Mesozoic Oceanic Anoxic Events Research Articles

Ammonium “nutrient capacitor” model for δ15N signatures associated with marine anoxic events

Abstract Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium “nutrient capacitor” model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.

Multiple diachronous “Black Seas” mimic global ocean anoxia during the latest Devonian

Abstract The Devonian-Carboniferous boundary (DCB) Hangenberg extinction in Europe (ca. 359 Ma) and interval of black shale deposition has been considered a global anoxic event, the cause of which remains debated. However, the lack of a prominent global carbon isotope excursion and the duration of anoxia (3–4 m.y. at least) is inconsistent with other global anoxic events (e.g., Mesozoic ocean anoxic events) throughout the Phanerozoic. We examined geochemical and biostratigraphic data along with radiometric dating from purported Hangenberg event sections across the DCB in western Canada and demonstrate that the onset of anoxic pulses in Laurentian seas started before 363 Ma, diachronous with those in Baltica (Europe). Thus, DCB black shales do not record a single global anoxic event. We suggest instead that black shales near the DCB record multiple, but diachronous, Black Sea–like basins around the globe, promoted by the Late Devonian tectonic configuration. This drove a gradual biotic crisis in the form of regional punctuated benthic faunal extinction events.

Nature of Devonian anoxic events based on multiproxy records from Panthalassa, NW Canada

Causes and controls of Middle-Late Devonian anoxic and biotic events are highly debated, and our results from the oceanographically open black-shale succession called the Horn River Group (HRG) provide elucidations. These strata are overall lean in siliciclastics and enriched in authigenic U, Mo, V, and planktogenic organic matter. We focus on three major event horizons within the HRG. Within new conodont constraints, these horizons are correlated to the global Frasnes-Manticoceras, basal punctata, and late punctata events. Two major characteristics of these three events are positive δ13Corg excursions of ~2.0–3.0‰ amplitude and U, Mo, V enrichment, which indicate pulses of severe anoxia within the basin. Trace-metal enrichment horizons are offset from associated isotopic excursions, indicating a time lag of ~80–125 ky between the onset of the δ13C excursion and the onset of severe anoxia. We demonstrate that these events are robust and traceable in off-bank anoxic facies across the entire basin, including remote locations ≥400 km away from the study area. The onsets of three major δ13C excursions are coupled with the thin spikes of elemental mercury and siliciclastic content. Our observations align with the hypothesis that Middle-Late Devonian anoxic events were triggered by pulses of large-scale volcanism through the same pathway as classical Mesozoic oceanic anoxic events. No rigorous evidence exists that land plants could drive shelfal seas into anoxia and mass extinctions.

Effects of redox variability and early diagenesis on marine sedimentary Hg records

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed “steady-state” environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity.

Molybdenum isotope evidence from restricted-basin mudstones for an intermediate extent of oxygenation in the late Ediacaran ocean

The Mo isotope composition of late Ediacaran global seawater and the global extent of ocean oxygenation are still debated due to the complex controls on sedimentary Mo isotope compositions and the rarity with which sediments directly capture global seawater Mo isotope compositions. Deep-water sulfidic sediments from modern severely restricted basins like the Black Sea have the best chance of capturing global seawater Mo isotope compositions. However, few studies have focused on sedimentary Mo isotope variations and their causes in late Ediacaran restricted basins. In this study, we present Mo isotope data for a new section from the Chengkou sub-basin of South China. The sedimentary rocks, mostly organic-rich mudstones, in this restricted basin have authigenic δ98Mo values from −0.01‰ to 1.27‰. Comparison of δ98Mo values with redox-sensitive elemental abundances reveals variation in the extent of Mn-oxide shuttling, related to enhanced hydrothermal activity and the inflow of open-ocean seawater, as the main cause of sedimentary Mo isotope variation in the Chengkou sub-basin. An unambiguous negative correlation between δ98Mo and Mn content points to a greater importance of Mn-oxide shuttling than Fe-oxide shuttling on the sedimentary Mo isotope signatures. Variations in the intensity of Mn-oxide shuttling were likely linked to changes in Mn inputs and the extent of basin restriction.A compilation of late Ediacaran sedimentary Mo isotope data suggests overall limited Mo isotope variation in (semi) restricted late Ediacaran basins, but local factors causing the variation are different and may result in the heaviest Mo isotopic signal in these settings being slightly lighter than that of open seawater. Using the observed negative linear relationships between δ98Mo values and the shuttling-sensitive proxies (V/TOC and Mn), the Mo isotope compositions recorded in the Chengkou sub-basin yield a reliable estimate of 1.3–1.6‰ for the δ98Mo of late Ediacaran global seawater. This value is slightly higher than the δ98Mo estimated for global seawater in the Cryogenian and early-middle Ediacaran, suggesting the extent of ocean oxygenation during the late Ediacaran Period was more similar to the oceans during Mesozoic Oceanic Anoxic Events than to the modern ocean.

