Signature Isotopic Signatures

Paleontologists get to dabble in a lot of different fields of science, which is one of the reasons the field appeals to me (and many others).  One area that has been getting more and more attention lately is chemistry, specifically isotopic geochemistry.  To understand this, you have to know a little about how atoms are put together, and what the heck an isotope is.  For those of you that don't remember your high school chemistry, here's a quick review.

Atoms are the primary components of elements, like oxygen or carbon.  Atoms have a nucleus made of relatively heavy particles - positively charged protons and electrically neutral neutrons.  Orbiting this nucleus are very light, negatively charged electrons.  In a neutral atom, that is one that has no net charge, there are an equal number of protons and electrons (so their charges will cancel out).  It is the number of protons or electrons, the atomic number, which gives elements their chemical properties - how they interact with other elements to make molecules during chemical processes.  The number of neutrons has no part in this; these neutral particles don't affect the chemical properties of atoms.  The atoms of most elements have a common number of neutrons.  For example, carbon, which has an atomic number 6 (so has 6 protons in the nucleus and 6 electrons in orbit around the nucleus), usually has 6 neutrons in the nucleus, giving it a total of 12 particles in the nucleus.  I say carbon "usually" has 6 neutrons, because there are some versions of carbon with more neutrons.  Carbon-13 and carbon-14 have 7 and 8 neutrons respectively.  These variations are called isotopes.  Some of these isotopes are radioactive, even if the 'usual' form is not.  Carbon-14 is radioactive, and it's this fact that allows for carbon dating (a different, but related, story).  There is a known ratio of the various isotopes of a particular element in nature, so when this ratio is off it can tell us something.

So how could the ratio be changed?  Now, I said that the number of neutrons doesn't affect the chemical properties of atoms, and that is mostly correct.  For instance, there are three stable (non-radioactive) isotopes of oxygen, oxygen-16, oxygen-17, oxygen-18.  Oxygen-16 is by far the most abundant of them.  Oxygen-17 isn't found much at all on Earth (it's mostly found in stars).  Oxygen-18 is common enough that we have a good idea of the natural ratio of oxygen-16 to oxygen-18.  Water made with these isotopes (each molecule of water, H2O, has one atom of oxygen in it) acts the same chemically, but there is a key difference.  Oxygen-18, with its 2 extra neutrons, is heavier.  This means that water with oxygen-18 moves a little slower.  When water is heated and starts to evaporate the lighter oxygen-16 will evaporate a littler more readily than oxygen-18, a process called 'fractionation'.  We can see the effects of this in comparing fresh and salt water.

There is far more salt water on the surface of the Earth than fresh water, so most of the moisture in the air comes from the evaporation of salt water (basically the oceans).  Because of fractionation, that evaporated water is going to be slightly enriched in oxygen-16, or you could say slightly depleted in oxygen-18.  The moisture in the air is what forms clouds, and eventually comes down as precipitation.  Most of the freshwater (in rivers, lakes, etc.) is the product of that precipitation, so it too has a higher proportion of oxygen-16 than salt water.

The oxygen cycle
The oxygen cycle (image from NASA)

Well this is all well-and-good, and a lovely story, but how does this relate to paleontology?  Well the obvious answer is that animals have isotopes of various elements in their bodies, and these isotopes, and more importantly, the ratios of the different isotopes of the same element, can get preserved in fossils and consequently can tell us something about how the animal lived.  Take the oxygen isotopes we were just talking about.  By looking at the isotopic oxygen ratios in fossil bones we can determine if an aquatic animal was living most of the time in fresh or salt water.  This sort of analysis was done for early whales, and demonstrates the transition from the earliest, amphibious whales, which lived mainly in freshwater, to the fully aquatic whales which spend most of their time in salt water.  There's a nice verification of this fact today, because some dolphins live in freshwater (so-called "river dolphins" found in the Amazon, the Ganges, and the Yangtze rivers).  They have oxygen isotopic ratios in their bodies similar to the freshwater they live in, that is their tissues are enriched in oxygen-16 relative to their saltwater relatives.  And the earliest whales look like modern freshwater dolphins, and you can follow the change through time as they became adapted to living in soltwater by watching the oxygen isotope ratios change.

Oxygen isotopes in fossil and extant whales
Oxygen isotope ratios in fossil and extant whales (from Thewiseen & Bajpai, 2001)

Oxygen isotopes allow us to gather another sort of information - global temperatures.  It starts the same way - the water that evaporates off the oceans is relatively enriched in oxygen-16, and so is the precipitation that results from that moisture in the air.  Normally, that water eventually makes it back to the ocean via rivers and streams, and there is an equilibrium set up by this cycle, i.e., the ratio of oxygen-16 to oxygen-18 in the oceans remains relatively constant.  The main process that disrupts this cycle is glaciation.  When precipitation gets locked up in glaciers (i.e., is frozen and doesn't make it back to the oceans), the level of oxygen-18 to oxygen-16 the oceans begins to rise.  If there is more glaciation, as happens during ice ages, then even more of that oxygen-18 depleted water will be stuck in glaciers.  So the ratio of oxygen-16 to oxygen-18 in the oceans can be used as a proxy for global temperatures.  Where do we get a record of oxygen isotope levels in the oceans?  Skeletons again, although this time its usually the skeletons of microscopic invertebrates found in cores of the ocean floor.  These cores can be dated, and the oxygen isotopes in the invertebrate skeletons can give us data on global temperatures in the past.  This is the primary way we know about the global warming that is happening today.

Oxygen Isotope ratios showing climate change
Oxygen isotope ratios over the last 500+ million years used to estimate past climates (from Wikipedia)

So these are just two ways to use isotopes in paleontological studies - figuring out habitats and inferring past temperatures.  There are countless other uses for isotopic data, and I'll try and touch on a few more in upcoming entries.

Posted: 9/3/2013 6:26:08 PM by oldbones | with 0 comments

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