetary Sciences, 39: 517-544. http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-040809-152402?journalCode=earth

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s