Frédéric Caupin is a professor of Lyon University and a member of the Liquids and Interfaces group at the Institute of Light and Matter. 

Frédéric and his group investigated the validity of Brillouin spectroscopy as a paleothermometer when used with Fluid Inclusions in quartz crystals. Their novel method overcomes previous theoretical and practical limitations and accesses information locked away in rock minerals that may prove vital in estimating the history of the earth’s surface temperatures. 

They used the Linkam THMS600 to apply the wide thermal cycles necessary for their study.

We spoke to Frédéric about his group’s research, which was recently published in the journal Scientific Reports:   

El Mekki-Azouzi, M. Tripathi, C.S.P. Pallares, G. Gardien, V. and Caupin, F. (2015).  Brillouin Spectroscopy of Fluid Inclusions Proposed As A Paleothermometer For Subsurface Rocks. Scientific Reports 5, 13168.

Q. What was/is the motivation for your research?

A. The main motivation of my research is the study of water anomalies. Indeed, water is the most familiar liquid, but also the most anomalous. For instance water expands upon cooling below 4°C. Water anomalies are due to the complex hydrogen bonded network present in water. These anomalies get even more pronounced in the metastable liquid, namely liquid water supercooled below 0°C, its equilibrium melting point, or liquid water stretched at negative pressure. 

The concept of negative pressure sounds surprising, but it's just a mechanical tension that is applied to the liquid, which is able to resist it thanks to the attractive forces between its molecules.

To generate these extreme conditions of negative pressure, I use water droplets (a few microns in size) trapped inside a quartz crystal. These samples have been known for a long time by geologists. It was applied to the study of metastable water by the groups of Austen Angell in 1991 ((Zheng et al. Science 254, 829-832 (1991), and R.J. Bodnar (Alvarenga, Grimsditch and Bodnar. J. Chem. Phys. 98, 8392-8396 (1993)) who later applied Brillouin spectroscopy to measure the speed of sound in metastable water. 

Thanks to generous funding from the European Research Council (ERC Starting Grant WASSR), I decided to revisit these experiments and extend them below 0°C, where we have recently found new anomalies (G. Pallares et al. Proc. Natl. Acad. Sci. USA 111, 7936-7941 (2014)). I have thus developed techniques to study fundamental properties of metastable water, as for me, the samples known by geologists were mere containers. Through discussing with my colleague, Véronique Gardien from the laboratory of Geology of Lyon, what I found interesting and realized is that I could give back to the geologist community, by using our tools to address questions in the field.
Q. What is gained from the results?

A. This collaboration between physicists and geologists provides a new tool for paleoclimate reconstruction. As the concern about global warming grows, it is crucial to improve our understanding of the past evolution of Earth’s climate, yet Modern climate archives date back only from the nineteenth century. 

To trace temperatures back to geological times (a discipline called paleothermometry), scientists have developed a range of techniques. One of them is based on water droplets trapped during the formation of minerals. These droplets can be found for instance in stalagmites, or in salt deposits left by evaporation of salty water. 

In some of these trapped droplets, a small bubble is present; its study brings information about the ambient temperature at the time the droplet was trapped. But what to do when the bubble is absent? By shining a laser on one of these small droplets, and analysing the light after its interaction with the liquid (Raman and Brillouin spectroscopy), it is possible to obtain the temperature sought for, without the need for a bubble. 

The paper ‘Brillouin spectroscopy of fluid inclusions proposed as a paleothermometer for subsurface rocks’ provides proof for the concept of this method, using inclusions of water in quartz crystals, for which it is easy to make a direct comparison between the new Brillouin method and the traditional bubble-based method. The two methods are in excellent agreement. 

Having established this, we are now going to apply the technique to samples relevant for paleothermometry. This will now be the goal of Emmanuel Guillerm’s PhD thesis, which has just started with funding from the Rhône-Alpes province.

Q. Why was the Linkam Stage useful?
A. The Linkam THMS600 stage was useful as a "turnkey" solution to apply the wide thermal cycles necessary to our study, while providing easy optical access to perform spectroscopic measurements. The sample-to-window distance of the stage is quite large, so we used appropriate long-working distance objectives and fitted them to the microscope setup.