Periodic Reporting for period 1 - CROSSROADS (Climate ReOrganizations at Synoptic Scale as Recorded in the Offshore Archives of the Dead Sea)
Reporting period: 2022-09-01 to 2024-08-31
The warming trend of the last decades has produced increased drought in the Mediterranean region (MR), and projections of the 6th IPCC report warn of even greater drought risks in this region in the future. Water stress is not only problematic for societies and their economies (agriculture), but also adds to political tensions, for example, the Middle East. Atmospheric circulation at synoptic scales (i.e. at scales beyond 1000 km) controls the transport of moisture and heat to landmasses, especially during winter when its influence reaches a maximum. In the Northern hemisphere (NH) midlatitudes (~30 to 65 °N), stronger-than-average and northward-shifted westerlies favor warm and wet winters in Northern Eurasia but leads to colder and drier winters in latitudinal bands further South. In some regions, including the Mediterranean, rainfall occurs quasi-exclusively during the cold season, making wintertime westerlies a critical factor for water availability, both because of temperature (through evaporation) and precipitation. Therefore, numerical models must produce realistic projections of changes in the westerlies induced by global climate change in order to assess the risks in hydrologically-sensitive regions. Climate models to date have reached sufficient accuracy to predict global increase in temperature, but they have lacked accuracy in finely reproducing changes in atmospheric circulation. Improved climate models have been based on comparisons of simulated variables to the same variables reconstructed with proxy data from past periods. This method, however, is limited by the biases of proxies, such as their dependence on multiple parameters.
In the CROSSROADS project, we investigate changes in the westerlies with a new approach. We (i) apply novel paleothermometry and paleo-brine composition methods and develop a new method for the reconstruction of lake levels to determine the two crucial variables that are affected by atmospheric circulation, temperature and hydrology; (ii) achieve these reconstructions at a location that is critical for understanding changes in the westerlies, the Dead Sea, and (iii) apply these results to the Holocene, a period for which a mismatch between simulations and proxy data is unresolved.
In the CROSSROADS project, we investigate changes in the westerlies with a new approach. We (i) apply novel paleothermometry and paleo-brine composition methods and develop a new method for the reconstruction of lake levels to determine the two crucial variables that are affected by atmospheric circulation, temperature and hydrology; (ii) achieve these reconstructions at a location that is critical for understanding changes in the westerlies, the Dead Sea, and (iii) apply these results to the Holocene, a period for which a mismatch between simulations and proxy data is unresolved.
During these first two years of the project, we have worked on multiple fronts to address the aforementioned issues.
First, we have worked on the development of the technique that uses fluid inclusions in halite (i.e. table salt, NaCl) as a "paleothermometer". A paleothermometer is a thermometer for past environments. Fluid inclusions are microscopic pockets of water trapped in crystals during their precipitation, and are thus so many archives of the brine in which the crystal grew. The density of the all-liquid fluid inclusion under atmospheric external pressure is an indicator of the temperature at which they were trapped. This density can be recovered by measuring the fluid inclusion "homogenization temperature", that is, the temperature at which a biphasic vapor-liquid inclusion becomes all-liquid -i.e. the temperature at which the vapor bubble disappears. On the theoretical side, we developed a physico-chemical model of fluid inclusions in halite. From the observed homogenization temperature of fluid inclusions, the model allows calculation of the true waterbody temperature upon entrapment, taking account of physical processes such as surface tension, mechanical and chemical interactions between the liquid content and solid container of the inclusion, and the pressure of the water column ovelying the crystal upon formation. This model led to a paper now submitted to the American Journal of Science. On the experimental side, we led field work in February 2023, during which we sampled halite that precipitated on the floor of the Dead Sea 35 meters below the water surface in the 1980s. These samples are interesting because they formed when the temperature of the lake was monitored, thus allowing comparison between reconstructed and monitored water temperatures. We nucleated vapor bubbles with a femtosecond laser and measured the homogenization temperature of hundreds of fluid inclusions from a dozen samples at the University of Bergen, Norway. After corrections using our physical model, we found an average temperature of the waterbody almost exactly identical to the monitored one within 0.1 °C, thus validating the accuracy of the paleothermometer. This experimental work was published in August 2024 in Chemical Geology.
Second, we applied the fluid inclusion paleothermometry technique to dozens of halites sampled in a core recovered from the deepest floor of the Dead Sea, during an experimental campaign at Bergen in March 2023. The sample ages span the last 12,000 years, the so-called Holocene period. We now have a timeseries of homogenization temperatures with an average time-resolution of ~400 years for the Holocene period.
