Paleocene–Eocene thermal maximum

The Paleocene–Eocene thermal maximum (PETM) was a short but important time in Earth's history when the planet got much warmer. About 55.8 million years ago, the average temperature rose by about 5 to 8 °C (9 to 14 °F) over a period of around 200,000 years. This warming happened right at the boundary between two geological periods called the Paleocene and the Eocene.

During the PETM, a lot of carbon entered the ocean and atmosphere. Scientists study this event to better understand how our planet handles large amounts of carbon and global warming. We can see evidence of this change in rocks and fossils from that time. For example, the carbon in these rocks shows a special pattern that tells us a big amount of carbon was released.

The PETM also brought big changes to life on Earth. In the oceans, many small creatures called benthic foraminifera died out, and new types of tiny ocean plants appeared. On land, many modern groups of mammals, including the first primates, showed up in places like Europe and North America. The PETM helps scientists understand how Earth’s temperature changes can affect plants, animals, and the whole planet.

Setting

The way oceans and continents were arranged was a bit different a long time ago during the early Paleogene period. The land that we now call the Panama Isthmus had not yet joined North America and South America, so water could flow freely between the Pacific and Atlantic Oceans. The passage known as the Drake Passage, which now separates South America and Antarctica, was closed back then, possibly helping keep Antarctica warmer. The Arctic region was also more limited in size.

During this time, Earth’s temperature was gradually getting warmer. Scientists think that the average temperature rose by about 6 °C (11 °F) from the late Paleocene to the early Eocene. This warming happened over a long period, but there were also shorter times when temperatures jumped even more — these are called "hyperthermals." One of these big jumps is known as the Paleocene–Eocene thermal maximum (PETM).

Studies from the year 2020 suggest that before the PETM, Earth’s average temperature was between 22.3 °C and 28.3 °C (72.1 °F and 82.9 °F). During the PETM, it rose to between 27.2 °C and 34.5 °C (81.0 °F and 94.1 °F). Scientists know about this temperature rise from several clues, like changes in the shapes and sizes of ancient plants, and the types of tiny sea creatures that lived at the time.

In some places, like Esplugafereda in Spain, temperatures shot up by about 8 °C (14 °F). In parts of China, temperatures went from 15.6 °C to 19.7 °C (60.1 °F to 67.5 °F). Even in Antarctica, some areas got as warm as 15 °C during parts of the year.

The ocean also got much warmer. In the tropics, sea surface temperatures may have reached over 36 °C (97 °F), which is very hot even for tough ocean creatures. Some places in the ocean, like near Tanzania, might have been even hotter — perhaps over 40 °C (104 °F). In the Pacific Ocean, tropical sea surface temperatures rose by about 4 °C to 5 °C. In the southwestern Pacific, near what is now New Zealand, temperatures jumped by more than 10 °C (18 °F). Even the Arctic was much warmer than today, with no ice covering the central Arctic Ocean.

Carbon cycle disturbance

Scientists found strong signs that a lot of carbon, which lacks a special type of atom called ¹³C, entered the world during the PETM. They saw this in two main ways. First, they noticed a big change in a special measurement of carbon called δ¹³C at many places around the globe. Second, they found that in deep parts of the ocean, some substances made of carbon dissolved during this time.

We still don’t know exactly how much carbon entered the oceans and air during the PETM. Some scientists think it was between 2,000 and 7,000 gigatons, but this number depends on where you look and what you measure. The change in δ¹³C was different in different places, and how much the ocean substances dissolved also changed from one ocean area to another. It was much more noticeable in parts of the north and central Atlantic Ocean than in the Pacific Ocean.

Timing of carbon addition and warming

The timing of changes in a special kind of carbon during the PETM is very important. This is because it helps us understand how our world's carbon cycle works and where the carbon came from.

