Source of rainfall above Mediterranean caves (Chauvet and Orgnac) and long-term trend of cave dripping oxygen isotopes based on 20 years monitoring records: Importance for speleothem-based climate reconstructions

Abstract

Understanding the factors that shape climate and influence the isotopic composition of precipitation is crucial for paleoclimate reconstructions, especially in regions with Mediterranean climates where rainfall is influenced by both Atlantic and Mediterranean moisture sources. This study examines the relationship between moisture origins, climatic variables, and the stable isotopic composition of precipitation and cave drip water in the Orgnac and Chauvet caves, located in southern France, over a 20-year period. The research reveals notable seasonal variations in rainfall δ¹⁸O values, driven by temperature and Rayleigh distillation processes. As shown in our previous work in Villars Cave (SW-France), temperature changes alone cannot fully explain the observed isotopic variability. We observed that winter precipitation tends to have lower δ¹⁸O values due to longer transport distances from distant oceanic sources, while summer precipitation displays higher δ¹⁸O values due to shorter transport paths. Additionally, the study highlights the influence of sea surface wind speeds and evaporation rates on water vapor isotopes, further shaping the seasonal δ¹⁸O patterns. As rainwater infiltrates the soil and percolates into the karst system, the seasonal δ¹⁸O signal in drip water is often dampened due to mixing in the reservoirs above the caves, which typically reduces seasonality. The key findings include: (1) a multi-year increasing trend in drip water δ¹⁸O, likely associated with reduced local water excess and the effects of global warming, with significant implications for speleothem isotope records, and (2) moisture from the Mediterranean Sea contributes to 10% of the total precipitation source, despite the region's proximity to the sea, especially during intense storm events. This study provides new insights into the complex interactions between moisture sources, temperature, and isotopic signatures in Mediterranean climate regions, with implications for improving speleothem-based paleoclimate reconstructions.