Iron microbial mats in modern and phanerozoic environments

Abstract

The recognition of iron microbial mats in terrestrial environments is of great relevance for the search for extraterrestrial life, especially on mars where significant iron minerals were identified in the subsurface. Most researches focused on very ancient microbial mats (e.g. BIFs) since they formed on Earth at a time where similar conditions are supposed to have prevailed on Mars too. However, environmental proxies are often difficult to use for these deposits on Earth which, in addition, may be heavily transformed due to diagenesis or even metamorphism.Here we present modern and phanerozoic iron microbial mats occurrences illustrating the wide variety of environments in which they form, including many marine settings, ponds, creeks, caves, volcanoes, etc. Contrarily to their Precambrian counterparts, Modern and Phanerozoic deposits are usually less affected by diagenesis and the environmental conditions likely to be better constrained. Therefore, their investigation may help for the search for morphological and geochemical biosignatures (e.g. iron isotopes) in ancient iron microbial occurrences on Earth but also on other Planets. In particular, many of the case studies presented here show that microstromatolithe-like morphologies may be valuable targets for screening potential biosignatures in various rock types.

Microbial activity records in Marinoan Snowball Earth postglacial transition layers connecting diamictite with cap carbonate (Otavi Group, NW-Namibia)

“…The main Fe oxide mineral in the filaments of our samples is goethite, but Raman analyses indicate that hematite is also present, similarly to those, reported by Rajabzadeh et al (2017). The fossilized Fe-rich biomats were rapidly and extensively encrusted by authigenic minerals, such as dolomite and silica, similar to what has been reported by Baele et al (2008). The amorphous silica can easily be transformed into more stable minerals, such as cristobalite, tridymite, and quartz (Herdianita et al, 2000).…”

“…In general, Fe-rich biomats are encrusted by dolomite and silica (Baele et al, 2008), which is also present in our samples. Fe-rich biomats are better preserved in diagenetic quartz, than in carbonate host material, which is well documented in our profile C2a.…”

“…Therefore, morphology in combination with mineralogical and chemical bioindicators point towards a complex interpretation (Baele et al, 2008), with microtextural characteristics (filamentous, tubular, coccoidal forms of iron-oxide hydroxides), the carbon isotopic composition, and the bulk rock geochemical data support this scenario. The enrichment of some elements, such as Fe, P, Zn, and Mn, can be related to biogenic activity, similar to the presence of negative carbon isotopic signatures (Polgári et al, 2012ab).…”

“…The amorphous silica can easily be transformed into more stable minerals, such as cristobalite, tridymite, and quartz (Herdianita et al, 2000). Silica precipitation is derived by either the destruction of organic complexes or the transformation of ferrihydrite (Baele et al, 2008). Muscovite (hydromuscovite) is common in the studied samples and was most likely formed by the diagenesis of cyanobacteria filaments in Fe-rich microbialites by the leaching of biofilm alkali elements (Na, Ka, Al, Mg) as reported by Ewers (1983).…”

Genesis of a regionally widespread celadonitic chert–ironstone bed overlying Upper Lias manganese deposits, Hungary

“…Biological activity can also mediate silica deposition (e.g., Hein et al 1999Hein et al , 2008. Silica precipitation can be enhanced by the release of silica from either the destruction of organic complexes or the transformation of ferrihydrite, which usually contains substantial silica (Baele et al 2008).…”