The Toarcian Oceanic Anoxic Event (T-OAE), occurring around 183 million years ago, was a significant perturbation of the Earth's atmosphere-ocean system, associated with a mass extinction and linked to the emplacement of the Karoo-Ferrar large igneous province (LIP). LIP emplacement triggered various environmental changes, including climate shifts, hydrological cycle intensification, continental weathering, increased marine productivity, and a considerable expansion of seafloor anoxia and euxinia. Despite the event's name emphasizing expanded anoxia, the global extent of total bottom water anoxia and the proportions of euxinic and non-euxinic anoxic conditions remain poorly understood. Previous studies using molybdenum isotopes (δ⁹⁸Mo) inferred an expansion of euxinia during the T-OAE, but limitations due to basin restriction and the indistinguishability of non-euxinic anoxia from other redox conditions complicate these interpretations. Thallium isotopes (δ²⁰⁵Tl) offer additional insights into ocean anoxia expansion, but they do not differentiate between various oxygen-deficient conditions. This research aims to address these limitations by employing a novel approach to quantify the extent of oceanic anoxia, differentiating between non-euxinic anoxic and euxinic seafloor areas during the T-OAE, using redox-sensitive elemental mass balances of Re and Mo.

Previous research on the T-OAE has utilized various geochemical proxies to reconstruct ocean redox conditions. Molybdenum isotope data (δ⁹⁸Mo) from organic-rich mudrocks (ORMs) in Europe suggested an expansion of euxinia during the early T-OAE, estimating euxinic seafloor coverage between 2-10%. However, concerns about basin restriction in the European sections challenged these interpretations. Thallium isotope data (δ²⁰⁵Tl) from western Canada indicated two expansions of ocean anoxia, but this proxy couldn't distinguish between different oxygen-deficient environments. Existing studies employing redox-sensitive elemental mass balances have focused on longer timescales (Proterozoic), neglecting the application of these models to shorter events like the T-OAE. This study addresses the shortcomings of previous work by combining Re and Mo mass balance models, offering a more comprehensive assessment of both total anoxia and euxinia during the T-OAE.

This study utilizes temporally calibrated Re and Mo elemental mass balances to estimate total anoxic and euxinic seafloor areas, respectively, using organic-rich mudrocks (ORM) from the Fernie Formation (Gordondale Member, British Columbia, Canada). The chosen section shows a clear negative carbon isotope excursion (N-CIE) characteristic of the T-OAE, confirming its relevance for the study. To establish the local paleoenvironmental redox conditions, redox-sensitive trace metal proxies (Re, Mo, U, V) were analyzed. Ratios of these metals normalized to aluminum (Al) provided robust paleoredox interpretations. Thresholds for different redox settings (oxic, dysoxic, suboxic, anoxic, euxinic) were determined using a compiled dataset from modern anoxic basins. Based on these ratios, samples were selected for the global mass balance models, focusing on intervals representing suboxic/anoxic (Re) and fully anoxic/euxinic (Mo) conditions. The mass balance model considers metal inputs (primarily riverine) and outputs (burial in sediments). Authigenic concentrations of Re and Mo scale inversely with the areas of anoxic and euxinic sediment deposition, respectively. The model is solved iteratively to estimate past seafloor areas. The impact of various parameters (BMAR, thermal maturity, riverine fluxes) on model solutions was assessed through sensitivity analyses. A conservative threefold increase in Re and Mo riverine fluxes was applied for the N-CIE period, reflecting increased continental weathering during the T-OAE. Hydrothermal fluxes were excluded due to their minor contribution compared to riverine fluxes. The model intervals were further subdivided according to local redox conditions to improve temporal resolution.

The mass balance model results revealed that before the onset of the N-CIE, global ocean redox conditions were similar to modern conditions. The model estimates suggest that at the onset of the N-CIE the maximum extent of global seafloor anoxia reached approximately 6.9% (Model Interval MI-3), predominantly euxinic. This expansion was followed by a contraction of anoxia towards the end of the N-CIE (MI-4), with total anoxic seafloor area declining to about 4.1%. Euxinic seafloor area estimates were 0.47% (MI-1) and 4.8% (MI-4) before and during the event, respectively. The model results for the MI-1 and MI-4 intervals, where both Re and Mo models were applicable, showed consistent estimates for total anoxia and euxinia, validating model calibration. The large uncertainty in MI-3 reflects the asymptotic nature of the model at high anoxia levels. The study's findings demonstrate that the expansion of anoxia during the T-OAE, primarily limited to continental margins, mirrors the collapse and recovery patterns of global ammonite and foraminiferal biodiversity, suggesting a link between anoxia and mass extinction. The contraction of anoxia appears earlier in the Re record than in Mo and sulfur isotope data, likely due to the shorter oceanic residence times of Re and Tl. The difference in N-CIE morphology between the studied section and other sections likely reflects local sedimentological processes.

The findings support the hypothesis that significant expansions of ocean anoxia occurred during the T-OAE. The maximum extent of anoxia at the event's onset, estimated at up to ~7%, primarily euxinic, was likely concentrated in continental margin settings rather than the open deep ocean. This expansion of anoxic conditions, especially euxinia, coincides with a major decline in marine biodiversity, suggesting that anoxia played a crucial role in the T-OAE mass extinction. The observed contraction of anoxia toward the end of the N-CIE mirrors the recovery pattern of marine biodiversity. The discrepancy between the timing of anoxia contraction in Re and Mo isotope data highlights the importance of considering element residence times when interpreting global redox changes. The study demonstrates the effectiveness of using a combined Re and Mo mass balance approach in understanding global ocean redox dynamics during short-lived geological events.

This study provides robust quantitative estimates of global seafloor anoxia and euxinia during the Toarcian Oceanic Anoxic Event using a novel combination of Re and Mo mass balance modeling. The results highlight the significant expansion of anoxic conditions, primarily euxinia, at the onset of the event and their subsequent contraction. These redox changes correlate with patterns of ammonite and foraminiferal biodiversity, supporting the hypothesis that widespread anoxia contributed to the T-OAE mass extinction. Future research could explore the spatiotemporal dynamics of euxinia in greater detail and refine the model by incorporating additional factors affecting metal cycling.

The model's reliance on mass balance calculations and assumptions about riverine fluxes introduces uncertainty into the absolute estimates of anoxic seafloor areas. Local variations in depositional conditions could influence the authigenic metal enrichments, affecting the accuracy of global extrapolations. Further, the study focuses on a single section, which may not fully capture the global heterogeneity of redox conditions. Finally, the model's resolution is limited by the availability of suitable samples and the temporal resolution of the studied section.