The Indian Institute of Science (IISc) has discovered a way to measure seawater temperature by examining the microscopic fish ear bones. Mapping early seawater temperatures is important for a better understanding of earth’s history, said the IISc researchers.

“Oceans cover three-quarters of the Earth’s surface and host many remarkable life forms. Scientists have been attempting to reconstruct the seawater temperature over time, but it is not easy to do so. When you go back in time, you do not have any fossilised seawater,“ said Ramananda Chakrabarti, Associate Professor at the Centre for Earth Sciences (CEaS), IISc.

Otoliths hold the clues

Fish bones or otoliths, are made of calcium carbonate and grow throughout a fish’s lifetime by accumulating minerals from seawater. Like tree rings, otoliths also hold clues to the fish’s age, migration patterns, and the type of water that the fish lived in.

They used a thermal ionisation mass spectrometer (TIMS) to analyse the ratio of various calcium isotopes in the otolith. The ratios of the observed calcium isotopes in the sample can be used to correlate with the temperature of the saltwater where the fish were taken. “We 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,” said Chakrabarti.

In addition to calcium isotopes, the team analysed the concentration of elements like strontium, magnesium, and barium, and their ratios in the same sample, and collated the data together to tease out a more accurate value for seawater temperature within a range of plus or minus one degree Celsius when compared to 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 fossilised samples. Chakrabarti said, “Mapping early seawater temperatures is important to better understand earth’s history, they say. What happened back in time, is key to our understanding of what will happen in the future.”