The early Toarcian (~183 Ma) Toarcian Oceanic Anoxic Event (T-OAE, or Jenkyns Event) is the best-studied Jurassic episode of widespread marine deoxygenation and organic-carbon (OC)-rich shale deposition. A 2024 synthesis by Ruebsam and Schwark demonstrated that OC-rich deposition was strongly diachronous and not restricted to the negative Toarcian carbon isotope excursion (nT-CIE), placed the T-OAE climax in the nT-CIE core interval, and attributed the spatiotemporal pattern qualitatively to interacting global and regional drivers: eustatic sea-level rise, freshwater stratification under an intensified hydrological cycle, riverine nutrient fluxes, sluggish ocean circulation, and upwelling. What remains unresolved is the quantitative mechanistic core: the relative magnitudes and interactions of these drivers in controlling when and where oxygen-deficient conditions and OC accumulation developed across the main early Toarcian paleogeographic settings. Existing modeling consists of global carbon-cycle box models (carbon source, weathering, global-mean anoxia) and circulation-focused general circulation model experiments for single shelves; reviews from 2022-2025 state the quantification gap explicitly.

Background

Oceanic anoxic events (OAEs) are geologically brief intervals (typically 10510^{5}-10610^{6} years) during which large parts of the ocean became depleted in dissolved oxygen and organic-carbon (OC)-rich sediments (source rocks of many petroleum systems) accumulated widely. The early Toarcian stage of the Early Jurassic (~183 million years ago) hosts the Toarcian Oceanic Anoxic Event (T-OAE), also called the Jenkyns Event, which is linked to massive volcanism of the Karoo-Ferrar large igneous provinces (LIPs; vast flood-basalt provinces emplaced in southern Gondwana and Antarctica), global warming of roughly 5 °C, a second-order mass extinction (one rank below the largest Phanerozoic extinctions in severity), and sea-level rise. The event is conventionally identified stratigraphically by a pronounced negative excursion in the carbon isotope ratio δ13C\delta^{13}\mathrm{C} of marine and terrestrial organic and carbonate carbon (the nT-CIE), which records the injection of isotopically light carbon into the ocean-atmosphere system; a subsequent positive excursion records enhanced burial of isotopically light organic carbon.

A central recent insight is that the T-OAE is not a single globally synchronous anoxic interval. Ruebsam and Schwark (2024) compiled OC contents and redox indicators (chemical signatures of bottom-water oxygen states: oxic, suboxic-anoxic, or euxinic, the last meaning anoxic and sulfide-rich) from sections across the northwestern Tethys Shelf (the Early Jurassic epicontinental seaway covering much of Europe), the Arctic, Panthalassa (the vast proto-Pacific Ocean), eastern Tethys, and lacustrine basins. They showed that OC-rich strata and oxygen-deficient conditions (ODC) appeared and ended diachronously at basin to sub-basin scale, that OC-rich deposition is not restricted to the nT-CIE interval, and that the stratigraphic expression of the event falls into recurring pattern types: open ocean and unrestricted shelf settings where deoxygenation is largely confined to the nT-CIE (type 1); restricted epicontinental shelves (best exemplified by the northern European shelf) where prolonged ODC persisted well beyond the nT-CIE (type 2); upwelling-influenced shelf settings along the western Laurentia margin (northeastern Panthalassa) where ODC and OC accumulation persisted throughout the entire early Toarcian, decoupled from the nT-CIE (type 3a); and a small number of marginal settings with purely local expression, likewise decoupled from the nT-CIE (type 3b). They placed the climax of the event (widest extent of OC-rich strata and ODC, coinciding with a positive thallium isotope peak) in the nT-CIE core interval.

The paper's account of why deoxygenation developed where and when it did is, however, entirely qualitative. The proposed drivers are: high eustatic (global) sea level, which flooded shelf areas and placed the seafloor below storm wave base in low-energy settings; freshwater stratification (buoyant low-salinity surface layers from river runoff and Arctic inflow through the Viking Corridor seaway, which inhibit vertical oxygen exchange) in the hydrographically restricted northern European shelf; riverine nutrient supply stimulating marine productivity and oxygen consumption by organic-matter respiration; sluggish ocean circulation under globally warm, ice-free conditions, which reduces oxygen supply to deep waters; and upwelling (wind-driven transport of nutrient-rich subsurface water to the surface) along the eastern Panthalassic margin.

