End-Triassic mass extinction: evidence for oxygen depletion on the ocean floor

For British geologists of my generation the Triassic didn’t raise our spirits to any great extent. There’s quite a lot of it on the British Geological Survey 10-miles-to-the-inch geological map (South Sheet) but it is mainly muds, sandstones or pebble beds, generally red and largely bereft of fossils. For the Triassic’s 50 Ma duration following the end-Permian extinction at 252 Ma Britain was pretty much a desert in the middle of the Pangaea supercontinent. Far beyond our travel grants’ reach, the Triassic is a riot, as in the Dolomites of Northern Italy. Apart from a day trip to look at the Bunter Pebble Beds in a quarry near Birmingham and several weeks testing the load-bearing strength of the Keuper mudstones in the West Midlands (not far off zero) in a soil-mechanics lab, we did glimpse the then evocatively named Tea Green Marl (all these stratigraphic names have vanished). Conveniently they outcrop by the River Severn estuary, below its once-famous suspension bridge and close-by the M5 motorway. Despite the Tea Green Marl containing a bone bed with marine reptiles, time didn’t permit us to fossick, and, anyway, there was a nearby pub … The formation was said to mark a marine transgression leading on to the ‘far more interesting Jurassic’ – the reason we were in the area. We were never given even a hint that the end of the Triassic was marked by one of the ‘Big Five’ mass extinctions: such whopping events were not part of the geoscientific canon in the 1960s.

Pangaea just before the start of Atlantic opening at the end of the Triassic, showing the estimated extend of the CAMP large igneous province. The pink triangles show the sites investigated by He and colleagues.

At 201.3 Ma ago around 34 % of marine genera disappeared, comparable with the effect of the K-Pg extinction that ended the Mesozoic Era. Extinction of Triassic terrestrial animals is less quantifiable. Early dinosaurs made it through to diversify hugely during the succeeding Jurassic and Cretaceous Periods. Probably because nothing famous ceased to be or made its first appearance, the Tr-J mass extinction hasn’t captured public attention in the same way as those with the K-Pg or the P-Tr acronyms.  But it did dramatically alter the course of biological evolution. The extinctions coincided with a major eruption of flood basalts known as the Central Atlantic Magmatic Province (CAMP), whose relics occur on either side of the eponymous ocean, which began to open definitively at about the same time. So, chances are, volcanic emissions are implicated in the extinction event, somehow (see: Is end-Triassic mass extinction linked to CAMP flood basalts? June 2013). Tianchen He  of Leeds University, UK and the China University of Geosciences and British and Italian colleagues have studied three Tr-J marine sections on either side of Pangaea: in Sicily, Northern Ireland and British Columbia (He, T. and 12 others 2020. An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction. Science Advances, v. 6, article eabb6704; DOI: 10.1126/sciadv.abb6704). Their objective was to test the hypothesis that CAMP resulted in an episode of oceanic anoxia that caused the many submarine organisms to become extinct. Since eukaryote life depends on oxygen, a deficit would put marine animals of the time under great stress. Such events in the later Mesozoic account for global occurrences of hydrocarbon-rich, black marine shales – petroleum source rocks – in which hypoxia thwarted complete decay of dead organisms over long periods. However there is scant evidence for such rocks having formed ~201 Ma ago. Such as there is dates to about 150 ka younger than the Tr-J boundary in an Italian shallow marine basin. The issue of evidence is compounded by the fact that there are no ocean-floor sediments as old as that, thanks to their complete subduction as Pangaea broke apart in later times and its continental fragments drifted to their present configuration.

But there is an indirect way of detecting deep-ocean anoxia, in the inevitable absence of any Triassic and early Jurassic oceanic crust. It emerges from what happens to the stable isotopes of sulfur when there are abundant bacteria that use the reduction of sulfate (SO42-) to sulfide (S2-) ions. Such microorganisms thrive in anoxic conditions and produce abundant hydrogen sulfide, which in turn leads to the precipitation of dissolved iron as minute grains of pyrite (FeS2). This biogenic process selectively excludes 34S from the precipitated pyrite. As a result, at times of widespread marine reducing conditions seawater as a whole becomes enriched in 34S relative to sulfur’s other isotopes. The enrichment is actually expressed in the unreacted sulfate ions, and they may be precipitated as calcium sulfate or gypsum (CaSO4) in marine sediments deposited anywhere: He et al. focussed on such fractionation. They discovered large ‘spikes’ in the relative enrichment of 34S at the Tr-J boundary in shallow-marine sedimentary sequences exposed at the three sites. Moreover, they were able to estimate that the conditions on the now vanished bed of the Triassic ocean that gave rise to the spikes lasted for about 50 thousand years. The lack of dissolved oxygen resulted in a five-fold increase in pyrite burial in the now subducted ocean-floor sediments of that time. The authors suggest that the oxygen depletion stemmed from extreme global warming, which, in turn, encouraged methane production by other ocean-floor bacteria and, in a roundabout way, other chemical reactions that consumed free dissolved oxygen. Quite a saga of a network of interactions in the whole Earth system that may hold a dreadful warning for the modern Earth and ourselves.

