The ubiquitous iron oxides and oxyhydroxides on Mars represent intriguing targets in the search for evidence of past microbial life on the Red Planet. So far, no studies have systematically investigated microbial fossils in fluviolacustrine ferricrete composed entirely of nanophase goethite; thus, our understanding of biosignatures in such deposits is limited. Here, we report exceptionally well-preserved microfossils in laminated nanophase goethite from the Miocene McGraths Flat Lagerstätte, New South Wales, Australia. Using scanning electron microscopy and energy dispersive spectroscopy, we identified a diverse range of fossil morphotypes, including bacteria and fungi. Syngenetic forms embedded in the sedimentary matrix and post-depositional endolithic colonizers were recognized. Our findings establish nanophase goethite as an excellent preservation medium for microfossils, capable of preserving subcellular details across multiple generations of microbial communities, and highlight nanophase goethite as a promising target for future Mars exploration missions.

Disc-shaped pyrite suns of the Pennsylvanian age Anna Shale are thought to have formed where pressure restricted pyrite crystal growth to a flattened disc shape during diagenesis at the Anna Shale and the underlying Herrin coal boundary. Others have proposed syndepositional involvement of sulfate-reducing bacteria in the depositional environment. We hypothesize that the first steps in pyrite sun formation occur in mudflats of proglacial Alaska, with cyanobacterial mats trapping glacier silts within extracellular polymeric substances where microbial communities interact with allochthonous hydrocarbons, sulfur, and iron to precipitate amorphous iron sulfide minerals. We compared pyrite sun morphology with pyrite sun precursor formations, used 16S rRNA amplicon sequencing to investigate putative pyrite-forming bacteria, and determined iron content and mineralogy using XRD and sequential iron extraction of Matanuska Glacier mudflats sampled in June 2023. Recovered 16S rRNA sequences include EPS-generating cyanobacteria (Aphanizomenon NIES81), sulfur cycling bacteria (e.g., Thiobacillus, Sulfuritalea, Desulfovibrio), and iron cycling bacteria (e.g., Rhodoferax, Geobacter). In our proposed model, methane and hydrogen sulfide generated within anoxic mud form gas domes in benthic silt resting in a disc shape at the air-water interface. Iron sulfides precipitate below the surface of the cyanobacterial mats and are later buried and transformed into pyrite crystals with diagenesis across the disc shape. This combination of high organic carbon availability with sulfur and iron cycling and resulting iron sulfide mineral precipitation across a sharp redox gradient in depositional silt is a close match to the ancient depositional environment of the pyrite sun containing unit within the Anna Shale.

Abstract. In many glacial settings, winter outflows of proglacial water create stratified domed ice structures on glacial forefields. These structures, called naledi, provide an opportunity to characterise the solute fluxes of hydrological systems in winter which are poorly constrained compared to summer outflows which are dominated by supraglacial meltwater input. To characterise the different hydrological systems feeding naledi and provide conceptual models of their formation, we sampled 12 overwinter naledi and 4 supercooled summer ice accumulations (accreted ice) from the forefield of Isunnguata Sermia, a western outlet glacier of the Greenland Ice Sheet, during four field campaigns. Major ions and stable water isotopes reveal complex geochemical signatures, where the composition of successive naled layers reveal fluctuations in water source and transport throughout winter. In comparison, core analysis of summer accreted ice shows uniform geochemistry throughout, suggesting freeze-on of water from one single source in quick discrete events. These findings are supported by Electrical Resistivity Tomography (ERT) geophysical surveys, which reveal shallow hydrological pathways (1–5 m below the surface) in the glacial foreland (surveyed 400 m from the glacier terminus). Here, meltwater is transported through a saturated sediment zone feeding proglacial upwellings. Our results inform the first conceptual models of summer vs. wintertime subglacial water routing and naled formation in the proglacial zone of the Greenland Ice Sheet. We show that wintertime naledi form from a combination of top-down (unconfined) and bottom-up (confined) freezing processes which incorporates subglacial discharge, overland flow and precipitation, whereas accreted ice forms from upwellings of high velocity, highly pressurised water which freezes upon release. These seasonal changes drive different geochemical and nutrient outputs, which are critical for evaluating glacier meltwater contributions to proglacial groundwater systems and the impact of glacial discharge on downstream ecosystems.

