Global Ocean Anoxia Research Articles

Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO2 release, marine faunal extinction, sea-level rise, a crisis in carbonate production related to ocean acidification, and elevated seawater temperatures. Despite recognition of the T-OAE as a potential analog for future ocean deoxygenation, current knowledge on the severity of global ocean anoxia is limited largely to studies of the trace element and isotopic composition of black shales, which are commonly affected by local processes. Here, we present the first carbonate-based uranium isotope (δ238U) record of the T-OAE from open marine platform limestones of the southeastern Tethys Ocean as a proxy for global seawater redox conditions. A significant negative δ238U excursion (~0.4‰) is recorded just prior to the onset of the negative carbon isotope excursion comprised within the T-OAE, followed by a long-lived recovery of δ238U values, thus confirming that the T-OAE represents a global expansion of marine anoxia. Using a Bayesian inverse isotopic mass balance model, we estimate that anoxic waters covered ~6 to 8% of the global seafloor during the peak of the T-OAE, which represents 28 to 38 times the extent of anoxia in the modern ocean. These data, combined with δ238U-based estimates of seafloor anoxic area for other CO2-driven Phanerozoic OAEs, suggest a common response of ocean anoxia to carbon release, thus improving prediction of future anthropogenically induced ocean deoxygenation.

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.

Did sea level change drive carbon isotopic trends in the Madison Shelf? Sequence stratigraphy and carbon isotopes in the Mississippian Lodgepole Formation of southwest Montana

The Lower Mississippian Lodgepole Formation of Montana and Wyoming records one of the largest positive carbon isotopic excursions of the Phanerozoic. This globally recognized up to 7‰ increase in δ13Ccarb values occurs across the North American Kinderhookian-Osagean boundary (referred to as the KO excursion). It has been argued to reflect significant organic carbon burial, possibly linked to the onset of the Late Paleozoic Ice Age. Previously proposed correlations between carbon isotopic patterns and the sequence stratigraphic framework within these strata suggests that changes in sea level could have played a significant role in the expression and/or magnitude of the KO excursion in the Madison Shelf. This study explores the relationship between carbon isotopic values and sea level change at multiple scales. To accomplish this, we provide a comprehensive overview of the sedimentological and stratigraphic framework and address uncertainty about the number of sequences in the Lodgepole Formation. Our results support a three-sequence model for the Lodgepole Formation. Based on the number of sequences and the placement of sequence stratigraphic surfaces, we see little evidence of statistically significant correlation between carbon isotopic trends and the sequence stratigraphic framework. We argue that sea level change was not the primary driving mechanism for carbon isotopic trends in the Madison Shelf, nor the KO excursion. Instead, we support models that invoke global ocean anoxia and/or destabilization of the global carbon cycle due to land plants.

New U-Pb baddeleyite ID-TIMS ages from the intrusive high-Ti-Sr rocks of the Southern Paraná LIP, Brazil: Implications for correlations with environmental disturbances during the Early Cretaceous

Large volumes of mafic igneous rocks are commonly emplaced during Large Igneous Province (LIP) eruptions, and these mafic rocks are often contemporaneous with periods of environmental disturbances, such as global ocean anoxia, and as a result, mass extinctions. The Paraná-Etendeka LIP is no exception, and has been previously correlated with, and interpreted to be the cause of the Valanginian oceanic anoxic event (OAE), a small global environmental disturbance. Here, we present new U-Pb ID-TIMS baddeleyite ages from high-Ti-Sr mafic intrusive rocks from the Paraná LIP, in Brazil. While these data are potentially complicated by the presence of Pb-loss and inheritance, it is possible to interpret geologically meaningful ages from them. The first high-precision age is reported for the type-locality of the Florianópolis Dyke Swarm, in North Santa Catarina Island, which yields an age of 132.53 ± 0.40/0.40/0.42 Ma. We also report an age of 132.07 ± 0.27/0.30/0.33 Ma for a dolerite sill, which intrudes organic-rich sedimentary rocks of the Paraná Basin. The emplacement of high-Ti-Sr magmas at ca. 132 Ma suggests that there was up to 2 Myr of intrusive magmatism in the southern part of the Paraná LIP. Further investigation on the mafic intrusive magmatism from the Paraná LIP through robust high-precision geochronology is required to elucidate the proposed linkage more clearly with environmental changes during the Early Cretaceous.

Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates

Widespread marine anoxia is hypothesized as the trigger for the second pulse of the Late Ordovician (Hirnantian) mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters. We test the anoxia hypothesis using δ238U values of marine limestones as a global seawater redox proxy. The δ238U trends at Anticosti Island, Canada, document an abrupt late Hirnantian ∼0.3‰ negative shift continuing through the early Silurian indicating more reducing seawater conditions. The lack of observed anoxic facies and no covariance among δ238U values and other local redox proxies suggests that the δ238U trends represent a global-ocean redox record. The Hirnantian ocean anoxic event (HOAE) onset is coincident with the extinction pulse indicating its importance in triggering it. Anoxia initiated during high sea levels before peak Hirnantian glaciation, and continued into the subsequent lowstand and early Silurian deglacial eustatic rise, implying that major climatic and eustatic changes had little effect on global-ocean redox conditions. The HOAE occurred during a global δ13C positive excursion, but lasted longer indicating that controls on the C budget were partially decoupled from global-ocean redox trends. U cycle modeling suggests that there was a ∼15% increase in anoxic seafloor area and ∼80% of seawater U was sequestered into anoxic sediments during the HOAE. Unlike other ocean anoxic events (OAE), the HOAE occurred during peak and waning icehouse conditions rather than during greenhouse climates. We interpret that anoxia was driven by global cooling, which reorganized thermohaline circulation, decreased deep-ocean ventilation, enhanced nutrient fluxes, stimulated productivity, which lead to expanded oxygen minimum zones.

Uranium isotope compositions of mid-Proterozoic black shales: Evidence for an episode of increased ocean oxygenation at 1.36 Ga and evaluation of the effect of post-depositional hydrothermal fluid flow

We report U isotope data for marine black shales of the early Mesoproterozoic Velkerri Formation (Roper Group) and late Paleoproterozoic Wollogorang Formation (Tawallah Group) from the McArthur Basin, Northern Australia. An average authigenic δ238U of 0.13±0.04‰ (1SD; relative to standard CRM145) was obtained for six U- and Mo-rich shales from a ∼1m interval that was deposited at 1361±21Ma (based on previous Re-Os geochronology). After correcting for a local U isotope fractionation of ∼0.60–0.85‰ associated with U removal to anoxic sediments, we infer that global seawater at 1.36Ga had a δ238U of ∼−0.47‰ to −0.72‰, which is ∼0.1−0.3‰ lighter than modern seawater (−0.39±0.01‰). Uranium isotope mass-balance modelling suggests that <25% of the seafloor was anoxic at 1.36Ga. This interpretation is consistent with high U and Mo enrichments in these samples compared with other Velkerri Formation and mid-Proterozoic black shales, which suggests a sizable dissolved oceanic Mo and U inventory developed in response to an episode of increased ocean oxygenation. Hence, a significant expanse of O2-bearing deep ocean waters may have existed at 1.36Ga. The O2 concentrations of those waters were not necessarily high given that a large expanse of weakly oxygenated deep waters is also consistent with the mass-balance model. A lower average authigenic δ238U of −0.08±0.18‰ (1SD) was obtained for comparatively U- and Mo-poor black shales from a ∼1m interval in the lower Velkerri Formation, deposited at 1417±29Ma. In contrast to the upper Velkerri interval, the mass-balance model permits widespread ocean anoxia during deposition of the lower Velkerri interval.Black shales of the ca. 1.73Ga Wollogorang Formation previously yielded an erroneously young Re-Os date of 1359±150Ma, likely due to post-depositional hydrothermal alteration at ca. 1640Ma. Higher δ238U is observed in samples closer to the base of the black shale unit where the greatest extent of open-system Re-Os behavior was observed. Hence, post-depositional hydrothermal fluid flow may overprint the depositional δ238U of black shales and cause erroneous estimates of ancient global ocean anoxia.

