Following the largest known Phanerozoic extinction, the end-Permian mass extinction, the Early Triassic is often considered an interval of slow biotic recovery. During this time, the minor Smithian-Spathian extinction occurred, further delaying the biotic recovery of this epoch. One mechanism linked to the slow recovery is widespread marine anoxic conditions, but the spatiotemporal changes to anoxia following the mass extinction remain poorly constrained. Here, we utilized local and global redox-sensitive metal proxies to track variations in (de-)oxygenation at Wallenbergfjellet, a section in Spitsbergen, Arctic Norway. Local proxies (e.g., Fe speciation and trace metal concentrations) indicate basinal anoxia and possibly euxinia prevalent in the Boreal Ocean during this interval. Importantly, thallium isotopes, a global paleoredox proxy sensitive to Mn oxide burial, have not previously been applied to the Early Triassic recovery interval that followed the end-Permian mass extinction, though basinal restriction may limit geographic extent of interpretations. Our results show a shift in ε205Tl values from ~ -4.0 in the Smithian to ~ -2.0 at the Smithian-Spathian boundary (SSB), consistent with a rapid expansion of global anoxia coincident with the boundary extinction. A rapid perturbation in thallium isotopes at the SSB (from -4.0 to -2.0 ε-units) indicates this interval experienced an expansion of anoxic conditions that can be linked to the minor extinction event during the recovery interval following the end-Permian mass extinction. This suggests that global anoxia persisted throughout the Early Triassic and contributed to delayed biotic recovery, with the SSB representing a major intensification of reducing conditions.
Microbial dolostones that now form the Arkaroola reef, northern Adelaide Rift Complex, and were deposited during the Cryogenian interglacial period, display a wide range of meso-scale stromatolite morphotypes in outcrop. Microcharacterisation involving optical microscopy, combined scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) and nanoscale secondary ion mass spectroscopy (NanoSIMS), was undertaken to identify microbial textures and relics in stromatolite samples. Microbial textures identified are micritic streaky and micropeloidal (grumous) microstructures as well as microcolumns, micritic threads and micritised tubular outlines resembling the Girvanella calcimicrobe. Within these microbial textures, multiple biotic components are discerned, including intact and flattened tubular bacterial sheaths, smooth and nanogranular non-crystalline extracellular polymeric substances (EPS), filamentous and spherical bacteriomorphs, as well as coccobacilli-like moulds. The biotic-mineralogical interface is characterised by microcrystalline rhombic dolomite, nano-sized pyrite spheroids and nano-sized flaky silicates in direct contact with biotic components. Interpreted as a mixed community of filamentous and coccoidal (cyano)bacteria, these microbes were important contributors to reef building. Preservation of microfossils was likely facilitated by synsedimentary silicification of EPS and bacterial sheaths. Microbial metabolic processes lowered kinetic barriers for authigenic precipitation of clay minerals on bacterial surfaces serving as nucleation sites. These synsedimentary clays strengthened initially soft microfabrics and potentially helped establish chemical conditions that promoted early diagenetic precipitation of protodolomite. The findings of this study show that carbonates may intricately preserve microfossils where associated with synsedimentary authigenic clays and microcrystalline dolomite. These results contribute to a deeper understanding of the formation and growth of microbial reefs during the Cryogenian interglacial period.
