A stalagmite-based multiproxy reconstruction of local environmental changes in the 20th century from Normandy, France

Speleothems are increasingly widely studied globally, but some regions remain poorly covered, including NW France. The cave-quarry of Caumont, located in Normandy develops in chalk limestone and contains speleothem formations from different time intervals. Chalk was extracted from this quarry for building stone since Roman times well into the 20th century. We collected several stalagmites from one of the excavated chambers with the aim to investigate their sensitivity to environmental signals. Of particular interest is stalagmite CCB-1 which was deposited over the last century and likely holds highly detailed climatic signals. We have tested whether stalagmite CCB-1 records hydrological or thermal seasonality, and if atmospheric circulation patterns can be reconstructed from this sample. To establish modern baselines we monitored temperature, drip rate, drip water isotopes, and collected modern carbonate precipitates between November 2019 and July 2021. The monitoring revealed that in-cave conditions are very stable all year round (T_{cave air}= 10.4 ±0.3°C; δ¹⁸O_drip and δD_drip change is ~0.1‰ and <0.8‰ respectively). Modern calcite samples fall mostly on the Coplen (2007) equilibrium regression line, suggesting that modern precipitation occurs near isotope equilibrium.

 

From sample CCB-1 we obtained three U/Th dates, counted 114 growth laminae and analysed 750 samples for stable isotope composition. The age model was constrained with laminae counting, historical inscriptions, and grey value based annual growth counting measurements and provides an annually resolved record for the last 114 years (1905 to 2019). The δ¹⁸O and δ¹³C signals from CCB-1 were compared with surface climatic parameters including precipitation, temperature, and the standardised precipitation evapotranspiration index (SPEI) for the last century. While δ¹⁸O of carbonate is influenced by surface temperature and the original isotopic composition in precipitation, the δ¹³C signal is governed by effective moisture supply and prior carbonate precipitation. Our reconstruction analysis reveals a significant change in the isotope record after ca. 1960. This change is best seen when comparing the δ¹³C time series with the SPEI and might be related to increasingly drier local conditions. A trend to stronger local evapotranspiration in recent decades is most likely linked to an increasing lack of effective rainfall.