Orbital pacing and secular evolution of the Early Jurassic carbon cycle

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.

Interdisciplinary Approaches to the Scandinavian Heritage of North-West England

“…2). δ 13 C wood values from Kaszewy are averagely and systematically heavier by about 1‰ than the corresponding values from δ 13 C TOC values from Mochras, likely due to contributions from marine organic matter to the Mochras curve(Storm et al, 2020), but what is important for chemostratigraphic correlation is concordance in general trends.This average δ 13 C contrast disappears considering the results of δ 13 C wood from Mochras. The correlation presented shows also a general concordance with other biostratigraphically constrainedδ 13 C curves from marine sections in Europe and North America, e.g.…”

“…Studies hitherto have largely focused on two events, homogenous woody material (δ 13 C wood ) are more reliable in interpreting global changes more directly, as they reflect atmospheric (i.e. global) processes (Gröcke 2002) and can therefore be used more confidently for chemostratigraphic correlations (see for Lower Jurassic examples: Hesselbo and Pieokowski 2011;Pieokowski et al 2016;Them et al 2017 b;Ruebsam et al 2020;Storm et al, 2020).…”

“…For comparison to Kaszewy we use the new compilation from the biostratigraphically constrained Mochras core (Storm et al, 2020) which covers the entire Early Jurassic, reproducing large-amplitude δ 13 C TOC excursions (>3 ‰) at the Sinemurian-Pliensbachian transition and in the lower Toarcian serpentinum Zone, as well as several previously identified medium-amplitude (~0.5-3 ‰) shifts in the Hettangian to Pliensbachian. In addition, δ 13 C wood (Storm et al, 2020) and δ 13 C carb (Katz et al 2005) data was also obtained (Fig. 2).…”

“…The section above the T-OAE is a special case because it has already been demonstrated that the lower Toarcian is commonly truncated in the Polish Basin beneath younger Toarcian strata(Pieokowski 2004, Hesselbo andPieokowski 2011). All conspicuous negative CIEs can be correlated with the δ 13 C curves from the Mochras borehole(Katz et al 2005;Storm et al 2020) and from the St Audrie's Bay section for the Rhaetian(Korte et al, 2009;Ruhl et al 2010); Fig. 2).…”

“…Expanded core sections, providing a continuous record of the Lower Jurassic series -such as the Llanbedr (Mochras Farm) core recovered in Wales, UK (Woodland, 1971;Katz et al, 2005;Hesselbo et al, 2013;Ruhl et al, 2016;Xu et al, 2018;Storm et al, 2020), hereafter referred to as Mochras, and Kaszewy borehole drilled in central Poland (Figs. 1, 2), are exceptionally valuable.…”