STABLE IRON ISOTOPES DATA ASSES MICROBIALLY MEDIATION IN THE ROSSO AMMONITICO (MID-LATE JURASSIC, VERONA AREA, ITALY)

Different pathways can be proposed to explain the origin of the red pigmentation of the Rosso Ammonitico of Verona (RAV) : (i) telogenetic alteration ; (ii) detrital input and (iii) role of iron-bacteria. The third hypothesis is supported by our data : iron is not dispersed at random but is concentrated in a number of specific sedimentary features. The most pertinent indication is the systematic presence of micronic filaments with Fe-rich sheaths attributed to iron-bacteria (Mamet & Préat, 2003). No filaments have been observed in the interstratified grey facies of the RAV. The paragenesis of the calcite of the RAV shows this sequence : 1° submicronic micrite deposition with picoeucaryotes, nanodebris and iron-bacteria ; 2° first matrix recrystallization (‘neomicrite') ; 3° microsparitization (in the grey facies); 4° residual pores filled by three types of calcitic cements, respectively non luminescent, luminescent and dull; 5° fractures filled by luminescent cement. Fe isotopic compositions were analyzed on residues as well as on leachates of red and grey RAV facies. d56Fe values are systematically lower in the red residues (varying from -1.19‰ to -0.34‰) compared to the grey ones (-0.27‰ to -0.09‰) indicating a accumulation of lighter isotopes in the red parts. In addition, the residues of the red samples (hematite, XRD analysis) show depleted d56Fe relative to their corresponding leachates. These consistent isotopic differences between the red and grey residues could reflect the activity of iron bacteria and fungi. Compared with the data of Beard et al. (2003) the RAV Fe isotopic compositions are in the range of subrecent Fe-Mn crusts of the Atlantic Ocean and present-days Fe-Mn nodules of the Arctic Ocean reflecting mobilization of ocean sediment Fe by bacteria at sediment-water interface. The transformation of the original iron (hydro-)oxides to hematite is a very rapid process (weeks to months, Cornell & Schwertmann, 1996). It occurs without temperature change and preserved the original isotopic composition.