PALAOA is a hydro-acoustic observatory, or in other words: a specially designed underwater microphone, called a hydrophone, ‘listens’ below the ice shelf and records the calls of seals and whales. The long-term observatory belongs to the Ocean Acoustics Group at the Alfred Wegener Institute in Bremerhaven.
The hydrophone is located near the “northern shore” where Polarstern dropped anchor in January. It was first lowered into the ocean through a hole in the ice shelf in 2005. There’s a previous press release with information on PALAOA, at the AWI website (Link: https://www.awi.de/en/about-us/service/press/single-view/palaoa-worldwide-unique-underwater-acoustic-observatory-celebrates-its-fifth-anniversary-live-sounds-of-seals-and-whales-from-antarctica.html). And anyone interested in whales, the sounds in the Southern Ocean and related topics can find plenty of further reading at the Ocean Acoustics Group website (Link: https://www.awi.de/en/science/climate-sciences/physical-oceanography/main-research-focus/ocean-acoustics-group.html).
Every two or three months, it’s time to switch out the memory cards and batteries in the recording box; as the station radio operator, that’s one of my jobs. My predecessor and I chose a nearly windless day in February to make the trek. The preparations began a few days earlier: requesting freshly charged batteries from the station electrician; inspecting the recording box, complete with memory cards (we always switch out the box and cards together); loading the GPS for the time synchronisation; gathering the necessary tools, and checking in with engineering to see which snowcat we could use.
On the morning of the trip we then loaded the memory cards, batteries (which are massively heavy!), tools, two shovels and some food onto the snowcat and “took off” at a whopping 13 km/h. By snowcat, PALAOA takes roughly an hour and a half to reach. On the way there we had a good view of the local icebergs; thanks to mirage effects, some of them even seemed to float above the horizon. Now that’s what I call a great commute!
Once we’d reached PALAOA, we parked the snowcat beside it to use as wind cover, and got out our shovels. Thanks to the bamboo marking rods, we soon found the spot we were looking for, and got down to work: digging until we hit the sheet of wood covering the metal crate with the recording box inside. We then carefully opened the crate, turned off and removed the old recording device and put new batteries in place. Next we lowered the new recording box with its empty memory cards onto the batteries and turned it on. Then it was time to close the crate, replace the sheet of wood, shovel a bit of snow on top, and the job was done. It may not sound like much, but when you’re wearing bulky polar snowsuits and thick gloves, even the simplest tasks suddenly become much more difficult and tiring.
Next we traced the hydrophone cable from the crate to where it disappeared in the ice. The cable seemed to be fine, which meant it shouldn’t be any problem leaving PALAOA alone again to record for the next few months.
After a ‘picnic on the beach’ (a cup-a-soup, tea and biscuits), despite our warm boots and gloves we slowly started getting cold, so we headed for home. Back at the station, we brought our precious cargo – the memory cards – and the old batteries inside.
It wasn’t until a few days later, when the weather conditions turned poor, that we found time to actually start reviewing what was on the memory cards. Amazing: You really can hear so many different animals. And most of them sound like something from outer space … I spent an afternoon having a closer listen to some of the best passages, and trying to identify what exactly I was hearing.
You can get a rough idea of when something’s happening just by opening up the recordings in any program that shows the changing volume levels. Then you know exactly at which points to listen. But the amplitude, i.e., the volume, can’t show you what you’re hearing. Thanks to a piece of friendly advice, I tried looking at the spectrogram (i.e., the sound frequencies) to help match the sounds to different animals. Soft sounds are blue, red is moderately loud and white is very loud. In addition, low sounds, like deep rumbling, make spikes pointing downwards, while higher ones like whistling make spikes upwards in the spectrogram.
And in fact, this approach allows you to clearly identify some animals. For me, the most distinctive one was the Ross seal, whose siren-like vocalisations produce an unmistakable spectrogram pattern. You can very clearly see how the red and white lines go up and down, up and down, reflecting the ‘siren’ sound that you hear.
With a bit of help from a biologist at the AWI who’s been working with marine sounds for years, I was able to identify some of the samples. Here are a few highlights:
Warm wishes from the ice on the ocean!