There is ample evidence that the Triassic was a dry period with huge temperature swings. The splitting up of Pangea in the Jurassic meant that larger parts of the Earth's surface became covered by sea, and larger areas got close to the sea. Temperatures continued to rise, humidity increased with the proximity of the sea, and deserts receded. In the unique climate of the Cretaceous, the world's temperature gradient from north to south was almost flat. Temperatures were about the same all over the planet. It was much warmer than today, at least 10-12℃ warmer.
Extensive volcanic activity in connection with the splitting of Pangea caused ten times as much CO2 in the atmosphere in the last half of the Jurassic and in the Cretaceous period as compared to earlier periods.
The dinosaurs evolved into animals of enormous size. Which necessarily must be because the combination of humidity, heat and a high content of CO2 in the atmosphere allowed an exceptionally large organic production both on land and in water, which created food for these enormous creatures.
It is commonly believed that a giant meteor that crashed near the Yucatan Peninsula in Mexico caused a global winter that wiped out all the huge animals at the same time. But it can in no way be ruled out that they slowly starved to death, so to speak, because both the temperature and the atmosphere's CO2 content gradually fell and thus also the large organic production, which was their food base.
1. Earth's middle ages
The Phanerozoic is the name given to the part of Earth's history where tangible life was visible. It is divided into Paleozoic, Mesozoic and Cenozoic, which are popularly called the Earth's antiquity, middle ages and recent times. The Mesozoic is the subject of this article.
Time progresses from right to left. In Hadal, the Earth's surface was a glowing chaos, marked by intense radioactivity and an atmosphere of toxic gases. In the Archean, the Earth's surface was solidified, the atmosphere consisted of nitrogen and methane, and the first cyanobacteria appeared. In the Proterozoic, methane and iron were oxidized and an atmosphere of oxygen was formed.
The Phanerozoic refers to the part of Earth's history in which visible tangible life existed, from the appearance of trilobites at the beginning of the Cambrian until the appearance of man and historical times.
The Phanerozoic is divided into the Paleozoic, Mesozoic and Cenozoic. The Paleozoic was the era of early life where plants, insects, fish, molluscs, corals and many more living organisms evolved. The Mesozoic was the age of the dinosaurs and described in this article, and the Cenozoic is the age of the mammals.
The Mesozoic is further divided into the periods Triassic, Jurassic and Cretaceous.
The start of the Mesozoic at the beginning of the Triassic was characterized by a harsh and dry continental climate on the giant Pangea continent.
The breakup of Pangea in the Jurassic paved the way for a hot and humid greenhouse climate with little temperature difference between low and high latitudes. The temperature in the Cretaceous was even higher, and most of the world was probably covered by dense and humid forests.
Atmospheric oxygen content through the Phanerozoic according to Robert Berner Yale University - but added the geological periods. Time progresses from left to right. Already at the end of the Proterozoic there seems to have been a considerable oxygen content in the atmosphere.
The Sun is a common yellow dwarf star in the main series of the Hertzsprung-Russell diagram. It will stay there for about 11 billion years, during which time it will increase its brightness three times overall. At the start of the Mesozoic, about 250 million years ago, the Sun had a brightness of about 97% of its current brightness. At the end of the Mesozoic 65 million years ago, the Sun had reached a good 99% of today's brightness.
According to Robert Berner of Yale University, the oxygen content of the atmosphere has varied between 16% and 27% during the Mesozoic.
Atmospheric content of CO2 in the Phanerozoic according to Robert Berner Yale University - added to the geological periods. Time progresses from left to right. It can be seen that there was a maximum in the Cambrian, and thereafter it has been decreasing except for a minimum in the Carboniferous period. There are several reconstructions of the CO2 content of the past atmosphere, all of which are different. But the trend is the same: for the vast majority of time, the concentration has been much higher than in today's atmosphere.
From the middle of the Triassic and a few million years into the Jurassic, the oxygen percentage of the atmosphere had dropped to a low point for the entire Phanerozoic of around 15-17% and then rose again to a maximum of around 27% at the transition from the Mesozoic to the Cenozoic.
Throughout the Mesozoic, the content of CO2 in the atmosphere has been significantly lower than in the Paleozoic, but still between two and nine times as high as in today's atmosphere.
