The Cryogenian global glaciations profoundly shaped the evolution of Earth's ecosystem. An active Cryogenian biosphere accompanied by key evolutionary innovations has been indicated by geochemical and phylogenetic studies, although fossil records from Cryogenian strata are limited. In this study, we report a silicified microfossil assemblage from the Cryogenian Datangpo Formation in an interglacial offshore setting of the Yangtze block, South China. The Datangpo assemblage majorly comprises coccoidal microfossils classified into three morphological types, with minor components of fragmented filamentous forms. Morphological and structural observations combined with Raman spectroscopic analysis indicate that this microfossil assemblage may represent a planktonic microbial community dominated by cyanobacteria. The exceptionally silicified taphonomic window in the Datangpo microfossil assemblage provides a snapshot of primary producers in an offshore environment between the two Cryogenian global glaciations.

Alkaline lakes are among the most bioproductive aquatic ecosystems on Earth. The factors that ultimately limit productivity in these systems can vary, but nitrogen (N) cycling in particular has been shown to be adversely affected by high salinity, evidently due to the inhibition of nitrifying bacteria (i.e., those that convert ammonic species to nitrogen oxides). The coastal plain of Coorong National Park in South Australia, which hosts several alkaline lakes along 130 km of coastline, provides an ideal natural laboratory for examining how fine-scale differences in the geochemistry of such environments can lead to broad variations in nitrogen cycling through time, as manifest in sedimentary δ15N. Moreover, the lakes provide a gradient of aqueous conditions that allows us to assess the effects of pH, salinity, and carbonate chemistry on the sedimentary record. We report a wide range of δ15N values (3.8‰-18.6‰) measured in the sediments (0-35 cm depth) of five lakes of the Coorong region. Additional data include major element abundances, carbonate δ13C and δ18O values, and the results of principal component analyses. Stable nitrogen isotopes and wt% sodium (Na) display positive correlation (R2 = 0.59, p < 0.001) across all lake systems. Principal component analyses further support the notion that salinity has historically impacted nitrogen cycling. We propose that the inhibition of nitrification at elevated salinity may lead to the accumulation of ammonic species, which, when exposed to the water column, are prone to ammonia volatilization facilitated by intervals of elevated pH. This process is accompanied by a significant isotope fractionation effect, isotopically enriching the nitrogen that remains in the lake water. This nitrogen is eventually buried in the sediments, preserving a record of these combined processes. Analogous enrichments in the rock record may provide important constraints on past chemical conditions and their associated microbial ecologies. Specifically, ancient terrestrial aquatic systems with high δ15N values attributed to denitrification and thus oxygen deficiency may warrant re-evaluation within the framework of this alternative. Constraints on pH as provided by elevated δ15N via ammonia volatilization may also inform critical aspects of closed-basin paleoenvironments and their suitability for a de novo origin of life.

During the end-Permian mass extinction, a global decline in seafloor sediment mixing and burrowing (bioturbation) provides critical evidence for the collapse of marine ecosystems, likely triggered by rapid ocean warming and deoxygenation. However, the decline and subsequent recovery of bioturbation after the extinction event may not only have been a symptom of environmental change but also a driver, influencing nutrient exchange and reductant burial across the sediment-water interface and thus water column oxygen availability and seafloor habitability more broadly. Here we test this hypothesis through combined analyses of bioturbation and sedimentary geochemistry, focusing on marine siliciclastic records of the Permian-Triassic transition from Svalbard. We find that total organic carbon, total sulfur, and organic phosphorus decrease with increasing bioturbation intensity, whereas inorganic reactive phosphorus phases (authigenic and iron oxide-bound phosphorus) increase. These differences are most strongly associated with biodiffusion (particle mixing) rather than bioirrigation (solute exchange). Our findings suggest that bioturbation primarily influenced sediment chemistry by enhancing organic matter oxidation, in contrast to some modern settings where downward mixing may promote organic matter preservation within the anoxic portion of seafloor sediments. The early return of shallow-tier bioturbators in this region < 200 kyr after the extinction event likely promoted a rapid restoration of efficient carbon and sulfur cycling within benthic ecosystems. In contrast, efficient phosphorus burial via sink-switching may not have resumed until deeper-tier bioturbators achieved pre-extinction levels of sediment mixing > 1 Myr after the mass extinction.

ABSTRACTThe widespread, stepwise oxygenation of Earth's atmosphere in the Precambrian led to a transformation of the global carbon (C) and nitrogen (N) cycles. While the temporal evolution of these nutrient cycles has been studied extensively in marine environments, lacustrine environments are understudied. This study first examines how water column oxygen conditions impact sedimentary carbon (δ13Corg) and nitrogen (δ15N) isotope signals in modern lakes. Subsequently, we use these patterns to interpret past changes in the geological record of lacustrine δ15N during atmospheric oxygenation. The compiled modern lake sediment dataset reveals average (± standard deviation) δ15N values of +2.9‰ ± 3.2‰ and δ13Corg values of −25.99‰ ± 3.77‰, as well as thresholds in δ13Corg for oxic versus anoxic conditions, and in δ15N for circumneutral versus alkaline pH conditions. In contrast to the stepwise oxygenation of the atmosphere, the lacustrine δ15N record does not directly reflect major oxygenation events, but instead increases gradually in response to the evolution of new aerobic N metabolic pathways, with a notable shift in the Phanerozoic. While we found that intrasite variability at a single modern anoxic lake is expected to remain within ~5‰ for δ15N, alkaline lakes in both the ancient and modern deviate from this range. We observe δ15N > +10‰ for approximately half of total ancient alkaline lake sediments and some modern lake sediments. This is consistent with previous applications of enriched δ15N as a basicity proxy. The lacustrine δ15N record aligns well with the evolution of microbial metabolic pathways in addition to providing information pertaining to environmental conditions of the depositional setting.

