Hydrothermal deposits that form in the presence of heat-loving microbial communities are analogs for one of Earth's earliest ecosystems. Molecular phylogenetic studies of extant life suggest that these earliest ecosystems were inhabited by hyperthermophiles (optimum growth temperatures > 80?C), microorganisms that define life's upper temperature limit. It remains debatable whether life evolved at high temperatures, or whether high temperature microorganisms were the only groups to survive the massive impact events of the 'late bombardment' period. In either case, study of the potential for hydrothermal deposits to serve as important paleobiological repositories supports NSF's goal to understand how the products of early life on Earth are preserved in the geological record. To maximize paleobiological interpretations of hyperthermophilic communities from ancient hydrothermal deposits requires an understanding of the processes by which hyperthermophilic biogenic signatures are initially preserved and subsequently altered by post-depositional diagenetic effects. Biogenic signatures, the products of life that are preserved in the rock record, include microfossils, stromatolites, and chemofossils. The primary objectives of the project are to identify the organosedimentary processes responsible for the preservation of hyperthermophilic biogenic signatures in high-temperature ecosystems, and to document how primary biogenic signatures are altered during early diagenesis.
To provide a framework within which the paleobiology of hyperthermophilic communities fossilized in siliceous hydrothermal deposits (thermal spring and especially epithermal deposits) can be properly interpreted, extant communities that occupy silica-depositing thermal springs will be characterized by molecular phylogeny and various electron beam-imaging methods. Hyperthermophilic microfossils that display morphological fidelity will be investigated using spectroscopic imaging, diffraction, and low dose transmission imaging and analytical (EELS/EDS) electron microscopy to identify the mechanisms of fossilization. To establish a baseline survey of the types of unique hyperthermophilic biomarker compounds preserved in natural siliceous hydrothermal deposits, samples with organically preserved hyperthermophilic remains will be analyzed for total organic carbon (TOC), 13C, lipid ratios, and biomarkers. The organosedimentary interactions responsible for the morphological development of high-temperature siliceous sinter stromatolites will be investigated from the submicroscopic to macroscopic scale. Field experiments, designed to produce sequentially biogenic and abiogenic siliceous hydrothermal deposits from natural hydrothermal fluids in a 'water tunnel' will be conducted at a geothermal power station in New Zealand. Direct comparison of biogenic and abiogenic hydrothermal deposits from the submicroscopic to macroscale will provide a robust set of criteria for distinguishing the biogenicity of siliceous hydrothermal precipitates. An improved ability to recognize and confidently assess in the rock record the biogenic signatures of hyperthermophilic communities will advance interpretations of the early fossil record of life on Earth and, potentially, on Mars.