When rain kick-started evolution

The end of the Palaeozoic Era was marked by the greatest known mass extinction at the Permian-Triassic boundary 252 Ma ago. An estimated 96% of known marine fossil species simply disappeared, as did 70% of vertebrates that lived on land. Many processes seem to have conspired against life on Earth although it seems that one was probably primary: the largest known flood-basalt event, evidence for which lies in the Siberian Traps. It took as long as 50 Ma for ecosystems to return to their former diversity. But, oddly, it was animals at the top of the marine food chain that recovered most quickly, in about 5 million years. There must have been food in the sea, but it was at first somewhat monotonous. The continents were still configured in the Pangaea supercontinent, so much land was far from oceans and thus dry. Oxygen was being drawn down from the atmosphere to combine with iron in Fe2O3 to form vast tracts of redbeds for which the Triassic is famous. From a peak of 30% in the Permian, atmospheric oxygen descended to 16% in the early Triassic, so living even at sea level would have been equivalent to surviving today at 2.7 km elevation today. Potential ecological niches were vastly reduced in fertility and in altitude, and Pangaea still had vast mountain ranges inherited from its formative tectonics as well as being arid, apart from in polar regions. Unsurprisingly, recovery of terrestrial diversity, especially among vertebrates, was slow during the early Triassic.

Triassic grey terrestrial sediments on the Somerset coast of SW England (credit: Margaret W. Carruthers; https://www.flickr.com/photos/64167416@N03/albums/72157659852255255)

Then, about halfway through the Triassic Period, it began to rain across Pangaea. Whether that was continual or seasonal is uncertain, although the presence of large mountains and high plateaus would favour monsoon circulation, akin to the present-day Indian monsoon associated with the Himalaya and Tibetan Plateau. How do geologists know that central Pangaea became wetter? The evidence lies in grey sedimentary strata between the otherwise universal redbeds, which occur in the Carnian Age and span one to two million years around 232 Ma (Marshall, M. 2019. Did a million years of rain jump-start dinosaur evolution? Nature, v. 576, p. 26-28; doi: 10.1038/d41586-019-03699-7). A likely driver for this change in colour is a rise in water tables that would exclude oxygen from sediments deposited recently. The red Iron-3 oxides were reduced, so that soluble iron-2 was leached out. Some marine groups, such as crinoids, underwent a sudden flurry of extinctions, as did plants and amphibians on land. But others received a clear boost from this Carnian Pluvial Event. A few dinosaurs first appear in older Triassic sediments, but during the Carnian they began to diversify from diminutive bipedal species into the main groups so familiar to many: ornithischians that lead to Stegosaurus and Triceratops and the forerunners of the saurischians that included huge long-necked herbivores and the bipedal theropods and birds. Within 4 Ma dinosaurs had truly begun their global hegemony. Offshore in shallow seas, the scleractinian corals, which dominate modern coral reef systems, also exploded during the Carnian from small beginnings in the earlier Triassic. It is even suspected that the Carnian nurtured the predecessor of mammals, although the evidence is only from isolated fossil teeth.

A Carnian shift in carbon isotopes, measured in Triassic limestones of the Italian Dolomites, to lower proportions of the heavier 13C suggests that a huge volume of the lighter 12C had entered the atmosphere. That could have resulted from large-scale volcanism, the 232 Ma old laves of the Wrangell Mountains in Alaska being a likely suspect. Such an input would have had a warming climatic outcome that would have increased tropical evaporation of ocean water and the humidity over continental masses. The once ecologically monotonous core of Pangaea may have greatly diversified into many more niches awaiting occupants, thereby stimulating the terrestrial evolutionary burst. Perhaps ironically, and fortunately, a volcanic near snuffing-out of life on Earth was soon followed by another with the opposite effect. Yet another negative outcome arrived with the flood basalts of the Central Atlantic Magmatic Province at the end of the Triassic (201 Ma), to be followed by further adaptive radiation among those organisms that survived into the Jurassic.