We can figure out how long the PETM lasted in several ways. One key place to study this is a core taken from the ocean floor in 1987 by the Ocean Drilling Program at a spot called Maud Rise in the South Atlantic Ocean. At this spot, the change in carbon from the start to the end of the PETM covers about 2 meters of sediment. By studying fossils and magnetic patterns in the sediment, scientists think the sediment built up at a rate of about 1.23 centimeters every 1,000 years. This suggests the whole event lasted around 200,000 years. Other studies using different methods have found similar or slightly shorter times, around 170,000 years.

Models of how carbon moves around the world also support a duration of about 200,000 years for this event. Some studies using a special kind of helium also point to a quick start and a faster return to normal conditions for the carbon changes.

Effects

Precipitation

The climate became much wetter, with more evaporation happening especially near the equator. More moisture reached places like the Arctic. Warm weather stretched far north. Fossils of floating ferns found in polar areas show that temperatures there were much warmer. East Asia got wetter, with China having dense forests and lots of rain. Central Asia also became wetter, and India saw more layers of soil from plants breaking down. In the Arctic and North Sea, rain increased. In some places like Normandy, there was a short dry period followed by much wetter conditions. Along the New Jersey Shelf and the Gulf Coast in Texas, rain patterns changed. In the Rocky Mountains, it got drier, while California had less rain overall but more in the summer. East Africa had dry spells with heavy rains at times. Italy saw alternating wet and dry periods.

Ocean

The Arctic Ocean had more freshwater from increased rain, especially in the Northern Hemisphere. A lot of freshwater flowed into the oceans, continuing after the PETM ended.

Anoxia

The PETM created the only oceanic anoxic event in the Cenozoic. Oxygen levels dropped because of warmer water, less mixing of water layers, and methane breaking down. In some ocean areas, especially the North Atlantic, there was no mixing of ocean layers. This might be because of low oxygen at the bottom or changes in ocean currents. In places like the Arctic and Tethys Oceans, oxygen levels were very low. The North Sea Basin and Gulf Coastal Plain also had low oxygen. The Tasman Sea saw less oxygen too. Tropical surface oceans stayed oxygenated.

It’s possible that low oxygen helped slow warming by burying carbon. More weathering and erosion happened, burying more carbon and helping cool the planet.

Sea level

With less ice and warmer water expanding, sea levels rose. In the Arctic Ocean, plant material changed, showing less land material and more sea material. In the Tarim Sea, sea levels rose by 20-50 metres.

Currents

Ocean currents changed quickly at the start of the PETM, reversing directions in under 5,000 years. This lasted for 40,000 years, moving warm water deep into the ocean and causing more warming. Changes in deep-sea creatures show this big shift in ocean currents.

Acidification

The ocean became more acidic during the PETM, with seawater pH dropping by about 0.46 units. This made it harder for creatures to build shells. Deep ocean waters became more acidic, especially in the North Atlantic. This acidity spread to other parts of the ocean. Acidification might have helped slow down carbon buildup by changing how carbon moved in the ocean. Some tiny ocean plants grew more shells in acidic water, helping balance the ocean’s chemistry.

Life

Tiny magnetic particles found in ocean sediments from the PETM show unique shapes, possibly from tiny ocean creatures. These particles grew larger, suggesting more iron-rich areas in the ocean.

Ocean

Shallow ocean areas saw big changes in tiny shell-making creatures. Many deep-water shell-making creatures died off, with 35–50% disappearing over about 1,000 years. In the Pacific Ocean, these creatures’ variety dropped by 30%, and in Spain, 55% died out. Some areas didn’t see as big a drop. Tiny shell-making creatures in the open ocean changed in size, maybe because of less food or warmer water. Some tiny creatures with special partners grew more. Many shell-making tiny plants died, but new ones appeared. In the Kerguelen Plateau, tiny plant growth dropped at first but went up later. Some tiny plant groups disappeared.

In the Atlantic Ocean, a special tiny plant group grew a lot. Radiolarians, another tiny ocean creature, grew bigger. Coral reefs, which need warm water, declined and were replaced by other shell-makers. Corals struggled because of more acidic water and extra nutrients. Deep-sea areas are hard to explain because many deep-sea creatures live in many places. Some think warmer water used up oxygen, or more acidic water hurt shell-makers. Warmer water might have made these creatures need more food, which wasn’t available because less food reached the deep sea. Overall, warmth was the big change. In the North Atlantic, low oxygen in deep water might have caused extinctions.

