Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Mochras Borehole, Cardigan Bay Basin, NW Wales

Abstract

Abstract. The Late Pliensbachian Event (LPE), in the Early Jurassic, is associated with a perturbation in the global carbon cycle (positive carbon isotope excursion (CIE) of ~ 2 ‰), cooling of ~5 °C, and the deposition of widespread regressive facies. Cooling during the Late Pliensbachian has been linked to enhanced organic matter burial and/or disruption of thermohaline ocean circulation due to North Sea doming. Orbital forcing had a strong influence on the Pliensbachian environments and recent studies show that the terrestrial realm and the marine realm in and around the Cardigan Bay Basin were strongly influenced by orbital climate forcing. In the present study we build on the previously published data for long eccentricity cycle E459 ± 1 and extend the palaeoenvironmental record to include E458 ± 1. We explore the environmental and depositional changes on orbital time scales for the Mochras core during the onset of the LPE. Clay mineralogy, XRF elemental analysis, isotope ratio mass spectrometry, and palynology are combined to resolve systematic changes in erosion, weathering, fire, grain size and riverine influx. Our results indicate distinctively different environments before and after the onset of the LPE positive CIE, and show increased physical erosion relative to chemical weathering. We also identify 5 swings in the climate, in tandem with the 405 kyr eccentricity minima and maxima. Eccentricity maxima are linked to precessionally repeated occurrences of a semi-arid, monsoonal climate with high fire activity and relatively coarser fraction of terrestrial runoff. In contrast, 405 kyr minima in the Mochras core are linked to a more persistent, annually wet climate, low fire activity, and relatively finer grained deposits across multiple precession cycles. The onset of the LPE +ve CIE did not impact the expression of the 405 kyr in the proxy records, however, during the second pulse of lighter carbon (12C) enrichment, the clay minerals record a change from dominant chemical weathering to dominant physical erosion.

Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes: Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK

“…The prominent expression of Carbon Isotope Excursions (CIEs) in Mochras with an amplitude of about one per mil in δ 13 C TOC (Xu et al 2018a) testifies to the expanded and relatively complete stratigraphic record therein, particularly in the early Hettangian and Sinemurian age part of the section, allowing for a distinction between intrinsic Earth processes and extrinsic solar system dynamics as the driving mechanisms for the Early Jurassic δ 13 C fluctuations (Xu et al 2018a;Storm et al 2020;Hollaar et al 2023;Hesselbo et al 2023).…”

Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes – Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK

“…The prominent expression of Carbon Isotope Excursions (CIEs) in Mochras with an amplitude of about one per mil in δ 13 C TOC (Xu et al 2018a) testi es to the expanded and relatively complete stratigraphic record therein, particularly in the lower Hettangian and Sinemurian part of the section), allowing for a distinction between intrinsic Earth processes and extrinsic solar system dynamics as the driving mechanisms for the Early Jurassic δ 13 C uctuations (Xu et al 2018a; Storm et al 2020;Hollaar et al 2023).…”