Needle Fibre Calcite (NFC): biogenic or physico-chemical in origin. Biogeochemical approach.

This study deals with the processes leading to calcium carbonate accumulation in soils and surficial sediments, in relationship with the vegetation and associated micro organisms. A particular habitus of calcite is studied: the needle fibre calcite (NFC). This study use biogeochemical techniques in order to evaluate the potential implication of fungi in NFC precipitation in pedogenic environment, and their possible role in the sequestration of atmospheric carbon. The objectives are:

1) to compare the stable carbon and oxygen compositions of NFC, purely physico-chemically precipitated calcite, the carbonate host rock and the dissolved inorganic carbon of the soil.

2) to determine three groups of microscopic morphologies, and to compare their isotope signatures.

3) to measure the incorporation of various elements (mainly Fe, Mn, Na) and REE (Rare Earth Elements) in NFC and more precisely in each microscopic morphology.

4) to trace the source of Ca involved in NFC precipitation, using the 87Sr=86Sr ratio, in order to understand the exact role of fungi in primary carbonate weathering, and to estimate the amount of atmospheric carbon stored in NFC.