MICROBIAL CARBONATES: DIAGENESIS AND ITS EFFECTS ON THE PRESERVATION POTENTIAL OF PALEOCEAN REDOX PROXIES (Invited Presentation)

The bulk and in situ trace metal and isotope contents of microbial carbonates are thought useful proxies for interpreting ancient ocean redox conditions. Moreover, during major paleoenvironmental perturbations, these features might also be valuable for mapping redox disequilibrium conditions developed in the narrow zone between peritidal and shallow subtidal waters at the time diverse stromatolitic morphotypes formed. Early during their burial history, however, microbial carbonates may well dissolve partially or entirely, re-precipitate, replace other minerals, and ultimately mix and exchange their original trace elements with those present in diagenetically evolved pore water fluids, which are both in organic and inorganic equilibrium with decaying metal-reactive biofilms and metastable minerals, respectively. Furthermore, in marginal environments physicochemical shifts linked to water mixing could lead to the development of temporally and spatially restricted diagenetic fronts, which result in variable rates of microbially induced precipitation and dissolution of authigenic minerals. For instance, two early diagenetic processes also influenced by the activity of benthic microbes that occur in parallel to, or shortly after, the onset of carbonate precipitation are: (i) reduction of (hydr)oxides, and (ii) oxidation of pyrite. These reactions significantly alter the ionic composition of the precipitation milieu that could be partitioned in multiple directions of mineral equilibrium, all of them with the ability to exert an effect over the rates of co-precipitation of other metals with Ca and Mg onto multigenic carbonates. Because the microbial communities that influence such dissolution reactions also thrive well in shallow burial diagenetic environments, paleocean chemical interpretations based on microbial carbonates should be accompanied by careful textural/mineralogical evaluations and concentrate on exceptionally well-preserved primary phases. Nonetheless, the small crystal size of such early formed carbonate minerals represents an analytical challenge that can only be undertaken by applying emerging spatially resolved micro-focused methods, capable of interrogating early diagenetic, fine-crystalline carbonate phases.