Northern glacial refugia are a hotly debated concept. The idea that many temperate organisms survived the Last Glacial Maximum (LGM; ~26.5 to 19 thousand years) in several sites across central and northern Europe stems from phylogeographic analyses, yet direct fossil evidence has thus far been missing. Here, we present the first unequivocal proof that thermophilous trees such as oak ( Quercus ), linden ( Tilia ), and common ash ( Fraxinus excelsior ) survived the LGM in Central Europe. The persistence of the refugium was promoted by a steady influx of hydrothermal waters that locally maintained a humid and warm microclimate. We reconstructed the geological and palaeohydrological factors responsible for the emergence of hot springs during the LGM and argue that refugia of this type, allowing the long-term survival and rapid post-LGM dispersal of temperate elements, were not exceptional in the European periglacial zone.

Hydrothermal environments—where chemical interactions between heated water and rock take place—are of great interest for astrobiology as they may create habitable environments and preserve signs of life. Several planetary bodies display evidence of hydrothermalism, including Mars, which has possessed various hydrothermal environments that could alter organic signals, complicating assessments of biogenicity and our understanding of the depositional environment and subsequent alteration processes. Organic molecules from Earth-based hydrothermal systems serve as an analogue for what we might find on Mars and are typically made up of complex mixtures of in situ and transported molecules that have been altered by diverse mechanisms. Improving our understanding of the processes that drive the preservation and circulation of organic molecules in Earth-based hydrothermal settings is crucial, as the sources and fates of these molecules in marine hydrothermal environments are different from those in subaerial (land-based) hydrothermal environments, even if many of the processes that govern the transport and alteration of this organic matter seem to be similar. To understand the origin of organic molecules found in samples from another world, we must critically evaluate the processes that alter these molecules in terrestrial hydrothermal samples. Hydrothermal environments are sites of high astrobiological importance. Examining the behaviour of organic molecules in hydrothermal analogues from Earth could help in interpreting observations of hydrothermal settings on other planetary bodies, especially Mars.

From Viking landers to Perseverance rover, Mars has been explored by several in situ missions capable of analyzing organic compounds. Results from the SAM and SHERLOC on Curiosity and Perseverance, respectively, support the detection of lean organic matter (at ppb-ppm levels) in the top surface samples, although the source (s) and preservation mechanisms are still ambiguous. Perseverance is currently exploring a fluvio-lacustrine system at Jezero crater and may explore an ancient volcanic terrain after exiting the crater. As Perseverance would collect samples for potential return to Earth, preparation is needed for sample return efforts through various means including i) the detection of trace organic compounds in various matrices, ii) validation of compounds identified by Martian rovers, and iii) better understanding of mechanisms of their production on Mars. On these returned samples, the community may be able to resolve the timing of organic matter formation and refine hypotheses regarding organic preservation in Martian soils despite the presence of numerous oxidants, salts, and pH-temperature intra and inter-site variations that are less conductive to long-term preservation of organic matter. For instance, acidic conditions promote clay catalyzed isomerization, but seem to benefit for the fatty acid preservation producing organic-salts or favoring salt dissolution in the matrix to protect organic compounds from radiations and water alteration. With a similar aim, we selected samples from Yellowstone National Park hot springs and silica sinters as analogs to locations visited by Curiosity and Perseverance or - in the future - Rosalind Franklin rover. The hot springs in this study developed over hundreds to thousands of years, providing optimal conditions (i.e., matrix composition, temperature, pH) of preservation for organic molecules, extremophilic and mesophilic cells. In our study, the most well preserved organic matter and biosignatures were detected in acidic silica sinters with a surface (water) temperature below 50 degrees C and a minor crystalline phase. The gas chromatography - mass spectrometry molecular analysis revealed a variety of organic compounds we classified as bioindicators (such as amino acids, nucleobases, and sugars), and biosignatures (such as long-chain branched and/or (poly)unsaturated lipids, secondary metabolites involved in the quorum sensing or communication between individuals). We validated with a SAM/MOMA-like benchtop extracting oven the organic matter extraction protocols performed with the SAM experiment. We identified using the different SAM and MOMA extraction protocols (pyrolysis and wet-chemistry derivatizations) eight microbial classes through a unique untargeted environmental metabolomics' method embracing space flight technology constraints. Additionally, we identified one (and likely two) agnostic biosignature(s): i) the concomitance of some elements and organic compounds in the analogs (correlation of organic matter elements: C, N, S, P and organic molecules colocated with essential biological elements: Fe, Mg, V, Mn and non-essential biological elements concentrated by

