Orbital pacing and secular evolution of the Early Jurassic carbon cycle

Jenkyns, Hugh C.; Storm, Marisa S.; Ruhl, Micha; Hesselbo, Stephen P.; Ullmann, Clemens V.; Xu, Weimu; Leng, Melanie J.; Riding, James B.; Gorbanenko, Olga

Abstract

Global perturbations to the Early Jurassic environment (∼201 to ∼174 Ma), notably during the Triassic–Jurassic transition and Toarcian Oceanic Anoxic Event, are well studied and largely associated with volcanogenic greenhouse gas emissions released by large igneous provinces. The long-term secular evolution, timing, and pacing of changes in the Early Jurassic carbon cycle that provide context for these events are thus far poorly understood due to a lack of continuous high-resolution δ13C data. Here we present a δ13CTOC record for the uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic), derived from a calcareous mudstone succession of the exceptionally expanded Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, United Kingdom. Combined with existing δ13CTOC data from the Toarcian, the compilation covers the entire Lower Jurassic. The dataset reproduces large-amplitude δ13CTOC excursions (>3‰) recognized elsewhere, at the Sinemurian–Pliensbachian transition and in the lower Toarcian serpentinum zone, as well as several previously identified medium-amplitude (∼0.5 to 2‰) shifts in the Hettangian to Pliensbachian interval. In addition, multiple hitherto undiscovered isotope shifts of comparable amplitude and stratigraphic extent are recorded, demonstrating that those similar features described earlier from stratigraphically more limited sections are nonunique in a long-term context. These shifts are identified as long-eccentricity (∼405-ky) orbital cycles. Orbital tuning of the δ13CTOC record provides the basis for an astrochronological duration estimate for the Pliensbachian and Sinemurian, giving implications for the duration of the Hettangian Stage. Overall the chemostratigraphy illustrates particular sensitivity of the marine carbon cycle to long-eccentricity orbital forcing.

Citation

Jenkyns, H. C., Storm, M. S., Ruhl, M., Hesselbo, S. P., Ullmann, C. V., Xu, W., …Gorbanenko, O. (2020). Orbital pacing and secular evolution of the Early Jurassic carbon cycle. Proceedings of the National Academy of Sciences, 117(8), 3974-3982. https://doi.org/10.1073/pnas.1912094117

• Journal Article Type: Article
• Acceptance Date: Jan 3, 2020
• Online Publication Date: Feb 10, 2020
• Publication Date: Feb 25, 2020
• Deposit Date: Feb 18, 2020
• Publicly Available Date: Feb 18, 2020
• Journal: Proceedings of the National Academy of Sciences of the United States of America
• Print ISSN: 0027-8424
• Electronic ISSN: 1091-6490
• Publisher: National Academy of Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 117
• Issue: 8
• Pages: 3974-3982
• DOI: https://doi.org/10.1073/pnas.1912094117
• Keywords: Multidisciplinary
• Public URL: https://nottingham-repository.worktribe.com/output/3983898
• Publisher URL: https://www.pnas.org/content/early/2020/02/04/1912094117