Third, we measured several physico-chemical parameters of the fluid inclusions. During a 1 month stay at the University of Lyon, France, in October 2022, we measured the density of the biphasic (liquid+vapor) halite fluid inclusions using Brillouin spectroscopy in the Holocene Dead Sea samples. Then, at Binghamton University -the host for the outgoing phase of the project-, we measured the relative concentration of all major elements in the fluid inclusions of the same samples except sodium and chlorine, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry.
Finally, we developed a physical lake model of the Dead Sea, written in Wolfram Language. This model incorporates physical equations of all the major heat fluxes, and a simple stratification module based on the empirical observations of the last 45 in the Dead Sea. The model reproduces accurately the temperature of the Dead Sea monitored during the last 45 years. A manuscript is currently in preparation. The final goal of this model is to be applied to past periods to allow interpretation of deep water temperature in terms of atmospheric temperature.
First, we have worked on the development of the technique that uses fluid inclusions in halite (i.e. table salt, NaCl) as a "paleothermometer". A paleothermometer is a thermometer for past environments. Fluid inclusions are microscopic pockets of water trapped in crystals during their precipitation, and are thus so many archives of the brine in which the crystal grew. The density of the all-liquid fluid inclusion under atmospheric external pressure is an indicator of the temperature at which they were trapped. This density can be recovered by measuring the fluid inclusion "homogenization temperature", that is, the temperature at which a biphasic vapor-liquid inclusion becomes all-liquid -i.e. the temperature at which the vapor bubble disappears. On the theoretical side, we developed a physico-chemical model of fluid inclusions in halite. From the observed homogenization temperature of fluid inclusions, the model allows calculation of the true waterbody temperature upon entrapment, taking account of physical processes such as surface tension, mechanical and chemical interactions between the liquid content and solid container of the inclusion, and the pressure of the water column ovelying the crystal upon formation. This model led to a paper now submitted to the American Journal of Science. On the experimental side, we led field work in February 2023, during which we sampled halite that precipitated on the floor of the Dead Sea 35 meters below the water surface in the 1980s. These samples are interesting because they formed when the temperature of the lake was monitored, thus allowing comparison between reconstructed and monitored water temperatures. We nucleated vapor bubbles with a femtosecond laser and measured the homogenization temperature of hundreds of fluid inclusions from a dozen samples at the University of Bergen, Norway. After corrections using our physical model, we found an average temperature of the waterbody almost exactly identical to the monitored one within 0.1 °C, thus validating the accuracy of the paleothermometer. This experimental work was published in August 2024 in Chemical Geology.
Second, we applied the fluid inclusion paleothermometry technique to dozens of halites sampled in a core recovered from the deepest floor of the Dead Sea, during an experimental campaign at Bergen in March 2023. The sample ages span the last 12,000 years, the so-called Holocene period. We now have a timeseries of homogenization temperatures with an average time-resolution of ~400 years for the Holocene period.
Third, we measured several physico-chemical parameters of the fluid inclusions. During a 1 month stay at the University of Lyon, France, in October 2022, we measured the density of the biphasic (liquid+vapor) halite fluid inclusions using Brillouin spectroscopy in the Holocene Dead Sea samples. Then, at Binghamton University -the host for the outgoing phase of the project-, we measured the relative concentration of all major elements in the fluid inclusions of the same samples except sodium and chlorine, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry.
Finally, we developed a physical lake model of the Dead Sea, written in Wolfram Language. This model incorporates physical equations of all the major heat fluxes, and a simple stratification module based on the empirical observations of the last 45 in the Dead Sea. The model reproduces accurately the temperature of the Dead Sea monitored during the last 45 years. A manuscript is currently in preparation. The final goal of this model is to be applied to past periods to allow interpretation of deep water temperature in terms of atmospheric temperature.
The CROSSROADS has already achieved important progress beyond the state of the art. It has made halite fluid inclusion paleothermometry a robust paleothermometer, applied this paleothermometer to reconstruct paleotemperatures in the Dead Sea during the Holocene, made several steps towards the reconstruction of the chemical composition, volume and lake level of the Dead Sea during the Holocene, and developed a thermal model that accurately reproduces the temperature of the modern Dead Sea. Next expected results are (i) a reconstruction of the absolute concentration of all major elements, by combining density, relative concentration and chemical modelling results; (ii) a reconstruction of the lake levels of the Dead sea during the Holocene; (iii) and a reconstruction of the temperature of the deep Dead sea throughout the Holocene, obtained by applying water column height correction to the halite fluid inclusion homogenization temperatures. This Holocene temperature and hydrological reconstruction will provide unprecedented insights into the quantitative evolution of temperatures and rainfall in the Eastern Mediterranean during the last 12,000 years, allowing for a better understanding of the parameters that drive atmospheric circulation changes in the area.