Active hot springs are dynamic geobiologically active environments. Heat- and element-enriched fluids form hot spring sinter deposits that are inhabited by microbial and macroscopic eukaryotic communities, but it is unclear how variable heat, fluid circulation, and mineralization within hot spring systems affect the preservation of organic matter in sinters. We present geological, petrographic, and organic geochemical data from fossilized hot spring sinters (<13 Ka) from three distinct hot spring fields of Yellowstone National Park. The aims of this study were to examine the preservation of hydrocarbons and discern whether the hydrocarbons in these samples were derived from in situ communities or transported by hydrothermal fluids. Organic geochemistry reveals the presence of n-alkanes, methylalkanes, hopanes, and other terpanes, and the distribution of methylheptadecanes is compared to published observations of community composition in extant hot springs with similar geochemistry. Unexpectedly, hopanes have a thermally mature signal, and Raman spectroscopy confirms that the kerogen in some samples has nearly reached the oil window, despite never having been buried. Our results suggest that organic matter maturation occurred through below-surface processes in the hotter, deeper parts of the hydrothermal system and that this exogenous material was then transported and emplaced within the sinter.

Snow is a critical component of the Earth system. High-elevation snow can persist into the spring, summer, and early fall and hosts a diverse array of life, including snow algae. Due in part to the presence of pigments, snow algae lower albedo and accelerate snow melt, which has led to increasing interest in identifying and quantifying the environmental factors that constrain their distribution. Dissolved inorganic carbon (DIC) concentration is low in supraglacial snow on Cascade stratovolcanoes, and snow algae primary productivity can be stimulated through DIC addition. Here we asked if inorganic carbon would be a limiting nutrient for snow hosted on glacially eroded carbonate bedrock, which could provide an additional source of DIC. We assayed snow algae communities for nutrient and DIC limitation on two seasonal snowfields on glacially eroded carbonate bedrock in the Snowy Range of the Medicine Bow Mountains, Wyoming, United States. DIC stimulated snow algae primary productivity in snow with lower DIC concentration despite the presence of carbonate bedrock. Our results support the hypothesis that increased atmospheric CO2 concentrations may lead to larger and more robust snow algae blooms globally, even for sites with carbonate bedrock.

ABSTRACT Snow is a critical component of the Earth system. High elevation snow can persist into the melt season and hosts a diverse array of life including snow algae. Due in part to the presence of pigments, snow algae lower albedo and accelerate snow melt which has led to increasing interest in identifying and quantifying the environmental factors that constrain their distribution. Dissolved inorganic carbon (DIC) concentration is low in supraglacial snow on Cascade stratovolcanoes and snow algae primary productivity can be stimulated through DIC addition. Here we asked if CO 2 would still be a limiting nutrient for snow hosted on glacially eroded carbonate bedrock (which could provide an additional source of DIC). We assayed snow algae communities for nutrient and DIC limitation on two seasonal snowfields on glacially eroded carbonate bedrock in the Snowy Range of the Medicine Bow Mountains, Wyoming, USA. DIC stimulated snow algae primary productivity in snow with lower DIC concentration despite the presence of carbonate bedrock, which alleviated DIC limitation in the other site. Our results support the hypothesis that increased atmospheric CO 2 concentrations may lead to larger and more robust snow algae blooms globally, even for sites with carbonate bedrock.

