Fin whale songs, one of the loudest animal calls in the ocean, can be used to learn about the structure of Earth’s crust.
Václav Kuna at the Institute of Geophysics of the Czech Academy of Sciences in Prague and his colleague John Nábĕlek at Oregon State University thought of the idea while they were recording seismic activity from earthquakes off the coast of Oregon.
“I was processing the data and found that there were some signals recorded which I didn’t recognise,” says Kuna. The mystery was solved when he realised the stations were recording songs produced by fin whales (Balaenoptera physalus).
Between 2012 and 2013, the researchers deployed 54 ocean-bottom seismometer (OBS) stations to record seismic activity. Four stations recorded six fin whale songs – patterns of repeated vocalisation – that ranged from 2.5 to nearly 5 hours long.
“The calls travel through the water and penetrate into the ground,” says Kuna. “They then bounce off the layers within the oceanic crust and come back to the surface where we record them at OBS stations.”
If you know the distance between the whale and station, which can be worked out from the frequency of the sound waves, you can measure the returning sound waves and determine the make-up and the thickness of Earth’s crust as they refract and reflect through different layers.
The researchers tested this on the seafloor surrounding the OBS stations and found that the whale songs could show the thickness of the top sediment layers. Their results matched thickness values previously observed by geologists for layers of the same crustal age.
Seismic airguns are conventionally used in a similar way to study Earth’s oceanic crust. These create one of the loudest human-made sounds in the ocean by releasing loud blasts of pressurised air, which can distress animals like whales and disrupt their vocalisations. “Airguns produce noise pollution in the ocean. It’s very expensive and it is not environmentally friendly,” says Kuna.
Although whale songs aren’t as effective as airguns, they could be used to complement existing methods. Airguns emit a broader range of frequencies that can model Earth’s oceanic crust at a higher resolution than the whale songs, but there is potential to use the songs of other whales, such as sperm whales, which have a broader frequency range.
Journal reference: Science Advances, DOI: 10.1126/science.abf3962
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