Curiously, one of the largest environmental disruptions during the Phanerozoic Eon (i.e. since 541 Ma ago) does not stand out in the way that the ‘Big Five’ mass extinctions do. Each of them killed off between 70 and 95% of all marine species. The Jurassic was a period of biological recovery from the End-Triassic extinction 201 Ma ago. Throughout its ~50 Ma duration extinction rates were below the average for the Phanerozoic, and they remained relatively low until the K-Pg mass extinction that drew the Mesozoic Era to a close at 66 Ma. Nevertheless, there were significant extinctions, such as the demise of several lineages of herbivorous dinosaurs towards the end of the Early Jurassic followed by the rise of the familiar, long-necked variety of eusauropods. Marine organisms that secreted hard parts made of calcium carbonate also experienced a collapse then. From time to time during the Jurassic and Cretaceous Periods the oceans lost a great deal of dissolved oxygen, increasing the chances of organic carbon being buried in marine sediments. Such oceanic anoxia resulted in the widespread deposition of hydrocarbon source rocks in the form of black bituminous muds. Overall, both the Jurassic and Cretaceous experienced greenhouse climatic conditions, with atmospheric CO2 levels rising to almost 3000 ppm and oxygen levels significantly lower than the modern 21%. Sea levels rose by up to 200 metres, thought to be due to fast sea-floor spreading and large areas of warm, buoyant oceanic lithosphere.
A notable ocean-anoxia event took place during the Lower Jurassic, around 183 Ma ago at the start of the Toarcian Age. This stratigraphic level was penetrated by a 1.5 km borehole sunk in 2015-2016 at Mochras in North Wales, UK, on the shore of Cardigan Bay. The core provided the thickest and most complete record ever recovered for this event, and has been analysed in exquisite detail using many techniques. The most revealing data have been published by a multinational team led by scientists from Trinity College, Dublin (Ruhl, M. et al. 2022. Reduced plate motion controlled timing of Early Jurassic Karoo-Ferrar large igneous province volcanism. Science Advances, v. 8, article eabo0866; DOI: 10.1126/sciadv.abo0866).
At the start of the Toarcian (183.7 Ma) the 187Os/186Os ratio of the samples begins to rise from 0.3 to almost 0.8 to fall back to 0.3 by 180.8 Ma. Osmium isotopes are a measure of continental weathering, and this ‘excursion’ surely signifies significant global warming and increases in atmospheric humidity and acidity that broke down rocks at the continental surface. Over the same period δ13C rises, decreases to by far the lowest value in the Lower Jurassic, rises again to gradually fall back. The start of the Toarcian seems to have experienced a major release of carbon then a profound sequestration of organic carbon, presumably through burial of dead organisms in the black mudstones that signify anoxic conditions. Remarkably, the 95 m thick Toarcian black-mudstone sequence also reveals a tenfold increase in its content of the element mercury, from 20 to 200 parts per billion (ppb), peaking at the same time (~182.8 Ma) as the most negative δ13C value was reached: the acme of carbon sequestration. A coincidence of massive organic carbon burial and increased mercury in marine sediments also happened at the time of the end-Permian mass extinction, although that does not necessarily imply exactly the same mechanism.
The early Toarcian geochemical trends, however, coincide with the initiation and duration of the Karoo-Ferrar large igneous province, which formed flood basalts, igneous dyke swarms and large volcanic centres in South Africa and Antarctica. That LIP may have emitted mercury, but so too may have increased chemical weathering of the land surface. Whichever, mercury forms an organic compound (methyl mercury) in water bodies. Readily incorporated into living organisms, that could explain the close parallel between the δ13C and Hg records in the Jurassic sediment core from Wales. The Karoo-Ferrar igneous activity itself presents a bit of a conundrum, as suggested by Ruhl et al. It happened at the very time that there was a 120° change in the direction of motion of the tectonic plate carrying along Africa and, indeed, the Gondwanaland supercontinent during the Jurassic. The directional change also involved local plate movement stopping for a while. According to the authors, it wasn’t a fortuitous coincidence of two mantle plumes from the core-mantle boundary hitting the bottom of the continental lithosphere below Africa and Antarctica at this tectonic ‘U-turn’. It is more likely that the pause gave existing plumes the opportunity and time to ‘erode’ the base of the continental lithosphere and rise. Decompression melting would then have produced the voluminous magmas. The two plumes were in place for a very long time and created seamount chains as plates moved over them. Both are still volcanically active: Tristan de Cunha on the mid-Atlantic Ridge, and Bouvet Island at a triple junction between South Africa and Antarctica.
So, a venture to unravel a period of profound environmental change during the Early Jurassic, which didn’t result in mass extinction, may well have spawned a new model for massive igneous events that did. Ruhl et al. suggest that the short-lived Siberian, North Atlantic and East African Rift LIPs each seem to have coincided with short episodes of tectonic slowing-down: LIPs may result in dramatic environmental change, but at the whim of plate tectonics.
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