Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation

The ~2.0 Ga Zaonega Formation (ZF) holds one of the oldest phosphorites in the geologic record, reaching >15% P2O5. Understanding the depositional conditions that enabled sedimentary phosphorus enrichment in this unit will thus help us to interpret the significance of the temporal distribution of phosphorites in Earth’s early history. Here we use an array of major and trace element data to constrain the redox conditions in the water column and extent of basinal restriction during deposition of the ZF. We also present new selenium (Se) abundance and isotopic data to provide firmer constraints on fluctuations across high redox potentials, which might be critical for phosphogenesis. We find that Se isotope ratios shift over a range of ~3‰ in the ZF, with the earliest stratigraphically-resolved negative Se isotope excursion in the geologic record, implying at least temporarily suboxic waters in the basin. Furthermore, we find that redox-sensitive element (RSE) enrichments coincide with episodes of P enrichment, thereby implicating a common set of environmental controls on these processes. Together, our dataset implies deposition under a predominantly anoxic water column with periodic fluctuations to more oxidizing conditions because of connections to a large oxic reservoir containing Se oxyanions (and other RSE’s, as well as sulfate) in the open ocean. This is broadly consistent with the depositional setting of many modern and recent phosphorites, thereby tying these ancient deposits to a common depositional mechanism. In light of these data, we propose that the broader prevalence of phosphogenesis in the Paleoproterozoic Era was driven by growth of the seawater oxidant reservoir (namely sulfate), thus enabling diagenetic apatite precipitation in basins with high rates of export production, particularly by facilitating the activity of sulfide-oxidizing bacteria. This suggests that the muted authigenic P burial observed in marginal, marine siliciclastic sedimentary rocks during other intervals of the Precambrian was not merely a result of low dissolved P levels in the global deep ocean, but also was influenced by sulfate scarcity and strongly reducing bottom-water conditions

Hydrothermal circulation and oil migration at the root of the heterogeneous micro-structure of carbonaceous material in the 2.0 Ga Zaonega Formation, Onega Basin, Russia

Organic-rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia, have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE). This formation has a complex history of alteration, involving pervasive hydrothermal circulation, hydrocarbon generation/migration, and mineral authigenesis. Several previous studies have focused on the description of these secondary effects, and the identification of primary geochemical signals in the carbonaceous phases. Migration and infiltration of organic-rich fluids appear to have had only limited effect on the primary carbon isotope record (δ13Corg). However, the structural variability of carbonaceous material (CM) appears to have been strongly affected, with a range of reported structures including carbon onion-shaped nanostructures and mineral-templated graphite films. Here we present a systematic Raman spectroscopy-based study of the structural variability of CM in a drill core representing the middle and upper strata of the Zaonega Formation. The Raman spectra of CM show a systematic difference in structural order between the bulk carbonaceous matrix (Matrix-CM) and the CM occurring near mineral contacts (Contact-CM), indicating that mineral templating was an important process affecting structural order in the formation. The templating effect was observed on the surface of a wide range of minerals. The difference in structural order between Matrix-CM and Contact-CM can be traced throughout the ca. 400 m stratigraphy. The structural order varied with the degree of alteration and hydrothermal circulation, from highly ordered structures directly above a large gabbro intrusion at the bottom of the stratigraphy to less ordered structures higher up in the sequence. This trend directly correlates with the δ18O trend of secondary calcite, and can be attributed to the decreasing influence and temperature regime of hydrothermal circulation upward in the stratigraphy. The results presented here suggest that organic-rich hydrothermal fluids can locally strongly enhance graphitization of carbonaceous materials, and cause sample-scale heterogeneities in the structural order of organic materials. This has implications for the interpretation of carbonaceous materials in other ancient rocks experiencing circulation of organic-rich hydrothermal fluids.publishedVersio

Increased barium levels in recent marine sediments from the Norwegian and Barents Seas suggest impact of hydrocarbon drilling and production

Barium (Ba) in recent marine sediments can originate from natural and anthropogenic sources including discharges from the oil and gas industry. In this study, we use data from the Norwegian and Barents Seas to assess whether Ba in recent marine sediments has increased due to these discharges. To account for Ba in detrital material, we normalise all samples with respect to aluminosilicate by calculating an enrichment factor. We use statistical modelling to control for parameters related to sedimentation. We present results that suggest increased Ba levels in recent sediments that coincide with the timing of hydrocarbon drilling and production. This is supported by geographical differences on a large scale that relate to proximity to hydrocarbon drilling and production. Among 243 sampling stations, we identify 73 locations exhibiting enrichment of Ba in the upper 6 of sediment. At these locations, Ba is 1.55 to 3.55 times higher than the levels that can be expected from the shale average when Ba in detrital matter is accounted for. Excess Ba is reported in sediment surface samples in areas important to fisheries like the Lofoten area and the western Barents Sea.publishedVersio

