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Trees under retreating glaciers

  1. Jan 14, 2009 #1
    Three years ago, I spend some days on the Swiss Alps.
    All the glaciers we visited were in clear recession. Ok, this is a global issue.
    But I was suprised by a little detail in the Ferpècle Glacier, in the Évolène valley, near Sion, Switzerland.

    There were several trees on the river bed, just below the ice edge. A clearly retreating ice edge, one has to say. On the first photo, you'll see the remains of the tree on the river bed.
    The second photo show a general view from a recent morraine. You'll see the same hole on the glacier from which melt waters flow in its context. The area is at 2000m above sea level and you'll see no trees.

    Can anyone explain where these trees came from? I say trees because there were several of them, and we could even see one inside the glacier.
     

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  3. Jan 14, 2009 #2

    turbo

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    Glaciers are slow, but not "frozen" in the sense that they do not move. When they advance, they pretty much knock down and engulf anything in the way. Since the trees knocked down by the last advance are encased in ice and protected from air, the wood can be preserved for a very long time (remember the "ice-man" found in a glacier years ago, with his clothing, tools, and weapons well-preserved?). No mystery.
     
  4. Jan 14, 2009 #3
    Ok. Agree, but let me clarify: nowdays, at 2000m (6500feet?) there are no trees on that area. And as you point, those trees were even higher when the glacier started to grow.

    So the question is: where were those trees growing when the glacier started to grow? And when the glacier started to grow?
     
  5. Jan 14, 2009 #4

    turbo

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    As temperatures change, the borders between boreal forest/tundra change. Apparently, at some time prior to the last glacial advance, it was once warm enough to allow trees to grow at high elevations.
     
  6. Jan 14, 2009 #5

    Xnn

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    About 10,000 years ago, the earths orbit was differant than it is now, in that the perihelion occured during the northern hemisphere summer. This made summers warm enough to melt high elevation glaciers and resulted in Alpine glaciers retreating significantly.

    Since that time, as the earths orbit shifted towards cooler NH summers, Alpine glaciers have been generally advancing. So, those trees most likely grew in the general area between after 10,000 years ago.

    At about 6,000 years ago, the glaciers started to advance and have buried many things.

    It has only been fairly recently, with global warming, that the glaciers have retreated, despite perihilion occuring in the winter, and started exposing areas that were buried.
     
  7. Jan 14, 2009 #6
    So do you really think that those trees are 10000 years old?
     
  8. Jan 14, 2009 #7

    turbo

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    Carbon-dating would tell, though if a previous glacial recession had left them uncovered for many years, they would have rotted. It's probably safe to assume that these trees were buried in the last glacial advance in that region.
     
  9. Jan 14, 2009 #8
    I was wondering if the trees were not so old, and covered by the glacier not much higher than where they are now. The alternative is that they are as old as it seems, but then they must come from much higher!

    Basically, if Little Ice Age was playing a role on this issue, or a proof of a warmer Europe when Rome was ruling.
     
  10. Jan 14, 2009 #9

    Xnn

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    Otzi the ice man was about buried 5300 years ago. So, those trees could be roughly between 5,300 to 10,000 years old; at least that is my guess.

    Besides Otzi, I believe they also found other human belongings under some other alpine glacier that were about 6,500 years old. Apparently, ice is a great preservative. Maybe somebody will publish an actual scientific study them and come up with a precise age.
     
  11. Jan 14, 2009 #10
    Really nothing to speculate before having carbon dates. It could be anything. Remember that there were more trees under the ice. Like this "needle" under the ice core of NGRIP

    [​IMG]

    However it turned out to be willow bark and it was beyond carbon dating. The ice around it was much older than 100,000 years.
     
  12. Jan 14, 2009 #11
    Of course carbon dating would tell, but I do not know if it has been dated.
    For me it is hard to imagine that such big trees suffering ice pressure for 10000 years apear in one piece. Melt waters on that area are milky due to glacier erosion on local rock.
     
  13. Jan 18, 2009 #12

    LURCH

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    Sort of an aside, but...Vivesdn; you're a lot younger than I'd assumed.
     
  14. Jan 19, 2009 #13
    Ok. I should have spent some seconds to crop the picture.
    That kid today is still 6.

    D
     

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  15. Jan 19, 2009 #14
    Really great photos, I love to visit spots like that.
     
  16. Jan 19, 2009 #15

    LURCH

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    I would guess (and that's all it is) that the glacier picked up those trees somewhere below the treeline, and dragged them to their current location.
     
  17. Jan 19, 2009 #16

    mgb_phys

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    Glaciers don't move up the valley as such, they melt at the end so the 'end point' moves up but it doesn't drag anything,
     
  18. Jan 20, 2009 #17
    It's even possible that the Gulf Stream was stronger in the recent past and that this brought even warmer weather than that of today to mainland Europe.
     
  19. Jan 20, 2009 #18

    mgb_phys

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    The climate for the last 20,000 years is pretty well known.
    Current glaciers are generally the remains of the glaciers formed in the last ice age.

    Glaciers gradually flow down the alley, bringing debris with them, as more snow falls at the top. The extent (position of the end) of the glacier does vary during the year (summer-winter) and year to year as temperatures change - although on average they have been retreating.

    The problem is that the ice preserves things rather well so without analysis it's difficult to tell if these trees are 1 year or 1000years old!
    It could be that a few warm decades allowed trees to grow in front of a retreating glacier which then expaned, or some trees might have grown in a sunny sheltered spot above the official tree line and were washed own into the glacier. Or they could even be the remains of some structure built last summer and have only just been covered.
     
  20. Jan 20, 2009 #19

    LURCH

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    Or (as I was speculating) the tree could have grown below the treeline in a valley somewhere "upstream," within the glacier, then been pushed uphill (but "downstream") to where they currently lie.
     
  21. Jan 20, 2009 #20

    mgb_phys

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    The current glaciers are well above the current treeline and presumably since the last ice age the glaciers and the tree line have both been moving up. Thats the confusion.
     
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