A brief introduction to... The Periglacial Environment



Great image of ice wedge polygons from physicalgeography.net



Well... not quite the post rate I'd expected after finishing Uni, but work and life take over and before you know it you've not posted anything in over a month! Before I get stuck in, I graduated last week with a 1^st, something I am especially happy with as I had resigned myself to a 2:1 following an uncomfortable feeling after an exam. In other news, I’ve not seen the sun in what feels like months as Britain drowns in the longest intermittent downpour I have ever been subject to. I’m also seriously considering further study/ research; my graduation reminded me how much I miss it already! Enough about me... If I remember rightly, I promised an introduction to the periglacial environment. When I say environment in this context, I mean the specific climatic and geomorphological conditions to allow for a specific suite of processes to operate.

First, (as I'm sure @Dawnitoes will delight in reading) we'll talk about the word "periglacial". The word literally means "near a glacier" and on its inception that is exactly what the word was used for. However, since the processes and features seen in these areas surrounding glaciers were identified in other cold areas in the absence of a glacier, the term eventually evolved. “Periglacial” is now the word used to describe a set of zonal processes that occur in cold environments due to the presence of ice and snow and the repeated freezing and thawing of water. Certain azonal processes can also exhibit distinctive characteristics in periglacial areas. I've visited permafrost before in a previous article (however not formally introduced you... how rude of me). Permafrost – an area of perennially frozen ground – is responsible for many of the processes and landforms associated with periglacial areas; however it is not the defining characteristic of a periglacial area. There is likely to be an article on permafrost in the near future.

So, we have rather hand-wavey, vague explanation of the factors controlling periglacial environments without explaining any processes or resultant landforms associated with places you may term “periglacial”. Or have we? I mentioned in the paragraph above about “the repeated freezing and thawing of water”, which is probably the most important process when we consider most specific periglacial processes and landforms.

In these periglacial environments, the temperature fluctuates about the freezing point of water often diurnally (daily) at the ground surface; and annually (with the seasons) for deeper freezing and thawing. Seasonal snow accumulation and subsequent melting and the movement of groundwater towards a freezing front are also important water processes. As a general statement, these water/ ice processes result in weathering processes acting upon bare rocks and transport processes acting upon sediments.

As I write this, I’m debating whether to name and dissect specific processes and resultant landforms, or whether they warrant their own “brief introductions”... I have an idea. I’ll recommend a reference for you to read (if you wish) and then follow this up, starting with a post on periglacial weathering processes and formations, I’ll formally introduce you to permafrost, finishing with sediment movement processes and distinctive landforms. Anything I don’t catch in either of these will more than likely land in a final summary post...

For your reading, I recommend you read the following book; a comprehensive summary of the processes and landforms typical of periglacial environments:

French, H. M. (2007). The Periglacial Environment. Thirdedition. John Wiley and Sons, Chichester.

A brief introduction to... Glaciation

Glen Nevis. www.thewalkingzone.co.uk

Back again! Today's brief intro is on the evolution of landscapes through the transition from interglacial to glacial. I'll try not to focus too much on global temperature changes or the causes of these, more on a hypothetical landscape. And we're off...

A gradual drop in temperature will result in periglacial conditions and the onset of permafrost. Periglacial processes occur on the surface and subsurface, resulting in patterned ground features, pingos, palsas (in wetlands), sorting of sediments and frost action processes on exposed rocks. 

Precipitation in winter months results in a build up of snow cover, with melting occurring during the summer months. Seasonality is considered one of the primary controls on glaciation. If the net snow accumulation is positive for an extended period of time, snow cover thickness increases. Pressure from the snow overburden causes the transition of snow into firn, the point where the pore spaces between ice crystals become enclosed (more on this in a further post), trapping the air at its present concentration within the firn. Under increasing pressure, firn becomes ice.

