Ice shelves fracture under weight of meltwater lakes

When air temperatures in Antarctica rise and glacier ice melts, water can pool on the surface of floating ice shelves, weighing them down and causing the ice to bend. Now, for the first time in the field, CIRES-led research shows that ice shelves don't just buckle under the weight of meltwater lakes -- they fracture. As the climate warms and melt rates in Antarctica increase, this fracturing could cause vulnerable ice shelves to collapse, allowing inland glacier ice to spill into the ocean and contribute to sea level rise.

"Ice shelves are extremely important for the Antarctic Ice Sheet's overall health as they act to buttress or hold back the glacier ice on land," said Alison Banwell, a CIRES scientist in the Earth Science and Observation Center (ESOC) and lead author of the study published today in the Journal of Glaciology. "Scientists have predicted and modeled that surface meltwater loading could cause ice shelves to fracture, but no one had observed the process in the field, until now."

The new work may help explain how the Larsen B Ice Shelf abruptly collapsed in 2002. In the months before its catastrophic breakup, thousands of meltwater lakes littered the ice shelf's surface, which then drained over just a few weeks.

To investigate the impacts of surface meltwater on ice shelf stability, Banwell and her colleagues from the University of Cambridge, University of Oxford, and University of Chicago traveled to the George VI Ice Shelf on the Antarctic Peninsula in November 2019. First, the team identified a depression or "doline" in the ice surface that had formed by a previous lake drainage event where they thought meltwater was likely to pool again on the ice. Then, they ventured out into the frigid landscape on snowmobiles, pulling all their science equipment and safety gear behind on sleds.

Around the doline, the team installed high-precision GPS stations to measure small changes in elevation at the ice's surface, water-pressure sensors to measure lake depth, and a timelapse camera system to capture images of the ice surface and meltwater lakes every 30 minutes.

In 2020, the COVID-19 pandemic brought their fieldwork to a screeching halt. When the team finally made it back to their field site in November 2021, only two GPS sensors and one timelapse camera remained; two other GPS and all water pressure sensors had been flooded and buried in solid ice. Fortunately, the surviving instruments captured the vertical and horizontal movement of the ice's surface and images of the meltwater lake that formed and drained during the record-high 2019/2020 melt season.

GPS data indicate that the ice in the center of the lake basin flexed downward about a foot in response to the increased weight from meltwater. That finding builds upon previous work led by Banwell that produced the first direct field measurements of ice shelf buckling caused by meltwater ponding and drainage.

The team also found that the horizontal distance between the edge and center of the meltwater lake basin increased by over a foot. This was most likely due to the formation and/or widening of circular fractures around the meltwater lake, which the timelapse imagery captured. Their results provide the first field-based evidence of ice shelf fracturing in response to a surface meltwater lake weighing down the ice.

"This is an exciting discovery," Banwell said. "We believe these types of circular fractures were key in the chain reaction style lake drainage process that helped to break up the Larsen B Ice Shelf."

The work supports modeling results that show the immense weight of thousands of meltwater lakes and subsequent draining caused the Larsen B Ice Shelf to bend and break, contributing to its collapse.

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Gullies on Mars could have been formed by recent periods of liquid meltwater, study suggests

A study led by Brown University researchers offers new insights into how water from melting ice could have played a recent role in the formation of ravine-like channels that cut down the sides of impact craters on Mars.

The study, published in Science, focuses on Martian gullies, which look eerily similar to gullies that form on Earth in the Dry Valleys of Antarctica and are caused by water erosion from melting glaciers. The researchers, including Brown planetary scientist Jim Head, built a model that simulates a sweet spot for when conditions on Mars allow the planet to warm above freezing temperatures, leading to periods of liquid water on Mars when ice on and beneath the surface melts.

The scientists found that when Mars tilts on its axis to 35 degrees, the atmosphere becomes dense enough for brief episodes of melting to occur at gully locations. They then matched the data from their model to periods in Mars history when the gullies in the planet's Terra Sirenum region are believed to have expanded rapidly downhill from high elevation points -- a phenomenon that could not be explained without the occasional presence of water.

