ABSTRACT
The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23 and +13.7, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30 for δ13Corg and +5 for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.
Subject(s)
Carbon Isotopes , Nitrogen Isotopes , Oxidation-Reduction , Nitrogen Isotopes/analysis , Carbon Isotopes/analysis , Geologic Sediments/chemistry , Canada , Carbon/analysisABSTRACT
The initial rise of molecular oxygen (O2) shortly after the Archaean-Proterozoic transition 2.5 billion years ago was more complex than the single step-change once envisioned. Sulfur mass-independent fractionation records suggest that the rise of atmospheric O2 was oscillatory, with multiple returns to an anoxic state until perhaps 2.2 billion years ago1-3. Yet few constraints exist for contemporaneous marine oxygenation dynamics, precluding a holistic understanding of planetary oxygenation. Here we report thallium (Tl) isotope ratio and redox-sensitive element data for marine shales from the Transvaal Supergroup, South Africa. Synchronous with sulfur isotope evidence of atmospheric oxygenation in the same shales3, we found lower authigenic 205Tl/203Tl ratios indicative of widespread manganese oxide burial on an oxygenated seafloor and higher redox-sensitive element abundances consistent with expanded oxygenated waters. Both signatures disappear when the sulfur isotope data indicate a brief return to an anoxic atmospheric state. Our data connect recently identified atmospheric O2 dynamics on early Earth with the marine realm, marking an important turning point in Earth's redox history away from heterogeneous and highly localized 'oasis'-style oxygenation.
Subject(s)
Atmosphere , Earth, Planet , Oxygen , Seawater , Atmosphere/chemistry , Geologic Sediments/chemistry , History, Ancient , Oceans and Seas , Oxidation-Reduction , Oxygen/analysis , Oxygen/history , Oxygen/metabolism , Seawater/chemistry , South Africa , Sulfur Isotopes/analysis , Thallium/analysis , Thallium/chemistryABSTRACT
The likelihood of finding pristine molecular biosignatures preserved in Earth's oldest rocks or on other planetary bodies is low, and new approaches are needed to assess the origins of highly altered and recalcitrant organic matter. In this study, we aim to understand the distributions and systematics of preservation of ancient polycyclic aromatic hydrocarbons (PAHs), as both free hydrocarbons and bound within insoluble macromolecules. We report the distributions of bound PAHs generated by catalytic hydropyrolysis from ancient biogenic kerogens and from insoluble organic matter (IOM) in high-temperature carbonaceous residues from pyrobitumens and synthetic coke. For biogenic kerogens, the degree of thermal maturity exerts the primary control on the preservation and distributions of the major five-ring and six-ring PAH compounds. This holds for both Precambrian and Phanerozoic rocks, thus source variation in primary biogenic organic matter inputs does not exert the major control on bound PAH. The IOM samples, predominantly residues from hydrocarbon cracking at high temperatures, preserve a bound PAH profile significantly distinct from ancient biogenic kerogens and characterized by an absence of perylene and higher abundance of large-ring condensed PAHs. Covalently bound PAH profiles offer promise as "last resort" molecular biosignatures for aiding the astrobiological search for ancient life.
Subject(s)
Hydrocarbons, Aromatic , Polycyclic Aromatic Hydrocarbons , Environmental Monitoring , Hydrocarbons , Polycyclic Aromatic Hydrocarbons/analysisABSTRACT
The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth's habitability1. Early atmospheric oxygenation occurred over a protracted period of extreme climatic instability marked by multiple global glaciations2,3, with the initial rise of oxygen concentration to above 10-5 of the present atmospheric level constrained to about 2.43 billion years ago4,5. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago4, which represents the estimated timing of irreversible oxygenation of the atmosphere6,7. Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron-sulfur-carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10-5 of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated.
