Characterization of moonmilk deposits as a potential natural recorder of organic matter and contaminant flows
Résumé
Moonmilk is a peculiar type of karst deposit, which is a porous, plastic deposit constituted by needle-fiber of calcite and more than 80% of water. Abiotic and biotic environment parameters are involved to explain moonmilk development [1-3]. Thus moonmilk deposits, which can be dated [1], could be potentially used to retrieve past environmental conditions [4]. Furthermore, walls coated by moonmilk were widely used by prehistoric Man for parietal representation (Chauvet cave for instance). Finally, because of their porosity, moonmilk deposits constitute a potential natural recorder of contaminant flow through karst systems. These three goals (environment proxy, conservation, natural traps to present flows) need to improve our knowledge of the link between the entrance fluxes of water coming from the cover soil and the role of moonmilk toward this water in order to better sort the influence of biotic and abiotic parameters in growth. The studied cave (Les Elaphes, Massif des Bauges, France, 1400 m) has the particularity to present 3 kinds of moonmilk containing more or less clay: pure white moonmilk (WM), brownish moonmilk (BM) and clayey moonmilk (CM) assumed to be more or less directly connected to soils sources. Each sample was analyzed with a large range of tools (XRD, oriented XRD for clay, SEM, grading, FTIR). A complete description of the organic matter (OM) entrapped in moonmilk and also contained in the soils above was done thanks to fluorescence spectroscopy measurement. Specific OM and pollutants, more particularly PAHs (Polycyclic Aromatic Hydrocarbons, contaminant produced by human activity since several centuries) could be transported with the flux as function of season or period of time. Estimation of the PAHs content was realized in the various fractions of moonmilk (in free water, adsorbed on clay and OM, entrapped in calcite matrix) and then compared to content in soil and streaming water in order to understand the flow of contaminant. In parallel, we analyzed (through DNA PCR based methods) the bacterial community structure in the 3 different moonmilk deposits. We used both molecular fingerprinting (DGGE), and cloning-sequencing (on 16S), to compare the structure of Eubacteria and Archae in these 3 samples. The objective was not to quantify the relative importance of the various bacterial phylotypes in the moonmilk formation but, rather, to provide first knowledge regarding the identity of the main bacterial groups in these samples, and the similarity (or not) in the bacterial composition from these 3 kinds of moonmilk.