Periodic Reporting for period 1 - FluidMICS (Fluid Inclusion Microthermometry in Speleothems) Reporting period: 2021-09-01 to 2023-02-28 Summary of the context and overall objectives of the project Ongoing global warming is rapidly moving us away from the climate states we are used to and understand from observational time series. This poses the urgent need to extend our “climate memory” by deciphering climate information stored in geologic archives. However, obtaining quantitative estimates for past climate changes is challenging, particularly in terrestrial archives due to common limitations in time coverage, resolution, and chronology. FluidMICS employs a novel technique to reconstruct past temperatures based on physical properties of relict drip water preserved in cave formations (speleothems). The behavior of such micrometer-scale fluid inclusions during cooling and heating is directly related to the temperature at which the inclusions were once closed off. This physical basis makes the method uniquely robust and distinguishes it from other paleo-thermometers that depend on empirical calibrations. Combined with the distinct advantages of speleothems, which cover long time periods and can be absolutely dated, this approach will lead to unprecedented insights into magnitude, timing, and distribution of past temperature changes, lifting paleoclimate research to a new level. Our pilot data show that the microthermometry method faithfully discloses past temperatures several hundred thousand years ago. In FluidMICS, we aim to generate a solid understanding of potential non-thermal effects, further increasing precision and accuracy of the reconstructed temperatures, streamlining the analysis, and extending the applicability of the method. These advances will enable us to generate uniquely accurate and precise terrestrial temperature records far back in time that are distributed over large areas of the globe. These new datasets will serve as invaluable resources to better understand the complexities of our climate system under different atmospheric CO2 concentrations and in times of rapid change, and to test climate models used for future projections. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In the beginning of the project, we assembled the team and expanded our laboratory facilities. We then started generating new records of past temperature changes in parallel with further testing and improving the method. The reconstructions are currently mainly focusing on generating new data from Borneo in the tropical West Pacific, a region that hosts the warmest ocean waters on the globe and is thus an important factor in the global climate system. We have also started investigating samples from other locations in order to test the applicability of our new method to different types of speleothems. We have found that the method works very well in some samples, but we have also seen cases where the data show unexpected results. We purposefully include samples that seem less ideal because all of these results allow us to better understand the non-thermal factors that can modify the signal. In addition to analyzing natural speleothems, we have started experiments to grow synthetic inclusions in the laboratory and are applying various methods to better characterize the fluid inclusions in the context of the confining calcite host. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The speleothem fluid inclusion microthermometry method is already unique regarding its precision and accuracy coupled with the superb age control in speleothems. In the FluidMICS project, we aim to significantly further improve the method by greatly refining our understanding of the fluid inclusion-calcite host system. Our insights will further improve the reliability and applicability of the method, for example by enabling us to assess which samples and inclusions do and do not fulfill the prerequisites of the method. The novel microthermometry approach in speleothems yields unique constraints into the precise magnitude and timing of past temperature changes on land, providing crucial new insights with every climate record we generate. We finalized and published a first temperature reconstruction for Borneo covering the transition out of the last ice age, where our data show that the tropics warmed substantially, and in-step with the rise of atmospheric CO2. We are currently generating similar data for other glacial-interglacial transitions and for intermediate climate states that are characterized by large temperature swings in the high northern latitudes. We aim to do the same for other low- to mid-latitude locations across the globe to derive a comprehensive picture of the role of low-latitude climate in global climate transitions.