Carbonate deposition in the Palaeoproterozoic Onega basin from Fennoscandia : a spotlight on the transition from the Lomagundi-Jatuli to Shunga events

Date of Acceptance: 08/05/2015 Date of online publication: 16/05/2015 Acknowledgements Elemental and isotopic data, thin and polished sections used in this contribution were obtained through two large umbrella-projects with grants provided by the Norwegian Research Council grant 191530/V30 to VAM and NERC grant NE/G00398X/1 to AEF. We thank A. Črne, the editor A. Strasser as well as one anonymous reviewer and D. Papineau for providing their valuable criticism and suggestions.Peer reviewedPostprin

Palaeozoic carbon isotope excursions and carbonate component analysis

Palaeozoic carbonate δ13C studies have mainly focused on bulk rock analysis in chemostratigraphic correlations and palaeoenvironmental interpretations, assuming limited isotopic heterogeneity of samples. Particular attention has been paid to positive δ13C excursions, which likely reflect climatic events. However, several studies have provided isotope data from different rock components demonstrating significant sample-scale variation of δ13C values. We describe the isotopic and elemental composition of individual components in carbonate rocks and analyse which components carry the primary depositional isotopic signatures and which components have been isotopically reset by diagenetic processes. The comparison of bulk rock and component-specific data allows us to assess the reliability of isotopic data used in environmental reconstructions and offers an opportunity to better reconstruct magnitudes of δ13C excursions and related dynamics of seawater dissolved inorganic carbon. Here we focus on the Hirnantian Isotopic Carbon Excursion (HICE) at the end of the Ordovician period and the Mid-Ludfordian Carbon Isotope Excursion (MLCIE) during the late Silurian period. Previous bulk rock studies have demonstrated the occurrence of the HICE in multiple Estonian drill cores corresponding to the Porkuni Regional Stage. The HICE samples from the Kamariku, Karinu, and Otepää drill cores were subsampled by micro-drilling, and isotope analysis of individual components was done by isotope-ratio mass spectroscopy (IRMS). Limited sample-scale heterogeneity and good correspondence with bulk rock δ13C values are seen in micritic carbonate samples, whereas different bioclastic and cement components in wackestone and grainstone samples exhibit up to 4Ⱐrange of δ13C values with some components having δ13C values up to 3Ⱐhigher than the bulk rock values. Lithologically heterogeneous grainstone from the Otepää core shows δ18O variability as much as 3Ⱐand ooids carrying the lowest, most overprinted values. Isotope results obtained by the Secondary Ion Mass Spectrometry (SIMS) on MLCIE samples from Lithuanian Vidukle drill core show up to 5Ⱐrange of δ13C values, and some values are up to 4Ⱐhigher than the bulk rock values. A similar range can also be seen in δ18O values. Our results demonstrate that multi-component wackestone and grainstone samples from HICE and MLCIE intervals are isotopically heterogeneous and that heterogeneity needs to be accounted for in reconstructions of the past carbon cycle and δ13C curves

Stable and Clumped Isotope Characterization of Authigenic Carbonates in Methane Cold Seep Environments

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ¹³C, δ¹⁸O, δ⁴⁴Ca, and Δ₄₇) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ₄₇-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ¹³C_(VPDB) values (−30 to −47‰), high δ⁴⁴Ca_(SW) values (−0.4 to −0.8‰), and Δ₄₇-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ¹³C_(VPDB) values (−18 to −58‰), lower δ⁴⁴Ca_(SW) values (−0.8 to −1.6‰) and a larger range in Δ₄₇-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ⁴⁴Ca_(SW) values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO₂ degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions

Boron concentrations and isotopic compositions in methane-derived authigenic carbonates : constraints and limitations in reconstructing formation conditions

The work is supported by Norwegian Research Council through the schemes PETROMAKS2-NORCRUST (grant number 255150 ) and Centre for Arctic Gas Hydrate, Environment and Climate (CAGE grant number 223259 ) as well as Lundin Norway AS. Cruise MSM57-1/-2 was funded by the German Research Foundation (DFG), the Research Center/Excellence Cluster “The Ocean in the Earth System” at MARUM–Center for Marine and Environmental Sciences, University of Bremen and funds from CAGE.The boron content and isotopic composition (δ11B), of marine carbonates have the potential to constrain CO2 chemistry during carbonate growth conditions. However, obtaining and interpreting boron compositions from authigenic carbonates in geological archives present several challenges that may substantially limit their application. In particular, contamination from non-carbonate phases during sample preparation must be carefully avoided, and a variety of controls on boron composition during authigenic growth conditions must be evaluated. To advance understanding of the use and limitations of boron in authigenic carbonates, we present data and modelling results on methane-derived authigenic carbonate (MDAC), a by-product of microbially mediated anaerobic oxidation of methane, taken from three cold seep sites along the Norwegian margin. We present a novel sequential leaching method to isolate the boron signals from the micritic (Mg-calcite) and cavity-filling (aragonitic) MDAC cements in these complex multi-phase samples. This method successfully minimizes contamination from non-carbonate phases. To investigate the factors that could potentially contribute to the observed boron signals, we construct a numerical model to simulate the evolution of MDAC δ11B and B/Ca ratios over its growth history. We show that diagenetic fluid composition, depths of precipitation, the physical properties of sediments (such as porosity), and mineral surface kinetics all contribute to the observed boron compositions in the different carbonate cements. While broad constraints may be placed on fluid composition, the multiple competing controls on boron in these diagenetic settings limit the ability to place unique solutions on fluid CO2 chemistry using boron in these authigenic carbonates.Publisher PDFPeer reviewe

The Palaeoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event

The study was supported from Estonian Research Agency grant PRG447 to KK, AL and KB.The Paleoproterozoic Francevillian succession of Gabon has figured prominently in concepts about Earth’s early oxygenation and genesis of a large positive excursion in carbon-isotope values, the Lomagundi-Jatuli event (LJE). Here we present a detailed study of a 139-m-long core of Francevillian rocks marked by carbonate δ13C (δ13Ccarb) values of 5‰–9‰ that decline upsection to near 0‰, a trend inferred by many workers as a fingerprint of the LJE and its termination. However, we show that the shift in δ13Ccarb values coincides with a facies change: shallow-marine facies are marked by the strongly positive values, whereas deeper-marine facies (below storm wave base) are at ~0‰. The most circumspect interpretation of such facies dependence of δ13Ccarb is that shallow-marine settings record the isotope effects of local physical and biochemical processes driving the ambient dissolved inorganic carbon (DIC) pool to heavier values, and the lighter values (~0‰) in deeper-water facies track the DIC of the open-marine realm where δ13C was largely unaffected by fractionations occurring in shallow-water settings. Further, a transgressing redoxcline created conditions for precipitation of Mn-bearing minerals and chemotrophic microbial biota, including methane cycling communities evident by organic δ13C (δ13Corg) values of –4‰ and Δδcarb-org values as high as 46‰. Thus, the Francevillian C-isotope profile reflects basin-specific conditions and is not a priori an indicator of global C-cycle disturbances nor of the termination of the LJE.PostprintPeer reviewe

Distribution of natural and anthropogenic radionuclides in sediments from the Vefsnfjord, Norway

Areas in central Norway were heavily contaminated with fallout from the Chernobyl accident in 1986. In this study, we assess 137Cs in surface sediments and sediment cores collected in the Vefsnfjord in Nordland county. Concentrations of 137Cs in surface sediments ranged from 159 to 191 Bq kg−1 dry weight (d.w.). Sub-surface peaks of 137Cs were observed in all cores, with a maximum concentration of 432 Bq kg−1 d.w. Given that little is known about the distribution of naturally occurring radionuclides in Norwegian fjords and coastal areas, a better understanding of the total burden of radioactivity is important for the Norwegian fishing and aquaculture industries. Therefore, analyses of the natural radionuclides 40K, 226Ra, 228Ra and 210Pb were included in the study. Analyses of total sulphur (TS), total carbon (TC), total organic carbon (TOC) and grain size distribution have been performed to provide a sedimentologic context for interpreting the radionuclide results.publishedVersio

Gas seeps in Norwegian waters – distribution and mechanisms

Gas seeps and fluid-flow related seabed features are found over the entire Norwegian exclusive economic zone (EEZ). Multibeam water-column data from c. 136 000 km2 has revealed more than 5 000 gas seeps. Most of the gas seeps seem to have biogenic, thermogenic or mixed origin; some may be of abiotic origin. The spatial distribution of the gas seeps appears to correlate with: 1 – structural highs with associated faulting, exposing hydrocarbon reservoir rocks at or near the seabed; 2 – faults serving as conduits for fluid flow; 3 – settings where reservoir rocks overlain by less permeable cap rocks sub-crop at the seabed. Other mechanisms involve seepage around abandoned exploration wells, and possible abiotic gas generation from serpentinisation of ultramafic rocks near mid-oceanic ridges. The gas seeping from the Norwegian cold seeps is mostly methane and has, in many places, led to the formation of methane-derived authigenic carbonate crusts, which give evidence for either extensive gas seepage in the past or long-lived seepage. Chemosynthetic communities are commonly associated with cold seeps and may form special habitats together with the carbonate crusts. Methane seepage has been proposed to contribute significantly to the global carbon budget and may be associated with gas hydrates giving rise to potential geohazards. Gas seeps have been identified and spatially mapped as acoustic gas flares, using multibeam echosounder systems, which have the ability to record reflections from both the water column and the seabed. Water-column data have been recorded in the MAREANO seabed mapping program since 2010, covering an area of c. 262 000 km2 , with a data volume in the order of 210 Tb. The observations of extensive gas flares in the Norwegian EEZ are available to the scientific community and other users through a dedicated MAREANO data and web access system