Global ocean redox changes before and during the Toarcian Oceanic Anoxic Event

Mesozoic oceanic anoxic events are recognized as widespread deposits of marine organic-rich mudrocks temporally associated with mass extinctions and large igneous province emplacement. The Toarcian Oceanic Anoxic Event is one example during which expanded ocean anoxia is hypothesized in response to environmental perturbations associated with emplacement of the Karoo–Ferrar igneous province. However, the global extent of total seafloor anoxia and the relative extent of euxinic (anoxic and sulfide-rich) and non-euxinic anoxic conditions during the Toarcian Oceanic Anoxic Event are poorly constrained. Here we present estimates of the global total anoxic and euxinic seafloor areas before and during the Toarcian Oceanic Anoxic Event based on rhenium and molybdenum enrichments, respectively, in organic-rich mudrocks of the Fernie Formation (British Columbia, Canada). We find that mass balance models depict an expansion of up to ~7% total seafloor anoxia, which was dominated by euxinia, at the onset of the Toarcian Oceanic Anoxic Event, followed by a contraction before the end of the event. The global ocean redox trends revealed by the rhenium data mirrors the collapse and recovery patterns of global ammonite and foraminiferal biodiversity.

What controls the duration and intensity of ocean anoxic events in the Paleozoic and the Mesozoic?

The sedimentological and geochemical expression of Mesozoic ocean anoxic events (OAEs) in the geologic record is reasonably well-characterized and the causal mechanisms driving these events are generally agreed upon. By contrast, OAE-type intervals in the Paleozoic are relatively understudied and less well constrained. Here, we review literature published on three intervals that may be termed OAEs in the Paleozoic: the Late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) event, the Late Ordovician – Early Silurian Hirnantian Ocean Anoxic Event (HOAE), and the Late Devonian Frasnian-Famennian, or Kellwasser events. Comparing them to Mesozoic OAEs reveals that Paleozoic OAEs are typically longer in duration than Mesozoic OAEs, and it is also likely that anoxic waters were more areally extensive during these Paleozoic events. With the traditional framework for OAEs the potential for a very extended timeframe of anoxia is problematic, given that more extensive bottom-water anoxia should accelerate reoxygenation of the atmosphere-ocean by promoting more efficient phosphorous recycling in the ocean and therefore boosting primary productivity. However, benthic phosphorous retention could have been enhanced under anoxic and iron-rich (ferruginous) marine conditions. Therefore, we propose a role for ferruginous conditions with more widespread anoxia in driving longer OAEs. Ultimately, a protracted increase in atmospheric oxygen concentrations through the Phanerozoic may have been a key factor controlling the expression of OAEs through time.

A Redox‐Based Model for Carbonate Platform Drowning and Ocean Anoxic Events

AbstractThe deposition of marine carbonate rocks is influenced by climate and seawater chemistry. Carbonate platforms usually keep pace with subsidence and sea level rise but “platform drowning” occurs when carbonate sedimentation slows or when siliciclastics replace carbonates. Identifying specific mechanism(s) behind platform drowning is critical for understanding global environmental changes such as Ocean Anoxic Events (OAEs). We developed a model for OAEs which couples ocean basin redox processes to rates of carbonate sedimentation. Well‐oxygenated oceans have steep gradients in saturation state such that deep‐ocean dissolution is balanced by carbonate “overproduction” in shallow water. Through anaerobic metabolisms, deep‐ocean anoxia reduces both dissolution and overproduction, leading to slower accumulation rates in shallow‐water environments. This quasi‐steady state response links carbonate sedimentation with longer timescales associated with redox changes. Redox‐based drowning may have acted alongside other mechanisms to create spatially diverse patterns of platform drowning during Mesozoic OAEs and other Phanerozoic hyperthermal events.

Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes

The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope data for the PETM. A lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. We use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia, covering <2% of seafloor area.

