Microbial EPS-mediated amorphous calcium carbonate–monohydrocalcite–calcite transformations during early tufa deposition

The Holocene Wolfenden tufa deposit in south-eastern British Columbia, western Canada, preserves a unique record of the earliest stages of calcium carbonate deposition resulting from microbial extracellular polymeric substances-mediated precipitation of amorphous calcium carbonate (ACC) with partial transformation to monohydrocalcite (MHC) and subsequently to nanocrystalline calcite. This is the first documentation of tufa mineralogy involving ACC transformation to MHC. Progressive dehydration triggered ACC–MHC–nanocrystalline calcite transformations on bryophytes, algae and cyanobacteria sheaths. The adsorption of extracellular polymeric substances matrix molecules into the ACC and ACC–MHC structures preserved polymorph mineralogy of incomplete transformation. Unusual concentrations of biofilm extracellular polymeric substances filaments provided nucleation sites for the ACC precipitation. The ACC nucleation calcified extracellular polymeric substances filaments and resulted in partially coalesced arrays of nanoscale ACC spheroids. Mesocrystalline structures of MHC reconfigured the concentric growth layers of ACC precipitate with bulbous ACC–MHC protuberances. Nanocrystalline rhombic faces of calcite developed within and on the surfaces of the ACC–MHC protuberances. Dehydration of these concentric growth layers of ACC–MHC resulted in the transformation into nanocrystalline calcite with substrates coalesced into micrite fabrics. Recrystallisation obliterated evidence of the calcified extracellular polymeric substances filaments and resulted in microcrystalline calcite spar domains as the widespread encrustation fabric. Localised magnesium adsorption during nucleation of the ACC within the biofilms resulted in needle calcite crystals without the precursor ACC–MHC transformation process. Microbial extracellular polymeric substances-mediated precipitation of ACC as a necessarily critical step in the earliest phase of the tufa deposition process, leading to the nucleation of calcite has been underappreciated and generally not considered. These earliest stages of calcium carbonate precipitation are proposed as a possible template for other tufa deposits, where the evidence of microbial extracellular polymeric substances-mediated precipitation of ACC with transformation to MHC and subsequently to nanocrystalline calcite has been obscured by recrystallisation into micrite and spar fabrics.