Mercury (Hg) enrichment and elevated ratios of Hg to total organic carbon (Hg/TOC) in sedimentary rocks have often been linked to volcanism from large igneous provinces (LIPs). Primary Hg and TOC ...contents of sediments can be altered by secondary processes like extreme weathering. These effects must be evaluated before tying Hg anomalies in weathered rocks directly to LIP events. However, the effects of incipient weathering on Hg contents and Hg/TOC ratios are not known. In this study, we elucidate the behavior of Hg during incipient weathering by investigating visually pristine black shales from outcrops of the Ravnefjeld Formation in East Greenland (GRL) and comparing them to drill core equivalent intervals acquired from the same outcrop area and correlative shales from the mid-Norwegian shelf (MNS). By using geochemical investigations and principal component analysis, we characterize the main host phases of Hg and relate the different Hg contents of pristine samples from GRL and MNS to different Hg inputs during shale deposition. Compared with pristine drill core samples, incipiently weathered outcrop shales have up to 77% lower Hg contents and up to 64% lower Hg/TOC ratios. Incipient weathering causes the early degradation of Hg signals, which masks the primary Hg and Hg/TOC signals in sedimentary rocks. Therefore, we suggest that the presence and effects of weathering in sedimentary rock should be evaluated before discussing Hg signals.
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The end of Permian time (252–251
Ma) hosts the largest mass extinction in Earth history, yet events heralding this global catastrophe remain intensely disputed. We present a chemostratigraphic ...marker, the
187Re/
188Os ratio, which soars to unprecedented levels approaching the Permo-Triassic boundary. These ratios are tied to profound trace element changes and a precise Re–Os time record at 252
Ma preserved in black shales from East Greenland and the mid-Norwegian shelf. Within a 36-meter shale section, an 80-fold increase in Re concentrations (two-fold for Os) signals seawater conditions that became increasingly inhospitable to life. Unwavering initial
187Os/
188Os ratios of 0.6 preclude mafic volcanism and meteorite impact as the direct cause of Late Permian anoxia. We argue that extraordinarily high
187Re/
188Os ratios are the hallmark of simultaneously rising ocean temperature and acidity, leading to loss of oxygen and the stifling of life in latest Permian time.
► Shales from Greenland and mid-Norwegian shelf yield precise Late Permian Re–Os ages. ► Dramatic trace element changes upsection capture rapid development of severe anoxia. ► Unwavering initial Os ratios at ~
0.6 acquit mafic volcanism as the cause of anoxia. ►
187Re/
188Os ratios soar to unprecedented levels approaching the P–Tr boundary. ► High ocean temperature, acidity and anoxia together stifled Late Permian marine life.
Two recent advances in instrumentation convey rhenium and osmium elemental and isotopic distribution in molybdenite at the micro‐scale: (1) NanoSIMS imaging which reveals heterogeneities that could ...preclude micro‐scale geochronology (spot dating), and (2) reaction‐cell LA‐ICP‐MS/MS which partially resolves overlapping masses of isobaric 187Re and 187Os which otherwise confound micro‐scale analyses. The results from each of these measurement principles led respective researchers to conclude that no apparent parent‐daughter decoupling occurs, at least in their chosen samples. This contradicts 20 years of prior macro‐ to micro‐scale Re‐Os molybdenite data. Given these contradictions, we compare their results to our macro‐scale isotope dilution (ID) N‐TIMS measurement results of NIST Reference Material 8599, Henderson molybdenite. We document the homogeneity of the NIST molybdenite, including model age variability, precision of ID N‐TIMS measurement results and statistical treatment. We demonstrate that parent‐daughter decoupling cannot be precluded by visual inspection of NanoSIMS isotopic maps. In addition, we prove mathematically that quantifying reaction‐cell reacted 187Os by LA‐ICP‐MS/MS is far too imprecise to preclude parent‐daughter decoupling at an extent that hinders high‐precision geochronology, although full 2‐D ablation of a molybdenite crystal surface may yield an accurate Re‐Os age. Nevertheless, LA‐ICP‐MS/MS is useful for documenting the spatial extent of heterogeneous parent‐daughter distribution within individual crystals and supports whole crystal ID N‐TIMS.
