PRESERVATION POTENTIAL OF CALCIFIED EXTREMOPHILE MICROBIAL FORMS THROUGH GEOLOGIC TIME

Recent controversy has rekindled interest in the question of whether we can accurately identify remnants of ancient life in the geologic record. As microbial extremophiles become widely recognized as major contributors to ecosystems once thought to be devoid of life, questions of preservation potential, and recognizable microbial features in ancient deposits becomes more relevant. This study examined tufa and travertine deposits from Recent to Devonian age rocks to document which calcified or silicified features, indicative of biological activity, are preserved in some recognizable form through diagenesis and lithification.

Although isotopic and biomarker research is valuable in discriminating between abiotic and biotic material, standard petrography and scanning electron microscopy are tools that are readily available to geoscientists and should be the first step in looking for remnants of ancient life. Tufas and travertines generally produce highly porous and permeable deposits that have a high degree of potential for diagenetic alteration. Ancient tufa and travertine deposits are typically cemented with calcite or silica, often with little remaining porosity or permeability. This study establishes a broad visual vocabulary for recognizing fossilized microbial material.

Recent and Pleistocene tufas and travertines contain a plethora of calcified microbial material dominated by microtubules, coccoids, filaments, and clots (see papers by Riding, Renaut, and Jones for examples). But how many of those features are retained through cementation and compaction? Preserved features in ancient deposits form 4 general categories: encrusted tubules, smooth-walled circular voids, encrusted filaments grading into knobs, and clusters of rounded micrite crystals. Depending on the specific geologic history of a deposit, more or less recognizable features from these categories will be present. Curiously, the commonly recognized clots or coccoid forms from Recent and Cenozoic deposits are rarely observable in ancient tufas or travertines. Filaments or tubules seem be better preserved as indicators of ancient microbial activity.