The average temperature of the globe in the Phanerozoic according to Anton Uriarte - however, the geological periods have been added. Time progresses from right to left. It can be seen that it has generally been slightly increasing until the middle of the Cretaceous period, after which it fell towards the Pleistocene ice ages. Furthermore, the graph has some similar minima at the other two ice ages in the Phanerozoic, namely the Andean-Saharan Ice Age at the transition between the Ordovician and Silurian, and the Karoo Ice Age in the late Carboniferous and early Permian.
The temperature of the past has been calculated based on the analysis of sediments on the bottom of the Arctic Ocean, as it is the case that the occurrence of certain oxygen isotopes depends on the temperature, when the sediment was formed. Throughout the Mesozoic, the globe's temperature has been significantly higher than the modern global average temperature of just over 14℃. It has been suggested that during the Cretaceous maximum the global average was around 20℃. For comparison, Denmark's average temperature today is 8℃.
2. Triassic
The first part of the Triassic was characterized by the supercontinent Pangea, which had already formed in the Permian period. In a relatively short period seen from a geological point of view, all the Earth's continents were united in a single gigantic land area.
The Triassic lasted from 248 to 213 million years before the present, a total of 35 million years.
Pangea was shaped like a C, the bay on the right side of the C is called the Tethys Sea, and the rest of the World Ocean is called the Panthalassic Sea. The Mediterranean, the Black Sea, the Caspian Sea, the Aral Sea, the Red Sea and the Persian Gulf are today the last remnants of the Tethys Sea. Photo user:Kieff Wikipedia.
The name Pangæa is derived from the Latin "pan", which means something all-encompassing, and the Greek "gaia", which is a name for the Earth, Pangæa thus means "the whole Earth". It was formed in the Permian period, but already in the Triassic it began to split, which is the process that has formed the Earth's current continents and determined their location until now.
Pangea was characterized by extreme climate differences because it was such a huge continent. The coasts around the equator had a monsoon climate with rain in the summer. The interior of the country, on the other hand, was dry and desert-like.
The interior of today's Eurasia, for example China's western Xin Jiang province or Mongolia has a typical continental climate. Summers are incredibly hot with temperatures of 40 ℃or more and winters are freezing cold with double digit minus temperatures. The reason is that Eurasia is such a large continent with a long distance from its interior to the sea's water reservoir with its high heat content.
Pangea was a much larger continent than Eurasia, and it is assumed that it had an even more distinctly continental climate. There can be no doubt that a winter in central Pangea must have been a very harsh experience, and the summers must have been blazing hot.
There were no real ice ages in the entire Mesozoic and therefore not in the Triassic either.
A team of researchers from the American Brown University has prepared a climate description for Pangea from the Triassic period using samples taken from the seabed and lakes from Nova Scotia to Georgia. They concluded that Pangea's summer temperature in the Triassic was 20℃ warmer than today, and the atmosphere's CO2 content was five to twenty times greater than today.
Reconstructed Pangean landscape from the Triassic - Painted by Mauricio Anton.
The monsoon is a steady wind that blows from sea to land or from land to sea due to temperature differences between land and sea.
The sun heats both land and sea in summer, but the temperature of forests, deserts and grasslands rises faster than the temperature of the sea with the same solar radiation. Rocks and soil have a poor thermal conductivity and small heat capacity, and therefore the temperature on land rises quickly. Water, on the other hand, can absorb much more heat from the same solar radiation with less temperature rise, because water has a good thermal conductivity and high specific heat capacity, and furthermore the heat will be quickly distributed to the deeper layers of the sea by currents and waves.
When land masses are heated in the summer, warm air will rise into the air and thereby create a low pressure over land. The warm moist air over the sea will flow in to seek to fill this low pressure and this wind is the monsoon. In winter, the sea is warmer than the land, and the monsoon then blows from the land and over the sea.
The modern Southeast Asian monsoon - In principle, monsoon winds can occur anywhere where land meets sea. However, the Southeast Asian monsoon is the best known because here a very large continent meets a very large and warm sea. Pangea was an even larger continent bordering an even larger ocean, which must have been the cause of an even stronger monsoonal wind. Photo Quora.