Key Points
Rhenium distribution in molybdenite is heterogeneous as visually demonstrated by NanoSIMS imaging (Barra et al., Scientific Reports, 2017).
Repeated traditional isotope dilution N‐TIMS molybdenite Re‐Os measurements produce ages with intermediate precision of 0.25% (2 RSD) (Markey et al., Chemical Geology, 2007).
New LA‐ICP‐MS/MS reaction cell measurements produce Re‐Os ages with intermediate precision of 6.2% (2 RSD) (Hogmalm et al., Mineralium Deposita, 2019).
Limited chronological information for Upper Jurassic rocks hampers precise age assignment of the Jurassic-Cretaceous (J/K) boundary. This study presents Re-Os geochronology, Rock-Eval pyrolysis and ...geochemical data for Upper Jurassic rocks in the Nordland VII area, offshore northern Norway. Four intervals of organic-rich shale were investigated, two from the Krill and one from the Alge Member of the Hekkingen Formation and one from the underlying Måsnykan Formation (drilled cores 6814/04/U-01 and 6814/04/U-02). Rock-Eval pyrolysis data indicate immature organic matter with mixed (Type II/III) oil-gas potential in these units. Among the four intervals, Re-Os isochroneity prevails only for the Hekkingen Formation samples; data from the shale interval in the otherwise sand-rich Måsnykan Formation, deposited in a shoreface-to-foreshore marine environment, are not isochronous. Non-isochroneity of the samples from relatively proximal setting is attributable to non-conservative elemental (desorption of Os from sediments to dissolved phase) and isotopic exchange of Os at this depositional site. The Re-Os isochron age for the Alge Member (153.2 ± 7.3 Ma (Model 3); MSWD = 29; n = 9) is consistent with its biostratigraphic Kimmeridgian age (152.1–157.3 Ma). A Re-Os isochron for the lower part of the Krill Member yields an age of 144.5 ± 1.4 Ma (Model 3; MSWD = 3.7; n = 10) with an initial 187Os/188Os ratio of 0.552 ± 0.007. Given the interval's lower Tithonian biostratigraphic age, the Re-Os age suggests that the J/K boundary must be a few Myr younger than its currently assigned age of ~145.0 Ma (2017/02 time scale; www.stratigraphy.org). A Re-Os model age for shales from the upper part of the Krill Member places an upper age limit for the J/K boundary at 142 (±2) Ma. Hence, the J/K boundary is between 142 Ma and 144.5 Ma.
Fe and S concentrations for the Hekkingen Formation show strong correlation and the S/Fe slope (~1.19) mimics the stoichiometric S/Fe ratio for pyrite (~1.15). A Fe-S-TOC ternary diagram for these samples points to an iron-limiting, sulphidic depositional environment. Anoxic to sulphidic depositional conditions for the Hekkingen shales are also evident from relatively higher authigenic enrichment of Mo compared to U. Unlike the Hekkingen Formation, shales from the more sandy Måsnykan Formation have lower S/Fe ratios and lower enrichment factors for redox-sensitive elements, indicating relatively oxygenated marine conditions during deposition.
•Re-Os age of organic-rich shales from the base of M21 sequence is 144.5 ± 1.4 Ma.•The Jurassic-Cretaceous boundary age is a few Myr younger than its currently assigned age (~145 Ma).•Re-Os isochroneity prevail only for proximal fine-grained shales and not for shoreface-to-foreshore settings.•Hekkingen Formation shales were deposited in an iron-limiting, sulphidic environment.
Petroleum systems are inherently complex, comprising multiple components and processes, which variably interact in space and time to form petroleum accumulations. Improved understanding of these ...complex systems requires a holistic approach of integrating diverse geological, geophysical and organic and inorganic geochemical data. Here, we present an applied and comprehensive Re-Os geochronological and geochemical study on the Brynhild petroleum system from the Norwegian North Sea. Essential components of this system include shale source rocks of the Mandal and Farsund Formations, reservoir sandstones of the Ula Formation and their bitumen extracts, and produced Brynhild crude oil.
