Unusual, basin-scale, fluid–rock interaction in the Palaeoproterozoic Onega basin from Fennoscandia : Preservation in calcite δ18O of an ancient high geothermal gradient

Acknowledgements We acknowledge financial support from ICDP for the drilling programme. AEF, ATB and ARP thank NERC for financial support through NE/G00398X/1. VAM thanks the Norwegian Research Council for financial support through 191530/V30. We are grateful for sample preparation and analyses to all the personnel at NGU lab. At SUERC we enjoyed exceptional analytical support from Julie Dougans. Anonymous reviewers and the editor provided comments that improved the final manuscript.Peer reviewedPostprin

Ocean acidification in the aftermath of the Marinoan glaciation

Boron isotope patterns preserved in cap carbonates deposited in the aftermath of the younger Cryogenian (Marinoan, ca. 635 Ma) glaciation confirm a temporary ocean acidification event on the continental margin of the southern Congo craton, Namibia. To test the significance of this acidification event and reconstruct Earth’s global seawater pH states at the Cryogenian-Ediacaran transition, we present a new boron isotope data set recorded in cap carbonates deposited on the Yangtze Platform in south China and on the Karatau microcontinent in Kazakhstan. Our compiled δ11B data reveal similar ocean pH patterns for all investigated cratons and confirm the presence of a global and synchronous ocean acidification event during the Marinoan deglacial period, compatible with elevated postglacial pCO2 concentrations. Differences in the details of the ocean acidification event point to regional distinctions in the buffering capacity of Ediacaran seawater

Petrography and geochemistry of carbonate rocks of the Paleoproterozoic Zaonega Formation, Russia : Documentation of C-13-depleted non-primary calcite

The Norwegian Research Council grant 191530/V30 to V.A. Melezhik fully funded the work of AEC, VAM and AL. ATB was supported by NERC grant NE/G00398X/1 to AEF and ARP. We are grateful for sample preparation and analyses to all the personnel at NGU lab. We appreciate the work on carbon and oxygen isotope analyses by Julie Dougans and Chris Taylor. Bojan Otoničar organized and helped with the CL work at the Karst Research Institute at Postojna. Arrangement of TOC, IC, and TC analyses at University of Münster is acknowledged to Harald Strauss.Peer reviewedPostprin

The Stac Fada “impact ejecta” layer: not what it seems

The Stac Fada Member (SFM) forms part of the Stoer Group of the Torridonian of NW Scotland. The SFM is unique in the Torridonian, being characterized by the presence of greenish altered glass clasts. Its origin has been debated for decades with several hypotheses being proposed but all invoking some connection with volcanic activity in the region. More recently, Amor et al. suggested that the SFM represents “a chord section through the continuous ejecta blanket surrounding an impact crater”. Here, we confirm the presence of shocked material within the SFM and then discuss its origin

Hydrothermal regeneration of ammonium as a basin-scale driver of primary productivity

Funding: This study was financially supported by a NERC research grant (NE/V010824/1), and Estonian Science Agency project PRG447 to K.KHydrothermal vents are important targets in the search for life on other planets due to their potential to generate key catalytic surfaces and organic compounds for biogenesis. Less well studied, however, is the role of hydrothermal circulation in maintaining a biosphere beyond its origin. Here we explored this question with analyses of organic carbon, nitrogen abundances, and isotopic ratios from the Paleoproterozoic Zaonega Formation (2.0 Ga), NW Russia, which is composed of interbedded sedimentary and mafic igneous rocks. Previous studies have documented mobilization of hydrocarbons, likely associated with magmatic intrusions into unconsolidated sediments. The igneous bodies are extensively hydrothermally altered. Our data reveal strong nitrogen enrichments of up to 0.6 wt.% in these altered igneous rocks, suggesting that the hydrothermal fluids carried ammonium concentrations in the millimolar range, which is consistent with some modern hydrothermal vents. Further, large isotopic offsets of approximately 10 ‰ between organic-bound and silicate-bound nitrogen are most parsimoniously explained by partial biological uptake of ammonium from the vent fluid. Our results, therefore, show that hydrothermal activity in ancient marine basins can provide a locally high flux of recycled nitrogen. Hydrothermal nutrient recycling may thus be an important mechanism for maintaining a large biosphere on anoxic worlds.PostprintPeer reviewe

Multiple Palaeoproterozoic carbon burial episodes and excursions

Organic-rich rocks (averaging 2–5% total organic carbon) and positive carbonate-carbon isotope excursions (View the MathML source and locally much higher, i.e. the Lomagundi-Jatuli Event) are hallmark features of Palaeoproterozoic successions and are assumed to archive a global event of unique environmental conditions following the c. 2.3 Ga Great Oxidation Event. Here we combine new and published geochronology that shows that the main Palaeoproterozoic carbon burial episodes (CBEs) preserved in Russia, Gabon and Australia were temporally discrete depositional events between c. 2.10 and 1.85 Ga. In northwest Russia we can also show that timing of the termination of the Lomagundi-Jatuli Event may have differed by up to 50 Ma between localities, and that Ni mineralisation occurred at c. 1920 Ma. Further, CBEs have traits in common with Mesozoic Oceanic Anoxic Events (OAEs); both are exceptionally organic-rich relative to encasing strata, associated with contemporaneous igneous activity and marked by organic carbon isotope profiles that exhibit a stepped decrease followed by a stabilisation period and recovery. Although CBE strata are thicker and of greater duration than OAEs (100 s of metres versus metres, ∼106 years versus ∼105 years), their shared characteristics hint at a commonality of cause(s) and feedbacks. This suggests that CBEs represent processes that can be either basin-specific or global in nature and a combination of circumstances that are not unique to the Palaeoproterozoic. Our findings urge circumspection and re-consideration of models that assume CBEs are a Deep Time singularity

