Chromium evidence for protracted oxygenation during the Paleoproterozoic

Accepted manuscript version, licensed CC BY-NC-ND 4.0. It has commonly been proposed that the development of complex life is tied to increases in atmospheric oxygenation. However, there is a conspicuous gap in time between the oxygenation of the atmosphere 2.4 billion years ago (Ga) and the first widely-accepted fossil evidence for complex eukaryotic cells . At present the gap could either represent poor sampling, poor preservation, and/or difficulties in recognizing early eukaryote fossils, or it could be real and the evolution of complex cells was delayed due to relatively low and/or variable O2 levels in the Paleoproterozoic. To assess the extent and stability of Paleoproterozoic O2 levels, we measured chromium-based oxygen proxies in a core from the Onega Basin (NW-Russia), deposited billion years ago—a few hundred million years prior to the oldest definitive fossil evidence for eukaryotes. Fractionated chromium isotopes are documented throughout the section (max. ‰ ), suggesting a long interval (possibly >100 million years) during which oxygen levels were higher and more stable than in the billion years before or after. This suggests that, if it is the case that complex cells did not evolve until after 1.7 Ga, then this delay was not due to O2-limitation. Instead, it could reflect other limiting factors—ecological or environmental—or could indicate that it simply takes a long time—more than the tens to >100 million years recorded in Onega Basin sediments—for such biological innovations to evolve

In memoriam mr. sc. Vesna Burić (1943. - 2002.)

The exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation deposited in the Onega Basin, NW Russia, have refined our understanding of Earth System evolution during the Paleoproterozoic rise in atmospheric oxygen. These rocks were formed in vent- or seep influenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13C-depleted organic matter. Here we report new isotopic (δ34S, Δ33S, Δ36S, δ13Corg) and mineralogical, major element, total sulphur and organic carbon data for the upper part of the Zaonega Formation, which was deposited shortly after the termination of the Lomagundi-Jatuli positive carbon isotope excursion. The data were collected on a recently obtained 102 m drillcore section and show a δ13Corg shift from -38‰ to -25‰. Sedimentary sulphides have δ34S values typically between +15‰ and +25‰ reflecting closed-system sulphur isotope behaviour driven by high rates of microbial sulphate reduction, high sulphate demand, hydrothermal activity and hydrocarbon seepage. Four intervals record δ34S values that exceed +30‰. We interpret these unusually 34S-enriched sulphides to be a result of limited sulfate diffusion into pore waters due to changes in sedimentation and/or periods of basinal restriction. Additionally, there are four negative δ34S and positive Δ33S excursions that are interpreted to reflect changes in the open/closed-system behaviour of sulphate reduction or availability of reactive iron. Our findings highlight the influence of basinal processes in regulating sulphur isotope records and the need for care before interpreting such signals as reflecting global conditions

The REE-composition and petrography of apatite in 2Ga Zaonega Formation, Russia: the environmental setting for phosphogenesis

The first significant P-rich deposits appear in the global rock record during the Paleoproterozoic around 2 Ga, however the specific triggers that led to apatite precipitation are still under debate. The ca. 2 Ga Zaonega Formation, Karelia, Russia contains P-rich intervals in its upper part with abundantly occurring apatite. These apatites have been studied for their Rare Earth Element (REE) composition using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with an emphasis on the environmental condition during phosphogenesis. Petrographic observations by scanning electron microscopy (SEM) integrated with LA-ICP-MS results allow recognition of variously recrystallized apatite, and distinction of best-preserved diagenetic apatite which presumably records syn-depositional REE characteristics. Diagenetic apatite exhibits moderately negative Ce anomalies that indicate an at least partially oxygenated water column. A variable but typically positive Eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation. We conclude that phosphatic sediments in the Paleoproterozoic Zaonega Formation record phosphogenesis in a vent/seep influenced setting that experienced fluctuating redox conditions at the sediment–water interface

Mineral-templated growth of natural graphite films

International audienceOrganic material in sediments is progressively altered during diagenesis and metamorphism, leading to the formation of kerogen and ultimately crystalline graphite. Bulk carbonaceous material in metamorphic terrains typically has attained an overall degree of structural order that is in line with peak metamorphic temperature. On a micron- to nano-scale, however, carbonaceous material can display strong structural variation. The main factor that drives this variation is the chemical and molecular heterogeneity of the precursor biologic material. Specific conditions during metamorphism, however, can also play a role in shaping the microstructure of carbonaceous material. Here we describe the structural variation of carbonaceous material in rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia. Raman spectroscopy indicates that bulk carbonaceous matter in these rocks has experienced peak temperatures between 350 and 400 °C consistent with greenschist-facies metamorphism. On a nano-scale, however, a strong structural heterogeneity is observed. Transmission electron microscopy (TEM) reveals the occurrence of thin films of highly ordered graphitic carbon at mineral surfaces. These graphite films - consisting of 20-100 individual layers - completely envelop quartz crystals and occur on specific crystal surfaces of chlorite. It is proposed that minerals can act as templates for the parallel ordering of carbon crystallites causing enhanced graphitization within narrow zones at mineral surfaces. Alternatively, oriented organic precursor molecules could have been adsorbed onto charged mineral surfaces, leading to thin graphitic films during later metamorphic heating episodes. Overall the presented observations demonstrate that mineral surfaces can initiate and accelerate localized graphitization of sedimentary organic material during metamorphism, and therefore cause distinct nano-scale variation in structural order

