Freezing the Antarctic

Records of seawater oxygen isotopes and its Ca/Mg ratio shows that a substantial permanent ice sheet first formed in Antarctica in the Oligocene Epoch, about 34 Ma ago.  The favoured explanation, until this month, was that the South polar continent became thermally isolated from the rest of the planet when circumpolar currents were able to flow around it, once South America and Australia had separated from Antarctica and opened the “gateways” of the Drake and Tasmanian Passages.  But what if atmospheric CO2 played a role?  A drop in the “greenhouse” effect and global cooling could have driven polar temperatures low enough for ice formation without an oceanographic influence.  Once established, the albedo effect of a large ice sheet would seal Antarctica into permanent freeze-up.  Factoring all the likely components in a general circulation model leads to a surprise (DeConto, R.M. & Pollard, D. 2003.  Rapid Cenozoic glaciation of Antarctica by declining atmospheric CO2, Nature, v. 421, p. 245-249).  The opening of the Drake and Tasmanian Passages was not accompanied by a sufficient depth of water to support massive current reorganisation until several million years after the ice cap left its clear imprint on the marine record.  DeConto and Pollard’s model shows that even with closed Passages an ice cap would have formed, if CO2 levels had fallen below three times those that prevailed in the Holocene, before industrial emissions began.  Global cooling had begun somewhat earlier than Antarctic freeze-up, following the high around the Palaeocene/Eocene boundary (~55 Ma), falling to a plateau about 40 Ma ago.  Undoubtedly CO2 concentrations had fallen globally for this to have happened.  Of course, there is no Oligocene ice, from which glaciologists might extract trapped bubbles and samples of ancient air with which to refute or confirm the model.  However, a decrease in carbon dioxide would also cause the acidity of rainfall to decrease as well as the amount of rainfall globally, and that might show up in changed weathering processes, especially in the tropics of the time. 

How patterned ground forms

Visiting flat areas of permanently frozen ground brings you face to face with truly bizarre patterns at the ground surface.  Some are perfect hexagons of stones around finer soils, others doughnut-like circles and then a perplexing range of other features that look for all the world as though they were built by humans.  Undoubtedly, they result from the forces at work when the top soil layer freezes and thaws annually, together with soil creep down extremely shallow slopes, repeated over millennia.  However, exactly how the patterned ground develops has eluded geomorphologists for more than a century.  Rejecting the reductionist approach that any landform’s evolution can be deduced from basic principles of physics seems to be the key (Kessler, M.A. & Werner, B.T. 2003.  Self-organization of sorted patterned ground. Science, v. 299, p. 380-383).  Kessler and Werner of the University of California modelled the two likely processes of ice lensing that sorts stones and finer soil, and the transport of individual stones along the lines of accumulated stones as freezing fines expand, building in elements of spatial and time scales plus other parameters such as surface slope.  Their model is self-organising, and proceeds to mimic many of the intricacies of patterned ground, even the most labyrinthine.  It might seem a little heavy handed to crunch numbers to help explain what are really quite minor features.  But having demonstrated the power of non-linear modelling here, the authors open up a novel approach to landscape evolution of every scale and antiquity.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s