Biomolecules from Fossilized Hot Spring Sinters: Implications for the Search for Life on Mars

Abstract

Hot spring environments are commonly dominated by silica sinters that precipitate by the rapid cooling of silicasaturated fluids and the activity of microbial communities. However, the potential for preservation of organic traces of life in silica sinters back through time is not well understood. This is important for the exploration of early life on Earth and possibly Mars. Most previous studies have focused on physical preservation in samples <900 years old, with only a few focused on organic biomarkers. In this study, we investigate the organic geochemistry of hot spring samples from El Tatio, Chile and the Taupo Volcanic Zone, with ages varying from modern to *9.4 ka. Results show that all samples contain opaline silica and contain hydrocarbons that are indicative of a cyanobacterial origin. A *3 ka recrystallized, quartz-bearing sample also contains traces of cyanobacterial biomarkers. No aromatic compounds were detected in a *9.4 ka opal-A sample or in a modern sinter breccia sample. All other samples contain naphthalene, with one sample also containing other polyaromatic hydrocarbons. These aromatic hydrocarbons have a thermally mature distribution that is perhaps reflective of geothermal fluids migrating from deep, rather than surface, reservoirs. These data show that hot spring sinters can preserve biomolecules from the local microbial community, and that crystallinity rather than age may be the determining factor in their preservation. This research provides support for the exploration for biomolecules in opaline silica deposits on Mars.

Arthropod entombment in weathering-formed opal: new horizons for recording life in rocks

“…The encrustation of insects in opal is a rare phenomenon. The few previously documented examples have originated in hydrothermal settings, where silicification rapidly entombs organisms [24][25][26][27]59 . Opal formation through continental weathering is a much more widespread phenomenon, but is also slower.…”

“…Amorphous silica is present on the surface of Mars 29,63,64 , and is already targeted as the most promising source for the search for extraterrestrial life. The fossil-preserving potential of amorphous silica has been mostly observed in samples that formed in hydrothermal environments 25,26,65 , whereas opaline silica deposits have been identified as originating from the weathering of its silicate crust 66,67 . The future landing site of NASA's next rover missions on Mars (Mars 2020) includes opaline silica that is thought to be of weathering origin 68 .…”

“…By contrast, entombment is the trapping of an organism in silica that occurs by a rapid silica precipitation on the external surface. This is mainly documented in hydrothermal settings [24][25][26] , where silica may preserve an entire ecosystem, as observed in the 407-million-year-old Rhynie Chert (Scotland), the oldest fossil terrestrial environment with a comprehensive preservation of flora and fauna 27 . During diagenesis (the hardening of sediments into rocks due to burial), the most amorphous phase of silica (opal-A: amorphous) will transform into more crystalline phases (opal-CT, constituted of nano-domains of cristobalite and tridymite) until it becomes quartz.…”

“…The transformation of opal into quartz leads to the formation of chert, a sedimentary rock that contains the oldest evidence of life on Earth 28 . Silica entombment associated with hot springs is thus highly studied for its similarity to early Earth environments, and the same process is suspected to have occurred in some locations on Mars 25,26,29 .…”

Fatty Acid Preservation in Modern and Relict Hot-Spring Deposits in Iceland, with Implications for Organics Detection on Mars

“…(Herdianita et al, 2000). This recrystallization process, specifically the transition to microquartz, reduces porosity and decreases the preservation of organic matter in siliceous sinter (Teece et al, 2020).…”