Carbon and oxygen isotope compositions of the needle fibre calcite (NFC)
from two soils observed in quarries (Villiers and Savagnier) in the
Swiss Jura Mountains are used to investigate the processes involved in
the formation of these calcite crystals in pedogenic environments. NFC
occurs in soils with different macroscopic and microscopic morphologies.
Three groups of microscopic morphologies are distinguished: simple
needles (SN), simple needles with overgrowths (SNO), and simple needles
with nanofibres (SNN), nanofibres being either organic or mineral in
nature. These groups correspond to different stages in the formation and
evolution of NFC. Evaluation of the delta C-13 and delta O-18 values of
NFC and late calcite cements (LCC) in relationship to the composition of
the carbonate host rock (CHR), and delta C-13(DIC) from the soil waters,
indicate that both NFC and LCC are precipitated in isotopic equilibrium
with the soil waters. Similar delta C-13 and delta O-18 values of the
elongated NFC and the rhombohedral calcite crystals of the LCC suggest
that the precipitation of these different calcite habits is not due to
changes in physicochemical conditions. The growth of NFC crystals inside
an organic mould formed by the fungal hyphae could explain the formation
of calcite crystals in the shape of a needle in isotopic equilibrium
with the local environment. The comparison of the delta C-13 and delta
O-18 signatures of the various NFC morphological groups suggests that
they likely evolve in the soil solution from calcite crystals liberated
during the degradation of fungal hyphae walls. The NFC may be a proxy of
the soil conditions (temperature and vegetation) during its
precipitation, and also an indicator of biogenic (fungal) activity. (C)
2010 Elsevier B.V. All rights reserved.