We know famously little about the ocean floor, but now scientists are getting a little help learning about it from a most unexpected direction – fin whales. The fin whale’s song is one of the loudest noises made by animals, and audible over immense ocean distances. These soundwaves also penetrate the Earth, and scientists have worked out how to use this to explore the thickness of ocean sediment.
Fin whales (Balaenoptera physalus) are less well known than the slightly longer (and much heavier) blue whales and show-off humpbacks. Yet these are amazing creatures with an astonishing turn of speed. Like other great whales, the males sing complex songs, apparently to attract mates, at frequencies partially too low for us to hear.
The increasingly noisy oceans are thought to be interfering with the fin whale’s recovery from widespread hunting. Among the noises that interfere with ocean life are seismic air-gun surveys Dr Václav Kuna and Dr John Nábëlek of Oregon State University wondered if it was possible to turn this around, to use the fin whale’s own noises to probe the ocean floor, avoiding the need to make more disturbing sounds.
In Science, Kuna and Nábelek report on their first efforts in this direction. In 2012, 54 ocean bottom seismometer stations were used to monitor the Blanco fault off the coast of Oregon. In addition to underwater earth tremors, these stations recorded many songs from nearby fin whales, sometimes lasting up to 10 hours.
Using six of these songs recorded at three stations Kuna and Nábelek compared the soundwaves that traveled to the stations directly and those that penetrated the seafloor to become seismic waves. The seismic waves bounced off boundaries between sediments and basalt, and basalt and lower crust, before reaching the stations. The delay in the reflected waves’ arrival reveals each layer’s thickness.
Helpfully, the whales were in constant motion during their songs, giving the researchers something to work with from almost directly above the seismometer to 20 kilometers (12.4 miles) away.
Putting all this data together the authors calculated sediment thickness of 380 to 650 meters (1,250-2,150 feet) for the seafloor around the stations, lying above 1.8 kilometers (1 mile) of basaltic rocks. The sediment estimate is thinner than conventional measurements taken further north, but the authors think this is unsurprising given the greater distance from the main sediment sources.
Higher pitched sperm whale songs could be used for higher resolution measurements, the authors believe. “Our study demonstrates that animal vocalizations are useful not only for studying the animals themselves but also for investigating the environment that they inhabit,” they said. Fin whales’ range is so broad their songs are widely available for research purposes.
Fin whale songs are so loud (exceeding the volume of a rocket launch) they were thought to be geophysical phenomena or some Cold War Russian activity when first recorded.