Vitamin B12-dependent biosynthesis ties amplified 2-methylhopanoid production during oceanic anoxic events to nitrification

Bacterial hopanoid lipids are ubiquitous in the geologic record and serve as biomarkers for reconstructing Earth's climatic and biogeochemical evolution. Specifically, the abundance of 2-methylhopanoids deposited during Mesozoic ocean anoxic events (OAEs) and other intervals has been interpreted to reflect proliferation of nitrogen-fixing marine cyanobacteria. However, there currently is no conclusive evidence for 2-methylhopanoid production by extant marine cyanobacteria. As an alternative explanation, here we report 2-methylhopanoid production by bacteria of the genus Nitrobacter, cosmopolitan nitrite oxidizers that inhabit nutrient-rich freshwater, brackish, and marine environments. The model organism Nitrobacter vulgaris produced only trace amounts of 2-methylhopanoids when grown in minimal medium or with added methionine, the presumed biosynthetic methyl donor. Supplementation of cultures with cobalamin (vitamin B12) increased nitrite oxidation rates and stimulated a 33-fold increase of 2-methylhopanoid abundance, indicating that the biosynthetic reaction mechanism is cobalamin dependent. Because Nitrobacter spp. cannot synthesize cobalamin, we postulate that they acquire it from organisms inhabiting a shared ecological niche-for example, ammonia-oxidizing archaea. We propose that during nutrient-rich conditions, cobalamin-based mutualism intensifies upper water column nitrification, thus promoting 2-methylhopanoid deposition. In contrast, anoxia underlying oligotrophic surface ocean conditions in restricted basins would prompt shoaling of anaerobic ammonium oxidation, leading to low observed 2-methylhopanoid abundances. The first scenario is consistent with hypotheses of enhanced nutrient loading during OAEs, while the second is consistent with the sedimentary record of Pliocene-Pleistocene Mediterranean sapropel events. We thus hypothesize that nitrogen cycling in the Pliocene-Pleistocene Mediterranean resembled modern, highly stratified basins, whereas no modern analog exists for OAEs.

Climate and paleoceanography controlled size variations of calcareous nannofossils during the Valanginian Weissert Event (Early Cretaceous)

Abstract Strata of Valanginian age (Early Cretaceous, 139.8–132.9 Ma) record a 1.5‰ positive carbon isotope excursion (CIE), the Weissert Event. It coincides with volcanic activity of the Parana-Etendeka large igneous province. Unlike the Mesozoic Oceanic Anoxic Events (OAEs), no organic-rich deposits documenting widespread perturbation of the marine carbon-cycle are associated with the Weissert Event. To study the impact of paleoceanographic changes in the Valanginian on calcareous nannofossils, this study analyzes coccolith size data of three species (Biscutum constans, Watznaueria barnesiae, Zeugrhabdotus erectus) from northern Germany and the western North Atlantic. Further it presents a new Valanginian carbon isotope record of organic material (δ13Corg) from northern Germany. The dataset records the Weissert Event CIE in the late Valanginian. At both locations biometric analyses reveal size decreases of B. constans coccoliths during the Weissert Event. A size reduction of ~1 μm has been observed for northern Germany (37°N) and of 0.4 μm for the North Atlantic (23°N). The different magnitudes of size decrease possibly reflect the different paleogeographic positions of the sites and the paleoceanographic response to the late Valanginian climate variations. Proxy data support a link between a humid climate with increased weathering, and the observed coccolith size variations. This impacted nannoplankton communities either through the availability of light or by eutrophication and, possibly, nutrient limitation. In addition to increased terrigenous input, emissions of the Parana-Etendeka volcanism possibly acted fertilizing. Furthermore, widely dispersed volcanic emissions possibly caused increased concentrations of toxic trace metals in the surface ocean which affected marine primary producers.

Persistent global marine euxinia in the early Silurian

The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ98Mo and δ238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs – notably longer than well-studied Mesozoic ocean anoxic events.

A molybdenum-isotope perspective on Phanerozoic deoxygenation events

The careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters. A synthesis and analysis of data from two Mesozoic ocean anoxic events and the Palaeocene-Eocene thermal maximum applies these techniques to constrain past ocean deoxygenation.

Multiple Palaeoproterozoic carbon burial episodes and excursions

Organic-rich rocks (averaging 2–5% total organic carbon) and positive carbonate-carbon isotope excursions (δC13>+5‰ 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.

