Underwater robotic gliders provide new insights into the impact of a melting megaberg
The pioneering use of underwater robotic gliders has provided new insights into the impact of a giant melting iceberg.
In an ambitious world-first mission, these vehicles were deployed close to iceberg A-68a in Antarctica, providing important new measurements of the effects of iceberg meltwater on the surrounding Southern Ocean and ecosystem.
The findings of the study, which was led by Dr Natasha Lucas, a physical oceanographer at Bangor University’s School of Ocean Sciences, have been published in the journal Nature Geoscience.
The researchers found that meltwater from the surface, sides and base of the giant iceberg cause large changes in the temperature and salinity structure of the upper part of the water column.
Importantly, melt from the base of the iceberg mixes with relatively warm and salty deep water and ‘upwells’, bringing with it nutrients from the deeper ocean and mineral-rich particles from within the iceberg itself.
This additional supply of nutrients stimulates the growth of phytoplankton (microscopic marine plants) in the surface, which forms the base of the highly productive Antarctic food chain.
As the number of calving icebergs is likely to increase due to the impact of climate change, understanding the complex physical and biological impacts on the ocean waters through which they transit is important to predicting future ocean circulation and the health of Antarctic ecosystems
Due to the inherent danger of iceberg proximity to ships, this is thought to be the first time scientists have collected measurements right next to a melting megaberg, robotic gliders sampling where ships cannot, giving an unprecedented window into the impact of meltwater on the surrounding Southern Ocean
Researchers from British Antarctic Survey (BAS), where Natasha was working at the time of the study and is now Honorary Researcher, and the National Oceanography Centre (NOC) deployed a robotic glider 23 km from one of the world’s largest icebergs, A-68a, in February 2021 close to the sub-Antarctic island of South Georgia.
Gathering data on icebergs is notoriously difficult. Large scale movements of giant icebergs can be tracked with satellites, but ships will not get close as smaller scale movements are currently unpredictable. This means the data needed by researchers to develop accurate models – critical for predicting future climate change – is often missing.
The glider collected data on the ocean’s salinity and temperature, along with chlorophyll (a proxy for productivity) and optical backscatter, which measures the particles suspended in the water.
The unique measurements revealed that as the iceberg melts from beneath – a process called basal melting – a layer of water called ‘Winter Water’ - formed in the Austral summer when warmer waters cap cooler winter waters below - is ‘eroded’.
This band of cold water, only present in this time period, provides a barrier between surface and deeper waters, restricting nutrients from reaching surface layers.
By eroding this barrier, nutrient-rich deep waters can rise towards the surface, along with mineral-rich particles, such as iron and silica, from the melting iceberg. These nutrients play a key role in stimulating primary productivity creating food for the charismatic animals that live in the Southern Ocean.
Dr Natasha Lucas said, “We believe this is the first time measurements have been made so close to an iceberg – so it’s really ground-breaking stuff! It was just really exciting to see the data come back and see how the ocean was changing so drastically.
“The number of giant icebergs is increasing with climate change so it’s important that we understand the physical and biological processes that happen as an iceberg of this size melts, often far from its source.
“By mixing up these ocean layers – which are normally very stable in the Antarctic summer – the ocean’s temperature, its salinity and the amount of nutrients are all changed. This ultimately impacts how much heat and carbon is exchanged between our ocean and atmosphere.”
The mission was high-risk; remotely operated robotic gliders are not normally deployed so near to icebergs.
Dr Lucas added, “This mission was far from straight forward. We were piloting the gliders remotely from over 12000 km away, each in our own ‘lockdown’ offices during COVID, relying on infrequent cloud-free satellite imagery to locate the iceberg and smaller icebergs around it.
“A-68a was constantly on the move, we sadly lost one glider, and the second glider got trapped under A-68a a few times. However, after it emerged victorious 17 days later with a wealth of data, we were able to quantify the processes involved as these giant icebergs melt.”
Since the A-68a calved in 2021, several more megabergs have made their way towards South Georgia. Most notable of these is A-23a, which grounded on the island’s continental shelf earlier this year, releasing freshwater and nutrients into the surrounding ocean and impacting the biosphere.