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Environment Focus: Scientists estimate the ancient temperature of sea water by probing tiny bones in the ears of fish

Scientists at the Indian Institute of Science (IISc) have identified a way to estimate the ancient temperature of sea water by probing tiny bones in the ears of fish. The oceans cover three-quarters of the Earth’s surface and host many remarkable forms of life. Earth scientists have tried to reconstruct the temperature of sea water over time, but it is not easy.

“When you go back in time, you don’t have any fossilized seawater,” explained Ramananda Chakrabarti, associate professor at IISc and corresponding author of the study published in ‘Chemical Geology‘ in a press release issued by IISc on Wednesday. So he and his PhD student, Surajit Mondal, in collaboration with Prosenjit Ghosh, a professor at CEaS, turned to “otoliths” – tiny bones found in the inner ear of fish.

Like corals, otoliths are made of calcium carbonate and grow throughout the fish’s life by accumulating minerals from the seawater. Similar to age rings, these otoliths also hold clues to the fish’s age, migration patterns, and the type of water in which the fish lived. For several years, Chakrabarti and his team observed calcium carbonate deposits found in tiny animals such as corals or foraminifera. In the current study, they chose otoliths because the researchers discovered fossilized otolith specimens dating back to the Jurassic period (172 million years ago). The researchers used six contemporary otolith samples collected from different geographic locations along the east coast of North America. They analyzed the ratio of different calcium isotopes in these otoliths using a thermal ionization mass spectrometer (TIMS).

By measuring the calcium isotope ratios in the sample, they were able to correlate it with the temperatures of the seawater from which the fish were collected. “We have shown that calcium isotopes are a strong indicator of water temperature, and Surajit’s efforts make our lab the only lab in the country that can actually measure these isotopic variations,” says Chakrabarti. In addition to calcium isotopes, the team also analyzed the concentration of other elements such as strontium, magnesium and barium and their ratios in the same sample and collected data to find a more accurate value of the seawater temperature within plus or minus one degree Celsius compared to the actual value.

Organisms living in the ocean are extremely sensitive to temperature. A temperature increase of two degrees could lead to the extinction of several species, the report says. Additionally, because the atmosphere and ocean “talk” to each other, says Chakrabarti, large amounts of carbon dioxide in the atmosphere eventually dissolve in the ocean, and this ability to dissolve carbon dioxide is also linked to seawater temperature—the lower the temperature, the more carbon dioxide is trapped .

 Like a carbonated drink that loses its fizz when heated, the ocean loses its ability to hold carbon dioxide as it warms. Because of the close correlation they found between calcium isotope ratios and temperatures, the authors are confident that their approach can now be applied to fossilized samples. Mapping early seawater temperatures is important for better understanding Earth’s history, they say. “What happened in time,” says Chakrabarti, “is the key to our understanding of what will happen in the future.”

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