The larger the land masses bordering the larger the sea, the more pronounced the monsoon winds will be. In principle, there can be monsoon winds all over the Earth, but the Southeast Asian monsoon is the best known because a very large continent borders a very large and warm sea.
Pangea was the largest continent ever, and it was surrounded by an equally huge ocean, the Panthalassic Ocean, which covered the entire rest of the Earth's surface. Therefore, it is believed that the coasts of Pangea must have been exposed to even very strong monsoon winds.
Today, the interior of the World's continents is dry and desert-like. The Sahara and Kalhari extend into the interior of Africa, the Gobi and Taklamakan are found in interior Asia, and the Great Sandy Desert fills the interior of Australia. Pangea was an even larger and even more massive continent, and therefore one must imagine that the interior of the supercontinent was even more filled with endless, barren and windswept deserts.
However, there were also forests in the southern part of Pangea, they consisted of various conifers.
When a moist wind blows from the sea and inland, it will release its rain when it is forced up over mountains, as we know it today from, for example, Western Norway. But the original continents that made up the Pangea of ??the Triassic period were still largely flat as pancakes. There were only very few and low mountains, such as the Appalachians and the Ural Mountains, and therefore one must assume that winds from the sea were not forced to deliver their rain to the same extent as today. For this reason, one must assume that there were very large areas in Pangea with desert and dry steppe. Around the equator, however, there were smaller areas with tropical vegetation, it is said.
Sediments formed by evaporation of water are very common in Triassic strata. They may have formed by lagoons being isolated from the sea due to the formation of sand dunes, then simply drying out in the rain-poor climate. Inland lakes may exhibit several layers of sediments formed by evaporation, they appear to have dried up several times.
Reconstruction of Hyperodapedon, which was a type of Rhynchosaur. Photo Nobu Tamura Wikiedia.
In Ischigualsto in Argentina, near the border with Chile, important finds from the Triassic period have been made. Giant petrified tree trunks, which have been named Protojuniperoxylon ischigualastianus, more than 40 meters high, testify to a lush vegetation in this place. Fossil ferns and toads have also been found.
Dinosaurs are, perhaps surprisingly, neither the most numerous nor the largest group of animal species from Ischigualasto. Only about 6% of all four-legged animals found there are dinosaurs. Fossilized bones of Rhyncosaurs and Cynodonts are far more common. This shows that dinosaurs were not an immediate success when they first appeared in the Triassic.
Rhynchosaurs from the Triassic have been found all over the world. They were stocky herbivores with short legs and a powerful beak. Later species developed a deep lower jaw, and when they closed their mouths, the upper jaw was clamped down into the lower jaw, like the blade of a folding knife is clamped down into the handle. This scissor-like motion probably enabled Rhynchosaurs to eat tough plant material. They were generally quite small, although some species could grow up to two meters long.
Reconstructed skeleton of the Cynodont Belesodon Magnificus in the "Staatliches Museum f r Naturkunde Stuttgart". Cyno-dont means dog-tooth. Photo Ghedoghedo Wikipedia.
Cynodonts had almost all the characteristics of mammals. Their teeth were fully separated, their brains bulged out at the back of their heads, and many of them were said to be able to stand upright. Like probably all other Mesozoic proto-mammals, cynodonts laid eggs. They are believed to be warm-blooded and covered in hair. Some cynodont species could grow a meter long.
At the end of the Triassic, Pangea began to crack. Between North America and Africa, volcanoes spewed large quantities of alkaline lava, which can be traced on both continents. It is believed that this volcanic catastrophe initiated another round of extinction of up to 80% of the Earth's species, thereby paving the way for the dinosaurs, who came to dominate the Earth for the next hundred million years.
Volcanic activity is always associated with large amounts of carbon being released from the Earth's interior into the atmosphere in the form of CO2, carbon dioxide. In fact, by far most of the CO2 in the atmosphere originally come from volcanic activity. Therefore, we must assume that the increase in atmospheric CO2 in the Triassic and Jurassic, which Robert Berner has demonstrated, came from extensive volcanic activity in connection with the break-up of Pangaea.
The greatly increased amount of CO2 in the atmosphere in the Triassic and Jurassic was a prerequisite for the luxuriant fast growing vegetation that came to cover the entire Earth in the last half of the Jurassic and the entire Cretaceous period, and which made it possible for such huge animals as the dinosaurs to develop and find food.