Microbially mediated manganese (Mn) oxidation is a key process in the biogeochemical cycling of Mn. While Shewanella is widely recognized for its metal-reducing capabilities, recent studies have shown that certain terrestrial strains can also oxidize Mn2+ during aerobic respiration. Nevertheless, the Mn2+-oxidizing capacity of marine Shewanella strains, which are ubiquitous in oceanic settings, has remained largely unexplored, and the influence of environmental factors on this capacity has not been systematically evaluated. Here, we investigated the ability of the deep-sea bacterium S. piezotolerans WP3 to oxidize Mn2+ under aerobic conditions and examined the effects of temperature (4°C-20°C) and hydrostatic pressure (0.1-20 MPa). We found that S. piezotolerans WP3 efficiently oxidizes Mn2+ to Mn oxides, predominantly forming bixbyite-like minerals and amorphous mixed-valence nanoparticles. This oxidation process was not attributable to a Mn2+-specific enzyme but to bacterially generated reactive oxygen species (ROS), with superoxide (O2 •-) playing the primary role. Both temperature and hydrostatic pressure significantly affected the final extent of Mn2+ oxidation by altering the production of O2 •-. Transcriptomic analysis revealed that exposure to high hydrostatic pressure induced the upregulation of genes involved in antioxidative stress, which likely accounts for the enhanced ROS-mediated Mn2+ oxidation observed in cultures incubated at 20 MPa. Under alternating aerobic and anaerobic conditions, strain WP3 mediated successive Mn oxidation and reduction, ultimately forming rhodochrosite as a secondary mineral. These results suggest that S. piezotolerans WP3 has the potential to mediate Mn redox cycling in marine sediments, coupling ROS-dependent oxidation with anaerobic Mn reduction.
Rare earth elements and yttrium (REY) signatures in stromatolitic carbonates have emerged as powerful geochemical proxies for reconstructing paleo-depositional environments of microbial habitats. The applicability of such a proxy relies on the assumption that REY substitutes for calcium into crystal lattices, directly reflecting the composition of the fluid from which the carbonate precipitated. The REY signatures of stromatolites are often similar to those of open ocean seawater. In restricted environments, however, REY fractionation can occur between waters and stromatolites, questioning their reliability to reconstruct microbial habitats through Earth's history. In this contribution, we determined the REY concentrations and partition coefficients (Kd(stromatolite-fluid)) of sub-recent stromatolitic carbonates and ambient waters from the hypersaline Hamelin Pool in the Shark Bay lagoon, Australia. Shale-normalized REY patterns of stromatolite morphologies show (except colloform morphologies) a middle REYSN enrichment relative to the light and heavy REYSN. These signatures differ from those of seawater and ambient waters, suggesting that today's waters cannot be directly compared with stromatolitic carbonates from Shark Bay, which formed over thousands of years in a complex microbial mat system. Stromatolite morphologies such as colloform, smooth, or pustular structures formed in supratidal and intertidal environments exhibit the most variable Kd(stromatolite-fluid) values for the REY. The most dominant process affecting REY geochemistry in the Shark Bay stromatolites is most likely organic matter degradation and subsequent REY release into porewaters from which the carbonates formed in a (semi)closed microbial mat system. Cerebroid structures of the deepest lagoonal environment in the subtidal zone show the most constant Kd(stromatolite-fluid) values throughout the REY series, reflecting the least closed microbial mat system and a direct relationship between stromatolite morphology and water chemistry.
Microbialites provide a unique insight into ancient microbial processes and environments, but trends in the diversity of unlaminated microbialites remain poorly understood. The ca. 745 Ma Callison Lake Formation in the Yukon (northwest Canada) features a range of microbialites which are diverse at both the mesoscale and microscale. Four microbialite facies are recognised from the Ramp member of the Callison Lake Formation, all of which include framework structures with syn-depositional cavities. The thrombolite facies of the Callison Lake Formation is typified by exceptional preservation, with isopachous primary marine cements and distinct microclots that we suggest formed through early, mimetic carbonate precipitation. However, based on fine-scale textural relationships within the rock, we infer that the other microbialite textures were affected by diagenetic crystal growth, which reaffirms the importance of considering paragenetic history when describing and comparing ancient microbialites. The trace and rare earth element geochemistry of primary marine cements in the Callison Lake Formation microbialites suggests that the unit was deposited in a stratified, dominantly anoxic basin, with oxygenation above a very shallow chemocline. The relative depletion of chalcophile elements implies a euxinic depositional setting. This case study highlights the complexity of unlaminated microbialites and emphasises the need for detailed documentation of microbialites at multiple scales.
