Terrestrial Hot Spring Systems: Introduction

Abstract

This report reviews how terrestrial hot spring systems can sustain diverse and abundant microbial communities and preserve their fossil records. Hot springs are dependable water sources, even in arid environments. They deliver reduced chemical species and other solutes to more oxidized surface environments, thereby providing redox energy and nutrients. Spring waters have diverse chemical compositions, and their outflows create thermal gradients and chemical precipitates that sustain diverse microbial communities and entomb their remnants. These environments probably were important habitats for ancient benthic microbial ecosystems, and it has even been postulated that life arose in hydrothermal systems. Thermal spring communities are fossilized in deposits of travertine, siliceous sinter, and iron minerals (among others) that are found throughout the geological record back to the oldest known well-preserved rocks at 3.48 Ga. Very few are known before the Cenozoic, but it is likely that there are many more to be found. They preserve fossils ranging from microbes to trees and macroscopic animals. Features on Mars whose morphological and spectroscopic attributes resemble spring deposits on Earth have been detected in regions where geologic context is consistent with the presence of thermal springs. Such features represent targets in the search for evidence of past life on that planet.

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.…”

“…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 .…”

Chasing Synthetic Life: A Tale of Forms, Chemical Fossils, and Biomorphs

“…As displayed in Figure , the resulting morphologies vary with both the pH and the concentration of calcium ions. These results are interesting as hot springs may sustain diverse microbial communities and preserve fossilized structures . Moreover, hydrothermal vents in seawater would have supported prebiotic scenarios as hot springs with ionic solutes are more conducive to self‐assembly leading to protocells …”

Life is everywhere in sinters: examples from Jurassic hot-spring environments of Argentine Patagonia

“…This study constitutes a detailed analysis of the Deseado Massif sinter deposits, which enabled construction of a more comprehensive facies model for sinters of the region, including geographically adjacent, hydrothermally influenced, silicified fluvial and lacustrine strata, that are broadly associated in time and space (cf. Des Marais & Walter, 2019;Hamilton et al 2019;Djokic et al unpub. data;Teece et al unpub.…”