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The Eocene was the hottest period in 50 million years. With temperatures 13 ° C hotter than today, with no ice on the poles and mammals just beginning to conquer the world, our planet was a very different environment. But something strange happened. About 50 million years ago the climate changed from a “greenhouse†to a cooler “ice houseâ€, which affected the evolutionary history of flora and fauna.
An ancient shark’s teeth might now help us understand what happened.
Researchers have worked on several theories about what is driving this climate change, with some focusing on Antarctica as it borders on tectonic gates and amplified temperature effects at high latitudes. Tectonic theory states that the expansion of the Australian and Antarctic tectonic plates resulted in increased volcanism and increased CO2 emissions.
Antarctica also played a key role in the eventual cooling – as the oceans around them began to freeze, Antarctic waters sent cold water and ice floes (Antarctica) broadened and deepened during this period.
Studies have shown that the wider and deeper passages would have been necessary so that the waters of the great oceans could come together and the Antarctic Circumpolar Current could form. This current, currently flowing around Antarctica, traps cold water in the Southern Ocean and keeps Antarctica cold and frozen.
In a new study, researchers from the University of California analyzed the chemical chemistry preserved in the teeth of the now-extinct sand tiger shark species Striatolamia macrota. The shark hunted in the waters off the Antarctic Peninsula ten million years ago and left well-preserved fossil teeth on what is now Seymour Island.
“By studying the chemistry preserved in these shark teeth, my colleagues and I found clues as to when the Drake Passage was opened, mixing the Pacific and Atlantic waters, and what the water felt like at that time,” Lead Author Sora Kim wrote in a comment. “The temperatures measured in shark teeth are among the warmest in Antarctic waters.”
Teeth full of information
For the study, the researchers examined 400 teeth from Seymour Island from sharks of all ages that lived 45 to 37 million years ago.
Sand tiger sharks have sharp teeth that protrude from their jaws to grab prey. They have hundreds of teeth in multiple rows and over the course of their lives they lose and regrow thousands of teeth. Every tooth contains important environmental information that is encoded in its chemistry and retained there for millions of years. In essence, a tooth can act as a proxy, giving researchers information about the water in which it was formed.
The outer layer of the teeth consists of an enamel-like layer with oxygen atoms from the water in which the shark lived. The researchers analyzed the oxygen to determine the water’s temperature and salinity, and found that the Antarctic waters remained warmer than previously estimated.
“Perhaps the difference was between waters closer to the surface and deeper on the ocean floor, or the sharks whose teeth we found that spent part of their lives in South America,” wrote Kim.
The modern analog of the sand tiger shark spends summer and early fall between the coasts of Massachusetts and Delaware. When the water cools, they migrate to North Carolina and Florida. The researchers believe that ancient sand tiger sharks also migrated as the Antarctic waters cooled and migrated north to warmer waters.
The carbon dioxide levels in the teeth were 3-6 higher than today, suggesting that the atmospheric C02 levels were also much higher than they are today – which fits an overall higher temperature.
Another finding arose from the element neodymium, which adsorbs and replaces other elements in the outer enamel-like material of the tooth during the early fossilization. His analysis provided researchers with the earliest chemical evidence that water flows through the Drake Passage, which is consistent with existing tectonic evidence. If conditions are stable the neodymium would stay stable over the years, but if the neodymium composition changes it means the oceanography is changing.
“The early opening of the Drake Passage, but the delayed cooling effect, suggests that there are complex interactions between Earth’s systems that affect climate change,” wrote Kim.
Although the sand tiger sharks eventually became extinct, other relatives managed to adapt to the changing conditions. However, current climate events differ from those in the Eocene because they happen much faster and leave less time for adaptation. In addition, current events are also linked to other stressors such as pollution and shrinking ecosystems.
The study was published in the journal Advance Earth and Space Science
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