Subject(s)
Atmosphere/chemistry , Oxygen/analysis , Oxygen/history , Carbon Isotopes/analysis , Carbonates/analysis , Climate , Ecosystem , Geologic Sediments/chemistry , History, Ancient , Oceans and Seas , Oxidation-Reduction , Seawater/chemistry , South Africa , Sulfur Isotopes/analysis , Time FactorsABSTRACT
Modern marine hydrothermal vents occur in a wide variety of tectonic settings and are characterized by seafloor emission of fluids rich in dissolved chemicals and rapid mineral precipitation. Some hydrothermal systems vent only low-temperature Fe-rich fluids, which precipitate deposits dominated by iron oxyhydroxides, in places together with Mn-oxyhydroxides and amorphous silica. While a proportion of this mineralization is abiogenic, most is the result of the metabolic activities of benthic, Fe-oxidizing bacteria (FeOB), principally belonging to the Zetaproteobacteria. These micro-organisms secrete micrometer-scale stalks, sheaths, and tubes with a variety of morphologies, composed largely of ferrihydrite that act as sacrificial structures, preventing encrustation of the cells that produce them. Cultivated marine FeOB generally require neutral pH and microaerobic conditions to grow. Here, we describe the morphology and mineralogy of filamentous microstructures from a late Paleoproterozoic (1.74 Ga) jasper (Fe-oxide-silica) deposit from the Jerome area of the Verde mining district in central Arizona, USA, that resemble the branching tubes formed by some modern marine FeOB. On the basis of this comparison, we interpret the Jerome area filaments as having formed by FeOB on the deep seafloor, at the interface of weakly oxygenated seawater and low-temperature Fe-rich hydrothermal fluids. We compare the Jerome area filaments with other purported examples of Precambrian FeOB and discuss the implications of their presence for existing redox models of Paleoproterozoic oceans during the "Boring Billion."
Subject(s)
Hydrothermal Vents , Arizona , Iron/analysis , Oceans and Seas , Oxidation-Reduction , Seawater , TemperatureABSTRACT
Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth's history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 - 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/Ce* data to estimate the partial pressure of atmospheric oxygen (pO2) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO2 of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).
ABSTRACT
The role that iron played in the oxygenation of Earth's surface is equivocal. Iron could have consumed molecular oxygen when Fe3+-oxyhydroxides formed in the oceans, or it could have promoted atmospheric oxidation by means of pyrite burial. Through high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments and laboratory grown pyrites, we show that the triple iron isotopic composition of Neoarchean-Paleoproterozoic pyrites requires both extensive marine iron oxidation and sulfide-limited pyritization. Using an isotopic fractionation model informed by these data, we constrain the relative sizes of sedimentary Fe3+-oxyhydroxide and pyrite sinks for Neoarchean marine iron. We show that pyrite burial could have resulted in molecular oxygen export exceeding local Fe2+ oxidation sinks, thereby contributing to early episodes of transient oxygenation of Archean surface environments.
ABSTRACT
The oxygen isotope composition (δ18O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid δ18O on the rocks' isotopic composition. A new δ18O record in marine iron oxides covering the past ~2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater δ18O over time was the primary cause of the long-term rise in δ18O values of marine precipitates. The 18O enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high- and low-temperature crustal alteration, or a combination of these and other factors.
ABSTRACT
Illitisation requires potassium incorporation into a smectite precursor, a process akin to reverse weathering. However, it remains unclear whether microbes facilitate K+ uptake to the sediments and whether illitisation was important in the geological past. The 2.1 billion-year-old Francevillian Series of Gabon has been shown to host mat-related structures (MRS) and, in this regard, these rocks offer a unique opportunity to test whether ancient microbes induced illitisation. Here, we show high K content confined to illite particles that are abundant in the facies bearing MRS, but not in the host sandstone and black shale. This observation suggests that microbial biofilms trapped K+ from the seawater and released it into the pore-waters during respiration, resulting in illitisation. The K-rich illite developed exclusively in the fossilized MRS thus provides a new biosignature for metasediments derived from K-feldspar-depleted rocks that were abundant crustal components on ancient Earth.
