Résumé
Fossil calcite granules have been found in loess sequences, in large amount in tundra gley horizons and in palaeosols. These granules, composed of rhomboedric calcite crystals and organized in a radial crystalline structure are produced by earthworms that released them in the first upper centimeters of the soil. Oxygen isotope compositions of earthworm calcite granules (ECG) from the Nussloch loess sequence (Rhine Valley, Germany) have already been used to reconstruct absolute mean soil and air temperatures during the warmest period of the Last Glacial interstadials. In this study, we explored, for the first time, the potential of this new bio-indicator as a climate proxy for precipitation. In loess sequences, palaeoprecipitation reconstructions are estimated from the delta (super 13) C of organic matter, which is not always well preserved in sedimentary sequences. ECG were extracted from the Nussloch loess sequence (17-m-thick) previously dated between 45 and 23 ka. 30 granules were selected from 3 tundra gley horizons and 2 brown soils. Carbon measurements were performed on each granule and duplicated. Throughout the studied section, delta (super 13) C values range from -15.4 to -10.3 ppm for tundra gleys and from -14.9 to -9.5 ppm for boreal brown soils. The isotopic fractionation factor between the carbon ingested by the earthworm and the carbon output in the granule is equal to -11.7+ or -1.5 ppm. Thus, we estimated the delta (super 13) C of the plants with a mean of -24.3+ or -2.4 ppm for tundra gley horizons and -24.1+ or -2.4 ppm for brown soils. We used two independent methods to calculate the yearly amount of rainfall: 1) an empirical method based on the relationship between the delta (super 13) C of plants and that of biogenic carbonate and 2) the BIOME4 inverse model. The mean annual paleo-precipitation estimated by the empirical equation is higher in tundra gley horizons (365+ or -124 mm/yr) than in brown soils (304+ or -115 mm/yr). BIOME4 output suggests that the vegetation of tundra gley horizons was mainly shrub tundra whereas the vegetation cover from brown soils was between shrub and steppe tundra. Amounts of rainfall inferred from BIOME4 are coherent with the estimates obtained by the empirical method. This original preliminary study highlights the potential of ECG delta 13C values as a new proxy for palaeoprecipitation during the Last Glacial interstadials in continental environments.