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/

An Early Archaean Waterworld

In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.