During the Paleocene-Eocene Thermal Maximum (PETM), there was an increase in global temperatures and emissions of isotopically depleted carbon, resulting in a negative carbon isotope excursion (CIE). This climatic event caused a widespread ocean deoxygenation, leading to substantial biotic turnover. Previous ichnological studies of deep-sea environments have suggested that bioturbating communities perished or diminished considerably during this event. In this study, we present an ichnological analysis of a well-known deep-sea outcrop (Rio Gor section; lower bathyal-upper abyssal depth; 1000-2000 m) from the southern Iberian margin. Contrary to previous studies, at this location, the PETM onset did not result in the extinction of the bioturbating community. In fact, high abundances of trace fossils were recorded during the PETM, indicating favorable paleoenvironmental conditions for the community. We discuss how sedimentary and climatic dynamics played a key role in regulating trace fossil abundance throughout the PETM. The paleogeographical position and deep-water circulation of the area appear to have played a crucial role in preventing low-oxygen deep water masses and the impoverishment of the bioturbating community. Overall, our findings reveal the PETM's positive impact on the bioturbating community at the southern Iberian margin. Given the essential ecological functions of these organisms on the seafloor-such as nutrient recycling and sediment mixing-we emphasize their potential importance in future warmer ocean scenarios.
Sulfate-reducing bacteria (SRB) drive the process of sulfate reduction in low-temperature sedimentary environments. Through the production of sulfide, they promote the formation of iron-sulfide (Fe-S) minerals when Fe(II) is available. The negative charge of the cell surface of bacteria can promote the binding of Fe(II), leading to the precipitation of Fe-S minerals at the surface of SRB when sulfide is released from cells. We evaluated interactions between Fe-S minerals and the surface of SRB using transmission electron microscopy (TEM) in cultures of Maridesulfovibrio hydrothermalis AM13 grown with 4 mM of Fe(II) over 1 month of incubation. On average, 18% ± 10% of cells were encrusted in cultures collected during the exponential phase. Fe-S mineral deposition occurred at the surface of cells while cells were growing and producing sulfide in the presence of Fe(II), but mineral crusts were removed from most cells shortly after deposition. Cells removed crusts from their surface through the formation of membrane vesicles, which were apparently only produced during growth. Mineralized and non-mineralized membrane vesicles were preserved in mineral aggregates in stationary-phase cultures. On average, 17% ± 7% of cells were encrusted in cultures collected during the stationary phase, indicating that Fe-S minerals precipitated during the exponential phase and removed from the cell surface did not aggregate back onto cells. On the contrary, they formed large aggregates away from cells. When Fe-S mineral precipitation occurred in non-growing cell suspensions that were first exposed to Fe(II) then to sulfide, the proportion of encrusted cells increased to 95% ± 6%, indicating that resting or non-growing cells were not able to remove mineral crusts from their surface. The metabolic status of SRB therefore plays a role in their ability to escape Fe-S mineral entombment.
Observations of morphology are commonly used to evaluate the biogenicity of terrestrial microfossils and could constitute a crucial line of evidence for extraterrestrial life-detection missions in the future. However, evaluating the origin of morphological features in the rock record can be problematic because naturally occurring abiotic structures can resemble biological morphologies, which may lead to false-positive detections of fossilised life. Iron-mineralised chemical gardens have been highlighted as potentially confounding abiotic structures because of their morphological and chemical resemblance to biomineralised filaments. Despite this, the potential for chemical garden structures to be preserved in the fossil record has not been thoroughly investigated. Here, we subjected abiotic iron-mineralised chemical garden structures to artificial maturation using hydrous pyrolysis, in order to evaluate their preservation potential. We found that these abiotic filaments were relatively resistant to degradation caused by maturation when compared with analogous biological material. Additionally, the transformation of ferrihydrite to crystalline iron oxides was found to be relatively inhibited, likely because of the influence of silica. These findings highlight the need for fossilised filamentous material to be distinguished from chemical garden structures before a biological origin can be confidently attributed, particularly when observed in significantly altered rocks.