3. Jurassic
Jurassic began 213 million years before the present and lasted until 144 million years before the present, a total of 69 million years.
The world in the Jurassic period - Pangea has disintegrated. Photo scotese.com.
The gradual break-up of Pangea, which began at the end of the Triassic, continued in the Jurassic period. The climate of the new and smaller continents became milder and rainier. Warm and humid tropical breezes blew through dense forests of ferns, conifers, ginkgo trees and various conifers.
As larger parts of the Earth's surface were covered by water and green plants, the planet's albedo decreased and more of the Sun's energy was absorbed as heat.
The fact that many ferns and gymnosperms are known from both polar regions shows that the climate in the Jurassic was generally warm and without great geographical variations. Fern species, whose modern relatives do not tolerate cold, were also widespread in northern latitudes. In the opinion of some Russian researchers, the fauna found indicates that the winter temperature in Siberia never fell below freezing. Fossils of crocodile-like lizards, whose modern relatives can only live in subtropical or tropical areas, have been found in Greenland, which also confirms that the climate was hot and humid over almost the entire globe.
The Jurassic petrified forest at Curio Bay in New Zealand. Photo Karora Wikipedia
Coral reefs from the Jurassic period have been found on the Sakhalin peninsula at 60 degrees north latitude, which is 30 degrees further north than where coral reefs are found today, as coral reefs require a minimum temperature of 20℃ to grow.
Findings of coal all over the globe, which were formed in the Jurassic period, also support the theory of the warm and humid climate over almost the entire Earth.
In a belt around the equator, which stretches 10-20 degrees north and south of the equator, you can however find layers of gypsum and salts, which originate from dried up lakes and sea bays, and testify to a fairly dry climate in these areas. Studies in the "Lower Jurassic Navajo Sandstone" of southwestern Utah testify to sand dunes in a dry windswept climate interrupted by heavy summer monsoon rains.
In the Jurassic, the disintegration of the supercontinent Pangea continued. Laurasia, the northern half, split into North America and Eurasia. Gondwana, the southern half, began to break up in the middle of the Jurassic. The eastern part, which consisted of Antarctica, Madagascar, India and Australia, split from the western part, which was Africa and South America. New oceans filled the spaces between the new continents. Mountains rose from the sea floor and pushed the water level of the World Ocean higher.
World ocean water levels in the Phanerozoic. As you can see, there are different opinions about when Pangea started to break up. Own work.
During the Jurassic period, larger parts of the Earth's surface than before were covered by oceans. Immediately, we would describe this as an increase in the water level of the world's oceans with the melting of the ice caps at the poles. Perhaps there have been smaller ice sheets in southern Pangea without an actual ice age. These smaller glaciers may have melted during the Jurassic and thereby in the short term - a few million years - contributed to the increased water level in the world ocean. But this alone cannot explain the apparent rise in world sea level.
Not all changes in the relationship between sea and land masses The water level of the world's oceans is connected with the formation or breakdown of the polar ice caps. Other, more long-term - hundreds of millions of years - mechanisms have been proposed which have contributed to changing this relationship.
The total volume of water on Earth must have been fairly constant over the past four billion years. Truly long-term changes in the relationship between the world's oceans and land masses for example, over a hundred million years must have been caused by changes in the shape of the ocean basin due to plate tectonic activity. Much like if we drop a brick into a bucket of water and see the water level rise.
When a supercontinent forms, some tectonic plates will be pushed under others, thereby reducing the volume of the landmass relative to the volume of the "water reservoir", thus causing the water level in the world's oceans to appear to fall. For example, it is believed that the fact that over the last 50 million years the Indian subcontinent has pushed itself under the Asian plate, thereby creating the Himalayan Mountains and the Tibetan Plateau, has been the cause of an apparent drop in sea level of approx. 70 m. through the last 50 million years - all other things being equal.
Formation of the Mid-Atlantic Ridge. own work.
Conversely, when a large continent is divided into several smaller ones, the length of the total coastline will increase. The volume of the continental shelves along the coastline, which had previously been depressed under another continent, will thus also increase, and the amount of seawater displaced by this will be the cause of apparently increased water levels. It is believed, for example, that the division of Pangea into several smaller continents has caused a changed relationship between sea and land masses, which can be interpreted as an increase in the level of the World Ocean of 100 m. over the last 200 million years.
