In 1970, a 34-minute album was released by bio-acousticians Katharine and Roger Payne composed entirely of humpback whale songs. A few minutes into the album, phrases begin to coalesce into conversations with voices rising and falling in a strangely familiar rhythm. Some phrases sound like whining teenagers. Others like an elder beginning a story or a family on a trip. At times, though, the songs sound completely otherworldly. The album is called Songs of the Humpback Whale, and it turned people’s ears towards the sounds of the oceans.
Today, whale songs have caught the attention of a different audience: geophysicists studying the ocean floor. Our modern understandings of bathymetry – or underwater topography – have largely come from studies using instrumental acoustic waves onboard ships. As a ship travels through the ocean, an instrument called a transducer sends sound waves through the water and receives the signals that bounce back. Imagine standing at the railing of a ship and flinging one end of a slinky down to the seafloor: when the slinky hits the bottom, it sends a pulse back up to your hand. Now imagine your slinky reaches a layer of mud on the seafloor. It will send a very different signal back to your hand than if it penetrated to a denser material, like the ocean crust. That return pulse can be analyzed to produce profiles of seafloor texture and thickness.
As powerful as this method – called the echo sounder method – is to the study of bathymetry, it comes with a major problem. Roger Payne and others helped us understand that marine mammals use sounds to socialize, navigate, find food, and identify mates. These are just a few possible interpretations of marine mammal vocalizations; it’s still a language we have yet to fully translate. What happens, though, when the ambient noise of the ocean from shipping increases? Global ship density increased by a factor of four from 1992 to 2012 which was linked to a 3 decibel per decade increase in ambient marine noise. That might not seem like a lot, but studies have found that maritime noise negatively interferes with marine mammals. As some biologists describe it, maritime noise is like smog; “it shrinks the perceptual world of whales, fish, and other marine life.” And it turns out, the echo sounder method and other acoustic methods used to study the seafloor can also be detrimental to marine mammals and cause temporary hearing loss, disorientation, and behavioral changes.
Is there a solution? Can geophysicists and oceanographers simultaneously study the seafloor while minimizing the impacts on marine life? From Science, authors Václav Kuna and John Nábĕlek introduce an innovative and less invasive technique for studying ocean seafloor structure by harnessing fin whale songs.
Fin whales (Balaenoptera physalus) are the second largest whale in our oceans, growing up to 75-85 feet long. Fin whales use baleen – hair-like plates made of keratin that hang inside whales’ mouths – to filter feed on krill and small fish. The most important characteristic of fin whales for Kuna and Nábĕlek’s research was their powerful vocalizations – often reaching up to 189 dB, which is louder than a jet engine. These vocalizations have been picked up at ocean-bottom seismometer (OBS) stations which monitor underwater earthquakes. Kuna and Nábĕlek discovered that these OBS stations were not only recording songs directly from fin whales but also the secondary waves reflected up from the seafloor.
Kuna and Nábĕlek studied six fin whale song recordings collected from three OBS stations located near an active fault in the north east Pacific, about 150 km off the coast of Oregon. The authors determined the whales’ paths and distances from the stations based off of the difference between the arrival times of two wave types – the direct wave, registering first at a higher velocity, and the waterborne multiple wave, a series of waves that bounce off the seafloor and sea surface before registering at an OBS station.
In addition to sound waves, the authors identified four distinct seismic waves based on their unique subsurface reflection patterns. The arrival times of these seismic phases at the OBS stations varied because their velocities differed depending on the composition of the layers they penetrated. Again, think of the slinky interacting with seafloor layers that have differing densities. Based on the arrival times of the seismic phases relative to the arrival times of the waterborne wave types, Kuna and Nábĕlek determined that the seismic velocities of the whales’ calls corresponded with wave interactions at sediment, basaltic, and lower crustal layers. These findings aligned well with geophysicists’ previous maps of the bathymetry in the region.
The significance of these results is nothing short of remarkable. Kuna and Nábĕlek harnessed a natural phenomenon – fin whale songs – to better understand geologic features of the seafloor that are completely unrelated to whales. They created a link between two distinct spheres of Earth that no one knew existed. Now as Songs of the Humpback Whale comes to an end, I’m left with an image of whales sharing secrets of the seafloor as they travel across ocean basins.