Hot spring cyanobacteria have long been model systems for examining ecological diversification as well as characterizing microbial adaptation and evolution to extreme environments. These studies have reported cyanobacterial diversification in hot spring outflow channels that can be defined by distinct temperature ranges.

Identifying microbial fossils in the rock record is a difficult task because they are often simple in morphology and can be mimicked by non-biological structures. Biosignatures are essential for identifying putative fossils as being definitively biological in origin, but are often lacking due to geologic effects which can obscure or erase such signs. As such, there is a need for robust biosignature identification techniques. Here we show new evidence for the application of trace elements as biosignatures in microfossils. We found elevated concentrations of magnesium, aluminum, manganese, iron, and strontium colocalized with carbon and sulfur in microfossils from Drummond Basin, a mid-Paleozoic hot spring deposit in Australia. Our results also suggest that trace element sequestrations from modern hot spring deposits persist through substantial host rock alteration. Because some of the oldest fossils on Earth are found in hot spring deposits and ancient hot spring deposits are also thought to occur on Mars, this biosignature technique may be utilized as a valuable tool to aid in the search for extraterrestrial life.

Glaciated environments are important sources of lithogenic nutrients due to mechanical and chemical weathering and meltwater transport influencing downstream ecosystems. High physical weathering rates in subglacial environments due to bedrock grinding results in significant chemical weathering due to increased reactive mineral surface area. Until recently, glaciers and ice sheets were considered a minor part of the global Si cycle, in part because near-freezing conditions were thought to limit silicate weathering rates. However, recent research has shown that silicate dissolution is the predominant chemical weathering process in alpine glaciated catchments in the volcanoes of the Cascades Range, USA [1]. Alpine glacier systems may provide large fluxes of bioavailable Si similar to other recent findings [2,3

Hydrothermal systems host microbial communities that include some of the most deeply branching members of the tree of life, and recent work has suggested that terrestrial hot springs may have provided ideal conditions for the origin of life. Hydrothermal microbial communities are a potential source for biosignatures, and the presence of terrestrial hot spring deposits in 3.48 Ga rocks as well as on the surface of Mars lends weight to a need to better understand the preservation of biosignatures in these systems. Although there are general patterns of elemental enrichment in hydrothermal water dependent on physical and geochemical conditions, the elemental composition of bulk hydrothermal microbial communities (here termed biocumulus, including cellular biomass and accumulated non-cellular material) is largely unexplored. However, recent work has suggested both bulk and spatial trace element enrichment as a potential biosignature in hot spring deposits. To elucidate the elemental composition of hot spring biocumulus samples and explore the sources of those elements, we analyzed a suite of 16 elements in hot spring water samples and corresponding biocumulus from 60 hot springs sinter samples, and rock samples from 8 hydrothermal areas across Yellowstone National Park. We combined these data with values reported in literature to assess the patterns of elemental uptake into biocumulus and retention in associated siliceous sinter. Hot spring biocumuli are of biological origin, but organic carbon comprises a minor percentage of the total mass of both thermophilic chemotrophic and phototrophic biocumulus. Instead, the majority of hot spring biocumulus is inorganic material—largely silica—and the distribution of major and trace elements mimics that of surrounding rock and soil rather than the hot spring fluids. Analyses indicate a systematic loss of biologically associated elements during diagenetic transformation of biocumulus to siliceous sinter, suggesting a potential for silica sinter to preserve a trace element biosignature.