When two tectonic plates move away from each other, it will cause volcanic activity and thus the associated emission of lava and the formation of volcanic rocks, which will displace seawater and thus cause an apparent increase in water levels in the world's oceans.
Thus, over the past two hundred million years, North America and South America have moved away from Europe and Africa at a rate of 2.5 cm. annually. The Mid-Atlantic Ridge is an undersea mountain range created by this volcanic process. The volume of this mountain range displaces sea water and thereby causes - other things being equal - an apparent rise in the level of the World Ocean.
The Mid-Atlantic Ridge stretches from Antarctica to Jan Mayen. The Azores, Iceland, Jan Mayen, and Saint Peter and Paul Rocks off Brazil were all created by volcanic activity associated with the creation of this undersea mountain range.
Basically, you cannot talk about decreased or increased water levels in the world's oceans without a fixed reference point or reference level, and since both the sea basin and the land masses have constantly changed, it is obvious that you do not have such a point or level.
But in any case there is a lot of evidence that a larger part of the Earth's surface was covered by sea during the Jurassic, which meant that many low-lying areas were transformed into shallow coastal areas, and at the same time the climate became more oceanic, i.e. wetter, warmer and with far less seasonal variation.
Brachiosaurus. Own work.
There is data indicating a shorter period of intense heat, about 183 million years before the present (Uriarte).
On land, the picture was totally dominated by the dinosaurs. The herbivorous sauropod Brachiosaurus, for example, could grow up to 16 m. high, 26 m. long and achieve a weight of over 50 tons. Another sauropod, which has been named Diplodocus, could be 27 m long. The sheer size of these giants may have protected them from attack by the predatory dinosaur Allosaurus.
Fossil of Archeopteryx lithographica found at Solnhofen in Germany. Photo Wikimedia Commons.
Modern gardeners can make their plants grow faster by increasing the CO2 content in the greenhouse's atmosphere. In the atmosphere of the Jurassic period, the CO2 content was 5-8 times greater than in today's, and this may explain how the fauna could produce enough biomass to feed such gigantic creatures.
There are different opinions about whether dinosaurs were cold-blooded or warm-blooded. But there are many indications that the birds are their descendants, and they are warm-blooded after all.
Reconstruction of Archeopteryx lithographica. Photo Pinterest
In the Jurassic, the birds began to compete with the flying lizards for air space. The earliest known bird is Archeopteryx lithographica, which was found in Jurassic limestone near Solnhofen, Germany.
Archeopteryx was the size of a crow with short, broad wings and a long tail. It had wings and feathers like today's birds, but also many features in common with the dinosaurs, for example jaws with sharp teeth, a head covered with scales and a long bony tail.
The dinosaurs and modern birds have several common characteristics that provide arguments for the birds being descended from the dinosaurs. It is about the porous air-filled bones, the typical bone that we call the "wishbone", they have the three toes and a "spur" on the "hind legs" in common, it has been proven that many dinosaurs had feathers - like the birds. Both birds and dinosaurs reproduce by laying eggs.
4. Cretaceous
Møns Klint - millions of small shellfish lived in the Cretaceous sea, died and sank to the bottom, thereby forming the enormous chalk layers that are found in the subsurface of Denmark and England from Møns Klint to "The White cliffs of Dover". Photo unknown origin.
The Cretaceous period began 145 million years ago and lasted until the demise of the dinosaurs 65 million years ago. During this period, a hot and humid climate prevailed almost everywhere on Earth. The thick layers of lime, created by microscopic calcareous algae that abounded throughout Earth's oceans, gave the period its name. Until then, lime deposits had been limited to shallow coastal waters.
At the middle of the Cretaceous period, about 100 million years ago, the average temperature on the planet's surface was between 6 and 12℃ higher than it is today. The annual mean temperature in the Arctic regions was approx. 10℃, which is about 20℃ warmer than today. In the tropical areas, the temperature in the surface water was approx. 5 to 10℃ higher than today.
Left: Lilac has smooth-margined leaves. Photo Nursery.
Right: Birch is not smooth-margined. Smooth-margined leaves on indigenous trees in Denmark is very rare. Photo unknown origin.
