Author: Laetitia Guibourdenche (Institut de Physique du Globe de Paris)
Winter 1957. Charcot Base. Adelia Land. Antarctica. 2400m above sea level.
Claude Lorius, a young glaciologist and his colleagues, spent 10 months in the Charcot Base, trying to unravel the secrets of the unexplored Antarctica. Working all day outside with temperature never exceeding -10°C, sampling ice core, measuring wind speed, daily temperature, size and characteristics of snow grains they were using all the available instruments to feel the pulse of this wild and mysterious continent.
In this inhospitable and cold land, where ice never completely melts, a new snow layer is formed every year coating and compacting the previous one into ice. Drilling deeper into the ice cover, Lorius was accessing to well preserved older ice, keeping the secrets of ancient snow falls and storms which would then be revealed by cautious analyses in the lab.
However, great discoveries sometimes happen outside the lab and it is after a long and glacial sampling day that he had one of his best idea.
One day, while he was observing the melting of a piece of ice core, he had put in his whisky to cool it, he noticed that little air bubbles prisoners of the ice and released in the whisky were certainly captive in the ice since the snow deposition.
He realized suddenly that these little bubbles of air were like fossils of the former atmosphere at the time of snow deposition (like the foraminifera of Francesca!) and thus should reflect the ancient atmospheric composition of Antarctica!!
Back to the lab, he was able to measure greenhouse gases concentration of these air bubbles and trace their evolution through time. Thus, becoming one of the first scientist making the link between human activities, greenhouse gases concentration and climate change. Pursuing geochemical analyses of ice cores with his colleague Jean Jouzel, they demonstrated that hydrogen isotopic composition of the air bubbles reflects the variations of temperature at the time of snow deposition, ages ago!
Hence, they created the first thermometer based on ice cores documenting past atmospheric temperatures.
But, how is it possible to create a thermometer of past environments with a simple piece of ice?
To understand these findings, I have to explain you what is an isotope, and how it can be useful to trace environmental changes.
Basically, two isotopes are two atoms of the same chemical element which have different weights. For example, carbon 14, carbon 13 and carbon 12 are three different isotopes of the carbon, 14C being the heaviest and 12C the lightest isotope.
Lorius and Jouzel were interested in hydrogen isotopes (1H and 2H) because hydrogen is one of the components of the water molecule (H2O) composing the ice core. Both hydrogen isotopes participate in the same chemical reactions but behave slightly differently thanks to their difference of mass.
For example, during water evaporation, the lighter isotope 1H will be preferentially incorporated in vapor, so the resulting cloud will be composed of a higher number of 1H than the original water. The snow falling from this cloud and accumulating in Antarctica ice layers through time reflects this evaporation process, keeping the hydrogen isotopic signature of the cloud.
By measuring the ratio between 2H and 1H, noted δD, in the water molecule (H2O) of the different ice layers, Lorius and Jouzel were able to document variations in abundance of 1H and 2H through time. Layers with a high amount of 1H (low δD) reflect periods of high evaporation rate and thus high temperature whereas those with a lower 1H amount (high δD) indicate a decrease in temperature. That’s how variations of hydrogen isotopic composition δD allow them to decipher temperature fluctuations through time.
As most of the chemical elements have several isotopes and different properties, repartitions and behaviors in the environment, they can be used to study the functioning of the Earth and its evolution. The Mendeleiev table thus represents a giant tool box for geochemists who can choose the most appropriate element to solve Earth enigmas. For example, the 14C is used for datation of archeological objects and hydrogen and oxygen isotopes are commonly studied to understand past climate variation.
What is interesting me in my PhD is to understand how Messinian old giant gypsum (CaSO4.H2O) deposits outcropping nowadays at the periphery of the Mediterranean Basin have been formed. The gypsum is composed of one calcium ion Ca2+, one sulfate ion SO42- and one molecule of water H2O. To precipitate from seawater, gypsum just needs high amounts of calcium (Ca2+) and sulfate (SO42-) (water is already fully available in…seawater 😉). Moreover, their naturel amount in seawater is too low to form gypsum spontaneously, so this mineral is ordinarily formed from very saline waters, in environments with high evaporation rates (allowing concentration of calcium and sulfate ions). However, against all expectations, recent studies have shown that the Messinian gypsum could have formed in waters with very low salinities! (More information about this problematic in p7 of the Saltgiant Newsletter).
So how could there be so many sulfate and calcium ions for gypsum to precipitate if the water was diluted? And more precisely what is the provenance of sulfate and calcium ions in sufficient quantities to precipitate gypsum, if they are not concentrated by evaporation? For now, I am working on the hypothesis that the sulfate was accumulated by the activity of micro-organisms which use sulfur compounds to live.
But how can I prove that these micro-organisms were active in the Mediterranean periphery environment 6 million years ago?
Luckily, I have something in my magic Mendeleiev table to help me! Sulfur isotopes have a very particular behavior when they are used by the micro-organisms I suspect to have been involved in the formation of my gypsum. This is why I am now analyzing the multiple sulfur isotopes (δ34S, Δ33S and Δ36S) of the sulfate ion (SO42-) contained in the Messinian gypsum (CaSO4.H2O).
For now, it is just the beginning of my investigations, but perhaps I should follow Lorius’s method and after a few evenings drinking whisky and looking at my samples of Messinian gypsum, I am sure I will have the answer…
- Jacquet, Luc. La Glace et le Ciel. Documentary. Production Eskwad and Wild-Touch. Distribution: Pathé and Wild Bunch. 2015, 89 minutes.
- Jouzel, J., Merlivat, L. & Lorius. Deuterium excess in an East Antarctic ice core suggests higher relative humidity at the oceanic surface during the last glacial maximum Nature. 299, pp 688–691 (1982)
- Jouzel J., C. Lorius, S. Johnsen and P. Grootes, Climate instabilities : Greenland and Antarctic records. Compte Rendu de l’Académie des Sciences de Paris. vol. t. 319, série II, 65-77, 1994
- Natalicchio M., Dela Pierre F., Lugli F., Lowenstein T K., Feiner S J., Ferrando S., Manzi V., Roveri M., Clari P., Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin(Italy). Geology . ISSN 0091-7613. – 42(2014), pp. 179-182.
- Evans N.P., Turchyn A. V., Gázquez F., Bontognali T.R.R., Chapman H.J., Hodell D.A.,
- Coupled measurements of δ18O and δD of hydration water and salinity of fluid inclusions in gypsum from the Messinian Yesares Member, Sorbas Basin (SE Spain),Earth and Planetary Science Letters, 430, pp 499-510, 2015,ISSN 0012-821X, https://doi.org/10.1016/j.epsl.2015.07.071.
- Roveri, M., Flecker, R., Krijgsman, W., Lofi, J., Lugli, S., Manzi, V., Sierro, F.J., Bertini, A., Camerlenghi, A., De Lange, G., Govers, R., Hilgen, F.J., Hübscher, C., Meijer, P.T., et al., 2014, The Messinian Salinity Crisis: Past and future of a great challenge for marine sciences: Marine Geology, v. 352, p. 25–58, doi: 10.1016/j.margeo.2014.02.002.