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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

Abstract : In the perspective of understanding the heterogeneity and intensity of the response of continents to climate changes, it is necessary to investigate a large number of climate archives displaying a dense spatial distribution. Unfortunately, contrary to the oceanic and polar realms, most landmasses are lacking such archives. Continuous deposits are rare, and the tools used by paleoclimatologists to decipher them all have their own biases, which are usually circumvented by means of a multi-proxy approach. This is not an easy task, especially in arid environments where the scarcity of water prevents potential organic archives from thriving and depositing. With this in mind, halite fluid inclusions (FIs) revealed very promising when Roberts and Spencer (1995), using the microthermometry technique, first showed their potential to hold past saline lakes temperature. Indeed, these micro-droplets of parent brine trapped inside incompressible cavities of the salt crystals happen to keep their initial density, and therefore the entrapment temperature, like the density of mercury indicating the temperature in the thermometer. However, Lowenstein et al. (1998) soon showed that FIs undergo damages during the requisite step of vapor phase nucleation in the freezer. Since then, paleoclimate studies using halite FIs have been few. During this thesis, we have developed a new methodology, based on Brillouin spectroscopy (BS), to bypass the limitations of microthermometry. This technique utilizes the inelastic interaction between light and spontaneous (thermal) density fluctuations in the fluid to measure its speed of sound, hence allowing for the determination of its density, ergo entrapment temperature. This non-destructive approach avoids submitting samples to large temperature gaps, as it does not need the presence of a bubble in the FI. As our method keeps FIs intact, we have restored their potential as an accurate paleothermometer. We show that BS on halite FIs reveals the entrapment temperature with an accuracy better than ±1 ◦C. We have established an empirical equation that defines the size-dependent threshold pressure beyond which FIs get damaged, and developed a model to calculate the FI pressure as a function of temperature and composition. We can thus determine the FIs size limit for safe paleothermometry measurements for a large variety of different natural samples. To illustrate the power of Brillouin thermometry, we sampled several tens of halite intervals from the 450 meters-long core 5017-1 drilled in the deep Dead Sea in 2010-2011 through the Deep Dead Sea Drilling Project (DSDDP). The application of Brillouin thermometry to this record provides a unique quantification of temperature changes in this region during the Last Interglacial (LIG, ~135,000 to 115,000 years ago). Furthermore, we show that Brillouin spectroscopy allows, at the same time, the quantification of the Dead Sea level and its evolution. Using the reconstructed lake level curve to quantify paleorainfall, we thus propose a complete temperature-precipitation 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. Contrary to previous estimations, the region never experienced extreme drought during the LIG, and only reached conditions as dry as today towards the end of the period. The clear connections with the Mediterranean and the Atlantic exhibited by the record, along with the clear climatic trends observed, lead us to suggest a strong orbital forcing of the atmospheric circulation over this part of the globe. 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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Submitted on : Thursday, March 26, 2020 - 11:34:49 AM
Last modification on : Wednesday, November 3, 2021 - 5:24:35 AM
Long-term archiving on: : Saturday, June 27, 2020 - 1:26:22 PM


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  • HAL Id : tel-02519728, version 1


Emmanuel Guillerm. 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. Earth Sciences. Université de Lyon, 2019. English. ⟨NNT : 2019LYSE1284⟩. ⟨tel-02519728⟩



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