Microbial Fossils and Sedimentary Facies in Terrestrial Hot Spring Deposits: Relevance for Early Life in the Archean

CAMPBELL, K.; WESTALL, F.; GUIDO, D.; FOUCHER, F.

Workshop; Early Earth, Hydrothermal Activity and Life, and the Implications for Life on Mars and Elsewhere; 2015

Terrestrial land-based hot springs today harbor diverse and distinctive microbial communities in varied habitats with well-defined temperature gradients, from near-boiling spring-vents to cooler apron terraces, pools and geothermally influenced marsh, fluvial, and lacustrine settings. Dissolved minerals in spring discharges enclose microbial remains in mainly siliceous sinter and travertine deposits, and also in less common iron-silica occurrences. Therefore, hot-spring deposits represent an array of microbially dominated ?extreme environments?, encompassing the upper temperature limit for life on land, with a geological record dating back to at least the Devonian. Recently a possible example of high-temperature vent geyserite was reported from 3.48 Ga strata in the Pilbara craton, Western Australia. Furthermore, an area of siliceous deposits (Home Plate) in Gusev Crater has been inferred as hot-spring related, and is being considered as a landing-site target for the Mars 2020 mission. Thus, hydrothermal systems are cited as analog settings for early life on Earth and possibly Mars. We have been investigating hot-spring deposits from New Zealand (Miocene-Recent) and Patagonia (Late Jurassic) for more than a decade. It is relevant to study these geologically young analogs in order to determine the controls on and character of microfossil preservation in definitive terrestrial hydrothermal settings, which preserve microbes in hydrothermal silica akin to several Archean examples in South Africa and Western Australia, and which clearly have a strong microbial component ranging from pristine to diagenetically altered. These regional to micron-scale contextural studies have established detailed paleoenvironmental reconstructions of paleo-geothermal fields including their paleo-hydrology. In addition, facies distributions have been mapped, including geothermal-epithermal (surface to shallow subsurface) transitions and structural relationships controlling discharge distributions. Moreover, distinct microbial fabrics in most geothermally influenced facies types have remained largely unchanged with respect to the geologic record of hot-spring deposits, as well as in relation to particular Archean hydrothermally influenced facies. We have confirmed that lagerstätte-style (exceptional fossil preservation) patches are affiliated with early silicification of sinters and travertines. We also have considered the effect of diagenesis on microfossils within the same hot-springs facies, particularly for the silica phase transition in sinters (i.e., freshly deposited amorphous opal-A to recrystallization to microcrystalline quartz), which may aid in a better  understanding of fossil preservation in Archean hydrothermal settings. We have found the most robust biomarkers of microbial activity to be macro- and micro-textures of microbial fossils, followed by laser micro-Raman characterization of carbonaceous material and hydrothermal minerals. Now that an inventory of varied hot-spring deposits and their microbial cargoes have been identified, we are beginning to focus research on high-resolution microbial fingerprints from localized lagerstätte, the enclosing hydrothermal signatures, and fine-scale diagenetic transitions in facies of particular relevance to habitable early Earth and martian settings. Some of the most promising comparisons between Phanerozoic hot-spring deposits and early life habitats may be found in local patches of subaqueous, hydrothermally influenced sediments, where we are discerning chemolithotrophic biofilms at the sediment-water interface from heterotrophic microbial fossils preserved within sedimentary pore spaces.