Microbially Induced Sedimentary Structures in Clastic Deposits: Implication for the Prospection for Fossil Life on Mars

Abundant and well-preserved fossil microbenthos occurs in siliciclastic deposits of all Earth ages, from the early Archean to today. Studies in modern settings show how microbenthos responds to sediment dynamics by baffling and trapping, binding, biostabilization, and growth. Results of this microbial-sediment interaction are microbially induced sedimentary structures (MISS). Successful prospection for rich MISS occurrences in the terrestrial lithological record requires unraveling genesis and taphonomy of MISS, both of which are defined only by a narrow range of specific conditions. These conditions have to coincide with high detectability which is a function of outcrop quality, bedding character, and rock type. Assertions on biogenicity of MISS morphologies must be based on the presence of microbially induced sedimentary textures (MIST), which are MISS-internal textures comprising replacement minerals arranged into microscopic biological morphologies, ancient carbonaceous matter, trace fossils, and geochemical signals. MISS serve as possible templates for the decryption of ancient life-processes on Mars. This article closes with a perspective on selected deposits and ancient environments in Meridiani Planum, Gale Crater, and Jezero Crater, Mars, regarding their potential for MISS occurrences. The earlier hypothesis of structures on Mars as potentially being MISS is revised

Turbulent Lifestyle: Microbial Mats on Earth’s Sandy Beaches—Today and 3 Billion Years Ago

Archean Earth history is very difficult to reconstruct. Until recently, only bacterial cells preserved in chert, microborings, and stromatolites provided the few clues to ancient life. Now, siliciclastic “microbially induced sedimentary structures” (MISS) are adding to our knowledge of both past life and paleoenvironments. MISS rise from the interaction of photoautotrophic microbial mats with physical sediment dynamics in siliciclastic, shallow-marine settings. Archean MISS can be understood through observations of living microbial mats and modern biotic-physical sedimentary processes. Such geobiological studies are key to the interpretation of the early evolution of prokaryotes. For example, the 2.9 Ga Pongola Supergroup, South Africa, includes MISS that possibly point to the oldest known cyanobacterial community preserved in Earth’s history

Volcanogenic Pseudo-Fossils From the ~3.48 Ga Dresser Formation, Pilbara, Western Australia

The ~ 3.48 billion-year-old Dresser Formation, Pilbara Craton, Western Australia, is a key geological unit for the study of Earth\u27s earliest life and the habitats it occupied. Here, we describe a new suite of spheroidal to lenticular microstructures that morphologically resemble some previously reported Archean microfossils. Correlative microscopy shows that these objects have a size distribution, wall ultrastructure, and chemistry that are incompatible with a microfossil origin and instead are interpreted as pyritized and silicified fragments of vesicular volcanic glass. Organic kerogenous material is associated with much of the altered volcanic glass; variable quantities of organic carbon line or fill the insides of some individual vesicles, while relatively large, tufted organic-rich laminae envelop multiple vesicles. The microstructures reported herein constitute a new type of abiogenic artifact (pseudo-fossil) that must be considered when evaluating potential signs of early life on Earth or elsewhere. In the sample studied here, where hundreds of these microstructures are present, the combined evidence permits a relatively straightforward interpretation as vesicular volcanic glass. However, reworked, isolated, and silicified microstructures of this type may prove particularly problematic in early or extraterrestrial life studies since they adsorb carbon onto their surfaces and are readily pyritized, mimicking a common preservation mechanism for bona fide microfossils. In those cases, nanoscale analysis of wall ultrastructure would be required to firmly exclude a biological origin

Evidence for Metabolic Diversity in Meso-Neoproterozoic Stromatolites (Vazante Group, Brazil)

