Petrography and the REE-composition of apatite in the Paleoproterozoic Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia

The first globally significant phosphorous-rich deposits appear in the Paleoproterozoic at around 2 Ga, however, the specific triggers leading to apatite precipitation are debated. We examine phosphorous-rich rocks (up to 8 wt% P2O5) in 1.98–1.92 Ga old Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia. Phosphates in these rocks occur as locally derived and resedimented sand-to-gravel/pebble sized grains consisting of apatite-cemented muddy sediments. Phosphatic grains can be subdivided into four petrographic types (A–D), each has a distinct REE signature reflecting different early-to-late diagenetic conditions and/or metamorphic overprint. Pyrite containing petrographic type D, which typically has a flat REE pattern, negative Ce anomaly and positive Eu anomaly, is the best preserved of the four types and best records conditions present during apatite precipitation. Type D phosphatic grains precipitated under (sub)oxic basinal conditions with a significant hydrothermal influence. These characteristics are similar to Zaonega Formation phosphates of NW Russia’s Onega Basin, and consistent with phosphogenesis triggered by the development of anoxic(sulfidic)–(sub)oxic redoxclines at shallow sediment depth during the Paleoproterozoic

Constraining the conditions of phosphogenesis : stable isotope and trace element systematics of Recent Namibian phosphatic sediments

This study was supported by the Estonian Science Agency project PRG447 and the Estonian Centre of Analytical Chemistry. K. Paiste was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 894831. We would also like to thank the organizers and participants of the Regional Graduate Network in Oceanography Discovery Camp 2015, funded by the Agouron Institute and the Scientific Committee for Oceanographic Research (SCOR), as well as the Namibian Ministry of Fisheries and Marine Resources and the captain and crew of R/V Mirabilis for access to the coring site.Modern phosphogenesis occurs on continental margins influenced by upwelling and high primary productivity. The formation of phosphatic sediments is coupled to global climate fluctuations, biological cycling of phosphorus and local redox conditions. Although the processes involved in phosphogenesis are well described, high-resolution data on the redox and stable isotope systematics in Recent in-situ phosphorites are scarce. In this contribution, we investigate the trace element and sulfur, nitrogen and organic carbon stable isotope composition of Recent in-situ phosphatic sediments off the coast of Namibia. Also, we examine the reliability of different widely used geochemical proxies in phosphatic sediments. Our results suggest a shift from sulfidic to suboxic conditions, coinciding with the maximum in solid calcium phosphate mineral concentration. This shift is accompanied by unidirectional changes in Mo and Re enrichments and TOC abundance. Relatively low pyrite δ34S values (ca -20‰) of phosphatic sediments indicate open system fractionation during phosphogenesis. The initiation of phosphogenesis is also accompanied by negative shifts in sedimentary δ13Corg and δ15N values. Phosphate associated sulfate (PAS) δ34S values are lower than modern seawater sulfate values, suggesting the involvement of chemolithotrophic sulfur oxidation. Our results show a shift in redox conditions from sulfidic to (sub)oxic, coupled with active sulfur cycling are prerequisites for phosphogenesis. Phosphatic sediments show substantial enrichments in U and V highlighting the complexity of using these elements, as well as V/(V+Ni) and V/Cr, as redox proxies particularly in phosphorites and phosphatic sediments.PostprintPostprintPeer reviewe

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

On the Preparation of Some Tertiary Amines Containing the 2-Furfuryl Group. Isomerization of Allyl-aryl( 2-furfuryl)-amines to N-Aryl-4H-5, 7 a-epoxyisoindolines

Six new tertiary 2-furfurylamines of the general formula 2-C 4H 30 · CH2 NRAr, w h ere R represents methyl, ethyl or ally!, and Ar phenyl, p-tolyl or p-methoxyphenyl groups, have been prepared by alkylation of the appropriate secondary aryl-(2-furfuryl)- amines with alkyl or ally! halides. It was found that the oily allyl-aryl-(2-furfuryl)-amines, on standing at room temperature, spontaneously isomerized to crystalline N-aryl-4H-5,7a-epoxyisoindolines, formed by a reversible intramolecular Diels-Alder reaction

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

Reconstructing the Paleoproterozoic sulfur cycle: Insights from the multiple sulfur isotope record of the Zaonega Formation, Karelia, Russia

Earth’s oxygen-rich atmosphere and its capacity to sustain complex life is the most paramount feature that distinguishes Earth from all other planets. The irreversible oxygenation of atmosphere occurred about half way through Earth’s history at ~2.3 Ga (the Great Oxidation Event – GOE) when for the first-time photosynthetic oxygen production surpassed the consumption by chemical reactions allowing it to accumulate in the atmosphere. Consequently, the GOE had a profound impact on the atmosphere-ocean system, intensifying continental weathering, and prompting an increase in the riverine inputs of phosphate, sulfate and metals to the oceans. Increased availability of macro- and micronutrients may have opened new ecological niches to be exploited by complex microbial communities who, in turn, impact biogeochemichal cycles leaving metabolic signatures in the sedimentary rocks. However, there has been significant debate about the exact nature of the GOE, the environmental changes that followed and how these changes are recorded in the rock record. The focus of this thesis is the ~2.0 Ga Zaonega Formation in the Russian Karelia. This formation is one of the finest archives from which to decipher the physical and chemical conditions, as well as the role of local- vs. global-scale processes in the aftermath of the GOE. The study applies organic carbon and sulfur isotope results to assess organic carbon fluxes, sulfur cycling and depositional conditions that indicate restructuring of the microbial communities at different stages of basin evolution. The results indicate that following the establishment of a substantial seawater sulfate reservoir after the GOE, its size and isotope composition may have remained stable for few 100s Ma. The thesis highlights the importance of evaluating spatial and temporal geochemical variability in the context of individual basinal history before reaching global-scale conclusions

Palaeoproterozoic oxygenated oceans following the Lomagundi–Jatuli Event

The approximately 2,220–2,060 million years old Lomagundi–Jatuli Event was the longest positive carbon isotope excursion in Earth history and is traditionally interpreted to reflect an increased organic carbon burial and a transient rise in atmospheric O2. However, it is widely held that O2 levels collapsed for more than a billion years after this. Here we show that black shales postdating the Lomagundi–Jatuli Event from the approximately 2,000 million years old Zaonega Formation contain the highest redox-sensitive trace metal concentrations reported in sediments deposited before the Neoproterozoic (maximum concentrations of Mo = 1,009 μg g−1, U = 238 μg g−1 and Re = 516 ng g−1). This unit also contains the most positive Precambrian shale U isotope values measured to date (maximum 238U/235U ratio of 0.79‰), which provides novel evidence that there was a transition to modern-like biogeochemical cycling during the Palaeoproterozoic. Although these records do not preclude a return to anoxia during the Palaeoproterozoic, they uniquely suggest that the oceans remained well-oxygenated millions of years after the termination of the Lomagundi–Jatuli Event