"We know from a lot of our research and other people's research that early on in Mars history, there was running water on the surface with valley networks and lakes," said Head, a professor of geological sciences at Brown. "But about 3 billion years ago, all of that liquid water was lost, and Mars became what we call a hyper-arid or polar desert. We show here that even after that and in the recent past, when Mars' axis tilts to 35 degrees, it heats up sufficiently to melt snow and ice, bringing liquid water back until temperatures drop and it freezes again."

The findings help fill in some of the missing gaps on how these gullies formed, including how high they start, how severe the erosion is and how far they extend down the side of craters.

Previous theories suggest Martian gullies were carved by carbon dioxide frost, which evaporates from soil, causing rock and rubble to slide down slopes. The height of the gullies made many scientists theorize that meltwater from glaciers had to be involved because of the distance they traveled down the slopes and how eroded the gullies looked. Proving liquid water could exist on Mars since it disappeared so long ago has been difficult because temperatures typically hover about 70 degrees below freezing.

The results from the new study suggest that gully formation was driven by periods of melting ice and by CO2 frost evaporation in other parts of the year. The researchers found this has likely occurred repeatedly over the past several million years with the most recent occurrence about 630,000 years ago.

They say that if ice was present at gully locations in the areas they looked at when Mars' axis tilted to about 35 degrees, the conditions would have been right for the ice to melt because temperatures rose above 273 degrees Kelvin, equivalent to about 32 degrees Fahrenheit.

"Our study shows that the global distribution of gullies is better explained by liquid water over the last million years," said Jay Dickson, the study's lead author and a former researcher at Brown who's now at California Institute of Technology. "Water explains the elevation distribution of gullies in ways that CO2 cannot. This means that Mars has been able to create liquid water in enough volume to erode channels within the last million years, which is very recent on the scale of Mars geologic history."

Despite doubts about meltwater being possible and scientists never being able to model the right conditions on Mars for ice to melt, the researchers were convinced that the meltwater theory was accurate because they had seen similar features firsthand in Antarctica. There, despite the cold temperatures, the sun is able to heat ice just enough for it to melt and for gully activity to occur.

The new study is a continuation of previous research the team started decades earlier looking at Martian gullies. In a 2015 study, for instance, the researchers showed it was possible that there may have been past periods on Mars when water was available to form gullies if Mars tilted on its axis enough. The findings encouraged them to model what that tilt was and match it with the locations and altitudes of gullies that have formed.

The paper raises anew the fundamental question of whether life could exist on Mars. This is because life, as it's known on Earth, goes hand in hand with the presence of liquid water. Mars will eventually tilt to 35 degrees again, the researchers said.

"Could there be a bridge, if you will, between the early warm and wet Mars and the Mars that we see today in terms of liquid water?" Head said. "Everybody's always looking for environments that could be conducive to not just the formation of life but the preservation and continuation of it. Any microorganism that might have evolved in early Mars is going to be in places where they can be comfortable in ice and then also comfortable or prosperous in liquid water. In the frigid Antarctic environment, for example, the few organisms that exist often occur in stasis, waiting for water."

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Meltwater influences ecosystems in the Arctic Ocean

In the summer months, sea ice from the Arctic drifts through Fram Strait into the Atlantic. Thanks to meltwater, a stable layer forms around the drifting ice atop the salty seawater, producing significant effects on biological processes and marine organisms. In turn, this has an effect on when carbon from the atmosphere is absorbed and stored, as a team of researchers led by the Alfred Wegener Institute has now determined with the aid of the FRAM ocean observation system. Their findings have just been published in the journal Nature Communications.

Oceans are one of the largest carbon sinks on our planet, due in part to the biological carbon pump: just below the water's surface, microorganisms like algae and phytoplankton absorb carbon dioxide from the atmosphere through photosynthesis. When these microorganisms sink to the ocean floor, the carbon they contain can remain intact for several thousand years. As experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) have now discovered, the meltwater from sea-ice floes can delay this process by four months.

