"Isotope-Hydrology": Influences of high reliefs on isotope hydrology and coupled climate proxies [funded by DFG]

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The overall aim of this subproject is to enhance our understanding of Corsica’s hydrologic cycle with special focus on isotope effects that directly influence the stable isotopic composition of precipitation. This is one of the major control factors in the interpretation of paleo-climate archives, such as oxygen isotope records from tree-rings. It will therefore enable the interpretation of biogeochemical archives from high relief regions with improved reliability.

The three main questions to be addressed in the isotope hydrology subproject are:


1. How do high relief topography and seasonal variations in the position of the planetary boundary layer (PBL) influence the isotope altitude effect?

    Seasonal changes in height above land surface of the PBL may influence the isotopic composition of precipitation in Corsica. A recent study by Kern et al. (2014) suggests that the position of the PBL has a so far unknown but important role in isotope hydrology, especially over high relief areas. Thus, Corsica is an ideal site with its steep topographic gradient. Although river samples of a previous study on Corsica did not reveal any anomalous elevation effects, several reasons lead to the conclusion that these samples may not be suitable to account for a precise investigation of this phenomenon and needs to be repeated with more ideal precipitation samples.


2. How do the isotope values of precipitation relate to the source region?

    The western Mediterranean region is located in the transition zone between west wind drift and the subtropical high-pressure belt with trade inversion (Bruno et al. 2001). Mainly during the cooler season Atlantic front systems can transport water vapour over long ranges from the West. The island of Corsica with its high relief represents a major obstacle in the way of these eastward moving systems. They bypass the island either north or south and cause regional advection of moisture from both westerly and easterly directions. However, the contribution of local vapour from the adjacent Mediterranean Sea areas to precipitation on the island is so far unknown. The reflection of these processes in the stable isotope signature should be explored within this study. The investigation of detailed climatic conditions and processes at the island are subject of the subproject CorsiClimAte. The origin of water vapour is usually reflected in the δ18O precipitation and deuterium excess (d-excess) values but also in the tritium signatures of rain (Gat et al. 2000). This holds especially true for event samples from individual precipitation events. However, the pristine δ18O moisture source signal can be masked by various effects including the continental effect, the afore-mentioned altitude effect or sub-cloud evaporation from falling drops. An option to delineate moisture source regions for event samples is by air-mass back trajectory models such as HYSPLIT. This allows to use overlapping geochemical (δ18O and δ2H, 3H), climate (T and P) and spatial information to identify the precipitation origin.


3. How is the δ18O precipitation value reflected in soil water?

For paleo-climate archives but also for hydrogeological studies on groundwater recharge conditions it is necessary to find out how the δ18Op translates to the soil and eventually to the groundwater. In Corsica, trees from high altitude locations that are typically used for dendrochronology studies will mostly rely on water from the unsaturated zone. On a global scale it has been recently shown that the δ18O of tree-rings generally correlate with δ18Op (Schubert and Jahren 2015) with different slopes of the correlation for different tree types. The same study also showed that intraannual variations could be resolved by this approach. However, so far little is known about specific conditions on the island of Corsica with its high mountainous terrain and its crystalline dominated geology with thin topmost soil and underlying regolith. One further aim of this proposed study is to investigate potential modifications of the δ18O value from precipitation to soil water for example by evaporation.

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