Bio-chemostratigraphy of the Barremian-Aptian shallow-water carbonates of the southern Apennines (Italy): pinpointing the OAE1a in a Tethyan carbonate platform

Abstract. Low biostratigraphic resolution and lack of chronostratigraphic calibration hinder precise correlations between platform carbonates and coeval deep-water successions. These are the main obstacle when studying the record of Mesozoic oceanic anoxic events in carbonate platforms. In this paper carbon and strontium isotope stratigraphy are used to produce the first chronostratigraphic calibration of the Barremian-Aptian biostratigraphy of the Apenninic carbonate platform of southern Italy. According to this calibration, the segment of decreasing δ13C values, leading to the negative peak that is generally taken as the onset of the Selli event, starts a few metres above the last occurrence of Palorbitolina lenticularis and Voloshinoides murgensis. The following rise of δ13C values, corresponding to the interval of enhanced accumulation of organic matter in deep-water sections, ends just below the first acme of Salpingoporella dinarica, which roughly corresponds to the segment of peak δ13C values. The whole carbon isotope excursion associated with the oceanic anoxic event 1a is bracketed in the Apenninic carbonate platform between the last occurrence of Voloshinoides murgensis and the "Orbitolina level", characterized by the association of Mesorbitolina parva and Mesorbitolina texana. Since these bioevents have been widely recognized beyond the Apenninic platform, the calibration presented in this paper can be used to pinpoint the interval corresponding to the Early Aptian oceanic anoxic event in other carbonate platforms of central and southern Tethys. This calibration will be particularly useful to interpret the record of the Selli event in carbonate platform sections for which a reliable carbon isotope stratigraphy is not available.

The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans

Despite important biological and biogeochemical consequences of extensive ocean anoxic events, their identification is controversial. The marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of Mo isotopes is known. Analysis of a set of rivers that account for 28% of global river runoff suggests more variable Mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes, suggesting near-total anoxia in the Proterozoic ocean and during Mesozoic ocean anoxic events. Despite its important biological and biogeochemical consequences1,2,3, the identification of extensive oceanic anoxia in the geological record is controversial. In particular, global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean, or in tectonically isolated basins such as the modern Black Sea. The marine isotope geochemistry of molybdenum (Mo) can help quantify the past oxygenation state of the ocean4,5,6,7,8,9,10,11,12,13,14, because to first approximation under oxic conditions lighter isotopes of Mo are preferentially removed to sediments, whereas in euxinic conditions quantitative removal leads to no fractionation. However, the isotopic composition of the Mo input from rivers, the main contributor of Mo to the oceans, is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4,5,6,7,8,9,10,11,12,13,14. Here we present an isotopic analysis of Mo in a set of rivers that together account for 22% of the global riverine water discharge. We find a broad range of variability in the Mo isotopic composition of these rivers, with almost all samples enriched in the heavy isotopes compared with continental rocks. Our data remove key uncertainties associated with the marine Mo isotope budget14 and strongly suggest near-total anoxia in the mid-Proterozoic ocean8 as well as during Mesozoic ocean anoxic events13.

Stable isotope record of the Eifelian–Givetian boundary Kačák–otomari Event (Middle Devonian) from Hungry Hollow, Ontario, Canada

The Kačák Event in the Middle Devonian (Eifelian–Givetian (E–G) boundary) is a period of apparent global anoxia coincident with widespread deposition of black shale in hemipelagic, pelagic, and some neritic facies. Conodont biostratigraphy in the North American Appalachian Basin has proven to be problematic in precisely demarcating the E–G boundary. In this study, we show that the E–G boundary may be defined more accurately through isotope stratigraphy (δ13C) in conjunction with a conodont faunal change across this boundary, identified as the Kačák– otomari Event. The Canadian Hamilton Group outcropping in Hungry Hollow, Ontario, is a 22 m sedimentary succession spanning the Middle Devonian. Conodont biostratigraphy for this section makes it difficult to define the E–G boundary, but the otomari Event can be detected. High-resolution isotopic analysis of bulk sedimentary carbonate and organic matter for this succession records a significant negative δ13C excursion (δ13Ccarb = up to 2‰; δ13Corg = ∼3.0‰) that is synchronous with total organic carbon (TOC) values up to 12.5%. We identify this negative δ13C excursion as a result of marine anoxia associated with the Kačák–otomari Event and suggest that the excursion is a global event driven by a source of isotopically light carbon, followed by a productivity event, similar to Mesozoic oceanic anoxic events. Such similarities between Devonian and Mesozoic oceanic anoxic events may become more evident with increased high-resolution isotopic and geochemical investigations of Devonian successions.

