Barbara Cavalazzi

A Silurian Cold-Seep Ecosystem From the Middle Atlas, Morocco

Abstract Geological and paleobiological attributes described in a carbonate unit from the upper Silurian of the Middle Atlas (Morocco) are interpreted as the result of chemosynthetic processes fueled by fluid seepage. These attributes include: the presence of authigenic carbonate accumulation embedded in siliciclastic rocks; tightly packed, monospecific megafaunal assemblages (atrypid brachiopods); microbial-derived products; precipitation of bio-induced minerals (especially hematite); and seep-derived carbonate phases. Evidence of in-situ growth of the dense brachiopod communities is that their shells are all articulated, and show a complete range of growth sizes. Products of microbial activity are shared with other fossil seep deposits and include clotted micrite concentrations, crusts, and laminated structures, microtufts, biofilms, and endolithic filaments. The best-developed microbial features were recovered in reddish mudstone beds below and within the brachiopod concentrations, where they consist of complex microbial frames morphologically comparable to the ones produced by Beggiatoa mats in modern chemosynthesis-based environments. These remnant mats are associated with early hematite precipitation, which has permitted their extensive mineralization and, consequently, the preservation of their stratigraphic relationships with the shell accumulations. Carbonate phases such as botryoids, splayed calcite, and stromatactoids form the infilling of cavity systems and microconduits. Repeated events of mineral precipitation and corrosion are documented in botryoids and stromatactoids, as well as in more extensive corrosion surfaces, as a result of changes in the ambient chemistry related to type and changing intensity of the seeping fluids. This carbonate unit is the oldest-known seep-derived unit, with a well preserved, voluminous, and diverse microbial component.

Potential Fossil Endoliths in Vesicular Pillow Basalt, Coral Patch Seamount, Eastern North Atlantic Ocean

The chilled rinds of pillow basalt from the Ampère-Coral Patch Seamounts in the eastern North Atlantic were studied as a potential habitat of microbial life. A variety of putative biogenic structures, which include filamentous and spherical microfossil-like structures, were detected in K-phillipsite-filled amygdules within the chilled rinds. The filamentous structures (∼2.5 μm in diameter) occur as K-phillipsite tubules surrounded by an Fe-oxyhydroxide (lepidocrocite) rich membranous structure, whereas the spherical structures (from 4 to 2 μm in diameter) are associated with Ti oxide (anatase) and carbonaceous matter. Several lines of evidence indicate that the microfossil-like structures in the pillow basalt are the fossilized remains of microorganisms. Possible biosignatures include the carbonaceous nature of the spherical structures, their size distributions and morphology, the presence and distribution of native fluorescence, mineralogical and chemical composition, and environmental context. When taken together, the suite of possible biosignatures supports the hypothesis that the fossil-like structures are of biological origin. The vesicular microhabitat of the rock matrix is likely to have hosted a cryptoendolithic microbial community. This study documents a variety of evidence for past microbial life in a hitherto poorly investigated and underestimated microenvironment, as represented by the amygdules in the chilled pillow basalt rinds. This kind of endolithic volcanic habitat would have been common on the early rocky planets in our Solar System, such as Earth and Mars. This study provides a framework for evaluating traces of past life in vesicular pillow basalts, regardless of whether they occur on early Earth or Mars.

The formation of low-temperature sedimentary pyrite and its relationship with biologically-induced processes

This contribution is an updated review on sedimentary pyrite and on its role in well-consolidated research topics, such as the biogeochemical cycles and the studies on sediment-hosted ore deposit studies, as well as new frontiers of research, such as astrobiology. Textural and compositional information preserved in sedimentary pyrite from sediment-hosted ore deposits has contributed to elucidate their environment of forzmation. In particular, the content of redox-sensitive elements such as Ni, Co, Mo, and V has implications for defining the syn- and post-sedimentary conditions. In addition, the stable isotope compositions are useful indicators of the pathways of both biogenic and abiogenic pyrite formation. Despite the longstanding research on pyrite and the mechanism of its formation, there are still significant gaps in our knowledge. In this nonexhaustive review, we briefly touch on different current aspects of research on sedimentary pyrite, exemplifying how sedimentary pyrite remains relevant to geoscientists, and becomes more and more relevant in understanding some basic aspects of knowledge, such as the origin of life and the search for extraterrestrial life, as well as aspect of classical applied science, such as the implications for ore deposition.

Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability

We investigated the equatorial layered deposits (ELDs) of Arabia Terra, Mars, in Firsoff crater and on the adjacent plateau. We produced a detailed geological map that included a survey of the relative stratigraphic relations and crater count dating. We reconstructed the geometry of the layered deposits and inferred some compositional constraints. ELDs drape and onlap the plateau materials of late Noachian age, while they are unconformably covered by early and middle Amazonian units. ELDs show the presence of polyhydrated sulfates. The bulge morphology of the Firsoff crater ELDs appears to be largely depositional. The ELDs on the plateau display a sheet-drape geometry. ELDs show different characteristics between the crater and the plateau occurrences. In the crater they consist of mounds made of breccia sometimes displaying an apical pit laterally grading into a light-toned layered unit disrupted in a meter-scale polygonal pattern. These units are commonly associated with fissure ridges suggestive of subsurface sources. We interpret the ELDs inside the craters as spring deposits, originated by fluid upwelling through the pathways likely provided by the fractures related to the crater formations, and debouching at the surface through the fissure ridges and the mounds, leading to evaporite precipitation. On the plateau, ELDs consist of rare mounds, flat-lying deposits, and cross-bedded dune fields. We interpret these mounds as possible smaller spring deposits, the flat-lying deposits as playa deposits, and the cross-bedded dune fields as aeolian deposits. Groundwater fluctuations appear to be the major factor controlling ELD deposition.

Silicified Biota in High-Altitude, Geothermally Influenced Ignimbrites at El Tatio Geyser Field, Andean Cordillera (Chile)

Biofilms and filamentous communities provided favorable sites for silica precipitation on deeply weathered ignimbrites that make up the substrate at the hydrothermal field of El Tatio (Andean Cordillera, Chile). The amorphous silica encrustation enabled the preservation of a variety of biotic and abiotic features. An integrated study based on optical/scanning electron microscopy and molecular methods of totally to partially silicified microbial communities and biofilms allowed a comparative evaluation of the microfacies and the microbial diversity in the siliceous sinters produced by the digression of a little braided stream departing from a hot spring pool. This study showed useful convergent identifications of certain groups of microbes, such as filamentous cyanobacteria attributed to the genera Phormidium and Rivularia. Together with these microbes, other presumably initial colonizers, such as the halophilic and thermophilic pennate diatoms Nitzschia and Synedra, were widely present and could have contributed to the formation of biofilms and mucus that, as potential home to early silicification, could have contributed to the preservation of microbiologically derived morphologies.

Fossil Microorganisms at Methane Seeps: An Astrobiological Perspective

The recent detection of methane in the martian atmosphere has stimulated a debate on its source, including speculations on a possible biological origin as in the Earth’s atmosphere, where methane is present as a trace gas and is mostly produced by life. Large amounts of methane seepage flows from the subsurface are documented on Earth since the lower Paleozoic by the formation of authigenic carbonate deposits. Methane-derived carbonates also precipitate in the modern continental slopes throughout the world with a great variety in size and shape, and document a still active methane advection from deep sources. The interest of seep carbonates in an astrobiological perspective relies on their relationship with microbiological communities that inhabit the methane seep ecosystems and establish the base of their food chain. They also might represent terrestrial analogues for martian environments and possible models for microbial life on other planets.

