WASHINGTON - Don't let the hobbling, wobbling, and blubber fool you into thinking elephantseals are merely sluggish sun bathers. In fact, scientists are benefiting from these seals'surprisingly lengthy migrations to determine critical information about Antarctic melting andfuture sea level rise.
A team of scientists have drilled holes through an Antarctic ice shelf, the Fimbul Ice Shelf, togather the first direct measurements regarding melting of the shelf's underside. A group ofelephant seals, outfitted with sensors that measure salinity, temperature, and depth sensors addedfundamental information to the scientists' data set, which led the researchers to conclude thatparts of eastern Antarctica are melting at significantly lower rates than current models predict.
"It has been unclear, until now, how much warm deep water rises below the Fimbul Ice shelf,and previous ocean models, focusing on the circulation below the Fimbul Ice Shelf, havepredicted temperatures and melt rates that are too high, suggesting a significant mass loss in thisregion that is actually not taking place as fast as previously thought," said lead author of thestudy and PhD student at the Norwegian Polar Institute (NPI), Tore Hattermann.
The Fimbul Ice Shelf - located along eastern Antarctica in the Weddell Sea - is the sixth largestof the forty-three ice shelves that dapple Antarctica's perimeter. Both its size and proximity tothe Eastern Antarctic Ice Sheet - the largest ice sheet on Earth, which if it melted, could lead toextreme changes in sea level - have made the Fimbul Ice Shelf an attractive object of study.
The team is the first to provide direct, observational evidence that the Fimbul Ice Shelf is meltingfrom underneath by three, equally important processes. Their results confirm a 20-year-oldtheory about how ice shelves melt that, until now, was too complex to be further investigatedwith models that had no direct observations for comparison. These processes likely apply toother areas of Antarctica, primarily the eastern half because of its similar water and windcirculation patterns, Hattermann said.
The scientists report their findings on June 22 in the journal Geophysical Research Letters, apublication of the American Geophysical Union.
Using nearly 12 tons of equipment, the scientists drilled three holes of an average depth of 230meters (820 feet) that were dispersed approximately 50-100 kilometers (31-62 miles) apart alongthe shelf, which spans an area roughly twice the size of New Jersey. The location of each holewas strategically chosen so that the various pathways by which water moves beneath the iceshelf could be observed.
What the team observed was that during the summer, relatively warm surface waters are pushedbeneath the ice shelves by strong wind-driven currents. While this happens, another processtransports warm water deeper in the ocean towards the coast and below the ice.
Combining with those effects is a process inherent to the cold ocean waters: The freezing pointof water depends on its depth. The deeper the water, the lower its freezing point. Water of aconstant temperature will freeze on the surface but remain liquid (or melt, if it was alreadyfrozen) at a given depth, like at the bottom of an ice shelf. Therefore, there is a slight butcontinuous melting of the Fimbul Ice Shelf's undersides due to this physical phenomenon.
To understand the extent to which these three processes interact and melt the ice shelf, scientistsneeded a detailed record of annual water cycles and circulation around eastern Antarctica. Enternine male elephant seals that swam 1,600 kilometers (about 1,000 miles) from Bouvet Island(written as Bouvetoya in Norwegian), in the middle of the Southern Ocean, to the outskirts of theFimbul Ice Shelf.
Hattermann and his team borrowed the "seal data" from biologists of the Norwegian PolarInstitute, who originally gathered the data during their Marine Mammal Exploration of theOceans Pole to Pole (MEOP) research project, part of the International Polar Year program.
"Nobody was expecting that the MEOP seals from Bouvetoya would swim straight to theAntarctic and stay along the Fimbul Ice Shelf for the entire winter," Hattermann said. "But, thisbehavior certainly provided an impressive and unique data set."
For nine consecutive months, the sensors atop the seals' heads read the temperature and salinityof the waters along the outskirts of the Fimbul Ice Shelf and recorded their changes over time. Tocollect the same amount of continuous data from a ship would not only incur far greater cost butwould be almost impossible during the winter months due to dangerous ice buildup.
From the "seal data", the scientists accumulated enough knowledge concerning the area's watercirculation and how it changes over the seasons to construct the most complete picture of whatand how the Fimbul Ice Shelf is melting from the bottom up.
It turns out that past studies, which were based on computer models without any direct data forcomparison or guidance, overestimate the water temperatures and extent of melting beneath theFimbul Ice Shelf. This has led to the misconception, Hattermann said, that the ice shelf is losingmass at a faster rate than it is gaining mass, leading to an overall loss of mass. The model resultswere in contrast to the available data from satellite observations, which are supported by the newmeasurements.
The team's results show that water temperatures are far lower than computer models predicted,which means that the Fimbul Ice Shelf is melting at a slower rate. Perhaps indicating that theshelf is neither losing nor gaining mass at the moment because ice buildup from snowfall haskept up with the rate of mass loss, Hattermann said.
"Our data shows what needs to be included in the next generation models, in order to be able todo a good job in predicting future melt rates," Hattermann said.
Because wind patterns and water cycles are similar for large parts of eastern Antarctica,Hattermann said, his team's results could help predict the next time when a section of the FimbulIce Shelf, or other ice shelves along the eastern coast of Antarctica, may break off. Because iceshelves are already submerged, their melting does not directly influence sea level rise. However,the rate that ice shelves are melting is still crucial to this issue, he said.
"Ice shelves act as a mechanical barrier for the grounded inland ice that continuously movesfrom higher elevation towards the coast," Hattermann said. "Once an ice shelf is removed, thisice flow may speed up, which then increases the loss of grounded ice, causing the sea level rise."
Source: American Geophysical Union