Calcareous nannofossils and Mesozoic oceanic anoxic events

Twenty-five years ago, Mesozoic oceanic anoxic events (OAEs) were documented and formalised as intervals of widespread to global deposition of organic matter. The Toarcian, Early Aptian (OAE1a) and latest Cenomanian (OAE2) OAEs are truly global in nature, commonly carbonate-poor, and typically represented by organic carbon-rich black shales. In some areas, these OAEs are also characterised by abundant radiolarian-sands and silts. They are associated with negative and positive excursions in the 87Sr/ 86Sr record, in addition to large global carbon-isotope anomalies in carbonate and/or organic matter, caused by a major perturbation of the global carbon budget. Increased rates of volcanism during the formation of the Ontong Java (and Manihiki) and Caribbean Plates, and the Karoo-Ferrar Traps, are believed to have caused the geological responses associated with OAE1a, OAE2, and the Toarcian OAE, respectively. Excess volcanogenic CO 2 in the atmosphere most probably turned the climate into a greenhouse mode, accelerating continental weathering and increasing nutrient content in oceanic surface-waters via river run-off. Higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes. Because Mesozoic OAEs are often represented by carbonate-poor sediments, quantitative studies of calcareous nannofossils have been applied to explore (a) the causes and effects of igneous/tectonic events and climate changes, relative to nannofloral increases and crises, as well as (b) dissolution events, and (c) diagenetic modifications. Characterization of calcareous nannofloras in OAE intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO 2 in the oceans. Types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events. In selected sections, it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pCO 2 by drastically reducing calcification. As in the modern oceans, during OAEs the increase of nutrients and atmospheric CO 2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep-photic zone nannoconids and schizosphaerellids. Similarly to the ‘nannoconid crisis’ preceding deposition of the Early Aptian OAE1a black shales, a ‘schizosphaerellid crisis’ is detected prior to the Toarcian OAE. Both OAEs are further characterised by a rapid nannofloral speciation, beginning approximately 1.5 myr before the OAE, but without extinctions. Global changes during the latest Cenomanian OAE 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions. This event, in fact, was a time of extinctions followed by originations within calcareous nannofossils. Precise timing of the events before, during and after OAE1a, OAE2 and the Toarcian OAE indicates that they were intervals of enhanced oceanic productivity and that anoxia/dysoxia post-dated biotic changes.

Further evidence for the development of photic-zone euxinic conditions during Mesozoic oceanic anoxic events

Periods in Earth history characterized by extensive organic carbon deposition, so-called oceanic anoxic events (OAEs), are the subject of considerable scrutiny. Insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene, a carotenoid found only in green sulphur bacteria and thus indicative of photic-zone euxinia (i.e. a water column containing free H 2 S), in North Atlantic deep-sea sediments deposited during the Cenomanian–Turonian boundary OAE2 (Bonarelli Event). In this study, we have used the distributions of chlorophyll and bacteriochlorophyll degradation products, maleimides (1- H -pyrrole-2,5-diones) and high molecular weight porphyrins, to examine further the Cenomanian–Turonian boundary OAE2 as well as other OAEs of the early Toarcian (Posidonienschiefer Event) and early Aptian (Selli Event). In particular, methyl isobutyl (Me, i -Bu) maleimide, on structural grounds, appears to be diagnostic of green sulphur bacteria. This compound occurs in five of seven examined marls that record the early Toarcian OAE (Marche–Umbria, Italy), further expanding the geographical range of Toarcian sediments where evidence for photic-zone euxinic conditions has been found. Me, i -Bu maleimide occurs in three of six black shales spanning the Livello Selli (Marche–Umbria, Italy), the type locality for the Aptian OAE1a, providing the first evidence for photic-zone euxinic conditions during this event. With respect to the Cenomanian–Turonian OAE, maleimide evidence for photic-zone euxinic conditions was found in all the North Atlantic sites investigated, including those characterized by relatively organic-lean sediments, several Tethyan sites and one site off western Australia. These data indicate that euxinic conditions were common in the water column during Mesozoic oceanic anoxic events.