Biosignatures in Deep Time

Life on the early Earth inhabited a planet whose environment was vastly different from the Earth of today. An anaerobic and hot early Earth was the birthplace of the first living cells but wide-spread small-scale physico-chemical diversity provided opportunities for a variety of specialists: alkalophiles, acidophiles, halophiles etc. The earliest record of life has been lost due to plate tectonic recycling and the oldest preserved terranes (~3.9–3.7 Ga) are heavily altered by metamorphism, although they may contain traces of fossil life. As of ~3.5 Ga, ancient sediments are so well-preserved that a broad diversity of micro-environments and fossil traces of life can be studied, providing a surprising window into communities of microbes that had already reached the evolutionary stage of photosynthesis. From the wide variety of traces of ancient life that have been reported from the Archaean geological record in Greenland, Canada, South Africa and Western Australia, we examine a few particularly pertinent examples. Biosignatures in the rock record include microfossils, microbial mats, stromatolites, microbially induced sedimentary structures, biominerals, biologically indicative isotopic ratios and fractionations, elemental distributions, organochemical patterns and other geochemical peculiarities best explained by biological mediation. Due to dynamic geological reprocessing over the billions of years since these fossils entered the rock record, identifications of very ancient traces of life have been subject to criticism, hence the often complex arguments regarding their biogenicity. We here highlight a range of unambiguously bona fide and widely supported examples of fossil biosignatures. Fossil biosignatures have great promise as analogues of life that might be detected on other planets. In this respect, the study of the early Earth is particularly pertinent to the search for life on Mars, given the planetary- and microbial-scale similarities that prevailed on both planets during their early histories, together with the lack of subsequent geological reprocessing on Mars, which may make it an ideal repository for a near-pristine fossil record.

Searching for Signs of Life on Other Planets: Mars a Case Study

Demonstrating the existence of simple life forms (past or present) on a cosmic body other than Earth is exceedingly challenging: (1) A naturally sceptic scientific community expects the evidence to be convincing—for example, several independent lines of analyses performed on a feature where the results can only be explained by a biological process. (2) Most bodies are difficult to explore in situ, just about the only way to achieve the above goal, and even then, typically, several missions are required to understand where to go and what to study. (3) Planets and moons that can only be observed remotely (e.g. exoplanets) or from orbit can at best provide some indirect hints of life potential. The actual verification of life would require studying samples containing biosignatures. With the exception of some active moons where jets and plumes may provide the means for satellites to analyse surface sourced material, most other cases require landing, exploring, collecting samples, and analysing them in situ—or bringing them back to Earth.

Astrobiology, the Emergence of Life, and Planetary Exploration

Astrobiology brings together scientific disciplines focused on deciphering the origin of life, its nature, evolution, and distribution in the universe. Exceptionally rapid progress in our understanding has been made over the past four decades, including new insights into how life could have emerged on Earth, the revelation that life can thrive in the most hostile terrestrial environments, evidence of the presence of liquid water throughout the universe, a controversial discovery of past life in a Martian meteorite that reinvigorated the search for life on Mars, and the discovery of Earth-like planets orbiting stars other than our Sun in the Milky Way. Although Earth is the only planet known to host life in our Solar System, continued advances in the field of astrobiology stimulate the search for life and its origin beyond our planet.

Microterracettes in Sabkha Oum Dba (Western Sahara, Morocco): Physical and Biological Interactions in the Formation of a Surface Micromorphology

Small-scale terracing (microterracettes) is a surface geomorphic feature that recurs under a range of environmental settings, such as those existing in high to low temperature geothermal springs and evaporitic environments, through the single or combined action of physicochemical agents and microbiological processes. Such morphology can also be observed in a confined sector of the Sabkha Oum Dba, which is an inland sabkha of the Western Sahara (Morocco), where field and laboratory investigations revealed that they primarily depend on the accumulation of naviculoid diatoms. Through their biofilm production ability, these benthic diatoms are able to stabilize surface morphologies and make organic alveolar frameworks where the precipitation of low Mg calcite occurs in areas subjected to active oxygenic photosynthesis. Because microterracettes arise in a specific set of environmental conditions, they have environmental significance and, thanks to a high fossilization potential due to mineral precipitation, they can be an effective source of biomorphological and chemical evidence for life. The relationship with aqueous environments, considered to be widespread on Mars especially during a period of intense hydrologic activity as in the late Noachian and Hesperian periods, make the understanding of surficial processes useful (such as the formation of microterracettes) whose formation is frequent in terrestrial analogues for martian environments, such as ephemeral saline continental lakes (sabkhas) and related to the products of bacterial and eukaryotic life, as in the case of biofilms, in search for similar life forms beyond Earth.