The Indian Institute of Science (IISc) has discovered a way to measure seawater temperature by examining the microscopic bones of fish ears. Mapping early seawater temperatures is important for a better understanding of Earth’s history, the IISc researchers said.
“The oceans cover three quarters of the Earth’s surface and are home to many remarkable life forms. Scientists have tried to reconstruct seawater temperature over time, but it’s not easy. When you go back in time, you don’t have fossilized seawater,” said Ramananda Chakrabarti, associate professor at the Center for Earth Sciences (CEaS), IISc.
Otoliths hold the clues
Fish bones or otoliths are made of calcium carbonate and grow throughout a fish’s life by accumulating minerals from seawater. Like tree rings, otoliths also contain clues to the age of the fish, its migration patterns and the type of water in which it lived.
They used a thermal ionization mass spectrometer (TIMS) to analyze the ratio of various calcium isotopes in the otolith. The ratios of calcium isotopes observed in the sample can be used to correlate with the temperature of the salt water where the fish were caught. “We have demonstrated that calcium isotopes are a powerful tracer of water temperature, and our lab is the only one in the country that can actually measure these isotopic variations,” Chakrabarti said.
In addition to calcium isotopes, the team analyzed the concentration of elements such as strontium, magnesium and barium, as well as their ratios in the same sample, and collated the data to determine a more precise value for the sea water temperature within a range of plus or minus one degree Celsius from the actual value.
With the close correlation found between calcium isotope ratios and temperatures, the authors are confident that their approach can now be used on fossilized samples. Chakrabarti said: “Mapping early seawater temperatures is important for better understanding Earth’s history, they say. What happened in time is the key to our understanding of what will happen in the future.
August 11, 2022