The fidelity of uranium isotopes (8238U) in marine carbonates as a paleoredox proxy relies on whether carbonates can record and preserve seawater 8238U. Although modern carbonate sediments deposited under oxic conditions have been shown to track seawater 8238U, it remains unknown whether this is true for carbonates deposited under anoxic conditions. This is a crucial question because many ancient carbonates were likely deposited or reworked under anoxic bottom waters. To better understand the behavior of uranium isotopes under this scenario, we investigated U isotope geochemistry in the meromictic Fayetteville Green Lake (FGL; New York, USA), where primary calcite is precipitated from oxic surface waters, sinks past the chemocline, and is deposited under anoxic bottom waters. We observed significant depletions of dissolved U concentration (from 2.7 to 0.9 ppb) and 8238U (from -0.55%0 to -0.96%0) below the chemocline in FGL. Parallel with these depletions, 8238U of sediment traps increased progressively from -0.51%0 to -0.16%0, suggesting that U(VI) reduction was occurring in the anoxic water column. Carbonate sediments deposited under anoxic bottom waters were enriched in U by 6-18x compared to primary calcite. Our data suggest that such significant authigenic U enrichments resulted from U(VI) reduction in the anoxic water column and below the sediment-water interface. The 8238U value in the top 0.25 cm of sediments was -0.29%0 +/- 0.10%0, overprinting original 8238U in primary calcite (-0.51%0 +/- 0.02%0). Future applications of carbonate 8238U as a paleoredox proxy should consider depositional environments (oxic vs. anoxic) of carbonates.

Exposure to high concentration geogenic arsenic via groundwater is a worldwide health concern. Well installation introduces oxic drilling fluids and hypochlorite (a strong oxidant) for disinfection, thus inducing geochemical disequilibrium. Well installation causes changes in geochemistry lasting 12 + months, as illustrated in a recent study of 250 new domestic wells in Minnesota, north-central United States. One study well had extremely high initial arsenic (1550 µg/L) that substantially decreased after 15 months (5.2 µg/L). The drilling and development of the study well were typical and ordinary; nothing observable indicated the very high initial arsenic concentration. We hypothesized that oxidation of arsenic-containing sulfides (which lowers pH) combined with low pH dissolution of arsenic-bearing Fe (oxyhydr)oxides caused the very high arsenic concentration. Geochemical equilibrium considerations and modeling supported our hypothesis. Groundwater equilibrium redox conditions are poised at the Fe(III)(s)/Fe(II)(aq) stability boundary, indicating arsenic-bearing Fe (oxyhydr)oxide mineral sensitivity to pH and redox changes. Changing groundwater geochemistry can have negative implications for home water treatment (e.g., reduced arsenic removal efficiency, iron fouling), which can lead to ongoing but unrecognized hazard of arsenic exposure from domestic well water. Our results may inform arsenic mobilization processes and geochemical sensitivity in similarly complex aquifers in Southeast Asia and elsewhere.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Elemental Relics: Biosignatures for Microbial Life in Terrestrial Hot Springs on Ancient Earth and MarsAuthorsAndrewGangidineMalcolmWalterJeffHavigAndrewCzajaDanielSturmeriDJeffreyHannonSee all authors Andrew GangidineCorresponding Author• Submitting AuthorCranbrook Institute of Scienceview email addressThe email was not providedcopy email addressMalcolm WalterUniversity of New South Walesview email addressThe email was not providedcopy email addressJeff HavigUniversity of Minnesotaview email addressThe email was not providedcopy email addressAndrew CzajaUniversity of Cincinnati Main Campusview email addressThe email was not providedcopy email addressDaniel SturmeriDUniversity of Cincinnati Main CampusiDhttps://orcid.org/0000-0002-9997-6702view email addressThe email was not providedcopy email addressJeffrey HannonUniversity of Wisconsin Madisonview email addressThe email was not providedcopy email address