Subject(s)
Bacteria/metabolism , Geologic Sediments/chemistry , Potassium/metabolism , Seawater/chemistry , Weather , Biofilms , Earth, Planet , Fossils , Gabon , Geologic Sediments/analysis , Minerals/analysis , Minerals/chemistry , Potassium/analysis , Silicates/chemistryABSTRACT
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
ABSTRACT
The Archean Eon was a time of predominantly anoxic Earth surface conditions, where anaerobic processes controlled bioessential element cycles. In contrast to "oxygen oases" well documented for the Neoarchean [2.8 to 2.5 billion years ago (Ga)], the magnitude, spatial extent, and underlying causes of possible Mesoarchean (3.2 to 2.8 Ga) surface-ocean oxygenation remain controversial. Here, we report δ15N and δ13C values coupled with local seawater redox data for Mesoarchean shales of the Mozaan Group (Pongola Supergroup, South Africa) that were deposited during an episode of enhanced Mn (oxyhydr)oxide precipitation between â¼2.95 and 2.85 Ga. Iron and Mn redox systematics are consistent with an oxygen oasis in the Mesoarchean anoxic ocean, but δ15N data indicate a Mo-based diazotrophic biosphere with no compelling evidence for a significant aerobic nitrogen cycle. We propose that in contrast to the Neoarchean, dissolved O2 levels were either too low or too limited in extent to develop a large and stable nitrate reservoir in the Mesoarchean ocean. Since biological N2 fixation was evidently active in this environment, the growth and proliferation of O2-producing organisms were likely suppressed by nutrients other than nitrogen (e.g., phosphorus), which would have limited the expansion of oxygenated conditions during the Mesoarchean.
ABSTRACT
Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232-235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023-1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351-381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment-water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).
Subject(s)
Biological Evolution , Fossils , Geologic Sediments/chemistry , Oxygen/chemistry , Atmosphere , Biota/physiology , Gabon , Oxidation-ReductionABSTRACT
The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆17O values (down to -0.88) observed in sulfates, except for those from the terminal Cryogenian period2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO2 and O2 and the photosynthetic O2 flux at this time3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean4-6 and Phanerozoic primary production7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2 over geologic time8.
Subject(s)
Ecosystem , Geologic Sediments/chemistry , Geologic Sediments/microbiology , Oxygen/analysis , Oxygen/metabolism , Sulfur/analysis , Sulfur/metabolism , Atmosphere/chemistry , Carbon Dioxide/analysis , History, Ancient , Ontario , Oxygen Isotopes/analysis , Oxygen Isotopes/metabolism , Partial Pressure , Photosynthesis , Probability , Sulfates/analysis , Sulfates/metabolism , Sulfides/analysis , Sulfides/metabolism , Sulfur Isotopes/analysis , Sulfur Isotopes/metabolismABSTRACT
Middle-to-late Ediacaran (575-541 Ma) marine sedimentary rocks record the first appearance of macroscopic, multicellular body fossils, yet little is known about the environments and food sources that sustained this enigmatic fauna. Here, we perform a lipid biomarker and stable isotope (δ15Ntotal and δ13CTOC) investigation of exceptionally immature late Ediacaran strata (<560 Ma) from multiple locations across Baltica. Our results show that the biomarker assemblages encompass an exceptionally wide range of hopane/sterane ratios (1.6-119), which is a broad measure of bacterial/eukaryotic source organism inputs. These include some unusually high hopane/sterane ratios (22-119), particularly during the peak in diversity and abundance of the Ediacara biota. A high contribution of bacteria to the overall low productivity may have bolstered a microbial loop, locally sustaining dissolved organic matter as an important organic nutrient. These oligotrophic, shallow-marine conditions extended over hundreds of kilometers across Baltica and persisted for more than 10 million years.
