FORMATION, ALTERATION, AND PRESERVATION OF HYPERTHERMOPHILIC BIOSIGNATURES

Biogeochemical interactions in high-temperature (>80°C) ecosystems produce a variety of biosignatures that include microbial fossils and microbially influenced accretionary growth structures (e.g., biogenic stromatolites). Studies of hydrothermally precipitated chert deposits and hydrothermally silicified sediments indicate that relicts of microbial biosignatures are preserved even in Archean rocks. Hyperthermophilic biosignatures, like all microbial biosignatures, are altered by taphonomic processes and diagenetic mineral transformations. Understanding how these processes affect hyperthermophilic biosignatures improves our ability to detect and interpret paleontological information from them in ancient and extraterrestrial hydrothermal deposits. As discussed here, a framework for understanding how hyperthermophilic biosignatures are preserved involves comparisons between modern hot spring deposits, hydrothermal sinters produced abiotically, and hydrothermal cherts preserved in older analog deposits.

Microbial communities play an important role in defining and modifying the surface characteristics of accretionary structures, and siliceous hydrothermal precipitates are no exception. Hyperthermophilic communities, dominated by distinctive, filamentous morphotypes, provide a continuous supply of substrates for the precipitation and adsorption of opaline silica. The distribution of the primary colonizers on the accretionary sinter surfaces imposes the most significant control on the subsequent development of geyserite microstructures. The structural and chemical fidelity of fossilized microbial cells depend upon the sequence of biogeochemical interactions that lead to their formation. Microfossil preservation was found to depend upon the intrinsic characteristics of the microorganisms, the extrinsic characteristics of their environment, and the relative timing of mineral precipitation vs. microbial degradation. Taphonomic changes alter the fidelity of geyserite stromatolites and microfossils. Key factors determining the pathway and degree of alteration include the rates of secondary infilling, mineral transformation, and the amount of water/rock interaction in the deposit during the earliest stages of diagenesis. Understanding how biosignatures of chemolithotrophic biofilms living near life's upper temperature limit are preserved improves our ability to interpret how microbial communities have interacted with their environments throughout geological time.