Abstract
Abstract Large igneous provinces (LIPs) are proposed to have caused a number of episodes of abrupt environmental change by increasing atmospheric CO2 levels, which were subsequently alleviated by drawdown of CO2 via enhanced continental weathering and burial of organic matter. Here the sedimentary records of two such episodes of environmental change, the Toarcian oceanic anoxic event (T-OAE) and preceding Pliensbachian–Toarcian (Pl-To) event (both possibly linked to the Karoo-Ferrar LIP), are investigated using a new suite of geochemical proxies that have not been previously compared. Stratigraphic variations in osmium isotope (187Os/188Os) records are compared with those of mercury (Hg) and carbon isotopes (δ13C) in samples from the Mochras core, Llanbedr Farm, Cardigan Bay Basin, Wales. These sedimentary rocks are confirmed as recording an open-marine setting by analysis of molybdenum/uranium enrichment trends, indicating that the Os isotope record in these samples reflects the isotopic composition of the global ocean. The Os isotope data include the first results across the Pl-To boundary, when seawater 187Os/188Os increased from ∼0.40 to ∼0.53, in addition to new data that show elevated 187Os/188Os (from ∼0.42 to ∼0.68) during the T-OAE. Both increases in 187Os/188Os correlate with negative carbon isotope excursions and increased mercury concentrations, supporting an interplay between terrestrial volcanism, weathering, and climate that was instrumental in driving these distinct episodes of global environmental change. These observations also indicate that the environmental impact of the Karoo-Ferrar LIP was not limited solely to the T-OAE.
Highlights
Oceanic anoxic events (OAEs) were times of abrupt carbon-cycle perturbations, driven by increases in atmospheric CO2 proposed to result, at least partially, from the emplacement of large igneous provinces (LIPs), and alleviated by the subsequent draw down of excess CO2 through enhanced continental weathering and widespread burial of organic matter (Jenkyns, 2010)
The Early Jurassic Toarcian OAE (T-OAE, 183 Ma) was one such event: geochemical evidence for LIP volcanism, oceanic anoxia, release of carbon to the atmosphere, and increased continental weathering rates are found in a number of sedimentary sections
Large-scale volcanism during the Toarcian oceanic anoxic event (T-OAE) has previously been inferred from elevated concentrations of sedimentary Hg of assumed volcanogenic origin (Percival et al, 2015), and attributed to the Karoo-Ferrar LIP, which is radiometrically dated as ca. 183 Ma and Toarcian in age (e.g., Rampino and Strothers, 1988; Duncan et al, 1997; Svensen et al, 2012; Burgess et al, 2015)
Summary
Oceanic anoxic events (OAEs) were times of abrupt carbon-cycle perturbations, driven by increases in atmospheric CO2 proposed to result, at least partially, from the emplacement of large igneous provinces (LIPs), and alleviated by the subsequent draw down of excess CO2 through enhanced continental weathering and widespread burial of organic matter (Jenkyns, 2010). There remains no single sedimentary record where evidence for volcanic activity, carbon-cycle perturbation, and enhanced weathering rates are compared using proxy data from the same succession. This situation is the case for a preceding event during the Pliensbachian–Toarcian (Pl-To) transition, which took place
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