Deciphering the evolution of ecological interactions among the metabolic types during the early diversification of life on Earth is crucial for our understanding of the ancient biosphere. The stromatolites from the genus Conophyton cylindricus represent a datum for the Proterozoic (Meso to Neoproterozoic) on Earth. Their typical conical shape has been considered a result of a competition between microorganisms for space, light and nutrients. Well-preserved records of this genus from the Paleontological Site of Cabeludo , Vazante Group, São Francisco Craton (Southern Brazil) present in situ fossilized biofilms, containing preserved carbonaceous matter. Petrographic and geochemical analyses revealed an alternation between mineral laminae (light grey laminae) and fossilized biofilms (dark grey laminae). The dark grey laminae comprise three different biofilms recording a stratified microstructure of microbial communities. These three biofilms composing the dark grey laminae tend to be organized in a specific pattern that repeats through the stromatolite vertical section. Iron and manganese are distributed differently along the dark and light grey laminae; X-ray absorption and luminescence data showed possible different areas with authigenic iron and iron provided from diagenetic infiltration. Cryptocrystalline apatite in the lowermost biofilms in each dark grey laminae may suggest past metabolic activity of sulfide-oxidizing bacteria. These findings suggest that the microorganisms reached a complex metabolic diversification in order to maintain an equilibrium situation between the three different biofilms along the vertical section of the structures, thus benefiting the whole microbial community. This means that the stromatolites from the Conophyton genus may have formed as a result of a greater complexity of interactions between microorganisms, and not only from competition between photosynthesizers

Surface Morphologies in a Mars-Analog Ca-Sulfate Salar, High Andes, Northern Chile

Salar de Pajonales, a Ca-sulfate salt flat in the Chilean High Andes, showcases the type of polyextreme environment recognized as one of the best terrestrial analogs for early Mars because of its aridity, high solar irradiance, salinity, and oxidation. The surface of the salar represents a natural climate-transition experiment where contemporary lagoons transition into infrequently inundated areas, salt crusts, and lastly dry exposed paleoterraces. These surface features represent different evolutionary stages in the transition from previously wetter climatic conditions to much drier conditions today. These same stages closely mirror the climate transition on Mars from a wetter early Noachian to the Noachian/Hesperian. Salar de Pajonales thus provides a unique window into what the last near-surface oases for microbial life on Mars could have been like in hypersaline environments as the climate changed and water disappeared from the surface. Here we open that climatological window by evaluating the narrative recorded in the salar surface morphology and microenvironments and extrapolating to similar paleosettings on Mars. Our observations suggest a strong inter-dependence between small and large scale features that we interpret to be controlled by extrabasinal changes in environmental conditions, such as precipitation-evaporation-balance changes and thermal cycles, and most importantly, by internal processes, such as hydration/dehydration, efflorescence/deliquescence, and recrystallization brought about by physical and chemical processes related to changes in groundwater recharge and volcanic processes. Surface structures and textures record a history of hydrological changes that impact the mineralogy and volume of Ca-sulfate layers comprising most of the salar surface. Similar surface features on Mars, interpreted as products of freeze-thaw cycles, could, instead, be products of water-driven, volume changes in salt deposits. On Mars, surface manifestations of such salt-related processes would point to potential water sources. Because hygroscopic salts have been invoked as sources of localized, transient water sufficient to support terrestrial life, such structures might be good targets for biosignature exploration on Mars

Geobiology: Microbial Mats in Sandy Deposits from the Archean Era to Today

A murmur is heard from the depths of time. Life and Earth are engaged in a dialog that has lasted for four billion years. Sometimes it’s a whisper, sometimes a roar. One part sometimes gets the upper hand, dominates the discussion and sets the agenda. But mostly the two have some kind of mutual understanding, and the murmur goes on. Most of us don’t listen. Nora does. She listens, and she tries to understand. Nora Noffke has focused her scientific career on the interaction between the living and the non-living. This is no mean task in an academic world where you are usually either this or that, such as either a biologist or a geologist. The amount of stuff you need to grasp is so large that it usually feels better to sit comfortably on one chair, rather than to risk falling between them. Geobiology is not for the faint of heart. Nora’s focus is on that all-important biological substance mucus, or EPS (ext- cellular polymeric substance). EPS is the oil in the machinery, the freeway to travel for many small animals and protists, the coat of armour for others, the mortar in the brick wall for yet others. For microbes such as cyanobacteria it may be the world they built, the world they live, eat, fight, multiply, and die in. [From Amazon.com]https://digitalcommons.odu.edu/oeas_books/1002/thumbnail.jp