The Jurassic is often considered “the” geological period that saw the reign of dinosaurs clearly established. The beginning of this period is in fact marked by the resilienceresilience ecosystems following the severe mass extinction which marked the end of the Triassic, around 200 million years ago. The climate, temperate and humid, even hot (5 to 10°C warmer than currently), is then conducive to evolutionary radiationevolutionary radiation new species, particularly among dinosaurs and mammalsmammals, which will take their place in the ecological niches left vacant by this recent biological crisis. THE landslands of supercontinentsupercontinent PangeaPangea, which begins to break up, are then covered with lush jungles. This could be considered the beginning of a golden age for rapidly growing species. However, the situation was not as idyllic as that.
Did you know ? Global warming was first highlighted in… 1856! Discover the story of the woman who demonstrated it in Science Hunters. © Futura
300,000 years of intense volcanic activity will change the climate
As always, the climate played the role of troublemaker. The beginning of the Jurassic, and more particularly the transition between the Pliensbachian and the Toarcian 183 million years ago, is marked by an extreme climatic episode. CO levels2 in L’atmosphereatmospherealready high at this period, will in fact jump under the effect of degassingdegassing huge quantities of makemake (more than 2.5 million cubic kilometers) emitted by the eruption of the large igneous provinces of the Karoo-Ferrar. The formation of this immense magmatic plateau which is located today in South Africa and AntarcticAntarctic will in fact release 20,500 gigatons of CO2 in the atmosphere over a period of 300,000 to 500,000 years. This evolution of the atmospheric composition will lead to a “super-greenhouse effectgreenhouse effect » which will result in a global warmingglobal warming overall. If the terrestrial environment is impacted (we note the disappearance of several cladesclades dinosaurs at that time), the marine environment is not spared either and will bear the brunt of these changes in environmental conditions.
Landscape of the Drakensberg (South Africa) where numerous basalt flows were superimposed at the beginning of the Jurassic, forming a plateau 1.4 kilometers high. © I, PhilippN, Wikimedia Commons, CC by-sa 3.0
Ocean acidification and anoxia cause mass extinction
The temperature of the oceans thus increases by 3 to 7°C depending on the latitudelatitude and increasing CO levels2 in the water will drastically change the chemistrychemistry oceans, causing mass extinctionmass extinctionparticularly among invertebratesinvertebrates sailors (brachiopodesbrachiopodes, bivalvesbivalves, ammonitesammonites…)). The oceans are becoming acidified while the massive influx of nutrientsnutrients via the increased alteration of continental surfaces will lead to a eutrophicationeutrophication middle. Of the alguesalgues et microorganismsmicroorganisms will grow en masse, producing large quantities of mattermatter organic which will accumulate in the sedimentsediment. The degradation of this organic matter will, however, lead to a high consumption of oxygen dissolved in the water. The oxygenation of the ocean will thus drop dramatically: this is anoxia.
This new dramatic episode in earthly history is thus known under the name of Event anoxicanoxic Toarcian oceanic (or T-OAE). Researchers were interested in the intensity and geographic extent that this state of anoxia in the ocean reached 183 million years ago. For this they based themselves on the quantity of certain isotopesisotopes of the’uraniumuranium in sediments dating from this period. The isotopic composition of uranium in the oceans is in fact linked to the oxygenation of the environment. When there is a lot of oxygen, uranium preferentially remains in soluble form whereas in a situation of anoxia, it will precipitate and enter the composition of the sediments.
The stratified sediments visible in this quarry (Mercato San Severino, Italy) are limestones deposited at the bottom of a now extinct ocean, 183 million years ago. © F. Tissot
We are heading straight towards a new anoxic ocean event
The results, published in the journal Pnas thus reveal that during the T-OAE, 6 to 8% of the ocean floor was in conditions of anoxia. An extension 28 to 38 times greater than that we know today. Because there are currently anoxic zones in our oceans. They represent 0.2% of the ocean floor. But current climate trends suggest that this figure could increase in the near future. If we are still far from CO levels2 emitted by the magmatic provinces of Karoo-Ferrar, this study nevertheless has something to alert us. Because in just 200 years, human activities have induced a emissionemission cumulative CO2 representing 12% of the total quantity emitted in 300,000 years by this magmatic activity. These figures are very worrying, because they reveal (once again) how quickly we are changing our environment in relation to natural factors. The study of past crises shows us that it does not take a major imbalance in the ocean to cause a severe extinction. Some 6% anoxic surfaces are sufficient. It is therefore high time to limit our CO emissions2before it is too late.