The laurentian record of neoproterozoic glaciation, tectonism, and eukaryotic evolution in Death Vally, California

Neoproterozoic strata in Death Valley, California contain eukaryotic microfossils and glacial deposits that have been used to assess the severity of putative Snowball Earth events and the biological response to extreme environmental change. These successions also contain evidence for syn-sedimentary faulting that has been related to the rifting of Rodinia, and in turn the tectonic context of the onset of Snowball Earth. These interpretations hinge on local geological relationships and both regional and global stratigraphic correlations. Here we present new geological mapping, measured stratigraphic sections, carbon and strontium isotope chemostratigraphy, and micropaleontology from the Neoproterozoic glacial deposits and bounding strata in Death Valley. These new data enable us to refine regional correlations both across Death Valley and throughout Laurentia, and construct a new age model for glaciogenic strata and microfossil assemblages. Particularly, our remapping of the Kingston Peak Formation in the Saddle Peak Hills and near the type locality shows for the first time that glacial deposits of both the Marinoan and Sturtian glaciations can be distinguished in southeastern Death Valley, and that beds containing vase-shaped microfossils are slump blocks derived from the underlying strata. These slump blocks are associated with multiple overlapping unconformities that developed during syn-sedimentary faulting, which is a common feature of Cyrogenian strata along the margin of Laurentia from California to Alaska. With these data, we conclude that all of the microfossils that have been described to date in Neoproterozoic strata of Death Valley predate the glaciations and do not bear on the severity, extent or duration of Neoproterozoic Snowball Earth events

The mesoproterozoic Stac Fada Member, NW Scotland : an impact origin confirmed but refined

Funding to GRO from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program and the Canadian Space Agency (CSA) Canadian Analogue Research Network and Field Investigation programs is gratefully acknowledged. Part of LF’s work was supported by the Department of Foreign Affairs and International Trade (DFAIT), Government of Canada.The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.PostprintPeer reviewe

Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate : insights from paired S and O isotopic analyses

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (#57548-ND2) to D.F. for partial support of this research and from the Estonian Research Council (#PUT611, #PRG836) to O.H and A.L.Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.PostprintPeer reviewe

Oxygenated conditions in the aftermath of the Lomagundi-Jatuli Event : the carbon isotope and rare earth element signatures of the Paleoproterozoic Zaonega Formation, Russia

This study was supported by Estonian Research Council project PRG447, and the Estonian Centre of Analytical Chemistry. K.P. and A.L. were supported by the Research Council of Norway through its Centres of Excellence funding scheme grant No. 223259. K.P. acknowledges the Estonian Research Council grant MOBJD542 and T.M. PUT611.The c. 2.0 Ga Zaonega Formation of the Onega Basin (NW Russia) has been central in efforts to understand what led to the initial rise (Great Oxidation Event, GOE) and postulated fall in free atmospheric oxygen and associated high-amplitude carbon cycle excursions, the Lomagundi-Jatuli Event (LJE) and subsequent Shunga Event during Paleoproterozoic time. The Formation accumulated shortly after the LJE and encompasses both the recovery in the carbon cycle and hypothesised contraction of the oceanic oxidant pool. However, interpreting the correct environmental context recorded by geochemical signatures in the Zaonega rocks is difficult due to a complex depositional and diagenetic history. In order to robustly constrain that history, we undertook a multiproxy study (mineralogy, petrography, carbon isotope and rare earth element composition) of carbonate beds in the upper part of the Zaonega Formation recovered in the 102-m composite section of the OnZap drill-cores. Our findings differentiate primary environmental signatures from secondary overprinting and show that: (i) the best-preserved carbonate beds define an upwards increasing δ13Ccarb trend from c. -5.4‰ to near 0‰; and that (ii) large intra-bed δ13Ccarb variations reflect varying contributions of methanotrophic dissolved inorganic carbon (DIC) to the basinal DIC pool. Rare earth element and yttrium (REYSN) patterns confirm a marine origin of the carbonate beds whereas a consistent positive EuSN anomaly suggests a strong high temperature hydrothermal input during accumulation of the Zaonega Formation. Importantly, the presence of a negative CeSN anomaly in the REYSN pattern indicates an oxygenated atmosphere-ocean system shortly after the LJE and indicates that models invoking a fall in oxygen at that time require reassessment.PostprintPeer reviewe