Multi-isotope approach (δ44/40Ca, δ88/86Sr and 87Sr/86Sr) provides insights into rhizolith formation mechanisms in terrestrial sediments of Nussloch (Germany)

Carbonates in loess-paleosol sequences are considered as a valuable paleoenvironmental proxy for climate change during the Quaternary period. In this study we propose a new dataset of δ44/40Ca, δ88/86Sr and 87Sr/86Sr to understand the formation of rhizoliths (roots encrusted with CaCO3) within these kinds of sequences from Nussloch region (Germany). Our results indicate that the leached fractions of rhizoliths (sampled at 3.25 m, 6.65 m, and 8.45 m depths and leached with HCl) have lower δ44/40Ca and δ88/86Sr, and higher 87Sr/86Sr isotopic values than the leached fractions of rhizosphere and root-free loess of the same stratigraphic levels. This suggests different mechanisms of carbonate formation and different sources of Sr and Ca. A three steps model of formation of rhizoliths from Nussloch has been proposed. First of all, the roots grow in an existent stratigraphic level and the biological activity associated with roots allows for the dissolution of carbonate and non-carbonate minerals. A part of the available Ca and Sr is taken up by roots, which absorb preferentially the light isotopes (e.g., 40Ca and 86Sr). A higher rate of water absorption by roots than nutrients, in association with CO2 released by biological respiration, allows to reach supersaturation with respect to CaCO3. The precipitation of rhizolith carbonate selects preferentially the light isotopes (e.g., 40Ca and 86Sr) leading to lower δ44/40Ca and δ88/86Sr isotope values than the ones recorded on the closest rhizosphere and loess. The higher 87Sr/86Sr signatures of rhizolith carbonates, which is not modified by root uptake or by precipitation processes, is the result of a mixture between 87Sr/86Sr signatures of carbonate and non-carbonate minerals digested around the roots. Nevertheless, grey zones remain, in this main hypothesis, about the very high contribution of residue materials to the isotopic signatures of rhizoliths. In this light, other mechanisms of downward transfer of Ca and Sr from upper stratigraphic level, that could imprint the leached fractions of rhizoliths, have been discussed.