Rock-Eval, trace metal and Re-Os data reveal the excellent oil-generation potential of the shaley source rocks (high contents of organic matter containing large amounts of type I kerogens) and constrain their depositional conditions (anoxic bottom waters in a high-productivity regime driven by enhanced supply of land-derived nutrients). Seven Re-Os shale ages between ~146 and ~134 Ma are consistent with existing Volgian–Ryazanian age estimates and constrain the initial 187Os/188Os of the shales to ~0.52 in the lowermost Farsund Formation and ~0.75 in the uppermost Mandal Formation. Solvent-extracted reservoir rocks of the Ula Formation contain insoluble organic matter with type III–type IV kerogens likely supplied by erosion of older (a precise 179 Ma Re-Os age with unrealistically low initial 187Os/188Os) coal-bearing sediments or organic-rich shales.
The Brynhild crude oil has relatively low 187Os/188Os and a generation age of ~44 Ma based on an asphaltene-only Re-Os isochron. Maltenes from the Brynhild crude oil are isotopically affected by interaction with organic-rich source rocks that represent the seal for the Brynhild oil field. Compared with the crude oil, bitumen from Ula Formation sandstones have contrasting Re-Os properties: high 187Os/188Os ratios and young generation ages between 8 and 0 Ma based on asphaltene-bitumen-maltene Re-Os triplets, asphaltene-only Re-Os regressions, and ReOs model ages for components with extremely high 187Re/188Os. The 44 Ma and 8–0 Ma Re-Os ages agree well with regional burial models that indicate oil generation from ~44 Ma to present day. A genetic link between crude oil and bitumen is indicated by their similar alkane patterns, V and Ni contents, and V/Ni and pristane/phytane ratios. Modeling of 187Os/188Os for individual organic components through time shows that shales of the Mandal Formation developed 187Os/188Os ratios that match those of the crude oil and bitumen at ~44 Ma and ~8 Ma, respectively, and therefore confirm the source for both hydrocarbon phases. We suggest that bitumen formed during in-reservoir mixing of a ~44 Ma old early mature, asphaltene-poor oil (Brynhild crude oil) with an ~8 Ma old asphaltene-rich oil.
Earth's most extreme extinction event near the end of the Late Permian decimated more than 90% of all extant marine species. Widespread and intensive oceanic anoxia almost certainly contributed to ...the catastrophe, though the driving mechanisms that sustained such conditions are still debated. Of particular interest is whether water column anoxia was a consequence of a ‘stagnant ocean’, or if it was controlled by increases in nutrient supply, primary productivity, and subsequent heterotrophic respiration. Testing these competing hypotheses requires deconvolving sedimentary/bottom water redox conditions from changes in surface water productivity in marine sediments. We address this issue by studying marine shales from East Greenland and the mid-Norwegian shelf and combining sedimentary redox proxies with cadmium-isotopic analyses. Sedimentary nitrogen-isotopic data, pyrite framboid analyses, and organic and inorganic shale geochemistry reveal sulfidic conditions with vigorous upwelling, and increasingly anoxic conditions with a strengthening upwelling in the Greenland and Norwegian sections, respectively. Detailed analysis of sedimentary metal budgets illustrates that Cd is primarily associated with organic carbon and records primary geochemical signatures, thus enabling reconstruction of surface water nutrient utilization. Cadmium-isotopic analyses of the authigenic shale fraction released by inverse aqua regia digestion yield an average δCd114/110 of +0.15±0.01‰ (2 SE, n=12; rel. NIST SRM 3108), indicative of incomplete surface water nutrient utilization up-section. The constant degree of nutrient utilization combined with strong upwelling requires increasing primary productivity – and not oceanic stagnation – to balance the larger nutrient fluxes to both study sites during the development of the Late Permian water column anoxia. Overall, our data illustrate that if bottom water redox and upwelling can be adequately constrained, Cd-isotopic analyses of organic-rich sediments can be used to provide valuable information on nutrient utilization and therefore past productivity.
•Past nutrient utilization efficiency constrained by sedimentary Cd-isotopic ratios.•Nutrient supply is reconstructed using trace metal and N-isotopic proxies.•Late Permian anoxia was driven by increasing productivity, not by oceanic stagnation.