Linking species habitat and past palaeoclimatic events to evolution of the teleost innate immune system.

Host-intrinsic factors as well as environmental changes are known to be strong evolutionary drivers defining the genetic foundation of immunity. Using a novel set of teleost genomes and a time-calibrated phylogeny, we here investigate the family of Toll-like receptor (TLR) genes and address the underlying evolutionary processes shaping the diversity of the first-line defence. Our findings reveal remarkable flexibility within the evolutionary design of teleost innate immunity characterized by prominent TLR gene losses and expansions. In the order of Gadiformes, expansions correlate with the loss of major histocompatibility complex class II (MHCII) and diversifying selection analyses support that this has fostered new immunological innovations in TLRs within this lineage. In teleosts overall, TLRs expansions correlate with species latitudinal distributions and maximum depth. By contrast, lineage-specific gene losses overlap with well-described changes in palaeoclimate (global ocean anoxia) and past Atlantic Ocean geography. In conclusion, we suggest that the evolvability of the teleost immune system has most likely played a prominent role in the survival and successful radiation of this lineage.

Increasing global ocean oxygenation and the Ordovician Radiation: Insights from Th/U of carbonates from the Ordovician of western Utah

The Cambrian and Ordovician radiations were the most important biodiversification events in the history of animal life, yet they were separated by intervals of recurrent anoxic sedimentation and regional biomere extinction events in the Late Cambrian and Early Ordovician. Recent work linked biomere extinction events to positive carbon isotopic excursions interpreted to represent enhanced organic matter burial under anoxic conditions. This led to the hypothesis that an increase in oceanic oxygenation following the last of the biomere extinctions paved the way for the rapid diversification of the Ordovician Radiation. Here we test the hypothesis of increasing ocean oxygenation using Th/U preserved in carbonate rocks from the Lower–Middle Ordovician succession near Ibex, Utah, USA. The global seawater uranium inventory is expected to vary inversely with the amount of deep-water anoxic deposition due to uranium sequestration in black shales. Therefore in shallow water carbonates higher Th/U ratios may reflect increased global ocean anoxia. Our data show that the highest Th/U values are coincident with the heaviest carbon isotopic values reported from the base of this succession, which is latest Tremadocian in age. Th/U then decreases and remains low through the Floian up through the contact with the Kanosh Formation, a shale-dominated unit with interspersed limestone units. The geochemistry of the carbonates within the Kanosh Formation likely represents local rather than global conditions in the marine environment. We interpret the overall decrease in Th/U ratios throughout the Tremadocian to Floian interval to represent an increase in global seawater uranium concentration. These data are consistent with the hypothesis that an increase in ocean oxygenation occurred prior to the major pulses of diversification associated with the Ordovician Radiation.

High-precision geochronology links the Ferrar large igneous province with early-Jurassic ocean anoxia and biotic crisis

Apparent synchrony between eruption/emplacement of large igneous province (LIP) magmas and mass extinction has led to the implication of magmatism as a primary trigger of global scale environmental change. Evaluating the efficacy of magmatism as a driver of global change depends on the relative timing of magmatism and environmental change, and the magma effusion/intrusion rate, both of which can be constrained by high-precision geochronology. Early Jurassic (Pliensbachian–Toarcian) global ocean anoxia and acidification, carbon isotope perturbations, and biotic crisis have been linked to “synchronous” eruption and emplacement of the Karoo and Ferrar LIPs. To better constrain the timing and duration of Ferrar magmatism, we apply the single crystal, chemical abrasion U–Pb ID-TIMS method to zircon crystals isolated from twenty Ferrar LIP sills and lavas, and the Dufek intrusion. Dates suggest that both intrusive and extrusive Ferrar magmatism occurred over an interval of 349±49 kyr, beginning with intrusive magmatism as early as 182.779±0.033 Ma. Lava eruption was synchronous with, and in some cases postdates intrusion. When coupled with existing geochronology on the Karoo province, our dates confirm broad synchrony between Karoo and Ferrar magmatism, though Karoo magmatism began demonstrably prior to Ferrar magmatism, starting as early as 183.246±0.045 Ma. The short-lived magmatic history of the Ferrar LIP makes it a plausible trigger for early-Jurassic environmental change.

Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic Event 2

Several periods of global ocean anoxia punctuated the Cretaceous period. Marine-sediment chemistry indicates that extensive volcanism at the beginning of Oceanic Anoxic Event 2 released sulphur to the oceans, triggering a biogeochemical cascade that led to enhanced surface productivity and depletion of oxygen in the underlying waters. During the Cretaceous period (∼145–65 million years ago), there were several periods of global ocean anoxia, each lasting less than one million years. These events, known as ocean anoxic events, were marked by significant increases in organic carbon burial1, and are generally attributed to increased primary productivity in surface waters2. The details underpinning the initiation, maintenance and termination of these events, however, remain equivocal. Here we present sulphur isotope data spanning the Ocean Anoxic Event 2 (about 94.5 million years ago) from sedimentary rocks in Colorado that were formed in the Western Interior Seaway; this seaway ran north–south, splitting North America during the Cretaceous. Sulphate levels increased rapidly from relatively low background levels at the onset of the event, most likely from the release of sulphur by massive volcanism, and fell during the anoxic event. We infer that the input of sulphate facilitated increased carbon remineralization, which enhanced nutrient recycling and increased global primary productivity, eventually resulting in widespread ocean anoxia. Our scenario indicates that Ocean Anoxic Event 2 may have persisted until sulphate levels were stabilized by the formation and burial of the sulphur mineral pyrite, which returned primary productivity to background levels. We suggest that fluctuations in sulphate levels may have regulated the marine carbon cycle during past periods of low oceanic sulphate concentration.

Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic

Relatively brief periods of severe paleoenvironmental change during the Jurassic and Cretaceous were associated with the widespread accumulation of organic-rich marine deposits, termed oceanic anoxic events (OAEs). These intervals involved abrupt global warming of ∼5–10 °C, higher rates of continental weathering, elevated extinction rates, and large-scale perturbations to the global carbon cycle. The major OAEs also overlapped temporally the emplacement of large igneous provinces. However, despite being known as OAEs, the extent of seawater anoxia at those times is undefined and the causative processes remain unclear. Here we show how changes in seawater molybdenum isotope ratios (a proxy for seawater anoxia) during the Toarcian (Early Jurassic) OAE define the onset and expansion of oxygen deficient conditions. Our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically, broadly in line with precession-driven changes in δ13 Corg. Despite their intermittent occurrence over geological history, OAEs have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day.

Potential links between ocean plateau volcanism and global ocean anoxia at the Cenomanian-Turonian boundary

The Cenomanian-Turonian boundary is marked globally by the extinction of marine invertebrates and an increase in the accumulation and preservation of organic carbon-rich sediments (black shales). The timing of this boundary also coincides with large-scale volcanism from at least three large igneous provinces: the Caribbean and Ontong-Java oceanic plateaus and the Madagascar flood basalts. In this paper, we assess the possibility that hydrothermalism associated with large-scale submarine magmatism was responsible for the reduction of dissolved O 2 in the oceans. We investigate two potential mechanisms: the oxidation of reduced material in hydrothermal effluents and the stimulation of primary productivity in the water column due to the injection of hydrothermal Fe into surface waters. With the first mechanism, we find that a 10,000-km 3 submarine basalt eruption (three orders of magnitude larger than recent midocean ridge eruptions) could release enough reduced hydrothermal material to consume at least 6 percent of the dissolved O 2 in seawater in a well-ventilated ocean (such as the present ocean). With the second mechanism, we calculate that even a small percentage of the hydrothermal Fe released from a single large flow could have led to a significant increase in primary productivity in areas where Fe was the biolimiting nutrient (such as open-ocean areas in the Pacific). The potential impact of both of these O 2 -reducing mechanisms at the Cenomanian-Turonian boundary would have been significantly greater if Cretaceous oceans were less well ventilated than the present oceans.