Experimental Simulation of Evaporation-Driven Silica Sinter Formation and Microbial Silicification in Hot Spring Systems

Abstract

Evaporation of silica-rich geothermal waters is one of the main abiotic drivers of the formation of silica sinters around hot springs. An important role in sinter structural development is also played by the indigenous microbial communities, which are fossilized and eventually encased in the silica matrix. The combination of these two factors results in a wide variety of sinter structures and fabrics. Despite this, no previous experimental fossilization studies have focused on evaporative-driven silica precipitation. We present here the results of several experiments aimed at simulating the formation of sinters through evaporation. Silica solutions at different concentrations were repeatedly allowed to evaporate in both the presence and absence of the cyanobacterium Synechococcus elongatus. Without microorganisms, consecutive silica additions led to the formation of well-laminated deposits. By contrast, when microorganisms were present, they acted as reactive surfaces for heterogeneous silica particle nucleation; depending on the initial silica concentration, the deposits were then either porous with a mixture of silicified and unmineralized cells, or they formed a denser structure with a complete entombment of the cells by a thick silica crust. The deposits obtained experimentally showed numerous similarities in terms of their fabric to those previously reported for natural hot springs, demonstrating the complex interplay between abiotic and biotic processes during silica sinter growth.

A Field Guide to Finding Fossils on Mars

“…Silicification of organic remains involves the bonding of silicic acid to organic cell walls or envelopes, ensuring long-term stability (e.g., Knoll, 1985). Experiments have confirmed that diverse archaeal and bacterial extremophiles and even viruses silicify readily in silica saturated solutions, with minimal dependence on cell/substrate type, pH, or salinity (Orange et al, 2009(Orange et al, , 2013(Orange et al, , 2014Westall et al, 1995;Westall, 1997). Such results suggest that silicification could outpace cell lysis and degradative processes in brines on early Mars (e.g., Harrison et al, 2016;Toporski et al, 2002;Yee et al, 2003).…”

Silicon isotope fractionation during silica precipitation from hot-spring waters: Evidence from the Geysir geothermal field, Iceland

“…Hence, the high degrees of supersaturation would predict homogeneous nucleation in the water and deposition of pre-constituted particles (Campbell et al, 2001;Lynne and Campbell, 2004;Orange et al, 2013).…”

The Case for Ancient Hot Springs in Gusev Crater, Mars

“…Most sinter deposits are produced from near-neutral pH alkali chloride springs, with accumulations that are centimeters to meters thick, but acid-sulfate-chloride springs with pH as low as 2.1 are also known to produce sinter deposits (Schinteie et al, 2007). Because hot springs commonly host extensive microbial communities, the role of microbes in facilitating silica precipitation has long been debated (e.g., Weed, 1889;Allen, 1934;White et al, 1956;Walter et al, 1972) and continues to be investigated (e.g., personal communication; Tobler et al, 2008;Orange et al, 2013;Murphy et al, personal communication). The potential for sinter deposits to entomb and preserve biomaterials and textures over geological timescales is well documented (e.g., Campbell et al, 2019;Campbell et al, 2001;Djokic et al, 2017;Guido et al, 2010;Rice et al, 1995;Teece et al, 2020 in this issue;Walter et al, 1996;White et al, 1989), which is a primary reason that they are a favored target in the search for ancient life on Mars (e.g., Walter and Des Marais, 1993;Farmer and Des Marais, 1999;Cady et al, 2018).…”