Once overburden reaches a critical point, the ice spreads out in all directions from this "ice cap". Just as a river will find the path of least resistance, ice will flow in a direction which is easiest, this could be a relict river valley or any natural depression in the land. The erosive power of a massive body of moving ice is huge. Imagine the base of the glacier includes fragments of rock like giant sand-paper. Through time, this ice carves the classic glacial troughs seen throughout the Yorkshire Dales, Lake District, Snowdonia and Glen Nevis (Ben Nevis is an amazing place to hike; see the picture above and follow the link for more). This is probably the most awe inspiring period to imagine; millions of tonnes of ice grinding away the rock it is forced over by more ice being produced at the ice cap it originated from.

The Last Glacial Maximum (LGM) was between 12 and 14 thousand years ago depending on where you were at the time. It is known as the Loch Lomond Stadial or the Younger Dryas, and was preceded by an interglacial, just as every other glacial within the Quaternary.

Just as these periods capture my fascination due to the pure scale of landscape change, I find the transition from glacial to interglacial, known as the paraglacial period (coined by Colin K Ballantyne), is by far the most interesting. Landscapes just go crazy! This will be covered in my next post, so make sure to check back for that!

I hope you found this interesting. Please have a go at the feedback below, let me know what you think and what you'd like to read about, I'd be grateful for the ideas!

Permafrost Thawing in Alaska and Global Implications

Air temperatures in Alaska across a number of different monitoring sites have increased since the late 1970s. As ice rich permafrost melts, a thermokarst landscape is produced, leaving marshy land interspersed with  shallow lakes due to the settlement of ground following permafrost thaw. In a recent study, Osterkamp (2007) tried to establish a link between mean annual air  temperature and permafrost thickness temperature.

   Osterkamp (2007) found that permafrost temperatures have increased by between 0.3 and >6oC in some areas of northern Alaska because of rising air temperatures and an increase in snow cover. He found complex relationships between surface cover (vegetation and snow), air temperature, local hydrology and geothermal activity with permafrost temperatures. It was observed during the research that thermokarst now exists in areas where it was absent in the 1980s in both northern and interior Alaska.

    At a regional scale, the thawing of permafrost causes problems for ecosystems and infrastructure which require the stability of permafrost. Thawing ground can be a major problem for roads, buildings and can significantly change local hydrology. This is an important issue for Alaskans in particular, with around 80% of Alaska underlain by permafrost (Osterkamp et al., 1998). On a global scale, it seems the thawing of permafrost should be factored into models of climate change and atmospheric carbon. There is thought to be more carbon stored within permafrost than is in circulation in the modern atmosphere (Bowden, 2010), and would form a significant positive feedback loop should this carbon be released into the atmosphere.

    Models produced by Schaefer et al. (2011) suggest a reduction in permafrost area of 29-59% by 2200. This thawing would result in an irreversible contribution of between 126 and 254 Gt of carbon into the atmosphere. It would seem that it's not only those at high latitudes that should be concerned by the thawing of permafrost.

    See also:    http://www.biology.ufl.edu/permafrostcarbon/index.html


    References


Bowden, W. B., 2010. Climate change in the Arctic- permafrost, thermokarst and why they matter to the non-Arctic world. Geography Compass. Vol 4 (10), p1553-1566.


Osterkamp, T. E., Esch, D. C., and Romanovsky, V. E.: 1998, ‘Chapter 10: Permafrost. implications of global change in Alaska and the Bering Sea region’, in Weller, G. and Anderson, P. (eds.), Proceedings of a Workshop at the University of Alaska Fairbanks, 3–6 June 1997, Published by the Center for Global Change and Arctic System Research, University of Alaska Fairbanks, p115–127.


Osterkamp, T. E., 2007. Characteristics of the recent warming of permafrost in Alaska. Journal of Geophysical Research. Vol 112, p1-10.


Schaefer, K., Zhang, T., Bruhwiler, L., Barrett, A. P., 2011. Amount and timing of permafrost carbon release in response to climate warming. Tellus. Vol 63B, p165-180.