Using light to describe the ancient world

Spinosaurus, the estuary-dwelling super predator. Photo by Kabacchi (2009)

Spinosaurus, a semi-aquatic dinosaur. Photo by Kabacchi (2009)

I am going to do something a little bit different with this post and predict a future scientific instrument. I got the idea for this post after reading an interesting list of inventions inspired by science fiction, compiled by Mark Strauss at the Smithsonian.

You see, I really enjoy science fiction. I like how, given enough time, technological or scientific predictions can become reality. Check out the early predictions of continental drift for a great example. I like that scientists are free to receive inspiration from anywhere, including fiction. So, on that note, allow me to make my own prediction. In the not-too-distant future, I think there will be a non-destructive device that measures the oxygen isotopic composition of phosphate in bone.

Allow me to explain.

Did you know that Spinosaurus, the record-breaking sometimes super-predator, spent much its life standing in water catching fish? We know this because Romain Amiot and colleagues did an amazing job of analysing the oxygen isotopic composition of phosphate in the ancient bones of Spinosaurus. Some background on oxygen isotopes:

  • two atoms of the same element (like oxygen) that differ in their numbers of neutrons are actually two isotopes of the same element
  • the atoms in any substance are actually a mixture of isotopes, and the ratio of isotopes can change from place to place (i.e. different isotopic compositions)
  • thanks to evaporation and precipitation, different bodies of water (rivers, estuaries, oceans) have different oxygen isotopic compositions
  • animals ingest water  – some of the oxygen from this ingested water is stored as phosphate and used by the animal for bone construction
  • ingested oxygen mostly retains it’s isotopic composition, so animals end up storing evidence about the bodies of water they associate with

I have always liked this Spinosaurus study for the simple fact that I now picture this gigantic dinosaur wading in a North African estuary. The obvious next question though, is “Why don’t we have these kinds of stories for every dinosaur?” Quite simply, this is expensive scienceThe process of extracting phosphate from fossil bone is destructive (…fossils are precious) and labor intensive, and analyses require dedicated and often pricey lab equipment. Wouldn’t it be great if there was another way of getting these data, so we could tell more Spinosaurus-like stories?

So here is my speculation. One day we will be able to analyse the isotopic composition of bone phosphate using a non-destructive device. This isn’t totally random – there is actually an existing instrument that does something similar (Note: feel free to skip the video – it’s an advertisement for a scientific instrument):

That video doesn’t do a great job of actually saying how the instrument works. In essence:

  • A spectrometer shines infrared light at a sample
  • Molecules or minerals in that sample contain atoms, and some of these atoms share covalent bonds
  • The atoms shared by the covalent bonds move relative to one another – you can think of the bond ‘stretching’, ‘bending’, ‘twisting’ etc.
  • Each ‘stretch’, ‘bend’ and ‘twist’ requires energy, which is gained by absorbing infrared light
  • Actually, only very specific wavelengths of infrared light are absorbed – the wavelengths absorbed match the energy needed to ‘stretch’, ‘bend’ or’twist’ the bond
  • The energy needed to ‘stretch’, ‘bend’ or ‘twist’ the bond is partly determined by the mass of the atoms sharing the covalent bond
  • Different isotopes have different masses –  the ‘vibrating’ bonds of different isotopes require different wavelengths of light
  • We can figure out which infrared wavelengths correspond to which isotope-bond systems
  • A detector in the spectrometer counts all of the wavelengths that are not absorbed by the sample, which in turn tells us which wavelengths are absorbed
  • By comparing the amounts of each isotope-informative wavelength that are absorbed, we can calculate the isotopic composition of the sample.

Simple enough, right? Well, right now this works well for CO2 because it is a carbon-and-oxygen isotope system. A CO2 molecule with two oxygen-16 isotopes has noticeably different vibrational frequencies compared with a CO2 molecule with two oxygen-18 isotopes, and so we can see distinct peaks for each isotopologue in an infrared spectrum (isotopologues are molecules that only differ in their isotopic compositions). Karl Dierenfeldt provided a great description of this phenomenon in this chemistry experiment (Dierenfeldt 1995). Yes, differences in peak positions from each CO2 isotopologue are subtle, but infrared spectrometers have long been sensitive enough to detect them.

So, what about the different isotopologues of phosphate in bone? First off, infrared spectrometers work really well on gases, liquids and translucent or highly polished solids. Of course, polishing a sample requires some destruction, which is what we would need to do to fossil bone. That’s fine, we can instead use Raman spectroscopy to get our spectrum without sample destruction. The real problem is resolution. At the moment we cannot resolve the peaks we get from a Raman spectrum of bone into phosphate isotopologues. Such fine levels of resolution might be possible one day, and on that day, we will have a device that can quickly give us ancient environmental data from dinosaur bones (even while they are on display in a Museum).

What a day that will be.
Dierenfeldt KE. 1995. Isotope ratio, oscillator strength, and band positions from CO2 IR spectra: a physical chemistry experiment. Journal of Chemical Education 72: 281-283.

Amiot R, Buffetaut E, Lécuyer C, Wang X, Boudad L, Ding Z, Fourel F, Hutt S, Martineau F, Medeiros A, Mo J, Simon L, Suteethorn V, Sweetman S, Tong H, Zhang F, Zhou Z. 2010. Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods. Geology 38: 139–142.


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