The quantitative modeling landscape is thin. Global carbon-cycle box models (models resolving the ocean-atmosphere system as a few well-mixed reservoirs) — mass-balance estimates of methane-hydrate or thermogenic carbon release, calcium- and osmium-isotope cycle models, and the COPSE Earth-system box model (which couples carbon, oxygen, phosphorus, and sulfur cycles) applied by Ullmann et al. (2020) — constrain the carbon source and weathering enhancement but carry no spatial resolution. Spatially resolved general circulation model (GCM) experiments are limited to: a Princeton Ocean Model (a widely used numerical ocean-circulation model) study of the Early Jurassic Laurasian Seaway (Bjerrum et al. 2001); FOAM (Fast Ocean-Atmosphere Model) simulations showing global warming, Arctic seawater freshening, and sluggish circulation (Dera and Donnadieu 2012); and FOAM/MITgcm (Massachusetts Institute of Technology general circulation model) experiments by Ruvalcaba Baroni et al. (2018) demonstrating a clockwise gyre that ventilated the southern European shelf while rough bathymetry isolated the northeastern basins, where anoxia and enhanced carbon burial (3-5% of the total carbon extracted from the ocean-atmosphere system) developed. No study has quantified the relative magnitudes and interactions of all five drivers, or predicted which pattern type of the event develops under which conditions.

On the quantitative-anoxia side, isotope proxies that track global rather than local redox have recently yielded first numbers: thallium isotopes (which track the global burial flux of manganese oxides and hence the areal extent of anoxic sinks) showed that deoxygenation began roughly 600 thousand years before the classically defined T-OAE interval and persisted after it (Them et al. 2018); rhenium and molybdenum enrichments (sedimentary accumulations of these dissolved trace metals, whose removal from seawater accelerates under anoxic bottom waters) imply up to ~7% of the global seafloor was anoxic (Kunert and Kendall 2023); and carbonate uranium isotopes (δ238\delta^{238}U) with a Bayesian mass-balance inversion give ~6-8% anoxic seafloor at the peak (Remírez et al. 2024). These global-mean severity estimates do not attribute causes, and their mutual differences illustrate that proxy calibration (which proxies record global versus local redox) is itself unfinished. Timing is equally unsettled: astrochronologies (time scales built by counting astronomically paced sedimentary cycles) give T-OAE durations near one million years (Ait-Itto et al. 2018; Tang et al. 2025 find the negative carbon isotope excursion lasting 350-1800 thousand years across sections), whereas new high-precision uranium-lead dates from Japan bracket the event at only ~300 thousand years and tie it to Ferrar magmatism (Kemp et al. 2024).

Problem Statement

Quantify the mechanistic controls on early Toarcian (Early Jurassic, ~183 Ma) marine deoxygenation and organic-carbon burial, validated against the geological record. Specifically, determine the relative magnitudes and interactions of the proposed drivers — eustatic sea-level rise, freshwater (salinity) stratification under an intensified hydrological cycle, riverine nutrient fluxes and productivity, sluggish ocean circulation, and eastern-margin upwelling — in controlling when and where oxygen-deficient bottom waters and organic-carbon-rich deposition developed, including under which combinations of boundary conditions each recurring stratigraphic pattern type of the event arises: open-ocean/unrestricted-shelf settings with deoxygenation largely confined to the nT-CIE; restricted epicontinental shelves with prolonged deoxygenation extending well beyond the nT-CIE; upwelling-influenced western Laurentian shelf settings where oxygen-deficient conditions and organic-carbon accumulation persist throughout the early Toarcian, decoupled from the nT-CIE; and locally controlled marginal settings. The problem is addressed by (i) quantifying, with quantified uncertainty, the relative magnitudes and interactions of the drivers in each setting; (ii) determining which combinations of boundary conditions produce each recurring pattern type of the event; and (iii) reproducing the observed timing and spatial pattern of organic enrichment and oxygen-deficient conditions in the organic-carbon-content and redox-indicator section data of Ruebsam and Schwark (2024) [reference 1], in each section's stratigraphic framework as reported there. The proxy-calibration and spatial-coverage gaps are optional supporting analysis, to be addressed only where they materially bound the attribution or its validation.

