Dhe climate in the prehistoric age of our planet was subject to constant changes, some of which were considerable. These natural fluctuations did not happen as quickly as the current man-made climatic capers. But they were still able to move the earth from very hot to extremely frosty states. However, the knowledge about the paleoclimate has so far been quite incomplete, and there have been strong fluctuations in the accuracy of the measured values.
For the first time, an international group of researchers has recorded a picture of the earth’s climate over the past 66 million years based on uniform measurements and consistent interpretations. In this epoch, the New Earth Era, the climate went through four distinctly different phases. In the hottest climatic phase, which climatologists call “hothouse”, the mean temperatures were up to 15 degrees higher than today. In comparison, from a geological point of view, the prevailing climate today resembles a freezer, also known as an “icehouse”.
The 24-member research group led by Thomas Westerhold from the Marum Center for Marine Environmental Sciences (Marum) at the University of Bremen used a total of fourteen drill cores for their investigations that had been drilled in the ocean sediments of various seas over the past two decades. These deposits cover the entire epoch of the New Earth Era, beginning 66 million years ago with the Paleocene Age up to the present day Holocene. Since there are no direct readings for past temperatures, Westerhold’s scientists were dependent on so-called climate proxies, from which earlier temperatures can be derived. Several such indirect indications are found in marine sediments.
Lime shells as temperature probes
In order to make their extensive analyzes as uniform as possible, Westerhold and his colleagues concentrated on the traces of oxygen and carbon isotopes in the calcareous shells of marine microorganisms. However, only two of the more than ten thousand species were of interest to the researchers. These are the foraminifera of the genera Cibicidoides and Nuttalides that live on the seabed. Because these two species existed largely unchanged throughout the Earth’s modern age, they allowed a uniform analysis of the paleoclimate for the entire period since 66 million years.
For their paleoclimatological analyzes, the researchers working with Westerhold use, among other things, a measurement method that goes back to the American Nobel Prize winner in chemistry, Harold Urey (1893 to 1981). It is based on the fact that the three natural isotopes of oxygen occur in three different concentrations. The most common isotope with an average of 99.76 percent is oxygen-16 (¹⁶O) with eight protons and eight neutrons in the atomic nucleus. Oxygen-17 (¹⁷O) with 17 neutrons, on the other hand, is the rarest variant with 0.04 percent. Oxygen-18 (¹⁸O) occurs in nature with an average of 0.2 percent. Since it has two more neutrons than ¹⁶O, it is also a little heavier. When evaporating seawater, oxygen-18 is at a disadvantage compared to ¹⁶O because of its slightly higher weight. In order for it to evaporate, the water temperature must be slightly higher than with the light oxygen variant ¹⁶O.
From this Urey had correctly concluded that cold seawater contains more oxygen-18 than warm water. If, for example, a cold period prevails, the ¹⁸O content in the marine sediment and thus also in the calcareous shells of the foraminifera is slightly higher than the average. If, on the other hand, the earth’s climate is dominated by a hot period, more oxygen-18 evaporates and the proportion of this isotope in the calcareous shells decreases. Differences in the ratio of the isotopes 16O and 18O can therefore be converted directly into the temperature prevailing during a cold and warm phase. You will thus become a climate proxy. Similar conclusions about the prehistoric climate can be drawn from the ratio of the two carbon isotopes ¹³C and ¹²C.
As the scientists working with Westerhold report in the journal “Science”, they have succeeded in using the relationships between the oxygen and carbon isotopes in the sediments to depict the climate throughout the entire modern period in a uniform manner. The global temperature was used as a measure.
The hot planet cooled off
It has been known roughly for a long time that it was particularly hot on earth at the beginning of the Eocene, for example around 45 to 55 million years ago. Since then, our planet has cooled continuously until it was four degrees colder on average during the Quaternary Ice Ages than it is today. From the latest analyzes, the researchers conclude that the mean temperature dropped by up to twenty degrees between the Eocene and the Ice Ages.
In total, the Westerhold scientists identified four distinctive climatic phases. In the first ten million years of the Earth’s Modern Age (Paleocene) and in the period between 50 and 35 million years later (Eocene), the “warm house” climate prevailed, in which it was between four and twelve degrees warmer than today. The extreme hot period during the Eocene was called the “hothouse” phase. At the beginning of the Oligocene, about 34 million years ago, there was then a clear, quite sudden cooling of the earth by about three degrees.
At this time, the ice armor that still exists today began to emerge in East Antarctica. This phase lasted about twenty million years until there was another drop in temperature in the Middle Miocene. The researchers call both epochs “coolhouse”. The coldest section of the New Age to date, the “Icehouse” phase, then begins with the Ice Ages about two million years ago and continues today – regardless of global warming as a result of man-made climate change.