Using light to describe the ancient world

A diverse group of penguins lived in Africa 10-12 million years ago. Dr. Dan Ksepka and I recently co-wrote an article describing these ancient penguins, and Dan has a great summary on his blog. I thought I would take this opportunity to show some of the ‘behind the scenes’ work that helped with the article but was not included in the final cut.

One of the first things we wanted to know was the age of the fossil penguin bones. Eventually we would solve this problem with stratigraphy – one of our first moves, though, was to see if the burial environment of the fossils we had just found was similar to the burial environment of fossil penguins that had previously been described. The burial environment for these already-described penguins is around five million years old.

The chemistry of fossil bones can be useful for describing different burial environments. Water that flows between grains of sediment can have very different chemical compositions in different burial environments, and water can alter the chemistry of a fossil bone in a distinct way. If two fossils have very similar elemental compositions, then you can start thinking about how they might have come from the similar burial environments. Likewise, if fossil bones have distinct chemical compositions, then it might be telling you that the bones also have different ages. Of course, fossils with the same age can be buried in different burial environments, so checking for similarities in burial environment is just a preliminary step.

Analysing the chemistry of a fossil bone is easy to do when you have access to a handheld x-ray fluorescence spectrometer. Two of the penguin bones that I analysed are shown below – most of a humerus from a ~5 million year old Inguza predemersus, and the head of a humerus from one of the newly found penguins (Sphenisciformes B). I collected XRF data from these specimens and found the same proportion of calcium and phosphorus in each fossil. This isn’t surprising – these are the two major ingredients of bone.

Fossil bones from the Western Cape of South Africa.

Fossil bones from the Western Cape of South Africa.

Both of these fossil bones are orange-brown-ish, and both have roughly the same proportion of iron. Iron oxides (rust) can produce orange-brown colours in fossils. So, no great differences in calcium, phosphorus or iron. Strontium, however, was a large component of the newly discovered bone, and represented a smaller proportion of the Inguza bone. Strontium can be fairly mobile, however, and bones from the same locality can have different amounts of strontium.

Energy dispersive x-ray fluorescence spectra from fossil penguin bones.

Energy dispersive x-ray fluorescence spectra from fossil penguin bones.

The most surprising and interesting results were at the higher end of the energy scale. A peak that might represent uranium was very clear in the spectrum from the newly discovered fossil penguin, and comparatively weak in the spectrum from Inguza. Likewise, a peak that might represent yttrium is distinct in the spectrum from Inguza, and weak in Sphenisciformes B. These trace elemental differences, combined with the variation in strontium concentration, are telling us that the two fossil bones have been altered by groundwater in different environments. We took this to mean that the bones were from different burial environments….

….and sure enough, the Inguza fossil was buried around 5 million years ago in a sandy river channel, and Sphenisciformes B was buried between 10 and 12 million years ago in a gravelly estuary. Of course, this conclusion was brought to us by sedimentology and stratigraphy, but it is very nicely supported by spectroscopy.

Thomas DB and Ksepka, DT. 2013. A history of shifting fortunes for African penguins. Zoological Journal of the Linnean Society. DOI: 10.1111/zoj.12024

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