Hot springs and the search for early life on earth and mars

CAMPBELL, K.; VAN KRANENDONK, M.; WESTALL, F.; GUIDO, D.; FARMER, J.

Conferencia; GSNZ Conference; 2018

Continental hot springs harbor diverse microbialcommunities in varied habitats with well-defined temperature gradients, fromnear-boiling spring-vents to cooler apron terraces, pools and geothermallyinfluenced marsh, fluvial, and lacustrine settings. Spring dischargescontain relatively high concentrations of dissolved ions and thus may entombmicrobial remains in mainly siliceous sinter and travertine deposits, as wellas in less common iron-silica occurrences. Therefore, they represent anarray of microbially dominated ?extreme environments?, encompassing the uppertemperature limit for life on land, with a geological record dating back to atleast the Devonian. They are cited as analog settings for early life onEarth and possibly Mars. We have been investigating hot spring deposits from NewZealand (Miocene-Recent) and Patagonia (Late Jurassic) for more than a decade. Studies of relatively young terrestrial hydrothermal spring deposits with strong microbialcomponents provide relevant information for Precambrian geobiologywith respect to controls on and character of microfossil preservation, rangingfrom pristine to largely obliterated. These regional tomicron-scale contextural studies have established: (1) detailedpaleoenvironmental reconstructions of paleo-geothermal fields including theirpaleo-hydrology (fluid flow directions, relative discharge volume); (2) mappingof well-exposed areal and stratigraphic facies distributions, includinggeothermal-epithermal (surface to shallow subsurface) transitions andstructural relationships controlling field locations;  (3) recognition of distinct microbial fabricsin most geothermally influenced facies types, largely unchanged forthe past ~150 m.y. and (4) identification of lagerstätte-style fossilpreservation ?patches? affiliated with early silicification of sinters andtravertines. We have evaluated diagenetic alteration of microfossils ofdifferent ages across the same hot- springs facies, particularly with respectto silica phase transitions (amorphous opal-A to microcrystalline quartz) insinters. The most robust biomarkers of fossil microbial activityare their macro- and micro-textures, with additionalimportant characterization of the carbonaceous material and hydrothermalminerals provided by laser micro-Raman analysis. Lipid biomarkers, whilepreserved in some Quaternary sinters, thus far do not extend meaningfullyinto the deeper time record we have studied.