The Americans Irving Bailey and Edmund Sinnott observed in 1915 that there is a correlation between the number of plant species with entire margins in an area and the climate of that area. Then the American geologist Jack Wolfe showed that this more specifically implies a correlation between the proportion of whole-edged leaves and the annual mean temperature of the area.
A large number of different leaves have been collected from original forests from all over the Earth, and a correlation has been established between the distribution of leaf types and the average temperature of the area; the uncertainty is plus or minus 4-6℃, depending on how many leaf types have been collected.
The temperature as a function of the proportion of whole-edged leaves - From 2006 Geoviden - Geology and geography no. 4.
This relationship can also be applied to fossil leaves. Over time, Danish geologists have found fossil leaves from 87 different plant species from the Cretaceous period on the island of Disko off the west coast of Greenland. Of these, 20 species are fully rimmed, which means 23%. By entering the curve, you can see that the average annual temperature in West Greenland during the Cretaceous period must have been around 9℃. In modern times, Disko Island's annual average temperature fluctuates between -2 and -9℃. The modern Danish annual average temperature is 8℃. In the Cretaceous period it was therefore warmer on Disko island than it is in Northern Europe today.
Analyzes of Cretaceous fossil leaves found in northern Alaska and
Russia shows that the annual average temperatures in these areas were around 9-12℃, and that the coldest month had an average temperature of approximately 5℃, which fits very well with the Greenlandic temperatures found.
However, there are also other methods for calculating past temperatures.
There are two isotopes of oxygen, namely oxygen-16 and oxygen-18. Almost all oxygen atoms on Earth have 8 protons and 8 neutrons and thus an atomic weight of 16. But 0.2% of oxygen atoms have 10 neutrons and thus an atomic weight of 18. Water molecules that contain oxygen-16 evaporate more easily than water with the heavier oxygen - 18.
In a warm period, the evaporated water will fall as rain or snow over land and rather quickly find its way back to the sea through rivers, so that the sea retains its original isotopic distribution.
But during a cold period, some of the evaporated water will fall as snow on glaciers and ice sheets and remain there, and therefore glacier ice will become poor in oxygen-18, while seawater will similarly become relative ice rich in oxygen-18.
In the oceans of the Cretaceous period lived a lot of microscopic algae that had a shell of chalk. When they died, they sank to the bottom of the sea. Their shells can still be found. The shells still contain the relative distribution of oxygen isotopes, which reflects the isotope distribution in the seawater at the time when the algae lived. It is possible to analyze the microscopic shells with regard to their isotopic distribution and in this way get an indication of the temperature of the Cretaceous ocean.
Left: Axel Heiberg Island. NordNordWest Wikimedia Commons.
Right: Cretaceous turtle fossil found on Axel Heiberg Island. The turtle is named Borealochelys axelheibergensis. Fossils from the dinosaur Plesiosaur and the crocodile Champsosaurus were found at the same site. Photo Research Gate uploaded by Donald B. Brinkman.
The "Cretaceous Research Centre" under the Geological Institute has taken samples from Stevns Klint and determined the temperature of the Cretaceous Sea to have been 14-22℃, which is considerably higher than today's in today's sea. The sea temperature in today's Denmark only reaches 16℃ in the summer months.
Left: Plesiosaur teeth found on Axel Heiberg Island. The scale is in cm. Research Gate uploaded by Donald B. Brinkman.
Right: Plesiosaurs painted by Heinrich Harder.
Animals and plants that today are typical of a warm climate, such as crocodile-like animals, were able to live at very high latitudes during the Cretaceous period. Finding fossils of million-year-old trees, sea turtles and the crocodile-like Champsosaurus from the Cretaceous period on Axel Heiberg Island in northernmost Canada is an even stronger indication that it was very warm indeed during the Cretaceous period, when today's crocodiles prefers water temperatures between 25 and 35℃.
Fossils of the crocodile-like Champsosaurus were also found on Axel Heiberg Island. North Dakota Heritage Center. Photo 2.bp.blogspot.com
Layers from Greenland have been found to contain fossils of leaves from the breadfruit tree, a plant which today only grows in a humid tropical climate.
Coal forms in a warm and humid climate, and coal deposits formed in the Cretaceous period are found all over the Earth at all latitudes, showing that it was indeed very hot all over the Earth. The large surface deposits that are exploited today in the western United States were thus formed in the Cretaceous period.
