Geobiology最新文献

Microbialites are organo-sedimentary structures formed throughout most of the Earth history, over a wide range of geological contexts, and under a multitude of environmental conditions affecting their composition. The carbon and oxygen isotope records of carbonates, which are most often their main constituents, have been used as a widespread tool for paleoenvironmental reconstructions. However, the multiplicity of factors that influence microbialites formation is not always properly distinguished in their isotopic record, in both ancient and modern settings. It is therefore crucial to refine our understanding of the processes controlling microbialites isotopic signal. Here, we analyzed the carbon and oxygen isotope compositions from bulk and micro-drilled carbonates as well as bulk organic carbon isotope compositions in microbialites from four Mexican volcanic crater lakes of increasing alkalinity. The survey of four lakes allows comparing microbialite formation processes and their geochemical record within distinct physico-chemical contexts. The geochemical analyses were performed in parallel to petrographic and mineralogical characterization and interpreted in light of the known microbial community composition for microbialites of the same lakes. Combining these data, we show that the potential for isotopic biosignature preservation primarily depends on physico-chemical conditions. Carbon isotope biosignatures pointing out to an autotrophic influence on carbonate precipitation are preserved in the lowest alkalinity lakes. By contrast, higher alkalinity lakes, where microbialites are more massive, favor carbonate precipitation in isotopic equilibrium with the lake water, with secondary influence of heterotrophic organic carbon degradation. From these results, we suggest that microbialite carbonate C isotope records can be interpreted as the balance between the microbialite net primary productivity and the amount of precipitation that relates to physico-chemical forcing. The signals of microbialite oxygen isotope compositions highlight a lack of understanding in the oxygen isotope records of relatively rare carbonate phases such as hydromagnesite. Nonetheless, we show that these signals are primarily influenced by the basins' hydrology, though biological effects may also play a (minor) role. Overall, both carbon and oxygen isotopic signals may record a mixture of different local/global and biotic/abiotic phenomena, making microbialites intricate archives of their growth environment, which should thus be interpreted with cautions and in the light of their surrounding sediments.

The evolution of oxygenic photosynthesis during the Archean (4–2.5 Ga) required the presence of complementary reducing pathways to maintain the cellular redox balance. While the timing of the evolution of superoxide dismutases (SODs), enzymes that convert superoxide to hydrogen peroxide and O₂, within bacteria and archaea is not resolved, the first SODs appearing in cyanobacteria contained copper and zinc in the reaction center (CuZnSOD). Here, we analyse growth characteristics, SOD gene expression (qRT-PCR) and cellular enzyme activity in the deep branching strain, Pseudanabaena sp. PCC7367, previously demonstrated to release significantly more O₂ under anoxic conditions. The observed significantly higher growth rates (p < 0.001) and protein and glycogen contents (p < 0.05) in anoxically cultured Pseudanabaena PCC7367 compared to control cultures grown under present-day oxygen-rich conditions prompted the following question: Is the growth of Pseudanabaena sp. PCC7367 correlated to atmospheric pO₂ and cellular SOD activity? Expression of sodB (encoding FeSOD) and sodC (encoding CuZnSOD) strongly correlated with medium O₂ levels (p < 0.001). Expression of sodA (encoding MnSOD) correlated significantly to SOD activity during the day (p = 0.019) when medium O₂ concentrations were the highest. The cellular SOD enzyme activity of anoxically grown cultures was significantly higher (p < 0.001) 2 h before the onset of the dark phase compared to O₂-rich growth conditions. The expression of SOD encoding genes was significantly reduced (p < 0.05) under anoxic conditions in stirred cultures, as were medium O₂ levels (p ≤ 0.001), compared to oxic-grown cultures, whereas total cellular SOD activity remained comparable. Our data suggest that increasing pO₂ negatively impacts the viability of early cyanobacteria, possibly by increasing photorespiration. Additionally, the increased expression of superoxide-inactivating genes during the dark phase suggests the increased replacement rates of SODs under modern-day conditions compared to those on early Earth.