Subject(s)
Biota , Fossils , Geologic Sediments/chemistry , Geologic Sediments/microbiology , Asia , Bacteria/growth & development , Biological Evolution , Biomarkers/analysis , Ecosystem , Europe , Geography , Hydrocarbons/analysis , Lipids/analysis , Prochlorococcus/growth & development , Synechococcus/growth & developmentABSTRACT
Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen.
ABSTRACT
Fungi have recently been found to comprise a significant part of the deep biosphere in oceanic sediments and crustal rocks. Fossils occupying fractures and pores in Phanerozoic volcanics indicate that this habitat is at least 400 million years old, but its origin may be considerably older. A 2.4-billion-year-old basalt from the Palaeoproterozoic Ongeluk Formation in South Africa contains filamentous fossils in vesicles and fractures. The filaments form mycelium-like structures growing from a basal film attached to the internal rock surfaces. Filaments branch and anastomose, touch and entangle each other. They are indistinguishable from mycelial fossils found in similar deep-biosphere habitats in the Phanerozoic, where they are attributed to fungi on the basis of chemical and morphological similarities to living fungi. The Ongeluk fossils, however, are two to three times older than current age estimates of the fungal clade. Unless they represent an unknown branch of fungus-like organisms, the fossils imply that the fungal clade is considerably older than previously thought, and that fungal origin and early evolution may lie in the oceanic deep biosphere rather than on land. The Ongeluk discovery suggests that life has inhabited submarine volcanics for more than 2.4 billion years.
ABSTRACT
The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, â¼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next â¼200 My until ≤2,250-2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.
ABSTRACT
The rise of oxygen on the early Earth (about 2.4 billion years ago) caused a reorganization of marine nutrient cycles, including that of nitrogen, which is important for controlling global primary productivity. However, current geochemical records lack the temporal resolution to address the nature and timing of the biogeochemical response to oxygenation directly. Here we couple records of ocean redox chemistry with nitrogen isotope (15N/14N) values from approximately 2.31-billion-year-old shales of the Rooihoogte and Timeball Hill formations in South Africa, deposited during the early stages of the first rise in atmospheric oxygen on the Earth (the Great Oxidation Event). Our data fill a gap of about 400 million years in the temporal 15N/14N record and provide evidence for the emergence of a pervasive aerobic marine nitrogen cycle. The interpretation of our nitrogen isotope data in the context of iron speciation and carbon isotope data suggests biogeochemical cycling across a dynamic redox boundary, with primary productivity fuelled by chemoautotrophic production and a nitrogen cycle dominated by nitrogen loss processes using newly available marine oxidants. This chemostratigraphic trend constrains the onset of widespread nitrate availability associated with ocean oxygenation. The rise of marine nitrate could have allowed for the rapid diversification and proliferation of nitrate-using cyanobacteria and, potentially, eukaryotic phytoplankton.
Subject(s)
Nitrogen Cycle , Oxygen/metabolism , Seawater/chemistry , Aerobiosis , Aquatic Organisms/metabolism , Carbon Isotopes/analysis , Cyanobacteria/metabolism , Geologic Sediments/chemistry , History, Ancient , Iron/chemistry , Nitrates/metabolism , Nitrogen/metabolism , Nitrogen Isotopes/analysis , Oceans and Seas , Oxidation-Reduction , Phytoplankton/metabolism , South Africa , Time FactorsABSTRACT
It has been proposed that an "oxygen overshoot" occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga-Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δ82/78Se values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O2 levels of >0.4 µM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δ82/78Se values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δ82/78Se. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δ82/78Se and Se/TOC values during the subsequent Shunga-Francevillian anomaly indicates a widespread decrease in surface oxygenation.
ABSTRACT
The Paleoproterozoic Era witnessed crucial steps in the evolution of Earth's surface environments following the first appreciable rise of free atmospheric oxygen concentrations â¼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.