The hyperthermals of the geological record

During the last 540 million years five mass extinction events shaped the history of the Earth. Those events were related to extreme climatic changes. The geological records show that large and rapid global warming events occurred repeatedly during the course of Earth history.
Our planet’s climate has oscillated between two basic states: the “Icehouse”, and the “Greenhouse”, and superimposed on this icehouse–greenhouse climate cycling, there are a number of geologically abrupt events known as hyperthermals, when atmospheric CO2 concentrations may rise above 16 times (4,800 ppmv). Although each hyperthermal is unique, they are consequence from the release of anomalously large inputs of CO2 into the atmosphere and are relatively short-lived (with the exception of the Permian–Triassic boundary).

A summary of the most significant hyperthermals in the last 300 Myr. From Foster et. al., 2018.

The emplacement of large igneous provinces (LIPs) is commonly associated with hyperthermals, for example, the Siberian Traps at the P–T boundary. The CO2 emissions caused global warming. The SO2 emissions on mixing with water vapour in the atmosphere, caused acid rain, which in turn killed land plants and caused soil erosion. Warmer oceans melted frozen methane located in marine sediments which pushed the global temperatures to higher levels. Additionally, the increased continental weathering induced by acid rain and global warming led to increased marine productivity and eutrophication, and so oceanic anoxia, and marine mass extinctions.

The hyperthermal at the P–T boundary was associated with the most severe terrestrial and oceanic mass extinction of the last 541 Myr, where 96% of species became extinct. It comprises two killing events, one at the end of the Permian (EPME) and a second at the beginning of the Triassic, separated by 60000 years. In terms of carbon isotope excursion, the P–T boundary hyperthermal and the PETM share many similarities, but the warming after the P-T boundary was more extreme and extended for longer than PETM.

Flow chart summarizing proposed cause-and-effect relationships during the end-Permian extinction (From Bond and Wignall, 2014)

The End-Triassic Extinction is probably the least understood of the big five. It has been linked to the eruption of the Central Atlantic Magmatic Province (CAMP), a large igneous province emplaced during the initial rifting of Pangea. Most mammal-like reptiles and large amphibians disappeared, as well as early dinosaur groups. In the oceans, this event eliminated conodonts and nearly annihilated corals, ammonites, brachiopods and bivalves. In the Southern Hemisphere, the vegetation turnover consisted in the replacement to Alisporites (corystosperm)-dominated assemblage to a Classopollis (cheirolepidiacean)-dominated one.

The early Toarcian Oceanic Anoxic Event (T-OAE; ∼183 mya) in the Jurassic Period is considered as one of the most severe of the Mesozoic era. The T-OAE is thought to have been caused by increased atmospheric CO2 triggered by Karoo–Ferrar volcanism. Results from the Paris Bassin indicates that the increasing greenhouse conditions may have caused acidification in the oceans, hampering carbonate bio-mineralisation, and provoking a dramatical loss in the CO2 storage capacity of the oceans.

Tentative changes in mid-latitude vegetation patterns during OAE2. (a) Araucariaceae, (b) other conifers incl. Cheirolepidiaceae, (c) Cupressaceae, (d) angiosperms incl. Normapolles-producing forms, (e) ferns. From Heimhofer et al., 2018.

The early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) represents a geologically brief time interval characterized by rapid global warming, dramatic changes in ocean circulation including widespread oxygen deficiency, and profound changes in marine biotas. During the event, black shales were deposited in all the main ocean basins. It was also associated with the calcification crisis of the nannoconids, the most ubiquitous planktic calcifiers during the Early Cretaceous. Their near disappearance is one of the most significant events in the nannoplankton fossil record.

The mid-Cretaceous Oceanic Anoxic Event 2 (OAE2, 93 Ma) marks the onset of an extreme phase in ocean temperatures known as the “Cretaceous thermal maximum”. It has been postulated that the OAE2 was triggered by a massive magmatic episode.

Comparison of the effects of anthropogenic emissions (total of 5000 Pg C over 500 years) and PETM carbon release (3000 Pg C over 6 kyr) on the surface ocean saturation state of calcite. From Zeebe, 2013

The Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago), was a short-lived (~ 200,000 years) global warming event attributed to a rapid rise in the concentration of greenhouse gases in the atmosphere. It was suggested that this warming was initiated by the melting of methane hydrates on the seafloor and permafrost at high latitudes. During the PETM, around 5 billion tons of CO2 was released into the atmosphere per year, and temperatures increased by 5 – 9°C. This event was accompanied by other large-scale changes in the climate system, for example, the patterns of atmospheric circulation, vapor transport, precipitation, intermediate and deep-sea circulation and a rise in global sea level. But unlike other hyperthermals, the PETM is not associated with significant extinctions.

Anthropogenic climate change and ocean acidification resulting from the emission of vast quantities of CO2 and other greenhouse gases pose a considerable threat to ecosystems and modern society. The combination of global warming and the release of large amounts of carbon to the ocean-atmosphere system during the PETM has encouraged analogies to be drawn with modern anthropogenic climate change. The current rate of the anthropogenic carbon input is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.


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