ABSTRACTMethanogenic archaea were likely among the earliest organisms to populate the Earth, perhaps contributing to the Archaean greenhouse effect; they are also widely discussed as analogues to any potential life on Mars. However, fossil evidence of archaea has been difficult to identify in the rock record, perhaps because their preservation potential is intrinsically low or because they are particularly small and difficult to identify. Here, we examined the preservation potential of a methanogen of the genus Methanobacterium, recently isolated from a low‐temperature serpentinizing system, an environment somewhat analogous to habitats on the early Earth and Mars. Notably, this organism has a cell wall composed of peptidoglycan‐like pseudomurein, which may imply a mineralisation potential similar to that of gram‐positive bacteria. Methanobacterium cells were placed in carbonate, phosphate, and silicate solutions for up to 3 months in order to assess the relative tendency of these minerals to encrust and preserve cellular morphology. Cells readily acquired a thick, uniform coating of silica, enhancing their potential for long‐term preservation while also increasing overall filament size, an effect that may aid the discovery of fossil archaea while hindering their interpretation. Phosphates precipitated from the medium in all experimental setups and even in parallel experiments set up with low‐phosphate medium, suggesting a hitherto unknown biomineralisation capacity of methanogens. Carbonate precipitates did not form in close association with cells.

In freshwater lakes and rivers, cyanobacteria belonging to the family Leptolyngbyaceae bore > 1 mm deep into limestone pebbles by dissolving carbonate at the tip of their 3-8 μm-thick filaments. The abundance of these borings decreases downward while it is so high at the rock surface that micrometric debris is formed. Moreover, the disintegrated material on the pebbles' surface can be easily removed, for instance, when pebbles are grinding against each other due to wave or current action or when insect larvae settle and scratch loosened grains from the surface while constructing their cases. After a larvae case has been abandoned, it decays with time and the surface benath it is colonized again by boring cyanobacteria. These processes can alternate repeatedly and lead to a sculptured appearance of the pebbles, especially because insect larvae tend to colonize already existing depressions where they are better protected from predation and where they can access suspended food more easily. In the sculptures entrenched by insect larvae, larvae of byssate bivalves like Dreissena polymorpha may settle. When growing, these bivalves also remove loosened carbonate from the bored surface. Thus, boring cyanobacteria play a pioneering, preconditioning role in the morphological evolution of limestone (pebble) surfaces by transforming an initially hard substrate into a firm- to softground that is subsequently colonized and structured by animals. Consequently, sculptured pebbles are the product of multiphase, preconditioned bioerosion. Ultimately, the synergistic effects of these bioerosive processes result in the dissolution of carbonate leading to a maximum take-up of approximately 0.5-0.8 kg CO2 per square meter and year, as a preliminary estimate indicates.

Whole microorganisms are rarely preserved in the fossil record but actively silicifying environments like hot springs provide an opportunity for microbial preservation, making silicifying environments critical for the study of microbial life through time on Earth and possibly other planetary bodies. Yet, the changes that biosignatures may undergo through lithification and burial remain unconstrained. At Steep Cone Geyser in Yellowstone National Park, we collected microbial material from (1) the living system across the active outflows, (2) the silicified areas adjacent to flows, and (3) lithified and buried material to assess the preservation of biosignatures and their changes across the lithification transect. Five biofabrics, built predominantly by Cyanobacteria Geitlerinema, Pseudanabaenaceae, and Leptolyngbya with some filamentous anoxygenic phototrophs contributions, were identified and tracked from the living system through the process of silicification/lithification. In the living systems, δ30Si values decrease from +0.13‰ in surficial waters to -2‰ in biomat samples, indicating a kinetic isotope effect potentially induced by increased association with actively growing biofabrics. The fatty acids C16:1 and iso-C14:0 and the hydrocarbon C17:0 were disentangled from confounding signals and determined to be reliable lipid biosignatures for living biofabric builders and tenant microorganisms. Builder and tenant microbial biosignatures were linked to specific Cyanobacteria, anoxygenic phototrophs, and heterotrophs, which are prominent members of the living communities. Upon lithification and burial, silicon isotopes of silicified biomass began to re-equilibrate, increasing from δ30Si -2‰ in living biomats to -0.55‰ in lithified samples. Active endolithic microbial communities were identified in lithified samples and were dominated by Cyanobacteria, heterotrophic bacteria, and fungi. Results indicate that distinct microbial communities build and inhabit silicified biofabrics through time and that microbial biosignatures shift over the course of lithification. These findings improve our understanding of how microbial communities silicify, the biomarkers they retain, and transitionary impacts that may occur through lithification and burial.

A three-dimensional tubular fabric known as "vermiform microstructure" in Phanerozoic and Neoproterozoic carbonate microbialites has been hypothesized to represent the body fossil of nonspicular keratose demosponges. If correct, this interpretation extends the sponge body fossil record and origin of animals to ~890 Ma. However, the veracity of the keratose sponge interpretation for vermiform microstructure remains in question, and the origin of the tubular fabric is enigmatic. Here we compare exceptionally well-preserved microbialite textures from the Upper Triassic to channel networks created by modern microbial biofilms. We demonstrate that anastomosing channel networks of similar size and geometries are produced by microbial biofilms in the absence of sponges, suggesting the origin for vermiform microstructure in ancient carbonates is not unique to sponges and perhaps best interpreted conservatively as likely microbial in origin. We present a taphonomic model of early biofilm lithification in seawater with anomalously high carbonate saturation necessary to preserve delicate microbial textures. This work has implications for the understanding of three-dimensional biofilm architecture that goes beyond the current micro-scale observations available from living biofilm experiments and suggests that biofilm channel networks have an extensive fossil record.