Present-day angiosperm plants produce a plethora of metabolites including pigments that serve for important functions such as photosynthesis, protection against light, attraction of pollinators, and defense against microbes and herbivores. However, little is known about phytochemical constituents of ancient angiosperms, their distribution in the fossil record, their stability in deep time, and diagenesis. Outstanding preservation of ancient angiosperms, including exceptional color preservation, has been reported, but chemical analyses of such valuable specimens are limited by the rarity of the fossil material and the small amounts of potentially preserved metabolites. Here we use highly sensitive targeted liquid chromatography-tandem mass spectrometry in multiple reaction monitoring mode to screen for nanogram quantities of intact ancient phytochemical metabolites and their products in exceptionally well-preserved, about 45-Ma-old leaves from the Eocene Geiseltal fossil Lagerstätte, Germany. We show that diverse chlorophyll derivatives and degradation products as well as polyphenolic pigments are preserved in green to yellow colored angiosperm leaves and the brown coal matrix from Geiseltal. Most interesting is the fossil occurrence of the "unstable" green chlorophyll derivative dihydro-132,173-cyclopheophorbide a-enol, since cyclopheophorbide-enols are otherwise known as unique non-fluorescent chlorophyll catabolites of microorganisms in modern aquatic environments. The monopyrrole hematinic acid is interpreted as a stable product of chlorophyll catabolism via linear tetrapyrroles. Moreover, polyphenolic compounds in the fossil angiosperms are represented by the flavonoid pigments apigenin and luteolin. Our results demonstrate the potential of paleometabolomic-like screening of individual plant fossils to trace the fate of phytochemical constituents and to understand the processes of fossilization at the molecular level.
The Ediacaran-Cambrian boundary, which precedes one of the most significant biotic diversification events in Earth's history, is associated with a global negative carbon isotope excursion termed the BAsal Cambrian carbon isotope Excursion (BACE). Late Ediacaran and early Cambrian changes in shallow marine oxygenation have been proposed to relate to the BACE as well as metazoan extinction and radiation. However, reconstructing paleoredox conditions at the Ediacaran-Cambrian boundary is limited by challenges in correlating carbonate strata due to sparse stratigraphic markers and non-unique chemostratigraphic correlations. These imprecise correlations have led to uncertainty in how redox changes across the BACE should be interpreted in relation to broader regional and global environmental patterns. Here, we present redox reconstructions from southwestern Laurentian carbonate successions that record the BACE, including the limestone-dominated Deep Spring Formation, southwestern USA, and the dolostone-dominated La Ciénega Formation, northern Mexico. We combine local (carbonate-bound iodine, I/(Ca + Mg) and cerium anomaly, Ce/Ce*) and global (carbonate-associated uranium isotopes, δ238Ucarb) redox proxies to investigate marine oxygenation in relation to the BACE. Contrary to previous suggestions that a global ocean oxygenation event coincided with the BACE, we do not observe a shift in δ238Ucarb concurrent with the carbon isotope excursion in either section. The δ238Ucarb values differ between two sections, likely reflecting distinct diagenetic offsets attributed to different diagenetic U reduction, but together provide a minimal constraint on the carbonate δ238U value and suggest a more anoxic ocean compared to today. The local proxy results at both sites suggest widespread low-oxygen surface waters with a transient and localized interval of shallow marine oxygenation at one site that coincides with the nadir of the BACE. Persistently low I/(Ca + Mg) ratios, below values observed in today's oxygenated oceans, suggest a broadly redox-stratified surface ocean. Negative Ce anomalies in the La Ciénega Formation were recorded during the BACE nadir, suggesting a short-lived interval of local oxygenation within otherwise low-oxygen conditions. In sum, we do not find evidence for major, widespread oxygenation coincident with the BACE, but a continuation of low-oxygen conditions punctuated by a short-lived oxygenation event in the shallow oceans. These brief fluctuations in oxygen levels, in turn, may have played a role in the onset of behavioral complexity among bilaterian invertebrates during this critical transition.
