Scientists sailing on a research cruise in the Amundsen Sea, off the coast of western Antarctica, have found evidence of massive, ancient loss of ice in the region, resulting from contact with warmer seawater. Sediment cores the team collected by drilling in front of the current Cosgrove Ice Shelf indicate that relatively warm ocean waters dissolved the vast ice shelf and even some of the glacier behind it about 2000 years ago, they recently reported. This melting occurred even though air temperatures were too cold to melt the shelf and neighboring ice shelves were stable or enlarging.
The new results indicate that the similar and seemingly unstoppable melting of huge swaths of the West Antarctic Ice Sheet (WAIS) today by relatively warm ocean waters has precedent in an earlier era, members of the research team and other scientists said. Prior to this new drilling, scientists lacked evidence of such melting from undersea warmth in WAIS’s past.
The clues found in sediments deposited during the late Holocene suggest that an ocean current that circles the southern polar region, known as Circumpolar Deep Water, flowed underneath the Cosgrove Ice Shelf and melted it. Today, the shrunken shelf covers an area nearly as large as the state of Rhode Island, but it was much larger before this melting event.
“This circumpolar water isn’t like a hot tub. It’s only about 2°C warmer” than the surrounding ocean, said lead research author Rebecca Totten Minzoni, a paleoclimatologist from the University of Alabama in Tuscaloosa. Still, that’s enough to have a major impact on ice dynamics, she explained.
Minzoni presented the findings, to be published in Holocene, on 15 December at the 2016 American Geophysical Union Fall Meeting.
The new work by Minzoni and her colleagues is extremely interesting and reveals past ocean-forced melting, according to Eric Rignot, a research scientist at NASA’s Jet Propulsion Laboratory and the University of California, Irvine, who studies ice shelf thinning around the Antarctic. Rignot, who was not involved in the new Cosgrove study, told Eos in an email that Minzoni’s study confirms that Circumpolar Deep Water “IS the major factor” for Antarctic ice shelf melting.
Scientists see in present-day West Antarctica that warmer ocean water can also reach beyond ice shelves and infiltrate beneath inland glacial ice. There, it can loosen the ice’s grip at locations, called grounding lines, where ice frozen to underlying outcrops of rock has long anchored the ice sheet to the land, Minzoni said. This detachment from the land can speed the flow of an ice sheet into the sea.
Importance of Ice Shelves
In the Amundsen Sea, ice shelves play an important role in preserving the Western Antarctic Ice Sheet. An ice shelf is a floating extension of a seaward flowing glacier that starts where the glacier reaches the water’s edge. The shelves act like stoppers for ice flow, slowing the glaciers’ drainage. Today, two of the ice sheet’s main conduits to the sea, the Pine Island and Thwaites glaciers, drain about a third of WAIS and end in ice shelves. “Once you lose the ice shelf, you lose the continental ice,” Minzoni noted, adding that accelerated continental ice flow to the sea will undoubtedly contribute to sea level rise.
The Cosgrove Ice Shelf doesn’t provide major drainage today for WAIS as it did earlier in the Holocene epoch, but it still has lessons to teach scientists about how outlet glaciers can react when exposed to warmer deep water, she added.
Proof Is in the Proxies
Minzoni and her fellow scientists had sailed to the Amundsen Sea to study other parts of WAIS, but bad weather turned into good luck, forcing their sediment sampling activities into a more protected area near the Cosgrove Ice Shelf. There, the team ended up collecting sediment cores at depths ranging from about 700 to 1400 meters. They also completed a bathymetric survey of the Cosgrove fjord. The sampling locations ranged from the innermost area of the fjord just seaward from the modern ice shelf to locations farther out to sea that the shelf would have covered 2 millennia ago.
To reconstruct the past evolution of the Cosgrove Ice Shelf, Minzoni and her team examined multiple characteristics of the core sediments, including grain size and layering, carbon and nitrogen levels, and species of diatoms and foraminifera. All of these serve as proxies used to reconstruct environmental conditions that prevailed when the sediments drifted down to the seafloor. The researchers found an increase in the amounts of organics and nitrogen around 2000 years ago, suggesting a phytoplankton bloom in more open water.
Over the same time period, the appearance of a deepwater foraminifera (Bulimina aculeata) in the sediment core indicates that warmer Circumpolar Deep Water flowed under the ice, melting the ice shelf from below. During the last ice age, glacial movements on the western Antarctic’s margins carved channels into the ocean floor that provided conduits for warmer water to easily infiltrate beneath the ice later, said Minzoni. She and her colleagues observed examples of such channels in bathymetric data they collected.
Accounting for Warm Water Intrusions
Until this new Cosgrove Ice Shelf research, scientists had not found indicators of intrusion by warmer water underneath Antarctic ice shelves in the geological record of the Holocene epoch, said Frank Nitsche, a research scientist at Lamont-Doherty Earth Observatory in Palisades, N.Y. He did not participate in the research. Those indicators were “the most important findings of Rebecca Minzoni’s study,” he said.
Nitsche also applauded the coring methodology the researchers used as a means of investigating the history of ice shelves for longer time periods than remote sensing and ship-based observations provide.
According to Minzoni, proxies like foraminifera that signify melting by Circumpolar Deep Water can provide boundary conditions for use in ice-ocean-atmospheric predictive models. “Studying past oceanography and glacial conditions can better constrain the variability of Circumpolar Deep Water and its role in ice shelf stability of the West Antarctic Ice Sheet,” she said.
She suggests that future shelf stability studies should consider the role of the ocean’s influence, like the effects of warm water pulses flowing under the Cosgrove Ice Shelf. “By understanding past ice shelf stability, we might get a glimpse into the future of ice shelves,” Minzoni said.
—Sarah Derouin (email: [email protected]), Science Communication Program Graduate Student, University of California, Santa Cruz