Thèses

Vendredi 6 Décembre 2019 à 14h00.

Turning Halite Fluid Inclusions into Accurate Paleothermometers with Brillouin Spectroscopy: Development of a New Method and Application to the Last Interglacial in the Dead Sea

''Determining past continental temperatures is a key aspect to understand past climatic trends, as continents are more sensitive than oceans to climate variations. Modest global changes may cause a rearrangement of the atmospheric circulation and may thus induce large changes in the distribution of heat and moisture over lands. However past continental temperature proxies ("paleothermometers") are scant, and calibrations on which they are based may be subject to controversy. During this thesis, we have developed a new paleothermometer, based on Brillouin spectroscopy (BS), to unravel the entrapment temperature of fluid inclusions (FIs) trapped in halite (NaCl) crystals. This technique utilizes the inelastic interaction between light and spontaneous density fluctuations in the fluid to measure its speed of sound and deduce its entrapment temperature. We have thus developed a physically-based paleothermometer and demonstrated the ability of halite fluid inclusions to record past temperatures, assumed for decades but never firmly evidenced so far.
To illustrate the power of Brillouin thermometry, we sampled several tens of halite intervals from a long core drilled at the bottom of the Dead Sea, Palestine. We focused on halite intervals deposited during the Last Interglacial (LIG, ~135,000 to 115,000 years ago)​, known as the most recent period when climate was comparable to today, and also as the moment when Homo Sapiens first stepped out of Africa across the Palestinian gateway. The application of Brillouin thermometry to this record provides a unique quantification of temperature changes in this region during the LIG. We also show that Brillouin spectroscopy allows, at the same time, the quantification of the Dead Sea level. Using the reconstructed lake level curve to quantify past rainfall, we thus propose a complete temperature-hydrology reconstruction that enable us to outline a radically new narrative for the climate of the region during this period. We show that the LIG winter temperatures were mostly lower than today, and precipitation were much higher, albeit on a drying trend. We suggest a mechanism linked to the Earth's precession to account for this climatic trend. The example of the Dead Sea shows that Brillouin spectroscopy on halite FIs is in position to provide valuable data to test the efficiency of climate models.
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