The Paleoproterozoic Earth underwent profound environmental changes, including multiple severe glaciations and fluctuations in atmospheric oxygen (O2) levels. However, the precise relationship between O2 evolution and the glaciations remains unclear. Here, we use a biogeochemical cycle model involving carbon, phosphorus, sulfur, and oxygen to investigate the redox dynamics of the ocean-atmosphere system following the climatic transition to a super-greenhouse state after deglaciation. Our stochastic analysis reveals that climatic recovery on a timescale of ~105 years from elevated atmospheric CO2 levels (> 0.2 atm) triggers an extensive oxidation of the atmosphere and oceans over the subsequent 106-107 years, aligning with the large sulfur isotope anomaly in buried pyrite after the third Paleoproterozoic glaciation (~2.3 Ga). This finding suggests that the third glaciation represented an extensively glaciated, snowball state, which would have required massive accumulation of atmospheric CO2 for deglaciation. Variation in the boundary conditions regarding the global redox budget, as represented by high reductant fluxes, may explain the return of atmospheric O2 to Archean-like levels following the first (Makganyene) glaciation, which is also considered a snowball Earth.
Bacteriohopanepolyol derivatives (BHPDs) and dia- and catagenetic products formed from these bacterial membrane modifiers are extensively used as biomarkers in molecular ecological and geoscience studies. Some BHPDs can be assigned to specific phylogenetic bacterial groups. With the vastly increasing availability of complete bacterial genomes, hopanoid production can be readily predicted by the presence of specific genes encoding biosynthetic enzymes involved in their production, but the time-consuming physiological confirmation remains a critical element in the interpretation of such data in a biosynthetic and paleontological context. Alphaproteobacteria (APB) have often been proposed as important BHPD producers in a wide variety of environments and produce indicative BHPDs containing an additional methyl group in the A-ring, complicating the assignment of 2-methyl hopanes to N2-fixing cyanobacteria in paleontological studies. Here we provide the first comprehensive study of the relationship between genotype and phenotype with respect to the production of C30 hopanoids and BHPDs by APB. Genome analysis of > 6000 reference genomes of APB revealed that ca. 23% possess the genetic capacity to produce BHPDs, which is substantially higher than for all bacteria. However, BHPD biosynthesis genes were unevenly distributed between taxonomic and phylogenetic groups and not consistently found in mono-phylogenetic clusters. To study the relationship of genotype and phenotype with respect to the production of BHPDs, we cultivated 52 strains (50 terrestrial and 2 marine species) of the three major orders of the APB: Hyphomicrobiales, Rhodospirillales, and Sphingomonadales. These include species of 29 genera that have not previously been examined for BHPDs. Intact BHPDs were analyzed by UHPLC-MSn, resulting in the identification of overall 63 different structures and a wide variety in BHPD distributions. These results were in line with those obtained from Rohmer degradation on intact cells, which were specifically used to accurately assess the degree of methylation at C-2 and C-3 of ring A of the BHPDs. This revealed a 1-2 orders of magnitude lower degree of methylation at C-2 of BHPDs than for tetrahymanol (which was detected in three species all belonging to the Nitrobacteraceae) and C30 hopanoids, which has important implications for the interpretation of the molecular fossil record. Our results also showed that the presence of BHPD biosynthetic genes, often organized in a biosynthetic gene cluster, in all cases results in actual production of BHPDs. Thus, the presence of BHPD genes is a good predictor for the actual production of BHPDs. However, the presence of genes encoding proteins that result in methylation at C-2 and C-3 of BHPDs does not always lead to the production of methylated BHPDs, complicating the interpretation of the presence of the hpnP and hpnR genes in their genomes. Rohmer degradation-derived BHPD concentrations in APB species that do produce hopanoids can vary by two orders of magnitude and are not directly related to a specific phylogenetic group, indicating that the origin of sedimentary BHPDs may be biased towards specific species that produce relatively high amounts of BHPDs. These findings constrain the use of BHPDs as biomarkers for specific groups of bacteria in environmental and palaeontological studies.
