Determination of palaeotemperatures and lapse rates in central Europe using water isotopes in speleothem fluid inclusions

Speleothems are secondary cave carbonate deposits and contain fluid inclusions (typically between 0.05 and 0.5 weight %) which are filled with cave drip water that was trapped in the host calcite at the time of growth. Speleothem fluid inclusions are therefore ideal natural repositories of past cave drip water and rainfall, respectively. Thus, hydrogen (δDfi) and oxygen (δ18Ofi) isotopes of fluid inclusion waters contain direct information about the hydrological cycle in the recent and distant past, allowing us to gain direct information on the precipitation amount, temperature or moisture source, depending on the geographical position of the cave. In central Europe, the δDfi alone or coupled with calcite oxygen isotope (δ18Oc) enables us to determine absolute paleotemperatures of mean annual or cold-season temperatures. Speleothem fluid inclusion-based paleotemperature records can therefore complement biological proxies such as pollen or tree ring records, which are seasonally biased towards spring and summer. In addition, speleothem water content is a new and rarely used potential proxy for the amount of rainfall above the cave, allowing us to study variations in both temperature and amount of rainfall within the same stalagmite. Speleothem fluid inclusion water isotopes are a key-proxy for paleotemperature, whereas only a limited number of studies has been published so far. Most of the fluid inclusion records are coarsely resolved because of analytical limitations. As a result, little is known about the water isotope evolution in the past, particularly in Europe and the alpine region. In Switzerland, only a very limited number of temperature reconstructions extend beyond the Last Glacial Maximum (LGM) as lake sediments containing pollen and chironomids formed after the LGM and older deposits are rare and affected by poor preservation. In contrast, speleothems in caves are protected against (glacial) erosion and therefore old samples can be easily found in caves in Switzerland and the neighbouring regions, providing a unique opportunity to develop long and continuous paleotemperature records for central Europe. The development of new analytical techniques using laser spectroscopy allows to conduct paired analyses of δDfi and δ18Ofi on water extracted from speleothem samples. Based on exceptional results on a 14,000 year-old stalagmite from Milandre Cave in the Jura Mountains, we know that fluid inclusion water isotopes can be used to develop quantitative temperature records with a high temporal resolution. Encouraged by these initial results and further pilot studies, we aim at developing Holocene and Late Pleistocene temperature records using a network of caves and speleothems from the alpine foreland (Jura mountains) and alpine region. Using a state-of-the-art analytical method developed by the applicants, two PhD projects will investigate, improve and apply the fluid inclusion water isotope paleo-thermometry to determine absolute temperatures for the Holocene and Pleistocene periods. More generally, they will investigate the modern and past hydrological cycle in order to generate comprehensive paleowater isotope datasets for Switzerland and neighbouring regions. For this research, the major achievements expected are the following. - Characterisation of speleothem fluid inclusions using digital image analyses. Size, spatial distribution, tightness and water content. - Water isotope and paleotemperature records covering several glacial and interglacial intervals for the alpine foreland and alpine region. - Determination of Holocene and Pleistocene isotope and temperature lapse rates in Switzerland and neighbouring regions. As this research area is practically unexplored, this project will (i) provide a thorough understanding of the use of speleothem fluid inclusions in paleoclimate research, (ii) generate of comprehensive water isotope datasets for the western Alps and pre-alpine areas, (iii) give new insights in the past water cycle and paleotemperature variations over several glacial and interglacial cycles.