References

  1. Ruebsam W, Schwark L (2024) Disparity between Toarcian Oceanic Anoxic Event and Toarcian carbon isotope excursion. International Journal of Earth Sciences 113(8):2065-2076. doi:10.1007/s00531-024-02408-8, https://doi.org/10.1007/s00531-024-02408-8
  2. Kemp DB, Suan G, Fantasia A, Jin S, Chen W (2022) Global organic carbon burial during the Toarcian oceanic anoxic event: Patterns and controls. Earth-Science Reviews 231:104086. doi:10.1016/j.earscirev.2022.104086, https://doi.org/10.1016/j.earscirev.2022.104086
  3. Ruvalcaba Baroni I, Pohl A, van Helmond NAGM, Papadomanolaki NM, Coe AL, Cohen AS, van de Schootbrugge B, Donnadieu Y, Slomp CP (2018) Ocean circulation in the Toarcian (Early Jurassic): A key control on deoxygenation and carbon burial on the European shelf. Paleoceanography and Paleoclimatology 33(9):994-1012. doi:10.1029/2018PA003394, https://doi.org/10.1029/2018PA003394
  4. Dera G, Donnadieu Y (2012) Modeling evidences for global warming, Arctic seawater freshening, and sluggish oceanic circulation during the Early Toarcian anoxic event. Paleoceanography 27(2):PA2211. doi:10.1029/2012PA002283, https://doi.org/10.1029/2012PA002283
  5. Bjerrum CJ, Surlyk F, Callomon JH, Slingerland RL (2001) Numerical paleoceanographic study of the Early Jurassic transcontinental Laurasian Seaway. Paleoceanography 16(4):390-404. doi:10.1029/2000PA000512, https://doi.org/10.1029/2000PA000512
  6. Zhang Y, Mills BJW, He T, Hu X, Zhu M (2023) Modeling hyperthermal events in the Mesozoic-Paleogene periods: a review. Frontiers in Ecology and Evolution 11:1226349. doi:10.3389/fevo.2023.1226349, https://doi.org/10.3389/fevo.2023.1226349
  7. Ullmann CV, Boyle R, Duarte LV, Hesselbo SP, Kasemann SA, Klein T, Lenton TM, Piazza V, Aberhan M (2020) Warm afterglow from the Toarcian Oceanic Anoxic Event drives the success of deep-adapted brachiopods. Scientific Reports 10:6549. doi:10.1038/s41598-020-63487-6, https://doi.org/10.1038/s41598-020-63487-6
  8. Them TR II, Gill BC, Caruthers AH, Gerhardt AM, Gröcke DR, Lyons TW, Marroquín SM, Nielsen SG, Trabucho Alexandre JP, Owens JD (2018) Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction. Proceedings of the National Academy of Sciences 115(26):6596-6601. doi:10.1073/pnas.1803478115, https://doi.org/10.1073/pnas.1803478115
  9. Kunert A, Kendall B (2023) Global ocean redox changes before and during the Toarcian Oceanic Anoxic Event. Nature Communications 14:815. doi:10.1038/s41467-023-36516-x, https://doi.org/10.1038/s41467-023-36516-x
  10. Remírez MN, Gilleaudeau GJ, Gan T, Kipp MA, Tissot FLH, Kaufman AJ, Parente M (2024) Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event. Proceedings of the National Academy of Sciences 121(27):e2406032121. doi:10.1073/pnas.2406032121, https://doi.org/10.1073/pnas.2406032121
  11. Kemp DB, Ramezani J, Izumi K, Al-Suwaidi AH, Huang C, Chen W, Zhu Y (2024) The timing and duration of large-scale carbon release in the Early Jurassic. Geology 52(12):891-895. doi:10.1130/g52457.1, https://doi.org/10.1130/g52457.1
  12. Ait-Itto F-Z, Martinez M, Price GD, Ait Addi A (2018) Synchronization of the astronomical time scales in the Early Toarcian: A link between anoxia, carbon-cycle perturbation, mass extinction and volcanism. Earth and Planetary Science Letters 493:1-11. doi:10.1016/j.epsl.2018.04.007, https://doi.org/10.1016/j.epsl.2018.04.007
  13. Tang W, Ma C, Liu Y, Wu S, Li Y, He K, Yu K, Ye L, Hou M, Zhang D (2025) Orbital controls and threshold effects on carbon isotope excursions during the Toarcian Oceanic Anoxic Event. Global and Planetary Change 254:105050. doi:10.1016/j.gloplacha.2025.105050, https://doi.org/10.1016/j.gloplacha.2025.105050
  14. Kemp DB, Han Z, Hu X, Chen W, Jin S, Izumi K, Yan Q, Baranyi V, Jin X, Dal Corso J, Ge Y (2024) Global hydroclimate perturbations during the Toarcian oceanic anoxic event. Earth-Science Reviews 258:104946. doi:10.1016/j.earscirev.2024.104946, https://doi.org/10.1016/j.earscirev.2024.104946
  15. Kemp DB, Chen W, Cho T, Algeo TJ, Shen J, Ikeda M (2022) Deep-ocean anoxia across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event in the Panthalassic Ocean. Global and Planetary Change 212:103782. doi:10.1016/j.gloplacha.2022.103782, https://doi.org/10.1016/j.gloplacha.2022.103782
  16. Remírez MN, Algeo TJ (2020) Carbon-cycle changes during the Toarcian (Early Jurassic) and implications for regional versus global drivers of the Toarcian oceanic anoxic event. Earth-Science Reviews 209:103283. doi:10.1016/j.earscirev.2020.103283, https://doi.org/10.1016/j.earscirev.2020.103283
  17. Chen W, Kemp DB, Jenkyns HC, Robinson SA, Jiang S, Pan C, Li C (2024) Widespread upper-ocean deoxygenation in the Alpine-Mediterranean Tethys during the Toarcian Oceanic Anoxic Event. Global and Planetary Change 243:104631. doi:10.1016/j.gloplacha.2024.104631, https://doi.org/10.1016/j.gloplacha.2024.104631