Formation of bauxite requires a warm tropical climate with regular seasonal rainfall, and bauxite from the Cretaceous has also been found at fairly high latitudes including Greenland. Which in turn suggests that a hot and humid climate prevailed almost everywhere on the globe.
Left: Petrified Ginko leaf from the Cretaceous period. Photo Fossilera.
In the middle: Petrified leaf from a breadfruit tree found in Vestrønland. Photo Geoviden 4.
Right: Fossilized leaf from the Cretaceous period, which looks like a leaf from a beech. Photo unknown origin.
Flowering plants reach their maximum photosynthetic efficiency at a CO2 level between 1,000 and 1,500 ppm. Flowering plants were at their peak during this period, making it likely that CO2 levels were indeed very high compared to today.
The determination of the CO2 content in the past atmosphere can be done in two ways.
Trees absorb CO2 for their photosynthesis through slit-openings on the underside of the leaves called stomata. Pressed and dried leaves collected from old herbariums over the last two centuries seem to indicate a decrease in the number of stomata openings on the underside of the leaves in recent times, when the CO2 content of the atmosphere is known to have increased slightly. Therefore, it is assumed that the number of stomata openings is inversely proportional to atmospheric CO2, as the trees seek to minimize the necessary number of openings to limit evaporation.
With the stomata method, the researchers will analyze the underside of petrified leaves and determine the number of stomata openings.
Microscopic stomata openings on the underside of a leaf. Photo Quora.
The density of stomata openings on fossil leaves from ginkgo trees shows that the CO2 concentration in the Cretaceous atmosphere was 4-5 times, and sometimes up to 7 times, higher than today.
Another way to determine the CO2 content of the past atmosphere is by analyzing the content of the carbon-13 isotope in layers from this time, as the level of carbon-13 varies with atmospheric CO2.
The high atmospheric CO2 concentration of the Cretaceous period was created by emissions from volcanic zones on the sea floor in connection with the continental plates that had made up Pangea starting to move apart. A process which started already at the end of the Triassic. The Caribbean Islands in the Atlantic, the Kerguelen Islands in the southern Indian Ocean, and the submarine Ontong Java Plateau near the Solomon Islands in the Pacific Ocean are all widespread oceanic basalt plateaus that were created in the mid-Cretaceous period by volcanic activity.
The Ontong Java Plateau is a huge oceanic plateau of Cretaceous volcanic origin, located in the Pacific Ocean north of the Solomon Islands and New Guinea. The plateau covers an area of ??about 2,000,000 km2, about the size of Alaska, and is said to be several kilometers thick. All the drilled samples from the sea floor appear to be chemically similar, and they are all of about the same age - about 122 million years old. The fossils found in the samples also appear to be from the same geological period, the Cretaceous. This is why scientists are reasonably certain that the Ontong Java Plateau was formed in a single, massive eruption of volcanic magma. The total volume of lava spewed would have been a million times greater than the largest volcanic eruption in recorded history. Map Anton Uriarte.
Some of the dinosaurs were of simply enormous size. One might think that the existence of these gigantic animals was only possible due to an abundance of biomass, created by a warm and humid climate and a high CO2 content in the atmosphere.
Fossilized bones of the sauropod Argentinosaurus were found in Argentina in South America. It is believed to have been 40 meters high with a weight of up to 100 tons, it is said. In any case, it has been a truly gigantic animal. As you can see, its brain was quite small, it must have been a huge eating machine that roamed lakes and coastal waters. One has to believe that the high CO2 content in the atmosphere has enabled the growth of all the biomass needed to feed such giants. Photo Fernbank Museum of Natural History Atalanta Georgia.
Today, the greenhouse effect created by the atmosphere's water vapor content is limited to tropical and subtropical regions. Because of the cold, the absolute humidity at higher latitudes is very low; for example, the water vapor content in the air in winter in Antarctica is about 0.00001%, while humid regions at the equator can have an absolute humidity of up to 4%.
In the Jurassic and Cretaceous periods, the air at the poles was much warmer, and the absolute humidity on the whole Earth was generally much higher than today, and consequently the greenhouse effect of water vapor was also considerably greater.
