A recent paper published in the Journal of Glaciology explores how a team of researchers studied waves in a Patagonian lake to detect glacier calving events at Glaciar Perito Moreno. Calving events occur when an iceberg detaches from the glacier front. Such events produce waves of different magnitudes as the glacier discharges into the ocean or an adjacent lake.
The paper’s lead author, Masahiro Minowa, told GlacierHub that while calving plays a key role in the recent rapid retreat of glaciers around the world, many processes related to calving are still poorly understood because direct observations are scarce and challenging to obtain.
Minowa and his team employed a creative methodology to observe calving events at a distance. Employing four time-lapse cameras and a water pressure sensor, they conducted fieldwork in three separate time periods, roughly one week to three weeks long between 2013 and 2016. 420 events were noted within this relatively short period of time. They also estimated the calving volume using the time-lapse images and maximum wave amplitude.
“We did our field works twice in summer and once in winter so that we could observe the seasonality of calving activity. We also wanted to understand mechanisms driving calving if there are any,” Minowa said.
The researchers categorized the time-lapse images by separating calving events into four groups: 1) Topple, an ice tower toppling into the lake; 2) Drop, an ice block dropping into the water; 3) Serac, a small piece of serac slipping down to the lake; and 4) Subaqueous, an underwater iceberg detachment that floats up to the lake surface.
These images were then scrutinized in great detail. For example, Topple and Drop events were distinguished based on whether crevasse widening occurred; while Subaqueous was differentiated from other subaerial events by noting a relatively large single iceberg appearing without any geometrical change on the glacier front and a lack of sediment inclusion on the surface.
The surface wave profiles corresponding to the events were also examined. Their signals were more complex, making it difficult in some cases to distinguish events on the basis of wave profiles alone.
“Initially, we expected a clear difference in wave frequencies between subaqueous and subaerial events. While we could see some difference in frequencies, we are unsure if this is a result of different calving style,” Minowa explained. Wave frequencies also vary based on the relative location of the event to the sensor, even if it is the same calving style. A larger sample of cases is thus required to confirm the wave patterns associated with different calving events.
However, Minowa stressed the importance of choosing a strategic location for the water pressure sensor, which vastly affects the results and findings of a glacier calving study. He warned that a problem may arise from the instrument’s location. “Since waves’ amplitude decay with distance, you will not be able to detect all of the calving events if you place the sensors too far. So, you need to be close enough to the glacier, and you will easily detect many of them,” he said. Yet, this might limit the scope of the area studied, requiring a balanced consideration.
From the data, the team could see the seasonality of calving activity. Their results showed that calving events were 2.6 times more frequent during the austral Summer (December-March) as compared to Spring (October). Subaerial calving events occurred 98 percent of the time, although Minowa conceded that the dataset was a bit short to confirm any trigger mechanisms.
Following the research, the team is now ready to install new water sensors for a year-round measurement around the glacier in the hope of further understanding calving processes through the use of surface-waves in glacier fronts. This is a step toward reducing glacier melting in Patagonia and the rest of the world.