New Ice Core Information

      Last week I attended a conference on the last interglacial.
      Sigfus Johnsen gave a talk on the recently retreived ice core
      from the greenland ice divide (the summit core).  For reasons
      of ice dynamics, a core from the summit will be less distorted
      by flow than the cores taken to date at places like
      Camp Century and Dye 3 which are areas of rapid flow and/or
      strongly sloping bedrock.

      One of the startling results is that in this core, forty thousand
      (yes, 40,000) annual layers can be counted before they grow too
      thin to distinguish.  Forty thousand years and no sign of
      a flood or catastrophic climate change (though plenty of rapid
      change including one cold snap with several degrees of
      cooling, lasting only 70 years or so.  This was also seen
      in camp Century, but not considered very credible by many.
      Now it has been confirmed).

      Deeper in the core we get to ice laid down in the last
      interglacial (circa 120,000 years ago).  According to both
      observations and theory (Milankovitch) this was a time in which
      the northern hemisphere was much warmer than today.  Sure enough,
      at this point the annual layers reappear.  Precipitation was
      much higher at this time (a consequence of the greater capacity
      of air to hold water vapour as temperature increases) and the
      annual layers are still 6mm thick despite the thinning
      induced by the flow of ice.  Roughly another ten thousand
      layers can be counted.  It is possible, though not yet confirmed,
      that more sophisticated isotopic analysis may allow annual
      layers to be counted between these two zones, perhaps back
      to around 70,000 years before present.  This will be quite
      difficult because these layers have been considerably thinned
      by ice flow, and unlike the last interglacial layers, they were
      never that thick to begin with.

      Very preliminary results on this core were published in Nature,
      volume 359, page 311.  Further results should come out in Nature
      in a few weeks.  I think this will make a nice addition to the
      ice core FAQ.

Department of Oceanography                  Bill Hyde
Dalhousie University,
Halifax, Nova Scotia
hyde@Ice.ATM.Dal.Ca or hyde@dalac