The geochemical record for the Permian–Triassic boundary in northern latitudes is essential to evaluation of global changes associated with the most profound extinction of life on Earth. We present ...inorganic and organic geochemical data, and Re–Os isotope systematics in a critical stratigraphic interval of pre- and post-extinction Upper Permian–Lower Triassic sediments from Opal Creek, western Canada (paleolatitude of ∼30°N). We document significant and long-lived changes in Panthalassa seawater chemistry that were initiated during the first of four magmatic or meteoritic inputs to Late Permian seawater, evidenced by notable decreases of Os isotopic ratios upsection.
Geochemical signals indicate establishment of anoxic bottom waters shortly after regional transgression reinitiated sedimentation in the Late Permian. Euxinic signals are most prominent in the Upper Permian sediments with low organic carbon and high sulfur contents, and gradually wane in the Lower Triassic. The observed features may have been generated in a strongly euxinic ocean in which high bacterioplankton productivity sustained prolific microbial sulfate reduction in the sediment and/or water column, providing hydrogen sulfide to form pyrite. This scenario requires nearly complete anaerobic decomposition of predominantly labile marine organic matter (OM) without the necessity for a complete collapse of primary marine productivity. Similar geochemical variations could have been achieved by widespread oxidation of methane by sulfate reducers after a methanogenic burst in the Late Permian. Both scenarios could have provided similar kill mechanisms for the latest Permian mass extinction.
Despite the moderate thermal maturity of the section, OM in all studied samples is dominantly terrestrial and/or continentally derived, recycled and refractory ancient OM. We argue that, as such, the quantity of the OM in the section mainly reflects changes in terrestrial vegetation and/or weathering, and not in marine productivity. At Opal Creek, a pyrite layer and <20-cm-thick siltstones that are lean in OM mark dramatic and long-lived inorganic geochemical and stable isotope changes. Initial Os isotope ratios decline markedly toward values of ∼0.35 in the pyrite interval, indicating a mantle-sourced or meteoritic trigger for the intensification and expansion of latest Permian anoxia. Subsequent and stronger magmatic or meteoritic pulses recorded by low initial Os isotopes followed the main extinction.
Re–Os isochron ages for black shales of the Hekkingen Formation in the Barents Sea constrain the onset (157.7±1.3 Ma) and termination (138.8±1.0 Ma), and thereby indicate a long duration (∼19 Myr) of ...widespread Jurassic–Cretaceous anoxia in the Arctic. Integration of these new Re–Os ages with published radiometric ages, ammonite biostratigraphy and geomagnetic polarity chrons shows shorter late Oxfordian–late Kimmeridgian and longer Berriasian stages relative to estimates in the 2012 and 2016 Geological Time Scales. Late Jurassic anoxia was likely the result of warming climate due to high atmospheric CO2 levels from increased oceanic crust production. Rising temperatures enhanced weathering and nutrient supply, increased productivity, and slowed ocean circulation before a sea-level rise brought anoxic waters onto continental shelves. Assessment of new and published Os- and Sr-isotopic data suggests that prolonged oceanic anoxia required a sustained CO2 source from fast spreading rates and/or longer subduction zones and spreading ridges to balance large burial of carbon in voluminous Upper Jurassic and Lower Cretaceous black shales.
•Hekkingen Formation shales were deposited in ∼19 Myr, from ∼157.7 Ma to ∼138.8 Ma.•Re–Os ages constrain the onset and duration of regional anoxia.•Radiometric ages refine the Jurassic time scale.•Late Jurassic anoxia is likely related to warm climate.•The long duration of anoxia required a sustained CO2 source.
Re–Os geochronology of oil may constrain the timing of oil formation and improve oil-source and oil–oil correlations. Typically, asphaltene (ASPH), the heaviest and most Re–Os rich oil fraction, from ...multiple oils within an oil field or a larger petroleum system are analyzed to obtain sufficient spread in Re–Os isotopic ratios, a mathematical necessity for precise Re–Os isochrons.
