The Jurassic (∼201–145 Myr ago) was long considered a warm ‘greenhouse’ period; more recently cool, even ‘icehouse’ episodes have been postulated. However, the mechanisms governing transition between so-called Warm Modes and Cool Modes are poorly known. Here we present a new large high-quality oxygen-isotope dataset from an interval that includes previously suggested mode transitions. Our results show an especially abrupt earliest Middle Jurassic (∼174 Ma) mid-latitude cooling of seawater by as much as 10 °C in the north–south Laurasian Seaway, a marine passage that connected the equatorial Tethys Ocean to the Boreal Sea. Coincidence in timing with large-scale regional lithospheric updoming of the North Sea region is striking, and we hypothesize that northward oceanic heat transport was impeded by uplift, triggering Cool Mode conditions more widely. This extreme climate-mode transition provides a counter-example to other Mesozoic transitions linked to quantitative change in atmospheric greenhouse gas content.

The Mesozoic Era featured emplacement of a number of Large Igneous Provinces (LIPs), formed by the outpouring of millions of cubic kilometres of basaltic magma. The radiometric ages of several Mesozoic LIPs coincide with the dates of Oceanic Anoxic Events (OAEs). As a result of these coincidences, a causal link has been suggested, but never conclusively proven. This study explores the use of mercury as a possible direct link between the Karoo–Ferrar LIP and the coeval Toarcian OAE (T-OAE). Mercury is emitted to the atmosphere as a trace constituent of volcanic gas, and may be distributed globally before being deposited in sediments. Modern marine deposits show a strong linear correlation between mercury and organic-matter content. Results presented here indicate departures from such a simple linear relationship in sediments deposited during the T-OAE, and also during the Pliensbachian–Toarcian transition (an event that saw elevated benthic extinctions and carbon-cycle perturbations prior to the T-OAE). A number of depositional settings illustrate an increased mercury concentration in sediments that record one or both events, suggesting a rise in the depositional flux of this element. Complications to this relationship may arise from very organic-rich sediments potentially overprinting any Hg/TOC signal, whereas environments preserving negligible organic matter may leave no record of mercury deposition. However, the global distribution of coevally elevated Hg-rich levels suggests enhanced atmospheric mercury availability during the Early Toarcian, potentially aided by the apparent affinity of Hg for terrestrial organic matter, although the relative importance of aquatic vs terrestrial fixation of Hg in governing these enrichments remains uncertain. A perturbation in atmospheric Hg is most easily explained by enhanced volcanic output. It is suggested that extrusive igneous activity caused increased mercury flux to the Early Toarcian sedimentary realm, supporting the potential of this element as a proxy for ancient volcanism. This interpretation is consistent with a relationship between LIP formation and a perturbed carbon cycle during the Pliensbachian–Toarcian transition and T-OAE. The recording of these two distinct Hg excursions may also indicate that the Karoo–Ferrar LIP released volatiles in temporally distinct episodes, due either to multiple phases of magmatic emplacement or sporadic release of thermogenic gaseous products from intrusion of igneous material into volatile-rich lithologies

Significance: The end of the Triassic Period (∼201.5 million years ago) witnessed one of the largest mass extinctions of animal life known from Earth history. This extinction is suggested to have coincided with and been caused by one of the largest known episodes of volcanic activity in Earth’s history. This study examines mercury concentrations of sediments from around the world that record this extinction. Mercury is emitted in gaseous form during volcanism, and subsequently deposited in sediments. We find numerous pulsed elevations of mercury concentrations in end-Triassic sediments. These peaks show that the mass extinction coincided with large-scale, episodic, volcanism. Such episodic volcanism likely perturbed the global environment over a long period of time and strongly delayed ecological recovery.

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 (δ13Cδ13C), 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

The end-Triassic mass extinction (~201.4 million years ago), marked by terrestrial ecosystem turnover and up to ~50% loss in marine biodiversity, has been attributed to intensified volcanic activity during the break-up of Pangaea. Here, we present compound-specific carbon-isotope data of long-chain n-alkanes derived from waxes of land plants, showing a ~8.5 per mil negative excursion, coincident with the extinction interval. These data indicate strong carbon-13 depletion of the end-Triassic atmosphere, within only 10,000 to 20,000 years. The magnitude and rate of this carbon-cycle disruption can be explained by the injection of at least ~12 × 10³ gigatons of isotopically depleted carbon as methane into the atmosphere. Concurrent vegetation changes reflect strong warming and an enhanced hydrological cycle. Hence, end-Triassic events are robustly linked to methane-derived massive carbon release and associated climate change.