Reconstructed Map of the World in the Cretaceous period - Europe was an archipelago and the West Siberian Lowlands and the Mississippi Valley were shallow seas that connected the Arctic Ocean with the World Ocean. North Africa was also flooded. Photo scotese.com
An important geographical reason for the hot and humid climate of the time was the distribution of seas and land masses. The break-up of Pangea into smaller and smaller continents favored a mild coastal climate, which was very different from the harsh continental climate that prevailed in the Triassic period's Pangea.
In relation to the Triassic, 20% of the existing land masses were covered by sea during the Jurassic and Cretaceous and transformed into shallow coastal areas. Many areas of Eurasia and Africa were flooded by the Tethys Sea. Europa was an archipelago in the Tethys Sea. The West Siberian Lowland and the Mississippi Valley were shallow seas that connected to the Arctic Ocean.
It is assumed that the temperature of the deep sea in the Cretaceous period was 1℃5, which is somewhat higher than the 2℃ that can be measured today.
It is assumed that the deep sea water in the Cretaceous period was warm. In the shallow coastal areas, which were very common during this period, large amounts of water evaporated, making the remaining water saltier and therefore heavier. The warm, salty and heavy water sank to the bottom and made its way to the deep sea as a warm bottom current. Drawing Anton Uriarte.
Today, the water of the deep sea comes from the polar oceans, where it is cooled and thus relatively heavier, it sinks to the bottom and then flows away from the northern and southern oceans as cold bottom currents. However, it is assumed that in the Cretaceous period the water of the deep sea came from coastal tropical waters. The evaporation in these seas caused the salt content to increase, thereby the heated water became relatively heavier, and it sank to the bottom and flowed into the deep sea as warm bottom currents.
A very similar process takes place at the Strait of Gibraltar today, where heated salt water flows out into the World Ocean along the bottom, while colder and less saline water flows into the Mediterranean as a surface current.
The Cretaceous-Tertiary boundary is clearly seen in Stevns Klint. It contains the so-called fish clay, which contains iridium from outer space. Nowhere else in the world is the Cretaceous-Tertiary boundary so beautifully exposed and easy to study. That is why Stevns Klint is today a UNESCO World Heritage Site. Photo 2020 All you need to know - mungfali.com.
During the latter part of the Cretaceous from about 80 to 65 million years before the present, the CO2 content of the atmosphere fell again to about twice the present level. This happened at the same time as the apparent water level in the World Ocean fell. The temperature dropped, but not so much that ice formed at the poles. The Arctic Ocean was still ice-free - at least in the summer - and the temperature in Greenland, Alaska and Antarctica was still substantially above today's.
Reconstruction of an ammonite, an animal that had lived in the Earth's sea for hundreds of millions of years. They disappeared in the K-T catastrophe along with the plankton they lived on. Ammonites were distant relatives of today's cephalopods, which swam around with a kind of snail shell. Photo gagaru.club.
At the end of the period, the K-T catastrophe occurred (Cretaceous-Tertiary, as it is named after the German Kreide-Tertiary), which was a mass extinction of a number of different species that had hitherto dominated life, in the sea as well as on land. Ammonites, which had lived in the seas since the Devonian, disappeared with a blow, along with many kinds of plankton. All dinosaur species on earth became extinct at one time.
Some believe that the extinction happened quickly, at most within a few thousand years, while others believe that the extinction of the many species happened gradually and started several million years before the end of the Cretaceous period.
Right: The K-T Catastrophe 65 million years before the present. Very beautiful and thought provoking painting but unfortunately unknown artist.
Most believers in a rapid extinction process support the theory that the Earth was struck by a giant meteor about 10 km in diameter that struck the shallow waters off the Yucatan Peninsula in Mexico near a place called Chicxulub, where it created a crater with a diameter of about 180 km. In connection with the impact, a thin layer of iridium from outer space was spread over the entire globe. This thin layer is indeed found all over the Earth in layers from the transition from the Cretaceous to the Tertiary (which in recent times has been divided into the Paleogene and Neogene) and marks the end of the Mesozoic.
But it can in no way be ruled out that the dinosaurs slowly died of starvation, so to speak, because both the temperature and the atmosphere's CO2 content gradually decreased and thus also the vegetable production, which was their food base.
