MSGID: 1:317/3 6274a479   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    In sediments below Antarctic ice, scientists discover a giant   
   groundwater system    
    Previously unmapped reservoirs could speed glaciers, release carbon    
      
     Date:   
         May 5, 2022   
     Source:   
         Columbia Climate School   
     Summary:   
         A team has mapped a huge, actively circulating groundwater system in   
         deep sediments in West Antarctica. They say such systems, probably   
         common in Antarctica, may have as-yet unknown implications for how   
         the frozen continent reacts to, or possibly even contributes to,   
         climate change.   
      
      
      
   FULL STORY   
   ==========================================================================   
   Many scientists say that liquid water is a key to understanding the   
   behavior of the frozen form found in glaciers. Melt water is known to   
   lubricate their gravelly bases and hasten their march toward the sea. In   
   recent years, researchers in Antarctica have discovered hundreds of   
   interconnected liquid lakes and rivers cradled within the ice itself. And,   
   they have imaged thick basins of sediments under the ice, potentially   
   containing the biggest water reservoirs of all. But so far, no one has   
   confirmed the presence of large amounts of liquid water in below-ice   
   sediments, nor studied how it might interact with the ice.   
      
      
   ==========================================================================   
   Now, a team has for the first time mapped a huge, actively circulating   
   groundwater system in deep sediments in West Antarctica. They say   
   such systems, probably common in Antarctica, may have as-yet unknown   
   implications for how the frozen continent reacts to, or possibly even   
   contributes to, climate change.   
      
   The research appears today in the journal Science.   
      
   "People have hypothesized that there could be deep groundwater in   
   these sediments, but up to now, no one has done any detailed imaging,"   
   said the study's lead author, Chloe Gustafson, who did the research   
   as a graduate student at Columbia University's Lamont-Doherty Earth   
   Observatory. "The amount of groundwater we found was so significant,   
   it likely influences ice-stream processes. Now we have to find out more   
   and figure out how to incorporate that into models."  Scientists have for   
   decades flown radars and other instruments over the Antarctic ice sheet   
   to image subsurface features. Among many other things, these missions   
   have revealed sedimentary basins sandwiched between ice and bedrock. But   
   airborne geophysics can generally reveal only the rough outlines of such   
   features, not water content or other characteristics. In one exception,   
   a 2019 study of Antarctica's McMurdo Dry Valleys used helicopter-borne   
   instruments to document a few hundred meters of subglacial groundwater   
   below about 350 meters of ice. But most of Antarctica's known sedimentary   
   basins are much deeper, and most of its ice is much thicker, beyond the   
   reach of airborne instruments. In a few places, researchers have drilled   
   through the ice into sediments, but have penetrated only the first few   
   meters. Thus, models of ice- sheet behavior include only hydrologic   
   systems within or just below the ice.   
      
   This is a big deficiency; most of Antarctica's expansive sedimentary   
   basins lie below current sea level, wedged between bedrock-bound land   
   ice and floating marine ice shelves that fringe the continent. They are   
   thought to have formed on sea bottoms during warm periods when sea levels   
   were higher. If the ice shelves were to pull back in a warming climate,   
   ocean waters could re-invade the sediments, and the glaciers behind them   
   could rush forward and raise sea levels worldwide.   
      
   The researchers in the new study concentrated on the 60-mile-wide   
   Whillans Ice Stream, one of a half-dozen fast-moving streams feeding the   
   Ross Ice Shelf, the world's largest, at about the size of Canada's Yukon   
   Territory. Prior research has revealed a subglacial lake within the ice,   
   and a sedimentary basin stretching beneath it. Shallow drilling into the   
   first foot or so of sediments has brought up liquid water and a thriving   
   community of microbes. But what lies further down has been a mystery.   
      
      
      
   ==========================================================================   
   In late 2018, a U.S. Air Force LC-130 ski plane dropped Gustafson, along   
   with Lamont-Doherty geophysicst Kerry Key, Colorado School of Mines   
   geophysicist Matthew Siegfried, and mountaineer Meghan Seifert on the   
   Whillans. Their mission: to better map the sediments and their properties   
   using geophysical instruments placed directly on the surface. Far from   
   any help if something went wrong, it would take them six exhausting   
   weeks of travel, digging in the snow, planting instruments, and countless   
   other chores.   
      
   The team used a technique called magnetotelluric imaging, which measures   
   the penetration into the earth of natural electromagnetic energy generated   
   high in the planet's atmosphere. Ice, sediments, fresh water, salty water   
   and bedrock all conduct electromagnetic energy to different degrees;   
   by measuring the differences, researchers can create MRI-like maps of   
   the different elements.   
      
