"Leaf wax n-alkanes are long-chained lipids that are vital components of plant cuticles. What makes leaf wax n-alkanes unique is that their stable hydrogen isotope composition (δD) contains information on precipitation and plant water relations. In addition, leaf wax n-alkanes are abundant in leaves, soils, sediments and even the atmosphere and can persist with their δD values over millions of years. With this exceptional combination of properties, leaf wax n-alkanes and their δD values are now being celebrated as the much-needed ecohydrological proxy that provides information on the hydrological cycle and plant water relations across spatial and temporal scales that range from leaves to biomes and from weeks to millions of years. Despite the enormous potential that leaf wax n-alkanes have as ecohydrological proxy for a range of different research areas, the exact type of hydrological information that is recorded in the δD values of leaf wax n-alkanes remains still unclear. This is because critical mechanisms that determine the δD values of leaf wax n-alkanes are not understood. This proposal will perform the experimental work that is now needed to resolve the key mechanisms that determine the δD values leaf wax n-alkanes. These experiments will set the basis to develop a new numerical model that will allow to ultimately test what the exact hydrological signal is that leaf wax n-alkanes record in their δD values: a mere hydrological signal reflecting the amount or origin of precipitation or, a plant-shaped signal indicating plant water relations such as evapotranspiration. Building on this new model, COSIWAX will set out to test the potential that leaf wax n-alkane δD values hold as new ecohydrological proxy for ecology and ecosystem sciences. If successful, COSIWAX will establish with this research leaf wax n-alkanes δD values as a new and innovative ecohydrological proxy that has extensive possible applications in paleoclimatology, ecology, earth system sciences."