Ferruginous (iron-rich) conditions have been prominent in oceans throughout the Earth's geologic history but now are reliably found only in a handful of permanently stratified lakes. Microbially mediated iron reduction in such anoxic environments competes with sulfate reduction, which promotes euxinic (sulfide-rich) conditions. Besides the shared demand for organic compounds, the competition is fostered by the produced hydrogen sulfide, which may reduce iron oxides abiotically or co-precipitate with dissolved iron as iron sulfides. Understanding why some environments develop ferruginous rather than euxinic conditions (or vice versa), as well as the attendant effects on methanogenic fermentation, is key to understanding both modern and ancient anoxic ecosystems. Here, we reproduce biogeochemical distributions in multiple anoxic, low-sulfate, meromictic lakes around the world using a biomass-explicit reaction-transport model with a fixed set of metabolism-specific microbial parameters. The results suggest that sulfate reduction and methanogenesis are ubiquitous even in iron-rich systems, and are reflected in microbial surveys. Ferruginous conditions typically develop for surface sulfate concentrations below ≃100 μM. Interestingly, there seems to be a dearth of stably stratified water bodies where sulfate concentrations can persist in the medium-sulfate range of several hundred μM. Rather, when sulfur burial into the sediments becomes iron limited, sulfate tends to accumulate in the water column to much higher (mM) concentrations. A similar mechanism could be suggested to have operated in the variably sulfidic and ferruginous water columns of early oceans. Model simulations also reveal the previously underappreciated role of physical transport in shaping biogeochemical distributions, as minor variations in mixing rates can lead to large variations in microbial abundances. Model applicability across multiple lakes points to an encouraging possibility that geochemical patterns in complex biogeochemical systems may be described from a small number of thermodynamic and kinetic principles using a minimum of fitting parameters.
Tasik Biru is a ~70 m-deep tropical lake in Malaysia, originating from a water-filled open pit mine. We investigated the biogeochemistry and microbial community of the lake as a modern model habitat to the stratified ancient ocean. We found that a sharp redoxcline exists at around 50 m depth, related to the decrease of O2 and pH (7.2-6.8) going down into the monimolimnion. Despite being relatively sulfate-rich (~320 μM), only a slight decrease of sulfate (to ~240 μM) was observed coupled with an increase of dissolved sulfide to 4 μM, attributed to microbial sulfate reduction in the monimolimnion. Comparatively, dissolved Fe and total Mn rose to ~50 μM in the anoxic layer with an unusual 1:1 concentration ratio. Other nutrients (PO4 3-, Si) and trace metal(loid)s (As, Mo, Sb, Co, U, and V) depth profiles increased or decreased across the chemocline, indicating controls via cycling of redox-sensitive elements. Microbial community analysis based on 16S rRNA amplicon sequencing reflects various metabolisms, from aerobic metabolisms in the mixolimnion to putative nitrite-dependent methane oxidation (e.g., by Methylomirabilis) at the chemocline, to sulfate reduction, methanogenesis, and fermentation in the monimolimnion. Tasik Biru is not in steady-state, and its anoxic water is predicted to shift from being Fe/Mn-rich to sulfide-rich, perhaps lending it as a model habitat to investigate biogeochemical changes from the metal-rich Archean to the Proterozoic oceans with expanding zones of sulfide-rich margins. An overview of the current biogeochemical cycles in the lake is presented, and open questions regarding partial sulfate consumption, methane, and Mn cycling and mineralogical distribution are highlighted to guide future studies.
