Micro-thermometry of liquid-vapor homogenization of fluid inclusions in stalagmites and its applications

Reconstructing past temperature variations with high accuracy has become one of the key scientific challenges in global environmental change research. Current temperature reconstructions face issues such as seasonal biases, insufficient representativeness, and differences between land and ocean. Stalagmites possess precise geochronology and record annual mean temperatures, making them essential for addressing the Holocene temperature conundrum. Fluid inclusions in speleothems experience fewer diagenetic processes, thereby preserving clearer paleoclimate information. Therefore, cave temperature can be determined by analyzing the physical or chemical properties of fluid inclusions. The micro-thermometry of liquid-vapor homogenization of fluid inclusions in stalagmite utilizes the temperature-density relationship of the trapped water in stalagmite fluid inclusions and determines the formation temperature of inclusion. It uses a femtosecond laser and microscope, allowing for the inclusion selection and inspection under a microscope. When measuring the stalagmite fluid inclusions in the laboratory, a single ultra-short laser pulse is used to stimulate vapor bubble nucleation after cooling the inclusion. Upon subsequent heating, the liquid phase expands at the expense of the vapor bubble, and ultimately, the inclusions homogenize to the liquid phase at Th(obs), i.e., the theoretical formation temperature of the liquid inclusion in the stalagmite. In practice, due to the surface tension effect and the micro size of the inclusion in the stalagmite, the observed homogenization temperature(Th(obs))is lower than the inclusion formation temperature(Tf=Th∞). To obtain the formation temperature of the stalagmite, it is necessary to measure the bubble radius under a specific temperature and homogenization temperature(Th(obs)). This method's reliability depends on preserving fluid inclusions in the stalagmite, which is closely related to the sample's storage temperature, calcite fabric, and preparation process. Furthermore, the method cannot be applied to the stalagmite whose formation temperature is below 9~11 ℃, thereby impeding the broad applications of this method to low-temperature paleoclimate reconstructions. Following the introduction, this study also presents a study applying the method for reconstructing past temperatures using a stalagmite(TF-R14)from Tianfu Cave(23°12′N, 99°17′E), Yunnan Province. The age of the stalagmite TF-R14 spans 14454±103 to 10589±52 a B.P., covering the Bølling-Allerød(BA), the Younger Dryas(YD), and the Early Holocene. A total of six layers were analyzed: layers 6 and 5 correspond to the BA, layers 4, 3, and 2 to the YD, and layer 1 to the Early Holocene. The results indicate that the annual mean temperature decreased from 14.9±0.4 ℃ to 13.6±0.4 ℃ from the Bølling-Allerød(BA)to Younger Dryas(YD), with a decrease of ca. 1.3±0.6 ℃. From the YD to the Early Holocene, the temperature increased from 13.6±0.4 ℃ to 15.7±0.4 ℃, with an increase of ca. 2.1±0.6 ℃. Considering the modern cave monitoring data, the reconstructed temperature changes from the last Deglacial to the Early Holocene are reasonable. This study validates the potential of the liquid-vapor homogenization temperatures for paleotemperature reconstruction and provides a technical approach for quantitative paleoclimate research.