A Lower Jurassic environmental crisis

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).

Plate boundaries around Gondwanaland and the Karoo-Ferrar large igneous province in the Early Jurassic (small yellow dots show dated localities) . Large pink dots: positions of Tristan de Cunha and Bouvet hotspots at the time (Credit: Ruhl et al. Fig 1A)

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.

See also: https://scitechdaily.com/surprising-discovery-shows-how-slowing-of-continental-plate-movement-controlled-earths-largest-volcanic-events/

Dust tied to climate

This TOMS image shows a record-setting Asian d...
Dust moving in April 2001 from arid areas in Central Asia and North Africa to the oceans. From NASA's Nimbus-7 satellite. Image via Wikipedia

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant foot chain. Oddly, even when commonly known nutrients are brought to the ocean surface far from land by deep-sourced upwellings the effect on near-surface biomass is far from that expected. The key factor that is missing is dissolved divalent iron that acts as a minor nutrient for phytoplankton: even in deep ocean waters any such ferrous iron is quickly oxidised and precipitated as trivalent ferric compounds. One of the suggested means of geoengineering away any future climatic warming is to seed the far-off oceans reaches with soluble iron in the hope of triggering massive planktonic blooms, dead organisms sinking to be buried along with the their carbon content in the ocean-floor oozes. Retrospectively, it has been suggested that the slight mismatch between changes in atmospheric CO2 concentration and climate changes may be linked to fluctuating availability of iron dissolved from dust in ocean-surface waters, but so far that hypothesis has not been robustly tested. It is well known, however, that global cooling is accompanied by drying of continental climates and thereby an increase in the delivery of dust, even to polar ice caps where cores have shown dustiness to fluctuate with temperature.

Recently an ocean-floor sediment core from around 42° S has revealed a high-resolution record of the deposition of dust and iron at that location over the last 4 Ma (Martinez-Garcia, A. et al. 2011. Southern Ocean dust-climate coupling over the past 4 million years. Nature, v. 476, p. 312-315). In it one proxy for dust is the amount of organic compounds known as n-alkanes that are a major component of the waxes shed from plant leaves. Others are iron, titanium and thorium concentrations in the ooze. Dust proxies tally with land-ice volumes shown by the fluctuating d18O measured in bottom-dwelling foraminifera found as fossils in the core to form a convincing link between dust and climate over the Southern Ocean. Those proxies also match nicely the record of dust delivered to Antarctica that emerged from the 0.8 Ma Dome C ice core that was extracted and analysed by the EPICA consortium. The record shows boosts in iron and dust deposition at 2.7 Ma, when ice first took hold of northern high latitudes, and at 1.25 Ma when larger ice sheets began to develop and climate shifts switched to 100 ka cyclicity. Although the match between marine and glacial dust accumulation in the latter part of this mid-Pleistocene Transition is an important step forward in palaeoclimatology, it is a surprise that the new ocean-floor data is not plotted with the record of atmospheric CO2 in Antarctic ice bubbles: if there was a clear relationship that would have iced the cake.