Identifying global vs. basinal controls on Paleoproterozoic organic carbon and sulfur isotope records

Paleoproterozoic sedimentary successions are important archives of the redox evolution of Earth’s atmosphere and oceans. Efforts to unravel the dynamics of our planet’s early oxygenation from this archive rely on various geochemical proxies, including stable carbon and sulfur isotopes. However, ancient metasedimentary rocks often experienced early- and late-stage (bio)geochemical processes making it difficult to discern primary environmental signals from bulk-rock δ13Corg and δ34S values. Such complexity in carbon and sulfur isotope systematics contributes to uncertainty about the redox structure of Paleoproterozoic oceans. A currently popular idea is that, following the Great Oxidation Event, global changes led to low-oxygen environments and temporally fluctuating ocean redox conditions that lasted until the Neoproterozoic. The volcano-sedimentary rocks of the Onega Basin have figured prominently in this concept, particularly the exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation. However, a growing body of evidence shows that local depositional processes acted to form the δ13Corg and pyrite δ34S records of the Zaonega Formation, thus calling for careful assessment of the global significance of these isotope records. Placing new and existing organic carbon and sulfur isotope data from the Zaonega Formation into the context of basin history and by comparing those results with key Paleoproterozoic successions of the Francevillian Basin (Gabon), the Pechenga Greenstone Belt (NW Russia) and the Animikie Basin (Canada), we show that the stratigraphic δ13Corg and pyrite δ34S trends can be explained by local perturbations in biogeochemical carbon and sulfur cycling without requiring global drivers. Despite their temporal disparity, we also demonstrate that individual successions share certain geological traits (e.g. magmatic and/or tectonic activity, hydrocarbon generation, basin restriction) suggesting that their pyrite δ34S and δ13Corg trends were governed by common underlying mechanisms (e.g. similar basinal evolution and biogeochemical feedbacks) and are not necessarily unique to certain time intervals. We further show that pyrites in these successions that are most likely to capture ambient seawater sulfate isotopic composition have consistent δ34S values of 15–18‰, which hints at remarkable stability in the marine sulfur cycle over most of the Paleoproterozoic Era

The Great Oxidation Event Recorded in Paleoproterozoic Rocks from Fennoscandia

With support of the International Continental Scientific Drilling Program (ICDP) and other funding organizations, the Fennoscandia Arctic Russia – Drilling Early Earth Project (FAR-DEEP) operations have been successfully completed during 2007. A total of 3650 meters of core have been recovered from fifteen holes drilled through sedimentary and volcanic formations in Fennoscandia (Fig. 1), recording several global environmental changes spanning the time interval 2500–2000 Ma, including the Great Oxidation Event (GOE) (Holland, 2002). The core was meanwhile curated and archived in Trondheim, Norway, and it has been sampled by an international team of scientists

The kaolinite shuttle links the Great Oxidation and Lomagundi events

The ~2.22–2.06 Ga Lomagundi Event was the longest positive carbon isotope excursion in Earth’s history and is commonly interpreted to reflect perturbations in continental weathering and the phosphorous cycle. Previous models have focused on mechanisms of increasing phosphorous solubilization during weathering without focusing on transport to the oceans and its dispersion in seawater. Building from new experimental results, here we report kaolinite readily absorbs phosphorous under acidic freshwater conditions, but quantitatively releases phosphorous under seawater conditions where it becomes bioavailable to phytoplankton. The strong likelihood of high weathering intensities and associated high kaolinite content in post-Great-Oxidation-Event paleosols suggests there would have been enhanced phosphorus shuttling from the continents into marine environments. A kaolinite phosphorous shuttle introduces the potential for nonlinearity in the fluxes of phosphorous to the oceans with increases in chemical weathering intensity

The onega basin

The main geological and stratigraphic features of the Onega Basin are discussed in Chap. 4.3. Given here is a brief geological outline to provide a scientific context and background information for the FAR-DEEP implemented in this area

Iron Isotopes Reveal a Benthic Iron Shuttle in the Palaeoproterozoic Zaonega Formation: Basinal Restriction, Euxinia, and the Effect on Global Palaeoredox Proxies

The Zaonega Formation in northwest Russia (~2.0 billion years old) is amongst the most complete successions that record the middle of the Palaeoproterozoic era. As such, geochemical data from the formation have played a central role in framing the debate over redox dynamics in the aftermath of the Great Oxidation Event (GOE). However, uncertainty over local redox conditions and the degree of hydrographic restriction in the formation has led to contradictory interpretations regarding global oxygen (O2) fugacity. Here, we provide new iron (Fe) isotope data together with major and trace element concentrations to constrain the local physiochemical conditions. The Zaonega Formation sediments show authigenic Fe accumulation (Fe/Al ≫ 1 wt.%/wt.%) and δ56Fe ranging from −0.58‰ to +0.60‰. Many of the data fall on a negative Fe/Al versus δ56Fe trend, diagnostic of a benthic Fe shuttle, which implies that Zaonega Formation rocks formed in a redox-stratified and semi-restricted basin. However, basin restriction did not coincide with diminished trace metal enrichment, likely due to episodes of deep-water exchange with metal-rich oxygenated seawater, as evidenced by simultaneous authigenic Fe(III) precipitation. If so, the Onega Basin maintained a connection that allowed its sediments to record signals of global ocean chemistry despite significant basinal effects