The Present is Key to the Past

By Elizabeth Peneycad

Portrait of Charles Lyell.
Public domain image from Wikimedia commons

When I tell people that I’m researching past environmental change, they probably think that I spend my time in the dusty corners of a lab, studying fossils or ancient mud. While this is partly true, what might not be immediately obvious is that understanding how modern environmental systems work is an essential first step to any research on past environments. The idea that the ‘present is the key to the past’ was first popularised by the famous geologist Charles Lyell during the early 1800s, who observed that the slow geological processes in operation today must have also occurred in the past. Although science has greatly advanced since Lyell’s time, the concept of studying modern environments in order to understand the past remains important to the present day, and has turned out to be a key part of my PhD.

When scientists seek to understand the nature of past environments, they turn to indicators (or proxies) of past environmental conditions, such as the fossil remains of mammals, plants and insects. These proxies can tell us a wealth of information about the climate and habitat in which the organisms lived. However, before a proxy can be used as an environmental indicator, we must first establish how the proxy responds to environmental changes, and how this is recorded within fossilised remains. This can be achieved by studying the proxy in a modern setting, and creating a model that describes the relationship between the proxy and a particular environmental factor, such as temperature or rainfall.

The proxy that I am investigating for my PhD is the ratio of oxygen isotopes in the mineral structure of rodent teeth. Isotopes are variants of a chemical element, in which the number of protons in the atom’s nucleus remains the same, but the number of neutrons and the overall mass of the atom vary. Naturally-occurring oxygen is comprised of two main isotopes: the oxygen-16 isotope and the oxygen-18 isotope, and the ratio of these two isotopes in a substance is known as its δ¹⁸O composition. Research has shown that the δ¹⁸O compositions of rodent teeth can provide an indication of the average δ¹⁸O of water from rivers and lakes in the rodents’ habitat^1,2. The δ¹⁸O composition of this water is, in turn, closely related to air temperature. This means that, in principal, the δ¹⁸O compositions of fossil rodent teeth can be used to reconstruct past climatic changes.

The two most abundant oxygen isotopes
Image copyright Elizabeth Peneycad.

So if I already know how my proxy records environmental conditions, what’s the problem? Well, the problem is that previous studies have shown that the δ¹⁸O compositions of rodent teeth from any one location are often quite variable^2-4. This may be caused by a range of factors, for example, the δ¹⁸O compositions of water in local rivers and lakes might vary, and these isotope variations are then recorded within the rodent teeth. At present, we don’t fully understand the importance of these factors in influencing the way in which the proxy correlates with environmental conditions.

This is where modern analogue studies come in useful, as they allow us to test the proxy under different environmental conditions that we can directly measure or control, and therefore take into account. During the past year of my PhD I have been putting these ideas into practice, as explained in my 3-step guide below.

A modern analogue study in 3 simple steps

Step 1: Sample collection and analyses

A modern barn owl pellet.
Image copyright Elizabeth Peneycad.

Well-preserved rodent remains from barn owl pellets are identified under the microscope.
Image copyright Elizabeth Peneycad.

The obvious first stage of any modern analogue study is to collect samples for analysis. But how can large numbers of teeth from modern wild rodents be obtained responsibly and ethically? The answer to this question is the Barn Owl. Just as a domestic cat coughs up fur balls, owls regurgitate the indigestible remains of their prey (fur, bones and teeth) in the form of a pellet. These pellets accumulate near to the owl’s roosting and nesting sites, and so with permission, are easy to collect for study. Barn Owls mainly feed on small rodents such as the short-tailed field vole (Microtus agrestis). This species, and other voles within the genus Microtus, are also very common in European fossil assemblages dating to the Pleistocene epoch (0.01-2 million years ago). This means that the short-tailed field vole is a useful modern analogue for understanding how the δ¹⁸O compositions of fossil teeth record past environmental conditions.

Since starting my PhD I have obtained several Barn Owl pellets from across Britain to examine how the δ¹⁸O values of vole teeth vary between different geographical locations. After collecting the pellets, I dissect them to extract the rodent teeth for analysis. Though not the most pleasant of tasks, the rewards of the data make this a worthwhile endeavour! Once the teeth have been identified, they are cleaned, crushed, and then analysed to produce oxygen isotope data.

Step 2: The Data – what does it all mean?

This is the one critical question that preoccupies every PhD student. But fear not! There are useful tools, such as statistical tests, that can be utilised to start tackling this question. In the case of my modern analogue study, I am using statistical tests to compare the isotope values of different teeth, and understand the importance of variations in tooth δ¹⁸O compositions.

Online databases and journal publications⁵ have also proved useful for finding modern oxygen isotope data for water collected from rivers and lakes across the world. These datasets allow me to compare the δ¹⁸O values of rodent teeth with the δ¹⁸O compositions of local water, and study the relationship between the proxy and the environmental factor of interest. I can then use this information as a model of how rodent teeth record environmental conditions.

Step 3: Looking to the past

Fossil rodent tooth
Image copyright Elizabeth Peneycad.

Once equipped with a model explaining how a proxy relates to the modern environment, we can then apply this model to interpret fossil data. Firstly, results from modern analogue studies can be used as a point of reference for determining how the environment in the past differed from the environment today.

Secondly, by establishing the modern relationship between a proxy and, for example, temperature, we are then able to estimate what temperatures were like in the past. In this way, I hope that my study investigating the oxygen isotope compositions of rodent teeth will provide a foundation that allows this proxy to be utilised for reconstructing past environments.

So, the next time you meet someone who’s researching past environmental change, keep in mind that we don’t just spend our time studying fossils or ancient mud; understanding the present is the key to understanding the past.

References

1. Royer, A., Lécuyer, C., Montuire, S., Amiot, R., Legendre, S., Cuenca-Bescós, G., Jeannet, M., Martineau, F., 2013. What does the oxygen isotope composition of rodent teeth record? Earth Planetary Science Letters, 361, pp. 258–271. DOI: 10.1016/j.epsl.2012.09.058

2. Navarro, N., Lécuyer, C., Montuire, S., Langlois, C., Martineau, F., 2004. Oxygen isotope compositions of phosphate from arvicoline teeth and Quaternary climatic changes, Gigny, French Jura. Quaternary Research, 62, pp. 172-182. DOI: 10.1016/j.yqres.2004.06.001

3. Gehler, A., Tütken, T. & Pack, A. (2012). Oxygen and carbon isotope variations in a modern rodent community – implications for palaeoenvironmental reconstructions. PLoS One, 7, e49531.

4. Velivetskaya, T. A., Smirnov, N. G., Kiyashko, S. I., Ignat’ev, A. V., Olenev, G. V., Evdokimov, N. G., 2014. Effects of environmental factors and species identity on oxygen and carbon isotope composition of teeth in recent small mammals of the Urals. Russian Journal of Ecology, 45, pp. 136–142. DOI: 10.1134/S106741361402009X

5. Darling, W., Bath, A., Talbot, J., 2003. The O and H stable isotope composition of freshwaters in the British Isles. 2: Surface waters and groundwater. Hydrology and Earth System Sciences, 7, pp. 183-195. DOI: 10.5194/hess-7-183-2003