From the summer of 2016 to the summer of 2018, the FRAM (Frontiers in Arctic Marine Monitoring) ocean observation system continually gathered data in Fram Strait (between Greenland and Svalbard). Dense clusters of moorings were installed at two sites in the strait in order to monitor as many aspects of the coupled physical-biological processes in the water as possible. Physical, biogeochemical and acoustic sensors throughout the water column and on the ocean floor, as well as devices that gathered water and sediment samples for subsequent laboratory analysis, were used. "For the first time, for two entire years we were able to comprehensively monitor not only the seasonal developments of microalgae and phytoplankton, but also the complete physical, chemical and biological system in which these developments take place," says Dr Wilken-Jon von Appen, a climate researcher at the AWI and first author of the study.

During this period, the sea-ice export reached two extremes: in the summer of 2017, an extraordinarily large amount of ice was transported out of the Arctic through Fram Strait. This produced a great deal of low-saline meltwater and a pronounced stratification of the water. In contrast, uncharacteristically little ice was transported out of the Arctic in the summer of 2018, which meant there was very little meltwater and therefore no pronounced, salinity-based stratification. The processes involved in the biological carbon pump progressed so differently during these two extremes that the experts refer to them as two different regimes: the meltwater regime (summer of 2017) and the mixed-layer regime (summer of 2018).

Meltwater regime in the summer of 2017

The first algal and phytoplankton blooms appeared on 15 May, when the atmosphere began warming the ocean. In the summer of 2017 a great deal of ice drifted through Fram Strait, producing large quantities of meltwater. "This low-saline water lay atop the saltwater without mixing," says von Appen. "And the stratification between 0 and 30 metres was ten times as intense as between 30 and 55 metres." Consequently, very few nutrients made their way upwards from the deeper water layers, while very little carbon made its way to the seafloor. Phytoplankton growth, which is the first step in the biological carbon pump, took place almost exclusively in the top 30 metres. This intense stratification only collapsed in mid-August, when the atmosphere no longer warmed the water's surface. The majority of the biomass drifted down from the upper layer between September and November, was more than three months old, and was too lacking in nutrients to interest fauna at the ocean floor. In the meltwater regime, during the bloom the microorganisms were able to fix up to 25 grams of carbon per square metre.

Mixed-layer regime in the summer of 2018

The spring and summer of 2018 were another story entirely: conditions were relatively ice-free, which meant less meltwater and less intense stratification of the seawater. A mixed layer formed to a depth of ca. 50 metres. With the first of May came the first diatom blooms; at the same time, the numbers of zooplankton, and of the fish that primarily feed on them, began to rise. Thanks to their faeces, only two to three weeks after the start of the bloom, organic carbon reached depths of up to 1200 metres. Four to seven weeks after the start of the bloom -- almost four months earlier than in the summer of 2017 -- the biomass reached the seafloor. This material was rich in nutrients, attracting five times more fish and benthic fauna than in the meltwater summer. During the bloom, the algae were able to fix roughly 50 grams of carbon per square metre, twice as much as in the meltwater regime.

Despite all these differences between the two regimes, the biological carbon pump wasn't necessarily more productive in the summer of 2018: "We found that, in the summer of 2017, the majority of the organic carbon didn't reach the seafloor until after September," says von Appen. "If you look at the period between early May and late November, the carbon export in the mixed-layer regime was only a third higher than in the meltwater regime." Rather, the pronounced stratification in 2017 promoted longer-term growth over several months, since carbon and nutrients were trapped in the upper layers. In contrast, the ice-free situation in 2018 produced a brief, intense bloom and rapid export, providing food and carbon for deep-sea ecosystems on the ocean floor. As such, the latter would seem to particularly benefit from the summertime conditions in the mixed-layer regime; in the meltwater regime, the intense stratification blocks nutrient input in the summer and deep water mixing in the winter.

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