Here we offer a new approach for Re–Os geochronology of oil based on isotopic analyses of different fractions within a single sample of crude oil. We studied three oils from the Gela oil field, southern Sicily, Italy, recovered from Triassic–Jurassic stratigraphic intervals (Streppenosa, Noto, and Sciacca Formations) within the Gela-1 well. ASPH (insoluble in n-alkane) and maltene (MALT, soluble in n-alkane) fractions of oil were separated using n-pentane, n-hexane, n-heptane and n-decane solvents. The ASPH contents of the Sciacca and Noto oils (26–33wt%) are notably higher compared to the Streppenosa oil (7–12wt% ASPH).
We present an optimized Re–Os procedure with sample digestion in a high-pressure asher, followed by isotopic measurements using negative thermal ionization mass spectrometry. Very high metal contents of Gela oils allowed acquisition of precise Re–Os data. Systematic variations between the type of solvent used for ASPH precipitation and the ASPH content of the oil (also known from the literature) and the Re–Os contents of the ASPH and MALT fractions (first observed in this study) provide important practical applications for Re–Os analyses of oil. Most Re and Os (∼96–98%) in the Noto oil are hosted in the ASPH fraction. In contrast, a significant portion of Re and Os (∼33–34%) is stored in the MALT fraction of the lighter, but heavily biodegraded Streppenosa oil.
Collectively, our new data on alkane distribution, hopane and sterane biomarkers, major and trace element contents, and Re–Os concentrations and isotopic ratios of the oils and their fractions support the presence of two oil families. Streppenosa oil, a heavily biodegraded oil generated at an early stage of thermal maturation from a shaly source rock, is clearly distinct from the Noto and Sciacca oils, both generated at peak maturation from a carbonate source rock with no subsequent biodegradation.
Two Re–Os ages were obtained. Crude oil and ASPH from Noto and Sciacca oil yield a Model 1 isochron age of 27.5±4.6Ma with an initial 187Os/188Os of 3.89±0.43 (MSWD=1.6, n=19). We interpret this Oligocene age as the time of initial oil generation, that was probably related to the onset of regional collisional tectonics in southern Sicily. ASPH and crude oil from the Streppenosa oil yield scattered data, perhaps related to effects of biodegradation. Streppenosa MALT yield a Model 1 age of 200.0±5.2Ma and initial 187Os/188Os of 1.39±0.11 (MSWD=0.52; n=4). This age and initial Os-isotopic composition are consistent with rapid oil generation shortly after Late Triassic source-rock formation, perhaps driven by magmatic-related heating.
Here we document a potentially powerful geochronological tool that can be applied to single samples of crude oil. Within each oil, the 187Os/188Os and most of the 187Re/188Os ratios in the MALT fractions are lower than in the corresponding ASPH fractions. Crude oils, being physical mixtures of ASPH and MALT, have intermediate Re–Os isotopic ratios. Hence, the needed spread in data points for a Re–Os isochron is obtained by analyzing components of a single crude oil. This approach eliminates pitfalls in interpretation of Re–Os data from multiple oils with possibly different generation, migration, or reservoir modification history.
Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Here we bring Rock‐Eval, major and trace element and Re‐Os isotopic ...data together to examine the environmental record at the Cambrian‐Ordovician Global Stratotype Section and Point (GSSP) at Green Point in western Newfoundland, Canada. The Green Point shales are oil mature and contain Type II organic material of marine origin. A Re‐Os isochron for the shales provides the first radiometric age for shale deposition at the GSSP at 484 ± 16 Ma, with an initial 187Os/188Os ratio of 0.74 ± 0.05 (Model 3 age; MSWD = 21; n = 13; 2σ uncertainties). Factor analysis of the geochemical data set shows association of most trace elements with total organic carbon (TOC) and S contents, ensuring an authigenic origin for most elements and hence, their validity for evaluating the paleo‐redox state. Relatively high‐enrichment factors for redox‐sensitive elements (e.g., Re, U, and Mo) compared to average shale, but comparatively low enrichment compared to modern Black Sea sediments, suggest deposition in anoxic, but not euxinic waters. Comparison of Lower Ordovician shale geochemistry data sets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica, whereas depositional basins with colder waters (e.g., Avalonia and Gondwana) were less reducing. These outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life.
Key Points
Re‐Os age for the Cambrian‐Ordovician GSSP is 484 ± 16 Ma
Geochemistry of Green Point shales shows their anoxic depositional condition
Important role of paleogeography in regulating seawater chemistry
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