Article: Pettit, E.C., Lee, K.M., Brann, J.P., Nystuen, J.A., Wilson, P.S., O’Neel, S. 2015. Unusually loud ambient noise in tidewater glacier fjords: A signal of ice melt. Geophysical Research Letters. DOI: 10.1002/2014GL062950
Marine noise pollution, and ocean noise in general, has received much attention from scientists. In fact, we have many previous Oceanbites posts about this very subject (like this one, or this one, or even more here). Human activity in the ocean is ever increasing and the trend is not likely one to stop. But despite all of this interest, there is a lot about ocean noise that still needs to be learned. For instance, we are only beginning to characterize what “normal” background noises in the ocean are.
A team of researchers, led by Dr. Erin Pettit from the University of Alaska, Fairbanks, set out to study what the ambient, or normal, level of noises are at the ends of glaciers in a fjord. Fjords are geological features found in higher latitude areas where massive glaciers carve out steep canyons and enter the ocean. Fjords that still contain glaciers (or glacierized fjords) are extremely dynamic and active – anyone who’s ever seen a calving glacier can attest to the sheer loudness that’s typical of a glacierized fjord.
With climate change on everyone’s minds, it’s even more imperative that scientists study these regions as they are quickly disappearing with rising temperatures. However, due to the dynamic nature of areas like these, it can be dangerous and very difficult to study these areas, leaving many of their secrets undiscovered.
How they did it
This study involved two major components: field deployment and laboratory measurements. Dr. Pettit and her colleagues use passive acoustic listeners (PALs) to study ambient noise. These PALs consist of underwater microphones (hydrophones) located 70m below sea level and were moored to the sea floor where they continuously measure sound waves produced in the area.
They deployed these PALs in Icy Bay, Alaska, only a couple of kilometers away from two major glaciers (Fig 1). They placed additional PALs in sites in Yakutat Bay, Alaska, and Andvord Bay, Antarctica with similar characteristics and in the open waters of the Bering Sea, which has no glacier or sea ice, for comparison. These PALs recorded sound measurements every 4.5seconds and were left to record data for a year. During deployment and retrieval, the researchers also used ship-based hydrophone measurements of the water near the air-sea interface, or surface water.
Dr. Pettit and colleagues also collected ice chunks from the glaciers and brought them back to the laboratory where they could study the noise-generating events more closely to try and identify the source of any sounds generated during ice melt. They submerged the ice chunks in sea water in a tank with a microphone and video camera for observations. They synced the audio and visual to get a sense of what events were creating what sounds.
What they found
Based on the PAL recordings, the yearly average noise level in Icy Bay, Alaska, was 113 to 127 decibels (dB) (Fig 2). Just for reference, the human ear can detect sound amplitudes from 0 dB (hearing threshold) to 140 dB (pain threshold). Building construction usually only get as high as 105 dB. While ocean noise can vary over a really wide range of loudness (from soft sounds like gently swimming plankton to very noisy events like heavy rain), this level of ambient noise is quite a bit louder than most other ocean noises that have been recorded (Fig 3).
They found that throughout the year, there was a distinctive signal within the ambient noise that fell between 1 and 3 kHz. This signal was also observed in Yakutat Bay where there is also a glacier terminus and Andvord Bay where there are abundant icebergs. Notably, this signal was absent from bays without ice, leading the researchers to conclude that the ice was the source of the ambient noise in Icy Bay.
Laboratory observation of melting ice synced with hydrophone measurement revealed bubbles of air trapped in the glacier was the source of the distinctive signal. As snow accumulates on a glacier, it compresses under the increasing weight and converts to solid ice. During this conversion process, air bubbles get trapped throughout the iceberg. When the glacier (or iceberg) ice melts, it releases those bubbles. The distinctive ambient noise signal is created when the bubbles pinch off from the ice and are released to the water (Fig 4). This pinching off event carries the distinctive 1-3 kHz signal observed in the field.
What does this mean?
This study bring us further along in our efforts to understand the nature of noise in the ocean. Much focus and attention has been placed on the adverse impact anthropogenic noise has on marine ecosystems. However, the authors note that the ambient or normal noises they recorded at glacierized fjords are extremely loud (much louder than some of the human activities that people are worried about).
This, unfortunately for us, does NOT mean that anthropogenic noise pollution is not an issue. Glacierized fjord environments are naturally very noisy and the organisms that occur there have adapted to that noise. In fact, many animals like seals may have actually taken advantage of this noise by using these fjords are refuges from their predators like orcas. These fjords are usually fairly turbid, making it hard for orcas to visually hunt, and the loud ambient noise makes it difficult for orcas to track the seals by hearing. Outside of this particular environment, organisms may not have had to adapt to such noisy conditions. Thus, anthropogenic noise pollution in areas besides glacierized fjords is likely to still be a major concern to marine organisms. But, if we have a better understanding of what ocean noise is like naturally, then we can better understand how anthropogenic noise influences marine systems in general.
Dr. Pettit and her colleagues developed a new approach to studying glaciers at the ocean/glacier interface. Through the process of glacier formation, these air bubbles become uniformly distributed throughout the ice, making it easy to measure and study various processes at this interface. There is a lot of potential for measuring changes in melt rates and ice cover of glacierized fjords simply based on the noises that the glacier makes! This is important, now more than ever, in the face of rapid climate change and increased melting.