In shallower waters, more carbon dioxide made the ocean less acidic, hurting corals a lot. Some tiny shell-makers might grow more shells in acidic water, but overall, changes in food and temperature mattered more. One tiny shell-maker did well because of more nutrients running off the land. More nutrients caused a drop in certain big shell-makers, but they came back later.

A study found that some fish did well in tropical areas during the PETM, but a group of pufferfish died out.

Land

More mammals appeared and many got smaller because of the warmth. Bigger mammals shrank the most, while smaller ones like certain spiky mammals didn’t change much. New groups of mammals like hyenas, even-toed hoofed animals, odd-toed hoofed animals, and primates appeared and spread around the world. Some older mammals declined. Humid conditions helped Asian mammals move north. It’s unclear exactly when these movements happened. Some think India was a center for mammals spreading to Africa and other continents, but others think mammals moved into India.

Lizards moved within continents, but not between them. Snakes in the genus Cheilophis spread during the PETM. Large bird-like dinosaurs called gastornithids didn’t change much in size.

Insects that eat plants increased, possibly because of warmer weather. Ants spread across Eurasia. Like mammals, tiny land creatures also got smaller. Plants changed a lot all over the world. The Arctic had palm trees and broadleaf forests. The Gulf Coast of Texas had tropical rainforests. Freshwater animals died off because of harmful algae blooms caused by the extreme heat.

Geologic effects

Sediment layers changed a lot during the PETM. More kaolinite, a type of clay, was found in sediments from weathering of older soils. Increased rain and erosion washed more material into rivers. In Spain, river systems grew and more sediment was laid down about 3,800 years after the PETM started.

In some deep ocean areas, less sediment was laid down because shells dissolved on the seafloor. In shallow ocean areas, more sediment was laid down because rivers brought more material.

Possible causes

It is hard to tell what caused the Paleocene-Eocene thermal maximum (PETM) because many things were happening at once. Temperatures were slowly rising, and something must have pushed them up very quickly.

One idea is that big volcanic eruptions released lots of carbon into the air. Another idea is that changes in Earth’s orbit caused the warming. Some scientists think a comet hitting Earth might have started it all.

There are also theories about methane gas trapped under the ocean floor being released when the water got warmer. All these different ideas try to explain how the PETM began and what kept it going for thousands of years.

Recovery

After the PETM, the Earth slowly began to cool and return to normal conditions. Scientists think this recovery took about 83,000 years, with the first 33,000 years being a quick change and the next 50,000 years a slower one.

One big reason for this recovery was that more plants and tiny sea creatures grew. They took in carbon from the air and water, helping to lower the amount of carbon in the environment. Warmer temperatures and more rain helped these plants grow better. Volcanoes may have also added nutrients to the soil, helping plants grow. In some places near the shore, warm water and nutrients from land helped many plants grow. There is also evidence that a special type of water plant called Azolla, found in the Arctic Ocean, may have helped by trapping carbon in the mud. Over time, these changes helped the Earth cool down again.

Comparison with today's climate change

Scientists study the PETM to better understand how our planet might change with today's global warming. One big difference is that during the PETM, Earth had no ice because important ocean passages were still open. Even though the PETM is often used as an example to study climate change, we still don’t fully know what caused it or how important it was.

During the PETM, carbon was added to the atmosphere more slowly than it is today. Scientists think the rate was between 0.3 and 1.7 petagrams of carbon each year, much slower than the over 10 gigatons we add each year now. Some believe that methane release from the ocean today might be similar to what happened during the PETM. Because we are adding carbon faster now, some worry that the effects could be even worse than what happened during the PETM.

Experts say that the PETM shows there are points where Earth’s climate can suddenly change, leading to even more warming. There is also debate about how sensitive Earth’s climate was during the PETM compared to today. Some studies suggest that large ancient seas helped slow down warming back then, while others think the climate was more sensitive to greenhouse gases.