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.
Highlights
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
Prominent carbon-isotope excursions (CIEs) are identified globally in strata from the Triassic–Jurassic boundary (∼201 Ma) and the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma), both of which are expressed in the δ13C values derived from various marine and terrestrial organic and inorganic materials [1,2,3]
These isotopic events express changes in the δ13C composition of the combined global exogenic carbon pool and are linked to the elevated release of isotopically light volcanic, and/or thermogenic, and/or biogenic carbon into the global ocean–atmosphere system and global increase in organic-carbon sequestration in marine and/or terrestrial environments. Bracketed by these globally recognized distinct large-amplitude δ13C events, numerous δ13C shifts of somewhat lesser magnitude have been identified in the Hettangian to Pliensbachian interval
Summary
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. These δ13C shifts have largely been interpreted as standalone events, linked to a release of 12C from as-yet-undefined sources, reduced organic productivity (leaving more 12C in the ocean–atmosphere system) and/or 13C-depleted carbon sequestration and orbitally forced environmental change affecting the carbon cycle on the scale of Milankovitch cyclicity [17, 20, 24, 25] Evidence for the latter is so far limited to the Hettangian to early Sinemurian and the early Toarcian, where high-resolution isotope records provide the basis for cyclostratigraphic analysis [17,18,19, 29,30,31]. The uppermost Pliensbachian and lower Toarcian strata are regarded as having been deposited in an unrestricted, openmarine setting [35]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
