Using light to describe the ancient world

Pleistocene horn cores from bovids (like the springbok shown here) were recently analysed using hyperspectral near infrared imaging. We were able to make spectral maps of the horn cores, and determine that the bones had been exposed to a substantial groundwater flow. Might this be bad news for isotope studies of these fossils? We shall have to find out.

Spectra can be richly informative, as I have hopefully shown in earlier entries. Take any fossil, position it under the spectroscopic instrument of your choice, and you will likely be rewarded with some otherwise invisible information. X-ray diffraction will reveal mineralogy, x-ray fluorescence will give chemistry, mid-infrared and Raman will give ionic composition. Most of the time, the information couldn’t be gleaned from that spot on the fossil using any other method. And most of the time, spectroscopic techniques reveal information about a single spot per analysis.

In the surge of every advancing technology, spectroscopic instrumentation is moving beyond single spot analyses and entering an era where entire surfaces are rapidly mapped. An example of this is hyperspectral near infrared imaging. We have recently published results from a SisuChema imaging system, administrated by Professor Alvaro Viljoen of the Tshwane University of Technology in Pretoria, South Africa. Using this instrument, we were able to collect near infrared spectra from the surfaces of Pleistocene bovid horn cores. Thousands of spectra. The instrument collected roughly 1000 spectra every square millimetre, across a 10 mm wide transect. By taking multiple transects we were able to prepare detailed maps of the horn cores that showed exactly where certain near infrared wavelengths were absorbed. Why was that important? The hyperspectral NIR maps revealed the suffusion of ancient groundwater.

A NIR spectrum of fossil bone is generally uninformative. Near infrared is only really useful for materials made from the lightest elements, which fortuitously includes carbonates and clays. The hyperspectral maps revealed that secondary minerals had been deposited deep inside the tiniest pores and cracks in the bovid horn cores, meaning that a substantial amount of groundwater had flowed into and through the bones. Groundwater is the agent of diagenesis, which means that these fossil bones may no longer carry vital information. The fossils we chose to study are from a suite of sites where geochemical signals have been used to understand the ancient environment. If anything, our data show that these sites may not be giving trustworthy answers. Our next step is to study the isotopic compositions and histology of these bones, to determine whether groundwater has stripped away any analytically useful signals.

So, hyperspectral NIR mapping of Pleistocene fossil bones is a great way to assess whether they have been diagenetically altered.

Thomas DB, McGoverin CM, Chinsamy A, Manley M. 2011. Near infrared analysis of fossil bone from the Western Cape of South Africa. Journal of Near Infrared Spectroscopy 19:151-159.

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