δ13C signal of earthworm calcite granules: A new proxy for palaeoprecipitation reconstructions during the Last Glacial in western Europe

Quantification of paleoprecipitation during the Last Glacial is a key element to reconstruct palaeoclimates. Recently, fossil calcite granules have been identified in loess sequences with high contents in specific horizons. In this study, we explored for the first time the potential of this new bio-indicator as a climatic proxy for precipitation in western Europe during the Last Glacial. We extracted 30 granules from eleven samples belonging to three tundra gleys and two brown soils from the Nussloch loess sequence previously dated between 50 and 20 ka. Stable carbon isotope measurements were performed on each granule and duplicated. Throughout the studied section, δ13C values range from −15.4 to −10.3‰ for tundra gleys and from −14.9 to −9.5‰ for brown soils. By taking into account the fractionation factor between the carbon ingested by the earthworm and the carbon output of the granules, the δ13C values of these granules reflect the composition of the C3 plant vegetation cover. Thus, we estimated the δ13C of the plants with a mean value of −24.3 ± 0.9‰ for tundra gleys and −24.1 ± 0.9‰ for brown soils, which are in agreement with values obtained from organic matter preserved in sediments. Palaeoprecipitation range over both tundra gley horizons and brown soils were estimated at about 333[159-574] mm/yr by using an empirical relationship determined between present-day plant leaf isotopic discrimination and the mean annual precipitation. This original preliminary study highlights the potential of earthworm calcite granule δ13C measurements as a new proxy for paleoprecipitation during the Last Glacial interstadials in continental environments.