Abstract
The late Early Jurassic Toarcian Stage represents the warmest interval of the Jurassic Period, with an abrupt rise in global temperatures of up to ∼7 °C in mid-latitudes at the onset of the early Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma). The T-OAE, which has been extensively studied in marine and continental successions from both hemispheres, was marked by the widespread expansion of anoxic and euxinic waters, geographically extensive deposition of organic-rich black shales, and climatic and environmental perturbations. Climatic and environmental processes following the T-OAE are, however, poorly known, largely due to a lack of study of stratigraphically well-constrained and complete sedimentary archives. Here, we present integrated geochemical and physical proxy data (high-resolution carbon-isotope data (δ13C), bulk and molecular organic geochemistry, inorganic petrology, mineral characterisation, and major- and trace-element concentrations) from the biostratigraphically complete and expanded entire Toarcian succession in the Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, Wales, UK. With these data, we (1) construct the first high-resolution biostratigraphically calibrated chemostratigraphic reference record for nearly the complete Toarcian Stage, (2) establish palaeoceanographic and depositional conditions in the Cardigan Bay Basin, (3) show that the T-OAE in the hemipelagic Cardigan Bay Basin was marked by the occurrence of gravity-flow deposits that were likely linked to globally enhanced sediment fluxes to continental margins and deeper marine (shelf) basins, and (4) explore how early Toarcian (tenuicostatum and serpentinum zones) siderite formation in the Cardigan Bay Basin may have been linked to low global oceanic sulphate concentrations and elevated supply of iron (Fe) from the hinterland, in response to climatically induced changes in hydrological cycling, global weathering rates and large-scale sulphide and evaporite deposition.
Highlights
The Toarcian Stage (∼183–174 Ma) is considered to have been the warmest interval of the Jurassic Period encompassing a tran-sient temperature rise of up to ∼7 ◦C in mid-latitudes (Dera et al, 2011a; Gradstein et al, 2012; Korte et al, 2015)
Earlier chemostratigraphic studies on the Toarcian in the Mochras core showed the characteristic overarching positive excursion interrupted by a negative carbon-isotope excursion (CIE) in δ13CTOC and δ13Ccarb records for the Toarcian Oceanic Anoxic Event (T-OAE) (Jenkyns and Clayton, 1997; Jenkyns et al, 2001, 2002; Jenkyns, 2003; Katz et al, 2005; van de Schootbrugge et al, 2005), as well as a negative CIE in δ13CTOC and δ13Ccarb at the Pliensbachian–Toarcian boundary
The maximum of the early Toarcian positive carbon-isotope excursion is interrupted by the stratigraphically extended early Toarcian negative CIE of ∼7h (−24 to −31h), which is marked by a step-wise onset at the tenuicostatum–serpentinum Zone transition (Fig. 2)
Summary
The Toarcian Stage (∼183–174 Ma) is considered to have been the warmest interval of the Jurassic Period encompassing a tran-sient temperature rise of up to ∼7 ◦C in mid-latitudes (Dera et al, 2011a; Gradstein et al, 2012; Korte et al, 2015). Significant burial of photosynthetically derived (12C-rich) organic matter took place, producing a positive (with higher δ13C) carbon-isotope excursion (CIE) of ∼3h in marine carbonate and ∼4h in bulk sedimentary organic matter, which lasted for ∼3.5 Myr, from the latest Pliensbachian (latest spinatum Zone) until midway through the Toarcian (bifrons Zone, fibulatum Subzone) (Jenkyns et al, 2002; Gradstein et al, 2012; Hermoso et al, 2013). The magnitude of the T-OAE negative CIE can be as great as ∼7h in marine and terrestrial organic carbonisotope records and ∼3–6h in pelagic and shallow-water carbonates and compound-specific biomarkers (Küspert, 1982; Jenkyns and Clayton, 1986; Hesselbo et al, 2000, 2007; Sælen et al, 2000; Schouten et al, 2000; Jenkyns et al, 2002; Kemp et al, 2005; Hermoso et al, 2009; Sabatino et al, 2009; Al-Suwaidi et al, 2010, 2016; French et al, 2014; Suan et al, 2015)
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