   The team planted their instruments in snow pits for a day or so at a   
   time, then dug them out and relocated them, eventually taking readings   
   at some four dozen locations. They also reanalyzed natural seismic   
   waves emanating from the earth that had been collected by another team,   
   to help distinguish bedrock, sediment and ice.   
      
   Their analysis showed that, depending on location, the sediments extend   
   below the base of the ice from a half kilometer to nearly two kilometers   
   before hitting bedrock. And they confirmed that the sediments are loaded   
   with liquid water all the way down. The researchers estimate that if all   
   of it were extracted, it would form a water column from 220 to 820 meters   
   high -- at least 10 times more than in the shallow hydrologic systems   
   within and at the base of the ice -- maybe much more even than that.   
      
   Salty water conducts energy better than fresh water, so they were also   
   able to show that the groundwater becomes more saline with depth. Key said   
   this makes sense, because the sediments are believed to have been formed   
   in a marine environment long ago. Ocean waters probably last reached   
   what is now the area covered by the Whillans during a warm period some   
   5,000 to 7,000 years ago, saturating the sediments with salt water. When   
   the ice readvanced, fresh melt water produced by pressure from above and   
   friction at the ice base was evidently forced into the upper sediments. It   
   probably continues to filter down and mix in today, said Key.   
      
   The researchers say this slow draining of fresh water into the sediments   
   could prevent water from building up at the base of the ice. This   
   could act as a brake on the ice's forward motion. Measurements by other   
   scientists at the ice stream's grounding line -- the point where the   
   landbound ice stream meets the floating ice shelf -- show that the water   
   there is somewhat less salty than normal seawater. This suggests that   
   fresh water is flowing through the sediments to the ocean, making room   
   for more melt water to enter, and keeping the system stable.   
      
      
      
   ==========================================================================   
   However, the researchers say, if the ice surface were to thin -- a   
   distinct possibility as climate warms -- the direction of water flow   
   could be reversed.   
      
   Overlying pressures would decrease, and deeper groundwater could begin   
   welling up toward the ice base. This could further lubricate the base of   
   the ice and increase its forward motion. (The Whillans already moves ice   
   seaward about a meter a day -- very rapid for glacial ice.) Furthermore,   
   if deep groundwater flows upward, it could carry up geothermal heat   
   naturally generated in the bedrock; this could further thaw the base   
   of the ice and propel it forward. But if that will happen, and to what   
   extent, is not clear.   
      
   "Ultimately, we don't have great constraints on the permeability of the   
   sediments or how fast the water would flow," said Gustafson. "Would   
   it make a big difference that would generate a runaway reaction? Or   
   is groundwater a more minor player in the grand scheme of ice flow?"   
   The known presence of microbes in the shallow sediments adds another   
   wrinkle, say the researchers. This basin and others are likely inhabited   
   further down; and if groundwater begins moving upward, it would bring up   
   the dissolved carbon used by these organisms. Lateral groundwater flow   
   would then send some of this carbon to the ocean. This would possibly   
   turn Antarctica into a so-far unconsidered source of carbon in a world   
   already swimming in it. But again, the question is whether this would   
   produce some significant effect, said Gustafon.   
      
   The new study is just a start to addressing these questions, say the   
   researchers. "The confirmation of the existence of deep groundwater   
   dynamics has transformed our understanding of ice-stream behavior,   
   and will force modification of subglacial water models," they write.   
      
   The other authors are Helen Fricker of Scripps Institution of   
   Oceanography, J.   
      
   Paul Winberry of Central Washington University, Ryan Venturelli of   
   Tulane University, and Alexander Michaud of Bigelow Laboratory for   
   Ocean Sciences.   
      
   Chloe Gustafson is now postdoctoral researcher at Scripps.   
      
      
   ==========================================================================   
   Story Source: Materials provided by Columbia_Climate_School. Original   
   written by Kevin Krajick. Note: Content may be edited for style and   
   length.   
      
      
   ==========================================================================   
   Related Multimedia:   
       * Maps_and_images_of_exploration_in_Antarctica   
   ==========================================================================   
   Journal Reference:   
      1. Chloe D. Gustafson, Kerry Key, Matthew R. Siegfried, J. Paul   
      Winberry,   
         Helen A. Fricker, Ryan A. Venturelli, Alexander B. Michaud. A   
         dynamic saline groundwater system mapped beneath an Antarctic   
         ice stream.   
      
         Science, 2022; 376 (6593): 640 DOI: 10.1126/science.abm3301   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2022/05/220505143225.htm   
      
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