Underwater gliders provide new insights into the impact of a melting megaberg

Underwater gliders from the National Oceanography Centre (NOC) have allowed, for the first time, scientists to collect measurements close to a giant iceberg, giving an unprecedented window into the impact of meltwater on the surrounding Southern Ocean and ecosystem. 

As part of a mission by a team of researchers from British Antarctic Survey (BAS) and NOC, a robotic glider was deployed 23 km from one of the world’s largest icebergs, A-68a, close to the sub-Antarctic island of South Georgia. 

Results from the mission, which used gliders from the UK’s National Marine Equipment Pool (NMEP), operated by and piloted remotely from NOC’s site in Southampton, have now been published in the journal Nature Geoscience. 

The data, gathered in February 2021, provide important new measurements of the effects of iceberg meltwater on the surrounding Southern Ocean.

A challenging Antarctic mission

Gathering data on icebergs is notoriously difficult, but important for understanding complex physical and biological impacts on ocean waters and the ability to predict future ocean circulation and the health of Antarctic ecosystems.

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 – are often missing.

To add to this challenges involved in this glider mission, it was in February 2021, with staff in 'lockdown' piloting the gliders remotely from over 12,000 km. 

“There were challenges, A-68a was constantly on the move, one glider was lost and the second got trapped under A-68a a few times,” says Steve Woodward, glider engineering manager within the Marine Autonomous and Robotic Systems (MARS) Group at NOC. “But it did what it should and emerged 17 days later with the data the scientists needed, to help understand what’s involved as these giant icebergs melt.

“It also achieved another step on the development pathway towards use of gliders close to and underneath icebergs, a high-risk, high-reward activity using autonomy in a very novel environment.” 

A-68 iceberg melting from beneath

The gliders were deployed from the Royal Research Ship James Cook, also operated by NOC. The measurements gathered 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 subsurface 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. 

The first time up so close to an iceberg

Natasha Lucas, a physical oceanographer at BAS and lead author of the study, said: “We think 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.”

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. 

Researchers on the RRS Sir David Attenborough recently collected samples from the iceberg as they transited past as part of the BIOPOLE 2 science mission. These will be analysed back in the UK.

Diver deeper

To read the full paper, visit Nature Geoscience: Giant iceberg meltwater increases upper-ocean stratification and vertical mixing by Lucas, N., et al is published in Nature Geoscience.