How old is a fossil from a sand dune? Fossils recovered from dune fields help us to reconstruct ancient environments. But how ancient is ancient? Thanks to Steno we know that the fossils are younger than the sand dunes they are buried in, and we know that the dune system is younger than the rocks underneath. But sand dunes are dynamic, shifting and reforming with every gust of wind and every drop of rain. Fossils can be winnowed out from depth and settle in newly formed layers, or simply be transported to the dune surface to sit alongside modern debris. Our usual methods for reading the age of a fossil are less reliable in dune environments – for an analogy, imagine pulling clothes out of a tumble dryer and trying to decide what order you first put them in.
We can try and solve this problem in a few different ways. One approach is to use relative dating. When we don’t have the ability to place an absolute age on a fossil assemblage – fossil A is 12,000 years old and fossil B is 10,000 years old – we can instead place fossils in an order of deposition – fossil A is older than fossil B. Relative ages are useful for tracking changes in ancient environments, and importantly, these ages can tell us about the natural disintegration of fossils over time.
Fossil ‘survivorship’ in the Geelbek Dune systems was the focus of a 2008 study by Nicholas Conard, Steven Walker and Andrew Kandel. Here the authors gathered an assemblage of fossils, placed them into size categories, and assigned a relative age to each one. Conard and colleagues observed that both dense and porous bones stood the test of time at Geelbek, in contrast to the standing paradigm, which states that dense fossil bones have a better chance of being preserved as fossils. Importantly, this means there is a good chance that tiny animals are also preserved from the ancient environment represented in the Geelbek Dunes assemblage. There is a big question looming over this conclusion though – how were the fossils assigned a relative age?
Conard and colleagues used colour, heft and other physical parameters to gauge the extent to which minerals had grown on and in the fossils. The authors reasoned that bones that had more minerals had been in the ground for longer, and hence where older. So, fossil bones were sorted into ‘mineralisation categories’ based on the perceived amounts of secondary minerals, and ‘mineralisation category’ became a proxy for burial duration.
This is where spectroscopy comes in. Professor Anusuya Chinsamy-Turan, my postdoc advisor at the University of Cape Town, spoke to Dr Kandel after he presented this work at a conference in Cape Town. Andrew and Anusuya agreed that studying the chemistry of the Geelbek fossils might make the ‘mineralisation categories’ (and the assessment of relative age in a dune system) a little more robust. Andrew brought the very precious fossils from Iziko South African Museum to the University of Cape Town, where I analysed the bones with a handheld x-ray fluorescence spectrometer. We were interested to see if the concentration of mineral forming elements agreed with the mineralisation category that had been assigned to each fossil.
Well, we found a loose correlation between the mineralisation categories and the XRF data (Thomas et al. 2012). To the original five categories we added a zeroth – modern bone. X-ray fluorescence spectra could distinguish the lower categories (zero to three) from the higher categories (four to five), but it couldn’t separate out individual categories. We were able to see a range of elemental concentrations, but the chemical data was not a strong match for the physically assessed ‘mineralisation categories’. From a chemical perspective, the ‘mineralisation categories’ did not reflect mineral accumulation.
So what does this mean? The original conclusion about bone ‘survivorship’ is mostly valid – there are both dense and porous fossils with very high elemental concentrations of mineral forming elements, and hence they have may been buried the longest. Unfortunately, I don’t think physically assessed ‘mineralisation categories’ can give accurate relative ages by themselves. Fortunately, x-ray fluorescence is non-destructive and portable, so we could easily supplement the physical assessments with chemical data. Together, these assessments can help assign relative ages to fossils recovered from sand dunes.
Conard NJ, Walker SJ, Kandel AW. 2008. How heating and cooling and wetting and drying can destroy dense faunal elements and lead to differential preservation. Palaeogeography, Palaeoclimatology, Palaeoecology 266: 236-245