Protodolomite formation in saline lakes remains enigmatic, despite favorable conditions for dolomite precipitation. To unravel this mystery, sediment samples were collected from two saline lakes in northeastern Inner Mongolia: Jibuhulangtu Nuur (JBHLT) and Dabusan Nuur (DBS). These samples were subjected to comprehensive analysis, including mineral composition detection, measurement of physicochemical variables, analysis of extracellular polymeric substances (EPS), and 16S rRNA sequencing. Results showed that protodolomite was exclusively observed in JBHLT, even though both lakes were supersaturated with respect to dolomite. Microbial communities in JBHLT were dominated by Gammaproteobacteria and Desulfuromonadia. Polysaccharide in EPS extracted from sediments was significantly enriched in JBHLT and positively correlated with microbial alpha diversity. pH was found to be the main factor significantly impacting microbial community composition, diversity, and functions. In DBS, increased pH led to the dominance of Halanaerobiaeota, while decreased microbial diversity and polysaccharide contents. The interplay of pH, microbial community structure, and sediment EPS content concurrently impacts protodolomite formation. Our findings highlight the interaction between environmental conditions and microbial communities and their consequence in terms of protodolomite mineralization.
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.
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.
Diagenetically mineralized fossil tissues represent invaluable paleobiological evidence of past life. Lipid biomarkers may be identified alongside fossils, yet the relationship between localized, diagenetic mineral precipitation, and lipid preservation remains underexplored. Coprolites (fossilized feces) attract a unique diversity of early diagenetic minerals including carbonates and phosphates, within individual samples, mediating molecular preservation of soluble lipid biomarkers alongside exceptional morphological preservation. Analysis of a well-preserved coprolite from the Carboniferous (307 ± 0.1 Ma) Mazon Creek assemblage, USA via time of flight-secondary ion mass spectrometry (ToF-SIMS) spatial compound mapping demonstrated the association of 5α,14α,17α(H) 20R cholestane, a C27 dietary sterane, with iron carbonate (and some pyrite) rather than phosphate minerals. Furthermore, Raman spectroscopic fingerprinting of a suite of organic-rich fossils spanning a number of biological species and preserved across the Mazon Creek site and other depositional settings was utilized to explore whether the localized preservation of steroids in carbonate phases represents a lagerstätten-specific or generalizable pattern. Our spectroscopic analyses demonstrate a significant positive correlation between signatures of lipid biomarkers and carbonates rather than phosphates across all soft-part samples at the Mazon Creek site and throughout Phanerozoic time and space. Early diagenetic carbonate measurably immobilizes otherwise labile lipid biomarkers and shields them against diagenetic stressors. Localized preservation identifies carbonate phases as a preferential resource for lipid-based biological information and reveals organomineral associations as a new frontier in understanding the survival of molecules in deep time.
The 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.
Large-scale geological processes shape microbial habitats and drive the evolution of life on Earth. During the Oligocene, convergence between Africa and Europe led to the opening of the Western Mediterranean Basin, a deep-ocean system characterized by fluid venting, oxygen depletion, and the absence of benthic fauna. In this extreme, inhospitable seafloor environment, fusiform objects known as Tubotomaculum formed, whose origin has long remained controversial. We show that these enigmatic mineralizations consist of nanosized, poorly crystalline, phosphorus-rich Mn-Fe compounds produced through microbial mediation. They preserve carbonaceous material together with morphological, chemical, and mineralogical biosignatures, including high Mn oxidation state (3.9 ± 0.15), cell envelopes, extracellular polymeric substances (EPS), cell-EPS partitioning of redox-sensitive Mn and Fe, cluster-assembled microbial cells, microbialite-like and branching structures, and channel networks for nutrient transport. Geochemical signatures indicate precipitation under suboxic to anoxic, non-sulfidic (post-oxic) conditions from mixed seawater-hydrothermal fluids, with exposure on the seafloor prior to burial. The fusiform architecture of these self-organized microbial populations suggests shaping by nutrient-rich bottom currents associated with venting activity. This study provides a detailed glimpse into initial benthic colonization of the nascent Western Mediterranean Basin and establishes Tubotomaculum as a model for investigating biomineralization and microbial adaptation in extreme environments, with implications for the search for life beyond Earth.
