Martin Homann

Laminae are millimeter-scale features in rocks created by physiochemical processes that can be influenced by the presence and activities of communities of organisms that occur as biofilms and microbial mats. The structure and composition of laminae reflect the processes involved in their formation and can be preserved in the rock record over geologic time; however, diagenetic and metamorphic alteration can lead to the loss of primary information and confusion over the interpretation of their origins. As potential records of ancient life, laminae can preserve evidence of microbial activity over billions of years of Earth’s history. On planetary bodies such as Mars, laminae in sedimentary rocks are common and represent significant features of interest that can record habitable conditions (e.g., the presence of liquid water) at the time of their formation. Here we review the significance of laminae as targets for astrobiological exploration. We discuss common mechanisms by which laminae form in natural environments on Earth, present arguments and evidence for laminae as potential biosignatures, and describe how such information is presented in the NASA Life Detection Knowledge Base.

There is ongoing debate as to when oxygenic photosynthesis evolved on Earth^1,2. Geochemical data from ancient sediments indicate localized or ephemeral photosynthetic O₂ production before the Great Oxidation Event (GOE) approximately 2.5–2.3 billion years ago (Ga), and currently suggest Archaean origins, approximately 3 Ga or earlier^{3,4,5,6,7,8,9}. However, sedimentary records of the early Earth often suffer from preservation issues, and poor control on the timing of oxidation leaves geochemical proxy data for the ancient presence of O₂ open to critique^10,11,12,13. Here, we report rare Earth element data from three different Archaean carbonate platforms preserved in greenstone belts of the northwest Superior Craton (Canada), which were deposited by the activity of marine photosynthetic bacteria 2.87 Ga, 2.85 Ga and 2.78 Ga. All three indicate O₂ production before the GOE in the form of significant depletions in cerium (Ce), reflecting oxidative Ce removal from ancient seawater, as occurs today¹⁴. Using ¹³⁸La-¹³⁸Ce geochronology, we show that La/Ce fractionation, and thus Ce oxidation, occurred at the time of deposition, making these the oldest directly dated Ce anomalies. These results place the origin of oxygenic photosynthesis in the Mesoarchaean or earlier and bring an important new perspective on a long-standing debate regarding Earth’s biological and geochemical evolution.

Shallow‐marine environments are thought to have been pivotal to the spreading, perhaps even the origin, of early life on Earth. The shallow‐marine Archean sedimentary record of early life, however, is biased towards carbonates; nearshore siliciclastic environments have not received proportional attention. Here we describe densely laminated, silicified and dolomitized fossil calcareous mounds in tidal‐facies sandstones of the Archean Moodies Group (ca. 3.22 Ga) in the Barberton Greenstone Belt, Eswatini. They vary between (1) cm‐ to dm‐scale, isolated, club‐ to pedestal‐shaped, nodular mounds on top of and within the conduits of fluid‐escape structures, and (2) mm‐ to cm‐scale, undulatory and wavily laminated structures, interbedded with well‐bedded silt‐ and sandstones. Geochemical indicators of a possible biogenic origin were largely obliterated by local hydrothermal alteration and regional lower‐greenschist‐facies metamorphism: In situ SIMS δ¹³C_carb isotope analyses from several traverses across the best‐preserved laminae of a mound and δ³⁴S_VCDT values from diagenetic rims of nearby detrital pyrite grains yield ambiguous isotopic evidence about biologic processing; TOC of putative laminae is too low to measure δ¹³C_org, and Raman spectroscopy of finely dispersed carbonaceous particles and of kerogenous laminae indicate mean maximum metamorphic temperature of ca. 500°C. Textural and regional evidence, however, suggests that the carbonate laminae represent metabolic products of microbial communities that took advantage of sand volcanoes from which nutrient‐rich fluids erupted episodically. We base this inference on the habitable depositional setting on a wave‐ or current‐swept photic‐zone tidal platform, the stromatolitic morphologies in two and three dimensions, the occurrence of in‐situ kerogen, the carbonate mineralogy, and the presence of comparable mound structures elsewhere in the Moodies Group. Although the metabolic strategies utilized by the microorganisms remain unknown, this occurrence places a novel ecologic niche in the Paleoarchean microbial colonization of coastal regions.

Stable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ ¹³ C and δ ¹⁵ N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogs for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses.

Palisade fabric is a ubiquitous texture of silica sinter found in low temperature (Leptolyngbya sp., were found to be incorporated into silica sinter by irregular cycles of wetting, evaporation, and mineral precipitation. Transmission electron microscopy analyses revealed that nanometer-sized silica particles are filling the pore space within individual cyanobacterial sheaths, giving rise to their structural rigidity to sustain a palisade fabric framework. Diagenesis experiments further show that the sheaths of the filaments are preferentially preserved relative to the trichomes, and that the amount of water present within the sinter is an important factor for overall preservation during burial. This study confirms that palisade fabrics are efficiently generated in a highly evaporative geothermal field, and that these biosignatures can be most effectively preserved under dry diagenetic conditions.

Stromatolites of the ~3.5 billion-year-old Dresser Formation (Pilbara Craton, Western Australia) are considered to be some of Earth’s earliest convincing evidence of life. However, uniquely biogenic interpretations based on surface outcrops are precluded by weathering, which has altered primary mineralogy and inhibited the preservation of microbial remains. Here, we report on exceptionally preserved, strongly sulfidized stromatolites obtained by diamond drilling from below the weathering profile. These stromatolites lie within undeformed hydrothermal-sedimentary strata and show textural features that are indicative of biogenic origins, including upward-broadening and/or upward-branching digitate forms, wavy to wrinkly laminae, and finely laminated columns that show a thickening of laminae over flexure crests. High-resolution textural, mineralogical, and chemical analysis reveals that the stromatolites are dominated by petrographically earliest, nano-porous pyrite that contains thermally mature, N-bearing organic matter (OM). This nano-porous pyrite is consistent with a formation via sulfidization of an originally OM-dominated matrix. Evidence for its relationship with microbial communities are entombed OM strands and filaments, whose microtexture and chemistry are consistent with an origin as mineralized biofilm remains, and carbon isotope data of extracted OM (δ¹³C_OM = –29.6‰ ± 0.3‰ VPDB [Vienna Peedee belemnite]), which lie within the range of biological matter. Collectively, our findings provide exceptional evidence for the biogenicity of some of Earth’s oldest stromatolites through preservation of OM, including microbial remains, by sulfidization.