Banded iron formations (BIFs) are chemical sedimentary rocks commonly utilized for exploring the chemistry and redox state of the Precambrian ocean. Despite their significance, many aspects regarding the crystallization pathways of iron oxides in BIFs remain loosely constrained. In this study, we combine magnetic properties characterization with high-resolution optical and electron imaging of finely laminated BIFs from the 2.7 Ga Carajás Formation, Brazil, to investigate their nature and potential for preserving ancient environmental conditions. Our findings reveal that magnetite, in the form of large 0.1–0.5 mm crystals, is the main iron oxide, with an overall averaged saturation magnetization (M_s) of 25 Am²/kg (corresponding to ~27 wt% of magnetite) over the studied 230 m of the sequence. Nevertheless, the non-negligible contribution of minerals with higher coercivity suggests variable proportions of hematite along the core. Additionally, we observe non-uniform behavior in magnetite grains, with distinct populations identified through low-temperature measurements of the Verwey transition. Petrographic observations indicate that the original sediment was an Fe–Si mud consisting of a ferrihydrite–silica mixture formed in the water column. This assemblage was rapidly transformed into nano-scale hematite embedded in silica as indicated by a honeycomb structure composed of Si-spherules distributed in a microscale hematite matrix. Textural relationships show that the nucleation of magnetite started during or soon after the formation of hematite, as indicated by the preservation of the Si-spherules within magnetite cores. Further magnetite overgrowth stages are characterized by inclusion-free rims, associated with continuous Si supply during the evolving diagenetic or early metamorphic stages. These findings, combined with existing literature, suggest that ferrihydrite precipitated alongside Si and organic material, later crystallizing as hematite on the seafloor. Anaerobic respiration by Fe(III)-reducing microorganisms likely contributed to early magnetite formation in a fluid-saturated, unconsolidated sediment. Subsequent low-grade metamorphism and Si mobilization led to palisade quartz precipitation and a second stage of magnetite growth likely formed at the expense of matrix hematite through thermochemical Fe(III) reduction. Low-temperature magnetic analyses revealed that the two generations of magnetite core and rim are associated with specific stoichiometry.

Due to their fast precipitation rate, sulfate evaporites represent excellent repositories of past life on Earth and potentially on other solid planets. Nevertheless, the preservation potential of biogenic remains can be compromised by extremely fast early diagenetic processes. The upper Miocene, gypsum-bearing sedimentary successions of the Mediterranean region, that formed ca. 6 million years ago during the Messinian salinity crisis, represent an excellent case study for investigating these diagenetic processes at the expense of organic matter and associated biominerals. Several gypsum crystals from the Northern Mediterranean were studied by means of destructive and non-destructive techniques in order to characterize their solid inclusion content and preservation state. In the same crystal, excellently preserved microfossils coexist with strongly altered biogenic remains. Altered remains are associated with authigenic minerals, especially clays. The results demonstrate that a significant fraction of organic matter and associated biominerals (notably biogenic silica) underwent early diagenetic modification. The latter was likely triggered by bottom sulfidic conditions when the growth of gypsum was interrupted. These results have significant implications for the interpretation of the Messinian Salt Giant.

The majority of large iron formations (IFs) were deposited leading up to Earth's great oxidation episode (GOE). Following the GOE, IF deposition decreased for almost 500 Myr. Subsequently, around 1.88 Ga, there was widespread deposition of shallow-water granular iron formations (GIF) within a geologically short time interval, which has been linked to enhanced iron (Fe) supply to seawater from submarine hydrothermal venting associated with the emplacement of large igneous provinces. Previous studies of Fe-rich, microfossil-bearing stromatolites from the ca. 1.88 Ga Gunflint Formation on the Superior craton suggested direct microbial oxidation of seawater Fe²⁺_(aq) by microaerophilic, Fe-oxidizing bacteria (FeOB), as a driver of GIF deposition. Although Fe-rich, microfossil-bearing stromatolites are common in 1.88 Ga GIF deposits on several cratons, combined paleontological and geochemical studies have been applied only to the Gunflint Formation. Here, we present new paleontological and geochemical observations for the ca. 1.89 Ga Gibraltar Formation GIFs from the East Arm of the Great Slave Lake, Northwest Territories, Canada. Fossil morphology, Rare Earth element (REE) concentrations, and Fe isotopic compositions support Fe oxidation by FeOB at a redoxcline poised above the fair-weather wave base. Small positive Eu anomalies and positive ε_Nd (1.89 Ga) values suggest upwelling of deep, Fe-rich, hydrothermally influenced seawater. While high [Fe²⁺_(aq)] combined with low atmospheric pO₂ in the late Paleoproterozoic would have provided optimal conditions in shallow oceans for FeOB to precipitate Fe oxyhydroxide, these redox conditions were likely toxic to cyanobacteria. As long as local O₂ production by cyanobacteria was strongly diminished, FeOB would have had to rely on an atmospheric O₂ supply by diffusion to shallow seawater to oxidize Fe²⁺_(aq). Using a 1-D reaction dispersion model, we calculate [O_2(aq)] sufficient to deplete an upwelling Fe²⁺_(aq) source. Our results for GIF deposition are consistent with late Paleoproterozoic pO₂ estimates of ~1%–10% PAL and constraints for metabolic [O_2(aq)] requirements for modern FeOB. Widespread GIF deposition at ca. 1.88 Ga appears to mark a temporally restricted episode of optimal biogeochemical conditions in Earth's history when increased hydrothermal Fe²⁺_(aq) sourced from the deep oceans, in combination with low mid-Paleoproterozoic atmospheric pO₂, globally satisfied FeOB metabolic Fe²⁺_(aq) and O_2(aq) requirements in shallow-marine subtidal environments above the fair-weather wave base.